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

Abstract

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 t’Sidhean 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. P–T constraints imposed by phase equilibria modelling imply conditions of 13–16 kbar at >900 °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 P–T 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. P–T conditions of 5.0–6.5 kbar at 520–550 °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. 2.7–2.8 Ga with subsequent fluid driven (Inverian) retrogression at c. 2.5–2.6 Ga, consistent with previous interpretations.

Petrogenesis of rare-metal pegmatites in high-grade metamorphic terranes: a case study from the Lewisian Gneiss Complex of north-west Scotland

“…Here, Archaean granulite-facies assemblages are well-preserved in the TTG gneisses, with limited Laxfordian reworking that is focused along discrete shear zones (Sutton and Watson, 1951;Tarney and Weaver, 1987;Kinny and Friend, 2005;Wheeler et al, 2010;Zirkler et al, 2012;MacDonald et al, 2015). Although the terrane is dominated by TTG gneisses, mafic to ultramafic and metasedimentary pods are found in a number of locations within the gneisses (Davies, 1974;Cartwright et al, 1985;Kinny and Friend, 2005;Goodenough et al, 2010).…”

“…Smaller amounts of metasedimentary rocks are also present in the Assynt terrane (Kinny and Friend, 2005), although not always well mapped; such metasedimentary rocks may have provided a volumetrically limited source close to Badcall. It is notable that metasedimentary rocks in both these areas had been metamorphosed to granulite facies, and subsequently retrogressed, prior to pegmatite formation (Hollis et al, 2006;Baba et al, 2012;Zirkler et al, 2012). Regionally, metasedimentary rocks are also found in the Loch Maree Group, although these have not been metamorphosed to granulite facies (Park et al, 2001).…”

“…The majority of the Lewisian gneisses were formed, deformed and metamorphosed to high grades during the Archaean (Sutton and Watson, 1951;Giletti et al, 1961;Kinny and Friend, 2005;Wheeler et al, 2010;Zirkler et al, 2012). Much of the region was subsequently affected by magmatism, amphibolite-facies metamorphism and deformation associated with collisional events during the Palaeoproterozoic Laxfordian orogeny (c. 1900-1670 Ma) (Sutton and Watson, 1951;Moorbath and Park, 1972;Kinny and Friend, 2005;Love et al, 2010;Goodenough et al, 2013).…”

“…negative Eu anomaly and MREE enrichment). Notably, small amounts of garnet-bearing pelitic rocks are also a feature of this part of the Assynt terrane (the Eanruig supracrustals (Kinny and Friend, 2005;Zirkler et al, 2012). Although no such supracrustals have been described at outcrop in the immediate vicinity of Badcall Bay, they typically form lenses and pods up to c. 100 m long contained within the TTG gneisses, and such pods at depth represent a potential source for the Badcall pegmatite.…”

High-precision U–Pb dating of complex zircon from the Lewisian Gneiss Complex of Scotland using an incremental CA-ID-TIMS approach

“…The Assynt block features TTG gneisses and metamorphosed mafic and ultramafic lithologies, some of which display granulite mineral assemblages formed during the Badcallian (Park, 1970). Granulite facies metamorphism has been estimated to have occurred at 1000-920°C and16-11 kbar (O'Hara, 1977;Cartwright and Barnicoat, 1987;Sills and Rollinson, 1987;Zirkler et al, 2012). Recent work on relic eclogite assemblages found within maficultramafic rocks from the Assynt block has revised these estimates to 1060-1040°C and 24-22 kbar thus making these the highest known pressures recorded in Archean crustal rocks worldwide (Sajeev et al, 2013).…”