Geochemical Characteristics and Sedimentary Environment of Ribbon Cherts in the Karaghol Area of the Southern Tianshan

Ribbon cherts constitute crucial sedimentological evidence for the existence of ancient oceans, and their depositional settings bear significant implications for constraining the nature of paleo-oceanic basins as well as reconstructing the regional stratigraphic framework of orogenic belts. This study focuses on radiolarian-bearing cherts from the Karaghol area of the South Tianshan, employing whole-rock major and trace element geochemical analyses to investigate their genesis and depositional environment. The cherts occurring as tectonic slices associated with pillow basalts, exhibit a cryptocrystalline texture and massive structure, and contain Late Devonian to Early Carboniferous radiolarians. The cherts contain SiO2 contents of 82.71%~89.29%, and Al2O3 contents of 3.72%~6.45%, with TiO2, Fe2O3, and MnO concentrations ranging from 0.14% to 0.33%, 2.45% to 4.44%, and 1.06% to 1.60%, respectively. The studied cherts exhibit relatively high Al2O3/(Al2O3+Fe2O3) (0.59~0.68) and Al/(Al+Fe+Mn) ratios (0.38~0.53). The trace element ratios V/Y and Ti/V range from 0.76 to 7.15 and 29.08 to 89.09, respectively. On the chondrite-normalized rare earth element (REE) diagram, all samples display light REE enrichment ((La/Yb)N = 5.1~9.2) with pronounced negative Eu anomalies (δEu = 0.63~0.75). In contrast, the North American Shale Composite (NASC)-normalized REE patterns are relatively flat and exhibit positive Ce anomalies, with (La/Ce)NASC ratios ranging from 0.57 to 1.04. Diagrammatic interpretations and geochemical ratios indicate that these ribbon cherts are predominantly of biogenic origin, with notable input from terrigenous material, pointing to a continental margin depositional setting. By integrating with lithological assemblages, we consider that the studied cherts not only represent a direct record of the northward subduction-accretion processes of the South Tianshan oceanic lithosphere, but also form a key component in reconstructing the South Tianshan accretionary complex.