Geochronological studies of Proterozoic crustal evolution in the northern Outer Hebrides

Abstract

New data on the Proterozoic thermal and tectonic crustal history of the Lewisian complex have been determined using Sm–Nd and 40Ar– 39Ar isotopic analyses. Igneous assemblages from two mafic/ultramafic masses indicate crystallization at 2.54 and 2.14 Ga from magmas with initial Nd isotopic composition lower than predicted depleted mantle at those times. Sm–Nd isotope systematics ( T DM=2.45 Ga) in two samples of metasediment from the Leverburgh Belt imply that they contain a significant post-Archaean component and cannot therefore be part of an Archaean stratigraphic sequence. Sm–Nd mineral isotope data on three samples confirm the Proterozoic age of high-pressure granulite facies metamorphism in the South Harris igneous complex; the most precise estimate is 1.827±0.016 Ga on a metagabbro which was intruded after deformation of the host anorthosite under granulite facies conditions. Localized granulite facies assemblages in Younger Basic bodies further north are significantly younger at 1.63 Ga. Whole rock Sm–Nd data on two samples from the 1.7 Ga granite injection complex of South Harris indicate ϵ Nd between −11.5 and −13.7 at the time of intrusion, providing a strong indication that they were produced by remelting of pre-existing crust similar to the exposed grey gneisses which had values close to −12 at that time. 40Ar– 39Ar data from 13 hornblendes generally show complex argon release patterns with indications of excess argon in most samples. Flat sections of the spectra range in age between 1.63 and 1.78 Ga. Previous Rb–Sr data on biotite showed a distinct break in age pattern across the Langavat Belt. There is no indication of such a break in the hornblende data and it is inferred that despite variations in the timing of peak metamorphism the whole of the northern Outer Hebrides cooled through hornblende closure temperatures in the interval 1.7–1.6 Ga.