Grenville orogenic affinities in the Red Mountain area, Llano Uplift

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Grenville orogenic affinities in the Red Mountain area, Llano Uplift, Texas
KAREN E. CARTER
Department of Geological Sciences, The Universiry of Texas at Austin, Austin, 7X 78713-7909, U.S.A
Received March 18, 1988
Revision accepted November 1, 1988
The Red Mountain area in the southeast comer of the Llano Uplift contains evidence of a Grenville-age, polyphase, noncoaxial deformational history. Four synmetamorphic deformational phases affected the Precambrian Click Formation of the
Packsaddle Gmuo. and a sinele shear-related deformation is observed in the Biz Branch and Red Mountain eneisses. The first
phase otdcformauon produced tlght folds u,ith an axial-planar metamorph!~l ~ ~ l ~ a t iThe
. ~ nsecond,
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and moit Intense. pervnslve deformatlonal phase pnrduced the regional meramorphlc layering. ~sncllnnlfolds, rransposition of brddin~.and mvl<~nltic
~extun!sin thc gneissrs. The third deformallon is c h a r . ~ ~ t c ~ nby~ ctlght
~ d to lsocl~nal.nonh-verging folds uiih sn sxtal -planar
difitrentlation crenulation cleavane.
- The founh ph3.c 01 Jch,ntvatiun ~rullc.Jin a nonh-nonhuest-trenJInC
- d~iicrent~:rti,~n
crenulation cleavage that is axial-planar to small east-nonheast-verging folds. The growth of codierite over all foliations
indicates that low-pressure metamorphism postdated all four deformational phases.
The second deformation had begun at the Latest by 1167 12 Ma, the age of metamorphism of the gneisses, and before the
intmsion of melarhyolite dikes dated at 1080 30 Ma. Thus, the omgenic event(s) responsible for the complex deformational
and metamorphic history is Grenvillian in age.
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L'etude de la &ion de Red Mountain, dans le secteur sud-est du soulbvement de Llano, dvsle qne son histoire est marquee
par une deformation non-coaxiale, polyphas& et d'zge grenvillien. Quatre phases de dkformation synmCtamorphiques ont
affect6 la Formation de Click, du Gmupe de Packsaddle, d'8ge pdcambrien, tandis qu'une seule deformation reliee au
cisaillement a kt6 observ6e dans les gneiss de Bin Branch el de Red Mountain. La vremibre vhase de dCformation a cdk des
plis fern& avec foliation mitamorp]iique axiale ;lanaire. C'est la deuxikme phase de dkformkion qui est la plus intense, elle
est &netrative et a pmduit le litage mitamorphique,
. . les vlis isoclinaux, la transwsition de la stratification et les textures
mylonit~quesdm.: lc.; pn:i\\ la t m ~ ~ ~ cdeformatton
nie
est caracl~nsr'epar dc* p l ~ \~s.ulinauxci fermks, de vcrgence nnrd,
Q C planaire J e dlffCrmciation-cri-nulation. La quatri2nic plyarc Jc Jcic~nndtiona p r l u l t un clivape de diffcrenavec C ~ ~ V ~axial
ciation-c&nulation de direction nod-nod-ouest. leouel va& d'axial D~anairei de vetits vlisde vereence eG-nord-est. La
cmissance de cordierite sur toutes les foliations indique que le mitamorphisme de basse-pression a postdate les quatre phases
de deformation.
LC &hut dv la dcuxli'mc' d6fomlation date d'au rnoins 1167 I12 Ma, soit 1'9gs du mkamorph~cmedes gneiss, et prCchlc
I'inlruston dcs dykes de mi-lsrhjolile datce a 1080 I 30 Ma. I)onc, lc ou les r'vCnemcnrs omzCniquef rcspon%hler .le ce
complexe de diformation et de I'histoire mitamorphique sont d'ige grenvillien.
[Traduit par la rewe]
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Can. I. Earth Sci. 26. 1124-1135
(1989)
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