MAGMA-MINGLING AND MELT MODIFICATION BETWEEN GRANITIC PIPES AND HOST DIORITE, GUERNSEY, CHANNEL ISLANDS

Within the Northern Igneous Complex of Guernsey cylindrical pipe intrusions, 20-40 cm in diameter, developed by buoyant emplacement of granite magma into overlying, denser diorite magma. The high viscosity of the partially crystallized magmas and lack of mechanical stirring inhibited physical mixing, and mesoscopically sharp, well-defined interfaces developed. The granitic pipes are surrounded by aureoles up to a metre in diameter which exhibit changes in bulk chemical composition and partial mineralogical re-equilibration. These formed in response to the modification of interstitial melt in the partially crystallized host resulting from residual melt migration and/or melt interdiffusion across the pipe/host interface. Such processes between partially crystallized magmas may, in part, provide an explanation for textural, mineralogical and chemical compositional features commonly exhibited by mafic enclaves within granites.