Multiple-Stage Neoproterozoic Magmatism Recorded in the Zhangbaling Uplift of the Northeastern Yangtze Block: Evidence from Zircon Ages and Geochemistry

Abstract

The Yangtze Block records Neoproterozoic magmatism and sedimentation related to the breakup of Rodinia and is an important piece in the reconstruction of the supercontinent. However, the tectonic setting and position of this block in Rodinia remain a subject of debate. In the present study, we report the zircon U-Pb ages and Hf isotopic composition of zircon and geochemical and Nd-Pb isotopic compositions for meta-volcanic rocks exposed in the Zhangbaling uplift of the NE Yangtze Block. The volcanic rocks, dominated by rhyolite and dacite, belong to the calc-alkaline series and show geochemical characteristics of arc rocks. Zircon U-Pb isotopic ages show that volcanic rocks in the Xileng Formation formed at ca. 790 Ma and ca. 760–700 Ma peaking at ~740 Ma. The late-stage volcanism was widely exposed in the uplift, characterized by a temporal-spatial trend becoming younger southwards. The old volcanic rocks have low initial εNd (−11.0) and εHf (−19.7 to −8.2) values and low Pb isotopic ratios, likely indicating an origin from ancient basement rocks underneath the Yangtze Block. The younger ones, being similar to continental arc andesite in trace element compositions, have relatively high initial εNd (mostly −4.6 to 0.5) and εHf (−0.4 to 8.8) values and high Pb isotopic ratios. These isotopic features point to an origin from the partial melting of juvenile crustal rocks. Sedimentary rocks of the Xileng Formation and the overlying strata also contain numerous zircon grains of ~700 Ma to ~630 Ma. The volcanic rocks in the Zhangbaling uplift might demonstrate long-lasting subduction along the northeastern margin of the Yangtze Block, probably active until ca. 700 Ma.