Ca. 830 Ma back-arc type volcanic rocks in the eastern part of the Jiangnan orogen: Implications for the Neoproterozoic tectonic evolution of South China Block

Abstract

The volcanic rocks in the eastern part of Jiangnan orogen provide an important suite to investigate magma petrogenesis and tectonic evolution of South China Block. We carried out a combined study of geochronology and geochemistry for the bimodal association of basalts and rhyolitic tuffs from the Shuangqiaoshan Group in south Anhui Province. Zircons from a basalt and two rhyolitic tuff samples yield crystallization ages of 829±12Ma, 832±9Ma and 832±10Ma, respectively. The basalts are characterized by left-sloping REE distribution patterns, enriched large iron lithosphere elements (LILEs, e.g. Rb, Ba, K) and depleted high field strength elements (HFSEs, e.g. Nb, Ta) relative to those in N-MORB, resembling those of modern Central Lau back-arc basin basalts (BABB). In addition, they have positive ɛNd(t) values (+4.6 to +5.7) with initial 87Sr/86Sr ratios in the range of 0.70706–0.70719. These geochemical and isotopic signatures indicate that the basalts were derived from an N-MORB-like source modified by slab-derived fluids. The rhyolitic tuffs have high SiO2 (72.9–78.3wt %) contents and high 10,000*Ga/Al ratios (mostly >2.6). Taking into account their aluminous nature (A/CNK>1.1), they are affinitive to aluminous A-type granites. They invariably show a relative enrichment in light-REE, and strong negative Eu anomalies (Eu/Eu*=0.16–0.31) and negative anomalies in HFSE, Sr and P compared with the neighboring elements in their trace element spidergrams. Their ɛNd(t) values range from −2.1 to +0.1. Detailed elemental and isotopic data suggest that the rhyolitic tuffs formed by partial melting of mafic lower crust triggered by asthenosphere upwelling. The occurrence of bimodal volcanic rocks with the basalts showing geochemical affinity to intra-oceanic BABB appears to support the development of a Neoproterozoic back-arc basin in the eastern part of Jiangnan orogen at ca. 830Ma. Combined with the published data from central and western part of Jiangnan orogen, it is inferred that the back-arc basin developed in the entire Jiangnan orogen during 850–810Ma. Considering the age of the strata overlying the folded basement sequence, we suggest that the final collision of the Yangtze and Cathaysia blocks happened during 820–810Ma along the Jiangnan orogen.