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Cherts, radiolarites, and radiolarian cherts are one of the most important components of accretionary complexes, and much used in the reconstruction of imbricated sections of ocean plate stratigraphy, and of the palaeogeographic and tectonic settings of ophiolitic mélanges. There has been little research on radiolarian cherts in the Kyrgyz South Tianshan, and the tectonic setting and temporal framework of the South Tianshan ocean are controversial. Here, we report well‐preserved radiolarians in ribbon‐bedded cherts in an ophiolitic mélange of the Atbashi accretionary complex (South Tianshan, Kyrgyzstan), which provide key evidence of ambient Palaeozoic oceanic conditions. The hydrofluoric acid technique was used to extract from the cherty rocks siliceous radiolarian, which yielded an age of late Early Carboniferous. Analyses of major, trace, and rare‐earth elements of four cherts indicate they are biogenic and deposited with little hydrothermal influence in a hemipelagic to continental margin environment. Our fossil ages and geochemical results of the radiolarian cherts from the Atbashi accretionary complex indicate that the South Tianshan ocean was still open in the Late Carboniferous, implying that it was a wide, long‐lived, major ocean that closed by northward subduction. [ABSTRACT FROM AUTHOR]

• Nong Pong Formation chert was deposited in a deep marine environment on continental crust. • Radiolarian indicate a late middle Permian age for the studied section. • Deposition was probably in a rift setting, flanked north and south by platform carbonates. The deepwater chert-shale-limestone-tuff Permian Nong Pong Formation (NPF) of the Saraburi Group is a major unit within the Triassic (Indosinian Orogeny) Khao Khwang Fold and Thrust Belt (KKFTB). Whether the NPF was deposited in a continental margin or ocean basin remains uncertain and impacts structural, sedimentary and stratigraphic interpretations of the KKFTB. This study investigates chert geochemistry from a 60 m section of the formation, near Muak Lek town, to determine its depositional environment. Radiolarian fauna from the chert beds includes Follicucullus porrectus Rudenko, (probable late Guadalupian, late middle Permian), indicating the NPF is time-equivalent to adjacent carbonate platform deposition (Khao Khwang and Khao Khad formations). High ratios of SiO 2 /(SiO 2 + Al 2 O 3 + Fe 2 O 3) and Fe-Al-Mn plots indicate a biogenic origin (radiolarians, sponge spicules) for the cherts, far from a hydrothermal source. Trace element ratios for U/Th, Sc/Th and Sr/Ba and Ce anomalies in the NASC-normalized REE distribution are ambiguous, and range between shallow to deep ocean basin, and continental margin settings. Major elements analysis of cherts show: (1) high ratios of SiO 2 /(SiO 2 + Al 2 O 3 + Fe 2 O 3), range: 0.60–0.82 (0.76 average), (2) Al/(Al + Fe + Mn), range: 0.67–0.82 and (3) average MnO/TiO 2 = 0.14. The MnO/TiO 2 and Al 2 O 3 /(Al 2 O 3 + Fe 2 O 3) ratio, 100 × Fe 2 O 3 /SiO 2 vs. 100 × Al 2 O 3 /SiO 2 and Fe 2 O 3 /(100-SiO 2) vs. Al 2 O 3 /(100-SiO 2) diagrams indicate deposition on a continental margin. The La n /Ce n versus Al 2 O 3 /(Al 2 O 3 + Fe 2 O 3) discrimination diagram indicates a pelagic, continental margin setting. Combined geochemical, structural and sedimentary facies analyses suggests the NPF was deposited in a deep marine, normal fault-bounded basin on a rifted, ribbon continent margin where carbonate platforms grew on structural highs. [ABSTRACT FROM AUTHOR]

Upper Ordovician (Katian) to Lower Silurian (Rhuddanian) strata in the Middle–Upper Yangtze Block, South China contain extensive chert beds, although the origin of microcrystalline silica in these cherts is largely unknown. In this study, the mineralogy and structure of chert as well as major, trace, and rare earth elements are used to determine the origin of microcrystalline silica in 29 chert samples from Longmaxi Formation (Rhuddanian) in Baizitian outcrop, west of Kangdian Uplift (KU). Results show that 21 of the 29 chert samples have a biogenic origin, 2 indicate a hydrothermal origin, and 6 show a mixed origin. Cherts from Baizitian outcrop are compared to contemporaneous cherts deposited east of KU. Cherts both east and west of KU are mainly biogenic in origin, which is attributed to high productivity in the paleo-seaway. Hydrothermal microcrystalline silica is relatively uncommon in the Longmaxi Formation across the Middle-Upper Yangtze Block, and only occurs in Baizitian outcrop. Terrigenous influx contributed to chert accumulation both east and west of KU; however, cherts east of KU received much more clastic material than those in Baizitian outcrop. A comparison of chert composition and geochemistry to geological reconstructions of the Middle-Upper Yangtze Block reveal that accumulation of hydrothermal chert is closely linked to tectonic evolution. During the Early Silurian, the Middle-Upper Yangtze Block was in a compressional tectonic regime with limited heat supply, and hence, low hydrothermal circulation; this is manifested in the paucity of hydrothermal microcrystalline silica in cherts east of KU. In contrast, hydrothermal microcrystalline silica in cherts west of KU is interpreted to have formed in an extensional tectonic regime with sufficient heat supply. This research demonstrates that chert source is, in part, tectonic controlled, and chert geochemistry can be used in reconstructing paleoenvironments. • Cherts in Longmaxi Formation (Fm) are mainly biogenic in origin. • Hydrothermal cherts in Longmaxi Fm only occur west of Kangdian Uplift (KU). • Hydrothermal chert west of KU was derived from active hydrothermal circulation. • East of KU there is no evidence of hydrothermal circulation. • Cherts east of KU received more clastic material than those west of KU. [ABSTRACT FROM AUTHOR]