Neoproterozoic S-type granites in the western Jiangnan Orogenic Belt, South China: Implications for petrogenesis and geodynamic significance

Abstract

ABSTRACT The Neoproterozoic Jiangnan Orogenic Belt in south China was formed by the amalgamation of the Yangtze and Cathaysia blocks, however, the timing of its formation is still debated. In this study, we present whole rock geochemistry, U-Pb zircon ages, Lu-Hf and Nd-isotope systematics on the Neoproterozoic Yexijiang and Baomuping granites occurring in the western segment of the Jiangnan Orogenic Belt. These granites mark the initiation of post-collision extension environment. Zircon U-Pb age data identify 807–805 Ma emplacement age for these granites. They have low and variable ΣREE concentrations (47–115 ppm) and show moderate to highly fractionated REE patterns ([La/Sm]N = 2.83–3.78) with low [Gd/Yb]N (1.34–2.31) and negative Eu anomalies (Eu* = 0.34–0.57). Geochemical features and presence of peraluminous minerals classify them as S-type granites and P-T parameters indicate low to moderate temperature (730–791 °C) and pressure (middle crustal level) conditions. These granites show consistent negative whole rock ɛNd(t) (−5.7 to −6.6) and magmatic zircon ɛHf(t) values from −0.3 to −3.8 while inherited zircon grains show variable ɛHf(t) values (+2.1 to −13.0) and two-stage Hf model ages (1.80 to 2.88 Ga). In particular, one Neoproterozoic (899 Ma) inherited zircon grain displays depleted Hf [ɛHf(t) = +5.4 and TDM2 = 1.34 Ga] isotopic characteristic. These lines of evidence indicate a sedimentary source for Yexijiang and Baomuping granites, possibly the Archean to Neoproterozoic crustal rocks, and emplacement in a post-collision tectonic environment triggered by mafic magma underplating during the amalgamation of the Yangtze and Cathaysia blocks. We infer that convergence in the Jiangnan Orogenic Belt occurred during 830–800 Ma therefore, unrelated to the global Grenvillian Orogeny.