Geochronology and geochemistry of bimodal volcanic rocks from the western Jiangnan Orogenic Belt: Petrogenesis, source nature and tectonic implication

Abstract

This study presents zircon SIMS U-Pb ages, whole-rock geochemistry and Nd-Hf-O isotope data of Neoproterozoic meta-volcanic rocks discovered from the Longsheng area of the western Jiangnan Orogenic Belt. Zircon SIMS U-Pb dating reveals that these rocks were emplaced during 820–806 Ma. The basaltic rocks are enriched in light rare earth element (LREE; [La/Yb]N = 3.38–8.40), slight negative Nb and Ta anomalies, and positive εNd(t) (+4.39 to +4.75) values. In addition, they possess relatively high zircon δ18O values of 7.24–10.78‰. These features, together with the SiO2 contents (41.26–51.76 wt%), and high Mg# of 54–68, Ti/V and Ti/Y ratios of 49–68 and 415–570, suggest that they could derived from a depleted mantle source metasomatized by subduction-related melts. The dacitic rocks are high-K calc-alkaline and exhibit the characteristics of A1-type granite: typical enrichment in alkaline element, LREE ([La/Yb]N = 5.74–15.16) and large ion lithophile element (LILE), and depletion in Nb, Ta, Sr and Ti. Coupled with their different and negative εNd(t) (−8.75 to −9.16) compared with the basaltic rocks. This indicates that the dacitic rocks were probably generated by partial melting of crust as a result of magma underplating. The geochemical and Nd-Hf-O isotopic characteristics, together with regional geology, imply that the formation of the Longsheng bimodal volcanic rocks involves an extensional regime associated with a subduction-related environment. These new data, combined with previous results, indicate a preliminary scenario for the tectonic evolution of the Jiangnan Orogenic Belt and the southeastern margin of the Yangtze Block, which can provide significant insight into the understanding the evolution of South China.