The petrology of mullite-bearing peraluminous xenoliths: Implications for contamination processes in basaltic magmas

A suite of high-level inclined sheets ranging in composition from basalt th rough to rhyolite is intruded into the Palaeogene lava field and underlying Moine Supergroup basement rocks around Loch Scridain, Isle of Mull, Scotla nd. Many of the sheets are highly xenolithic, containing a wide variety of crustal xenolith types derived front the Moine metasedimentary rocks, along with various gabbroic cumulate xenoliths. The most common xenolith types a re almost pore quartzites and a variety of mullite-bearing aluminous buchit es, many of the latter having thick crystalline selvages of plagioclase, co rundum and aluminous spinel. The plagioclase is highly calcic (up to An(87) ), and adjacent to the host basalt is commonly oscillatory zoned, implying crystallization from a melt. Trapped between plagioclase crystals are pocke ts of quenched, contaminated basic melt, which contain skeletal plagioclase and clinopyroxene, and preserve evidence of local mixing between the host basalt and the aluminous crustal melts. Sr and Nd isotope values of the the buchite cores [e.g. (Sr-87/Sr-86)(55) = 0.7074-0.7115], plagioclase selvag es [e.g. (Sr-87/Sr-86)(55) = 0.7137-0.7148], and associated trapped melts [ e.g. (Sr-87/Sr-86)(55) = 0.7126-0.7128], imply a complex series of magma-xe nolith interactions. The textural characteristics, mineral chemistry and is otype geochemistry of these rims suggest that they have crystallized from a hybrid liquid formed by the complex interaction of the aluminous liquids w ith basic magmas. Such interaction proceeded via liquid-liquid diffusion, p hysical mixing of melts and a variety of reactions between the crystallizat ion products of the buchites and the basaltic liquids. These crustal xenoli ths pressure a detailed record of mineral-melt reactions within a suite of basaltic sheets, dominated by both the production of granitic melts and the 'bulk' melting of Al-rich micaceous lithologies.