Neoproterozoic arc-related mafic–ultramafic rocks and syn-collision granite from the western segment of the Jiangnan Orogen, South China: Constraints on the Neoproterozoic assembly of the Yangtze and Cathaysia Blocks

摘要

The Jiangnan Orogen is considered as the trace of the collisional suture between the Yangtze and Cathaysia Blocks in South China. Here we investigate the mafic–ultramafic suites and granitoids exposed in the Yuanbaoshan area along the western segment of the Jiangnan Orogen. We present new LA-ICP-MS zircon U–Pb data, Hf isotopes and whole rock geochemistry from a suite of granite, gabbro diorite and mafic–ultramafic rocks from this area. Zircons in the mafic rocks yield a U–Pb age of 854.7 ± 5.3 Ma, whereas those from the granites show ages of 822.6 ± 5.2, 830.7 ± 5.0, 823.8 ± 5.3 and 833.0 ± 5.9 Ma. The Neoproterozoic mafic–ultramafic rocks display dominantly tholeiitic features and are characterized by negative Ba, Nb, Ta, Zr, Hf and Sr anomalies and LREE enriched patterns, with a minor negative Eu anomaly. The gabbro diorite also exhibits similar geochemical characters. The mafic suite plots in the field of volcanic arc basalt in tectonic discrimination diagrams and displays distinctive geochemical signatures of an arc setting. The zircons from these rocks show εHf(t) values in the range of 3.9–13.8. The granitoids in northern Guangxi are typical S-type granites with high ACNK values (1.15–1.40), and are classified as collision-related granites in tectonic discrimination diagrams. The zircons from these rocks show negative εHf(t) values of ?18.99 to ?0.84. We correlate the granitoids with the continent–continent collisional orogeny between the Yangtze and Cathaysia Blocks. Our new results suggest a ～854 Ma volcanic arc and an 840–820 Ma collisional setting in the western segment of the Jiangnan Orogen, which is different from previous proposals of plume and bimodal magmatic settings and place the timing of assembly of the Yangtze and Cathaysia Blocks at some time ca. 850–800 Ma.