Neoproterozoic tectonic evolution of the Jiuling terrane in the central Jiangnan orogenic belt (South China): Constraints from magmatic suites

Abstract

The formation and tectonic evolution of the Jiangnan orogenic belt in South China have been debated in the context of plate subduction-collision versus mantle plume. Here we present new results from petrology, whole rock geochemistry, zircon U-Pb chronology and Hf isotopes, on three magmatic suites ranging in composition from felsic to mafic from the Jiuling terrane of the central Jiangnan orogenic belt in the southeastern margin of Yangtze Block. Suite 1 including meta-diorites and meta-basites are dated at 849.9±4.9, 848.8±4.5 and 846.3±3.5Ma. The meta-diorites and some of the meta-basites display enrichment of LREE, Rb and K and depletions of Nb, Ta and Ti anomalies, with distinctive geochemical features of continental arc setting. The εHf(t) values of zircons from meta-diorites range from 0.17 to 6.10. The other meta-basites have LREE depleted REE patterns, similar to N-MORB and intra-oceanic island arc settings, suggesting a fore-arc environment. Suite 2 consists of foliated granitoids dated at 816.8±4.2, 814.6±3.6, 812.5±3.5 and 810.3±3.5Ma, with the A/CNK values >1.1, and are classified as post-collisional granites in the Rb/(Y+Nb) diagram. Zircon grains from the granitoids show εHf(t) values in the range of −1.99 to 8.49 and Hf model ages of 1835–1041Ma. Suite 3 is composed of meta-diabase dated at 792.1±2.8Ma, with zircon εHf(t) values of 8.46–13.83. This suite shows E-MORB REE patterns, and plot within continental rift field. Their zircon Hf model ages mainly clustered at 1046–811Ma, indicating magma sourced through partial melting of early Neoproterozoic lithosphere mantle. These data suggest that the three Neoproterozoic Jiuling magmatic suites in the Jiuling terrane correlate with (1) the continental arc, fore-arc, (2) collision and (3) continental rift settings in the interval from 850Ma to 790Ma. These tectonic settings are consistent with a geodynamic process from the oceanic subduction beneath the Yangtze Block (850Ma), collision between Cathaysia and Yangtze Blocks (820Ma) and intracontinental rift (790Ma). This tectono-magmatic cycle in the Jiangnan orogenic belt is >200Ma later than the global Grenville orogeny in response to Rodinia supercontinent assembly, and challenges some of the existing models.