扬子陆块南部新元古代煌斑岩锆石U-Pb年龄及构造背景

Zircon U-Pb Geochronology and Tectonic Settings of Neoproterozoic Lamprophyre in Southern Yangtze Block

鹤峰煌斑岩大地构造上位于江南造山带北缘、扬子陆块南部,是鄂西南地区唯一的岩体,本文采用LA-ICP-MS锆石U-Pb定年及微量元素测试方法对其形成时代进行厘定,并利用锆石微量元素探讨其形成构造环境。结果表明:(1)岩体^(206)Pb/^(238)U加权平均年龄为839±5 Ma(MSWD=2.1,n=24),形成时代为新元古代;(2)岩体锆石都具有较高稀土元素总量、重稀土元素(HREE)富集、轻稀土元素(LREE)亏损、强烈的Ce正异常和Eu负异常特征,与岩浆锆石的特征一致;在Y-U/Yb图中显示形成于靠近洋壳的陆壳锆石区域,在锆石微量元素构造环境判别图解中,大部分样品点落入火山弧环境区域,少量样品点落入板内环境区域。综合分析认为,鹤峰煌斑岩形成于晋宁期弧后盆地伸展环境,可能与华夏陆块向扬子陆块俯冲过程中发生俯冲板片的折断、拆沉引发深部地幔上涌的动力学机制有关。

Hefeng lamprophyre,as the only intrusion in southwestern Hubei province,is tectonically located in the northern margin of the Jiangnan orogenic belt and the southern part of the Yangtze Block.LA-ICP-MS U-Pb dating and trace element testing on zircons of Hefeng lamprophyre are used to determine its formation age and discuss its tectonic environment in this paper.The results show that:(1) The ^(206)Pb/^(238)U weighted average age of the intrusion is 839±5 Ma (MSWD=2.1,n=24),indicating it had intruded in Neoproterozoic;(2)The zircons in intrusion are characterized by high total rare earth elements,enrichment of heavy rare earth elements (HREE),depletion of light rare earth elements (LREE),strong positive Ce anomalies and negative Eu anomalies,which are consistent with the characteristics of magmatic zircons.The Y-U/Yb diagram shows that it was formed in the continental crust zircon area near the oceanic crust.In the zircon trace element tectonic environment discrimination diagram,most rock samples fall into the volcanic arc environment area while a small number of samples into the intraplate environment area.Comprehensive analysis shows that Hefeng lamprophyre was formed in the extension environment of the Jinning Period back arc basin,which may be related to the dynamic mechanism of deep mantle upwelling caused by the fracture and delamination of subduction plates during the subduction of the Cathaysian Block to the Yangtze block.