Tectonic evolution of the western Jiangnan Orogen: Constraints from the Neoproterozoic igneous rocks in the Fanjingshan region, South China

摘要

The Jiangnan Orogen in South China formed through collision between the Yangtze and Cathaysia blocks during the Neoproterozoic. The Fanjingshan Group, located in the western part of the Jiangnan Orogen, consists of the deformed volcano-sedimentary sequences intruded by the ca. 830 Ma S-type granites. The volcanic rocks are mainly composed of rhyolitic tuffs associated with minor basalts. The basalts are rich in LILEs (e.g. Th and U) and LREEs and depleted in HFSEs (e.g. Nb and Ta), and have high La/Nb (1.69-2.86) and Th/Yb ratios (0.92-3.34), as well as negative whole rock eNd values (-4.2 to -0.4), suggesting that they were derived from a subduction-modified lithospheric mantle. The rhyolitic tuffs show variable SiO2 (63.43-79.60 wt%), MgO (0.22-4.44 wt%) and Ni (17-41 ppm), and have negative epsilon Nd (-7.7 to -6.1) and large variable epsilon Hf values (-6.3 to + 10.4). Modeling calculations reveal that the rhyolitic tuffs were likely produced by mixing between basaltic and felsic melts. The S-type monzogranites show high FeO/MgO (1.33-95.4) and (K2O + Na2O)/CaO (7-64), and low Zr/Hf (11-18) and Nb/Ta ratios (1.6-3.9) that are indicative of strong fractionation and significant hydrothermal alteration. Their negative epsilon Nd (-13.0 to -7.3) and epsilon Hf values (-2.7 to + 0.7) and Paleoproterozoic two-stage Hf-isotope ages (1.69-1.87 Ga) suggest that the granites were partial melts of the ancient crustal rocks. The Fanjingshan Group is proposed to have been deposited in a forearc basin, and its volcano-sedimentary rocks were deformed and metamorphosed due to the collision between the Yangtze and Cathaysia blocks at ca. 830 Ma. The subsequent peraluminous granites were partial melts of the ancient crustal basement during or post the collision event.