南天山北缘榆树沟麻粒岩的变质作用及其锆石SHRIMP年龄的研究

Metamorphism process and SHRIMP dating of granulite at Yushugou,northern margin of South Tianshan

南天山北缘榆树沟麻粒岩主要为基性麻粒岩,由多个变质程度不同(低、中、高压麻粒岩相)的岩片被构造作用拼贴到一起。本文对不同麻粒岩开展详细的岩相学研究,识别出的变质作用有:进变质角闪岩相、低压麻粒岩相、中压麻粒岩相、高压麻粒岩相和退变质的中压麻粒岩相、角闪岩相、绿片岩相变质作用。对于有成熟温压计的中压麻粒岩相(即Grt-Opx-Cpx-Plg-Qtz组合)矿物组合的样品进行了温压估算,获得的温度和压力范围为724～826℃和0.64～0.88GPa。对原岩为火山岩的中压麻粒岩相岩石的锆石核部的研究表明,锆石的来源复杂;SHRIMP锆石U-Pb年代学测定显示,锆石的变质生成边的年龄稳定,为390～401Ma,代表了中压麻粒岩相进变质作用的时代。

Granulite facies terrane along northern boundary of South Tianshan is mainly composed of basic granulites,divided into various slabs of different extents of metamorphism.Careful petrographical observation recognized following metamorphic stages： Progressive metamorphism facies of amphibolite,low-pressure granulite,medium-pressure granulite,high-pressure granulite and retrogressive metamorphism facies of medium-pressure granulite,amphibolite and green schist.Based on a reliable geothermobarometer（Grt-Opx-Cpx-Plg-Qtz mineral assemblage）,the pressure-temperature conditions are estimated for two medium-pressure proxene granulite samples,with the P-T results of 0.64~0.88GPa and 724~826℃,in accordance with the medium-pressure conditions as indicated by mineral assemblage.According to the zircon morphology and SHRIMP dating from the granulite whose protolith was volcanic rock,the cores of zircon grains are variable,and the age of rims of zircon grains is concentrative,ranging between 390~401Ma,showing the age of progressive medium-pressure granulite facies metamorphism.