阿尔金淡水泉早古生代泥质高压麻粒岩及其P-T演化轨迹

P-T path of Early Paleozoic pelitic high-pressure granulite from Danshiiiquan area in Altyn Tagh.

南阿尔金构造带淡水泉一带出露的含石榴石蓝晶石黑云母片麻岩是一套典型的泥质高压麻粒岩,其峰期特征矿物组合为石榴子石+蓝晶石+钾长石+金红石+石英。根据矿物内部一致性热力学数据和Thermocalc 3.23程序计算,确定其峰期变质温压条件为T>850℃和P>11kbar。结合岩相学研究和P-T视剖面图计算,可识别出该岩石经历了4个阶段的变质演化,构成了一个早期快速等温降压,后期近等压降温的顺时针型的退变质P-T演化轨迹,为与陆壳俯冲碰撞有关的高压变质事件的产物。该岩石锆石阴极发光图像显示其内部具有明显的核-边结构,核部为残留的原岩碎屑锆石,边部则表现为面状或扇状生长的变质锆石的特征。微区原位LA-ICP-Ms微量元素分析表明,核部测点的重稀土含量较高,对应Th/U接近于0.4,具有岩浆锆石的特征;边部测点的重稀土相对亏损,重稀土配分曲线平坦,对应Th/U比值均小于0.1,显示与石榴子石平衡共生的变质锆石特征。LA-ICP-MS微区定年获得其变质年龄为486±5Ma,该年龄值与阿尔金江尕勒萨依和英格利萨依两地超高压变质岩石的变质年龄相近,进一步证明沿阿尔金构造带南缘断续存在一条早古生代的高压-超高压变质岩带。另外,本次研究在获得该泥质高压麻粒岩峰期变质时代的同时,还获得该岩石原岩的形成时代上限值约为719Ma,从而限定阿尔金构造带南缘阿尔金群的形成时代可能不属古元古代,而应属新元古代。

The peak metamorphic mineral assemblage ( g + sill + ksp + ru + (1) in garnmet-kyanite-bearing biotite gneiss from Altyn Group of Qingsbuiquan area, South Altyn, indicates that the gneiss is a suit of high-pressure pelitic granulite, and the metamorphic condition is T > 850 degrees C and P > 11kbar according to the THERMOCAI.C 3.23. Four stages metamorphic evolution are described by means of P-T pseudosections for selected metapelite specimens, showing a clockwise retrograde metamorphic P-T path with rapid exhumation and fast cooling. Zircon Cl, images show the Uniform internal structure of core-rim microstructure, the cores preserve magmatic zone and the rims display sector and planar structure. LA-ICP-MS in situ trace element analyses indicate that spots from the core have high HREE concentration, and their Th/U ratio are mostly close to 0.4, showing feature of magmatic zircon; and spots from the rim show slightly depleted HREE pattern, the corresponding Th/U ratios are all lower than 0.1, indicating their metamorphic growth coexisting with garnet. LA-ICP-MS in situ dating yields the metamorphic age of 486 +/- 5Ma from the rims, and the detrital protolith ages varying from 719Ma to 1780Ma from the cores. The metamorphic age of 486 +/- 5Ma obtained in this paper is nearly the same as the existing metamorphic ages of Jianggalesayi and Yinggelasayi UHP terrane, it provides us further evidence that there is an Early Paleozoic HP-UHP metamorphic belt along the south of Altyn Tagh. In addition, the youngest protolith age of 719Ma undoubtly indicates that the Altyn Group formed during Neoproterozioc instead of Paleoproterozoic.