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Geological Society, London, Special Publications; 1987; v. 33; p. 255-269;
DOI: 10.1144/GSL.SP.1987.033.01.18
© 1987 Geological Society of London

Early and Middle Proterozoic Volcanic Suites of the Laurentian and North Atlantic Shields

Geochemistry, petrogenesis and tectonic significance of the early Proterozoic Loch Maree Group amphibolites of the Lewisian Complex, NW Scotland

Y. A. Johnson, R. G. Park & J. A. Winchester

Department of Geology, University of Keele, Staffordshire, ST5 5BG, UK

The early Proterozoic Loch Maree Group (LMG) is a supracrustal sequence occupying two belts, 5 km and 3.5 km wide respectively, at Gairloch and Loch Maree in the southern region of the Scottish Lewisian Complex. The LMG at Gairloch consists of about one-third metasediments, mainly metagreywackes, and two-thirds amphibolite. The LMG amphibolites are of igneous origin, possess a tholeiitic chemistry, and can be divided into two petrogenetic groups. Group A consists of the main amphibolite sheets, believed to represent lavas, which show little or no LREE enrichment and flat incompatible element profiles, with some LILE depletion. Their chemical signatures are similar to those of ocean-floor (MORB) basalts. Group B consists of thin amphibolites and biotite-hornblende schists, believed to be sills, which show LREE and LILE enrichment trends quite different from group A and more similar to those of the well-documented Lewisian Scourie dykes.

It is suggested that the voluminous group A amphibolites represent primitive basalts extruded rapidly, directly from the mantle, after a period of extensional rifting had caused significant crustal thinning. Subsequently, after the cessation of extension and the filling of the rift basin, later magmas, represented by the group B amphibolites, became subject to crustal contamination before being emplaced as sills within the supracrustal pile. An early, pre-rifting stage of extension is represented by the Scourie dykes, which possess normal continental basalt geochemistry.





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Geological Society, London, Memoirs, 2002; 26: 73 - 76.
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