冈底斯岩浆弧东段晚白垩世花岗岩的变质作用:新生地壳生长与再造  

Metamorphism of Late Cretaceous granite in the eastern Gangdese magmatic arc: crustal growth and reworking

作　　者：李文坛 张宁 张泽明[3] LI Wentan;ZHANG Ning;ZHANG Zeming(School of Earth Science and Resources,China University of Geosciences,Beijing 100083,Chinas;Beijing Zhongdihua'an Engineering Co.,Ltd,Beijing 102601,China;Institute of Geology,Chinese Academy of Geological Sciences,Beijing 100037,China)

机构地区：[1]中国地质大学(北京),地球科学与资源学院,北京100083 [2]北京中地华安环境工程有限公司,北京102601 [3]中国地质科学院地质研究所,北京100037

出　　处：《地质学报》2022年第3期881-896,共16页Acta Geologica Sinica

基　　金：国家自然科学基金项目(编号91855210,41941016-2);中国地质调查项目(编号DD20190059)资助成果。

摘　　要：青藏高原南部的冈底斯岩浆弧形成于中生代新特提斯大洋岩石圈俯冲过程中。在冈底斯岩浆弧东段出露的中-高级变质岩代表岩浆弧的中-下地壳组成,是探索大陆地壳形成与演化的窗口。作为这些中-高级变质岩原岩主要组成部分的里龙岩基由晚白垩世辉长岩、闪长岩和花岗岩组成。本文对由里龙岩基上部花岗岩变质形成的片麻岩进行了岩石学与年代学研究,探讨其原岩时代、成因、变质作用条件、时间及构造意义。所研究的片麻岩由斜长石、钾长石、石英、黑云母、绿帘石和白云母组成,含或不含角闪石,SiO;含量为61.94%~74.39%,铝饱和指数(A/CNK)为0.89~1.03,属于高钾钙碱性、准铝质到弱过铝质岩石。这些岩石具有轻稀土元素富集和重稀土元素亏损的配分模式,并表现为富集大离子亲石元素和相对亏损高场强元素的特征。片麻岩中的锆石由继承的岩浆核和变质边组成,岩浆核获得了92~86 Ma的结晶年龄,变质边获得了81~72 Ma的变质年龄,锆石岩浆核具正的ε;(t)值(+10.2~+12.1)。这些片麻岩的变质条件为740~750℃和0.5~0.6 GPa。本文和现有研究表明,里龙岩基是形成在晚白垩世新特提斯洋俯冲过程中具有亏损地幔地球化学特征的弧岩浆岩,经历了强烈的结晶分异作用,所研究的花岗岩是残余岩浆结晶产物。笔者认为,晚白垩世早期新特提斯洋中脊俯冲过程中巨量幔源岩浆的增生导致了冈底斯弧发生了显著的新生地壳生长;在晚白垩世晚期残余新特提斯洋岩石圈平缓俯冲过程中的弧地壳强烈缩短加厚使里龙岩基被运移到中下地壳,并发生了中-高级变质和部分熔融。冈底斯岩浆弧新生地壳在大洋岩石圈俯冲晚期经历了明显再造。The Gangdese magmatic arc in the south of the Tibet Plateau was formed during the Mesozoic subduction of the Neo-Tethyan oceanic lithosphere. Middle-to high-grade metamorphic rocks exposed in the eastern Gangdese magmatic arc represent the middle-lower crust components of the arc, which provide a rare opportunity to study the formation and evolution of continental crust. The Lilong batholith, forming as main part of the middle-high metamorphic rocks, is composed of meta-gabbro, meta-diorite and meta-granite. In this paper, we conducted a petrological and geochronological study for the meta-diorite and meta-granite(orthogneisses) from the upper part of the Lilong batholith in order to reveal their protolith, metamorphic ages and P-T conditions, and tectonic significance. The orthogneisses are mainly composed of plagioclase, K-feldspar, quartz, biotite, epidote, and with or without amphibole. Their SiO_(2) contents and alumina saturation indexes(A/CNK) range from 61.94% to 74.39%, and from 0.89 to 1.03, respectively. They show enrichment of LREE and LILE and depletion of HREE and HFSE. The zircon from the orthogneisses consists of magmatic core and metamorphic rim. The zircon core yielded the crystallization age of 92-86 Ma, while the zircon rim yielded the metamorphic age of 81-72 Ma. The ε_(Hf)(t) values of zircon magmatic cores range from +10.2 to +12.1. These gneisses underwent metamorphism and partial melting under P-T conditions of 740-750℃ and 0.5-0.6 GPa. By combining available results, we conclude that the Lilong batholith is composed of arc magmatic rocks with geochemical characteristics of the depleted mantle, and the granite studied were the crystallization products of fractionated magma. We suggest that the accretion of voluminous mantle-derived magma during the early Late Cretaceous subduction of the Neo-Tethyan mid-oceanic ridge led to significant juvenile crustal growth of the Gangdese arc. In the latest Late Cretaceous, the flat subduction of the young Neo-Tethyan lithosphere resulted in intense

关 键 词：晚白垩世 里龙岩基 变质和部分熔融 地壳生长与再造 冈底斯岩浆弧 

分 类 号：P588.3[天文地球—岩石学] P542[天文地球—地质学]