MIXING BETWEEN GRANITIC AND DIORITIC CRYSTAL MUSHES, GUERNSEY, CHANNEL-ISLANDS, UK

The contact zone between the Cobo Granite and Bordeaux Diorite Complex of Guernsey (Channel Islands, UK) displays numerous features which re sult from the interaction of these two penecontemporaneously emplaced intermediate to felsic magmas. Initial interaction resulted in the for mation of chilled mafic enclaves in granite magma. As thermal equilibr ium was approached, some physical mixing took place to produce a heter ogeneous ''Marginal Facies''. Continued interaction resulted in incorp oration of previously mixed magma into intruding magma. The early mixe d material is locally preserved as enclaves, but more commonly underwe nt disaggregation promoted by its incompletely crystallised nature, th e mineral components becoming distributed as xenocrysts and often as m icroenclaves, or glomeroxenocrysts, into surrounding magma. Further mo dification within the contact zone was brought about by the infiltrati on of melt through the interconnected pore space of the magma mushes o n the scale of centimetres to hundreds of metres. These processes prod uced geochemical profiles which do not exhibit perfect mixing trends. The petrographic and geochemical features described not only demonstra te the efficacy of mixing between partially crystallised magma mushes of broadly similar composition, but also provide criteria by which suc h interaction may be recognised elsewhere. Many features, in particula r mineral scale disequilibria and small scale modal heterogeneity, bea r striking similarities to those which occur widely in granite plutons where obvious evidence for magma mixing is absent. As such, it is pos sible that many granite bodies preserve a subtle record of hitherto ov erlooked mixing events.