The lithospheric mantle and lower crust-mantle relationships under Scotland: a xenolithic perspective

In the British Isles the majority of volcanic rocks bearing upper mantle and lower crustal xenoliths occur in Scotland. Most of the occurrences are of Carboniferous-Permian age and provide unusually early examples of basalt-hosted mantle xenoliths. The paper presents new data on the mineral chemistry of spinel lherzolite xenoliths from fourteen localities spanning the five principal tectonic terranes composing Scotland. These supplement previously presented trace element and isotopic data that have demonstrated the broad lateral heterogeneity of the sub-continental lithospheric mantle (SCLM), e.g. Menzies & Halliday, (1988). Compositional variations among the four principal minerals emphasize the heterogeneity. The remarkable range of Al2O3 vs CaO exhibited by the clinopyroxenes compared to data from other ‘xenolith provinces’ emphasizes the extremely complex magmatic history of the Scottish lithosphere. The general increase in age from southern and central Scotland (Southern Upland and Midland Valley Terranes) to the older Northern Highland and Hebridean Terranes of the north and north-west, with concomitant complexity of geological history is reflected by trace element and isotopic studies. Reaction relationships in lherzolites from Loch Roag, Hebridean Terrane, due to pervasive metasomatism, involve secondary growth of sodic feldspar. These are related to the passage of carbonatitic melts as deduced by Long et al., (1991). The relationship between the upper mantle and lower crust is reviewed in the light of xenolith evidence. The majority of the pyroxenitic xenoliths are inferred to have formed as cumulates from underplating magmas. It is deduced that these compose a basal crustal layer with a generally sharp discontinuity to the dominantly lherzolitic mantle rocks beneath. A second discontinuity is inferred to separate the pyroxenitic layer from overlying, broadly cognate meta-gabbroic cumulates (‘basic granulites’). The two discontinuities are considered to represent a petrological and a seismic Moho respectively