Mineralogical and geochemical characteristics of K‐bentonites from the Late Ordovician to the Early Silurian in South China and their geological significance

Abstract

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 206 Pb/ 238 U 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/TiO 2 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 SO 2 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.

Geochemical characteristics of siliceous rocks of Wufeng Formation in the Late Ordovician, South China: Assessing provenance, depositional environment, and formation model

“…From the Late Ordovician to Early Silurian, the Caledonian Orogeny reached a maximum intensity caused by the accretion of the Cathaysia Block to the Yangtze Block, and related subduction resulted in orogenesis associated with voluminous intermediate to felsic magmatism (Su et al, ). Intensified volcanic ash eruptions occurred, as shown by numerous K‐bentonite beds present in the upper Yangtze Block, including our sections (Ge et al, ; Hu, Zhou, Song, Li, & Sun, ; Su et al, ; Su, He, Wang, Gong, & Zhou, ). Massive volcanic ashes fall into the ocean, which stimulate the abundance of phytoplankton.…”

“…Among the above elements, cerium has been considered as a marker element for sedimentary environment. Based on cerium anomaly (Ce/Ce*) and total REE abundance (∑REE), Murray et al (1990) (Ge et al, 2018;Hu, Zhou, Song, Li, & Sun, 2008;Su et al, 2009;Su, He, Wang, Gong, & Zhou, 2003 On the basis of geological field observation, petrology studies, the presented geochemical data, and the geodynamic evolution of this study area at the Ordovician-Silurian transition, we constructed a deposition model for the studied siliceous rocks (Figure 15). This the organic matter of these organisms by sulphate-reducing bacteria favoured the early pyritization at the same time that silica dissolved (Reolid, 2014).…”

Large mass-independent sulphur isotope anomalies link stratospheric volcanism to the Late Ordovician mass extinction

“…The Kuanyinchiao Bed is composed mainly of dark brown mudstone intercalated with thin carbonate lenses that preserve abundant shelly fossils. In both areas, numerous K-bentonite beds with thickness of 1-1.5 cm but many are <3 mm, have been found from D. complexus to upper P. pacificus Zone, indicate volcanic eruptions during the Late Ordovician in South China 24,25 (Fig. 1a, b).…”

Geochemistry and U–Pb geochronology of K‐bentonites from the Pingliang Formation of the Upper Ordovician in Gansu, North China, and their tectonic implications

“…The REE patterns correspond closely to the typical dacitic and trachyandesitic patterns (Roberts & Merriman, ) and are consistent with subduction‐related volcanic activities (Batchelor & Clarkson, ; Huff, Bergström, Kolata, & Sun, ). Negative Eu anomalies may be inherited from the original magma or may be the result of late diagenesis and low‐temperature metamorphism of the K‐bentonites (Ge et al, ).…”