Neoproterozoic granitic magmatism and tectonic evolution of the

Journal of African Earth Sciences, Vol. 20, No. 2, pp. 103-118, 1995
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Neoproterozoic granitic magmatism and tectonic evolution of the northern
Arabian Shield: evidence from southwest Jordan
K.M. IBRAHIM t and W.J. McCOURT 2
tGeology Directorate Natural Resources Authority, A m m a n HK, Jordan
2British Geological S u r v e y International Division, Nottingham, UK
(Received 22 March 1994 : accepted 29 N o v e m b e r 1994)
Abstract - The crystalline basement outcrop exposed in southwest Jordan, is subdivided into two broad lithosITatigraphicdivisions, the older Aqaba complex and the Araba complex separated by a regional imconformity. The
Aqaba complex principally comprises calc-alkaline plutonic rocks of probable age range 630580 Ma cut by extensive regional dyke swarms while the Araba complex is characterized by alkaline rhyolitic volcanics and minor
coeval granites with an approximate age of 550540 Ma. The plutonic rocks of the Aqaba complex are interpreted
to be the products of subduction at, or dose to, a continental margin, while the Araba complex rocks were most
probably formed in an extensional setting. The Aqaba complex granitoids are isotopically primitive and were derived from a depleted source region like the mantle. The Arabian Shield of southwest Jordan provides an example
of rapid crustal growth during the Neoproterozoic and the new data gives no support to models invoking the
presence or involvement of significantly older basement in its development.
R6sum~ - Le sode cristallin affleurant dans le sud-ouest de la Jordanie est subdivis~ en deux complexes lithostratigraphiques principaux qui sont s ~ a r ~ par une discordance r~gionale: le complexe d'Aqaba, le plus ancien, et
le complexe d'Araba, le plus jeune. Le complexe d'Aqaba comprend principalement des roches plutoniques calcoalcalines d'ages probables compris entre 630 et 580 Ma qui sont recoulxSespar de grands faisceaux de filons r6gionaux. Le complexe d'Araba se caracKnisepar des rhyolites alcalines et des granites contemporains en quantit~
mineure d'ages compris approximativement entre 550 et 540 Ma. Les roches pluton/ques du complexe d'Aqaba
sont interpr~t~es en tant que produits de subduction sur ou pros d'une marge continentale alors que lea roches du
complexe d'Araba se sont probablement formcSesdana un contexte extensif. Lea granito~des du complexe d'Aqaba
posstsdent une signature isotopique primitive et proviennent d'une source appauvrie tel le manteau. Le BouclieT
arabique du sud-ouest de la Jordanie foumit un exemple de croissance crustale rapide/~ la fin du Prot~rozoIque.
Ces nouveUes donn6es ne confirment donc pas les mod6les invoquant la pr6sence ou la participation d'un socle
significativement plus vieux darts la formation de ces deux complexes.
INTRODUCTION
likely to have been f o r m e d a n d to consider h o w they
fit into established models of c o n v e r g e n t margin accretionary processes and collisional tectonics for the
plutonic evolution of the Neoproterozoic Arabian
Shield.
Plutonic rocks of N e o p r o t e r o z o i c age comprise the
bulk of the Precambrian b a s e m e n t sequence exposed
in southern Jordan a n d are d o m i n a t e d b y postorogenic calc-alkaline granitoids cut b y n u m e r o u s
dyke swarms. As part of the Jordan National Mapping Project the systematic 1:50 000 m a p p i n g of the
basement, which covers a b o u t 1400 k m 2, was u n d e r taken between 1984-1988 a n d included a detailed
sampling p r o g r a m m e for whole rock (WR) geochemistry and geochronology investigation of the main
plutonic units. The p u r p o s e of this s t u d y was to establish the petrographic a n d geochemical characteristics and intrusive ages of the granitoid plutons and
establish a m o d e l for their tectonic setting a n d plutonic evolution. The aim of this p a p e r is to present the
results of this investigation s u m m a r i z i n g the major
and trace element geochemistry of the b a s e m e n t
granitoids and associated felsic volcanic rocks, to assess the petrogenetic processes by which they are
classification a n d n o m e n c l a t u r e
The petrographic classification and formal nomenclature of the plutonic rocks of the Neoproterozoic
basement of southwest Jordan covered in this paper,
follows the recommendations of the IUGS (Streckeisen
1976; Le Maitre 1989; Le Bas and Streckeisen 1991) and
the North American Commission on Stratigraphic
Nomenclature (NACSN, Steinmetz 1983). The detailed
lithostratigraphic subdivision of the rocks into units,
suites etc. is based on the field orientated techniques of
Cobbing et al. (1977) within the guidelines of the
NACSN. Primary mappable plutonic units, the lithodemesne of the NACSN, are defined on the basis of
field characteristics and composition utilizing such
factors as texture, grain size, mineralogy, xenolithic
Plutonic
103