Neoproterozoic magmatic events in the South Qinling Belt, China: Implications for amalgamation and breakup of the Rodinia supercontinent

Abstract

Neoproterozoic magmatic rocks in the South Qinling Belt of China provide important clues for understanding the mechanism and timing of the amalgamation and breakup of the Rodinia supercontinent. Here we report new geochemical and high-precision LA-ICP-MS zircon U–Pb–Hf isotopic analyses on magmatic suites from the Liuba and Zhashui areas in the South Qinling Belt. Our data show that the crystallization ages of the granitic intrusions from Tiefodian and Tangjiagou in the Liuba area are 863±22Ma and 794±11Ma, respectively, whereas those of the dioritic and gabbroic intrusions at Chishuigou in the Zhashui area are 925±28Ma and 832.6±4.0Ma, respectively. The diorites at Chishuigou display arc-related geochemical affinity, characterized by strong depletion in Nb, Ta, P and Ti, and enrichment in large-ion lithophile elements (i.e., Rb, Ba, Th and U), indicating a subduction-related arc setting at ca. 925Ma. The Tiefodian granitic rocks have high SiO2 (68.46–70.98wt.%), Na2O (3.87–4.51wt.%), and low K2O (1.34–2.61wt.%) contents with TTG affinity. However, their Cr, and Ni contents and Cr/Ni, Nb/Ta ratios are similar to those of continental crust, and together with high negative εHf(t) values (−4.87 to −14.84), suggesting a continental margin arc at ca. 863Ma. The gabbros at Chishuigou have high TiO2 content (2.74–3.14wt.%), Zr/Y (3.93–4.24), Ta/Yb (0.19–0.25) ratios and low Zr/Nb ratios (11.37–13.17), similar to the features of within-plate basalts, indicating an intra-continental rift setting at ca. 833Ma. The granitoids at Tangjiagou exhibit enrichment of LREE, K and Pb, and depletion of Nb, Ta, P and Ti, suggesting an extensional tectonic environment at ca. 794Ma.The results indicate that Neoproterozoic magmatic rocks in the South Qinling Belt formed before ca. 833Ma and might represent the amalgamation of the Rodinia supercontinent in an arc-related subduction environment, whereas the magmatic events with the peak ages at ~740Ma during ca. 833–680Ma represent the breakup of Rodinia. Integrating our new data with those from previous works, we propose a new tectonic model for the evolutionary history of the South Qinling Belt in the Neoproterozoic, including four key stages: 1) an ocean that separated the South Qinling Belt and the Yangtze Block in the Early Neoproterozoic (ca.1000–956Ma); 2) bidirectional subduction of the oceanic lithosphere during ca. 956–870Ma; 3) subduction and collision between the South Qinling Belt and the Yangtze Block during ca. 870–833Ma, thus suggesting that the South Qinling Belt was as a part of the Yangtze Block from this period; and 4) intra-continental rifting during ca. 833–680Ma, although the blocks were not entirely rifted apart.