北秦岭造山带的早古生代多期变质作用

北秦岭造山带的秦岭岩群以高级变质岩石为特征,主要包括少量榴辉岩、高压麻粒岩和区域上广泛分布的麻粒岩-角闪岩相变质岩石。年代学研究显示秦岭岩群中不同岩石记录了多期变质作用。已有的定年资料给出北秦岭官坡地区的榴辉岩的年龄为500Ma左右,代表榴辉岩相的变质时代。结合岩相学资料,对两个高压麻粒岩样品的SHRIMP和LA-ICPMS U-Pb测定分别获得504±7Ma 和506±3Ma的年龄,应代表高压麻粒岩相变质时代。这表明高压麻粒岩和相邻的榴辉岩有相近的变质时代,但形成在造山带中不同的构热造环境中。西峡地区的角闪二辉麻粒岩的U-Pb定年给出两组早古生代年龄,一组为440±2Ma,可能代表了中低压麻粒岩相的变质时代,另一组为426±1Ma,应代表区域角闪岩相的变质时代。桐柏山北部的石榴二辉麻粒岩的U-Pb定年数据给出436±1Ma的年龄,为中压麻粒岩相的变质时代。这些资料表明北秦岭造山带经历了早奥陶世的俯冲和地壳增厚作用,并在晚志留世遭受了广泛的巴罗式区域变质作用。

Ployphase Early Paleozoic metamorphism in the northern Qinling orogenic belt.

High-grade metamorphic rocks in North Qinling orogen, traditionally regarded as the Qinling Group, consist of minor eclogites, high pressure granutlite and widely distributed medium-low P/T granulite-amphibolite facies metamorphic rocks. Radiometric data indicate that ployphase Early Paleozoic metamorphisms are recorded in different rocks. The previous U-Pb datings of eclogites and associated rocks in the Guanpo area of the North Qinling orogen gave an age of ca.500Ma, interpreted as the time of eclogite facies metamorphism. In combination with petrological data, SHRIMP and LA-ICPMS U-Pb geochronology on two HP granulite samples in the Songshugou area of the North Qinling orogen yield ages of 504±7Ma and 506±3Ma, respectively, representing the time of high pressure granulite metamorphism. This implies that HP granulite-facies conditions in high pressure granulite and eclogite facies conditions in eclogites developed at the same time but they formed in different thermal environments. U-Pb datings of zircons of amphibole-two-pyroxene granulite from the Xixia area reveal two Early Paleozoic age groups: 440±2Ma represents the age of granulite metamorphism whereas the ca. 426±1Ma reflects the time of amphibolite facies metamorphism. The U-Pb data of zircons of garnet-two-pyroxene granulite from the North Tongbai area yield an age 436±1Ma, reflecting the time of granulite facies metamorphism. These data suggest that the North Qinling orogen experienced an Early Ordovician subduction and crustal thickening, followed by an Early Silurian widespread Barrovian-type metamorphic event as observed in many collisional orogen.