北秦岭超高压榴辉岩中长英质脉体的锆石U-Pb年龄及其地质意义

北秦岭地区的早古生代超高压变质作用是整个秦岭-桐柏-红安-大别-苏鲁造山带内最古老的超高压变质记录, 代表华南与华北板块之间最早的一期增生碰撞事件, 然而目前对于该地区早古生代的构造演化过程仍存在较大争议.对北秦岭官坡超高压榴辉岩中的一个长英质脉体开展了详细的锆石形态学、微量元素和U-Pb年代学研究.结果表明, 脉体中的锆石呈自形的棱柱状晶形, 发育弱的振荡环带、面状分带或无明显分带特征, 具有高的HREE、Y、U含量, 低的Th含量和Th/U比值, 说明锆石生长自含水熔体.另外, 这些变质锆石具有LREE亏损、HREE相对富集的配分模式以及明显的Eu负异常, 表明含水熔体形成于角闪岩相退变质过程.锆石的206Pb/238U加权平均年龄为494±10 Ma (MSWD=2.2), 与北秦岭超高压变质作用的峰期年龄(490.4±5.8 Ma)在误差范围内基本一致.北秦岭地体经历深俯冲作用之后发生快速折返, 并在折返过程中发生角闪岩相退变质作用诱发俯冲板片部分熔融产生含水熔体. 

Zircon U-Pb Ages of A Felsic Vein in Ultrahigh-Pressure Eclogite from North Qinling Terrane and Their Geological Implications

The Early Paleozoic high pressure (HP) to ultrahigh pressure (UHP) metamorphism in the North Qinling (NQ) terrane preserves the oldest UHP record in the whole Qinling-Tongbai-Hong'an-Dabie-Sulu orogenic belt and thus registers the first accretionary collision process between the South China block (SCB) and the North China block (NCB).However, its evolution processes remain the subject of heated debate.This paper presents an integrated study of CL image, trace elements, and U-Pb age of zircons from a felsic vein within a UHP eclogite outcrop from the Guanpo area, NQ terrane.Zircons from the vein are euhedral in shape, displaying oscillatory, planar or no zoning, high U but low Th contents resulting in low Th/U ratios, indicating that they precipitated from hydrous melts responsible for the vein formation.In addition, these metamorphic zircons are characterized by low LREE contents, relatively high HREE patterns, and significant negative Eu anomalies, suggesting the hydrous melts formed during the amphibolite-facies retrogression.The weighted mean 206Pb/238U age of 494±10 Ma of these zircons is similar to the age of the peak UHP metamorphism (490.4±5.8 Ma) of the NQ unit, within the analytical error.Based on the above observations, we suggest that the NQ terrane underwent fast exhumation after deep subduction in the Early Paleozoic, and released hydrous melts derived from partial melting during exhumation, which was triggered by the significant amphibolite-faices retrogression.