Petrogenesis of late Early Cretaceous adakitic rocks in the offshore Bohai Bay Basin, Eastern China, and tectonic implications

Adakitic rocks provide important data on Early Cretaceous magmatism and the tectonic setting of the eastern North China Craton. This study reports new data on adakitic alkali rhyolites in the offshore Bohai Bay Basin, eastern China. The eruption age of the rhyolites is represented by a weighted mean(206)Pb/U-238 age of 114.4 Ma. The geochemical characteristics of the samples are similar to those of adakitic rocks, with high Sr (307-1686 ppm) and low Y (3.18-11.3 ppm) and Yb (0.30-1.39 ppm) contents, as well as high Sr/Y (40-70) ratios. Moreover, the samples show relatively uniform epsilon Nd(t) values (-15.48 to -16.51) and older T-DM2 values (2253-2170 Ma). These geochemical features indicate that they were mainly derived from partial melting of the lower continental crust. Amphibole rather than garnet is the main residual phase in the source, indicating that the magma originated from the normal lower crust (<40 km) rather than from the thickened lower crust. Our data, along with regional geology, lead us to suggest that the lithosphere was being stretched and thinned before ca. 114 Ma in the eastern North China Craton, and the region was already under an intraplate tectonic setting associated with a large mantle wedge at ca. 114 Ma. The tectonic regime was triggered by the rollback of the Paleo-Pacific Plate during the late Early Cretaceous.