Discovery and geological implications of the Early Cretaceous basaltic andesites in SK2 borehole

A continuous sedimentary-volcanic core of Songliao Basin has been obtained through ultra-deep drilling of the SK2 borehole, which provides excellent materials for systematically investigating the formation of the Songliao Basin and the tectonic evolution history of Northeast China. In this study, we conducted a combined study of zircon U-Pb dating, whole-rock geochemical and Sr-Nd isotopic analysis on basaltic andesite samples from SK2 borehole with depths ranging from - 6035m to - 6084m. These SK2 basaltic andesite samples belong to Huoshiling Formation and they were formed at Early Cretaceous (141. 6 +/- 1. 4Ma) according to the zircon U-Pb dating results. The SK2 basaltic andesites are enriched in large ion lithophile elements, depleted in high field strength elements, and their geochemical signatures are consistent with arc magmas. These rocks show depleted Sr-Nd isotope characteristics ((Sr-87/Sr-86)(i) =0. 70496 similar to 0. 70478, epsilon(Nd)(t) = 1. 05 similar to 1. 61), suggesting a depleted mantle source. In the Early Cretaceous, the evolution of Northeast China was mainly controlled by the southward subduction of the Mongol-Okhotsk oceanic plate and the western subduction of the Paleo-Pacific Plate. Previous study has shown that the subduction of Mongolia-Okhotsk Ocean induced coeval magmatism in the Great Xing'an Range, however, it is not clear whether its influence extends into the Songliao Basin. The initial subduction of Paleo-Pacific lithosphere may induce Late Mesozoic magmatism in the west of Songliao Basin. The Early Cretaceous formation and evolution of Songliao Basin were mainly affected by the activities of Mongolia Okhotsk Ocean or Paleo-Pacific lithosphere remains controversial. The Th/Hf (1. 10 similar to 2. 87) and Zr/Y ratios (10. 1 similar to 18. 7) of the SK2 basaltic andesites are similar to the within-plate basaltic magmas, we therefore suggest that SK2 samples belong to intraplate magmatism. The samples also show higher Zr, Hf and Ti contents, higher Nb/Ta ratios and more depleted Nd isotope compositions than the coeval mafic rocks in the Great Xing'an Range, indicating the SK2 samples formed in a different tectonic background. Accordingly, the Early Cretaceous volcanic rocks in Songliao Basin are most likely occurred by rollback of the subducting Paleo-Pacific Plate, while incidental lithospheric extension may promote the formation of Songliao Basin.