大别造山带超高压变质岩石中部分熔融和变质分异的区分

Partial melting of metamorphic rocks during continental subduction and exhumation provides important evidences in understanding the fluid/melt action in continental subduction zones. Migmatic leucosomes and pegmatite veins are the results of partial melting at high degrees. However, metamorphic segregation at subsolidus conditions can also produce rocks that have leucocratic and melanocratic layers. .This project focuses on the migmatite-like rocks, such as banded metamorphic rocks and felsic veins in the ultrahigh pressure (UHP) metamorphic rocks in the Dabie orogen. Distinction between migmatization by partial melting and metamorphic segregation is critical when dealing with the migmatite-like rocks as they would take place at much different P-T conditions. Firstly, carefully petrographic observations can be used to distinguish the melt-related microstructures such as melt-producing or melt-consuming structures and melt inclusions, or metamorphic microstructures such as lepidoblastic, nematoblastic or granoblastic. Then a combined study of whole-rock geochemistry, in-situ analysis of major and trace elements in minerals, and U-Pb & Lu-Hf isotopes, and trace elements in zircons will be employed to investigate the mechanisms of migmatite-like rocks. .The banded UHP metamorphic rocks or felsic veins are the results of partial melting during the exhumation of continental crust, or metamorphic segregation at subsolidus conditions? Different mechanisms will give much different constraints on the P-T-t reconstruct and fluid/melt action during continental subduction and exhumation. In addition, different degrees and scales of partial melting may exist between low-T/UHP South Dabie metamorphic zone, mid-T/UHP Central Dabie metamorphic zone, and high-T/UHP North metamorphic zone. Thus, the distinguishing between partial melting and metamorphic segreagtion in the migmatite-like UHP rocks will provide petrologic and geochemical implications on the thermodynamics and kinetics during continental collision.

大陆深俯冲岩石在折返过程中的部分熔融是研究大陆俯冲带流体/熔体活动的重要内容。本项目以大别造山带超高压变质岩石中的条带状岩石和长英质脉体为研究对象，拟通过系统的岩相学(特别是矿物中熔体成因微结构和熔体包裹体的识别)、全岩地球化学、矿物微区主量元素和微量元素以及变质锆石学(岩相学、微量元素、U-Pb和Lu-Hf同位素)的研究，区分条带状岩石和长英质脉体的成因机制，它们是超高压岩石在板片折返过程中部分熔融形成的混合岩化，还是变质岩原岩在压力驱动下发生了亚固相条件的变质分异。开展大别造山带超高压变质岩中的长英质脉体和条带状岩石的成因研究，探讨大别造山带不同变质变质带经历部分熔融的时间、规模和范围，进一步理解大陆俯冲带岩石在超高压变质过程中的流体活动机制以及发生部分熔融的变质温压条件，为全面地认识大陆深俯冲和折返的热力学和动力学机制提供岩石学和地球化学依据。

本项目以大别-红安造山带中条带状片麻岩和榴辉岩为主要研究对象，开展了系统的全岩主微量元素和矿物主微量元素剖面及包裹体分析、以及变质锆石学和热力学相平衡计算的综合研究，取得了一系列重要进展和成果，发表了5篇与之相关的论文，其中国际SCI论文4篇，EI论文1篇。主要进展包括：(1) 发现大别造山带不同变质带之间或不同变质带内部部分熔融程度存在显著差异，中大别碧溪岭地区花岗片麻岩仅经历了超高压变质条件下的变质分异，而双河地区花岗片麻岩经历了低程度折返熔融，北大别花岗片麻岩经历了大范围的三叠纪部分熔融作用。(2) 超高压变质岩石中识别出过程中不同成因的石榴石，包括岩浆成因石榴石的残留、高压-超高压变质过程中的变质生长石榴石、以及部分熔融过程中形成的转熔石榴石。其中转熔石榴石的鉴别，为超高压变质岩石的折返熔融提供了重要的矿物学证据。（3）发现中大别碧溪岭和双河两地的榴辉岩及其围岩中锆石具有明显差异，暗示在大陆深俯冲和折返过程中，变质岩的原岩性质是控制高压-超高压条件下变质流体含量和特征的重要因素，不同类型片麻岩围岩中释放的流体量不同，会引起岩石发生不同程度的变质脱水和部分熔融。（4）在红安造山带内发现两种不同类型的榴辉岩，大洋型榴辉岩和大陆型榴辉岩。两类榴辉岩的共存，表明红安造山带保存了华北与华南板块间古特提斯洋闭合时从洋壳俯冲向陆壳俯冲的过渡阶段。因此红安造山带既不同于大别-苏鲁造山带，也不同于秦岭-桐柏造山带，是一个独立的造山带，它为寒武纪-三叠纪时期华北板块和华南板块间板块汇聚的构造演化提供了重要依据。