GENESIS OF ANDESITES - EVIDENCE FROM STUDIES OF MELT INCLUSIONS IN MINERALS

A bimodal distribution of natural magmatic melts (one mode at SiO2 con tents of 50-52 wt % and the other at 72-74 wt % SiO2) was detected by studying the composition of glasses in homogeneous melt: inclusions in minerals from all types of igneous rocks (similar to 3500 analyses). It was detected that magmatic melts of intermediate (andesitic) compos ition are relatively rare. The overwhelming majority of andesites orig inated by the crystallization of dacitic, or even rhyolitic, magmatic melts. Andesites are,in this case, cumulative rocks, whose silica cont ents are determined by the mixing of less silicic melts with more basi c phenocrysts. Basaltic and dacitic melt inclusions can be occasionall y encountered simultaneously in phenocrysts of andesites, a fact provi ding evidence for possible genetic relationships between more acid mag matic melt, which are parental for andesites, and basaltic melts.