U-Pb zircon systematics at Gruinard Bay, northwest Scotland: implications for the early orogenic evolution of the Lewisian complex

Abstract

The Gruinard Bay area of the mainland Lewisian complex comprises a metamorphosed suite of Archaean trondhjemites and minor granites enclosing remnants of older tonalitic gneiss and mafic to ultramafic enclaves. The U-Pb zircon dating yields ages of 2731 ±14 Ma and 2728 ±2 Ma for two trondhjemite and 2732 ±4 Ma for one granite sample, also revealing the presence of large amounts of inherited xenocrystic zircons. Although the region has been pervasively overprinted by retrogressive events in amphibolite to greenschist facies, the textural relations between biotite, hornblende, quartz and titaniferous minerals indicate that these minerals are pseudomorphs of pyroxene and high-Ti amphibole formed in hornblende-granulite facies. Structural relations link this metamorphism to a steep northeast-trending fabric coeval with the intrusion of the trondhjemites, dated at 2730 Ma. Dating of zircon in amphibolite and tonalite enclaves yields complex internal isotopic relations with apparent ages ranging from 2825 to 2740 Ma. This age range reflects new growth during the 2730 Ma metamorphic/metasomatic events, superimposed on older zircon phases which include combinations of xenocrystic cores, and magmatic and/or metamorphic growth phases whose mode of formation cannot clearly be resolved by imaging techniques (e.g. cathodoluminescence) alone. A pegmatitic vein that escaped the D3 strain and related isotopic disturbances yields a precise age of 2792 ±2 Ma, which constrains to some degree the earliest orogenic events in the area. Age relationships displayed in the central block at Scourie–Badcall, and in the Gruinard Bay area indicate that petrogenetic events in both areas were comparable about 2800 Ma and that both areas underwent trondhjemitic magmatism about 2730 Ma. In contrast, at Gruinard Bay there is no isotopic evidence for a period of high-grade metamorphism and magmatism at 2490–2480 Ma that drastically affected the Scourie block indicating that at this stage the two regions occupied different levels of the crust.