The nature of basement beneath the Songliao Basin in NE China: geochemical and isotopic constraints

Abstract

The Songliao Basin, located in northeastern China with an area of about 260,000 km2 and having the Daqing Oil Field as the most important center of the petroleum industry in China, is much debated about the basement nature. According to the drill holes, its basement is mainly composed of granites and the Paleozoic strata, some of which have undergone weak metamorphism and somewhat deformation. Gneiss has also been identified, but the petrographical studies indicate that the gneiss is in fact deformed granitic intrusions. Therefore, the granites are the major rock types beneath the Songliao Basin. Geochemical research indicates that these granites are peraluminous, and generated mainly by partial melting of juvenile crust. Geochronological studies show that one undeformed granitic sample gives an U-Pb age of 305 ± 2 Ma, and the mylonitic granite yields an U-Pb age of 165 ± 3 Ma; neither sample contains inherited zircon, which suggests that the basement of the Songliao Basin is not Precambrian, but part of Phanerozoic orogenic belt. With respect to Sr-Nd isotopes, these basement granites are characterized by low ISr (≈0.705), high εNd(t) (−2.2–+2.4) and young Nd model ages (690 to 1160 Ma with most less than 1000 Ma) which is the same as that of the granites in the surrounding orogenic belts such as the Zhangguangcai, Great Xing'an and Lessor Xing'an Ranges, but distinctly different from granites emplaced in the adjacent Precambrian Jiamusi Massif. The simple mass balance calculation indicates that the mixing between the depleted mantle and ancient crust could not reasonably explain the Sr-Nd isotopic features, but the mixing modeling between the juvenile and pre-exited ancient crust suggests that the granites would have about 90% of the juvenile crustal component. Therefore, the Sr-Nd isotopic studies suggest that the basement of the Songliao Basin is juvenile crust, identical to the surrounding orogenic belts, and that there is no Precambrian basement to the Songliao Basin.