The Cader Idris granophyre and its associated rocks

Abstract

Summary The Cader Idris granophyre and its surroundings have been remapped, using vertical aerial photographs on the scale of 18 inches to the mile, and the information recompiled on the scale of six inches to the mile. A petrographic study has been made of the rocks present, and a selection of these analysed for major elements. The results suggest that the granophyre was emplaced as magma but caused some mobilization and metasomatism of country rocks near its contacts, as well as in xenoliths and rafts of country rock trapped and surrounded by granophyre in a discordant vent-like portion of the intrusion on Mynydd Moel. It is believed that the associated quartz-diabase sill and certain rocks of intermediate composition occurring within the Upper Acid Group originated at depth by such processes of thermal mobilization and metasomatism. It is shown that the granophyre lifted its roof more than 1500 feet and that it is physically connected with the uppermost rhyolites of the Upper Acid Group. This fixes the age of the granophyre and suggests that its intrusion was associated with earth-movements of pre-Bala age.