Appraising the basement nature of Junggar Basin through borehole core and deep seismic reflection data

The basement nature of Junggar Basin is an important topic concerning the basin evolution and continental growth of CAOB, but it still remains highly controversial, with views varying from the existence of pre-Cambrian basement as its continental block to a basement of Paleozoic oceanic crust or oceanic island arc complexes. Here, we focus on the deep architecture of Junggar Basin and its nature, using deep seismic reflection together with zircon Hf isotopic analysis carried out on Late Paleozoic strata, in order to provide new constraints on the basement nature of Junggar Basin. Most Carboniferous volcanic rocks, obtained from seven wells within Junggar Basin, have positive ε_Hf(t) values except for minor negative ε_Hf(t) values in the western Junggar Basin, suggesting that the Junggar Basin is mainly dominated by juvenile crust without the large-scale pre-Cambrian basement, if exist, it is limited and only located in the western part of Junggar Basin. Moreover, the 2D seismic profile suggests that Junggar Basin has duplex basement structure according to the differences in wave velocity. The upper part is Hercynian folded basement, whereas the lower part is the ancient crystalline basement. Furthermore, the deep seismic reflection profiles and drilling data confirm that the basement of Junggar Basin is chiefly composed of Hercynian folded basement. These Hercynian volcanic rocks have typical arc-like geochemical characteristics with low TiO₂ contents, enrichment in LILEs and depletion in HFSE, suggesting that they are products of subduction-related magmatism. These results, in combination with previous data in the East and West Junggar terrane, imply that the Junggar Basin probably have a collaged basement of Paleozoic juvenile crust with limited pre-Cambrian basement.