Provenance of Late Ordovician sedimentary rocks in the SE Yangtze block: Implications for deposition in an active continental margin

Abstract

The Jiangnan Orogen separates the Yangtze block from the Cathaysia block to the southeast. Several competing models have been proposed for the evolution of both blocks and the intervening orogen, which mainly focus on aspects of igneous and metamorphic rocks. Limited research has been conducted on the sedimentary rocks in this region. Provenance of the Late Ordovician sedimentary rocks in the northeastern Jiangnan Orogen has been constrained based on systemic analyses of petrology, heavy mineral assemblages, geochemistry, and detrital zircon U-Pb ages, which provide vital clues for reconstructing the evolution of this belt during the early Paleozoic. Late Ordovician deposits in the northeastern Jiangnan Orogen consist of conglomerates, sandstones, siltstones, shales, calcareous mudstones, and limestones. These strata were deposited in a shallow to deep sea environment, showing water depth deepening to the northwest. The sandstone contains lithic graywackes and feldspar-lithic graywackes, with low compositional and textural maturities. They consist, on average, of quartz clasts (27%), feldspar clasts (18%), and lithic fragments (55%). The lithic fragments are angular, including andesites, rhyolites, tuffs, granites, slates, phyllites, quartzites, siltstones, and chert, which also occur in conglomerate form. Minor pyroxene, chromite, magnetite, and garnet grains occur in the sandstone. Geochemical analyses show that the sandstone and siltstone have widely variable Fe2O3T + MgO (4.43–12.05%), Cs, V, Ni, and Ti contents. They also exhibit a distinct negative Nb-Ta trough compared to the average upper continental crust and have light rare earth element (LREE) enrichment patterns with obvious Eu anomalies, similar to subduction-related rocks in an arc setting. Detrital zircon U-Pb ages demonstrate a mixed source from ca. 784 Ma and ca. 455 Ma. Paleocurrent indicators point to a southeastward origin. These results demonstrate that Late Ordovician sedimentary deposition occurred in a northwestward-facing active continental margin along the southeastern Yangtze block.