Petrogenesis and geochronology of Cretaceous adakitic, I- and A-type granitoids in the NE Yangtze block: Constraints on the eastern subsurface boundary between the North and South China blocks

Abstract

The position of the subsurface boundary between the North China block (NCB) and South China block (SCB) has been debated, mainly using evidence from surface geology and geophysical observations. Here, petrochemical and geochronological data on four early Cretaceous granitic plutons from a focused area east of the Tan–Lu fault, NE Yangtze block, are reported to provide constraints on their petrogenesis and tectonic affinity. The Chuzhou intermediate intrusions consist mainly of quartz monzonite and granodiorite, and formed at ca. 125–127Ma. They have high MgO, Al2O3, Sr, and low Rb, Y and Yb contents, together with high Sr/Y and La/Yb ratios, indicating an adakitic affinity. They also show strongly negative whole-rock εNd(t) and zircon εHf(t) (−26 to −16) values, and old Hf crustal model ages (2.2–2.8Ga). Coupled with their negative Nb–Ta anomalies, high K2O/Na2O (0.91–1.18) and low Ce/Pb (1.08–5.40) ratios, these data suggest that the Chuzhou adakites were derived from thickened Archean to Paleoproterozoic lower crust. In contrast, the Fanchang, Qingyang and Huangshan intermediate–silicic complexes are dominantly composed of I-type granodiorite and quartz monzonite and A-type monzogranite and alkali feldspar granite. The I-type intrusions (126–138Ma) were emplaced slightly earlier than the A-type intrusions (121–129Ma), and distributed around A-type intrusions with clear intrusive boundaries. The I- and A-type rocks have lower MgO, CaO, Co, Sr, and higher Rb, Nb, Th and HREE contents than the adakitic rocks, with strong depletion in Ba, Sr, P, Eu and Ti for the A-type rocks. They have higher whole-rock εNd(t) (−9 to −5) and zircon εHf(t) (−13–0) values, and younger Nd model ages (TDM2=1.1–1.4Ga) and zircon Hf model ages (Tcrust=1.2–2.0Ga) than the adakitic rocks. These features indicate that the I- and A-type granites originated by partial melting of Mesoprotorozoic–Neoproterozoic lower crust, followed by fractional crystallization of plagioclase, K-feldspar and ferromagnesian minerals. The completely different isotopic compositions suggest that the Chuzhou adakites and three I- and A-type complexes were derived from different lower crustal domains, the former with NCB affinity and the latter with SCB affinity. Thus, a subsurface boundary between the NCB and SCB in the area east of the Tan–Lu fault can be constrained to a position between the Chuzhou and Ningwu–Fanchang area, possibly near Nanjing city. The association of Cretaceous bimodal volcanic rocks, within-plate mafic rocks and rift basins, suggests that the early Cretaceous rocks in the NE Yangtze block were produced in a lithospheric extensional setting, probably related to mantle-derived magma underplating and lithospheric thinning.