山西南部中条山古元古代花岗质片麻岩 白云母40Ar/39Ar测年及其地质意义

中条山前寒武纪岩石是洞悉华北克拉通前寒武纪基底构造演化的重要窗口之一,该区的前寒武纪岩系主要由涑水杂岩、绛县群、中条群、担山石群,以及西阳河群和汝阳群组成.本文以中条山地区涑水杂岩中古元古代花岗质片麻岩为研究对象,挑选其中的变形变质白云母进行激光40Ar/39Ar和常规40Ar/39Ar测年分析.激光40Ar/39Ar法获得的白云母等时年龄1830Ma±20Ma为白云母氩封闭温度年龄的最小估计,常规40Ar/39Ar法给出的白云母坪年龄1852Ma±11Ma为白云母氩封闭温度年龄的最佳估计.白云母1852Ma± 11Ma与先前获得的独居石电子探针U-Th-Pb主峰值年龄相近,并且与华北克拉通中部带的变质年龄一致,表明中条山地区涑水杂岩中古元古代花岗质片麻岩记录了古元古代晚期的一次变质作用事件.这一事件与华北克拉通中部怀安-吕梁-恒山-五台-赞皇等地的变质变形作用同时发生,揭示华北克拉通东、西部陆块沿中部带的碰撞拼合应发生在古元古代晚期,而非新太古代.

Muscovite 40Ar/39Ar dating of the Palaeoproterozoic granitoid gneiss in the Zhongtiao Mountains and its geological significance

The Zhongtiao Mountains as one of the significant components of the Trans-North China Orogen, located in the southernmost part of Shanxi Province, comprise a basement of the Sushui Complex, Jiangxian Group, Zhongtiao Group, and a cover of Danshanshi Group dominated by conglomerate, and Xiyanghe and Ruyang Groups, separated from each other by unconformities and tectonic contact. The metamorphic rocks in the Zhongtiao Mountains are of vital importance in understanding the Precambrian tectonics and evolution of the North China Craton. However, the time of metamorphism so far is still poorly constrained. Herein, we present laser and conventionally 40Ar/39Ar analyses on fabric-forming metamorphic muscovite from a Palaeoproterozoic granitoid gneiss from the Sushui Complex in the Zhongtiao Mountains. The laser 40Ar/39Ar dating returned an isochron age of 1830Ma±20Ma, and the conventionally 40Ar/39Ar data yield a plateau age of 1852Ma±11Ma, which can be considered as minimum and optimal estimates of the muscovite argon closure temperature age, respectively. Furthermore, the plateau age of 1852Ma±11Ma is in excellent agreement with published U-Th-Pb Electron microprobe monazite prominent peak age, indicating that metamorphic muscovite recorded a late Palaeoproterozoic metamorphic event. Those metamorphic ages are also consistent with the metamorphic and deformation events occurred in the Huai'An-Lüliang-Hengshan-Wutai-Zanhuang regions of the Trans-North China Orogen, revealing that the Eastern and Western blocks of the North China Craton was stitched together along the Trans-North China Orogen at late Palaeoproterozoic, but not Neoarchean.