Geochronological definition on Shipu biohermal limestone and its regional stratigraphic attribution in Xiangshan, eastern Zhejiang Province

There is a set of volcanic-sedimentary rock series in Shipu area, Xiangshan County, eastern Zhejiang Province, which was previously named as Shipu Formation, including upper and lower members. The lower member is interbedded with mudstone, sandstone and stromatolite-wormtube biohermal limestone, while the upper member is characterized by bimodal volcanic rock. For a long time, there have been disputes about the stratification attribution of Shipu Formation and the age of the bioreef limestone. LA-ICP-MS zircon U-Pb ages of the basaltic interlayers newly discovered in rhyolite and sedimentary rocks of the Shipu Formation are obtained, which are (117.8±2.2) Ma and (116.1±1.0) Ma respectively, showing that Shipu Formation formed in late early Cretaceous. According to the regional stratigraphic correlation study, the stratigraphy of Shipu Formation and Guantou Formation is similar, both belongs to the contemporaneous heterogeneous sequence strata.