Geochemical studies of the Lewisian complex of the western assynt region, N.W. Scotland.
thesisposted on 19.11.2015, 09:03 by Jane D. Sills
This work examines the petrogenesis of layered ultramafic- gabbro bodies from the Scourian and the metamorphic evolution of the Assynt district of Sutherland. The layered bodies comprise ultramafic rocks (amphibole-spinel- lherzolites) and garnetiferous gabbros derived from the same tholeiitic magma which had about 15-20 MgO. The ultramafic rocks are partial cumulates formed by olivine and orthopyroxene after settling; with the gabbros being derivative liquids. The flat to LREE enriched patterns and trace element levels suggest the magma formed by 30-40% partial melting of undepleted mantle. The bodies are fragments of Archaean oceanic crust invaded by tonalitic magma generated at a convergent plate boundary followed by ductile deformation and metamorphism to granulite facies. The gabbros show evidence of two periods of granulite facies mineral growth, the first producing a clinopyroxene-garnet plagioclase assemblage at about 12-15 kb and 1000C. Uplift caused partial breakdown of the garnet to orthopyroxene-plagioclase spinel amphibole symplectites at about 800-900 C and 9-14 kb. Garnet stability depends on both P-T conditions and whole rock Fe/Mg ratio. The Lewisian complex in Assynt suffered widespread retrogression during the Inverian caused by the influx of large volumes of mantle- derived hydrous fluids associated with the development of NW-trending monoclinal folds. During retrogression the development of a uniform hornblende-plagioclase quartz assemblage in mafic and intermediate gneisses caused a redistribution of elements. Hornblende and biotite compositions depend on whole-rock composition and paragenesis. The assemblages developed suggest retrogression occurred with falling temperatures in the range 700-500C remaining above 500 C for the duration of the Inverian and Laxfordian events. The style of deformation became more brittle with time as the complex was uplifted. Amphibolite dykes from Clashnessie Bay are tholeiitic andesites formed from a tholeiitic magma by hornblende fractionation. They were deformed and metamorphosed before the intrusion of the main Scourie dyke swarm. The Canisp shear zone was the site of deformation over a protracted period and there may have been 5-10 km of right-lateral displacement.