Post-collisional ca. 800 Ma A-type felsic volcanic rocks in the eastern Jiangnan orogen, South China

Neoproterozoic igneous rocks in the Jiangnan orogen can provide important information to unravel the tectonic evolution of the South China Block and are thus important for the reconstruction of Rodinia. Dacites and rhyolites in the Shangshu region, the eastern Jiangnan orogen, contain zircon grains with U-Pb ages of ca. 800 Ma. These felsic rocks have high Ga/Al ratios (2.3-3.7) and HFSE and HREE contents relative to the primitive mantle, thus resembling A-type granitoids. Dacites have negative zircon epsilon Hf(t) values (-2.8 to -18.7), indicative of derivation from primary magmas generated by partial melting of ancient continental crust. In contrast, rhyolites show positive epsilon Hf(t) values (+1.9 to +8.8) and are explained as derived from partial melting of juvenile lower mafic crust. Felsic volcanic rocks in Shangshu were formed in a post-orogenic extension setting shortly after the final amalgamation of the Yangtze and Cathaysia blocks at around 830 Ma. Combining our results with the previously published ones, we demonstrate that the 800 Ma extension-related magmatic activity was likely the earliest phase of the Neoproterozoic rifting in the South China Block. The tectonic transition from continental collision to a post-orogenic setting should have occurred after ca. 800 Ma.