Volcanic event at the Ordovician-Silurian boundary:The message from K-bentonite of Yangtze Block.

There are several clay beds recognized from the Wufeng Formation,the top of the Ordovician and,Lungmachi Formation,the bottom of the Silurian in the Yangtze Block.These clay beds were taken as K-bentonites,which was produced by volcanic ash-fall deposits after diagenic metamorphism in marine environments.These K-bentonites are widely distributed along the margin of the Yangtze Block,such that can provide important constraints on the plate tectonic setting of the Yangtze Block as well as major geological events in the Early Paleozoic.In this contribution,we carried our mineralogical and geochemical studies on K-bentonite samples collected from two Ordovician-Silurian boundary sections,located in Yichang,Hubei Province and Tongzi, Guizhou Province.The results show that in addition to clay minerals,the clay beds also content quartz,feldspar,biotite,apatite, zircon,etc.,which are all common minerals in intermediate to silicic volcanic rocks.These clay beds should be typical K-bentonite. Most of the K-bentonite samples show typical characteristics of arc volcanic rocks,which were probably related to the subduction and closure of the Qinling Ocean in the Early Paleozoic.Alternatively,there might have been another subduction to the southeast of the South China Block.Sample YC0711 have no obvious Nb anomaly,but show Ti negative anomaly.They fall close to arc fields in a Th/ Yb-Nb/Yb diagram,suggesting that the protoliths were likely to special types of arc rocks,e.g.,Nb-enriched basalt.The peak age of the volcanic activity was Hirnantia stage,which is very close to the mass extinction at Ordovician-Silurian boundary,the second largest mass extinction in the Earth's history,which was generally attributed to the Gondwana glaciation.Considering that K-bentonite is widely distributed in South China,North America and Europe,the volcanic event at the Ordovician-Silurian boundary was a global event.In addition,most of these volcanic rocks are silicic,which are usually volatile enriched,and thus might have major effects on the atmosphere and biosphere.Here we propose that these volcanic events were the triggers of mass extinction at the Orodocician-Silurian boundary as well as the Gondwana glaciation.