K-bentonites in Ordovician–Silurian transition from South China: Implications for tectonic evolution in the northern margin of Gondwana

Numerous K-bentonites from the Ordovician-Silurian (O–S) transition in South China record important information about the geodynamics of volcanic activity in the northern margin of Gondwana. A series of K-bentonites beds have been identified in the Zhoujiaxi Group (Early Silurian) in foreland basins from central Hunan Province, China. They are dominantly composed of illite with minor kaolinite. Volcanogenic minerals include quartz, feldspar, biotite, and lesser apatite and zircon. U–Pb zircon ages from three beds, 442.8 ± 1.8, 442.2 ± 1.9 and 441.6 ± 2.0 Ma are compatible with the age of the O–S global boundary. Geochemical results indicate 24 calc-alkaline felsic magmas derived from continental crust and erupted in a subduction-related collisional environment. Isopach schemes and grain size reveal that the volcanic ashes were sourced from a volcanic event likely with a volcanic explosivity index of 8, and transported 300~1000 km away from the paleo south-southeast. The volcanism was associated with northwestern subduction of the Zhenghe–Dapu ocean beneath the southeastern South China Block (SCB) on the northern margin of Gondwana. The Zhenghe–Dapu fault might be a suture zone, implying the consumption of Zhenghe–Dapu ocean during collision between the Nanhai terrane and the SCB. This study supports that Wuyi–Yunkai orogenic belt is a collision-type orogen rather than an intraplate belt.