Neoproterozoic tectonic evolution of the northwestern margin of the Yangtze Block (southwestern China): Evidence from sandstone geochemistry and detrital zircon U-Pb ages of the Hengdan Group

Abstract

The Hengdan Group in the northwestern part of the Bikou Terrane preserves sedimentary archives for the debated early to middle Neoproterozoic tectonic evolution of the northwestern margin of the Yangtze Block (YB), while its depositional age, provenance and tectonic setting remain controversial. This research has reported the geochemical and geochronological studies on the sandstones in the Hengdan Group. Our work indicates that the sandstones in the Hengdan Group experienced low degree of chemical weathering and sedimentary recycling, and are mainly sourced from acidic to intermediate igneous rocks. In addition, their elemental contents (i.e., La, Ce, Th and ΣREE) and ratios (i.e., LaN/YbN and Eu/Eu*) are comparable to those deposited in an active continental margin setting. All samples yield similar age distribution, having a dominated age range at ca. 850 to 720 Ma. Together with the dominantly angular to subangular morphology and clear oscillatory or broad zoning internal structure of these aged detrital zircons, indicating that the Hengdan Group should be proximally sourced from the early to middle Neoproterozoic igneous rocks. In addition, the youngest group of detrital zircon U-Pb ages of the sandstones in the Hengdan Group suggest that the onset and ending of the deposition of this group is later than ca. 741 Ma and ca. 722 Ma, respectively. Combining with previous work, the Hengdan Group was most likely deposited during ca. 741 to 717 Ma in a forearc basin setting at the northwestern margin of the YB, and with its primary provenance probably from nearby acidic to intermediate igneous rocks. In addition, all these above further supports that the northwestern margin of the YB should be a long-lived active continental margin setting during early to middle Neoproterozoic (ca. 835 to 717 Ma) and, the South China Block should be located at the relative peripheral location of the Rodinia supercontinent.