The petrogenesis of the Loch Scridain xenolithic sill complex, Isle of Mull
Intruded into the Palaeocene lava field and underlying Moine (Neoproterozoic) crystalline basement rocks around Loch Scridain, Isle of Mull, Scotland, is a suite of high-level, inclined, xenolithic sheets, ranging in composition from basalt, through andesite and dacite, to rhyolite. These sheets, associated with the Mull central volcano, were emplaced at approximately 58Ma. Three distinct (magma) groups are recognised. Group I consists of fine-grained, aphyric basalts and basaltic andesites, the most primitive of which has MORB-like chemical affinities. Group II predominantly comprises aphyric two-pyroxene andesites, and glassy plagioclase- and pyroxene-phyric dacites (pitchstones). Group III consists solely of fine-grained rhyolites. Many of the sheets are composite. The phase relationships and major-element geochemistry suggest that many of the Group I specimens evolved via fractional crystallization of olivine + palgioclase + augite, followed by plagioclase + augite + low-Ca pyroxene, from the more primitive samples. Evidence for this fractionation scheme is supported by the occurrence of gabbroic cumulate xenoliths in many of the Group I and Group II sheets. Group II evolved via the mixing between basic magmas and crustally derived silicic melts, followed by the fractionation of plagioclase + pigeonite. However, Sr-Nd isotope analyses and trace-element characteristics, in particular the LREE enrichment and Nb depletion of all but the least evolved samples, suggest that both Groups I and II contain significant amounts of crustally derived material. Strontium initial ratios correlate positively with degree of fractionation throughout Groups I and II, with the least evolved members of Group I (Mg# ˜ 0.3) having (87Sr/86Sr)58 of around 0.7037. More evolved members of Group I and the dacites of Group II have (87Sr/86Sr)58 in excess of 0.7167. Phenocryst population evidence and phase relationships suggest that some Group I samples are the result of mixing between evolved and more basic magmas, implying that the magma-storage reservoir for the Loch Scridain Sill Complex (LSSC) was continually being replenished with fresh batches of basic magma.