冈底斯岩浆弧东段的变质−深熔−岩浆作用：大陆地壳的生长与再造

The Gangdese magmatic arc, located at the southern Lhasa terrane, south Tibetan Plateau, was derived from the Mesozoic subduction of Neo-Tethyan oceanic lithosphere and the Cenozoic collision between the Indian and Asian continental plates, representing a natural laboratory of studying the metamorphism and magmatism of subduction and collisional zones, and the growth of continental crust. The applying project will conduct a regional geological survey and corridor geological mapping, and petrological, geochronological and geochemical study for the different crustal levels in the Linzhi and Milin area, eastern Gangdese arc. The aims of the project are (1) to reveal the metamorphic conditions of the metamorphic rocks and crystallization conditions of the plutonic rocks, and their spatial changes, to determine the crustal depths of formation of these rocks, and the component, protolith, chemical composition, formation time and petrogenesis of the rocks from different crustal levels of the magmatic arc, and to establish the rock component and geochemical profiles of the magmatic arc; (2) to reveal P−T conditions, timescales and P−T−t paths of the high-grade metamorphism and associated partial melting in the thickened lower crust of magmatic arc during the oceanic subduction, syn- and post-continental collision, to establish the relationship between the melt compositions and anatectic condition, timescale, degree and the protolith composition, and then to probe the genetic relationship between the metamorphic and anatectic process and magmatism of the magmatic arc root; (3) to reveal the growth and reworking of the crust and tectonic dynamics of the composite magmatic arc, and finally to provide new insights into the timing and nature of the oceanic lithosphere subduction and continental collision.

青藏高原南部的冈底斯岩浆弧形成在中生代新特提斯洋俯冲和新生代陆−陆碰撞造山作用过程中，是研究俯冲−碰撞带变质与岩浆作用和大陆地壳生长的天然实验室。本申请项目拟对冈底斯弧东段林芝−米林地区不同地壳层次的岩石进行区域地质调查和廊带填图，岩石学、年代学和地球化学研究，（1）查明研究区变质作用条件和岩浆结晶条件及空间变化，确定岩石形成的地壳深度，揭示岩浆弧不同地壳层次的岩石组成、原岩类型、化学成分、形成时代与成因，建立岩浆弧地壳组成和化学成分剖面；（2）揭示岩浆弧加厚下地壳（弧根）在洋壳俯冲、陆−陆碰撞和后碰撞构造环境下变质与部分熔融的温压条件、起始与持续时间及P−T−t轨迹，熔体成分与熔融条件、时限、熔融程度和原岩成分关系，建立岩浆弧根部变质−深熔与岩浆作用之间的成因联系；（3）揭示复合型大陆岩浆弧地壳生长与再造作用及动力学机制，为新特提斯洋俯冲和印度−亚洲大陆碰撞的时限和性质提供重要制约。

本项目在冈底斯和喜马拉雅造山带东段开展了区域地质调查和地质填图，进行了系统的岩石学、地球化学和年代学研究。在中国地质调查局项目的联合资助下，我们完成了冈底斯岩浆弧东段地质图，冈底斯岩浆弧东段色季拉山地区1:5万地质图，冈底斯弧东段米林地区岩浆弧下地壳地质图，冈底斯弧东段岩浆弧中-上地壳剖面图。到目前为止，项目组成员作为第一作者已经在国内、外核心期刊发表论文30篇。本项目取得的重要成果如下：（1）揭示出冈底斯弧地壳的组成与特征。冈底斯岩浆弧下地壳主要由混合岩化石榴角闪岩和正片麻岩组成，总体上为中性成分，且富水和富铜，很可能是后碰撞含铜和多金属矿斑岩的物质源区。冈底斯弧的中-上地壳主要新生代的花岗岩组成，总体上具有酸性岩成分。（2）揭示出冈底斯弧的地壳生长与再造作用。冈底斯弧经历了从晚三叠到中新世的多期岩浆、变质与再熔融作用。岩浆弧地壳的岩石组成、化学成分和厚度是随时间变化的，具有从初生弧地壳向成熟大陆地壳转变的特征。（3）揭示出喜马拉雅造山带东段的变质-熔融作用与花岗岩成因。造山带加厚下地壳中的各种岩石在其进变质过程中经历了长期（45-15Ma）的脱水熔融，形成了不同时代的花岗岩，原岩成分与熔融程度的差异，以及熔体混合、分离结晶和同化混染作用导致了不同成分花岗岩的形成。（4）建立了喜马拉雅造山带变质作用与构造演化模式。造山带核部的变质岩具有相同的早始新世（~47 Ma）变质年龄，但从西向东变质压力逐渐降低，而变质温度逐渐增高，表明在大陆碰撞初期，印度大陆地壳的西北缘深俯冲到地幔，经历了超高压变质作用，而印度大陆地壳的东缘平俯冲到拉萨地体（冈底斯弧）之下，经历了高压榴辉岩相到麻粒岩相变质作用和部分熔融，并构成了造山带的加厚下地壳。