超高压榴辉岩的P-T-t-变形-熔融演化过程及其流变学意义

Dehydration and Partial melting of UHP metamorphic rocks has important contributions to arc magamatism. Partial melting of UHP eclogite has rarely been documented from natural examples, 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 rocks, making it one of the frontier projects for the study of UHPM rocks. 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 on the field evidence for large scale partial melting of UHP eclogite. Nevertheless, among these reported examples, the scale of partial melting, P-T conditions for melting, hydrous envrionment system and melt component are various. Therefore, it is necessary to strengthen the field investigation and mechanism of partial melting on natural UHP eclogites. The applicants recently identified migmatized eclogite and petrological evidence on its partial melting at Taohang, Central Sulu Belt, which have both similaries and varieties from the characteristics of migmatized eclogite in Yangkou and General’s Hill newly reported by her. The applicants 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, structural deformation and metamorphism of eclogites. The purpose is to establish the P-T-t-Deformation-Melting path of the partially eclogite, estimate the composition of melt products, investigate the petrofabrics and deformation mechanisms of UHP-HP minerals within the migmatized eclogite, which are comparable with that of Yangkou and General’s Hill. Combine with published P-T conditions along the Sulu belt, this work will provide good data constrain on the P-T- melt conditions change along strike of Sulu belt. Then further probe the correlation between partial melting with deformation and metamorphism, deformation mechanism of partially melted eclogite, contribution of the melt to the growth of continental crust. The final aim is to supply the data to help decipher the rheological consequences of partially melted eclogite in the deep crust, in subduction zones, and model the role it played in the exhumation mechanism of the Sulu UHP metamorphic rocks after deep continental subduction.

超高压变质岩脱水和部分熔融对岛弧岩浆来源有重要贡献，可显著影响深俯冲地壳的构造变形和流变学行为，加速俯冲板片快速折返，是超高压变质岩前沿研究课题。天然榴辉岩大规模熔融罕见，且世界范围已报道的有限几例天然榴辉岩部分熔融规模、温压条件、水环境体系和熔体成份各有不同，因此需加强天然榴辉岩的熔融机制研究。申请人新近从苏鲁桃行地区发现识别出榴辉岩部分熔融的宏观和微观证据，与最近报道的苏鲁仰口榴辉岩部分熔融特征既有相似也有明显不同。因此提出对该区开展关键露头区高精度构造-岩石填图，区分熔融-变形-变质阶段，确定熔融和变形发生的温度、压力、时间和熔体组分，研究其熔融岩石的变形组构和机制，并与仰口超高压榴辉岩部分熔融机制对比，探索部分熔融榴辉岩与构造变形变质之间的关系、变形机制对大陆壳生长的贡献及熔体对超高压变质岩流变学性质的改变，最终为理解超高压变质岩折返机制和俯冲带深部地壳流变行为提供重要约束。

后期熔体迁移混合、结晶分异、熔体抽离以及退变质过程的叠加，使准确限定天然榴辉岩的初始熔体成分和部分熔融机制面临挑战。最近在苏鲁桃行地区识别出首例发育原位浅色体囊的超高压榴辉岩。由斜长石、自形角闪石、细粒残余绿辉石（Jd含量25–45 mol%）及少量钾长石和绿帘石组成的原位浅色体囊保存于大颗粒石榴石、黝帘石和绿辉石等矿物围限区域；榴辉岩中大颗粒绿辉石Jd含量为39–56 mol%。47个原位浅色体囊的主量元素成分和14个浅色体囊的微量元素成分分析显示：浅色体囊具中等SiO2，高Na2O、Al2O3以及相对低Fe2O3T、MgO、CaO、K2O和TiO2含量；浅色体囊与基质中大颗粒绿辉石微量组成相似，均富集LILE、U、Pb、Zr、Hf和亏损Th、Ta。岩相学特征、浅色体囊的微量元素配分特征和较高Na2O含量均指示绿辉石分解主导该榴辉岩部分熔融，是榴辉岩部分熔融机制的最新发现。部分熔融显微构造证据仅存于非双矿物榴辉岩且含水矿物发生脱水分解，表明其产生的流体对绿辉石分解熔融发挥关键作用。绿辉石分解部分熔融温压条件为1.2–1.6 GPa和690–780 ℃；随后熔体穿越湿固相线时（0.8–1.1 GPa；530–650 ℃）冷却结晶。含浅色体囊的榴辉岩变质年龄为215 Ma，锆石边部结晶温度（567–639 ℃）与浅色体囊结晶温度近似，表明浅色体囊结晶时间约为215 Ma。