江南造山带中部存在约860 Ma隐伏花岗岩:来自湖南宁乡云影窝钾镁煌斑岩捕虏晶的证据

The existentce of the ca.860 Ma concealed granite in the central Jiangnan Orogen:Evidence from the xenocrysts in lamproite at Yunyingwo,Ningxiang,Hunan Province

为探寻江南造山带中部深部物质信息,本文以湖南宁乡云影窝钾镁煌斑岩Ⅰ号岩体为研究对象,对岩体中捕获的磷灰石、锆石以及围岩中的碎屑锆石开展了U-Pb定年和微量元素分析。结果显示,围岩中的碎屑锆石最年轻一组的^(207)Pb/^(206)Pb加权平均年龄为(769±19)Ma,指示岩体侵入围岩为新元古界板溪群五强溪组。钾镁煌斑岩捕获的磷灰石^(207)Pb矫正加权平均年龄和锆石^(207)Pb/^(206)Pb加权平均年龄分别为(859±9)Ma和(854±21)Ma,二者在误差范围一致,表明钾镁煌斑岩可能捕获了一期约860 Ma的深部岩浆岩。捕获约860 Ma锆石的微量元素特征指示其源岩为S型花岗岩。根据江南造山带中部同时存在约860 Ma的S型花岗岩、变质事件和基性岩浆活动的事实,本文提出江南造山带中部在约860 Ma时处于大洋板片后撤背景,暗示其在新元古代的洋-陆俯冲可能早于860 Ma。

In order to explore the information of deep magmatic record in the central Jiangnan Orogen,this contribution,taken the No.I Yunyingwo lamproite dike of the Ningxiang area,Hunan Province as a′lithoprobe′,and conducted U-Pb geochronological and trace element studies of apatite and zircon xenocryts in the lamproite dike and detrital zircons in wall rocks.The results show that the youngest group of detrital zircons from wall rocks has a^(207)Pb/^(206)Pb weighted average age of(769±19)Ma,which represents the maximum sedimentary age of ca.770 Ma for the Wuxiangxi Formation of the Neoproterozoic Banxi Group.The weighted average ages of apatite and zircon xenocrysts in the lamproite are(859±9)Ma and(854±21)Ma,respectively,which are consistent within the standard deviation range.It is suggested that the lamproite could have captured a deep magmatic rock with age of about ca.860 Ma.Furthermore,the trace element characteristics of xenocrystic zircons indicate those zircons were sourced from the S-type granite.Based on facts that the S-type granite,metamorphic event,and mafic magmatism in the central Jiangnan orogen,it is suggested that the central Jiangnan orogen was experienced an oceanic slab rollback,implying that the Neoproterozoic oceanic-continental subduction of the Yangtze Block occurred possibly before 860 Ma.