Radiolarian age and geochemistry of cherts from the Atbashi accretionary complex, Kyrgyz South Tianshan

摘要

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 melanges. 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 melange 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.