Petrogenesis of ca. 830 Ma Lushan bimodal volcanic rocks at the southeastern margin of the Yangtze Block, South China: Implications for asthenospheric upwelling and reworking of juvenile crust

Neoproterozoic bimodal volcanic rocks are widespread in the South China Block and provide information on crust-mantle interaction, the thermal and chemical status of the deep mantle, and the growth and reworking of continental crust. In this paper, we present sensitive high-resolution ion microprobe (SHRIMP) zircon U-Pb ages and whole-rock geochemistry for the Lushan bimodal volcanic rocks in northern Jiangxi Province, South China Block. Our SHRIMP dating indicates that the Lushan bimodal volcanic rocks were erupted at 835-820 Ma. The least-evolved mafic rocks have high epsilon Nd(t) (+8.3) and arc-like Nb/La and Nb/Th ratios, thier derivation from fluid-modified depleted mantle. The melts that formed these mafic rocks underwent fractional crystallization involving mineral assemblages of olivine + plagioclase -> clinopyroxene + plagioclase at MgO < similar to 6.0 wt % -> clinopyroxene + plagioclase + Fe-Ti oxides at MgO < similar to 5 wt%, and crustal contamination during ascent. eNd(t) values of the felsic rocks (-1.39 to +1.64) differ from those of the contemporaneous mafic rocks (eNd (t) = +2.19 to +8.33), arguing against an origin by fractional crystallization of the mafic magmas. The felsic rocks are characterized by high (10,000xGa)/Al ratios and enrichment in high-field-strength elements (Nb + Ce + Y+ Zr), low P2O5 contents (< 0.1%), a peraluminous nature (A/CNK > 1.2), and a lack of typical alkaline mafic minerals, suggesting that they are aluminous A-type granitic rocks. Therefore, the felsic rocks probably originated from partial melting of fertile felsic meta-igneous rocks in the shallow crust. The similar to 830 Ma bimodal volcanic rocks indicate Neoproterozoic rift-related extensional thinning of the crust associated with asthenospheric upwelling along the southeastern margin of the South China Block. The geochemistry of the mafic rocks indicates two distinct mantle sources beneath the southeastern margin of the Yangtze Block at ca. 830 Ma: a depleted asthenospheric mantle with epsilon Nd(t) > +8, and an enriched mantle plume source with epsilon Nd (t) < 0. The rift-related asthenospheric mantle upwelling associated with mantle-plume-related mafic magmatism enhanced the geothermal gradient of the crust and conducted mantle heat into it, ultimately resulting in reworking of 860-830 Ma juvenile crust at various levels.