Zircon ages and Nd-Hf isotopic composition of the Zhaertai Group(Inner Mongolia):Evidence for early Proterozoic evolution of the northern North China Craton

【摘要】：正The Mesoproterozoic rifting and accompanying magmatism and sedimentation in the North China Craton are interpreted to be related to the breakup of the Paleoproterozoic supercontinent Columbia.This study presents zircon ages and Nd-Hf isotopic data of the low-grade Mesoproterozoic Zhaertai Group exposed along the northern margin of the North China Craton.Sandstones of the Zhaertai Group contain detrital zircon populations of about 2500 Ma,which originated from the late Archean basement rocks underlying this group.A close match between the U-Pb ages and the T_(DM)(Hf) ages with high initialε_(Hf) values(at 2500 Ma) for the detrital zircon grains imply a juvenile origin for protoliths of the basement rocks.Sedimentation of the Zhaertai Group likely began around 1750 Ma tentatively constrained by the formation time of an intercalated volcanic layer.The volcanic rocks have low initialε_(Nd) values of -8.0 to -8.5(at 1750 Ma),similar to those of contemporaneous mafic dyke swarms within the North China Craton,which have been interpreted to be related either to the breakup of the supercontinent Columbia or to the post-collisional collapse of the Trans-North China Orogen.A progressive increase ofε_(Nd) values of whole-rock samples upwards in the stratigraphic sequence likely indicates an increase of juvenile material with time probably related to the contribution of Mesoproterozoic mantle-derived rocks.