Age and petrogenesis of the Yingyangguan volcanic rocks: Implications on constraining the boundary between Yangtze and Cathaysia blocks, South China

The Yingyangguan (YYG) Group is largely exposed in the possible suture zone between Yangtze and Cathaysia blocks, therefore crucial in understanding the amalgamation history of the South China Block. The LA-ICP-MS zircon UPb dating of andesitic and rhyolitic rocks from this Group yield crystallization ages of 822–816 Ma and ca. 765 Ma, respectively. The whole rock geochemistry indicates that the YYG andesitic rocks are analogous to the high-Mg sanukitoids from the Setouchi volcanic belt. They are significantly enriched in large ion lithophile elements (LILEs, eg., Rb, Ba, Pb) and light Rare Earth Elements (LREE), and depleted in high field strength elements (HFSEs, eg., Nb, Ta, Ti) and heavy Rare Earth Elements (HREE). They show enriched Nd isotopic signatures (εNd(t) = −7.3 to −0.9), and highly variable zircon εHf(t) characteristics (−12.4 to +6.8). High La/Ta, (Hf/Sm)N, Th/Yb and Th/La ratios underline derivation of the primary magma from a subduction melt metasomatized lithospheric mantle, indicating an arc-setting. Geochemical characteristics of ca. 765 Ma rhyolitic rocks point toward S-type granite affinity and enriched whole-rock Nd isotopic (εNd(t) = −7.9 to −6.9) and zircon Hf isotopic (εHf(t) = −12.9 to −5.0) signatures suggest an upwelled asthenosphere triggered partial melting of a recycled sedimentary source and intra-plate rift setting. Therefore, volcanic rocks from the YYG Group record a transition from a subduction setting during 822–816 Ma to a rift environment at ca. 765 Ma, coeval with the tectonic transition events recorded along the southeastern margin of the Yangtze Block, but distinct from the Cathaysia Block. Together with published geophysical data and the pieces of geochemical evidence, the similarity in the Neoproterozoic tectonic history between the YYG Group and southeastern Yangtze margin implies the existence of the southwestern boundary between Yangtze and Cathaysia blocks, to the east of the YYG Group.