Building of the Deep Gangdese Arc, SouthTibet: Paleocene Plutonism and Granulite-Facies Metamorphism
Building of the Deep Gangdese Arc, SouthTibet: Paleocene Plutonism and Granulite-Facies Metamorphism
复制标题
藏南冈底斯深部弧的构建：古新世岩成作用与麻粒岩相变质作用
DOI：
10.1093/petrology/egt056
复制
发表时间：
2013
影响因子：
3.9
通讯作者：
Xiang H
中科院分区：
文献类型：
--
作者：
Zhang ZM;Dong X;Xiang H
林芝杂岩位于西藏南部拉萨地体南部，构成冈底斯岩浆弧东段，主要由深圳湾及其变质等效物组成。这些岩石与一些变质沉积单元一起记录了新特提斯洋岩石圈向欧亚大陆俯冲过程中多期中、新生代岩浆和变质事件。岩石学和年代学研究表明，林芝杂岩经历了强烈的古新世俯冲相关的岩浆作用和几乎同步的麻粒岩相变质作用，伴随着S型花岗岩的形成。与俯冲作用有关的I型花岗岩具有典型的大陆岩浆弧地球化学特征，锆石的206 Pb/238 U年龄为c。65 toc. 56 Ma，通常显示正的εHf（t）值，范围从−1·7到+13 stats 0。岩浆绿帘石的出现以及同侵入高级变质作用，表明岩体侵位于拉萨地区的中低地壳深度。伴生的S型花岗岩为过铝质，含石榴子石和白云母，锆石206 Pb/238 U年龄为c. 66 toc. 55 Ma，它们的εHf（t）值明显不同，但大部分为负值，从−18·4到+2 statist 1。变质沉积岩中的锆石包括碎屑和变质两种类型，碎屑锆石的206 Pb/238 U年龄变化范围为c. 2910 toc。235 Ma，最大沉积时代为三叠纪。变质深成岩和变质沉积岩中的变质锆石年龄为c。67至52 Ma。相平衡模拟结果表明，林芝杂岩在800-830°C、9-10·5 kbar的条件下经历了麻粒岩相变质高峰期和部分熔融，然后等压冷却至℃。下地壳深度>30公里处的温度为700°C。我们认为，在早第三纪时期的新特提斯洋板的平俯冲回滚导致了收缩造山和大量的幔源岩浆的侵入，这导致了大规模的地壳加热，部分熔融和麻粒岩相变质作用在冈底斯弧的地壳深部。林芝杂岩代表了冈底斯岩浆弧的出露下地壳，并将麻粒岩相变质作用与古新世弧增生过程中的岩浆活动和地壳生长联系起来。
The Nyingchi complex, forming the eastern segment of the Gangdese magmatic arc, occurs within the southern Lhasa terrane in south Tibet, and is composed dominantly of plutons and their metamorphosed equivalents. Together with some metasedimentary units these rocks record multiple Mesozoic and Cenozoic magmatic and metamorphic events during northward subduction of Neo-Tethyan oceanic lithosphere beneath the Eurasian continent. Petrological and geochronological studies reveal that the Nyingchi complex experienced intense Paleocene subduction-related magmatism and almost synchronous granulite-facies metamorphism accompanied by the formation of S-type granites. Subduction-related I-type granitoids show geochemical features typical of continental magmatic arcs; zircon separates from them yield206Pb/238U ages fromc. 65 toc. 56 Ma, and commonly display positive εHf(t) values ranging from −1·7 to +13·0. The occurrence of magmatic epidote, as well as the syn-intrusion high-grade metamorphism, indicates that the plutons were emplaced at middle to lower crustal depths within the Lhasa terrane. Associated S-type granitoids are peraluminous and contain garnet and muscovite; their zircons yield206Pb/238U ages ranging fromc. 66 toc. 55 Ma, and these have distinct but mostly negative εHf(t) values from −18·4 to +2·1. The zircons from the associated metasedimentary rocks include both detrital and metamorphic types; the detrital zircons yielded variable inherited206Pb/238U ages ranging fromc. 2910 toc. 235 Ma, constraining the maximum depositional age to the Triassic. The metamorphic zircons from the metaplutonic and metasedimentary rocks yielded ages fromc. 67 to 52 Ma. Phase equilibria modeling shows that the Nyingchi complex experienced peak granulite-facies metamorphism and partial melting under conditions of 800–830°C and 9–10·5 kbar, and then cooled isobarically toc. 700°C in the lower crust at depths of >30 km. We argue that rollback of the flat-subducted Neo-Tethyan oceanic slab during Early Paleogene times resulted in a contractional orogeny and intrusion of voluminous mantle-derived magmas, which caused large-scale crustal heating, partial melting and granulite-facies metamorphism within the deep crust of the Gangdese arc. The Nyingchi complex represents the exposed lower crust of the Gangdese magmatic arc, and links the granulite-facies metamorphism with silicic magmatism and crustal growth during Paleocene arc accretion.