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

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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.

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