Generation of leucogranites via fractional crystallization: A case study of the Jurassic Bengbu granite in the southeastern North China Craton

The magmatic processes that form leucogranites are closely related to plate collision. As the products of crustal anatexis, leucogranites provide clues to intracrustal differentiation and the regional tectonic evolution. The Bengbu area is located along the southeastern margin of the North China Craton (NCC), 150 km north of the Dabie Orogen, and is bounded by the Tancheng–Lujiang Fault Zone to the east. Late Jurassic leucogranites are widely distributed in the Bengbu area. We collected samples of the Bengbu leucogranite (BLG) and analyzed data on whole-rock geochemistry, zircon U–Pb geochronology, zircon in situ Lu–Hf isotopes, quartz in situ O isotopes, and total bulk Sr–Nd–Pb isotopes. Zircon U–Pb dating yielded formation ages for these leucogranites of ∼160 Ma. The samples are characterized by high SiO2 (72.4 wt.%−76.9 wt.%), alkali (7.3 wt.%−9.4 wt.%), and Al2O3 (13.4 wt.%−15.5 wt.%) contents. In addition to two samples exhibiting the rare-earth element tetrad effect, all rocks have high differentiation index values (95–99), extremely low iron–magnesium–titanium oxide contents (total Fe2O3 < 1.4 wt.%, MgO < 0.3 wt.%, TiO2 < 0.2 wt.%), and low Zr/Hf (20–50) and Nb/Ta (5–25) ratios. These data suggest that the BLG is a highly fractionated granite. Furthermore, the samples contain Neoproterozoic and Triassic zircons, have low Ti-in-zircon temperatures (682–697 °C), low Rb contents (<192 ppm), high Sr contents (48–791 ppm), low Rb/Sr ratios, low radiogenic Pb isotopic compositions (206Pb/204Pb(t) = 17.169–17.297), and highly enriched Sr–Nd–Hf isotopic compositions (87Sr/86Sr(t) = 0.7082–0.7104, εNd(t) = −16.6 to −12.3, and zircon εHf(t) = −14.5 to −21.7). Taken together, the geochronological and geochemical results suggest that the BLG was formed by high-degree fractional crystallization after hydrous partial melting of the North Dabie granitic gneisses, which have been subducted beneath the Bengbu area by Triassic continental subduction. Following collision between the South China Block and the NCC, Late Jurassic granitoids in the eastern NCC, including the BLG, were generated in the resulting post-orogenic extensional environment and were also influenced by ongoing subduction and rollback of the Izanagi Plate.