ORIGIN OF GARNET IN SILICIC IGNEOUS ROCKS OF THE WESTERN CAUCASUS

Abstract

Garnet is rather common in intermediate and silicic plutonic igneous rocks but rare in volcanic rocks, particularly those of mafic composition. Finds of garnet of igneous origin are of great interest because they are a source of information on the conditions under which the parent magmas were generated, evolved, and crystallized. Garnet-containing rocks occur in the Caucasus only in the Jurassic, where they are widespread. They make up the drainage divides between the basins of the Tuapsinka and Pshisha rivers and between the Pshenako and Pshiyakho rivers. All of the garnet-containing rocks of the Western Caucasus are rich in alkalis. The composition of garnet and biotite was determined. The garnet present in silicic igneous rocks of the Western Caucasus most probably was not formed in the mantle, but near the surface, at low pressures and temperatures, during the crystallization of calc-alkalic melts high in water, as is indicated by the ubiquitous occurrence of hydrated biotite in the rocks.