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The Sichuan Basin in Southeastern China contains extensive bedded cherts dating back to the Late Ordovician–Early Silurian period. To investigate the origin and depositional environment of these cherts, we conducted a comprehensive study using field observations, thin sections microscopic, silicon isotope analysis, and major and trace element geochemistry of samples from three sections. Our results indicate that the cherts from Wufeng and Longmaxi formations are non-hydrothermal, normal biogenic seawater deposits mainly affected by terrigenous input and slightly associated with volcanic ash. Al2O3/(Al2O3 + Fe2O3T) and LaN/CeN ratios, δCe values and Fe2O3T/TiO2 − Al2O3/(Al2O3 + Fe2O3T), Fe2O3T/(100 − SiO2) − Al2O3/(100 − SiO2), 100 × (Fe2O3T/SiO2) – 100 × (Al2O3/SiO2), and LaN/CeN − Al2O3/(Al2O3 + Fe2O3T) discrimination diagrams indicated that the bedded cherts deposited in the continental margin environment. [ABSTRACT FROM AUTHOR]

The petrography, major, and trace elements concentration of the sandstones from the Zhanjin Formation in Qiwu area, south Qiangtang Basin, have been studied to determine their provenance, intensity of paleo-weathering and depositional tectonic setting. Petrography indicates that the composition and texture maturity of the sandstones are moderate. The detrital composition of Zhanjin Formation samples are dominated by quartz (70.1–80.3, average 74.1%) and feldspar (18.3–26.5, average 21.9%), but is low in lithic fragment (1.4–8.6, average 3.9%). The sandstone can be classified as arkose and litharenite arkose. The detrital model composition reflects that these sandstones are probably derived from a passive continental margin. The index of chemical variability (ICV), and Th/Sc vs. Zr/Sc discrimination diagram suggests that the compositional maturity and recycling were moderate to low. The index of alteration (CIA) and the A–CN–K diagram indicated that the intensity of weathering in the source area was low. The Al2O3/TiO2 vs. SiO2, Th/Sc, Co/Th, La/Sc, Cr/Th ratio values of the Zhanjin sandstones indicated that the sandstones were mainly derived from felsic source rocks. The Gondwana passive continental margin belt is probably the primary provenance area as evidenced by petrography and geochemistry features of the Zhanjin Formation. All kinds of tectonic discrimination diagrams based on major elements showed a rift setting for Zhanjin sandstones, which is consistent with the general views of study areas. [ABSTRACT FROM AUTHOR]

Collision of the South and North China cratons was an important event in eastern Asia and contributed to the formation of the Central Orogenic Belt in China. While the timing of collision has been documented to the west (at the West Qinling Belt) and east (at the Dabie Mountains), little is known regarding the timing of collision in the middle sector of the collision zone. We present field, sedimentological, mineralogical, and geochemical data from the Late Permian Dalong Formation and Early Triassic Daye Formation (lower part) in the Western Hubei Trough which is located in the northern South China Craton. Based on sedimentary facies, SEM, and XRD data, we suggest that the Dalong Formation, which is mainly composed of chert and carbonaceous mudstone, was deposited in a deep shelf, and the lower part of the Daye Formation, which is mainly composed of mudstone and carbonate rocks, was deposited in a shallow shelf. Geochemical data imply that the source of the mudstone in these formations was the Qinling active continental margin of the North China Craton. These data support a scenario that the South and North China cratons may have collided by the end of the Late Wuchiapingian of Late Permian time at the southern margin of the East Qinling Belt. The collision event may have led to the formation of the Western Hubei Trough. [ABSTRACT FROM AUTHOR]

• The Qinling Orogen provided the source for sediments in the Western Hubei Basin. • The eastern part of the Mianlue Ocean was closed earlier than its western part. • The South China Block may have become a part of Pangea during the Late Permian-Middle Triassic. It is thought that the North China Block collided with Pangea earlier than the closure of the Mianlue Ocean between the South and North China Blocks. Therefore, the closure of the Mianlue Ocean, which induced the formation of the Qinling-Dabie Orogen, led to the amalgamation of the South China Block and Pangea. Yet, the timing of closure remains poorly constrained. We present detrital zircon U-Pb ages, trace element and Hf isotopic data for an upper Permian-lower Triassic sedimentary succession in the Western Hubei Basin, northern Yangtze Block, South China. One major age population of 250–330 Ma and four minor age populations of 400–650 Ma, 1030–1300 Ma, 1600–2000 Ma and 2300–2800 Ma were identified. Trace element and Hf isotopic data indicate zircons in the major age population may have sourced from active continental margins. When integrated with tectono-magmatic events of adjacent blocks, our data support that these detrital zircons were mainly sourced from the Qinling Orogen in the southern North China Block. Combined with continental subduction-related metamorphic events in the Dabie Terrane and syn -collisional granitoids in the West Qinling Belt, as well as the paleomagnetic data, our data support that the Mianlue Ocean experienced a scissors-like closure from the Late Permian in the east to the Middle Triassic in the west. In the middle of the Qinling-Dabie Orogen, the closure of the ocean took place earlier than ~255 Ma. This study supports the hypothesis that the South China Block became a part of Pangea during the Late Permian-Middle Triassic. [ABSTRACT FROM AUTHOR]

The organic-rich shales of the upper Permian Dalong Formation in the middle Yangtze area are considered an important shale gas exploration horizon. However, the origin of siliceous matter and the mechanism of organic matter enrichment are still controversial. Based on geochemical data, we studied the provenance, paleoclimate, paleoenvironment, primary productivity and hydrological conditions of the Dalong Formation in western Hubei Province and discussed the mechanism of organic matter enrichment. The shales of the Dalong Formation can be divided into two units, organic-rich and organic-lean shales, with average TOC contents of 5.81 wt% and 0.45 wt%, respectively. The western Hubei Basin was a strongly restricted basin during deposition, and the provenance of the Dalong Formation had a continental margin sedimentary background, mainly from mixed felsic and basic sources. Terrigenous clastic quartz, biological quartz and hydrothermal quartz are recognized in the shales of the Dalong Formation. Based on U, V, and Mo concentrations and V/(V + Ni) ratios, the organic-rich shales were deposited in anoxic-sulfurized conditions. In contrast, the organic-lean shales were deposited under oxic-suboxic conditions. The Dalong Formation had moderate to high primary productivity and controls on the redox conditions in the basin. The decrease in primary productivity in organic-lean shales may be related to the transition from a warm and humid climate to a hot and dry climate. The sedimentary model shows that the formation of organic-rich siliceous rocks in the Dalong Formation was controlled by primary productivity. The decrease in primary productivity driven by the hot and dry climate in the latest Permian is the main factor controlling the enrichment of organic matter and the formation of bioquartz. • Dalong shale contains terrigenous clastic quartz, biological quartz and hydrothermal quartz. • The anoxic-sulfurized water were widely distributed on the organic-rich shales deposited. • Hot and dry climate caused the reducing primary productivity. • Organic matter enrichment controlled mainly by the primary productivity. [ABSTRACT FROM AUTHOR]