Origin of the Upper Ordovician-lower Silurian cherts of the Yangtze block, South China, and their palaeogeographic significance

Volcanic rocks, black shales, calcareous sandstones and cherts occur in the Wufeng and Longmaxi formations, which cross late Katian through Rhuddanian exposed along the northern and central part of the Yangtze block, South China. The evidence provided by radiolarite occurrences, sedimentological features and immobile elements indicate that the silica in these rocks is sedimentary and not of hydrothermal origin. Precursors of chert were deposited on a continental margin, or in a deeper basin on the shelf, during blooms of radiolarians and times of decreased clastic input. Interbedded volcanic tuffs of rhyolitic to andesitic composition suggest the presence of a continental volcanic arc to the east-southeast. The large regional extent of bentonites documents explosive volcanicity resulting in silica saturation of waters, leading to high bioproductivity and radiolarian blooms. Deepening of the sedimentary environment is most probably the result of orogenesis generated by an accretion of the Cathaysia Block to the Yangtze Block during the Ordovician-Silurian transition. Increasing water depth together with a decrease in clastic input contributed to the development of the radiolarian cherts in the basin.