The Loch Scridain Xenolithic Sill Complex, Isle of Mull, Scotland: Fractional Crystallization, Assimilation, Magma-Mixing and Crustal Anatexis in Subvolcanic Conduits

Abstract

The petrogenesis of a suite of high-level minor intrusions which intruded the Palaeogene lava field and older rocks in the vicinity of Loch Scridain, Isle of Mull, NW Scotland, has been investigated using mineral chemistry data together with whole-rock major and trace element and radiogenic Sr–Nd–Pb isotope data. Three distinct magma groups are present. Group I consists of aphyric tholeiitic basalts and basaltic andesites, the most primitive of which have MORB-like chemical affinities. Group I rocks are markedly xenolithic, containing both crustal and cognate gabbroic xenoliths. Group II comprises plagioclase- and pyroxene-phyric andesites and dacites. Group III consists solely of fine-grained rhyolites. Compositional trends displayed by the Group I basic magmas can be explained in terms of relatively low-pressure fractionation of the assemblage olivine + clinopyroxene + plagioclase, followed by plagioclase + clinopyroxene + low-Ca pyroxene. The extreme incompatible-element enrichment, the high initial Sr isotope ratios, and radiogenic Pb isotope ratios of the more evolved members of Group I can, however, only be explained through a process of combined fractional crystallization and crustal assimilation of the underlying Moine pelitic schists. The isotope and trace element data imply that the contaminant was a high-Nd partial melt of Moine metasediments rather than a bulk addition of Moine pelite. The major, trace element and Sr–Nd isotope geochemistry of the Group III rhyolites is consistent with their derivation mainly from the partial melting of a pelitic crustal source. The geochemical characteristics of the Group II intermediate rocks suggest that they represent simple mixtures between basic magma and Group III silicic melts.