Zircon U-Pb geochronology and its geological significance of Late Cretaceous volcanic breccias in Shuiwen Basin,Guangxi
The formation age,characteristics of zircons and the source of breccia in the lower section of Xidong Group volcano breccias in the upper Cretaceous Formation from Shuiwen Basin(Guangxi)and its geological sig-nificance are studied via the zircon U-Pb geochronology and petrography.The volcanic breccias are mainly composed of abundant sandstone breccias and metamorphic sandstone breccias,magma fragments,glasses,and fine felsic minerals.The U-Pb dating results of zircons are mainly from 418 to 469 Ma,with the younger ages at 107,104 and 90 Ma.The Th/U ratios of zircons are greater than 0.1,and chondrite normalised REE pat-terns of zircons for volcanic breccias has obvious positive Ce and negative Eu anomalies,indicating that they are magmatic zircons.The study of zircon morphology shows that the older zircons have a high psephicity,with their zonings not clear,the younger zircons a low psephicity,but their zonings relatively developed.Regional geolog-ical data show that the U-Pb ages of clastic zircons obtained from the metamorphic sandstones originally divided into the Neoproterozoic are from 393 to 456 Ma.Therefore,this paper believes that the zircons at 418-469 Ma in the lower part of Xidong Group in Upper Cretaceous Formation are not the diagenetic ages of the volcanic breccias.The zircons in this age range may come from the sandstone breccia of the Lower Cretaceous or from the metamorphic sandstone breccia that was once thought to be of the Neoproterozoic.The zircon age of 90 Ma prob-ably represents the age of deep magmatic eruption activity before the formation of volcanic breccia in the lower part of Xidong Group in the Upper Cretaceous.It is 7 Ma earlier than that of the quartz porphyry in the upper part.
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