Provenance of the Triassic Songpan–Ganzi flysch, west China

The Songpan–Ganzi Basin of western China is a triangular structure on the northeast corner of the Qinghai–Tibet Plateau filled with a great thickness of Triassic flysch, surrounded to the north by the Eastern Kunlun Orogenic Belt that extends farther eastwards as the Qinling–Dabie Belt, to the southwest by the Ganzi–Litang Ophiolite and Yidun Volcanic Arc and to the east by the Longmenshan Thrust Belt. We have carried out the most extensive petrographic and geochemical study so far undertaken on typical turbidites from the Songpan–Ganzi Basin including major element, trace element, Rb/Sr and Sm/Nd isotopic analyses in the Baryan Har region on the northern side of the basin. Petrographic study identifies the sandstones as typical graywackes whose matrix is fine-grained detrital material, recrystallized by greenschist facies metamorphism. Chemical weathering indices confirm that the fine-grained turbidites were little altered during transport and deposition, so compositions accurately represent their source areas. Major, trace and isotope analyses displayed on various discriminant diagrams indicate that the flysch from the Baryan Har region is derived from the Eastern Kunlun Belt to the north, a conclusion supported by published paleocurrent directions. Comparison of the Baryan Har turbidites with published results from Western Sichuan on the eastern side of the Songpan–Ganzi Basin suggests that the latter is derived from a source area of varied sedimentary rocks and a few igneous rocks. The most likely source is South Qinling to the northeast. Presently available data do not support derivation of material in the Songpan–Ganzi Basin from the Yidun Volcanic Arc or the Dabie Ultra-High Pressure Metamorphic Belt. The present provenance studies on the Triassic Songpan–Ganzi flysch support a westward opening model for the Paleotethyan Ocean and reveal different evolutional histories for the Qilian–Qinling–Dabie and Eastern Kunlun Orogens.