冈底斯岩浆弧的形成与演化

Formation and evolution of the Gangdese magmatic arc,southern Tibet

位于青藏高原南部的冈底斯岩浆弧是新特提斯大洋岩石圈长期俯冲导致的中生代岩浆作用的产物,而且在印度与亚洲大陆碰撞过程中叠加了强烈的新生代岩浆作用,是世界上典型的复合型大陆岩浆弧,也是研究增生与碰撞造山作用和大陆地壳生长与再造的天然实验室。基于岩浆、变质和成矿作用研究成果,我们将冈底斯弧的形成与演化历史划分5期,即新特提斯洋早期俯冲、新特提斯洋中脊俯冲、新特提斯洋晚期俯冲、印度-亚洲大陆碰撞和后碰撞期。第1期发生在晚白垩世之前,是以新特提斯洋岩石圈的长期俯冲、地幔楔部分熔融形成钙碱性弧岩浆岩为特征。长期的幔源岩浆作用导致了整个冈底斯弧发生显著的新生地壳生长,并在岩浆弧西部形成了一个大型的与俯冲相关的斑岩型铜矿。第2期发生在晚白垩世,活动的新特提斯洋中脊发生俯冲,软流软圈沿板片窗上涌,使上升的软流圈、地幔楔和俯冲洋壳发生部分熔融,导致了强烈的幔源岩浆作用和显著的新生地壳生长与加厚,并以不同类型和不同成分岩浆岩的同时发育和伴随的高温变质作用为特征。第3期发生在晚白垩世晚期,为新特提斯洋脊俯冲后残余大洋岩石圈的俯冲期,以正常的弧型岩浆作用为特征。第4期发生在古新世至中始新世,伴随印度与亚洲大陆的碰撞,俯冲的新特提斯洋岩石圈回转和断离引起软流圈上涌,诱发了强烈的幔源岩浆作用。在此阶段,大陆碰撞导致的地壳挤压缩短和幔源岩浆的底侵与增生,使冈底斯弧经历了显著的地壳生长和加厚,新生和古老加厚下地壳的高压、高温变质和部分熔融,幔源和壳源岩浆岩的共生和强烈的岩浆混合。所形成的I型花岗岩大多继承了新生地壳弧型岩浆岩的化学成分,并多显出埃达克岩的地球化学特征。在岩浆弧北部形成了一系列与起源于古老地壳花岗岩相关的Pb-Zn矿床。第5期发生在晚渐新世到早-中中新世的后碰撞挤压过程中,以地壳的继续加厚,加厚下地壳的高温变质、部分熔融和埃达克质岩石的形成为特征。在岩浆弧东段南部形成了一系列与起源于新生加厚下地壳埃达克质岩石相关的斑岩型Cu-Au-Mo矿。冈底斯带的多期岩浆、变质与成矿作用为其从新特提斯洋俯冲到印度-亚洲大陆碰撞的构造演化提供了重要限定。

The Gangdese magmatic arc,southern Tibet,was products of the Mesozoic and Cenozoic magmatism during the longlasting subduction of the Neo-Tethyan oceanic lithosphere and the subsequent India-Asia collision,respectively,and therefore,it is a natural laboratory for studying accretionary and collisional orogenesis,as well as growth and reworking of the continental crust.Based on a synthesis of available results of magmatism,metamorphism and mineralization in this region,the formation and evolution history of the Gangdese arc is divided into five stages,namely,the early subduction of the Neo-Tethyan lithosphere,the subduction of the NeoTethyan mid-oceanic ridge,the late subduction of remnant Neo-Tethyan lithosphere,the collision of India and Asia,and the postcollisional stage.The first stage,lasting from Late Triassic to Middle Cretaceous,is characterized by the normal subduction of the NeoTethyan oceanic lithosphere and the formation of subduction-related arc magmatic rocks,and during this stage,the long-term mantlederived magmatism resulted in the significantly growth of juvenile crust throughout the Gangdese arc,together with the generation of a giant porphyry Cu deposit in the western segment of the Gangdese arc.The second stage,happened in Late Cretaceous,was related to the subduction of the active Neo-Tethyan mid-oceanic ridge.In this stage,the upwell of the asthenosphere along the slab window resulted in extensive partial melting of upwelling asthenosphere,subduction slab and hinging-wall mantle wedge,which in turn,resulted in formation of diversity magmatic rocks and high-temperature metamorphic rocks.At the same time,underplating and accretion of the voluminous mantle-derived magma induced in the significantly growth and thickening of the Gangdese arc crust,and high-temperature and high-pressure metamorphism and partial melting of the thickened lower crust.The third stage,the latest Late Cretaceous,is characterized by the subduction of remnant Neo-Tethyan oceanic lithosphere and the normal arc magmatism.The fourth stage is represented by Paleocene to Middle Eocene magmatic flare-up,which was induced by the roll-back and breakoff of the subducted Neo-Tethyan slab during the Indo-Asian collision.This stage is characterized by the significantly thickening and partial melting of juvenile and old crusts,and extensive mixing of the mantle-and crust-derived magmas.The generated I-type granites inherited the chemical compositions of arc-type magmatic rocks,and also have the geochemical features of adakites.A series of large and giant Pb-Zn deposits related to the old crust-derived granites formed in the northern part of the Gangdese arc.The latest postcollisional stage is characterized by the continuous thickening and formation of the thickened lower crust-derived adakitic rocks during the LateOligocene to Middle Miocene and many large and giant adakitic rock-related porphyry Cu-Au-Mo deposits formed in the eastern Gangdese arc in this stage.The multistage magmatic,metamorphic and mineralization processes provide excellent constraints for the Gangdese tectonic evolution from the Neo-Tethyan ocean subduction to India-Asia continental collision.