ZIRCON U-PB GEOCHRONOLOGY, HF ISOTOPIC COMPOSITION AND GEOLOGICAL IMPLICATIONS OF THE NEOPROTEROZOIC SEDIMENTARY ROCKS IN THE WESTERN JIANGNAN OROGEN, SOUTH CHINA

Abstract

The Jiangnan Orogen, located at the southeastern margin of the Yangtze Block, South China, records the complete history of assembly and evolution between the Yangtze and Cathaysia blocks. Precambrian sedimentary sequences in the western Jiangnan Orogen, include the Neoproterozoic Lengjiaxi and Banxi Groups and the overlying Nanhua Sequence. A combined study of Lu-Hf isotopes and U-Pb ages for detrital zircons from Lengjiaxi and Banxi Groups can provide information on the crustal evolution of sedimentary provenances. Detrital zircons from the Lengjaixi Group and the lower Banxi Group have generally similar age populations, with two major peaks at 0.83-0.89Ga, 1.54-1.91 Ga and one minor at 1.01-1.39 Ga, 2.34-2.53 Ga. Detrital zircons from the upper Banxi Group have three major U-Pb age populations of about 2.0Ga, 2.5Ga and 0.82Ga and three subordinate age groups of about 0.91Ga, 2.4Ga and 2.65Ga. Grenville magmatism and juvenile crust were insignificant in the western Jiangnan Orogen. The magmatic activities of Paleoproterozoic ~ 2.0 Ga are distributed in the Yangtze, such as the area of Kongling, Jinshan, Liantuo, South Qinling and other places (Zhang et al., 2006; Sun et al., 2008; Wu et al., 2012; Yin et al., 2013; Wang et al., 2016; Nie et al., 2016), which records the tectonic heat events of Kongling complex and the crustal reconstruction event in the Yangtze Block (Zheng et al., 2007 and therein). Archean ~ 2.5Ga zircon age mainly from Yichang Kongling trondjemitic gneiss and migmatite (Zheng et al., 1991; Ling et al., 1998) and Hu Yang Po group and invasion of potassium granite in Zhongxiang (Wang et al., 2013), and the current study reported a large of ~2.5 Ga in Douling of South Qinling (Hu et al., 2013; Wu et al., 2014). Our new data, together with paleogeography, suggest that the in the early stage of banxi, the material source came from Cathaysia block, inversely, in the late stage of banxi the material source came from Yangtze block. This indicates that the differential subduction and non-synchronous uplift in the southeast margin of Yangtze block.