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Polymetamorphism in the mainland Lewisian complex, NW Scotland - phase equilibria and geochronological constraints from the Cnoc an t'Sidhean suite

Journal article
Authors A. Zirkler
T. E. Johnson
R. W. White
Thomas Zack
Published in Journal of Metamorphic Geology
Volume 30
Issue 8
Pages 865-885
ISSN 0263-4929
Publication year 2012
Published at Department of Earth Sciences
Pages 865-885
Language en
Keywords fluid-driven retrogression, granulite facies, lewisian complex, phase equilibria modelling, polymetamorphism, partial melting equilibria, loch-maree group, u-pb ages, northwest scotland, gneiss complex, shear zones, system k2o-feo-mgo-al2o3-sio2-h2o-tio2-fe2o3, petrological interest, mineral assemblages, central australia
Subject categories Earth and Related Environmental Sciences, Geology, Solid earth geology and petrology


The metamorphic evolution of rocks cropping out near Stoer, within the Assynt terrane of the central region of the mainland Lewisian complex of NW Scotland, is investigated using phase equilibria modelling in the NCKFMASHTO and MnNCKFMASHTO model systems. The focus is on the Cnoc an tSidhean suite, garnet-bearing biotite-rich rocks (brown gneiss) with rare layers of white mica gneiss, which have been interpreted as sedimentary in origin. The results show that these rocks are polymetamorphic and experienced granulite facies peak metamorphism (Badcallian) followed by retrograde fluid-driven metamorphism (Inverian) under amphibolite facies conditions. The brown gneisses are inferred to have contained an essentially anhydrous granulite facies peak metamorphic assemblage of garnet, quartz, plagioclase and ilmenite (+/- rutile, K-feldspar and pyroxene) with biotite, hornblende, muscovite, chlorite and/or epidote as hydrous retrograde minerals. PT constraints imposed by phase equilibria modelling imply conditions of 1316 kbar at >900 degrees C for the Badcallian granulite facies metamorphic peak, consistent with the field evidence for partial melting in most lithologies. The white mica gneiss comprises a muscovite-dominated matrix containing porphyroblasts of staurolite, corundum, kyanite and rare garnet. Previous studies have suggested that staurolite, corundum, kyanite and muscovite all grew at the granulite facies peak, with partial melting and melt loss producing a highly aluminous residue. However, at the inferred peak PT conditions, staurolite and muscovite are not predicted to be stable, suggesting they are retrograde phases that grew during amphibolite facies retrograde metamorphism. The large proportion of mica suggests extensive H2O-rich fluid-influx, consistent with the retrograde growth of hornblende, biotite, epidote and chlorite in the brown gneisses. PT conditions of 5.06.5 kbar at 520550 degrees C are derived for the Inverian event. In situ dating of zircon from samples of the white mica gneiss yield apparent ages that are difficult to interpret. However, the data are permissive of granulite facies (Badcallian) metamorphism having occurred at c. 2.72.8 Ga with subsequent fluid driven (Inverian) retrogression at c. 2.52.6 Ga, consistent with previous interpretations.

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