碰撞造山带超高温变质作用及构造意义——以泛非造山带为例  

Ultrahigh temperature metamorphism in collisional orogen and its tectonic significance:An example from the Pan-African orogens

作　　者：滕霞 张建新[1] TENG Xia;ZHANG JianXin(Key Laboratory of Deep-Earth Dynamics,Ministry of Natural Resources,Institute of Geology,Chinese Academy of Geological Sciences,Beijing 100037,China;School of Earth and Space Sciences,Peking University,Beijing 100871,China)

机构地区：[1]中国地质科学院地质研究所,自然资源部深地动力学重点实验室,北京100037 [2]北京大学地球与空间科学学院,北京100871

出　　处：《岩石学报》2020年第10期2963-2982,共20页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(41630207)资助。

摘　　要：超高温(≥900℃)变质作用发生在自太古代以来的各个地质历史时期,目前极可能也正发生在青藏高原地壳深部。同时,它也是以冈瓦纳为代表的超大陆在最终拼合时的显著标识,这一关联指示了超高温变质作用与碰撞造山带的密切关系。本文总结了东冈瓦纳内与泛非造山作用有关的典型超高温变质岩的分布、岩石学特征、峰期变质条件、P-T轨迹及形成时代,并简要介绍我们在柴达木地块西段新识别出的泛非期超高温变质作用的基本特征。结合东冈瓦纳超高温变质作用特征和造山带热模拟研究的新进展,本文获得以东冈瓦纳超高温变质作用为代表的碰撞造山带超高温变质作用的几点认识:1)东冈瓦纳麻粒岩地块中的超高温变质岩和普通麻粒岩记录了相似的变质年龄、P-T轨迹以及呈过渡变化的峰期温度,两者可能是同一构造事件的产物,共同组成一个高温-超高温变质岩单元;2)超高温变质作用在东冈瓦纳内部持续了至少超过30Myr,但未见呈大规模的同期或近同期基性岩岩浆出露,指示此处需要的长期热源不是地幔来源岩浆;3)虽然数值模拟能成功呈现加厚地壳被放射元素衰变热加热至超高温条件的情况,且加热及持续时间与东冈瓦纳超高温变质约束的结果相当,但是模拟中需要的高生热值暗示,在自然界中,完全只靠放射性元素衰变生热或许不能让碰撞造山带内达到超高温条件;4)碰撞造山带经历了长期的构造演化,这一过程中,造山带内地壳不太可能同时达到超高温变质条件,这一特征可能反映在P-T-t轨迹的差异上,对这些轨迹的系统研究有助于对超高温变质作用的构造-热过程的理解。Ultrahigh temperature(UHT,≥900℃)metamorphism has witnessed the Earth’s history since Archean,and is probably occurring at the deep crust beneath the modern Tibetan Plateau.UHT metamorphism is also a key feature of the final assembly of supercontinents such as Gondwana,resulting in an intense scientific curiosity on likely connections between the UHT metamorphism and collisional orogenesis.This paper reviews the distribution,petrography,peak condition as well as P-T path,and geochronology of UHT metamorphic rocks within East Gondwana in the context of the collisional orogeny,with a brief introduction of a newly identified Pan-African UHT metamorphic belt within the Qaidam block,western China.Combing with the information from related numerical modeling,this paper draws some insights about UHT metamorphism within East Gondwana,which are probably common for other collision-related UHT metamorphism.Firstly,the UHT rocks within granulite terranes share similar clockwise P-T paths and metamorphism ages with normal granulite;UHT rocks recorded higher but not exclusively higher peak temperature than normal granulite did;they together constitute a high temperature(HT)-UHT metamorphic unit response to the same tectonothermal event.Secondly,collective information from UHT metamorphic rocks within East Gondwana suggests that UHT condition beneath East Gondwana lasted for more than 30 Myr,in absence of coeval occurrence of large basic magmatic bodies,implying the lasting heat source is not mantlederived magma.Thirdly,although modeling shows that thickened crust can be heated to UHT condition by radioactive decay alone within the time period comparable to the duration of Gondwana UHT metamorphism,the high heat production value necessary for modeling indicates that heat solely from radioactive decay is probably unable to cause UHT metamorphism.Last but not least,during a prolonged orogenic evolution of collisional orogen,different parts of crust beneath collisional orogen unlikely reach their peak condition concurrently,as

关 键 词：碰撞造山 冈瓦纳 泛非造山带 超高温变质作用 柴达木地块 

分 类 号：P588.3[天文地球—岩石学]