Mineral banding and appinites in the Bon Repos meladiorite, Guernsey, Channel Islands

Abstract

Mineral banding of an unusual nature occurs within a small boss of amphibole meladiorite which invades granite gneiss on the south coast of Guernsey. The intrusion, which can be divided into three concentric zones, is also characterised by appinites containing cored amphiboles. The mineral banding and appinites are both restricted to the intermediate zone which separates an inner zone of uncontaminated meladiorite from an outer zone of contaminated meladiorite and hybrid granodiorite. The unusual nature of the banding rests in the manner in which amphibole-rich bands, separated by thicker bands more mesocratic in character, sometimes split into several branches, some of which either further bifurcate or join with adjacent bands to form a network pattern. Examination of the mechanisms proposed for the origin of mineral banding in magmatic rocks of other areas reveals that no one of these processes satisfies the various aspects of the banding at Bon Repos. The development of the appinites can be related to the accumulation of water within the intermediate zone during the later stages of crystallisation of the meladiorite magma.