Origin and solidification age of Proterozoic Baidrag batholith, Tsagaan Tsahir Uul, Bayanhongor, central Mongolia

Abstract

Geochemical and geochronological investigations have been carried out on the Baidrag batholith, Tsagaan Tsahir Uul, Bayanhongor, central Mongolia to characterize the batholith, and to investigate its origin. The batholith mainly consists of biotite granite and muscovite-biotite granite. Major mineral constituents are plagioclase, potassium feldspar, quartz, biotite±muscovite. The batholith has experienced deuteric alteration and chloritization. Mineral compositions were changed to some extent during those episodes, but bulk rock chemical change seems insignificant except for highly chloritized samples. Field occurrence, mineral assemblage and chemical and isotopic characteristics of the granite, including the high initial ⁸⁷Sr/⁸⁶Sr ratio, demonstrate that the rock has the S-type characteristics. They show Nb-depletion in MORB normalized diagram, and abundances of other trace element suggest that the parental material might be mixture of subduction-related igneous rocks similar to TTD and old continental materials. The determined age of the Baidrag batholith is 647±10 Ma by the Rb-Sr whole rock isochron method.