Cretaceous extension of the Ganhang Tectonic Belt, southeastern China: constraints from geochemistry of volcanic rocks

New geochemical and isotopic data are reported from Cretaceous volcanic rocks of the Ganhang Tectonic Belt, southeast China. Two periods of basaltic eruptions (143 ± 1.1–139 ± 0.7 Ma and 105–98 Ma), each representing results of previous tectonic events, are recognized in the belt. Rock types associated with the eruptions are significantly different in mineral and chemical composition. For the 143 ± 1.1–139 ± 0.7 Ma basalts, 87Sr/86Sr ratio (0.7041–0.7048) and Sm–Nd isotopic compositions (ɛNd = +3.78 to +4.38) suggest that the magma came from the depleted mantle and had not mixed with crustal material. During the second episode of magmatic and tectonic activities (105–98 Ma), volcanism was accompanied by intense regional extension and the development of a series of “red basins”. The geochemical characteristics of these basalts exhibit both tholeiite and alkaline olivine-types, indicating inhomogeneous mixing of magma with crustal material. The tholeiites have a medium–high 87Sr/86Sr ratio (0.7054–0.7070) and a low ɛNd value (1.69–0.70), indicating that the magma of these basalts came from an unchanged mantle. The alkaline olivine basalts, on the other hand, have a relatively higher 87Sr/86Sr ratio (0.7076–0.7080) and a relatively lower value of ɛNd (−6.43 to −5.18), suggesting that these magmas came from mantle enriched by mixing with subducted crustal material. Based on comprehensive geological analyses, it is concluded that an extensional environment existed in the region from early in the Early Cretaceous, and the Ganhang Belt represents the main rift zone in southeast China during much of the Early Cretaceous.