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The Late Ordovician-Early Silurian period witnessed the Phanerozoic mass extinction, glacial events, and volcanic events. Paleoweathering indexes chemical index of alteration (CIA), chemical index of weathering (CIW), and plagioclase index of alteration (PIA) indicated that the source area weathering changed from weak to moderate to intense. CIA values in the upper Zhongbao formation ranged from 66.71% to 73.97%, indicating a drier and colder climate. Upward, the CIA values on the bottom of the Mayinggou formation returned to the high value quickly (from 73.86% to 81.31%), suggesting that the ice age ended, the climate became warmer and wetter, and the sea level rose. The Al2O3-(CaO*+Na2O)-K2O triangular plots, Hf-La/Th, and SiO2-Al2O3/TiO2 bivariate plots inferred that the source of the siltstones in the two formations is mostly from the felsic igneous rocks. The samples from the Zhongbao-Mayinggou formations have chondrite-normalized rare earth element (REE) patterns similar to that of the North Qilian volcanic arc rocks. Geochemical discrimination plots displayed that the sediments of the Zhongbao-Mayinggou formations came from the active continental margin setting. [ABSTRACT FROM AUTHOR]

The North Qilian Orogenic Belt is on the northeastern margin of the Tibetan Plateau. It is connected with the Alxa Block in the north and the Tethyan orogenic assemblage in the south. The Lower Silurian Angzanggou Formation, deposited in the northern area of the North Qilian Orogenic Belt, is the most important for revealing the architecture and orogenic processes of the North Qilian Belt. Provenance analysis of the Angzanggou Formation can reveal not only the tectonic evolution of the central orogenic belt of China but also Palaeozoic Asia plate reconstructions. Petrographic analysis indicated that the compositional and textural maturity of the sandstones was low. The detrital composition of the Angzanggou Formation samples consists of quartz (8–14 %), feldspar (6–29 %) and lithic fragments (56–86 %). The sandstones could be classified as litharenites or feldspathic litharenites. The detrital modal composition suggests that these sandstones were probably deposited in a fore-arc basin. The element ratios and some discrimination diagrams based on geochemistry indicate that felsic and intermediate rocks were the main source rocks. The SiO2/Al2O3 ratio, the index of chemical variability and the Th/Sc versus Zr/Sc discrimination diagram suggest that the compositional maturity and degree of recycling were moderate to low. The index of alteration (CIA) and the A–CN–K diagram indicate the intensity of weathering was moderate. The discrimination diagrams based on major and trace elements and petrographic discrimination diagrams imply that the Angzanggou Formation rocks were derived from a continental island arc, and a sedimentary cover probably overlaid the volcanic arc. Therefore, we infer that during Early Silurian time the North Qilian Belt sediments accumulated in a fore-arc basin. [ABSTRACT FROM AUTHOR]

The Late Ordovician-Early Silurian transition was a critical interval in Earth's history, marked by mass extinction, the Gondwana glaciation and volcanic events. Here, we present mineralogical and geochemical characteristics across the Ordovician-Silurian boundary (Wufeng, Guanyinqiao and Longmaxi formations) from the Huadi No. 1 well, in the Sichuan Basin, in order to explore the paleoweathering, provenance, tectonic setting and paleoclimatic conditions of the source area. Thin sections, XRD (X-ray diffraction) data and major element results reveal that the rocks were abundant in clay minerals, quartz, minor feldspar, pyrite and carbonate minerals. The samples have similar chondrite-normalized REE patterns to those of PAAS, which are characterized by slight LREE enrichments and accompanied by flat HREE trends with weakly negative Eu anomalies. Paleoweathering indices (CIA and PIA) suggest that the sediments experienced weak to moderate chemical weathering in the source area. CIA values in the lower Wufeng formation range from 67.11 to 72.17, indicating a warm and humid climate, the CIA values in the upper Wufeng formation decrease from 70.35 to 66.15, and the CIA values in the Guanyinqiao and lower Longmaxi formation are persistently low (56.64–62.87), suggesting a cold and dry climate, which is consistent with the widespread presence of glaciation in the Hirnantian period. Further upward in the upper Longmaxi formation, the CIA returns to a high value (65.59), which may reflect that the climate started getting warm and glaciation had finished. Fluctuations in CIA values reflect that the climate shifted from warm to cold and then to warm again during the end Ordovician-early Silurian time period. The A-CN-K triangular diagram and SiO 2 -Al 2 O 3 /TiO 2 , TiO 2 -Zr, and La/Th-Hf bivariate diagrams indicate that the provenance of the rocks from the three formations was primarily felsic igneous rocks. Discriminant-function diagrams and La-Th-Sc, Th-Co-Zr/10 and Th-Sc-Zr/10 triangular diagrams show that the clastic sediments of the Wufeng-Longmaxi formations were derived most likely from an active continental margin. Finally, across the Late Ordovician-Early Silurian, tectonic activity and the end of the Ordovician glaciation dominantly controlled the deposition of black shales and siltstones in the Wufeng-Longmaxi formations of South China. • CIA shows that climate shifted from warm to cold, and to warm during Late Ordovician-Early Silurian. • A-CN-K and elements diagrams indicate that the shales were primarily from felsic igneous rocks. • Tectonic activity and end Ordovician glaciation controlled the deposition of black shales. [ABSTRACT FROM AUTHOR]

There are numerous K‐bentonites interlayered with black shales from the Late Ordovician‐Early Silurian that are widely distributed in South China. In this paper, we carried out mineralogical and geochemical investigations on K‐bentonite samples collected from 6 sections in South China. The petrological features of thin sections, X‐ray diffraction data, and major element results show that in addition to clay minerals, which are dominated by illite/smectite mixed layer and illite, the K‐bentonites also contain quartz, microcline, albite, pyrite, and zircon. Zircon U–Pb dating for two K‐bentonite beds (WXP‐BT2 and LBP‐BT1) by the LA‐ICP‐MS method yielded two weighted mean 206Pb/238U ages of 443.5 ± 1.9 and 440.4 ± 5.6 Ma, respectively. The concentrations of rare earth elements (REEs) range from 141.28 to 854.44 ppm, chondrite‐normalized REE patterns display a negative Eu anomaly and an enhanced enrichment in light REE. Samples plot in the fields of trachyte, trachyandesite, rhyodacite (dacite), and andesite in a plot of Nb/Y against Zr/TiO2 suggest that the K‐bentonites are most probably derived from felsic magmas with subalkaline to alkaline affinities. The discrimination diagrams (Y‐Nb, Y + Ta‐Rb, Y + Nb‐Rb) show that the tectonic setting of the source volcanoes ranges from a volcanic arc to within‐plate setting. The widely distributed bentonites suggest intensified volcanic ash eruptions in the early Late Ordovician, which released huge amounts of volcanic ash. The SO2 emission and the weathering of the volcanic ashes probably resulted in global climate cooling and indirectly caused the Hirnantian glaciation and the mass extinction at the end of the Ordovician Period. [ABSTRACT FROM AUTHOR]

The North Qilian orogenic belt is the key to figure out the evolution and assembly of Asia. The Upper Silurian Hanxia Formation which is deposited in the north area of North Qilian Orogenic Belt is of utmost important to reveal the architecture and orogenic process of the North Qilian belt. So provenance analysis of the Hanxia Formation is of significance to reveal not only that the tectonic evolution of the central orogenic belt China, but also that Paleozoic Asia plate reconstruction. The ratios of elements and some discrimination diagrams based on geochemistry indicate that felsic rocks constitute their main source rocks. The chemical index of alteration is less than 80, indicating that the source rocks are relatively fresh and of low maturity. On the Th/Sc versus Zr/Sc scatter plot, samples from Hanxia Formation occur along the magmatic compositional variation trend of rocks, indicating that the rocks did not undergo obvious sedimentary sorting and recycling. The major and trace elements discrimination diagrams imply that Hanxia Formation rocks were derived from continental island arc. Recent studies show that the North China plate subducted southwards and produced subduction-related arc magmatism along the southern margin of the North Qilian Terrane during the Silurian. Therefore, we infer that in the late Silurian period the subduction-related arc became accreted to the Central Qilian terrane to the south, forming a composite continental arc, and the North Qilian belt accumulated in a fore-arc basin. [ABSTRACT FROM AUTHOR]