PETROGENETIC SIGNIFICANCE OF METASTABLE MELTING OF GRANITE IN BASALT MELT

Abstract

We studied metastable melting and subsequent syntexis in the Ichetuy volcanotectonic structure, located in southwestern Transbaikal on the left bank of the Dzhida River, USSR. Lower and Middle Jurassic volcanic rocks here cover an area of 3500 km2. They consist mainly of potassic basalt, andesite-basalt and trachyte. Various subvolcanic bodies such as dikes, sills and stocks are also widespread, concentrated in dike belts as long as 30 to 40 km. The results convince us that metastable melting of granite in contact with large bodies of mafic magma may lead to the sweating out of melts relatively low in silica, but enriched in alkalies, primarily potassium. Because such melts are formed at contacts, they will readily mix with mafic magma, thereby forming a wide range of hybrid varieties. In nature, the phenomena studied seem to be characteristic of volcanic processes that are more dynamic than plutonic and their scale is fairly modest.