MAGMATIC DIFFERENTIATION AND ALTERATION IN THE ST. AUSTELL GRANITE

Abstract

Mineralogical and chemical evidence enables the St. Austell granite to be divided into four main varieties, of which the earliest probably constitutes a separate intrusion. The distribution of minerals and chemical components makes it clear that variation is due to magmatic differentiation. Suggestions as to the causes of albite enrichment and fine grain in the later varieties are offered. Further evidence shows that the granite has been altered by both late magmatic and post-magmatic processes. The latter are related to the joint system, which forms part of a regional pattern. Greisening, a late magmatic process, was initiated by aqueous residual solutions containing small amounts of boron and fluorine and was controlled by pressure, temperature, acidity and the concentration of various elements. Kaolinization, the important post-magmatic process, resulted from the attack on the solid rock of solutions, doubtless from unconsolidated magma below, escaping through the joints. These solutions, also aqueous, did not apparently contain any of the agents of alteration usually attributed to them. Reaction was controlled by pressure, temperature, acidity, concentration and also by ionic potential of the elements involved and the ion-exchange capacity of the newly formed clay minerals.