Petrology and gecochemistry of the Silurian sicliciclastic rocks: constrain on the provenance and tectonic setting of deposition in SE Yangtze block

Abstract:

The Jiangnan Orogen is a suture zone between Yangtze block and Cathaysian fold belt, which is a huge tectonic melange belt in the south China and one of the most important tectonic-magmatic-metallogenic belts in China. Knowledge of the provenance and tectonic setting of the lower paleozoic in NW Zhejiang region closely adjacent to the Jiangnan Orogen eastern section is essential to understanding the mechanism and timing of the amalgamation between the Yangtze and Cathaysia blocks. In this paper sandstone detrital fragments, detrital heavy mineral assemblage, geochemistry of siltstone and sandstone, paleocurrent data are used to constrain the provenance and depositional setting of the Silurian clastic sedimentary rocks from SE Yangtze block.

Silurian deposits in the SE Yangtze block consist of sandstone, siltstone and shale. They were deposited in a shallow environment, showing water-depth deepening northwestward. Petrographic features show that the lithic sandstones and arkose lithic sandstones, which have low compositional maturity and textural maturity. In the sandstone samples, the average content of quartz clasts are 27%, feldspar clasts are 17%, rock debris are 56%, among them, quartz is mainly single crystal quartz, feldspar is mainly plagioclase, and a small amount of potassium feldspar. The composition of rock debris mainly includes rhyolite, slate, and a small amount of andesite, etc. In the discriminant diagram of the tectonic background of the source area of sandstone, the sandstone sample falls into the transitional island arc area, which indicates that the debris comes from the source area of the island arc background. Minor pyroxene, chromite, magnetite and garnet grains occur in the sandstone. Geochemical analysis shows that sandstone and siltstone have a wide variable content of Fe₂O₃^T+MgO, Cs, V, Ni, and Ti. They also show a distinct negative Nb-Ta trough compared to the average upper continental crust (UCC) and have a light rare earth element (LREE) enrichment pattern with obvious Eu anomalies, similar to subduction-related rocks in an arc setting. Paleocurrent indicators point to a southeastward origin. These results demonstrate that the Late Ordovician deposition took place in a northwestward-facing active continental margin along the southeast Yangtze block.