COGNATE GABBROIC XENOLITHS FROM A THOLEIITIC SUBVOLCANIC SILL COMPLEX- IMPLICATIONS FOR FRACTIONAL CRYSTALLIZATION AND CRUSTAL CONTAMINATION PROCESSES

Intruded into the Palaeogene lava field and underlying Moine (Neoprote rozoic) crystalline basement rocks around Loch Scridain, Isle of Mull, Scotland, is a suite of high-level, inclined, xenolithic sheets, rang ing in composition from basalt, through andesite and dacite, to rhyoli te. These sheets, associated with the Mull central volcano, were empla ced post 55 Ma. As well as numerous crustal xenoliths, the more basic members of the complex contain a diverse suite of ultrabasic and basic xenoliths. Xenolith types include feldspathic peridotite with cumulus olivine, pyroxenite, gabbro with cumulus plagioclase and cumulus clin opyroxene, and pure anorthosite. Mineralogical data, coupled with whol e-rock major- and trace-element data from a small number of the xenoli ths suggest that the xenoliths represent early-formed cumulates cognat e with their host basalts. Sr and Nd isotope data from the xenoliths c onfirms the cognate origin, and also shows that the basic magmas suffe red crustal contamination at an early stage.