Abstract

The Cadomian diorites of SE Jersey contain two principal types of layering: primary gabbroic layering, formed as igneous cumulates in a gabbroic magma, and differentiated diorite sheet layering, which developed from gabbroic layering during metasomatic alteration of gabbro to diorite, probably by hydrous fluids associated with several pulses of granite emplacement. Primary gabbroic layering is preserved only in a small area near Le Nez and, though only a small proportion of relict grains of original pyroxene and calcic plagioclase remain, many of the textures and structures typical of primary layering are still preserved. The differentiated diorite sheets have the same orientation as the primary layers and each grades upwards from meladiorite to leucodiorite or quartz diorite. Intrusive structures at sheet contacts indicate that they, like the gabbroic layers, formed essentially horizontally, though they are now inclined NE at up to 65°, probably as a result of block rotation in granite. Chemical variation within the differentiated sheets suggests that there has been considerable modification of the original gabbro. The most basic part of the sheets occurs several tens of centimetres above the base because of reaction across the quartz diorite-meladiorite boundary. The layered diorites, and probably much of the homogeneous diorites as well, have originated by metasomatic alteration of layered gabbro, rather than having formed as cumulates in a primary dioritic magma.

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