Neoproterozoic tectonic evolution of South Qinling, China: Evidence from zircon ages and geochemistry of the Yaolinghe volcanic rocks

摘要

The South Qinling terrain is located at the northern margin of the Yangtze block. Neoproterozoic volcanic-sedimentary sequences of the Yaolinghe Group play the most significant role in studying the tectono-magmatic evolution of the South Qinling. Based on temporal and spatial geochemical variations the Yaolinghe Group can be divided into three volcanic cycles. Basaltic lavas of cycle 1 crystallized at 847 ± 8 Ma and exhibit typical island arc basalt characteristics with enrichment of LILEs (Th, U) and LREEs,depletion of HFSEs (Nb,Ta,Zr,Hf,Ti) and homogeneous initial ~(143)Nd/~(144)Nd ratios ranging from 0.51162 to 0.51172 corresponding to initial ε_(Nd) values of +1.6 to +3.5. These features suggest that they were derived from a depleted mantle source during a subduction period. Volcanic rocks of cycle 2 show a complete lithological evolution from basalt through andesite to rhyolite without a significant gap of SiO_2content. They formed between 731 ± 11 Ma and 768 ± 7 Ma and display trace element signatures similar to arc-volcanics with positive LILE and negative HFSE anomalies. These basalts have lower initial ~(87)Sr/~(86)Sr ratios from 0.7030 to 0.7052 and higher initial ~(143)Nd/~(144) Nd ratios from 0.51172 to 0.51194 with positive initial ε_(Nd) values ranging from +0.9 to +5.3, whereas intercalated felsic rocks show lower initial ~(143)Nd/~(144)Nd ratios from 0.51150 to 0.51177 with initial ε_(Nd) valuesvaryingfrom?4.5to+1.9,suggestive of metasomatic alteration of lithospheric mantle by slab-derived fluids/melts. Cycle 3 lavas show undepleted HFSEs (Nb, Ta, Zr, Hf) and slightly enriched LILEs (Th, U) and Ti concentrations. They were generated in a rift-related setting between ～680 Ma and 650 Ma. Geochronological and geochemical evidence from lavas of the Yaolinghe Group suggest that the Panxi-Hannan arc system extended to northern South Qinling and the magmatism in the northern margin of the Yangtze block evolved from arc to rift magmatism during the Neoproterozoic.