Characteristics and genesis of continental back-arc A-type granites in the eastern segment of the Inner Mongolia-Da Hinggan Mountains orogenic belt

Abstract：Late Paleozoic A-type granites are widely distributed in Hegenshan-Heihe segment of the Inner Mongolia-Da Hinggan Mountains orogenic belt. Their tectonic environments and petrogenesis remain controversial. A-type granites in Hegenshan Heihe segment were emplaced in Late carboniferous-Permian period (325～260Ma), and the magmatic activities entered into the peak period in Permian. The A-type granite belt is metaluminous or slightly peraluminous (A/CNK=0.89～1.13) and belongs to the high-K, calc-alkaline series. On the chondrite-normalized REE diagram, all plutons are enriched in LREE, but show significant negative Eu anomalies. Their primitive-mantle normalized spidergrams display negative Ba, Sr, Ce, P, Ti and positive Th, Pb, La anomalies. Researches show that the ancient Asian oceanic plate still subducted into the Siberia plate between Late carboniferous and Permian, so the subducton-related back-arc extensional environment is thought to have been responsible for the formation of Hegenshan-Heihe A-type granites. The continental back-arc extension was probably related to the subduction slab rollback or the break-off induced upwelling of hot asthenospheric mantle. This process provided the thermal flux for melting the overlying lower crust at a high temperature, and also resulted in the mixing of the crust and the mantle-derived magmas. The mixed magma emplacement along the continental back-arc extension fault system after fractional crystallization in the depth was responsible for the formation of the Hegenshan-Heihe A-type granites belt.