Mixing between granitic and dioritic crystal mushes, Guernsey, Channel Islands, UK

Abstract

The contact zone between the Cobo Granite and Bordeaux Diorite Complex of Guernsey (Channel Islands, UK) displays numerous features which result from the interaction of these two penecontemporaneously emplaced intermediate to felsic magmas. Initial interaction resulted in the formation of chilled mafic enclaves in granite magma. As thermal equilibrium was approached, some physical mixing took place to produce a heterogeneous “Marginal Facies”. Continued interaction resulted in incorporation of previously mixed magma into intruding magma. The early mixed material is locally preserved as enclaves, but more commonly underwent disaggregation promoted by its incompletely crystallised nature, the mineral components becoming distributed as xenocrysts and often as microenclaves, or glomeroxenocrysts, into surrounding magma. Further modification within the contact zone was brought about by the infiltration of melt through the interconnected pore space of the magma mushes on the scale of centimetres to hundreds of metres. These processes produced geochemical profiles which do not exhibit perfect mixing trends. The petrographic and geochemical features described not only demonstrate the efficacy of mixing between partially crystallised magma mushes of broadly similar composition, but also provide criteria by which such interaction may be recognised elsewhere. Many features, in particular mineral scale disequilibria and small scale modal heterogeneity, bear striking similarities to those which occur widely in granite plutons where obvious evidence for magma mixing is absent. As such, it is possible that many granite bodies preserve a subtle record of hitherto overlooked mixing events.