Early Cretaceous tectonic transition in South China: Insights from felsic igneous rocks and mafic dikes, eastern Guangdong Province

Large-scale magmatic activity and associated mineralization in South China are linked to the subduction of the palaeo-Pacific Plate, although specific processes have remained controversial including the timing of subduction initiation and lithospheric extension. We present new geochemical data and zircon U-Pb and Lu-Hf isotope measurements on volcanic rocks, granites and mafic dikes from coastal south-eastern China along the Eurasian continental margin. The biotite monzogranites show I-type granitoid affinity and fractional crystallization with zircon e(Hf)(t) values ranging from -10.15 to +0.50, suggesting magma derivation from the melting of lower crust with the addition of juvenile crustal or mantle components. The biotite granites are typical of highly fractionated A-type granites with e(Hf)(t) values ranging from -3.63 to +1.35, indicating magma hybridization between anatectic granitic and mantle-derived mafic magmas. The diabase dikes have zircon e(Hf)(t) values ranging from -2.99 to +1.71, with high La/Nb and La/Ta ratios. This suggests derivation from partial melting of a lithospheric mantle metasomatized by fluid from a subducting-slab, with contributions of depleted asthenospheric mantle components. Our data, combined with those from previous studies, suggest that the shallow subduction of the palaeo-Pacific Plate beneath the Eurasian continent had begun by ca. similar to 170 Ma at least and the subduction orientation changed to oblique with respect to the continental margin. Steepening of the subduction angle likely caused a slab rollback during the Early Cretaceous, resulting in a tectonic transition from compression to extension in South China at ca. 145 Ma.