Granitoid magmatism of Western Sangilen (to the problem of genesis of S- and A-granites)

Granitoids of Western Sangilen occur mostly in high-temperature zones (sill imanite and hypersthene-K-feldspathic) of the Lower Ordovician low-pressure granulite metamorphism. According to the degree of displacement, the grani toids are subdivided into the following groups: autochthonous (leucosome of migmatites), paraautochthonous (small bodies to a few tens of square meter s in area), and allochthonous (rather large intrusive massifs). Analysis of the chemical composition, geologic setting, and age of these groups sugges t 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 f orm a sequence: from paraautochthonous hyperaluminous S-granites to allocht honous weakly hyperaluminous (Khorumnug complex), subaluminous (Ulor comple x), and transitional subaluminous-metaaluminous A-granites (Ukhadag and Mat ut complexes). In this sequence the index of alumina content of rocks decre ases and alkalinity increases. Nevertheless, all granitoids have common geo chemical features: elevated contents of Sr and Ba and reduced contents of R b, U, and Th. These properties are also peculiar to metapelitic rocks, whic h, along with the similarity of correlation between oxides in the S- and A- granites and spatial coincidence of their occurrences, suggests that all gr anitoids of Sangilen resulted from melting of the same metasedimentary subs trate. In this case generation of A-granites required supply of some amount of alkalies, Sr-86, etc. into the crust. The supply can be related to gabb ros and diorites of elevated alkalinity which exist in the region.