广西水汶盆地晚白垩世火山角砾岩锆石U-Pb年代学及其地质意义

Zircon U-Pb geochronology and its geological significance of Late Cretaceous volcanic breccias in Shuiwen Basin,Guangxi

利用锆石U-Pb年代学和岩相学对广西水汶盆地上白垩统西垌组下段火山角砾岩的形成时代、锆石特征和角砾来源进行了研究,并探讨了它们的地质意义。该火山角砾岩主要由大量砂岩和变质砂岩角砾、浆屑、玻璃质和细小长英质矿物组成。锆石U-Pb定年结果主要集中在418~469 Ma,较年轻的为107、104和90 Ma;锆石Th/U值均大于0.1,且稀土元素球粒陨石标准化配分模式具有明显的Ce正异常和Eu负异常,表明其为岩浆成因锆石。锆石形态研究显示,年龄老的锆石磨圆度较好,环带不清晰;年龄新的锆石磨圆度较差,环带较发育。区域地质资料显示,原来划分为新元古界的变质砂岩获得的碎屑锆石U-Pb年龄为393~456 Ma。因此,本文认为上白垩统西垌组下段火山角砾岩中418~469 Ma的年龄不是该火山角砾岩的成岩年龄,该年龄段的锆石可能来自早白垩世砂岩角砾,也可能来自曾被认为是新元古界的变质砂岩角砾。90 Ma的锆石年龄代表了上白垩统西垌组下段火山角砾岩形成之前,深部岩浆喷发活动的年龄,该岩浆活动时限比上段的石英斑岩早约7 Ma。

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.