大巴山南部房县东河辉绿玢岩LA-ICP-MS锆石U-Pb测年及其构造意义

LA-ICP-MS Zircon U-Pb Age of Diabase Porphyry from the Donghe Area,Fangxian in South Daba Mountain and Its Tectonic Significance

大巴山构造带位于秦岭造山带和扬子板块的过渡部位,形成于印支-燕山期,定型于喜山期。前人对北大巴山地区基性岩年代学已有研究,但对大巴山南部地区基性岩却很少高精度年龄报道。本文对大巴山南部房县东河地区的辉绿玢岩脉进行LA-ICP-MS锆石U-Pb年龄测定,结果表明,东河辉绿玢岩形成于439.3Ma,相当于早志留世。与北大巴山镇坪地区辉绿岩,紫阳-岚皋地区基性岩及火山岩形成时代基本一致。结合前人的研究结果表明,大巴山南部地区在早志留世处于张裂状态,东河地区的基性岩为岩石圈处于拉张状态下大陆裂谷作用产物。此处439.3Ma的年龄值,也说明此时扬子板块北缘普遍处于拉张的构造环境,代表了在早志留世大巴山南部裂谷盆地的伸展裂陷幅度达到最大。本次研究所获得的年龄数据,确定了围岩竹山组形成不晚于早志留世,限定地层时代为寒武纪-奥陶纪。

Daba Mountain tectonic belt is located in the transitional position of the Qinling orogenic belt and the Yangtze plate,formed in Indosinian-Yanshanian,taken the shape in Himalayan period.Previous studies focused on geochronology of North Daba Mountain,less reserach work was reported in high precision age in South Daba Mountain.The dating of LA-ICP-MS zircon U-Pb fordiabase porphyry dike sample from the Donghe area in Fangxian,South Daba Mountain shows the that the Donghe diabase porphyry was formed at 440.6~442.2Ma,equivalent to the early Silurian System.It is almost consistent with that of diabase in the south area of North Daba Mountain,（Zhenping area）,and with that of the basic dike and volcanic rochks in the middle-northern area of North Daba Moutain,（Ziyang-Langao area）.Combined with previous studies,the authors suggest that South Daba Moutain was a rift during the middle-late stage of Early Silurian,and the diabase porphyry in the Donghe area was the product of continental rift.The age of439.3Ma represents that the north margin of Yangtze plate was generally in the extensional tectonic environment,suggesting the extension of this rift to its peak in Early Silurian.The age date obtained in this study determines the surrounding rock Zhushan Formation formed no later than early Silurian,defining the strata are Cambrian-Ordovician in age.