西大别浒湾高压变质带变沉积岩锆石U-Pb年龄及地质意义

U-Pb Ages of Detrital Zircon from Meta-Sedimentary Rock from the Huwan Shear Zone,Western Dabie Terrain and Their Geological Significance

西大别山浒湾高压变质带是研究秦岭-大别山-苏鲁造山带演化的关键区域,但是该地区榴辉岩相变质时间还存在较大争议.本文通过对浒湾变质带熊店附近的一个变沉积岩样品进行LA-ICPMS锆石U-Pb定年和微量元素分析,来确定该地区榴辉岩相变质作用和碎屑锆石的年龄,并确定该变质带的演化历史.该样品的变质锆石具有弱分带或无分带、低Th/U比值、重稀土含量低且相对平坦以及无明显的Eu负异常等特征.反映这些变质锆石形成时有石榴石,而缺乏长石,其形成对应于榴辉岩相变质作用条件.该样品中变质锆石的206Pb/238U加权平均值为310±3Ma,这一年龄代表了该地区榴辉岩相峰期变质作用的时间,因此浒湾地区榴辉岩相变质作用时间为石炭纪.碎屑岩浆锆石得到了3组近谐和的U-Pb年龄,其206Pb/238U年龄变化范围分别为:400±5Ma～419±5Ma、426±5Ma～449±5Ma以及496±5Ma～524±7Ma,还有1个颗粒给出了580±7Ma的较老年龄.这些结果表明变沉积岩中的碎屑锆石可能大部分来自于华北板块南缘,少量来自于扬子板块.

The Huwan high-pressure （HP） metamorphic zone is a key region to investigate the evolution process and timing of the Qinling-Dabieshan-Sulu orogenic belt. However, the timing of eclogite-facies metamorphism in this zone has not been well constrained. In order to constrain the time of eclogite-facies metamorphism, zircons from a metasediment sample HXD98-9 near Xiongdian were selected for LA-ICPMS U-Pb dating and trace element analysis. Metamorphic zircons are characterized by weak or none zoning, low Th/U ratios, low and/or flat HREE patterns, and an insignificantly negative Eu anomaly. These reflect that the metamorphc zircons crystallized with the presence of garnet and the absence of plagioclase feldspar, which occurred in eclogite-facies conditions. The weighted mean ^206Pb/^238U age of 310±3 Ma for these metamorphic zircons is thus the best estimated age of the eclogite-faeies metamorphism in this HP metamorphic zone. Three groups of U-Pb age were obtained from the inherited magmatic zircons, ranging of （400±5 Ma）-（419±5Ma）, （426±5Ma）-（449±5Ma）, and （496±5 Ma）-（524±7 Ma）, respectively. One inherited grain has a U-Pb age of 580.7±7.3 Ma. These indicate that the inherited zircons were mostly derived from southern part of the North China block, while some from the Yangtze block.