Late Mesoproterozoic continental rifting during the assembly of Rodinia: Evidence from low-δ18O A-type granites and associated volcanic rocks in the Tieshajie Complex, eastern South China

Late Mesoproterozoic tectonics of South China is characterized by near synchronous intraplate magmatism and orogeneses, which are linked with the early formation of the Rodinia Supercontinent. Our finding of low-δ18O zircons from magmatic rocks in the Tieshajie Complex, oldest basement unit of the northeastern Jiangnan (Sibao) Orogen, provide constraints on the late Mesoproterozoic geodynamic setting of South China and its paleogeography in Rodinia. New zircon U-Pb dating reveals an extended, probably pulsed bimodal magmatism at 1.17–1.15 Ga and 1.12–1.10 Ga in the Tieshajie Complex. The 1.12–1.10 Ga meta-mafic rocks are alkali and characterized by variably enriched LREE and Nb-Ta-Ti, similar to oceanic island basalts. They have positive εNd(t) values (0.54 to 3.55), relatively low La/Ta ratios and high (Gd/Yb)N ratios, indicating derivation of their parental magma from partial melting of depleted asthenospheric mantle in the spinel‐garnet facies field. Rhyolitic and granitic rocks of both ages are highly siliceous and show an A-type granitoid affinity with high total alkalis, Ga/Al ratios and REE abundances. These felsic rocks have similar patterns of REE and trace element and predominantly positive εHf(t) (mostly between 0 and 5.18) and εNd(t) (0.20 to 3.30), and show co-variations of major elements, suggesting they were generated by remelting of a common juvenile crustal source with linked fractional crystallization processes. Both rhyolitic and granitic rocks show variable zircon δ18O values (3.49 to 6.43 ‰) falling mostly within the range of mantle-derived zircons, but the occurrence of low-δ18O zircons (δ18O < 4.7 ‰) is reflective of possible small-scale O-isotopic exchange during interaction between surface water and the felsic parental magmas at relatively shallow depths, facilitated by high geothermal gradient and crustal fracturing under crustal extension. Thus, we interpret these granitic and bimodal volcanic rocks in the Tieshajie Complex as products of intraplate magmatism in rift settings. A scrutiny of the regional geology indicates that the late Mesoproterozoic continental rifting may have affected wide areas of South China. This rifting was probably linked with coeval LIP event in India, suggesting they were close neighbors during early assembly of the Rodinia Supercontinent.