部分熔融榴辉岩及其对深大陆俯冲的流变学意义

Partial melting of UHP metamorphic rocks is poorly documented yet prominently affects the deformation behavior and rheological properties of deeply subducted crust, and plays a key role to trigger the fast exhumation of UHP making it one of the frontier projects for the study of UHPM rocks. The partial melting of UHP country gneiss in Dabie-Sulu has been widely recognized and reported. Experimental petrology and microstructural evidence has indicated that the dehydration partial melting of UHP eclogite also occurred during its early stage of exhumation. However, there are very few reports so far on the field evidence for partial melting of UHP eclogite. Therefore, it is necessary to strengthen the field investigation and identification of partial melting of UHP eclogite from natural field examples. The applicants have recently identified field evidence of partial molten eclogite. The eclogite syn-deformational melting structures, include migmatitic and melt-enhanced flow structures. Therefore we propose to carry out high precision petro-structural geological mapping in this area and complete detailed classification and description of different stages of partial melting of eclogites. On the basis of field mapping, we will collect samples at key locations, carry out geochronological, petro-geochemistry of the rock units within the partial melting system. We also propose to combine microstructural-ultramicrostructural studies, rock fabric and quantitative fabric analysis of the eclogite , restite and melts. The purpose is to determine the timing of the partial melting, source and mechanism of melt, recover the P-T-t conditions of the partial melting, deformation mechanisms of partial melting and its influence on the rheological properties of the UHP metamorphic rocks. The final aim is to supply the data to help decipher the rheological consequences of partially melted eclogite in the deep crust, in subduction zonce, and model the role this played in the exhumation mechanism of the Sulu UHP metamorphic rocks after deep continental subduction.

超高压变质岩部分熔融会显著影响深俯冲地壳的构造变形行为和流变学性质，对启动超高压板片快速折返起关键作用，是研究超高压变质岩的前沿课题。苏鲁地区超高压片麻岩发生部分熔融已被广泛认可，实验岩石学和岩相学证据表明超高压榴辉岩也可在折返早期发生脱水部分熔融，因此加强榴辉岩部分熔融的野外识别是必然研究趋势。申请人最近在苏鲁地区新发现和识别出榴辉岩发生部分熔融的重要野外证据，榴辉岩与熔体发生同步协调变形，显示混合岩化和熔体促进的流动构造。因此提出对该区进行关键露头高精度构造岩石填图和部分熔融阶段划分，部分熔融体系内岩石年代学、岩石地化、显微-超显微构造以及岩石矿物组构定量化研究。确定部分熔融发生时间、熔体来源、恢复其温压条件，探索部分熔融与构造变形之间的关系、部分熔融榴辉岩的变形机制及其熔体对超高压变质岩流变学性质的改变，最终为理解俯冲带深部地壳流变行为的影响和超高压变质岩折返出露机制提供重要约束。

超高压变质岩部分熔融会显著影响深俯冲地壳的构造变形行为和流变学性质，对启动超高压板片快速折返起关键作用，是研究超高压变质岩的前沿课题。实验岩石学和显微构造证据表明超高压榴辉岩可在折返早期发生脱水部分熔融，但天然榴辉岩发生大规模熔融还罕见报道。..项目执行人最近在苏鲁造山带仰口-将军山地区通过关键露头高精度构造岩石填图，在将军山新发现和识别出世界上首例榴辉岩发生大规模部分熔融形成混合岩化的重要野外证据，榴辉岩与熔体发生同步协调变形的流动构造。通过显微结构、变质岩石学、地球化学和年代学综合分析，在峰期榴辉岩、浅色体和残留体（退变榴辉岩）样品内分别发现粒内、粒间熔体重结晶以及原位发生部分熔融的显微构造证据；该混合岩展示出部分熔融的连续发育过程，从最初产生的粒间熔体囊，到3D空间相连的熔体网状结构，然后汇聚到低应力区（如褶皱转折端），最后迁移形成熔体通道或岩脉，将熔体运移到岩石圈浅部。证明苏鲁造山带内深俯冲到地下100-200公里的榴辉岩在228-219百万年前折返至30-90公里的过程中发生了含水矿物脱水分解导致的部分熔融， 形成熔体和熔融后残留体相互复杂混合在一起的混合岩。该研究成果为大陆壳的早期形式——岛弧岩浆来源和下地壳地震剖面检测出的部分熔融导致的“亮点”的实际存在可提供地质依据，对下地壳物质流变强度改变和俯冲带超高压变质岩快速折返及其岛弧岩浆的产生具有重要指示意义。..仰口地区超高压榴辉岩中亲流体重晶石矿物和熔流体结晶的显微构造特征、岩石学、地球化学综合分析表明，大陆深俯冲进变质至折返过程中，5期重晶石生长记录了榴辉岩内部显微尺度富水流体、超临界流体和含水熔体随俯冲-折返过程温压条件改变而演化的轨迹，并对微量元素如Ba、Sr在俯冲隧道内迁移提供关键证据。苏鲁带其他地区包括仰口地区熔融尺度相对将军山地区非常有限，这表明苏鲁造山带超高压榴辉岩熔融的机制还存在多样性和多期性等差异，需要进一步深化对苏鲁造山带熔融机制和差异成因的研究。