Dyke swarms - time markers of crustal evolution

Transcription

FIFTH INTERNATIONAL DYKE CONFERENCE 31.7 – 3.8.2005 – ROVANIEMI, FINLAND
FIFTH INTERNATIONAL DYKE CONFERENCE
"Dyke swarms - time markers of crustal evolution"
Pohtimolampi Wilderness Hotel Polar Circle - Rovaniemi - Finland
31 June – 3 August 2005
PRE-CONFERENCE FIELD TRIP A GUIDEBOOK:
Southern and Central Finland
edited by
Luttinen, A.V.
Department of Geology
P.O.Box 64, FIN-00014 University of Helsinki
Vuollo, J.I.
Geological Survey of Finland
P.O.Box 77 FIN-96101 Rovaniemi
ISBN 951-690-927-2
Organising Committee
Dr. Jouni Vuollo (Geological Survey of Finland)
Dr. Satu Mertanen (Geological Survey of Finland)
Prof. Olav Eklund (University of Turku)
Prof. Eero Hanski (University of Oulu)
Dr. Hannu Huhma( Geological Survey of Finland)
Dr. Pentti Hölttä (Geological Survey of Finland)
Dr. Arto Luttinen (University of Helsinki)
Prof. Tapani Rämö (University of Helsinki)
Dr. Saku Vuori (Geological Survey of Finland)
IDC5 Congress Office
Ms. Marja-Leena Porsanger & Ms. Päivi Mäkikokkila (Rovaniemi-Lapland Congresses, University of Lapland)
Sponsors
Scientific
Large Igneous Provinces Commission
University of Oulu
University of Helsinki
University of Turku
Technical sponsors
University of Oulu
University of Helsinki
University of Turku
Tulikivi
Kivikeskus
Financial sponsors
Academy of Finland
City of Rovaniemi & Municipality of Rovaniemi
European Diamonds PLC
Scandinavian Gold Limited
Kiviteollisuusliitto
Table of contents
Introduction
Itinerary and stop descriptions
Proterozoic mafic dyke swarms of southern and central Finland
1. General geological background
2. Mafic magmatism
2.1. Rifting and breakup of the Archaean craton (2.5–2.0 Ga)
~2.45 Ga dyke swarms
~2.32 Ga dyke swarm and intrusions
~2.2 Ga layered sills and dykes
~2.1 Ga dyke swarms
~1.98 Ga dyke swarm
2.2. Seafloor spreading (1.95 Ga)
2.3. Rapakivi-related mafic magmatism (1.7–1.4 Ga)
Plutons
Dyke swarms and basalts
2.4. Rifting at 1.2 Ga
2.5. Dyke emplacement related to Sveconorwegian orogny (~1.1 Ga)
2.6. Dykes related to the Caledonides (~430 Ma)
2.7. Palaeozoic kimberlite magmatism
Day 1 – STOPS 1.1. – 1.3.
Day 2 – STOPS 2.1. – 2.10.
Day 3 – STOPS 3.1. – 3.3.
Day 4 – STOPS 4.1. – 4.4.
Day 5 – STOPS 5.1. – 5.9.
Day 6 – STOPS 6.1. – 6.8.
References
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Introduction
This fieldtrip was organized as a part of the 5th International Dyke Conference to be held in
Rovaniemi, Finland, July 31 – August 3 (Fig. 1). The purpose of the excursion is to highlight the
geology of various Proterozoic dyke systems, including diabase dyke swarms, the Åva ring complex, microtonalites, kimberlites, and dykes belonging to the Jormua ophiolite complex, in southern
and central Finland.
We depart from Helsinki downtown on July 26. On our 300 km drive to Rauma, we will first
examine the ~1.65 Ga Kopparnäs dyke. In the afternoon we make two stops in order to study the socalled Postjotnian 1.2 Ga dolerite sills and dykes in the Satakunta region. We stay overnight in
Rauma.
On July 27, we take a ferry to the Åland archipelago. Our first target is the 500 m wide 1.6 Ga
old bimodal Korsö dyke in the Åland–Åboland dyke swarm. In the afternoon we focus on the bimodality between granite and shoshonitic magmas and related lamprophyre dykes in the postcollisional 1.8 Ga Åva ring complex. We stay overnight in Uusikaupunki.
The main objective of the third day, July 28, is driving some 500 km to the east. Along our
way, we make quick stops to examine the ~1.7 Ga Häme swarm; one dyke near Orivesi contains
well-preserved basaltic glass. We recover from the strenuous drive in our peaceful lodging at
Kivennapa.
We begin our fourth excursion day, July 29, by examining ~1.85 Ga microtonalite dykes at
Kivennapa and then continue with Finnnish kimberlites at Kaavi. In the afternoon, we visit the
Tulikivi quarry and factory (soapstone products) in Nunnanlahti. We spend the night in Koli.
On July 30, we focus on the ~2 Ga dykes in the Koli area and Varpaisjärvi. We also visit the
Finnish Rock Centre at Juuka before arriving at our lodging in Kajaani.
On July 31, we visit some of the key outcrops of the Early Proterozoic Jormua Ophiolite
complex in the morning and then head towards Rovaniemi. A quick stop to examine the oldest rock
of Europe at Siurua will make the 300 km transit more bearable. We will arrive at the conference
venue, Rovaniemi, in time to participate to the IDC5 ice-breaker party at the Pohtimolampi Wilderness Hotel.
Arto Luttinen and Jouni Vuollo
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Itinerary and stops
DAY 1 - 26 JULY:
07:45
08:00
11:00
12:00
18:00
Name call at Hotel Arthur, Helsinki
Minibuses leave for Kopparnäs
Lunch at Mustio Castle
Fieldtrip in Satakunta area
Lodging and dinner at Hotel Kalatorin Majatalo, Rauma
DAY 2 - 27 JULY:
07:00
08:00
11:05
11:40
18:30
21:00
Breakfast at Kalatorin Majatalo
Minibuses leave for Åva
Ferry to Brändö, Åland
Fieldtrip and lunch on Brändö
Ferry to mainland
Lodging and dinner at Hotel Aquarius, Uusikaupunki
DAY 3 - 28 JULY:
07:00
08:00
11:00
18:00
Breakfast at Hotel Aquarius
Minibuses leave for fieldtrip
Fieldtrip and lunch in Orivesi area
Lodging, sauna, dinner, and lecture at Hotel Kivennapa
DAY 4 - 29 JULY:
07:00
08:00
09:00
09:30
14:00
17:30
Breakfast at Hotel Kivennapa
Fieldtrip starts at Kivennapa
Minibuses leave for Kaavi
Fieldtrip in the Kaavi area
Lunch and guided tour at Tulikivi, Nunnanlahti
Lodging and dinner at Hotel Koli
DAY 5 - 30 JULY:
07:00
08:00
13:00
15:00
18:00
Breakfast at Hotel Koli
Fieldtrip at Kaunisniemi
Lunch and guided tour at Finnish Rock Centre, Juuka
Fieldtrip at Varpaisjärvi
Lodging and dinner at Hotel Kajaani
DAY 6 - 31 JULY:
06:30
07:30
08:00
13:00
18:00
19:00
Breakfast at Hotel Kajaani
Minibuses leave for Jormua
Fieldtrip and lunch in Jormua region
Minibuses leave for Rovaniemi
Arrival at Conference venue
Ice-breaker party
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Stop descriptions
26.7.2005 (Guide: Arto Luttinen –University of Helsinki)
Stop 1.1. Kopparnäs dyke (AL)
Stop 1.2. Hinnerjoki crosscutting dyke (AL)
Stop 1.3. Reposaari dyke (AL)
27.7.2005 (Guide: Olav Eklund –University of Turku)
Stop 2.1. Ring dykes near Björnholma (OE)
Stop 2.2. Bimodal lamprophyre–granite dykes at Åva harbour (OE)
Stop 2.3. Roof pendants, monzonite-granite interactions, pegmatites (OE)
Stop 2.4. Thin tholiitic dykes south of Brändö (OE)
Stop 2.5. Korsö dyke (OE)
Stop 2.6. Leuconorite and labradorite megacrysts (OE)
Stop 2.7. Vesicular granite, hybrid, and snow-flake plagioclase (OE)
Stop 2.8. Gabbroic autoliths (OE
Stop 2.9. Pyterlitic rapakivi granite (OE)
Stop 2.10. Järppilä quartz-feldspar porphyry dyke (OE)
28.7.2005 (Guide: Arto Luttinen –University of Helsinki)
Stop 3.1. Proterozoic basaltic glass in Orivesi dykes (AL)
Stop 3.2. Flow-differentiated Vehkajärvi dyke (AL)
Stop 3.3. Quartzite cobbles in Maijaanvuori dyke (AL)
29.7.2005 (Guides: Matti Tyni; Olli Äikäs –Geological Survey of Finland)
Stop 4.1. Microtonalite dykes at Kivennapa (OÄ)
Stop 4.2. Kimberlite boulders (MT)
Stop 4.3. Kimberlite pipe 2 (MT)
Stop 4.4. Tulikivi soapstone factory
30.7.2005 (Guides: Jouko Parviainen and Jouni Vuollo –Geological Survey of Finland)
Stop 5.1. Olivine (-chromite) cumulate (JV)
Stop 5.2. Layered olivine, chromite, and clinopyroxene cumulates (JV)
Stop 5.3. Clinopyroxenite (JV)
Stop 5.4. Magnetite clinopyroxenite and magnetite gabbro (JV)
Stop 5.5. Layered and laminated magnetite gabbro (JV)
Stop 5.6. Granophyre, marginal types, and basement granite (JV)
Stop 5.7. The 1.97 Ga dyke (JV)
Stop 5.8. The Finnish Stone Centre, Juuka
Stop 5.9. Varpaisjärvi dyke (JP)
31.7.2005 (Guides: Hannu Huhma, Asko Kontinen and Katja Lalli –Geological Survey of Finland)
Stop 6.1. Clinopyroxenite and hornblendite-garnetite dykes (AK)
Stop 6.2. “OIB” metalamprophyre and “E-MORB” dykes (AK)
Stop 6.3. Sheeted dykes (AK)
Stop 6.4. Harzburgite and metarodingitic gabbro dyke (AK)
Stop 6.5. Gabbros, ferrogabros and plagiogranites (AK)
Stop 6.6. Podiform chromitite (AK)
Stop 6.7. Pillow lavas (AK)
Stop 6.8. The oldest rock of Europe at Siurua (HH, KL)
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Figure 1. General geology of southern and central Finland showing the areas to be visited.
Red star depicts the conference venue at Hotel Pohtimolampi.
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FIFTH INTERNATIONAL DYKE CONFERENCE 31.7. – 3.8.2005 – ROVANIEMI, FINLAND
Proterozoic mafic dyke swarms of southern and central Finland
1. General geological background
The Finnish bedrock makes up roughly one third of the Fennoscandian Shield. The bedrock
is divided into an Archaean complex (3.50–2.50 Ga) in the north and east and an Early Proterozoic
(1.92–1.77 Ga) Svecofennian domain which dominates in the central and southern Finland (Fig. 1).
The Archaean basement is mainly composed of ~2.7 Ga granodiorites and tonalities and
interfingered ~2.8 Ga greenstone belts. The 2.58 Ga Siilinjärvi carbonatite complex near Kuopio is
among the oldest carbonatites world-wide.
A sequence of continental sediments and lavas, designated as the Karelian Supergroup, was
deposited on the Archaean basement during successive periods of rifting within the Archaean
craton. At the base there are felsic lavas (Sumi), followed by immature conglomerates, sandstones
and intermediate volcanites (Sariola). Subsequently, a thick aluminium-rich weathered crust was
developed, which indicates low-latitude palaeoposition of the Archaean continent. The hiatus is
overlain by recycled weathered material and quartz-rich sediments (Kainuu) which were deposited
in fluvial systems in small grabens and on the flanks of Archaean crustal blocks. Advanced rifting
led to sedimentation of up to 3–4 km thick quartz-rich sandstones in fluvial and shallow marine
environments (Jatuli) and, finally, marine clay-rich sandstones, clays, iron formations, and
metalligenous black schists (Kaleva).
The Archaean basement and the sedimentary cover were repeatedly intruded by mafic
plutons and dykes during the Early Proterozoic time (2.50–1.98 Ga). Based on current
geochronological data, the intrusions may represent six, or possibly seven, distinctive magmatic
events and indicate periodic intracontinental rifting with intervals of ~50–100 Ma between
subsequent rifting events. These rifting events culminated in the formation of a passive continental
margin as the Archaean craton broke up at ~2 Ga. The now-separated “western” cratonic block has
not been identified, although Archaean blocks in northern Sweden may represent reassembled parts
of it. Obducted remnants of oceanic crust (eg. 1.95 Ga Jormua ophiolite) manifest a transition from
continental rift-related magmatism to seafloor spreading after the break up.
Isotope geological studies have established a suture between the Archaean and the
Svecofennian domains. The origin and evolution of the 1.9–1.8 Ga Svecofennian orogenic crust
has remained somewhat controversial. The Svecofennian domain of central and southern Finland
has been traditionally divided into three tectonic units based on lithological criteria and U-Pb
zircon age data. These units form 1) the primitive arc complex of central Finland adjacent to the
Archaean craton, 2) the accretionary arc complex of central and western Finland, and 3) the
accretionary arc complex of southern Finland (e.g. Korsman et al., 1997; Nironen, 1997) (Fig. 2).
Much of the accretionary arc complex of central and western Finland is occupied by the 40 000
km2 Central Finland Granitoid Complex (Elliott et al., 1998; Nironen et al., 2000). The generally
accepted concept of a semi-continuous Svecokarelian/Svecofennian orogeny has been recently
rejected by Lahtinen et al. (2003) who have put forward a more complex orogenic model for the
Early Proterozoic evolution of the Svecofennian domain. The model involves >2.0 Ga
microcontinents, pre-1.92 Ga island arcs, and 1.90–1.82 arcs that accreted to the Archaean craton
to form the Fennoscandian Shield. The following pre-1.92 Ga components have been proposed:
Karelian, Kola and Norrbotten Archaean cratons; Keitele, Bergslagen and Bothnia >2.0 Ga
microcontinents; Kittilä ~2.0 Ga island arc; and Savo, Knaften, Inari and Tersk ~1.95 Ga island
arcs. The assembly of the cratons and microcontinents at 1.92–1.88 Ga to form a larger
Fennoscandian continental plate was followed by a continental extensional stage along the
Archaean-Proterozoic boundary and in the central part of the continent at 1.88–1.85 Ga. Collisions
of Fennoscandia with Sarmatia and Amazonia have been envisaged at 1.85–1.79 Ga. The orogenic
cycle was terminated by an orogenic collapse and stabilization stage (1.79–1.77 Ga).
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Figure 2. Schematic map of the Finnish bedrock (Korsman et al., 1999)
The Middle Proterozoic (1.65–1.54 Ga) rapakivi granites of southern Finland represent the
predominant post-orogenic rock type. Little is known about the ~100 Ma interlude between the
Early Proterozoic orogenic stage and the emplacement of the bimodal rapakivi associations. The
Svecofennian mountain chain had already been eroded down to its roots when processes initiated in
the Earth’s mantle within Fennoscandia led to the generation and ascent of voluminous mafic
magmas and anatectic rapakivi magmas into the crust and to the surface. The emplacement of
rapakivi associations was periodic and took place between 1.65 and 1.53 Ga; some of the diabase
dykes may be marginally older at 1.67 Ga. The principal magma source of the rapakivi plutons has
been ascribed to lower Svecofennian crust (Rämö, 1991). Mixing and mingling of mafic and felsic
magmas has been limited on large scale apart from the Jaala-Iitti complex (Salonsaari, 1995).
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Figure 3. General geology of southern and central Finland showing the distribution of ~2.5
Ga to 0.4 Ga intrusions. The pre- and post-symposium excursion routes are indicated by blue
and red lines, respectively. Red stars depict excursion stops.
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2. Mafic magmatism
The mafic dyke swarms in Finland have traditionally been divided into three main groups
(e.g., Aro & Laitakari, 1987): (1) the Jatulian diabase dykes and sills (>1.9 Ga), (2) the Subjotnian
diabase dykes (~1.6 Ga), and (3) the Postjotnian diabase dykes (~1.3 Ga) (Fig. 3).
The Jatulian magmatism was associated with multiple rifting episodes which culminated in
the break up of the Archaean craton. The onset of rifting was manifested by the emplacement of
mafic dykes and voluminous layered intrusions at 2.50 Ga and 2.45 Ga in eastern and northern
Finland and in the Kola Peninsula. Dykes and plutons which have been dated at 2.32 Ga, 2.2 Ga,
2.1 Ga, 2.05 Ga, and 1.98 Ga have been associated with a sequence of intrusive events which
eventually led to the generation of a passive continental margin.
Obducted remnants of ~1.95 Ga oceanic lithosphere in Jormua provide an insight into a
period of seafloor spreading which was terminated by the accretion of juvenile volcanic arcs to the
rifted margin of the Archaean craton at ~1.9 Ga. During the Svecofennian orogen (1.9–1.8 Ga),
several lithospheric terranes were amalgamated and provided the essential building blocks of the
Fennoscandian Shield.
After some ~100 Ma of erosion, a distinctive period of rifting and anorogenic bimodal
magmatism commenced at 1.7 Ga and lasted until 1.4 Ga. The last major period of crustal growth
took place at ~1.3 Ga when the Jotnian sedimentary basins were intruded by voluminous diabase
sills and dykes. This event and the subsequent intrusion of ~1 Ga diabase dykes in the northern
Finland (e.g. Salla dyke; site 7.3. post-symposium excursion) may have originated in response to
the Sveconorwegian orogeny.
2.1. Rifting and breakup of the Archaean craton (2.5–2.0 Ga)
Several episodes of dyke emplacement during the period 2.5–1.98 Ga have been indicated by
U-Pb zircon geochronology (cf. Vuollo, 1994). The following groups have been identified based on
age data, geochemical composition, and mode of occurrence: 2.5, 2.45, 2.32, 2.2, 2.1, and 1.98 Ga.
The oldest group is only found in the Kola Peninsula, whereas the others are found across the
Karelian province. The ~2.45 Ga dyke swarms can be divided into five subgroups based on their
field relationships and geochemical and isotopic characteristics: (1) NE-trending boninite-noritic
dykes, (2) NW-trending gabbronorite dykes, (3) NW-trending tholeiitic dykes, (4) NW- and Etrending Fe-tholeiitic dykes, and (5) E-trending orthopyroxene-plagioclase phyric dykes. The
younger dyke swarms (2.32–1.98 Ga) form a homogeneous group in terms of geochemical
composition and, in general, resemble continental tholeiitic basalts.
~2.45 Ga dyke swarms
The most conspicuous products of the ~2.45 Ga magmatism are undoubtedly the mafic
layered intrusions, which are known for their Cr and PGE ores and include the TornioNäränkävaara belt and the Koitelainen and Akanvaara intrusions in Finland and also the Oulanka
complex (Lukkulaisvaara, Tsipringa, and Kivakka intrusions) near the Finnish border in Russia,
and the Burakovsky intrusion east of Lake Onega (Alapieti et al., 1990). Mafic dykes of the same
age are also found throughout the Archaean areas. One distinctive feature of the 2.45 dyke swarms
compared with the younger dyke swarms is their geochemical variability. The boninite-norite
dykes, the gabbronorite dykes, and the orthopyroxene-plagioclase phyric dykes have been grouped
as the BN dykes based on their calc-alkaline affinities and a general compositional similarity. The
BN dykes have much higher SiO2 (50–57 %) and generally higher Mg-number than the tholeiitic
dykes. They are characterised by variably high LREE/HREE ratios, with (La/Yb)N of 3–11. Results
of Sm-Nd isotopic studies show that the BN dykes (no data for the orthopyroxene-plagioclase
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phyric dykes are available) have negative initial ,Nd values (– 2.4 to –1.8), which correspond to
values previously obtained for the layered mafic intrusions (Huhma et al., 1990; Turchenko et al.,
1991; Amelin et al., 1996; Saini-Eidukat et al., 1997; Hanski et al., 2001). The tholeiitic dykes are
usually slightly LREE-enriched, but may also show flat or LREE-depleted patterns. Isotopic data
for the tholeiites show slightly positive initial ,Nd values (+0.3 to +1.7). These results suggest that
the ~2.45 Ga dykes may be consanguineous with the 2.45 Ga intrusions.
~2.32 Ga dyke swarm and intrusions
Until recently, there have been few indications of ~2.3 Ga magmatic events in the eastern
part of the Fennoscandian Shield (Paavola, 1988). However, Huhma et al. (1990) obtained a SmNd isochron age of 2330±180 Ma for the Runkaus metalavas in the Peräpohja area and new
unpublished age determinations (Vuollo et al., 2000; Juha Nykänen, pers. comm., 2003; Jorma
Paavola, pers. comm., 2003) indicate that the ~2.32 Ga magmatism represents a significant
magmatic event in the eastern part of the shield. The areal distribution of the 2.32 Ga dykes and
intrusions is difficult to estimate because of few available ages and the fact that these igneous rocks
are geochemically inconspicuous.
~2.2 Ga layered sills and dykes
The ~2.2 Ga “karjalitic” layered sills and intrusions form a conspicuous mafic-ultramafic
magmatic suite that is wide-spread in the eastern and northern parts of Finland, including all the
Karelian schist belts. These differentiated sills and intrusions have been called by a variety of
names: karjalite (Väyrynen, 1938; Vuollo & Piirainen, 1992), hypabyssal spilite (Piirainen 1969),
and the gabbro-wehrlite association (Hanski, 1986). Various alternatives have also been proposed
for the parental magma, including olivine basalt (Meriläinen, 1961), tholeiite (Piirainen, 1969), Fepicrite (Hanski, 1986), and low-Al tholeiite (Vuollo & Piirainen, 1992). Age determinations for
karjalitic sills from Peräpohja, central Lapland, Kuusamo, Kuhmo, and North Karelia allow us to
conclude that the emplacement of the karjalitic sills and dykes took place at ~2220 Ma.
Stratigraphically, the 2.2 Ga sills and intrusions are restricted to the vicinity of the unconformity
between the Archaean basement and Early Proterozoic metasediments, and they are found within
both of them. Most often they lie parallel to the overlying Karelian metasediment series (Karelian
belt) and are thus referred to as sills. Most typically, they are found as sills varying from a few
kilometres up to 150 km in length and are quite thin (200–400 m) relative to their length. Later
tectonic movements have fragmented the originally continuous intrusions. As exemplified by the
Koli layered sill, the karjalitic intrusions contain only one magmatic cycle and were crystallized
from a single magma pulse that crystallized into a highly differentiated structure with a simple
internal stratigraphy.
Geochemically, the karjalitic sills form a peculiar magmatic series. The composition of the
cumulates, granophyre, and the chilled margin is consistent with an overall LREE-enriched
character of the parental magma. This was a low-Al tholeiite (~10 % Al2O3) rich in iron (~13 %
FeOtot) and LREE [(La/Yb)N = 5.8]. A low Al2O3/TiO2 ratio of 5 to 6 is also a characteristic
feature of the 2.2 Ga layered sills.
~2.1 Ga dyke swarms
Emplacement of the ~2.1 Ga Fe-tholeiitic dykes was a wide-spread magmatic event that
affected all parts of central and northern Finland. It gave rise to a dense, predominantly NW- and
E-trending dyke network that now intersects the Karelian formations and the Archaean basement
(Fig. 3). New ages from the Kuhmo block and central Lapland show that there were many stages of
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dyke intrusion between 2.1 and 1.98 Ga. Regional differences can be observed; e.g., the
nortwesterly orientation is dominant in the North Karelia and Kainuu schist belts, whereas in the
Archaean basement of the Kuhmo block, the main orientation is to the east (Anttila et al., 1991;
Kilpelä, 1991). The scarcity of outcrops mean that the distribution of the dykes in Lapland must be
estimated based on their stratigraphic position and geochemistry. Available material suggests that
the Fe-tholeiitic dykes are preponderant also in Lapland. In the Archaean basement (Kuhmo block),
the Fe-tholeiitic dykes form a highly regular swarm in which individual dykes vary from a few
centimeters up to 200 m in width. They can be followed for a few hundred meters to several tens of
kilometers along the strike. On a large scale, the dyke swarms have an ‘en echelon’ structure. The
dykes are homogeneous in their modal composition, contain principally hornblende and
plagioclase, and have been referred to as metadiabases (e.g., Piirainen, 1969; Pekkarinen, 1979). In
the northern part of the Kuhmo block (on the eastern side of the Kuhmo greenstone belt), these
~2.1 Ga Fe-tholeiitic dykes have preserved their primary mineral composition. Geochemically, the
Fe-tholeiitic dykes form a relatively homogeneous group from North Karelia through Kainuu to
Lapland (Vuollo et al., 1992; Vuollo, 1994). They are quartz-normative, sub-alkaline tholeiitic
basalts and form a set of continental dyke swarms of the type frequently found in shield areas.
Several U-Pb ages from North Karelia, Varpaisjärvi, and Kuusamo point to an overall age of
~2.1 Ga for the emplacementof the Fe-tholeiitic dykes. New U-Pb ages from central Lapland
(Rastas et al., 2001; Räsänen & Huhma, 2001) show that a significant magmatic event occurred
also at ~2.05 Ga. This event is also registered by the Keivitsa and Otanmäki intrusions.
~1.98 Ga dyke swarm
Previous studies (Vuollo, 1994) show that the ~1.98 Ga dyke swarm is not as voluminous as
the ~2.1 Ga dyke swarm. However, recent geochronological (Vuollo & Huhma, 2004) and field
studies, combined with aeromagnetic data, show that the ~1.98 Ga dykes are found throughout the
Archean basement and the Karelian formations (Fig. 3). The swarm consists of up to 70 m wide,
coarse- to medium-grained, ophitic dykes that form prominent NW-trending, linear features >120
km in length (Kuhmo block). The 1.98 Ga dyke swarm is the youngest and least deformed Early
Proterozoic mafic dyke swarm observed within the Fennoscandian Archean craton and is closely
connected to the 1.95 Ga ophiolites (Kontinen, 1987; Vuollo et al., 1992; Hanski, 1997). A U-Pb
age data are somewhat scattered (1972±5 Ma to 1995±9 Ma; Vuollo et al., 1992 and Rastas et. al.,
2001, respectively) but show that the ~1.98 Ga swarm is present throughout the eastern part of the
Fennoscandian Shield. According to Vuollo et al. (1992), the 1.98 Ga dykes are Fe-tholeiitic to
tholeiitic in composition, have continental affinities, and are geochemically indistinguishable from
the older tholeiitic dyke swarms. The 1.98–1.95Ga mafic magmatic events were extremely
significant for the ore-forming processes of the Fennoscandian Shield, as indicated by the orebearing ophiolite formations (a breakup event at ~2.0 Ga) in Finland.
2.2. Seafloor spreading (1.95 Ga)
Opinions differ with regard to the exact timing of the final break up of the Archaean Craton.
At 1.95 Ga, the continental rifting and associated magmatism had given way to seafloor spreading.
The Jormua ophiolite complex (Kontinen, 1987; Peltonen et al., 1996) is a rare example of Early
Proterozoic oceanic lithosphere. It consists of two 1–4 km wide and >25 km long tectonic blocks
that interfinger with the Archaean basement and its Early Proterozoic dedimentary cover. The
~1000 m thick sequence includes most of the stratigraphic parts of oceanic lithosphere including:
1) serpentinite, 2) gabbroic intrusions, 3) sheeted dykes, 4) pillow lavas, and 5) pelagic sediments.
Metatronhjemites akin to modern oceanic granitoids have also been found at Jormua. The
6
serpentinites host various mafic dykes, some of which exhibit affinities to ocean island basalts
(OIB).
2.3. Rapakivi-related mafic magmatism (1.7–1.4 Ga)
Large volumes of mafic magma intruded the Svecofennian crust during the rapakivi
magmatic period at 1.7–1.4 Ga. Only a few pluton-size intrusions are visible at the current erosion
level, whereas rapakivi-associated dykes have been found across southern Finland. They are most
conspicious to the northwest of the Wiborg rapakivi massif and in the Åland archipelago where
they comprise distinctive swarms.
Plutons
The majority of the mafic rapakivi-related plutonic rocks are leucocratic gabbros. Troctolitic
and anorthositic varieties have also been discovered. The mafic plutonic rocks are found either as
inclusions within the rapakivi plutons or as small intrusions on their margins. Geochronological
studies indicate indistinguishable intrusion ages for the felsic and mafic plutonic rocks of the
rapakivi areas (Suominen, 1991; Vaasjoki et al., 1991; Alviola et al., 1999). The Ahvenisto
gabbro–anorthosite complex is the largest of the exposed plutons and surrounds the Ahvenisto
rapakivi pluton at Mäntyharju northwest of the Wiborg rapakivi area. The complex is typified by
coarse-grained leucogabbronorite, which contains scattered large (up to 10 cm) plagioclase
crystals. Olivine-bearing mafic plutonic rocks (olivine leucogabbronorite and troctolite) are found
in the northwestern part of the complex. Anorthosite is found as lens-shaped, well-defined (sharp
contacts) up to 200 m long bodies in the more mafic rock types of the complex. Recent seismic
studies indicate a voluminous gabbro–anorthosite pluton beneath the Wiborg batholith (Korja,
1995). Based on magnetic and gravimetric surveys, Elo et al. (1996) have suggested that the
Ahvenisto gabbro–anorthosite complex is a small outcropping part of this pluton. Relatively small
rapakivi-associated mafic plutonic rocks are also found (~0.2 – ~2 km) within the central part of
the Wiborg batholith and are associated with the Suomenniemi pluton, the Laitila batholith, and the
Åland batholith.
Monzodiorites, quartz monzodiorites, and dykes from the Ahvenisto gabbro–anorthosite
complex (Johanson, 1984; Alviola et al., 1999) are rare examples of rapakivi-associated
intermediate rock types. Based on cross-cutting relationships, the intermediate dykes are younger
than the gabbros and anorthosites, but older than the rapakivi granites. Intermediate rock types
have been also described from the Åland batholith (Eklund et al., 1994).
Dyke swarms and basalts
Evidence for rapakivi-related volcanic activity is found on the Island of Suursaari (Hogland),
near the southern margin of the Wiborg massif. The Early Proterozoic Svecofennian bedrock is
discordantly overlain by an undeformed conglomerate, a thin layer of pyroclastic material, basaltic
and andesitic lavas, and a second layer of pyroclastic material underneath a capping, over 100 m
thick unit of quartz-feldspar porphyries.
Rapakivi-related dyke rocks are wide-spread in southern Finland (Fig. 4). Northwest of the
Suomenniemi rapakivi pluton is a NW-trending, over 80 km long swarm of tholeiitic diabase dykes
(Rämö, 1991). Some of the intrusions belonging to the Suomenniemi dyke swarm contain alkali
feldspar ovoids, which suggests that the magmas have passed through a silicic magma chamber.
Locally, mingling structures indicate passage of mafic and felsic magmas through the same
fissures. The 1.65 Ga Lovasjärvi mafic intrusion is a ~5 km long and 800–1500 m wide vertical
sheet-like body that is composed of diabase, olivine diabase, and melatroctolite (Alviola, 1981;
7
Siivola, 1987). The Häme dyke swarm extends ~150 km NW of the Wiborg rapakivi batholith.
Two sets of dykes have been identified based on different strikes and compositions, and
geochronological data. The older 1665 Ma old (Vaasjoki & Sakko, 1989) set is ~100 km long,
strikes west-northwest, and is typified by abundant olivine (Laitakari, 1987). The younger ~1645
Ma old set is ~150 km long, strikes northwest, and is typified by phenocrysts, megacrysts (up to
<20 cm), and fragments of plagioclase (Laitakari, 1969). The younger dykes are mainly olivine
tholeiites, but they are more evolved than the older dykes and include also quartz tholeiites.
Noncrystalline glass has been discovered in the tails of some dykes (Lindqvist & Laitakari, 1980).
Mafic dyke rocks have been found near the small rapakivi plutons around Helsinki (Onas, Bodom,
Obbnäs) (e.g. Törnroos, 1984; Kosunen, 1999). Correlation of these undated dykes with the
rapakivi granites and the Häme and Suoenniemi swarms is not straightforward, but geochemical
and palaeomagnetic data (Satu Mertanen, personal communication, 2005) lend support to the
interpretation that they were emplaced coevally with the rapakivi granite magmas.
Numerous mafic dyke rocks between Åland and Turku (Ehlers & Ehlers, 1977; Suominen,
1987, 1991), in the southwestern part of the Åland rapakivi batholith (Eklund, 1993), in the Laitila
batholith (Haapala, 1977), and near the town of Pori north of Laitila (Pihlaja, 1987) are believed to
be coeval with rapakivi granite plutons. One of the best known examples is the 35 km long Föglö
swarm between the Åland and Kökarsfjärden rapakivi plutons. The en echelon dykes have been
dated at ~1570–1540 Ma. Similar to the younger set of diabase dykes in the Häme swarm, many of
the Föglö dykes contain large plagioclase fragments, even several tens of centimeters in diameter.
Figure 4. Distribution of rapakivi granites and Subjotnian diabase dyke swarms in southern
Finland.
2.4. Rifting at 1.2 Ga
The emplacement of bimodal rapakivi associations was the latest significant period of crustal
growth in Finland and the Fennoscandian Shield in general. Evidece for younger igneous activity is
sparse. The late evolution, which has been influenced by the Sveconorwegian and Caledonian
orogenies, is divided into distinctive phases on the basis of characteristic rock record and tectonic
significance. After the rapakivi magmatic cycle, thick fluvial deposits began to fill the developing
intracratonic rift basins. Remnants of these so-called Jotnian sequences are preserved in the
tectonic depressions and graben structures at Satakunta and Muhos. The intrusion of diabase sills
and dykes into the Jotnian sedimentary rocks at 1265 Ma is the most important post-rapakivi
8
magmatic event. These “Postjotnian” intrusions represent the eastern part of a wider ~1300 Ma
mafic igneous province that extends to central Sweden. The origin of this province has been
associated with initial crustal extension of the Sveconorwegian orogeny (~1300–900 Ma).
In the Satakunta region, extensive, vertical to subhorizontal ~1265 Ma “Postjotnian” dykes
cut the Svecofennian granitoids and metamorphic rocks, the rapakivi granites, and the Satakunta
sandstone. Mafic hypabyssal rocks of this age group (Suominen, 1991; Vaasjoki, 1996) are also
exposed in the Åland region (Bergman, 1979) and in the Vaasa region (Aro, 1987). The subhorizontal dykes are usually several tens of meters thick and some contain megaophitic parts.
Geochemically, they are fairly evolved transitional between olivine tholeiites and alkali basalts.
Isotopic analyses indicate radiogenic Nd compositions with initial ,Nd (1265 Ma) values from +1.3
to +3.5 and depleted mantle model ages (DePaolo, 1981) of 1.71–1.44 Ga. It is not clear whether
the lower ,Nd values of the dykes reflect crustal contamination associated with low-pressure
fractional crystallization (cf. Rämö, 1990) or indicate mantle source compositions (cf. Patchett et
al., 1994).
2.5. Dyke emplacement related to Sveconorwegian orogny (~1.1 Ga)
The ~1.1–1.0 Ga diabase dykes of Lapland manifest igneous activity that followed the
cratonisation of the Fennoscandian Shield. Their origin is temporally, and possibly also
petrogenetically, related to the closing stages of the Sveconorwegian orogey (Jarmo Kohonen,
personal communication, 2005). The diabases that crosscut the Salla greenstone belt area are
among the youngest igneous rocks in Finland. They are cutting all the volcanites, metasediments
and granitoids. The Salla dyke swarm can be followed laterally ~150 km across the boarder from
Russia (Tuutijärvi) to Salla and at least to village of Sodankylä. The en echelon Salla dyke is nearly
vertical and highly differentiated. The width varies from 60 to 100 m. The age of the Salla dyke is
1122±5 Ma (Lauerma 1995).
Other, presumably slightly younger swarms of basaltic dykes are found in the far northern
part of the country and in adjacent Norway (Fig. 5). Mertanen et al. (1996) reported Sm-Nd wholerock mineral ages of ~1050 Ma for samples belonging to the Laanila and Kautokeino diabases. The
en echelon Laanila dyke strikes NE and extends for >100 km from Laanila to Ristijärvi in Finland
and into northeastern Norway (Fig. 5) In the southwest, the diabases cut the Early Proterozoic
Lapland granulite belt and in the northeast, in Ristijärvi, they cut the Archaean basement. The
granulite belt is a high-grade metamorphic belt overthrust onto the Karelian basement in the
southwest. The Kautokeino dykes can be followed for ~50 km on low altitude aeromagnetic maps.
Individual dykes are 20–50 m wide and their length ranges from hundreds of metres to tens of
kilometres.
Geochemical data for the 1100–1000 Ma mafic dykes are relatively few. According to Pihlaja
(1987), the Laanila dyke is an olivine tholeiite with 46 to 49.% SiO2 and 5.4–6.8% MgO. The Nd
isotopic data of Mertanen et al. (1996) indicate notably positive initial ,Nd values (+4.8 to +5.8) for
the Laanila and Kautokeino dykes. Given that the diabase magmas transected an Early
Proterozoic–Archaean crustal domain, the effect of crustal contamination on the Nd isotopic
compositions has apparently been minimal.
2.7. Dykes related to the Caledonides (~430 Ma)
In the northwesternmost reaches of Finland, dykes belonging to the Caledonides have been
found. One baddeleyite age determination (Sipilä, 1992) yielded an age of 434 Ma. Dykes of this
age group are mainly exposed outside Finland.
9
2.8. Palaeozoic kimberlite magmatism
More than twenty-four intrusive bodies, mostly kimberlitic diatremes, but also including
hypabyssal kimberlites, olivine lamproites and ultramafic lamprophyres have been discovered
within the Archaean Karelian Craton in Finland. These Paleozoic intrusions, two pipes have K-Ar
ages of 590 and 430 Ma (Tyni, 1997), form the Eastern Finland Kimberlite Province which
includes the Kaavi and Kuopio clusters. The pipes and dikes intruded into 3.1–2.6 Ga gneiss
complexes of the Archaean Karelian craton and allochthonous 1.9–1.8 Ga metasedimentary cover
rocks thrust onto the craton during the Svecofennian orogeny (Kontinen et al., 1992). The
intrusions are dominated by textural and mineralogical variants of archetypal kimberlites and range
from purely hypabyssal kimberlite dikes to multiphase pipes of diatreme facies rocks. The Finnish
kimberlites generally contain abundant diamond indicator minerals and almost all contain at least
trace amounts of microdiamonds. In addition to the kimberlites, a spatially distinct and presumably
older group of dikes located elsewhere in the Karelian Craton comprise ultramafic lamprophyre
and rocks that have both olivine lamproite and Group II kimberlite affinities.
Figure 5. Generalised geological map of the northern Fennoscandian Shield showing the
distribution of ~1 Ga Laanila and Kautokeino diabases (Mertanen et al., 1996).
10
Day 1: Subjotnian Kopparnäs dyke and Postjotnian diabases
Subjotnian diabase dykes
Arto Luttinen
Department of Geology, P.O.Box 64, FI-00014 University of Helsinki
A fairly continuous swarm of diabase dykes stretches ~300 km from Lappeenranta, southeast
Finland, to the north of Tampere, central southern Finland (Fig. 4). The diabases are undeformed,
crosscut the Svecofennian gneisses, and are collectively designated as Subjotnian diabase dykes.
Although the dykes clearly cluster in two subsets, known as the Suomenniemi swarm and the
Häme swarm, which extend ÑW of the Wiborg rapakivi massif in a radial fashion, the actual
boundaries of the Subjotnian dyke system are somewhat ambiguous; diabase dykes of the same
type are found sporadically in nearly all directions around it. Some of the ~2 cm up to ~1 km wide
individual dykes can be followed laterally up to >10 km. Two dykes, representing NNW and NW
subsets of the Häme swarm, have been dated at 1667±9 Ma (Vaasjoki & Sakko, 1989) and 1646±6
Ma (Laitakari, 1987), respectively.
The Häme swarm has been petrografically studied in great detail by Laitakari (1969), but
geochemical data (trace elements and isotopes in particular) are sparse, apart from the Vehkajärvi
dyke (see STOP 3.2.). The existing data indicate subalkaline compositions with quartz and olivine
tholeiitic affinities with TiO2 ranging from 1.0 to 3.3 %. Most of the analysed dykes are fairly
evolved with MgO of 8.3–3.9. The so-called spotted olivine diabase has unusually high MgO of
12.4 %, but this probably reflects accumulation of olivine which is ~Fo75, based on 2Vmeasurements. Data on Nd isotopic ratios for two dykes indicate slightly radiogenic to marginally
non-radiogenic initial compositions (,Nd +0.6 to –0.2) (Rämö, 1990).
The Suomenniemi swarm has been studied in some detail by Rämö (1991). They are in many
respects similar to the Häme dykes; i.e. evolved tholeiites with TiO2 ranging from 1.8 to 3.2 % and
MgO from 5.6 to 2.3 %. They are enriched in LREE (La/Yb)N ~6) and other highly incompatible
trace elements, but, as in many continental tholeiites, the Nb/La ratios are fractionated (~0.3–0.6)
giving the dykes a “lithospheric affinity”. Most of the dykes have marginally non-radiogenic Nd
ratios (–0.2 to –1.2), but a few of them have radiogenic Nd and initial ,Nd up to +1.6.
STOP 1.1. The Kopparnäs dyke
The Kopparnäs basalt dyke is exposed over a distance of 2 km in the coast of the Gulf of
Finland ~5 km west of the Obbnäs rapakivi granite intrusion (Fig. 4). The dyke is composed of
numerous en echelon segments; the overall strike of the near vertical dyke is E-W. The maximum
width of the generally lens-shaped segments is ca. 1.5 m. The Kopparnäs dyke cross-cuts various
granitoids, gneisses, and gabbroids. Inclusions of wall-rock are fairly abundant and probably
mainly represent dislocated bridges. Recent palaeomagnetic measurements (Satu Mertanen,
personal communication, 2005) imply that the undated dyke is one of the wide-spread ~1.64 Ga
mafic dykes (Häme swarm) which are associated with rapakivi magmatism in southern Finland.
The Kopparnäs dyke is in mainly aphanitic and aphyric. Some wide segments have a
porphyritic appearance due to weakly oriented plagioclase laths. Geochemically, the dyke is
notably uniform and broadly similar to the dykes which belong to the Häme swarm, but it can be
distinguished from them on the basis of unusually low SiO2 (~45.6 %) and high FeOtot (~17 %) at
MgO of ~4.8 %. Although the dykes plot within the alkaline field in TAS diagram, normative
hypersthene and olivine indicate an olivine tholeiitic affinity. Concentrations of incompatible
elements, such as Nb (42 ppm) and TiO2 (4.8 %) are the highest so far reported for rapakivi-related
magmas. High (La/Sm)N (2.4), (Sm/Lu)N (3.7), and (Nb/La)N values (0.7) produce a fairly smooth
11
mantle-normalised incompatible element diagram that is not typical of the Häme swarm and
resembles that of modern OIB. Preliminary Nd isotopic data show marginally negative initial ,Nd
(1.64 Ga) values (~ –1) which are consistent with a hotspot source, but do not preclude crustal
contamination. The geochemical OIB-affinities of the Kopparnäs dyke concur with models that
associate the bimodal rapakivi suite of southern Finland with hotspot activity beneath the
Mesoproterozoic continent.
Sampling is only allowed with permission from the excursion leader!
The wet rock face can be extremely slippery –watch out!
Postjotnian diabase dykes
Arto Luttinen
Department of Geology, P.O.Box 64, FI-00014 University of Helsinki
The Postjotnian diabases were intruded into a sandstone-filled graben in southwestern Finland
during a relatively short period of magmatism between 1.25 and 1.27 Ga (Patchett et al., 1981;
Suominen, 1987). They are found as extensive, partly horizontal dykes cutting yje Laitila rapakivi
granite and the Jotnian sandstone in the graben (Hämäläinen, 1987) (Fig. 4). Adjacent to intrusive
contacts, rheomorphic dykes and partially melted xenoliths manifest interaction and contamination
of basaltic magma with wall-rock material. The geochemically rather uniform Postjotnian diabases
are transitional between alkali basalts and olivine tholeiites; the amount of normative hy is
typically <10 %. SiO2 ranges from 46 to 48 % and the diabases plot almost invariably within the
alkaline field in TAS-diagram. Similar to the older Subjotnian diabases, most of the Postjotnian
intrusions are fairly evolved with MgO of 7.7–4.6 % and TiO2 of 1.4–3.7 %, apart from coarsegrained patches in which SiO2 is up to 59 % and TiO2 up to 5 %. The dykes show variable
(La/Sm)N (0.9–2.6). The La concentration level is low (typically 7–30 ppm) in comparison with
that of Subjotnian diabases (typically 30–50 ppm). Published and unpublished Nd isotopic data
indicate radiogenic initial compositions with ,Nd from +1.5 to +3.6 (Rämö, 1990; Rämö, personal
communication, 2005).
STOP 1.2. The Hinnerjoki crosscutting dyke
Geological mapping and aeromagnetic images indicate fairly abundant, roughly N-S trending
dykes that crosscut the Laitila rapakivi batholith and the Postjotnian diabase dykes and sills in the
Satakunta region. Several of such crosscutting dykes are exposed in a road cut some 4 km east of
Hinnerjoki village (Fig. 6). Age data for these dykes are lacking, but unpublished elemental
geochemical and isotope geochemical analyses (Tapani Rämö, personal communication, 2005)
show similarities to the 1.25 Ga dykes and sills.
In the road cut, a half a dozen up to ~30 cm wide dykes crosscut a medium-grained
Postjotnian sill. One of such dykes have been analysed for major and trace elements and Nd
isotopic ratios. The dyke is highly evolved, marginally ne-normative alkali basalt, with MgO of 3.3
%, TiO2 of 3.4 %, and total alkalis of 5.9 %. Concentrations of incompatible elements, such as
P2O5 (0.93 %), Nb (20 ppm), Zr (338 ppm), and La (36 ppm) are high and those of Ni and Cr low
(7 and 5 ppm, respectively). Low Ti/Zr (60) suggests fractionation of Fe-Ti oxides. The radiogenic
initial Nd isotopic composition (,Nd at 1265 Ma= +1.8) is indistinguishable from those of the
Satakunta diabases (+1.4 to +3.6) and indicates a depleted source component. Low Nb/La (0.6) is
similar to those of Satakunta diabase dykes and infers a lithospheric component, however.
12
Figure 6. Distribution of Postjotnian diabases, Jotnian sandstone, and rapakivi granite in
Satakunta area (Hämäläinen, 1987). Excursion sites are indicated.
STOP 1.3. The Reposaari dyke
Postjotnian diabase crosscuts and overlies the rapakivi granites of the Eurajoki stock and
Laitila batholith and the Jotnian sandstone on Reposaari, Satakunta (Fig. 6). The diabases are
mainly coarse-grained, dark grayish of brownish black dykes and sills. Megaophitic varieties and
diabase pegmatoids occur. Heating by diabase magma melted parts of the host rock at the contacts.
In some places, the melt formed rheomorphic (palincenic) dykes in joints of already solidified
diabase. In other places, the rheomorphic melt has mixed with the diabase melt, forming hybrid
rocks of different types.
13
The Siikaranta outcrop on Reposaari is composed of rapakivi granite and diabase (Fig. 7).
The intrusive contact between the diabase and the granite is exposed near the shoreline. Close to
the contact, there are rheomorphic dykes and partially melted granite xenoliths. In the southwestern
part of the outcrop (A in Fig. 7), the diabase is homogeneous and medium-grained. Patches of
coarse-grained megaophitic portions occur here and there irregularly. A fine-grained aphanitic
diabase with small granite xenoliths suggests proximity to a contact with granite (B). In the central
part of the outcrop, small hybridized granite xenoliths (C) can be seen, unless the sea level is high.
The rapakivi granite (D & E) contains subspherical potassium feldspar phenocrysts (2–3 cm).
Rheomorphic dykes, from 1 to 10 cm wide, are found near the contact (F, G, H) in the central and
northwestern parts of the outcrop. Geochemically, the Reposaari dyke is a typical Postjotnian
diabase. It is marginally alkaline (ne-normative) basalt with MgO of 6.2 %, FeO2O3tot of 15.1 %,
and Al2O3 of 16.7 %. The initial ,Nd value is +1.7 and well within the range of values for other
Satakunta diabases.
Figure 7. Scematic map of the contact between Postjotnian diabase and rapakivi granite on
Reposaari. See text for explanation for A through H.
14
Day 2: Åva ring complex and Korsö dyke on Brändö, Åland
Introduction to the geology of southwestern Finland
Olav Eklund1, Sören Fröjdö2, Alexey Shebanov1 & Elin Siggberg2
1
Department of Geology, University of Turku, FI-20014, Turku
2
Department of Geology and Mineralogy, Åbo Akademi University, FI-20500, Turku
The accretionary arc complex of southern Finland and Russian Karelia
Southwestern Finland belongs to the southern Svecofennian arc complex of the Svecofennian
domain. The magmatic history of this arc complex begins with the pre-collisional volcanic-arc
magmatism dated to 1.90 Ga. Volcanites with more mature arc signatures formed at 1.88 Ga. The
collision of the arc complex towards the central Finland arc complex took place at 1.88–1.86 Ga.
These ages are obtained from the syn-collisional calc-alkaline granitoids in the area (Väisänen &
Mänttäri, 2002). The syn-collisional granitoids intruded simultaneously with the regional D2.
During post-collisional convergence (D3) at 1.84–1.82 Ga, regional high-T and low-P
metamorphism was associated with extensive migmatisation and production of melts in the crust,
mainly of S-type (Väisänen & Hölttä, 1999). Semi-simultaneously to the metamorphic culmination,
the Svecofennian crust of southern Finland and Russian Karelia was intruded by at least 14 small,
P-, F-, Ba-, Sr- and LREE-enriched, bimodal, shoshonitic intrusions (Eklund et al., 1998; Väisänen
et al., 2000). These small intrusions occur in a 600 km long belt extending from Lake Ladoga in
Russian Karelia to the Åland archipelago (Fig. 8). Based on geochemistry, Eklund et al. (1998)
concluded that the shoshonitic magmas stem from an enriched lithospheric mantle. Age
determinations indicate that this magmatism occurred between 1815 and 1770 Ma, most ages
centre ~1800 Ma (Eklund et al., 1998; Väisänen et al., 2000). The oldest shoshonitic rocks were
emplaced at mid-crustal levels (~4.5 kbar; Väisänen et al. 2000) and the younger in the upper crust,
at approximately 2 kbar and less (Eklund et al., 1998; Eklund & Shebanov, 2005).
After the emplacement of the 1.8 Ga post-orogenic intrusions, there was a magmatically quiet
period of 100–200 Ma until the voluminous anorogenic A-type rapakivi granites and associated
anorthositic rocks and tholeiitic mafic dyke swarms invaded the crust at roughly 1.6 Ga. The 1.58
Ga anorogenic rapakivi granites and associated anorthosites and mafic dyke swarms in SW Finland
were emplaced under approx. 1–2 kbar (Shebanov and Eklund, 1997). Consequently, exhumation
and uplift between the metamorphic peak and the post-collisional intrusive event must have been
fairly rapid, around 3 kbars in 30 Ma based on estimated spread in age from 1815±10 Ma to
1770±2 Ma, compared to almost no exhumation in the following 200 Ma.
Post-orogenic magmatism in southern Finland
At least 14 small (1–11 km across) 1.8 Ga Svecofennian post-collisional bimodal intrusions
occur in the accretionary prism of southern Finland extending from the Åland islands to the NW
Lake Ladoga region (Fig. 8). The intrusions form a shoshonitic rock series with total alkalis >5%,
K2O/Na2O>0.5, Al2O3>9% over a wide range of SiO2 (32–78%) (Fig. 9). The end members of this
rock series are ultramafic, calc-alkaline, apatite-rich high-K lamprophyres and peraluminous
HiBaSr granites. All rocks in the association are strongly enriched in the LILE Ba and Sr, in the
LREE and depleted in the HFSE Ti, Nb and Ta. The trace elements follow a fractionation trend
were LILE and LREE are depleted with increasing SiO2. For the lamprophyres and for the granites,
the initial Sr values are similar and low (~ 0.7033–0.7047) and the ,Nd values are slightly positive
and overlapping (+0.7 to +0.3, Andersson et al in press).
15
The Åva ring complex
Geochemistry, petrography and age
The Åva ring complex is one of three bimodal shoshonitic ring complexes situated along a
northeast-trending shear zone in soutwestern Finland; Åva, Seglinge and Mosshaga (Fig. 8). The
Åva ring complex is approximately 7 km across. The complex comprises hundreds of ring dykes of
coarse porphyritic (HiBaSr) granites, monzodiorite, monzonite, quartz-monzonite and granodiorite
pillows having shoshonitic geochemical affinity. These more mafic rocks are collectively referred
to as “monzonite”. The ring complex is cut by shoshonitic lamprophyres in a radial pattern (Fig. 8).
By using the geochemical method developed by Liégeois et al. (1998), it is possible to separate the
rocks with alkaline affinity from those whith shoshonitic affinities (Fig. 9b). The former are
relatively enriched in Zr, Ce, Sm, Y and Yb while the latter are relatively enriched in Rb, Th, U
and Ta compared to a reference rock series. The post-collisional ring complexes in southwestern
Finland plot within the field of shoshonitic rocks and may thus, according to Liégeois et al., (1998)
be interpreted that they stem from a previously enriched phlogopite- and amphibole-bearing source
in the lithospheric mantle.
Figure 8. (a) Geological sketch map and distribution of postcollisional shoshonitic intrusions
in southern Finland and Russian Karelia. (b) The Åva ring complex in the Åland archipelago
exposed on islands and skerries.
16
Fig. 9. (a) K2O vs SiO2 plot for postcollisional intrusions in SW Finland showing that
almost all these rocks plot in the field for shoshonitic rocks. (b) Trace element diagram
separating rocks with shoshonitic affinity from rocks with alkaline affinity. The values of the
trace elements are normalized to the Yenchichi-Telabit rock series in the Tuareg shield
(NYTS normalization) after Liégeois et al. (1998). The postcollisional rocks follow a welldeveloped shoshonitic trend with most of the mafic rocks having a prominent shoshonitic
affinity, while some of the granites (encircled) are marginally alkaline.
The granite carries K-feldspar megacrysts up to 3 cm in size. The megacrysts are rounded,
corroded and, sometimes, subhedral (tabular) in shape. They contain small, fairly rare inclusions of
variable amphibole and mica, coexisting plagioclase, sphene and accessory apatite and zircon.
Plagioclase occurs as small phenocrysts or as a mantle around the K-feldspar megacrysts. The
matrix consists of quartz (often bluish) and biotite, minor plagioclase, K-feldspar and Fe-Ti oxides
(mainly titanomagnetite). Occasionally, amphibole is present as inclusions in mica within the
matrix of the granite. Both K-feldspar megacrysts and groundmass contain apatite, pyrite, sphene,
zircon and fluorite as accessory phases.
In places, HiBaSr and sometimes S-type granites also form dykes radial to the ring-complex
(Skyttä, 2002). The HiBaSr granite was emplaced semi-simultaneously with palingenic crustal
melts, forming a mingled area between these two granites in the central part of the intrusion. Ehlers
& Bergman (1984) and Bergman (1986) suggested that the Åva intrusion centres on a gneissic ring
structure formed by diapiric emplacement of a late-orogenic S-type microcline granite pluton that
deformed the surrounding gneisses into a steep inward dipping ring.
The monzonite in Åva was dated at 1799±13 Ma and the granite at 1803±10 Ma (U-Pb in
zircon, Suominen, 1991). Field evidence of magma mingling indicates that monzonite and granite
magmas intruded contemporaneously. Suominen (1991) also reported younger sphene ages. The
reported 207Pb/206Pb sphene ages are between 1754 and 1789 Ma for the monzonites, and between
1759 and 1782 Ma for the granites. Recent SIMS age determinations on zircons situated in
different textural positions in the granite evidence a deep mineral assemblage that crystallized at
~1790 Ma and a shallow assemblage that crystallized at ~1760 Ma (Eklund & Shebanov, 2005).
In the Åva area, boulders of a rock consisting of plagioclase, orthopyroxene, biotite,
amphibole, apatite and minor clinopyroxene have been found. A SIMS age determination shows
that rims of zircons have the same age as the monzonite. Mineralogical correlations considering
substitution mechanisms in micas and hornblendes imply that the boulders belong to the Åva
complex.
17
Eklund et al. (1998) and Rutanen (2001) have suggested that the HiBaSr granite formed by
crystal fractionation of amphibole, biotite, plagioclase, apatite, sphene and magnetite from mafic
shoshonitic magma. In this scenario, it is most probable that the boulders represent the high-density
cumulus part of a stratified magma chamber deep in the crust.
The ring structure is cut by 33 known radial lamprophyre dykes. However, lamprophyres
were also emplaced prior to the ring dykes and some lamprophyres were intruded into semicrystallised granite (Kaitaro, 1953). Petrographically the lamprophyres are spessartites, vogesites
and minettes (Hollsten, 1997). Based on geochemistry (Eklund et al., 1998) and mineralogy
(Eklund & Shebanov, 2005), it has been concluded that the monzonites principally have the same
composition as the lamprophyres; hence they are considered to be plutonic varieties of the
lamprophyres.
Petrogenesis
It is suggested that the lithospheric mantle was metasomatised during the subduction stage of
the Svecofennian orogeny. An increased thermal gradient melted this phlogopite and amphibole
enriched lithospheric mantle at roughly 1.80 Ga, and shoshonitic magmas intruded the juvenile
crust and were emplaced at mid-crustal levels. There are some speculations about what caused the
melting of the enriched lithospheric mantle. Both Väisänen et al (2000) and Eklund & Shebanov
(2002) have suggested that the thermal rise was a consequence of slab breakoff (evidenced by rapid
uplift in combination with shoshonitic magmatism). However, since the crustal heating around 1.80
Ga seems to have affected major parts of the Baltic shield, an extensive lithospheric delamination
event may have generated the heat.
Magma differentiation took place in a mid-crustal magma-chamber (4–7 kbar) about 30 Ma
before the time of emplacement of the ring complex in the upper crust (deep assemblage ~1790
Ma, shallow assemblage ~1760 Ma). When about 50% of biotite, amphibole, plagioclase,
magnetite, apatite and sphene were fractionated from the initial magmas, peraluminous HiBaSr
granites were produced. Crustal assimilation during the fractionation process is considered to be
low. The alkali activity was elevated and the environment was relatively reducing in the midcrustal chamber when compared to the conditions of emplacement in the upper crust. It appears
that the juvenile Svecofennian crust was invaded by pulses of shoshonitic magmas from an
enriched lithospheric mantle over a long period. Some of these magmas were stored and
differentiated in the middle crust before transportation to the upper crust.
STOP 2.1. Ring dykes near Björnholma
The outcrop shows two ring dykes representing the outer part of the ring-intrusion. The dykes
are between 40 and 100 cm across and dip steeply towards the centre of the intrusion. They are
composed of coarse porphyritic (K-feldspar) granite. The K-feldspars are euhedral, but often
mechanically abraded. Towards the contacts, the K-feldspar megacrysts are cumulated.
STOP 2.2. Bimodal lamprophyre–granite dykes at Åva harbour
This outcrop shows a mixture of three components: 1) monzonite, 2) amphibolite fragments,
and 3) porphyritic granite. The monzonite forms pillows elongated in the dyke direction in the host
porphyritic granite. The amphibolite fragments impinge the monzonite pillows, indicating that the
monzonite was plastic. K-feldspar megacrysts from the porphyritic granite are encaptured into the
monzonite as well as blue quartz grains forming ocelli texture. These structural and textural
features indicate that the monzonitic and granitic magma intruded coevally into the upper crust.
18
STOP 2.3. Roof pendants, monzonite-granite interactions, pegmatites
This stop requires a ~2 hour walk during which we will be able to study roof pendants of
country rock in the central part of the Åva intrusion. Contact metamorphism has altered calcite to
wollastonite in some of the roof pendants. We will also see evidence of an intimate relationship
between a monzonitic magma and a cooling granitic magma. Such evidence includes: 1) small
mafic and hybrid magmatic enclaves (<5 cm) in in the granite, 2) mafic magmatic enclaves (up to
several m) in the granite, 3) lamprophyre dykes which intruded the semi-solid granite, 4)
crosscutting lamprophyres, and 5) boulders of deep mafic assemblages. Various vesicles (up to ~1
m with scapolite filling), pegmatites, and post-emplacement mylonites give an insight into the late
stage processes in the granite ring intrusion.
Anorogenic magmatism in SW Finland
About 200 Ma after the intrusion of the postcollisional shoshonitic intrusions, anorogenic
magmatic activity commenced and the Middle Proterozoic rapakivi complexes and associated
diabase dyke swarms were formed. The large ~1.58 Ga rapakivi granite intrusions of Åland,
Vehmaa and Laitila, and the smaller Kökar and Fjälskär intrusions dominate the anorogenic rocks
in southwestern Finland (Fig. 8). The Åland rapakivi granites are estimated to cover more than
5500 km2 and comprise a great variety of granite types ranging from plutonic to volcanic
(Bergman, 1981). The rapakivi massifs are accompanied by about 500 NE trnding tholeiitic dykes
in the Åland–Åboland diabase dyke swarm which traverse the archipelago from the Kökar area
towards the Vehmaa rapakivi massif. Prevalent in the swarm are narrow and fine-grained
hornblende-pyroxene diabases (Ehlers & Ehlers, 1977) of both olivine and quartz normative
compositions (Lindberg et al., 1991). Similar diabase dykes are associated with other rapakivi
massifs in the Svecofennian domain; e.g., the Häme swarm associated with the Wiborg rapakivi
complex in SE Finland (Rämö & Haapala, 1995). In addition to the fine-grained diabase dykes, the
anorogenic mafic rocks in the Åland region include leucogabbros and anorthosites (Bergman,
1981). Although the outcrops of these gabbro-anorthosites are small (less than 5 km2 in all), the
presence of these rock types in the anorogenic rapakivi-association is important. Age
determinations for the felsic anorogenic magmatism in southwestern Finland cluster at about
1575±10 Ma (Suominen, 1991).
Field relations evidence the contemporaneity of felsic and mafic activity; e.g. composite
dykes, and mingling of mafic and felsic magmas have been described from southwestern Åland
(e.g. Eklund, 1993; Lindberg & Eklund, 1988). Fragments of leucogabbro and anorthosite are
common in the studied diabases, and plagioclase megacrysts (labradorite) in mafic magmatic
enclaves within rapakivi granite varieties of the Åland complex are similar in composition to
plagioclase within the gabbro-anorthosites nearby (Fellman, 1994; Lindberg & Eklund, 1992;
Shebanov & Eklund, 1997). Bergman (1981) also noted the occurrence of fragments of coarse
ophitic diabase (texturally similar to the Korsö high-Al gabbros) within the fine-grained dykes.
Commonly the fine-grained diabases and gabbro-anorthosites are considered to be of the same age.
However, at all occurrences the gabbro-anorthositic xenoliths are angular fragments, apparently
indicating that they were crystalline when incorporated into the diabases, and thus, that the
intrusion of fine-grained mafic dykes and rapakivi granites postdates the formation of the
anorthositic rocks.
The Korsö dyke.
Of the 500 dykes in the Åland–Åboland dyke swarm Korsö dyke is one of the most extensive
and chemically least evolved. The dyke is bimodal (Fig. 10). In its central part it contains a
19
phenocryst free A-type granite variety (rapakivi-type?) with miarolitic cavities. Occurence of
autoliths of anorthosite and norite (senso stricto) is restricted to the chilled margin and fine grained
parts of the dyke. Between the granite and the diabase there is a hybrid zone with sinuous contacts
towards the granite as well as to the diabase.
The Korsö dyke carries several types of plagioclase cumulates, such as noritic fragments and
plagioclase megacrysts, and displays a chilled margin which is one of the most primitive mafic
rocks found in the region. The diabase, or leucogabbroic rock, is occasionally very rich in
plagioclase, the volume proportion of plagioclase being about 70±5%. The plagioclase is about
An65±5, the platy laths are usually ~2.5 cm long. Plagioclase spherulites, indicative of plagioclase
over-saturation or supercooled liquids are also found. The observed “stellate” (star-like)
plagioclase texture is similar to the “snowflake” textures in the mafic dykes of the Suomenniemi
complex, Finland (Rämö, 1991). There is thus ample evidence for the occurrence of high-Al basalts
(as noted in the literature) and close association of this texture and massif-type anorthosites—as it
is brought forth by plagioclase supersturation in mafic magmas of similar age as the anorthosites.
Polybaric evolution
In a fine-grained part of this dyke a noritic fragment is found in which Al-rich orthopyroxene
(Al2O3=4–6%; Mg# ~57) encloses laths of plagioclase (An55). The high Al content indicates
crystallization at mid to lower crustal depths, which also is consistent with the lower An content of
the plagioclase (Longhi et al., 1993) as compared to the dyke in whole. This indicates that
plagioclase was on the liquidus at conciderable depth followed by orthopyroxene. The dyke also
contains large euhedral plagioclases (labradorite in composition) up to 7 cm in size.
Thermobarometrical investigations on mineral inclusions in these megacrysts indicate a
crystallization pressure around 5 kbar, i.e. mid-crustal levels (Shebanov & Eklund 1997).
According to phase diagrams developed by Shkodzinsky (1985) magmatic boiling in phenocrystfree A-type granitic melts occurs at pressures less than 0.5 kbar. The presence of miarolites in the
granitic part of the dyke indicates that the emplacement level of the dyke was at 0.5 kbar or less. It
may thus be concluded that the Korsö dyke presents a record of polybaric evolution through the
whole crust.
Figure 10. Schematic map of the bimodal Korsö dyke showing the distribution of various
mafic and felsic rock types and autoliths.
20
STOP 2.4. Thin tholiitic dykes south of Brändö
This locality enables us to study thin (<1 mm to 30 cm) tholeiitic dykes typical of the Åland–
Åboland dyke swarm. The dykes represent low-viscous tholeiitic melts which intruded the country
rock and crystallized at a low pressure of <1 kbar.
STOP 2.5. Korsö dyke
The outcrop reveals a major phase of the Korsö dyke. The dyke exhibits a coarse-grained,
ophitic texture with plagioclase laths and intercumulus of olivine, clinopyroxene, orthospyroxene
and pigeonite. Accumulated plagioclase laths and fragments of leuconorite are found near the
contact. The crosscutting dykes are enriched in Ti, P, Fe, and incompatible trace elements.
STOP 2.6. Leuconorite and labradorite megacrysts
Leuconorite fragments with euhedral plagioclase crystals and high-Al opx and labradorite
megacryts are found at this outcrop.
STOP 2.7. Vesicular granite, hybrid, and snow-flake plagioclase
At this outcrop we see even-grained A-type granite with vesicles and hybrid varieties
between the granite and the Korsö diabase. The diabase contains snowflake plagioclase.
STOP 2.8. Gabbroic autoliths (optional)
Different gabbroic autoliths and large, euhedral plagioclase crystals are found in the Korsö
dyke at is outcrop.
The 1583 Ma Vehmaa rapakivi batolith
Five different rapakivi types can be discerned in the Vehmaa batolith: pyterlite (rounded Kfeldspar ovoids, rarely having plagioclase rims), two varieties of porphyritic rapakivi granite, evengrained rapakivi granite and porphyritic aplite. They all have different textural, mineralogical and
geochemical characteristics. The batolith exhibits a concentric pattern, with pyterlite along the
margins and the younger porphyritic granites in the central areas. The porphyritic aplites intruded
the pyterlite as minor bodies, except for the semicircular occurrence between two porphyritic
rapakivi varieties (STOP 2.10.). Even-grained rapakivi granites (Uhlu granite) form, together with
porphyritic aplite, a satellite intrusion east of the main batolith (Linderg & Bergman 1993).
STOP 2.9. Pyterlitic rapakivi granite (optional)
At the harbour of Vousnainen (Kustavi), we can see the uppermost part of the Vehmaa
rapakivi batolith. The pyterlite variety contains abundant melt-depleted xenoliths of country rock,
aplite dykes with pegmatic pockets, and areas of degassing of the batolith.
STOP 2.10. Järppilä quartz-feldspar porphyry dyke (optional)
This coarse-grained porphyritic aplite dyke intruded between two rapakivi varieties. It
contains large (up to 5 cm across) megacrysts of K-feldspar and micaceous clots in a fine-grained
(aplitic) matrix. The mineralogy of the clots and mafic silicates inside the K-feldspar megacrysts
21
differs from the mineralogy of rapakivi granites in general. The micas have low Fe-index and a
high content of Ba (usually the mafic silicates in rapakivi granites have high Fe-index).
Mineralogically, the clots thus have a “post-orogenic” rather than an “anorogenic” character.
Texturally, the porphyry consists of scattered K-feldspar megacrysts (ovoids), small
micaceous clots, and plagioclase megacrysts, scattered in a fine-grained granitic matrix.
Geochemically, the porphyries are remarkably enriched in Ba (1900–3700 ppm), Sr (420–780
ppm), and TiO2 (0.68–1.38 %) compared with rapakivi granites in the batolith. These “primitive”
features disconcert with late intrusive nature of the Järppilä dyke and are caused by the rather
unusual composition of the abundant mica.
The studied biotites are characterized by brittle mica substitions (Ba, Ca, Sr) operative in
interlayer site, apparent deficiency in tetrahedral site, and relatively low Fe/(Fe + Mg)-ratio
(average 0.69). Substitution of Ti 4+ in the tetrahedral site of the most Ba-rich micas is suggested to
maintain charge balance. These characteristics differ markedly from the well-restricted
compositional space of rapakivi biotites elsewhere. However, Järppilä biotites are nearly identical
to barian biotites recognised in the postorogenic (1.77–1.80 Ga) bimodal granite-lamprophyre
complexes with shoshonitic affinity, which are often spatially related to the rapakivi batholiths in
the area. This fact suggests a relic origin for barian-titanian biotite-bearing assemblages, pointing to
postorogenic suite as suitable protolith for the Järppilä porphyries. Aimed to verify that, U-Pb
dating by using IMS-facility (Nordsim) has been undertaken.
Two distinguishable populations of zircons have been investigated: a) nearly stoichiometric,
Hf-poor zircons found mainly as inclusions within K-feldspar ovoids and b) zircons exhibiting
elevated Zr/Hf and peculiar disturbances in stoichiometry; these zircons predominately occur in the
matrix. The first population sometimes contains inclusions of Ba-enriched minerals and yields a
discordia with an upper intercept of 1616±7 Ma, MSWD = 1.2. The second group does not
assemble with barian mica-bearing paragenesis and yields an emplacement age as defined by a
separate discordia with an upper intercept of 1579±6 Ma, MSWD = 0.47. These two events seem to
be reflected in the clustering of different mineral assemblages in their isotope compositions. Rb-Sr
isotope data for mineral inclusions within the ovoids plot on a reference line with an age of
1604±30 Ma and initial 87Sr/86Sr= 0.7018±0.0004, whereas the rest forms a separate errorchrone
defining an age of 1577±19 Ma, initial 87Sr/86Sr = 0.7040±0.0001 (MSWD = 1.26). The former
value indicates a presence of the mantle-derived component for the first assemblage encapsulated
within ovoids. This accords with significantly more positive time integrated ,Nd(1.58 Ga)= +2.0
for the first assemblage compared to the second assemblage that yielded ,Nd(1.58 Ga)= –1 0 to –
4.0. The latter is equal to common bulk-probe characteristics for Finnish rapakivi granites.
In sum, data on the relic mineral assemblage in the Järppilä rapakivi porphyries favour a
mantle-derived initial source (late postorogenic?) which experienced short residence in the crust
not long before the anorogenic 1.58 magmatic event. Our data suggest that postorogenic magmatic
activity might have continued up to approximately 1.62 Ga.
22
Day 3: The Subjotnian Häme swarm
Arto Luttinen
Department of Geology, P.O.Box 64, FI-00014 University of Helsinki, Finland
The following description of the Häme swarm and the excursion stops is largely based on the PhD
thesis and other published work of Ilkka Laitakari.
STOP 3.1. Proterozoic basaltic glass in Orivesi dykes
In the Häme dyke swarm, undevitrified Precambrian glass has been found (Lindqvist &
Laitakari, 1980). It appears in the narrow (<2 cm) tails of ~10 m long and ~10 cm wide diabase
apophyses. The parent diabase is 2.7 m wide, vertical, and strikes NNW. The glass is found ~25 m
from the dyke in an apophysis striking NNE and dipping 60 ESE in a road cut near Aihtianjärvi,
Orivesi. The apophyses cut a Svecofennian hypabyssal granitoid.
The fresh broken surface of the glass is lustrous black, but in thin section it is brown. Altered
parts are greenish blue and slightly birefringent. Traces of devitrification can be seen along tiny
cracks and at the contacts. The apophyse tails are hyalophitic and the glass content is up to 90%.
Two generations of plagioclase (An55–60) and sporadic olivine are found as phenocryst phases in the
samples studied by Lindqvist & Laitakari (1980), elsewhere, there are also larger, up to ~2 cm long
plagioclase macrocrysts. The large ~1 mm plagioclase phenocrysts are equidimensional, whereas
the smaller ones are ~0.3 mm long laths. The laths show flow orientation. Compositionally distinct,
yellow domains are found within the brown glass. These patches are relatively poorer in SiO2, CaO
and Na2O and higher in MgO and FeOtot compared with the enclosing brown glass. Euhedral,
partly serpentinized olivine has been found in the yellow patches (Fig. 11). The origin of the
yellow patches has been ascribed to immiscibility between two silicate liquids by Lindqvist &
Laitakari (1980).
Geochemically, the glass is a subalkaline, tholeiitic basalt and quite similar to the assumed
parent diabase dyke. The composition is typical of an evolved tholeiite with low MgO (4.2 %), Ni
(45 ppm) and Cr (46 ppm) combined with relatively high FeOtot (13.5 %), TiO2 (2.8 %), and
incompatible trace elements, such as Zr (230 ppm). The notably high water content of the glass (8.8
%) is believed to be of magmatic origin and may have fascilitated the formation of a glass
apophysis by lowering the viscocity of the basaltic melt.
In the same area (at Leväslahti), an amygdaloidal diabase has been found. This dyke is 8 m
wide, strikes N85W and dips 70N, and has a subophitic texture. The amygdules are spheroidal,
about 2 mm in diameter, and consist of calcite and euhedral quartz. There is a 40 cm wide
amygdule-free zone below the upper contact. The presence of amygdules has been regarded to
indicate that the present erosion level is close to that of ~1600 Ma ago.
Sampling is NOT allowed on this outcrop!
The road has heavy traffic -watch out!
STOP 3.2. Flow-differentiated Vehkajärvi dyke
The diabase dyke of Vehkajärvi is exceptionally well-exposed in a road cutting and,
accordingly, has been fairly intensively studied in the past (Laitakari, 1969; Boyd, 1972; Mäkipää,
1979). The 3 m wide dyke has sharp contacts with the wall-rock greywacke schist. It is black, finegrained and has glassy margins with small, needle-like plagioclase laths. There is a 50 cm wide
zone in the centre of the dyke which is markedly rich in up to 10 cm long megacrysts. The
23
plagioclase megacrysts have cores of An58, enclosed in a narrow rim of An62 and an outer rim of
An52 (Boyd, 1972).
Geochemical traverses of the Vehkajärvi dyke have been reported by Boyd (1972) and
Mäkipää (1979). The data for 13 whole-rock and 109 groundmass analyses indicate a uniform
subalkaline basaltic composition across the dyke. High concentration of plagioclase megacrysts in
the centre of the dyke is reflected by the increased Al2O3 (~19 %) and CaO (~8.6 %) contents in the
whole-rock data (Fig. 12), while the groundmass data indicate a slight decrease in these
components. The groundmass in the Vehkajärvi dyke is quartz-tholeiitic and fairly low in MgO
(3.3 %) and high in TiO2 (2.6 %), P2O5 (~0.8 %), and Zr (~340 ppm). It is actually one of the most
evolved diabase dykes so far analysed in the Häme swarm. The major element composition
corresponds to those of basalt liquids that are in equilibrium with olivine, plagioclase, and
clinopyroxene at one atmosphere pressure (Boyd, 1972). According to Mäkipää (1979), the large
megacrysts have not crystallized from a melt corresponding to the composition of the surrounding
matrix.
Sampling is only allowed with permission from the excursion leader!
STOP 3.3. Quartzite cobbles in Maijaanvuori dyke (optional)
Phenocrysts, megacrysts and large fragments of plagioclase (An55–60) are common in the
Häme swarm; they have been observed in 83 of the 109 studied dykes. The occurrence of
phenocrysts, megacrysts, and fragments is limited to the younger 1646±6 Ma set of dykes, however
(Laitakari & Leino, 1989). In 39 dykes at least some of them exceed 5 cm in diameter; the largest
crystals attain ~30 cm in length (e.g. Maijaanvuori) and typically are almost euhedral. In 19 dykes
there are autoliths. They are usually <30 cm in diameter. Some of them resemble coarse-grained
diabase or diabase pegmatite and others are anorthositic. Plagioclase (An48–53) is the predominant
mineral and interstitial hypersthene is the main mafic mineral. One dunitic inclusion has also been
discovered. Judging from the presence of interstitial plagioclase (An56), it is probably a lowpressure cumulate and does not have a mantle origin. The similarities between the autoliths and
megacrysts of the dykes to the rocks of the Ahvenisto gabbro–anorthosite complex and the fact that
dykes become narrower towards the northwest have led to the interpretation that the transport of
magma was essentially lateral and that the Ahvenisto pluton may well represent the magma source
of the Häme dykes (Laitakari & Leino, 1989).
Many of these dykes (n=12) also contain quartzite xenoliths which can comprise up to ~20 %
of the diabase (Fig. 11). Quartzite-bearing dykes are found over ~90 km long area west of Lake
Päijänne. The xenoliths are usually angular, but the Maijaanvuori dyke contains completely
rounded xenoliths. The inclusions are typically 2–10 cm in diameter, but the largests exceed 20 cm.
Two varieties of quartzite have been identified: Gray ones are most common, but the white type
includes most of the large xenoliths. The gray xenoliths have a blastoclastic texture and have minor
sericite, biotite, plagioclase, opaque minerals, apatite, and zircon. The grain size is ~0.1–0.2 mm.
The source of the quartzite xenoliths is unknown; no quartzite of this type is known in the
area. The minor quartzite occurrences are clearly schistose and more deformed than the xenoliths.
Two obvious solutions that have been considered by Laitakari and his co-workers are: derivation of
quartzite from sources 1) below or 2) above the present erosion level. Laitakari & Leino (1988)
envisaged that many of the Häme diabase dykes reached the surface in Proterozoic time, that
“magma ran in the fissures like water in a river”, and that the quartzite xenoliths fell into the lava
stream. In this scenario, the dykes may well have acted as feeder systems of fissure eruptions not
unlike the Laki eruption of Iceland in 1783–1785.
24
Figure 11. Photomicrograph of glassy Orivesi dyke showing brown glass (b), plagioclase (pl),
and a partly serpentinized euhedral olivine (ol) in yellow glass (y) (LEFT). Quartzite
xenoliths in Jouttijärvi diabase (RIGHT).
Figure 12. Al2O3, CaO and MgO variation and the variation of the grain size of the
Vehkajärvi dyke. The numbers below the photos give distance from the contact (Mäkipää,
1981).
Be ready for mosquitos at this outcrop!
25
Day 4: Microtonalite dykes, kimberlites, and soapstone factory
Intermediate magmatism at the craton border zone, central Finland
Olli Äikäs1 & Arto Luttinen2
1
Geological Survey of Finland, P.O.Box 1237, FIN-70211 Kuopio
2
Department of Geology, P.O.Box 64, FIN-00014 University of Helsinki
The western border zone of the Archaean bedrock complex with the overlying Early
Proterozoic supracrustal rocks are cut by numerous intrusions ranging from felsic to mafic in their
chemical compositions. Intermediate magmatic rocks make up one distinct group among these
intrusions, predominantly occurring in a relatively narrow zone from the line Leppävirta–
Tuusniemi–Kaavi up to the area west of Lake Oulujärvi. Tentatively named as microtonalites
(Huhma, 1981), these rocks comprise a texturally, structurally and compositionally varying series
of intermediate rocks, which occur mainly as dykes.
Dating of this intermediate magmatism has been problematic, mainly due to contamination.
According to Huhma (1981), the microtonalite dykes in the Kaavi area postdate the 1.86 Ga
Maarianvaara granite intrusion, but predate the 1.83 Ga lamprophyre dykes of the area. A U-Pb
sphene age of 1.857 Ga for a sample from Kaavi (A940, Fig. 13) has been considered to give a
minimum age for the microtonalite dykes. Another dyke from Murtolahti, Nilsiä yields ages of
1829±13 Ma for sphene, and 1835–1939 Ma for a heterogeneous population of zircon (Irmeli
Mänttäri, personal communication, 2000). Microtonalite dykes are numerous also in the Syväri
region in Nilsiä, between Tahkomäki and Lastukoski, where the Archaean basement is dominated
by paragneisses (Paavola, 1984). On the basis of field observations mainly in the Juankoski map
sheet area (Äikäs, 2000), several pulses exist, although the emplacement ages may be
indistinguishable using radiometric dating. The dykes can be grouped in the following fashion: 1)
Conformable or semiconformable dyke segments in the Archaean bedrock; 2) Dykes that crosscut
Early Proterozoic mica gneiss but have been deformed and broken apart into boudins during the
early stages of the Svecofennian orogeny; 3) Dykes that crosscut the Archaean bedrock,
Proterozoic quartzite, metadiabases, and the 1869±5 Ma (Hannu Huhma, personal communication,
2005) Juurus tonalite; and 4) Dykes that are associated with NW-trending granite pegmatite dykes
and, in places, crosscut other microtonalite dykes.
According to Rautiainen (2000), microtonalite dykes sampled at the craton border zone are
fine-grained, massive, light to dark gray, and homogeneous. They have sharp, often jagged contacts
with the wall-rock; in many places, the tonalite dykes brecciate the wall-rock. Apophyses are
common. The dykes are 4–5 m wide and strike NE-E. Many show weak magmatic banding. The
dykes have a hypidiomorphic texture and consist mainly of subhedral plagioclase, quartz, biotite,
and hornblende. Epidote, sphene, apatite, and opaque minerals are accessory mineral phases. The
dykes can be classified as tonalite and subordinate quartz diorite; some plot in diorite/gabbro field,
but they are collectively referred to as microtonalite. Plagioclase phenocrysts in the microtonalite
dykes are 0.5–2 mm long and show zoning from An49 to An15. The roundness and the resorbed
appearance of phenocrysts, needle-like apatite, crystal aggregates, micrographic texture, and
quartz-plagioclase ocelli are characteristic indicators of mixing processes.
The intermediate dykes include bimodal dykes with mafic enclaves within felsic “tonalite”
(Fig. 14). Phenocrysts in the mafic enclaves and in the felsic parts are compositionally similar. The
enclaves range from basalt to basaltic andesite with SiO2 of 43–55 %, whereas the felsic host rock
is andesite to rhyolite with SiO2 of 61–72 %. Compositions of the homogenous dykes are
intermediate between the mafic and felsic end-members. Overall, the enclaves, the felsic endmember, and the homogeneous dykes define linear arrays in geochemical diagrams: CaO, FeOtot,
MgO, TiO2, MnO, and P2O5 decrease (9–4 %, 14–4 %, 6–2 %, 2–0.6 %, 0.23–0.05%, 1–0.2 %,
26
respectively) when SiO2 increases from 43 to 64 %. Combined with petrographic evidence of
magma mixing and mingling, the linear trends indicate that the microtonalite suite was generated as
a consequence of interaction between a basaltic magma and a felsic magma. High La/Nb (1.6–3.2)
and Ba/La (18–20) of the most mafic samples (SiO2 51–48 %) are typical of subduction-related
magmas. In AFM-diagram, these samples plot within the calc-alkaline field.
Figure 13. Simplified geological map of the western North Karelia from Huhma (1981): 1 –
Archaean rocks, 2 – Early Proterozoic rocks, 3 – microtonalite dykes, 4 – lamprophyre dykes.
In the upper left corner elements from the geological map sheet area 3333 Juankoski (Äikäs,
2000) have been added: grey – Archaean; white – Early Proterozoic; crosses and spots in
black – microtonalite outcrops; symbols in magenta – kimberlite; red dot – location of the
Kivennapa field stops. The symbol for kimberlite NE of Kaavi denotes the Lahtojoki pipes
and the Kaavi kimberlite cluster in general.
27
Figure 14. Pillow-like mafic enclaves in felsic material from Honkamäki, Nilsiä, 10 km SW of
Kivennapa (Rautiainen, 2000). The length of the tag is 10 cm. Photo: J. Rautiainen.
Figure 15. Typical composite dyke of microtonalite intruding the Juurus tonalite in the
"Saunaranta" outcrop at Kivennapa, Juankoski. Photo: O. Äikäs.
STOP 4.1. Microtonalite dykes at Kivennapa
The bedrock exposures at Kivennapa provide an insight into the complex setting of the
microtonalite dykes (Figs. 15 and 16). Composite dykes with leucotonalitic seams between the
mafic parts intrude and brecciate the Juurus tonalite. Later dykes of the leucotonalitic material
crosscut the darker microtonalite. More complex outcrops show lithologies from the migmatitic
and folded and boudinaged Early Proterozoic mica schist to late granite pegmatite with successive
intrusive phases of Juurus tonalite and dykes of microtonalite, granodiorite, and granite. In
addition, a large erratic boulder of microtonalite presents an eaxample of the porphyritic texture
typical of numerous dykes in the Juankoski map sheet area.
28
Figure 16. Detailed map of the "Porch" outcrop at Kivennapa, Juankoski. Map by O. Äikäs.
STOP 4.2. The Muuruvesi church
Various small-scale features, such as interaction between mafic and felsic magma, can be readily
examined on the stone walls of the Muuruvesi church (Fig. 17), 12 km SE of Kivennapa.
Figure 17. Blocks of microtonalite (from composite dyke; below) and Juurus tonalite (above)
in the wall of the Muuruvesi church. Photo: O. Äikäs.
29
Eastern Finland Kimberlite Province
Hugh O´Brien1 & Matti Tyni2
1
Geological Survey of Finland, P.O.Box 96, FIN-02151 Espoo
2
Korvenkuja 7, 62310 Voltti
Intrusion morphology and Facies
The kimberlites from northeastern Finland range from <1 hectare to almost 4 hectares. The
intrusions range from circular to elliptical pipes to dikes up to 0.5 km in length (Fig. 18). Surface
exposure of the pipes is poor, however, Pipes 1 and 2 are presently exposed due to recent
exploration and diamond evaluation activities. Tuffisitic diatreme facies rocks are dominant,
although several of the intrusions are composed entirely of hypabyssal material (Table 1). Evidence
for possible crater facies material is limited to pipe 5 which contains volcaniclastic rocks that vary
from breccias to ash-rich sandstones. It is unclear whether the sandstones represent downrafted
blocks of crater facies material or simply more completely comminuted diatreme material.
Kimberlite samples
Pipe 1, the "strange rock" discovered in 1964, is the only pipe that does not contain breccia,
is almost xenolith-free, and represents the least contaminated kimberlite in this suite of samples. As
it is also by far the least altered, it makes an ideal endmember with which to compare the remaining
samples. Containing up to 40% olivine as macrocrysts and phenocrysts in a matrix of monticellite
microphenocrysts, perovskite, chromite zoned to magnesian ulvöspinel-magnetite (MUM), calcite,
serpentine, and kinoshitalite mica, the main phase of the pipe is mostly massive but about 10% has
a carbonate segregation-texture. The marginal phase of the pipe is quite distinct and is formed of
autoliths of the kimberlite magma cemented into a carbonate matrix (25% by volume). These
autoliths contain abundant spheres of calcite that likely record the separation of an immiscible
carbonate magma that coalesced near the margin of the pipe to form the carbonate matrix.
The freshest samples of the other nine kimberlites have the same matrix minerals as those in
Pipe 1, but in most cases only pseudomorphs of monticellite remain, the bulk of olivine has been
serpentinized and there is more sphene as opposed to MUM. An additional important component in
the other kimberlites is varying amounts of xenoliths (described below), xenocrystal peridotite
detritus (esp. olivine, chromite, chrome pyrope, chrome diopside), megacryst suite garnet, ilmenite,
pyroxene and abundant crustal detritus in the diatreme breccias. Some parts of pipes 5 and 14 in
particular are crystallinoclastic, comprising essentially disaggregated peridotite minerals in
minimal kimberlite matrix.
Xenoliths
Ultramafic to mafic mantle xenoliths are relatively common in pipes 5, 9, 10 and 14 and
comprise a suite ranging from garnet websterite to lherzolite, harzburgite and eclogite. Pipe 10
contains a rich variety of eclogites, phlogopite-bearing peridotites and Cr-rich to Cr-poor
lherzolites. Abundant xenoliths of lower and upper crustal wall-rocks range from granites to
sulfide-rich black schists, quartzites and two pyroxene granulites. Typically the largest population
in any given pipe represents the highest level wall-rocks encountered, but in all cases xenoliths of
felsic gneisses derived from Archean gneiss basement occur. Spectacular tuffisitic breccias from
pipe 3 contain abundant xenoliths of quartzite, gneisses and granitoids. However, the xenolith
population of the tuffisitic rocks varies dramatically among drill cores located only a few tens of
meters apart, and the quartzite xenoliths do not occur in any other drill core intercepts.
30
Figure 18. Surface plan maps of kimberlite pipes from Kaavi (LEFT) and distribution of
kimberlites in the Kaavi region (RIGHT).
Mineral chemistry
Olivine–Fresh olivine exists only in pipes 1, 5, 9 and 14. The forsterite-rich olivines from
pipes 1 and 14 have a restricted compositional range, from Fo87 to Fo92 which includes olivine from
peridotite xenoliths, rounded macrocrysts and smaller euhedral to subhedral phenocrysts that
presumably crystallized from the kimberlite magmas. The larger olivine grains are typically
unzoned but it is common for the smaller phenocrysts to show core Fo90 to rim Fo88 variations.
Olivines from pipe 5 and 9 appear to have a bimodal compositional distribution with a Fo93–89
population like those described above and a Fo86–83 population representing megacryst-suite
olivine.
Monticellite–Monticellite is common in Pipe 1, rare in Pipe 14 and found only as inclusions
in mica and sphene in pipe 10. It is absent from the remaining pipes which are all in less pristine
condition. Euhedral monticellite grains, mostly from 10 to 50 micrometers in size, form the main
component of the groundmass in the Pipe 1 samples. Those that occur in Pipes 10 and 14 are
partially altered microphenocrysts, quite similar in size and composition to those in Pipe 1.
Mica–Ba-rich mica (kinoshitalite) occurs in the groundmass of virtually all of the Finnish
kimberlite samples. Its abundance ranges from very sparse as in Pipe 9, to as much as 10% of the
matrix, as in Pipe 10. The Finnish examples range to extremely Ba-rich compositions (up to 17.8%
BaO), especially those from Pipe 1, and also contain a large amount of fluorine.
Spinel–Pipe 1 samples contain up to 5% spinel, which uniformly have spectacular atoll
structures. In thin section translucent dark red chromite cores are surrounded by an amorphous
serpentine-like material which is in turn mantled by magnesian ulvöspinel and surrounded by an
outer thin rim of magnetite (Fig. 19a). However, in rare cases it is possible to find remnant
pleonaste spinel that originally filled the "lagoon" in all of these atoll spinel grains (Fig. 19b). In
detail the chromite cores are typical kimberlite spinel magmatic trend 1 (Pasteris, 1983, Mitchell,
1986) titanian aluminous magnesian chromite (TIMAC) but are abruptly zoned to titanium-bearing,
nearly chrome-free pleonaste spinel, quite away from the normal zoning in Trend 1 spinels
(op.cit.). The MUM mantles are practically unzoned and a few, in addition to an outer rim of
magnetite, have an inner, discontinuous magnetite rim of the same composition (see Fig. 19b).
Pipes 2, 3 and 5 also contain atoll spinels analogous to those described above although they are less
31
abundant and suffer from the relatively altered condition of the host rocks. Somewhat similar atoll
spinels with considerably thinner pleonaste rims and titanomagnetite cores instead of chromite
were described from the De Beers kimberlite (Pasteris, 1983).
Additional information on the growth of the pleonaste spinels comes from mantles developed
on large olivine macrocrysts and rounded olivine-monticellite-spinel cumulate fragments that form
what we have termed "orbicules" in several Pipe 1 samples. In many respects these are similar to
the "spheroids" described by Reid et al. (1975) from the Igwisi Hills.
Ilmenite–Magnesium-rich ilmenite (picroilmenite), a typical mineral found in kimberlites and
representing one of the most important diamond indicator minerals, is abundant in pipes 4, 5, 6, 9,
10, 14 and 23. In a similar fashion to the spinels, ilmenite compositional zoning patterns in the
Finnish kimberlites are complex. Thus far three main types of zoning have been identified.
a. Decreasing Cr with increasing Mg toward rim. High Cr (ca. 5 wt% Cr2O3) picroilmenite
macrocrysts from Pipe 14 have homogeneous cores, but show decreasing Cr within 20 microns of
the crystal edge (Fig 20a). The lowest Cr, highest Mg picroilmenites occur just inside the very edge
of the crystals, adjacent to a few micrometer wide, outer MUM rim.
b. Increasing Cr and Mg toward rim.This is the predominant type of zoning developed in
picroilmenite macrocrysts with low Cr cores. Well documented in kimberlite picroilmenites
(Mitchell, 1986), it is manifested as diffuse zoning across most of the crystal at a relatively
constant Cr content and from 10 to 12 % MgO. However, near the crystal edge a much more abrupt
zoning pattern is developed, with a large increase in MgO and Cr2O3 over distances of tens of
micrometers. At the very edge of the crystal a ca. 5 mm reaction rim of MUM spinel occurs on
these grains. The high Cr ilmenite reaction zone mimics the shape of the MUM rim and where
there are embayments of MUM farther into the ilmenite crystal, the high Cr picroilmenite reaction
zone rather uniformly outlines the embayment. However, in some cases the core composition
occurs right up to the present crystal edge, which probably reflects fracturing of the crystal shortly
before pipe emplacement.
In the same sample, another version of zoning with an increase in Cr and Mg toward the rim
occurs in picroilmenites from peridotite microxenoliths. The microxenolith consists of a large Fo89
olivine, a very magnesian (~14% MgO) ilmenite and between them a patch of chalcopyrite and
associated djerfisherite. Olivine included in the picroilmenite is also Fo89, and this, coupled with
the rounded shape of the microxenolith, indicates that the fragment represents a mantle sample.
Developed only at the outer edge of the microxenolith, where the ilmenite was exposed to
kimberlite magma, is distinct zoning to more MgO-rich compositions and a sharply defined MUM
outer rim. There is no zoning in the ilmenite toward the area of serpentinized olivine, indicating
that this late stage process had no effect on ilmenite compositions. Chrome and Mg-rich
picroilmenite inclusions are relatively common in olivine macrocrysts in this sample and have very
similar compositions to the microxenolith ilmenite. There is no difference between those inclusions
that occur in fresh olivine and those that occur in serpentinized olivine further demonstrating that
late stage serpentinizing fluids had no affect on the ilmenites and in particular did not contribute to
the development of the spinel reaction rims.
c. Increase in Mg, no increase in Cr toward rim. A completely different zoning trend from b.
is developed in many low Cr picroilmenite cores and derived fragments; one of increasing Mg at
nearly constant Cr toward the grain rim. The example shown in Fig. 20b has a relatively wide
reaction zone and a correspondingly wide rim of MUM spinel. Remnant patches of ilmenite within
the MUM rim attest to the formation of the latter by the reaction ilmenite + liquid = MUM.
Supporting this is the fact that the spinel rim compositions reflect their ilmenite precursors
systematically, i.e., the highest Cr ilmenite has the highest Cr spinel reaction rim and so on.
Distinct from the ilmenites described above are the extremely Cr-rich examples from Pipe 9.
These contain up to 12 wt% Cr2O3 and consist of an ilmenite host with submicron Cr-rich planar
domains that appear to be chromite exsolution lamellae.
31
Apatite and Perovskite–Abundant apatite occurs as acicular grains commonly in radiating
stellate clusters in the groundmass and as larger more prismatic grains grown primarily within
calcite segregations. They are relatively Si-rich (0.7–1.1% SiO2) and Sr-poor (<1% SrO),
characteristic of Group I kimberlite apatite.
The majority of the perovskite occurs as euhedral to subhedral discrete grains (rarely as
aggregates) that are 0.02 to 0.1 mm across. Our limited data on perovskite shows them also to be
typical of Group I kimberlites with ~1.3% FeO, ~1.5% Nb2O5 and 0.1–0.3% SrO.
Figure 19. (a) Typical complexly zoned spinel from Pipe 1. Core of grainis titanian aluminous
chromite zoned to chrome spinel with a magnesian ulvöspinel magnetite mantle and a very
outer rim of titanian magnetite. (b) Rare example of pleonaste growth layer still partly intact.
Figure 20. (a) Example of high Cr picroilmenite core zoned to a lower Cr rim. (b) Small
fragment of a low Cr, low Mg picroilmenite core that is zoned to higher Mg (dark orange to
purple in this BSE image) without an increase in Cr. Relatively wide spinel reaction zone
retains some remnants of the precursor ilmenite (notably in the lower left).
Geochemistry
32
Major elements of the Finnish kimberlites show that the samples vary from being unaltered
and uncontaminated (e.g., Pipe 1) to highly altered and/or contaminated (e.g., tuffisitic diatreme
samples from Pipes 4 & 6). Only a few of the samples are uncontaminated or slightly
contaminated; the geochemical arrays point in the direction of increasing contamination for Pipes 3
and 14. Not unexpectedly, granitoid material appears to be the most significant contaminant.
Despite the indication for contamination from major elements, incompatible trace elements
show that this contamination mostly had a dilutive effect. Three samples from Pipe 1 have nearly
identical trace element compositions and represent the closest example to an uncontaminated
kimberlite. Relative to this Pipe 1 composition, the more contaminated samples have lower
concentrations of nearly all of the incompatible trace elements except K and Rb (Fig. 21).
Importantly, there is no evidence that the shape of the trace element profile was changed
significantly by the assimilation process. This is probably simply a consequence of the very high
trace element concentrations in the original kimberlite magma relative to the crustal assimilant.
Compared to average Group I kimberlites, the Finnish examples have very similar
incompatible element concentrations (Fig. 21). The only systematic differences are the
considerably lower concentrations of Hf and Zr and higher Ba in the latter. This is shown clearly in
a plot of Zr-Nb (Fig. 22) which compares kimberlites from various localities around the world. The
Finnish kimberlites plot with the relatively low Zr Koidu kimberlites of W. Africa.
Initial 87Sr/86Sr isotopic compositions of the Finnish kimberlites range from 0.7033 to 0.7055
calculated for ages of 434 Ma (Pipe 1), 593 Ma (Pipes 2 &3) and 450 Ma for the remainder. Initial
,Nd values range from +1 to –2.4. The Sr compositions overlap with and extend to more radiogenic
values than Group I kimberlites, but the ,Nd values, centered on bulk earth composition, are slightly
lower (Fig. 23). It must be noted that the Finnish kimberlites are not yet well dated (only two K-Ar
ages) leaving open the possibility that this field may shift slightly once more age dating has been
completed.
Figure 21. Primitive mantle-normalized incompatible element diagram showing selected
Finnish kimberlites and average Group IA and IB kimberlites (values from Smith et al., 1985
and Mitchell, 1986; primitive mantle from Sun & McDonough, 1989).
33
Figure 22. Nb vs. Zr for the Finnish kimberlites and related rocks which show that the least
contaminated kimbelirte compositions plot in the Koidu kimberlite field (Taylor et al., 1994)
and have lower Zr than the Group I kimberlites from South Africa. Also shown is lamproitelike Dyke 16.
Figure 23. Sr vs. Nd isotopic diagram for Finnish kimberlites. Note overlap with the Koidu
kimberlites.
34
Discussion and conclusions
A basic tenet of kimberlite petrology is that kimberlites are very complex and represent
mixes of various batches of magma and their phenocrysts along with abundant mantle xenocrysts
(Mitchell, 1986). They therefore do not represent true magmatic liquids.
The kimberlites from Finland exhibit typical mineralogy of Group I kimberlites from
elsewhere in the world including abundant macrocrysts of olivine, picroilmenite, Cr-diopside,
pyrope garnet; phenocrysts of olivine and microphenocrysts of monticellite, perovskite and spinel
in a calcite + serpentine matrix. Notably phlogopite phenocrysts are not abundant, and there are no
indications of Group II kimberlite (orangeite) affinities in these rocks. Detailed studies of the
complex zoning in constituent minerals reveal wide variations in the compositions of the magmas
that mixed to form the kimberlites discussed here. Major elements indicate that there is a crustal
component in some of the samples although trace elements show that this assimilation has had
mostly a dilutive effect. The Finnish kimberlites form a compositionally cohesive group of magmas
more akin geochemically to the Koidu type kimberlites of West Africa than the Group I kimberlites
from South Africa. Sr-Nd isotopic compositions either reflect derivation from a near bulk-earth
mantle or represent mixing of Group I kimberlite isotopic sources and a more radiogenic Karelian
Craton lithospheric mantle.
Figure 24. Topographic map showing the distribution of kimberlite boulders (stop 4.2.) and
Pipes 2 (stop 4.3.) and 3.
35
STOP 4.2. Kimberlite boulders
Late in the 1970´s, glacial boulders of well-preserved “almond rocks” were found. The name
referred to the numerous white almond-shaped inclusions that were later proven to be altered
peridotite xenoliths. Samples of these strange rocks were sent to diamond companies where they
were correctly identified and found to contain microdiamonds.
STOP 4.3. Pipe 2 Mäntyjärvi Niilonsuo
Pipe 2 was located by Malmikaivos Oy under a small swamp, about 1 km up-ice from the
glacial boulders (Fig. 24). Pipe 2 is an elongate body, roughly 300 m in length (Fig. 18). It contains
593 Ma hypabyssal kimberlite and volcaniclastic breccias. The hypabyssal rocks contain serpentine
pseudomorphs after olivine macrocrysts in a matrix of perovskite, serpentine, and calcite.
STOP 4.4. Soapstone factory (Tulikivi)
This combined lunch-break and excursion stop gives us an opportunity to learn about the
history and present activities of the Tulikivi Corporation at Juuka. Tulikivi Corporation and its
subsidiaries form the Tulikivi Group, which is the world’s largest and most technologically
advanced soapstone processing company as well as the largest manufacturer of industrially
produced heat-retaining fireplaces in the world. The Group is among the five largest stone
processing companies in Europe. In 2004, the Group’s revenue amounted to MEUR 55.3. The
Group has six production facilities, and its personnel consists of approximately 515 employees.
The Tulikivi Group focuses on stone processing as well as on the development and customeroriented manufacture of natural stone products and services. Tulikivi’s Fireplace Business
comprises soapstone quarrying, production, design, sales and marketing. The products include
soapstone fireplaces, bakeovens, cook stoves, custom-made products and sauna stoves. In addition,
soapstone lining stones to European heater manufacturers are supplied. Tulikivi Corporation’s
Architectural Stone Business comprises household interior decoration stone products and deliveries
of stone materials for construction sites. The products are made of soapstone, granite, marble,
limestone and other natural stones. The interior decoration stone products are suitable for various
household uses, such as for the kitchen, the bathroom, floors, stairs and wall tiling – and as
architectural stones, too.
Soapstone's reputation as a "wonder stone" first reached Helsinki in 1893, when a group of
distinguished industrialists founded the Finnish Soapstone Company. Soapstone's fame spread
rapidly to the imperial courts of St. Petersburg and Moscow. The stone was shipped from Lake
Pielinen via the Saimaa Canal to Russia to become building and fireplace stone material for the
palaces. A village formed around the quarry. Hundreds of stone workers moved into the area along
with their families. The Art Nouveau period was the golden age of soapstone. In the hands of
skilled sculptors, soapstone yielded to splendid columns, reliefs, facade decorations and stairways
in important buildings in Finland, still visible today. With the Russian revolution the export ceased.
After the Second World War the quarrying activity in Nunnanlahti came to a standstill. The
village was left deserted for twenty years. In the summer of 1979 all that was left in Nunnanlahti
was an abandoned quarry surrounded by a thick row of shrubs. In 1980 the old enterprise reopened
its operation, this time under the guidance of new entrepreneurs, Eliisa and Reijo Vauhkonen. The
company soon began to use the name Tulikivi, which means "fire stone" in Finnish.
36
Day 5: Koli layered sill, 1.97 Ga dykes, Stone centre, and Varpaisjärvi
dykes
The Koli layered sill in North Karelia, Eastern Finland
Jouni Vuollo
Geological Survey of Finland, P.O. Box 77, FIN-96101, Rovaniemi
Introduction
The Proterozoic quartzite hill of Koli is a famous tourist centre which offers beautiful views
over the Archaean bedrock terrain to the northeast and the Proterozoic domain to the southwest.
We also propose that the name of the hill should be applied to our present topic – the peculiar
layered sill which runs NW–SE, passes the hill of Koli beneath Lake Pielinen, and reaches the land
at Kaunisniemi, where it is very clearly visible. The sill has been under investigation by members
of the University of Oulu (Piirainen, 1969; Honkamo, 1972; Hanski, 1982, 1984, 1986; Vuollo,
1988; Vuollo & Piirainen, 1992) for a long time as a part of a theme "Basic igneous activities and
related ore deposits in Finland", being an example of the Jatulian magmatism of age about 2.2 Ga.
The sill is peculiar in many respects. It has Al203/TiO2 and Ti/V ratios of 5–6 and 31,
respectively, Fe and LREE are enriched and Al depleted, and the Mg' value is 0.59. Its cumulus
minerals are olivine, (chromite), clinopyroxene, ilmenomagnetite, (apatite), plagioclase and
intercumulus mineral hornblende, clinopyroxene, phlogopite, plagioclase and ilmenomagnetite
from bottom to top in this order. Since the plagioclase is always albite, the rocks have been called
"albite diabase or albite gabbros" in Finland, but we prefer here the term "karjalites' to cover the
rock types of this sill in general. The sill has all the features of a layered intrusion, forming a rock
series from wehrlite through clinopyroxenite and gabbro to granophyre below an upper layer of
cumulates and a chilled margin against the Archaean granite. Enrichment of PGE occurs in a
disturbed zone between the wehrlite and the clinopyroxenite.
General geology
The Koli layered sill (Fig 25) is located on the north eastern edge of the North Karelia Schist
belt, lying concordantly with the Proterozoic quartzites but in the Archaean basement granitoids, in
distance of about 200 m from the unconformity between the granitoids and quartzites. The surface
section of the sill is about 60 kilometres long and it is approximately 350 m thick. It has been
estimated to cover hundreds of square kilometres in area.
The Archaean basement granitoids surrounding the sill are tonalites, granodiorites and
granites, which vary in age from 2.9 Ga to 2.6 Ga, while the Proterozoic sequences discordantly
overlying the basement date from about 2.3 Ga to 1.97 Ga, beginning with Sariolan glacigenic
formations (Marmo & Ojakangas, 1984) and continued with Jatulian sequences (Vesivaara Fm,
Koli Fm, Jero Fm, Puso Fm after Kohonen et al., 1992), and ending with Kalevian metaturbidites.
Deposition of the Jatulian sequences was accompanied by the 2.2 Ga magmatism (Vuollo et al.,
1992), and the emplacement of the the Koli sill. The Koli sill and the Jatulian sequences are
intersected by 2.1 Ga and 1.97 Ga tholeiitic diabase dykes (see Pekkarinen, 1979 and Vuollo et al.,
1992, respectively). Deposition of the Kalevian turbidites was followed by mantle diapirism, as
evidenced by the Outokumpu ophiolite complex (Vuollo & Piirainen, 1989).
Finally, all the Karelian deposits were deformed during the Svekofennian orogeny about 1.9–
1.8 Ga ago. The metamorphic degree reached the lower amphibolite facies in the eastern part of
area, while the higher amphibolite facies was dominant in the western part, where granitoids of age
1.9–1.8 Ga intruded (Huhma, 1975).
37
Figure 25. General geological map of the eastern part of the North Karelia schist belt and the
excursion site at Kaunisniemi on the Koli sill.
Internal stratigraphy
The internal stratigraphy of the sill is similar to that of many layered intrusions, including
Marginal zones, Layered series and granophyre. The lower Marginal zone against the footwall
contact has not been found, but the other units, including the upper Marginal zone, are clearly
visible (Figs. 26 and 27).
The Layered series contains different ultramafic and gabbroic rocks. The lowermost parts are
composed of olivine (-chromite) and olivine-clinopyroxene cumulates. These rocks are then
followed stratigraphically by clinopyroxene, clinopyroxene-magnetite and plagioclaseclinopyroxene-magnetite cumulates in ascending order. The last-mentioned grades upwards into
granophyre which is found below the upper Marginal zone.
The upper Marginal zone separates the sill from the Archaean granitoids, the external contact
being very sharp. It is composed of a fine-grained chilled margin, noncumulate-textured
pyroxenitic rocks, and clinopyroxene cumulates. The differentiation of the Layered series is normal
from bottom to top but 'reversed' in the upper Marginal zone. The contacts between the layers are
relatively sharp, except for those between the gabbro and granophyre and between the granophyre
and the upper Marginal zone.
38
Figure 26. Geological map of the Koli sill from the Kaunisniemi section and the stop
numbers.
Cryptic layering (Fig. 27) is seen in a restricted form in the ferromagnesian silicates of the
Layered series, which are predominantly magnesium-rich in composition. One distinctive feature
throughout the sill is the presence of albitic plagioclase.
Petrography and mineralogy
The Layered series begins with a 50 to 70m thick wehrlite layer, the rocks of which are
composed of olivine (-chrornite) cumulates in which clinopyroxene (altered to tremolite) and
brown, edenitic hornblende (Table 1; 14) occur as intercumulus minerals and form oikocrysts
several centimetres in diameter. The Fo content of the olivine varies from 74 to 81 (Table 1; l-4).
Chromite occurs in euhedral grains which have been altered to Fe-chromite (Table 1; 5-8).
Intercumulus apatite (Table 1; 23-24) is preserved in the lowest part of the wehrlite layer. In the
terminology of Irvine (1982) the olivine (- chromite) cumulates are poikilitic orthocumulates. The
upper part of the wehrlite is heterogeneous and includes alternating layers of olivine (-chromite )
and olivine-clinopyroxene cumulates, which exhibit a rhythmic layering pattern in some places.
The primary minerals, excluding the intercumulus edenite (Table 1; 15), have been altered to
secondary minerals.
39
Figure 27. Detailed stratigraphy of the Koli layered sill at Kaunisniemi. Solid line = cumulus
mineral, dashed line – intercumulus mineral, plus sign = subsolidus mineral. Circled
numbers depict excursion stop numbers.
The wehrlites grade upwards rapidly to clinopyroxene mesocumulates. The clinopyroxenites
contain fresh, well preserved diopsidic clinopyroxene as cumulus crystals (Table 1; 19–21), and the
intercumulus minerals are albitic plagioclase (Table 1; 10) edenitic hornblende (Table 1; 16) and
ilmenomagnetite. Ilmenomagnetite becomes the cumulus mineral in the upper part of the
clinopyroxenite layer and reaches over 20 %, and apatite (Table 1; 25) appears as a cumulus
mineral alongside it. The magnetite clinopyroxenite is approximately 30 m in thickness.
40
Table 1. Mean chemical composition of minerals from Kaunisniemi section.
The contact between the magnetite clinopyroxenite and magnetite gabbro layers is marked by
a sudden increase in the proportion of plagioclase (Table 1; 11) as a cumulus mineral. Other
cumulus phases in the magnetite gabbro are augitic clinopyroxene (Table 1; 22) and
ilmenomagnetite (Table 1;9). In the upper part of the magnetite gabbro is encountered magmatic
lamination and also weak rhythmic layering.
The magnetite gabbro, rich in plagioclase, grades upwards to granophyre. The rock contains
large amounts of albitic plagioclase (Table 1; 12) and quartz (< - 25 %), forming a granophyric
texture, and also apatite (Table 1; 26), zircon and allanite as accessory minerals.The granophyre is
about 10 m thick. The total thickness of the upper Marginal zone is approximately 10 m. It begins
with a fine-grained chilled margin against the Archaean granite and grades rapidly to subophitic,
noncumulate-textured pyroxenite and on to coarse-grained cumulus-textured clinopyroxenite.
Except for the edenitic hornblende (Table 1; 18) the primary minerals have been altered in the
upper Marginal zone.
41
Geochemistry
Selected chemical analyses for the rock types are presented in non-volatile form together
with their CIPW norms in Table 2. In order to illustrate the trends in differentiation, the results of
the analyses are also plotted on diagrams in Figs. 28–30.
Fig. 28 shows variations in the modified differentiation index (von Gruenewaldt, 1973) and
in MgO and Cr content as a function of stratigraphic height. The MDI indicates a distinct reversed
fractionation trend for the upper Marginal zone, while the trend for the Layered series is normal,
from bottom to top. The extent of the variation, from 20 to 80, shows unusually marked
differentiation considering the thickness of the sill. The same fractionation trends are detectable in
the magnesium and chromium variations as in the MDI.
Figure 28. Variations in MDI and MgO and Cr content in the rocks of the Kaunisniemi
section, plotted against stratigraphic height.
The results presented in the CMA diagram (Fig. 29a) define an almost continuous, strongly
curved trend from the magnesium corner to the aluminium corner. The line to the calcium corner is
due to the dominant role of clinopyroxene among the cumulus phases. The AFM (Fig. 29b) and
Al2O3 vs. (FeO*/(FeO*+MgO) diagrams (Fig. 29c) show that all the samples analysed are situated
in the tholeiitic field, although differing from the normal tholeiitic trends. The cation plot of Jensen
(1976) for the Koli sill (Fig. 29d) projects along a trend line lying above the komatiitic and normal
tholeiitic suites, because of an enrichment of iron and depletion of aluminium in the karjalites. The
chondrite-normalized rare earth element patterns are seen in Fig. 30. Overall rare earth element
concentrations in the cumulates, the granophyre and the chilled margin of the Koli layered sill are
consistent with the light rare earth element-enriched character of the parental magma.
42
Figure 29. Analyses of the rocks of the Kaunisniemi section plotted on a) CMA, b) AFM, c)
Al203 vs. FeO*/( FeO*+MgO) and d) Jensen's diagrams.
The composition of the chilled margin (Table 2; 9) indicates the parent magma of the Koli
sill. This is in equilibrium with an olivine of Fo82, which corresponds to the olivines in the wehrlite
layer. The calculated mg' value for the liquid in the Koli sill is 0.59.
Platinum group elements (PGE)
A PGE concentration has been found between the wehrlite and clinopyroxenite layers (Table
3: 5), where the olivine (-chromite) and olivine-clinopyroxene cumulates form a rhythmic layering
pattern. In addition to the host, the zone contains small amounts of sulphides (pyrrhotite and
pentlandite) and PGE minerals (<< 10 µm), among which only froodite (Table 4; 1,2) has been
identified.
The concentration between the wehrlite and clinopyroxenite layer is not high, only 156 ppb
in the richest sample. The samples are enriched in Os, Ir, Ru, Rh, and especially in Pd, compared
with the chilled margin, while Pt is depleted (Fig. 31a and b). It is also significant that the S content
rises, but Ni, Cu and Cr remain at the same levels as in the adjacent cumulates (Fig. 32). The
(PGE)N pattern of the sill has features which are typical of primitive magmas (see Barnes et al.
1985).
43
Figure 30. REE pattern for cumulates from the Kaunisniemi section.
Conclusions
The Koli sill is an intrusion about 350 m thick and several hundreds of square kilometres in
area lying concordantly with Proterozoic formations in the Archaean basement, at a distance of
about 200 m from the unconformity. The intrusion has features in common with layered intrusions
composed of Marginal zones, Layered series and granophyre.
The parent magma of the sill, analysed from the upper chilled margin, is practically tholeiitic,
but differs in being rich in iron and poor in aluminium. The volatile content and fO2 were very high
compared with normal tholeiitic magma. The parent magma was strongly differentiated the MDI
varying from 20 to 80. The differentiation is "reversed" in the Marginal zone but normal in the
Layered series, proceeding from wehrlite (o(c)Cah, oaCh) through clinopyroxenite (aCp, amCp) to
gabbros (apmC, pamC) and granophyre below the upper Marginal zone.
PGE mineralization has occured between the wehrlite and clinopyroxenite in the disturbed
zone, where oC and oaC form a rhythmic layered pattern. The clinopyroxene is strongly altered to
amphibole in this zone and below it, but is fresh above it. The mineralized samples contain olivine,
clinopyroxene and edenitic hornblene and in addition to these ilmenite, Fe-chromite, polyrnineralic
sulphides (pentlandite-pyrrhotite-magnetite) and froodite.
44
The PGE content is low (156 ppb) in the richest sample, which is enriched in Os, Ir, Ru, Rh
and Pd and depleted in Pt compared with the upper chilled margin. The (PGE)N pattern has features
typical of primitive magmas. Alteration of clinopyroxene to tremolite only below the mineralized
zone proves that the fluid pressure was high during crystallization of the wehrlite layer but
suddenly decreased before the cumulation of the clinopyroxenite layer. This may indicate a mapa
eruption and fluids boiling from the sill. This boiling will have reduced the solubility of the sulphur
in the magma, causing sulphide and PGE secretion.
Figure 31. Chondrite normalized PGE pattern for a) the chilled margin and cumulates, b) the
zone enriched in PGE and its adjoining cumulates.
STOP 5.1. Olivine (-chromite) cumulate ( Fig. 33a)
A fairly well preserved olivine(-chromite) cumulate is visible in the lower part of the
exposure (Table 2; l), containing intercumulus clinopyroxene and edenitic hornblende. The same
cumulate in the upper parts contains larger amounts of poikilitic clinopyroxene, which has been
altered to an amphibole containing opaque pigment, as seen in the form of dark, raised nodules on
the weathering surface. The other intercumulus minerals are phlogopite, apatite, ilmenite and small
amounts of sulphides. The Fo content of the olivine is approx. 75 % in the lower part and increases
to approx. 80 % by the middle of the series. The olivine has been altered, chiefly
pseudomorphically, to lizardite and antigorite, but - < 20 % of it has remained unaltered in places,
while the cumulus chromite has been altered to Cr-magnetite. There is scarcely any primary
clinopyroxene remaining, it having virtually all been altered to amphibole and dark pigment. Large
amounts of the magmatic brown amphibole have been preserved, however, and it is only in places
that it has been altered to green or colourless amphibole. The olivine cumulates are texturally
poikilitic orthocumulates.
45
Table 2. Selected whole rock and trace element composition for the rock types together with
their CIPW norms and MDI indexes in the Koli layered sill.
STOP 5.2. Layered olivine (-chromite) (o(c)Cah), olivine-clinopyroxene (oaCh) (Fig. 33b) and
clinopyroxene (aCph) cumulates
The olivine-clinopyroxene cumulates (Table 2; 2) occur in a rhythmic unit of up to 2 m in
thickness in the uppermost part of the wehrlite layer, alternating with the olivine (-chromite)
cumulates. No primary minerals have been preserved in either with the exception of edenitic
hornblende. The olivine-clinopyroxene cumulates are practically black in colour at their weathering
surface and the olivine (-chromite) cumulates brown. This exposure also demonstrates the sharp
nature of the contact or unconformity between these cumulates and the dark green, almost entirely
unaltered clinopyroxene cumulate layer.
Slightly elevated PGE concentrations have been found about 1 m below the oC / aC contact
in the lower part of this rhythmically layered unit (Fig. 31; samples 30 and 463), and froodite has
been identified in one sample from the same unit (Table 4).
STOP 5.3. Clinopyroxenite (aCph) (Fig. 33c)
A pure clinopyroxenite (aC, Table 2; 3) in which the primary clinopyroxene has been
preserved practically unaltered is to be found in the lowermost parts of the series, where it has the
46
composition of a diopside-endiopside. Texturally this rock is a mesocumulate, the lower parts of
which contain up to 90 % clinopyroxene. The intercumulus materia1 consists of an albitic
plagioclase, which shows up as a lighter coloration at the weathered surface, and a little
ilmenomagnetite and edenitic hornblende. The plagioclase in the lowermost parts is well preserved
and has only a little epidote on its surface.
Tables 3 and 4. PGE and Au analyses (ppb).
Figure 32. Variations in S, Cu, Ni, Cr and PGE content in the wehrlite (o(c)C/oaC) and lower
parts of the clinopyroxenite layer.
47
STOP 5.4. Magnetite clinopyroxenite (amCph) and magnetite gabbro (apmC) (Fig. 33d)
Located above the clinopyroxenites (aC) are magnetite clinopyroxenites (amC, < 50 m) in
which ilmenomagnetite can account for up to 20 %. These magnetite clinopyroxenites (Table 2; 4)
contain very much larger amounts of chalcopyrite and pyrite than the other units. Primary
clinopyroxene has been encountered in the extreme upper parts of the magnetite clinopyroxenite,
but has been entirely altered to secondary amphibole further up in the series. The plagioclase is also
fairly extensively altered and contains large amounts of epidote. The other intercumulus minerals
are edenitic hornblende and sulphide ore minerals.
The magnetite clinopyroxenites (amC) are sharply divided from the magnetite gabbros above
them (apmC, Table 2; 5), in which plagioclase appears in the cumulus facies and increases
markedly in amount, to >50 %. This rock has once contained approx. 40 % clinopyroxene, but it
has now been entirely altered. Magnetite drops in amount to 5–10 % at this point. The intercumulus
minerals are edenitic hornblende and sulphides, and a little apatite is found as a cumulus mineral.
STOP 5.5. Layered and laminated magnetite gabbro (pamC) (Fig. 33e)
This rock is mineralogically similar to the magnetite gabbro above but here presents distinct
signs of magmatic lamination. The plagioclase and pyroxene grains in the cumulate are oriented in
the direction of the sill, and a rhythmic layering is detectable, with alternating layers richer in
plagioclase and pyroxene.
STOP 5.6. Granophyre (Fig. 33f), the upper Marginal zone (upper clinopyroxenite, Fig. 33f)
and chilled margin (Fig. 33g) and basement granite
The coarse-grained magnetite gabbro is seen to be overlain by approx. 10 m of a quartz-rich
granophyre (Table 2; 7) with a maximum content of 25 % quartz. The albitic plagioclase is also
coarse-grained, and accounts for 60–80 %. Typical of this unit is the presence of quartz-feldspar
'granophyric' intergrowths. Other minerals present are biotite, allanite, apatite, ilmenite, some
sulphides, and edenitic hornblende.
The granophyre grades steadily to a pegmatoid clinopyroxenite (Table 2; 8) with maximum
grains of 10 cm, which forms the lowermost part of the upper Marginal zone. The clinopyroxene
has been entirely altered to amphiboles (actinolite and hornblende), and the intercumulus
plagioclase to coarse-grained epidote. Small amounts of apatite, edenitic hornblende, ilmenite and
pyrite are also found.
A fine-grained chilled margin (Table 2; 9) and subophitic pyroxenite of a non-cumulate
texture occur at the contact with the Archaean granitoids. The principal minerals are a green
actinolite hornblende and an albitic plagioclase.
48
Fig. 33. a) Olivine-(chromite) cumulate, 26-JIV-85, 24.30 m, b) the contact between olivine-(chromite)
and olivine-clinopyroxene cumulates, 33B-JIV-85, 36.20 m, c) clinopyroxene curnulate, 35-JIV-85,
48.00 m, d ) the sharp contact between clinopyroxene-magnetite and clinopyroxeneplagioclasemagnetite cumulates, 43-JIV-85, 231.60 m, e) laminated plagioclase-clinopyroxene-magnetite
cumulate, 47-JIV-85, 263.40 m, f) granophyre, 51-JIV-85, 312.60 m, g) upper clinopyroxenite, 53-JIV80, 325.40 m, h) chilled margin, 430-JIV-80, 330.00 m.
49
STOP 5.7. 1.97 Ga dyke
The ~1.98 Ga dyke swarm is not as voluminous as the ~2.1 Ga dyke swarm (Vuollo, 1994).
However, recent geochronological (Vuollo & Huhma, 2004) and field studies, combined with
aeromagnetic data, show that the ~1.98 Ga dykes are found throughout the Archean basement and
the Karelian formations. The swarm consists of up to 70 m wide dykes that form prominent NWtrending (320–340°), linear features >120 km in length (Kuhmo block).
Tholeiitic dykes of ~1.97 Ga age have been found in connection with Proterozoic quartzites
in the Koli area (Vuollo et al., 1992). The dykes are only slightly altered and contain primary
olivine, pyroxenes, and plagioclase and have an ophitic texture. The zircon age (1965±10 Ma) is
supported by the Sm-Nd mineral age of 1985±80Ma. The 1.97 Ga dykes include both quartz and
olivine normative compositions. They are lower in Fe, P, Ti, Zr, and Y and higher in Mg, Cr, and
Ni than the 2.1 Ga dykes in the Koli region. Mg´ values range from 71 to 49. The REE-patterns
show slight enrichment with (La/Yb)N= 1.6–2.6 and REE concentrations range from 5 to 28 times
chondrites. Petrogenetic modelling implies that the dykes have been generated by 20–30% partial
melting of a LREE-enriched mantle peridotite. The diabases have slightly positive initial ,Nd values
(+1.8) and show geochemical affinitities to island arc tholeiites.
STOP 5.8. The Finnish Stone Centre, Juuka
The Finnish Stone Centre aims at increasing the recognition of Finnish natural stone,
promoting the competitiveness of the natural stone sector, and launching new businesses. The
world of stone is introduced from the viewpoint of art, science and construction.
Exhibition–“Of Many Forms” is an exhibition that tells how Finnish design has revised and
renewed sculpture. Though internationally renowned, Tapio Wirkkala and Timo Sarpaneva, as
designers, were not accepted in the field of the arts. Nonetheless, they had strong impact on the
development of modern sculpture in Finland. The new concepts emerged in the works of Kain
Tapper, among other sculptors. The works on display are from the prime creative period of the
artists. The exhibition focuses on their sculptural works in wood, glass, bronze and stone. The
works combine the features of Finnish nature and peasant craft traditions with international
modernism.
Soapstone art–In the span of several decades, German honorary consul Karl Heinz Arnold
has gathered a unique and extensive collection of soapstone art. The collection includes over 6000
miniature sculptures, of which more than 460 items are displayed at the Finnish Stone Centre. The
items in the collection originate from all around the world: human figurines from the Congo, Inuit
sculptures portraying hunting scenes, oil lamps from Afghanistan and home altar vases from China.
The items on display include cylindrical seals and printed clay tablets from 2950 BC that were
found in the area currently known as Iraq. The oldest of the works date from over 5000 years ago.
Stone Shop–The Stone Shop at the Finnish Stone Centre offers a wide variety of stones and
stone products for sale. Their assortment includes rough stones, soapstone products by Hukka
Design, stone jewellery, gifts and utility articles as well as ceramics. Curiosities include petrified
wood, sand roses, fossils and dinosaur bones.
Stone Park–The Stone Park is an oasis in the Stone Village. The various terraces of the park
have been bordered with pavings and groups of plants while taking pre-existing vegetation into
consideration. The Stone Centre faces the park, and a shallow pool has been constructed in front of
the main façade. Between the pool and the river lies an artificial island, which has been landscaped
using earth and rubble. The Stone Park has been designed in co-operation with teams of students
from the University of Art and Design, Helsinki (Taideteollinen korkeakoulu), and from North
Karelia Polytechnic (Pohjois-Karjalan ammattikorkeakoulu).
Studio A&G Mining–Studio A&G creates Finnish jewellery in the Finnish Stone Centre in
Nunnanlahti, Juuka. The jewellery selection has been inspired by the nature of Eastern Finland,
50
where the jewels used for them also originate from: diamonds, garnets, diopsides and ilmenites
from the municipality of Kaavi. The jewellery has been created by the prominent Finnish designers
Marja Kurki, Kaisa Blomstedt, Jussi Louesalmi and Timo Nupponen. Special mention is due to
Outi Toivanen, a talented young designer working for the company whom one can see at work
every day in our studio at the Stone Centre. Individual pieces of jewellery for the company have
also been designed by Finnish celebrities Tommy Tabermann, Kata Kärkkäinen, Maarit and Sami
Hurmerinta, Åke Lindman and Juice Leskinen.
2.1 Ga Fe-tholeiitic dykes of Varpaisjärvi
Jorma Paavola
Geological Survey of Finland, P.O.Box 1237, FI-70211 Kuopio
Early Proterozoic, NW-trending mafic dykes are abundant in the Iisalmi block. In areas
where they cut the Archean basement granulites, the primary magmatic mineralogy is often wellpreserved, presumably due to the dry mineralogy of the wall-rock granulites. Early Proterozoic
diabase dykes are abundant in the Sonkajärvi–Varpaisjärvi area, Central Finland (Fig. 34). The
dykes have intruded into the Archaean (3.1–2.7 Ga) bedrock. The predominant strike of the more
or less vertical dykes is NW-SE and their width varies from some meters up to ~150 meters.
Based on 50 elemental geochemical dyke analyses across the area, the 2.1 Ga dykes comprise
a relatively evolved, coherent group which is characterised by tholeiitic trends: Concentrations of
TiO2 (0.8–2.5 %) and Fe2O3tot (11–19 %) increase and those of Al2O3 (16–11 %) and CaO
decrease (12–8 %) along with decreasing MgO (8–4 %). These correlations, the positive
correlations between MgO, Ni, and Cr, and the nearly constant SiO2 contents (48–51 %) are readily
ascribed to fractional crystallization of a gabbroic (plagioclase, clinopyroxene, olivine, Cr-spinel)
mineral assemblage (Toivola, 1988). Chondrite-normalised REE-patterns indicate variably LREEenriched compositions; (La/Yb)N ranges typically from 1.1 to 2.5 but LREE-depleted patterns with
(La/Yb)N of 0.48 have also been found. The dykes show affinities to oceanic basalts in terms of
TiO2-K2O-P2O5-Zr -ratios and -concentrations. Their overall compositional uniformity suggests
derivation from a common parental magma.
STOP 5.9. Varpaisjärvi dyke
The Nieminen dyke in the Jonsa area, Varpaisjärvi, is a good example of the 2.1 Ga dykes.
Dykes within the Jonsa granulite block are abundant and strike ~W; elsewhere in the Iisalmi–
Varpaisjärvi area the predominant strike is roughly NW. The Nieminen dyke is well-exposed in a
presently abandoned dimension stone quarry. Both contacts are visible; the width of the dyke is
25–30 m. The dyke is homogeneous and unaltered. The main silicate constituents are plagioclase
and clinopyroxene and minor orthopyroxene. Secondary minerals, such as light-green amphibole,
biotite, and epidote occur sporadically on the grain surfaces.
Geochemically, the Nieminenn dyke is a subalkaline, tholeiitic basalt and similar to the other
dykes in the region with e.g. TiO2 of 1.2 % and MgO of 6.2 %. Repeated (n=12) Sm-Nd isotopic
analyses of minerals in sample A1223 from the quarry yield an isochron that corresponds to an age
of 2127±42 Ma and an initial ,Nd of +2.5. In places, the dyke contains relatively more coarsegrained and more plagioclase-rich patches (sample A1368). A small amount of zircon was obtained
from such material and facilitated eight U-Pb analyses. The results have been controversial: A few
brownish, long crystals with relatively low density yielded a slightly discordant 207Pb/206Pb age of
1866 Ma. The relatively heavier zircons yielded variably discordant but consistent 207Pb/206Pb ages
of ~2.1 Ga; rejection of one poor analysis results in a chord with intercepts at 2100±3 Ma and 256±150 Ma (MSWD=0.85). The average 207Pb/206Pb age of 2106±6 Ma provides the best estimate
51
of the age of magmatic zircon. The few brownish zircon grains with significantly younger
207
Pb/206Pb-age are quite exotic and form only very minor population (all grains were used for
analysis C). It remains obscure whether there have been some later veining.
The geochronological studies of the Nieminen dyke indicate that the intrusion of the
Nieminen dyke, and, presumably, the other Fe-tholeiitic dykes in the region, took place at ~2.1 Ga.
The positive intial ,Nd value (+2.5) indicates that the magma was derived from a depleted mantle
source (,Nd (2.1 Ga) of DM is +3.3) and precludes strong interaction of the magma with the
Archaean granulite wall-rock, which has ,Nd (2.1 Ga) values on the order of –10 (leucosome in
mafic granulite) to –6 (Varpasijärvi enderbite) (Hölttä et al., 2000).
Figure 34. Sketch map showing the distribution of the diabases in the Sonkajärvi–
Varpaisjärvi area. The locality of the Nieminen dyke is depicted by the square. Symbols: 1.
Archaean amphibolite-banded tonalitic–trondhjemitic migmatite or granitoid /
corresponding hypersthene-bearing rock. 2. Proterozoic intrusive/medasediment. 3. Fracture
or fault. 4. Diabase dyke. (Toivola et al., 1991)
52
Day 6: Jormua ophiolite and the oldest rock in Europe
Dykes in the Jormua ophiolite
Asko Kontinen
Geological Survey of Finland, P.O.Box 1237, FIN-70211 Kuopio
The Jormua Ophiolite is an allochthonous mafic-ultramafic rock complex that at ~1.9 Ga was
thrusted onto the Archaean craton margin, from a formative setting within a passive margin
environment ~100 km southwest from its present position (Kontinen, 1987) (Fig. 35). The complex
exposes a unique fragment of Proterozoic Red Sea-type oceanic crust. The complex comprises two
distinct units: 1) fragments of Archaean subcontinental lithospheric mantle that became exposed
from beneath the Karelian craton by detachment faulting, following its complete break-up
(Peltonen et al., 2003), and 2) alkaline and tholeiitic igneous suites that were emplaced within and
through the lithospheric mantle at ~2.1 Ga and 1.95 Ga, respectively. At the prerift stage of the
continental breakup (~2.1 Ga), residual lithospheric peridotites became intruded by alkaline melts
that formed “dry” clinopyroxene cumulate dykes. Slightly later, this same piece of mantle became
extensively intruded by hydrous alkaline magmas that resulted in formation of high-pressure
hornblendite-garnetite cumulates deep in the ophiolite stratigraphy and fine grained OIB-type
dykes at more shallow levels. Simultaneously, the residual peridotites became metasomatized due
to porous flow of the melt in the peridotite matrix. The alkaline magmatism was soon followed by
lithospheric detachment faulting that exposed the subcrustal peridotites at the seafloor, where they
at 1.95 Ga became covered by tholeiitic (E-MORB) pillow and massive lavas and intruded by
coeval, tholeiitic (E-MORB) dykes and gabbros. Since transitional contacts between all main
ophiolite units can be demonstrated, the Jormua Ophiolite Complex is interpreted to represent a
practically complete sample of seafloor from an ancient ocean-continent transition (OCT) zone,
strikingly similar to that reported from younger similar tectonic settings, such as the Cretaceous
West Iberia nonvolcanic continental margin.
STOP 6.1. Serpentinite and clinopyroxenite and hornblendite-garnetite dykes
The western block of Jormua ophiolite complex, best exposed at Hannusranta, comprises a
0.5–1.5 km wide, over 10 km long sliver of lherzolitic-harzburgitic mantle peridotites transformed
into antigorite±olivine metaserpentinites (Fig. 36). The peridotites and mantle foliations in them
are truncated by abundant clinopyroxenite and hornblendite dykes. Some of the hornblendite dykes
comprise (locally abundant) garnet pseudomorphs. Two distinct subtyps of clinopyroxeneamphibolegarnet mantle dykes can be distinguished on the basis of their cross-cutting relationships,
colour of weathering surfaces, grain size and mineral modes. Individual Type A mantle dykes
(clinopyroxenite) are generally 0.2–1 m wide, locally forming dyke-in-dyke sets up to >10 m wide.
They were originally medium-grained “dry” clinopyroxene ortho- or mesocumulates that contain
subordinate amounts of Al-rich calcic amphibole. Most of the Type B mantle dykes are coarser
grained (typically >10 mm) than Type A dykes, being “hornblendites” originally with or without
garnet. Less common Type B subtypes include pegmatitic varieties with amphibole up to 10 cm,
garnetite dykes, and magnetite+ilmenite+zirconolite-rich clinopyroxene cumulates. Single zircon
SIMMS and Sm-Nd isotope data from the clinopyroxenite dykes suggest they emplaced ~1.95 Ga
ago and that the host mantle would be a piece of late Archaean (>3 Ga) subcontinental lithospheric
mantle (Peltonen et al., 2003). All the main types of mantle dykes in the western block of the
Jormua ophiolite will be seen. The Hannusranta stop requires 1.5 km of walking, partly along a
clear path, partly in a bushy, rocky terrain. Waterproof boots are recommended.
53
Figure 35. Generalised geological maps of the central part of the Fennoscandian Shield (inset)
and the Jormua Ophiolite. Suture zone on the inset map indicates the approximate location of
the tectonic boundary between the 2.7–3.1 Ga craton and the 1.88 Ga Svecofennian arc
complex.
STOP 6.2. Serpentinite and “OIB” metalamprophyre and “E-MORB” dykes
The Lehmivaara outcrops are located in the middle part of the central block of the Jormua
ophiolite (Fig. 36). Characteristic for the middle block are myriads of metadolerite to metabasaltic
dykes of E-MORB chemical affinity. The E-MORB dykes frequently occur in metres to tens of
metres wide dyke-in-dyke sets, inside of which the individual dykes are mostly 20–120 cm wide.
Geochemically, they resemble the pillow and massive lavas of the Jormua Ophiolite (Fig. 37a,c),
but have notably lower SiO2 and alkali contents, and high MgO and FeO* values (Peltonen et al.,
1996). These differences, however, have been ascribed to interaction with the wall-rock
serpentinites. Based on Zr/TiO2 vs. Nb/Y relationships, all of the E-MORB dykes are subalkaline
basalts. In addition, there are older, markedly LREE-enriched dykes (Fig. 37d), being crosscut by
the E-MORB dykes. These “OIB” dykes have high Nb/Y values and are usually discrete (5–150
cm wide) and far fewer in number than the E-MORB-dykes. They are strongly altered but probably
mostly of ultramafic lamprophyre origin. SIMMS zircon data from the OIB dykes demonstrate
presence of Archaean zircons, probably inherited from the host peridotites. An origin as Archaean
SCLM seems likely also for the Jormua central block peridotites. Doing the stop 2 involves 2 km
walking, partly along a forest road, partly in an open-cut forest. Sturdy shoes or boots are
recommended. Given a sunny day, an added bonus at this stop is a wide, blue view over the eastern
part of the lake Oulunjärvi, the forth-largest lake in Finland.
54
Figure 36. Stratigraphic reconstruction of the Jormua Ophiolite. The western block,
extensively intruded by clinopyroxene+amphibole±garnet mantle dykes, is compositionally
and structurally distinct from the other main blocks.
Figure 37. Chondrite-normalised REE-patterns for the E-MORB-type (“Main Suite”) pillow
lavas (a), sheeted dykes (b), and “deep dykes (c) and the OIB dykes (“Early dykes”) (d). Inset
in (b) illustrates the fractionated character of an anomalous dyke sample.
55
STOP 6.3. Sheeted dykes
After mantle peridotites, sheeted dykes are the second most voluminous lithological
component in the Jormua Ophiolite. Large outcrops composed 100% of mostly 20–120 cm wide
metadolerite-metabasalt dykes are present in the Sammakkolampi area. The dykes are of E-MORB
tholeiite composition (Fig. 37b), partly plagioclase phyric, partly nearly aphyric. Most of the
classical features of sheeted dyke units of ophiolites are seen in the Sammakkomäki outcrops.
There are several generations of broadly subparallel dykes, the older being intruded and split by the
younger resulting in “marginless” (septa without chill margins) and “half” (septa with one chill
margin) dykes. Branching of the dykes and apophyses along their contacts are common. All these
features are well visible due to the nicely ice polished nature of outcrops at Sammakkomäki.
The Sammakkolampi stop poses a walk of 2 km, for most part along a forest road. Sturdy
shoes make the visit comfortable.
STOP 6.4. Harzburgite and metarodingitic gabbro dyke
Jormua peridotites are all pervasively altered and metamorphosed to mostly
nonpseudomoprphic antigorite serpentinite. In these metaserpentinites, the only mantle mineral
preserved, to some extent, is chromite. The Kontiomäki Shell outcrop is an example of those rare
Jormua metaserpentinites that still show some pseudomorphic mantle peridotite textures. Pale-grey
coloured spots in the weathering surface of the outcrop are pseudomporps after orthopyroxene,
while the intervening, slightly darker coloured serpentine mass is after olivine. In thin sections of
the metaserpentinite it can be seen that the orthopyroxene pseudomorps comprise bastite replaced
by antigorite, and that the intervening antigorite mass contains remains of magnetite dust meshes
typical of low-T serpentine after olivine. Oxide grains, largest of which have chromite cores, define
a weakly developed foliation of an obvious mantle origin.
A metre or so thick very coarse-grained gabbro dyke crosscuts the harzburgite. Typical of
lower amphibolite facies metarodingites, the gabbro dyke consists of clinopyroxene (diopside)
variably replaced by amphibole, while the original magmatic plagioclase has been replaced largely
by epidote minerals. In nearby outcrops similar metarodingic gabbro dykes comprise locally
abundant grossular garnet. A U-Pb zircon age of 1953±2 Ma has been obtained for the mantle
gabbro dykes of the Jormua eastern block. The stop 6.4. outcrop is located just 100 m to the south
of Shell Kontiomäki, our luncheonette while visiting Jormua.
STOP 6.5. Gabbros, ferrogabros and plagiogranites
A thick unit of gabbros, layered mafic-ultramafic for its basal parts, which is characteristic of
many classical ophiolites, is distinctly missing in the Jormua Complex. Instead all gabbro
occurrences in Jormua represent stocks and dykes intrusive into the mantle unit. Best outcrops to
study the variety of the Jormua gabbros and related intermediate-felsic rocks are at Sarvikangas
locality within the western block of the ophiolite.
Eastern part of the Sarvikangas area comprises greyish green, coarse-medium grained
gabbros intruded by many metadolerite-basalt dykes, often in dyke-in-dyke sets and frequently
showing chill margins against the host gabbros. In the western part of the area gabbro, ferrogabbro
and minor microgabbro-diorite-leucotonalite (plagiogranite) are found in close association. The
gabbros are mostly of “isotropic” character, while the ferrogabbros range “isotropic” to “variedtextured” (heterogenous, coarse-pegmatoid). Transitions between the gabbros, ferrogabbros and
microgabbro-diorite-leucotonalites are mostly abrupt. There are outcrop features that suggest
emplacement of the microgabbro-diorite-leucotonalite suite was a multistage progressive process
involving mingling and various pulses of magma.
56
Mineral assemblages in Jormua gabbroic rocks are usually wholly metamorphic, with the
exceptions that occasionally relicts of primary An-rich plagioclase are preserved in gabbros and
primary ilmenite commonly in ferrogabbros. The diorite-leucotonalites at Sarvikangas have the
chemical characters of ocean ridge granites being e.g. very high in Na2O but very low in K2O and
high in Y and Nb.
Zircons from one leucotonalite sample from Sarvikangas yield a somewhat unprecise U-Pb
age of 1954±12 Ma. Nd (1.95 Ga) for this sample is +1.9, noting that Jormua lavas and dykes yield
an average Nd (1.95 Ga) of +2, this suggests that the Sarvikangas gabbroic and plagiogranitic rocks
were intimately related to the 1.95 Ga oceanic crust-forming process.
Doing the stop 2 involves about 1 km walking in a bushy, rocky terrain. Once again, sturdy
shoes or boots are recommended.
STOP 6.6. Podiform chromitite
Typical of nonsuprasubduction zone ophiolites, chromitites are rare in Jormua ophiolite.
They have been located only within one small peridotite slice (200x700 m) at Pitkänperä, which
comprises a few small pods of massive chromitite. Stop 5 has to show the largest of the known
pods, which is ca. 1 m thick and at least 5 m long. The core part of the Pitkänperä pod consists of
coarse-grained well-preserved, moderately aluminous chromite (25% Al2O3, 45% Cr2O3), while
chromite at the margins of the pod show heavy fracturing and alteration into carbonate and chlorite.
Some of the larger chromites in the core of the pod preserve tiny, roundish inclusions of sodic
tshermakitic amphibole, probably after melt inclusions. The host peridotite shows intense prepeakmetamorphic carbonate alteration. This is a roadside stop.
STOP 6.7. Pillow lavas
The extrusive unit of the Jormua ophiolite comprises mainly pillow lavas of E-MORB
composition (Fig. 37a). These are met about only in the eastern part of the ophiolite, where they
occur in tectonic slices up to a couple of hundreds of metres thick and several kilometres long. The
Kylmä lava slice has a maximum thickness of ca. 400m. It is underlain by upper Kaleva
metawackes-black schists and overlain by a thin sheet of serpentinite-talc-carbonate rocks. The
Kylmä sequence is 100% metavolcanic without any terrigenous or cherty intercalations. Most of
the flows are pillowed with occasional massive basal parts. The Kylmä pillows often preserve
distinct gas discharge channels and thin chill margins. Minor pillow breccia and hyaloclastite
developments occur at the top parts of flows. In one locality there are clinopyroxenite-gabbro and
dolerite dykes inside of the pillow lava sequence. Visting the Kylmä stop involves some 600 m of
waking in an open-cut, rocky forest. During the walk some wet places may be crossed.
The oldes rock of Europe at Siurua
Katja Lalli1 & Hannu Huhma2
1
Geological Survey of Finland, P.O.Box 77, FIN-96101 Rovaniemi
2
Geological Survey of Finland, P.O.Box 96, FIN-02151 Espoo
The poorly known Pudasjärvi Complex consist mainly of Archaean gneisses and granitoids
and abundant amphibolites. Proterozoic granites and diabase dykes have intruded the Arhcaean
gneisses. The N-S trending Pudasjärvi Granulite Belt is located in the middle of the gneiss
complex. It is >50 km long and is cut by several fault and shear zones into smaller pieces. The
metamorphic grade between the blocks varies from amphibolite to granulite facies. West from
Pudasjärvi Granulite Belt lies the Oijärvi greenstone belt with associated gold deposits.
57
STOP 1.2 The oldest rock of Europe at Siurua (3.5 Ga)
The Siurua trondhjemite gneiss is the dominant rock in a large outcrop area (Fig. 38). The
rock is granoblastic, medium-grained and consists of plagioclase, quartz, potassium feldspar, dark
biotite and accessory fluorapatite, zircon, monazite, ilmenite, magnetite and metamict allanite.
Dark green hornblende and garnet occur in one sample. The chemical composition is close to that
of average Archean TTG gneisses. The major differences are the depletion of phosphorus and
strong enrichment of Th and LREE. The REE-patterns of the Siurua gneiss and felsic granulite of
the Pudasjärvi Granulite Belt are similar and exhibit a negative Eu anomaly; however, the REE
concentrations of the Siurua trondhjemite are about an order of magnitude higher. Gneissic banding
is seen on outcrop, but penetrative deformation is not noticeable under microscope. Gneissosityparallel veins of granite pegmatite and roundish boudins of mafic granulite appear on outcrop,
although the gneiss is not in its present state on granulite facies.
The zircon population has primarily a magmatic and relatively homogenous appearance. Ionmicroprobe U-Th-Pb analytical data appear to be reverse discordant. Nevertheless, out of 18
analysed grains 15 yielded 207Pb/206Pb ages of 3.4–3.5 Ga (Mutanen & Huhma, 2003). The best
estimate for igneous crystallization of the Siurua gneiss is ca. 3.5 Ga, which reveals that the
trondhjemite gneiss is the oldest rock so far identified in the Fennoscandian Shield. Signs of an
even older crustal contribution are obtained from a 3.73 Ga old zircon core.
Figure 38. Location map of Siurua showing the distribution of outcrops and the age dating
site. Base map © National land survey of Finland, licence No 13/MYY/05.
58
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Dyke swarms - time markers of crustal evolution

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