South African granites and their boron content

Abstract

Very few determinations of boron in granites outside the Fennoscandian shield exist. It is surprising to find on what scanty chemical data our knowledge of this important petrological agent rests and it is opportune to collect data from another Shield area. Data from about 40 granites from South Africa are presented, the spectrochemical determination being made on original chemically analysed powders from a number of previous and current studies. Care was taken to select representative samples of the more important granitic masses which are all of Pre-Cambrian age, but may cover a large period of the Earth's history. Most of the granites are particularly poor in boron, especially the ancient eroded batholiths. The wide difference in boron content of granitic rocks suggests that this element—rather than many other trace elements—deserves attention in studies on the origin of granites and on palingenesis. goldschmidts boron abundance figure for granites, viz. 0.001% B 2O 3 is considered too low. As a first approximation the value 0.005% is suggested for South African granites. The chemical composition of the granite masses examined approaches normal granite i.e. 1.4.2.3 as well as alkali granite, 1.4.1.3, to a large extent.