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

• South Qinling consists of three magmatic cycles in Neoproterzoic.
• The Panxi-Hannan arc system in NW Yangtze probably extends into South Qinling.
• Young magmatic rocks of ∼680–650 Ma are rift-related in northern Yangtze.

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 143Nd/144Nd ratios ranging from 0.51162 to 0.51172 corresponding to initial ɛNd 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 SiO2 content. 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 87Sr/86Sr ratios from 0.7030 to 0.7052 and higher initial 143Nd/144Nd ratios from 0.51172 to 0.51194 with positive initial ɛNd values ranging from +0.9 to +5.3, whereas intercalated felsic rocks show lower initial 143Nd/144Nd ratios from 0.51150 to 0.51177 with initial ɛNd values varying from −4.5 to +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.