北秦岭造山带西段早古生代榴辉岩相变质岩石的发现及其地质意义SCIEI北大核心CSCD

西秦岭地处中国中央造山系东西转换衔接部位,随着东秦岭以及祁连、柴北缘和东昆仑早古生代高压-超高压变质岩石的陆续发现和深入研究,西秦岭造山带变质作用研究吸引了大家的普遍关注。本文在大范围野外地质调查基础上,在北秦岭造山带西段天水南部的秦岭岩群长英质片麻岩中发现了一套石榴子石斜长角闪岩(榴闪岩),并开展了详细的锆石形貌和内部结构、微区微量元素和U-Th-Pb同位素研究。CL图像显示榴闪岩锆石普遍具有核-幔-边或核-边结构,部分存在原岩残留锆石。定年结果得到榴闪岩原岩残留锆石年龄为710±52Ma,同时得到497±3Ma、452±3Ma和423±7Ma三期变质年龄。其中497±3Ma变质年龄来自锆石核部并显示出轻稀土亏损、重稀土平坦且没有明显负Eu异常的稀土配分曲线特征,表明该时期的矿物组合中有石榴子石但没有斜长石,与榴辉岩相变质锆石特征一致;452±3Ma变质年龄来自锆石幔部或边部,对应测点重稀土元素的分异加大,并出现弱的负Eu异常,说明此时石榴子石被消耗且出现少量斜长石;423±7Ma变质年龄来自锆石的最边部,对应稀土配分曲线表现出更明显的负Eu异常和更大的重稀土元素分异特征,指示此时岩石中石榴子石含量更少,斜长石含量更多。上述结果表明该榴闪岩可能经历了~500Ma的榴辉岩相变质作用,并在~450Ma和~420Ma叠加了两期退变质改造。天水地区榴闪岩无论是其野外产状,还是变质锆石的形貌和内部结构、稀土配分曲线特征及其所记录的原岩和三期变质年龄都与北秦岭造山带东段大陆俯冲型高压-超高压变质榴辉岩近乎一致,指示早古生代时期,北秦岭造山带西段与东段经历了相似的大陆(深)俯冲和折返过程,它们共同构成一条统一的早古生代高压-超高压变质岩带。

Discovery of Early Paleozoic eclogite-facies metamorphic rocks in the western part of North Qinling Orogen and its geological significance

The western part of North Qinling Orogen (NQO) is located at the conjunction area of several Paleozoic high pressure-ultrahigh pressure (HP-UHP) belts in the central China continent, the North Qinling UHP belt in the east and the North Qaidam and the East Kunlun UHP belts in the west, thus its metamorphism has drawn broad scientific interest. We document here an integrated study of zircon morphology, microstructure, in-situ trace element compositions and U-Th-Pb dating on a newly discovered garnet amphibolite in southern Tianshui, the western part of NQO. The garnet amphibolite occurs as a lensoid block in the felsic gneiss of the Qinling Complex, and mainly consists of amphibolite (50%~60%), garnet (10%~15%), quartz (~10%), plagioclase (5%~10%), muscovite (~5%) and ilmenite (~3%). Zircons from the garnet amphibolite display obvious core-mantle-rim or core-rim structures with a few protolith zircon relicts. LA-ICP-MS U-Pb dating yielded a protolith age of 710±52Ma and three metamorphic ages of 497±3Ma, 452±3Ma and 423±7Ma. The 497±3Ma metamorphic cores display flat heavy rare earth elements (HREE) patterns without obvious Eu anomalies, indicating they were formed in presence of garnet but absence of plagioclase under eclogite-facies condition; The 452±3Ma metamorphic mantles or rims show shallow HREE slopes with moderately negative Eu anomalies, indicating that these zircons grew accompanied by initial appearance of plagioclase and decomposition of garnet; The 423±7Ma metamorphic rims exhibit steep HREE patterns and marked negative Eu anomalies, indicating massive crystallization of plagioclase and decomposition of garnet during their formation. These results suggest that the garnet amphibolite probably experienced eclogite-facies metamorphism at~500Ma and two stages retrograde overprint at ~450Ma and ~420Ma. The occurrence, zircon microstructures, protolith and multiple metamorphic ages of the garnet amphibolite from Tianshui area are basically consistent with the continental-type HP-UHP rocks of the eastern part of NQO. Therefore, we suggest that both western and eastern part of NQO experienced similar continental deep subduction and exhumation processes and thus they form a united Early Paleozoic HP-UHP metamorphic belt.