Isotopic evolution of the Lewisian Complex of Tiree, Inner Hebrides and correlation with the mainland

Abstract

Synopsis We present the first Sr, Pb, and Nd isotopic data from the Lewisian gneisses of Tiree, Inner Hebrides. Depleted mantle Sm–Nd model ages ( t DM ) from the granulite facies gneisses of SW Tiree range from 2.63 to 2.84 Ga (weighted terrane average of 2.79 Ga) and confirm a late Archaean gneiss protolith age. Pb–Pb data define a c. 2.7 Ga regression line with a typical mantle μ 1 value of 7.8. The absence of a good isochron relationship for these gneisses is attributed to U/Pb fractionation during metamorphism without complete Pb-isotopic homogenization. Multi-stage Pb-isotopic modelling results are consistent with late Archaean high-grade metamorphism; the alternative of an early Laxfordian (South Harris equivalent) granulite facies event at c. 1.9–1.7 Ga is rendered unlikely by the high μ 1 values required. Rb–Sr data from the Caoles migmatite complex of east Tiree yield a regression age of 1.68 ± 0.06 Ga with an initial 87 Sr/ 86 Sr ratio of 0.711 ± 0.003. The values for ɛ Nd (1.7) of c . −10, the elevated 87 Sr/ 86 Sr(1.7) ratio, and the recently documented presence of probable Scourie dykes (Muir et al . 1993) suggest that these migmatites formed during Laxfordian reworking of a late Archaean protolith. In addition, the higher 87 Sr/ 86 Sr(1.7) values of the migmatitic gneisses, relative to the SW Tiree granulites, suggests either that the late-Archaean high-grade event did not cause pervasive large ion lithophile element (LILE) depletion throughout Tiree, or that the east Tiree amphibolite facies terrane represents a different structural level and/or distinct terrane. Our data allow a correlation of gneiss-forming and reworking events on Tiree with those of the mainland Lewisian and thus answer long-standing questions concerning the nature of the Lewisian in the Inner Hebrides.