The Geology of South-west Jersey, Channel Islands

Abstract

South-west Jersey is formed of a granite complex emplaced in sediments of presumed Pre-Cambrian age. The granites, considered to be Armorican in age, are of four main types: (i) The marginal facies of an older medium-grained biotite-horn-blende granite in contact with the sediments of the St. Aubin area; (ii) the older biotite-hornblende granite which extends from Corbière to Noirmont; (iii) a younger micro-granite occupying the approximate centre of the area and (iv) a porphyritic granite surrounding the younger granite mass. Basic dykes of various ages traverse Pre-Cambrian sediments and granites alike in north-south and east-west directions. Mica lamprophyres are distributed sporadically. In the St. Aubin area the fine-grained marginal granite, characterised by its variable porphyritic texture and its contamination by the sediments, forms three lobes in slightly indurated Pre-Cambrian sediments. At Belcroute Bay, the type locality, the marginal granite is over a hundred and forty yards in width, but at La Carrière, in the north-west, it is only a few feet in width. The degree of contamination in the marginal granite decreases away from its contact with the Pre-Cambrian. The former is characterised by crystalloblastic textures; porphyroblastic oligoclase, perthite and quartz are commonly developed, and there is considerable micro-brecciation. Sedimentary xenoliths are abundant in the marginal granite and common in the main biotite-hornblende granite. Large xenoliths of altered Pre-Cambrian sediments, orientated approximately east-west, occur in a few localities. The origin of such xenoliths, and of other xenoliths within the granites, is discussed. The porphyritic granite, which is extremely variable in composition and texture, is considered to be altered older granite surrounding the younger microgranite masses. Micrometric and textural analysis of un-orientated specimens collected along traverses across the porphyritic granite (from the microgranite to the older granite) have revealed zoned variations in composition and replacement textures. The extent of alteration in the older granite round cupolas and domes of microgranite has been found to depend mainly upon the size of such microgranite masses. These zonal variations, which have been found both vertically and horizontally round a small microgranite mass at Portelet, are considered to be the result of cyclic metasomatism, both during and immediately after the emplacement of the microgranites.