The Moine amphibolite suites of central and northern Sutherland, Scotland

Abstract

SummaryThree groups of amphibolites within the Moine rocks of Sutherland have been analysed for twenty-six elements by XRF analysis. (1) Very rare epidotic hornblende schists within pelite, possibly derived from volcanic ash deposited penecontemporaneously with the pelite (2 analyses). (2) Major and minor bodies of ‘early Moine’ schistose garnetiferous amphibolite intruded early in the deformational sequence (41 analyses). (3) Small bodies of ‘Loch a' Mhòid metadolerite’, intruded later in the deformational sequence (23 analyses). Rare Moine ultrabasites are associated with these metadolerites and probably formed from the same magma as olivine cumulates. These ultrabasites (5 analyses) are chemically distinct from Lewisian ultrabasites (both in the basement sheets within the Moine rocks of Sutherland and in the foreland Lewisian rocks) and from the Glen Urquhart ultrabasic body (3 analyses).Using variation diagrams it is inferred that chemical variation in these amphibolites is due largely to pyroxene and plagioclase crystallization in the early amphibolites and olivine, pyroxene, and plagioclase crystallization in the Loch a' Mhòid metadolerites and ultrabasites, followed in both cases by high-level fractionation. This has led to high values of incompatible elements in many of the early amphibolites. Although geochemical discrimination diagrams tend to give ambiguous results for many of these rocks, the early amphibolites are shown to be tholeiitic in nature with a chemistry transitional between within-plate and island-arc type basaltic magma. The Loch a' Mhòid metadolerites are mildly alkaline within-plate type basaltic magma.