松科二井早白垩世早期玄武安山岩的发现及地质意义

松科二井超深钻探获取了松辽盆地连续沉积-火山岩心，为系统研究中生代松辽盆地形成机制、揭示中国东北地区构造演化历史提供了极好的研究材料。本文对取自松科二井井深-6035~-6084m的玄武安山岩开展了系统的锆石U-Pb定年、全岩主微量元素和Sr-Nd同位素分析测试。研究结果表明，松科二井玄武安山岩隶属于火石岭组底部，其形成时代为早白垩世早期（141.6±1.4Ma）。玄武安山岩样品富集大离子亲石元素，亏损高场强元素，具有弧岩浆的地球化学特征。这些样品显示了一定程度的轻重稀土元素分异（（La/Yb）_N=8.40~11.88），具有轻微亏损的Sr-Nd同位素特征（（⁸⁷Sr/⁸⁶Sr）_i=0.70496~0.70478，ε_Nd（t）=1.05~1.61），表明它们很可能源自于岩石圈地幔。早白垩世时期中国东北地区主要受控于蒙古-鄂霍茨克洋的南向俯冲和东侧古太平洋俯冲过程，目前研究揭示了前者影响了同期的大兴安岭地区的火山活动，但其影响范围是否延伸至松辽盆地内部尚不清楚。古太平洋岩石圈开始俯冲后很可能诱发了松辽盆地西侧晚中生代火山岩的形成。早白垩世松辽盆地演化主要受蒙古-鄂霍茨克洋还是古太平洋板块的活动影响仍存在较多争议。松科二井玄武安山岩具有与板内玄武岩相近的Th/Hf比值（1.10~2.87）及Zr/Y比值（10.1~18.7），表明它们具有板内岩浆的地球化学特征，且松科二井样品比同时代盆地西部的大兴安岭地区玄武岩具有更高的Nb/Ta比值，更高的Zr、Hf与Ti含量，以及更亏损的Nd同位素特征，可能指示与大兴安岭地区玄武岩形成于不同的构造背景。松辽盆地内早白垩世火山岩很可能为古太平洋板片回转诱发形成，指示早白垩世古太平洋板片回转诱发的岩石圈伸展促进了松辽盆地的形成。

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 ((87Sr/86Sr)i=0.70496~0.70478, εNd(t)=1.05~1.61), 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 (1.10~2.87) and Zr/Y ratios (10.1~18.7) 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.