PETROLOGY OF GRANITE-GNEISS DOMES OF THE KIRGIZ TIEN SHAN

Abstract

In the Kirgiz Tien Shan, hitherto regarded as a classical orthomagmatic granite province, recent work revealed granitoid rocks formed by metamorphic and ultrametamorphic processes. In their geologic position, structure, texture and mineralogy, these rocks consist of granite-gneiss domes of two different types. Domes of the first type lie among Precambrian metamorphic complexes and consist dominantly of various gneisses. Most granite-gneiss domes of the second type are large batholithic plutons with a subzonal structure. Our data show that granite-gneiss domes of types I and II are the geologic end products of petrogenetic processes that differed in the dynamics of variation of their physicochemical parameters. Granite-gneiss domes of type I develop as these parameters change gradually. When there is a spontaneous and significant change in physicochemical conditions, though, the resulting products will be granite-gneiss domes of type II.