Ages and tectonic implications of Neoproterozoic ortho- and paragneisses in the Beishan Orogenic Belt, China

Abstract

The Beishan Orogenic Belt (BSOB) in China plays a pivotal role in understanding the tectonic evolution of the southern Central Asian Orogenic Belt. Despite numerous studies primarily emphasizing on Phanerozoic events, the existence and significance of Precambrian rocks in the BSOB still remain controversial. Here we report new geochemical and zircon U–Pb–Hf isotopic data for the Gubaoquan ortho- and paragneisses in the BSOB and the comparable Xingxingxia orthogneisses in the Central Tianshan Orogenic Belt to constrain their age, petrogenesis and tectonic implications.New LA-ICPMS U–Pb zircon dating results of zircon reveal that the protoliths of the Gubaoquan orthogneisses crystallized at 905–871Ma, while the Gubaoquan paragneisses yielded an age peak at 1040–910Ma with a maximum depositional age of ∼871Ma. They are the oldest rocks discovered in the BSOB, mainly distributed in the southern units and interpreted as primary sources for the local Phanerozoic igneous and sedimentary rocks. The intrusion age of the Xingxingxia orthogneisses was ∼1014 Ma. All the Gubaoquan and Xingxingxia orthogneisses show low A/CNK, Rb/Sr and Rb/Ba ratios and similar depletion of Nb, Ta and Ti, akin to I-type granitoids related to subduction. However, the former have much lower Sr contents and Sr/Y ratios, lower light to heavy rare earth element fractionation (LREE/HREE=4.43–10.12), and weaker negative Eu anomalies, related to a back-arc setting at low pressures, whereas the latter formed at high pressures in a continental arc setting. Zircon Hf-isotope compositions in the Gubaoquan ortho- and paragneisses exhibit a large range of ɛHf(t) values from −3.77 to +5.29, and two-stage model ages (TCDM) of 1.98–1.44Ga, while those in the Xingxingxia orthogneisses are clustered with slightly higher ɛHf(t) values (+4.02 to +8.53) and younger TCDM ages (1.60–1. 34Ga). The protoliths of both the Gubaoquan and Xingxingxia orthogneisses were derived from the mixing of basaltic magmas and those derived from the partial melting of meta-greywackes. Based on a review of Neoproterozoic events, the BSOB tends to closely link with the Chinese Tianshan Orogenic Belt and Tarim Craton. They were most likely located in the periphery of the already assembled Rodinia supercontinent.