Layered ultramafic-gabbro bodies in the Lewisian of northwest Scotland: geochemistry and petrogenesis

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Abstract

Layered ultramafic-gabbro bodies occur widely in the Archaean of northwest Scotland. They were metamorphosed at granulite or high amphibolite facies and were tectonically thinned and broken up during deformation. They comprise repeated ultramafic-gabbro layers, locally with Ni-poor sulphide-rich tops, each rhythmic unit showing decreasing MgO, Ni and normative anorthite with stratigraphic height. Major, trace and rare earth element data are presented for the range of rock types. In ultramafic rocks, MgO varies from 22 to 37 wt.%, Ni from 1000 to 2500 ppm and TiO2 from 0.08 to 0.40 wt.%, while the MgO content of the gabbros ranges from 14 to 6 wt.%. The REE patterns are flat to LREE enriched with no significant Eu anomalies. In ultramafic rocks REE are from 4 to 10 times chondrite, and in the gabbros LREE range from 8 to 30 times chondrite and HREE from 6 to 15 times chondrite. Study of incompatible elements (Ti, Zr, Y) which are relatively immobile during metamorphism shows that neither garnet nor hornblende were involved in fractionation. Trace element modelling shows it is improbable that the ultramafic rocks represent primary MgO-rich liquids even though their incompatible element contents are quite high. The chemical trends are interpreted in terms of olivine and pyroxene settling from a tholeiitic high-Mg magma with 15–20 wt.% MgO derived by 30–40% partial melting of an undepleted mantle. The ultramafic rocks are the cumulates and the gabbros the derived liquids.

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    This paper is published by kind permission of the Director of the Institute of Geological Sciences (NERC).

    1

    We would like to thank the following for analytical facilities: Drs. J.V.P. Long and N.R. Charnley at Cambridge, P. Suddaby and M.T. Frost at Imperial College and R.N. Wilson at Leicester for electron microprobe facilities; Dr. R. Parker and G. Bullen for help with XRF analysis of the Scouriemore samples and Dr. G. Hendry (University of Birmingham) for running the remaining XRF analyses; and the University of London Reactor Centre for INAA facilities. We would like to thank Dr. Barry Weaver, Dr. Mike Norry and Prof. John Tarney for discussions of a previous draft of the manuscript.

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