Lewisian pyroxene gneisses from Barra and the geochemistry of the Archaean lower crust

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Abstract
Synopsis: Tonalitic, pyroxene gneisses from the pre- 2.6 Ga Lewisian complex of Barra are characterized by: (1) low K, Rb and Th; high Sr and Ba; high K/Rb ratios and low Rb/Sr and K/Ba ratios; (2) enriched light REE and depleted heavy REE; (3) positive Eu anomalies. These features, when considered in conjunction with evidence that a possible Archaean lower/upper crust interface may occur on Barra, are compatible with two models for lower crustal evolution: (A) the purging of hydrous tonalitic precursors in the lower crust of H 2 O and K, Rb and Th by mantle-derived, CO 2 -rich fluids carrying Sr and Eu during granulite facies metamorphism; (B) the primary igneous crystallization under high P co 2 of new tonalitic additions to the crust, from which the hydrous residual melt fraction was expelled upwards. Pyroxene gneisses may be the repository of the extra Eu in the crust which is needed to complement the deficiency of that element in post-Archaean sediments and the upper crust.