Development of Songliao Basin by Palaeo‐Pacific slab rollback: Evidence from Early Cretaceous rhyolites in SK2 Borehole, NE China

Abstract

As the largest petroliferous basin in northeast (NE) China, Songliao Basin preserves continuous Cretaceous sedimentary-volcanic records, providing an excellent opportunity to recover the palaeo-environment. The Yingcheng Formation in the Songliao Basin contains ample gas reservoirs that attract widespread attention. Whereas the timing, petrogenesis and geodynamic mechanism of the volcanic rocks in the Yingcheng Formation are still controversial, which largely constrain our understanding of the formation and evolution of the Songliao Basin. Here, we present an integrated investigation of zircon U-Pb ages and Hf isotope, as well as whole-rock elements and Sr-Nd isotopes data for a suite of rhyolites of Yingcheng Formation from the SK2 Borehole of NE China. Zircon U-Pb age dating results of nine samples have yielded a concentrated age of $110 Ma. These Yingcheng rhyolites are characterized by high SiO 2 (66-78 wt%) and alkali (Na 2 O + K 2 O = 7.80-11.70 wt%) content, high 10000*Ga/Al (1.26-3.82, mostly >2.6) and FeO T /(FeO T + MgO) ratios (0.81-0.95), which show geochemical affinities with A-type rhyolites. They have relatively low Y/Nb (0.69-1.78, average 1.19) and Rb/Nb (1.58-6.52, average 4.34) ratios, suggesting that the Yingcheng volcanic samples belong to A 1 -type rhyolites which formed in an intraplate environment. These Yingcheng rhyolites show depleted Nd-Hf isotopic compositions (ε Nd (t) = 2.43-4.87 and zircon ε Hf (t) = 4.22-9.88) comparable with the Early Cretaceous A-type and I-type rhyolites in the Songliao Basin, suggesting that they were originated from a juvenile continental crust. They were most likely derived from the partial melting of anhydrous lower crust instead of differentiation of mantle-derived or mixing with alkaline basaltic magma, due to their low Mg # and absence of the coeval mafic rocks. Combined with the previous studies, A-type rhyolites in the Yingcheng Formation erupted lasting at least 10 Myr and were widely distributed in the rifts of the Songliao Basin. We propose that the Yingcheng rhyolites were most likely generated from the rollback of the Palaeo-Pacific Plate in the Early Cretaceous, thus highlighting the significance of the subduction of the Palaeo-Pacific Plate during the secular evolution of the Songliao Basin.