Fluid escape structures in metagabbroic ‘sediments’, SE Jersey, Channel Islands

Abstract

Layered diorites in SE Jersey have been derived from an earlier layered gabbro by metasomatic recrystallization caused by granite intrusions. Relict patches of metagabbro still possess much of the primary mineralogy and cumulate textures and structures, the most distinctive of which is small-scale or ‘microrhythmic’ layering. Disrupted microrhythmic layering appears analogous to the dish structures described from some clastic sediments and a similar origin, by fluid escape from a compacting, unconsolidated crystal mush, is proposed. This process of continuous crystal compaction and return of pore fluid to the main magma body has some important implications for modern cumulus theory, especially the model of in-situ crystallization from a static boundary layer.