桐柏秦岭岩群的两类变质作用

Two types of metamorphism in the Qinling Complex,Tongbai area,Henan Province

本文重点对河南桐柏地区的秦岭岩群进行了观察与研究,根据野外地质、岩相关系及同位素测年资料,提出该区秦岭岩群具有明显不同的两类变质作用,一是较早期的高温麻粒岩相变质作用,以包体或长透镜群、甚至巨型条块状局限于中部郭庄组的花岗质片麻岩之中。根据伟晶岩、片麻岩及麻粒岩锆石年龄的综合限定,该变质作用的时间可能为-498Ma,多数人主张的445-430Ma的麻粒岩相变质年龄实际上是早期锆石被后期岩浆或变质事件引起的同位素体系重启年龄。另一种是相对晚期的角闪岩相变质作用,变质程度以角闪岩相为主,局部达高角闪岩相,没有任何早期高温或高压变质的残留迹象,形成秦岭岩群中主导类型的变质作用。同样,采用伟晶岩及有关片麻岩和麻粒岩中锆石测年限定,角闪岩相变质时间可能为-472Ma。高温麻粒岩的产出具有其特殊机制,大量的花岗质岩浆侵位过程中把地壳深部的高温麻粒岩裹挟上升至浅部层次,随后一起遭受区域上的角闪岩相变质作用。

The paper is stressed on geology of the Qinling Complex at Tongbai area,Henan Province. According to the field geology,petrographic observation and isotopic age measurement, the complex has been discerned with two distinct types of metamorphism. The earlier high temperature granulite facies metamorphism,which is confined to the enclaves,lensoids or megaslices distributed in the granitic gneisses of the Guozhuang Formation of the Qinling Complex. With zircon age constraints from the pegmatite intrusive,the surrounding gneisses and granulite,the granulite facies metamorphism may occur at - 498 Ma. The conventionally believed 445 - 430 Ma of the peak granulite facies metamorphism actually resulted from resetting of the earlier zircons during late magmatism or metamorphism events. The late amphibolite facies metamorphism is dominated by amphibolite and local upper amphibolite facies,without any relics of the earlier high pressure or high temperature metamorphism,and is overwhelming in the Qinling Complex. The time of amphibolite facies metamorphism is constrained at -472 Ma. It is the magmatic intrusive that carry the high temperature granulites of the lower crust up to the shallow level and then both underwent the regional amphibolite facies metamorphism together.