Petrology and geochemistry of Upper Jurassic Weimei Formation sandstones in southern Tibet: implications for provenance and tectonic setting

Abstract

Located within the centre of the Tethys Himalayas, the Weimei Formation of the Rongbu area in southern Tibet is a littoral- neritic sedimentary association composed of clastic rocks and a few shales. Petrologic characteristics and detrital component statistics indicate that sandstone types include lithic quartz sandstones, with the average content of quartz, feldspar and debris being 77%, 9% and 14%, respectively. The provenance of the Weimei Formation is a recycled orogeny, and the aforementioned sandstones generally have high SiO2 levels, low CaO, K2O and Na2O levels and wide ranges of CIA and CIW values. The primitive mantle-normalised trace element spider diagrams showed unstable content of some elements and losses of Ba, P, Sr, K, Ti and other elements in partial samples. Chondrite-normalised rare-earth element (REE) patterns indicate that the sandstones are characterised by an enrichment of LREE, significant fractionation of LREE and HREE and negative Eu anomalies. Further, ratios of Sm/Nd and Cr/Zr and discriminant diagrams (i.e., La/Yb versus ΣREE, ΣREE versus SiO2/Al2O3, La/Th versus Hf and function discrimination diagrams) reveal that sandstones from the Weimei Formation were derived from ancient quartzose sedimentary rocks and volcanic rocks of continental island arc. Many important parameters (i.e., La, Ce, Pb, Th, Hf, Zr, ΣREE, La/Yb, (La/Yb)N, ΣLREE/ΣHREE, Eu/Eu*, Rb/Sr and Zr/Hf) of samples and tectonic setting discriminant diagrams (i.e., Th versus Co versus Zr/10, La versus Th versus Sc, La versus Th and function discriminant diagrams) indicate that the tectonic setting of the Weimei Formation sandstones from the Tethys Himalayas has both characteristics of a passive margin and continental island arc. By comparing the detrital zircon U-Pb ages and Hf isotopic characteristics of the Weimei Formation with the strata whose provenances have been identified and analysing the weathering, petrography and geochemistry characteristics of Weimei Formation sandstones and the paleogeography conditions of the northern margin of the India craton within the Late Jurassic epoch, we inferred that Weimei Formation sandstones were mainly sourced from the Precambrian metamorphic rocks of the India craton, simultaneously, the sandstones also received minor sedimentary materials from earlier Tethys Himalaya strata whose provenances contained volcanic rocks of the nearby Gangdese volcanic belt. After experiencing weak to moderate weathering, denudation and long-distance transport, the sedimentary materials eventually migrated to the shelf and the top of the continental slope on the northern margin of the Indian continent, where the materials later participated in diagenesis.