Geochronology, geochemistry and petrogenesis of the early mesozoic granites in the Sunwu-Jiayin area, Heilongjiang province

To determine petrogenesis and tectonic background of the early Mesozoic granites in the Sunwu-Jiayin area, Xiao Hinggan Mountains, U-Pb dating and Hf isotopic composition in zircon grains, major and trace element characteristics in different granites have been analyzed. Due to their zircon U-Pb ages, ages of granites in this area can be classified into two periods: the Late Triassic period (~210 Ma) and Early Jurassic period (187-181 Ma). Geochemical data indicate that the Late Triassic alkali feldspar granite belongs to the aluminous A-type granite, and was originated from partial melting of the newly accreted Neoproterozoic basic igneous crust. The Early Jurassic tonalite-granodiorite and monzogranite have similar geochemical characteristics as the adakite, and were formed by partial melting of the thickened lower crust. The Early Jurassic syenogranite and alkali-feldspar granite are of significantly different characteristics from the cotemporaneous adakite, and were derived from the basaltic crust accreted from depleted mantle during the Mesoproterozoic period. Combining with the regional researching results, it's suggested that the Late Triassic granite was resulted from the post-orogenic extension after the collision between the North China plate and Siberia plate, whereas the Early Jurassic granites formed in a compressive tectonic setting related to the subduction of the Pacific plate.