The pegmatite problem

Abstract

Because of their unusual features and their economic importance pegmatites have attracted the attention of geologists since a very early date. Hauy, writing at the beginning of the 19th century, first gave the rocks their present name and since then a vast volume of literature has accumulated without providing a conclusive answer to the question of origin and mode of formation˙ To a certain extent this position appears to be due to overextrapolation and generalisation by investigators who have studied only one area restricted either in space or time, or who have considered only certain aspects of the rocks in their region, An attempt is therefore made here to provide a concise picture of present day knowledge of pegmatites. It is not proposed to give an historical survey of pegmatite literature, since this would not in any way help the method of treatment adopted in this paper, but would be a mere repetition of chapters from earlier writers. The known facts about the occurrence and nature of pegmatites are outlined in brief in this communication, including their relationship to aplites, quartz-veins and other rocks with which they are commonly associated. Although views on the origin of pegmatites vary considerably in detail, they fall into one of two main groups: (1) The formation of pegmatites associated with the emplacement of magma; (2) their formation through complete reconstitution of pre-existing rocks without magmatic intervention. The arguments put forward by these two schools of thought are considered critically and upon the validity and relative importance of these arguments a comprehensive and ordered view of pegmatites and their origin Is presented in the hope that such will guide and facilitate further research on problems connected with pegmatites.