Magmatic and metasomatic differentiation in the petrogenesis of Archaean metabasites from Tigharry, North Uist

Abstract

Synopsis The polymetamorphic supracrustal sequence at Tigharry, North Uist is associated with amphibolites, garnet–amphibolites, biotite–amphibolites, cummingtonite–garnet amphibolites, actinolitic hornblende–chlorite rocks and dolomite–actinolite–anthophyllite–chlorite rocks. These are a cogenetic group of tholeiitic basic and ultrabasic rocks belonging to the Archaean “banded basic” suite of the Lewisian complex in the Outer Hebrides. Diverse metabasite assemblages and cryptic variations of mineral composition are controlled by strong, predominantly magmatic chemical differentiation and the geochemistry is very similar to that of layered ultramafic/gabbro bodies of the mainland Lewisian complex. Volumetrically less important anthophyllite/gedrite–andesine–kyanite–phlogopite–quartz rocks and hornblende–anthophyllite–garnet–labradorite–staurolite–quartz rocks which have been described previously as metasediments are texturally and compositionally members of the basic/ultrabasic suite. The chemistry of these orthoamphibole-bearing rocks is not purely magmatic and reflects metasomatism (essentially Ca-depletion) of basic material either before or during metamorphism. Chlorite and biotite-rich assemblages including some with large garnets may also be of partially metasomatic derivation. The prograde assemblages of the basic rocks equilibrated during Proterozoic (Laxfordian) metamorphism under P–T conditions of 600–650°C and 6–9 kb. The associated metapelitic rocks contain kyanite and compositionally heterogeneous garnets whose complex zoning and inclusion fabrics may reflect growth during both the Laxfordian and Scourian metamorphic cycles.