Home – Home – Jornals – Russian Geology and Geophysics 2001 number 6

Granitoids of Western Sangilen occur mostly in high-temperature zones (sillimanite and hypersthene-K-feldspathic) of the Lower Ordovician low-pressure granulite metamorphism. According to the degree of displacement, the granitoids are subdivided into the following groups: autochthonous (leucosome of migmatites), paraautochthonous (small bodies to a few tens of square meters in area), and allochthonous (rather large intrusive massifs). Analysis of the chemical composition, geologic setting, and age of these groups suggest that they resulted from granite-forming processes which took place in the Ordovician. It is beyond doubt that metamorphism and granite formation are the consequence of the same major thermal event.
According to mineralogic and petrochemical features, the studied granites form a sequence: from paraautochthonous hyperaluminous S-granites to allochthonous weakly hyperaluminous (Khorumnug complex), subaluminous (Ulor complex), and transitional subaluminous-metaaluminous A-granites (Ukhadag and Matut complexes). In this sequence the index of alumina content of rocks decreases and alkalinity increases. Nevertheless, all granitoids have common geochemical features: elevated contents of Sr and Ba and reduced contents of Rb, U, and Th. These properties are also peculiar to metapelitic rocks, which, along with the similarity of correlation between oxides in the S- and A-granites and spatial coincidence of their occurrences, suggests that all granitoids of Sangilen resulted from melting of the same metasedimentary substrate. In this case generation of A-granites required supply of some amount of alkalies, ⁸⁶Sr, etc. into the crust. The supply can be related to gabbros and diorites of elevated alkalinity which exist in the region.