The Contaminated Granite of Bibette Head, Alderney

Abstract

THE Bibette Head granite outcrops along the western portion of the north coast of Alderney. The outcrop, as seen at present, is about five-eighths of a mile long in an E.-W. direction and about one quarter of a mile wide. The greater part of the intrusion must, however, be under the sea. The rocks into which this granite is intruded are a rather variable series of diorites, and the contact is a sharp one, although there is a certain amount of veining by the granite. Over a large part of the outcrop the granite is much decomposed and reddened. This would appear to be due largely to the action of iron-bearing solutions, derived from the magma, coming up along fissures which are parallel to the main joint planes. These run in a direction 17° west of north. The same series of solutions were probably responsible also for the epidotization of the granite which is a marked feature in various parts. Recent blasting operations at Bibette Head have exposed a large quantity of fresh granite, and although specimens were collected from all over the outcrop the bulk of the material with which the present investigation is concerned came from there.