Detrital zircon record of Neoproterozoic active-margin sedimentation in the eastern Jiangnan Orogen, South China

Abstract

Precambrian sedimentary sequences in the eastern Jiangnan Orogen, South China, include the Neoproterozoic Shuangqiaoshan and Xikou Groups and the overlying Nanhua Sequence. U–Pb ages of detrital zircons provide tight constraints on the deposition of the Shuangqiaoshan and Xikou Groups at 831–815Ma and 833–817Ma, respectively. Rocks from the lower parts of these strata contain dominantly Neoproterozoic detritus with a pronounced age peak at 830–850Ma, interpreted to be derived from a magmatic arc that has been mostly eroded. This magmatic arc was associated with sedimentation characterized by compositionally immature sediments with dominant early-Neoproterozoic (∼860–810Ma) zircons that have variable ɛHf(t) values of −21.5 to 14.8. The upper parts of these strata, uncomfortably underlain by the low units, are molasse-type assemblages with additional input of pre-Neoproterozoic detritus, representing sediments accumulated in a retro-arc foreland basin associated with the Jiangnan Orogen.Combining four samples from the Nanhua Sequence, three major magmatic activities at 2.50–2.35Ga, 2.10–1.90Ga and 980–810Ma in the source areas are identified. The older two events are characterized by reworking of pre-existing continental crust. Grenville magmatism and juvenile crust were insignificant in the eastern Jiangnan Orogen. Neoproterozoic zircons reflect generation of juvenile crust at 980–860Ma, involvement of both juvenile and recycled materials at 860–810Ma and reworking of pre-existing crust at 805–750Ma, corresponding to island arc, continental magmatic arc and post-collisional rifting stages in the eastern Jiangnan Orogen.