MESOZOIC GRANITOID MAGMATISMS IN THE CENTRAL ASIA OROGENIC BELT

The Mesozoic granitoids in the CAOB can be basely divided into Early (250-190 Ma) and Late (190-100 Ma) Mesozoic groups. The Early Mesozoic granitoids can be broadly classified into two groups according to zircon U–Pb ages: an early-stage group covering the time span from Early to Middle Triassic (250–230 Ma) and a late-stage group emplaced during Late Triassic to Early Jurassic (ca. 230–190 Ma). These granitoids show extremely large variations of whole-rock Sr–Nd and zircon Hf isotopes, demonstrating strong isotopic heterogeneity of the source regions that are mainly dominated by juvenile components with significant old crustal participation. Early Mesozoic granitoids in the western CAOB were emplaced in a post-orogenic or intracontinental setting, whereas those in the eastern CAOB were related to subduction of the Mongol–Okhotsk ocean.

Late Mesozoic granitoids mainly occur in the eastern CAOB. According to published zircon U-Pb ages, these late granitoids were mainly emplaced during 200-180 Ma, 180-165 Ma, 165-145 Ma, 145-135 Ma and 135-100 Ma. Combined with regional geology and geochemistry, it is proposed that the Jurassic granitoids are likely melted from contractionally thickened lower continental crust, whereas the Cretaceous granitoids melted from an extensional thickening crust. The Early Jurassic granitoids along the Okhotsk belt formed in subduction/collision setting related to the closure of Mongol–Okhotsk Ocean, and Late Jurassic granitoids in Great Xing’an Range and northern North China Craton might be in a collisional setting superposed by the far-field affect of subduction of Pacific plate. Early Cretaceous granitoids in these areas formed in response to the post-orogenic collapse of the Mongol–Okhotsk belt, supported by bake-arc extension related to the Pacific oceanic subduction.