日照地区地幔楔橄榄岩蛇纹石化过程中FME的富集与迁移机制

Mantle wedge serpentinite from global subduction zones has become an important research object to tracer the migration of H2O-rich fluids and deep elements, and the process of island-arc magmatism, and the relevant dynamics models were established. However, the precise description is not understood accurately about the enrichment and migration process of fluid-mobile elements (FME) in the serpentinization process of mantle wedge peridotites. A large number of peridotite is exposed at Rizhao region, which has derived from ancient lithospheric mantle of the North China Craton and undergone profoundly water-rock interaction at mantle wedge. The serpentinizated peridotite is good example for understanding the migration mechanism of FME and H2O-rich fluids in subduction zone..This project adopts geological observations, mineral chemistry and geochemical analysis. First, we’ll determine the source and properties of FME and H2O-rich fluids in the subduction zones; mainly tracer the micro-precise descriptions on the differences enrichment characteristics for primary minerals and reflect the microscopic serpentine process; identify the correspondence relationship between the serpentinization & dehydration process and the migration of FME and H2O-rich fluids & internal magmatic activity of mantle wedge; finally, we’ll try to establish a mechanism for the enrichment and migration mechanisms of FME during the serpentinization of mantle peridotites. The research findings will provide new constraints on the dynamics processes of deep subduction between the YZC and the NCC, and also provide available comparative information and new clues for the research on the serpentinites from global subduction zones.

全球典型俯冲带中地幔楔蛇纹岩已成为富H2O流体与深部元素迁移过程、岛弧岩浆形成机制的关键示踪窗口，并建立了相关动力学模型。然而，地幔楔橄榄岩蛇纹石化过程中，易溶流体迁移元素（FME）的富集与迁移过程尚缺乏清晰的微观描述。日照地区出露大量源自华北克拉通古老岩石圈地幔，并于地幔楔经历了深刻水-岩反应的橄榄岩，为研究俯冲带FME与富H2O流体的迁移机制提供了理想实例。.项目拟通过地质观测、岩石地球化学和矿物化学研究，识别FME与富H2O流体的来源属性；重点利用蛇纹石微区痕量元素示踪，查明原生矿物富集FME的差异性特征，反映微观蛇纹石化过程；分析蛇纹石化-深部去蛇纹石化与FME-富H2O流体深部迁移、地幔楔内岩浆活动间对应联系；建立地幔楔橄榄岩蛇纹石化过程中FME的富集与迁移机制。研究成果将对扬子与华北间的深俯冲动力学过程提供新的约束，也为全球典型俯冲带蛇纹岩研究提供对比信息和线索。

苏鲁超高压变质带作为全球最受关注的构造岩浆混杂岩带（或拼合带）发展起来的陆-陆碰撞造山带，保留大量源自地幔的蛇纹石化橄榄岩和蛇纹岩。本项目以日照地区（胡家林-娑罗树）及邻区（仰口）出露大量源自华北克拉通古老岩石圈地幔的蛇纹石化橄榄岩为研究对象。蛇纹岩化橄榄岩和蛇纹岩的原岩为方辉橄榄岩和纯橄岩，其在大陆俯冲通道中其经历了差异化蛇纹石化作用，并与源自扬子大陆的变质岩(花岗质片麻岩)一同折返至地表。它们在蛇纹石化过程中均富集流体易迁移元素，如Sb(亏损地幔值的42~333倍)和Pb(亏损地幔值的30~476倍)。富集程度与喜马拉雅造山带(Himalayan)中弧前蛇纹岩相似，但明显高于马里亚纳(Marianas)弧前蛇纹岩，表明大陆俯冲地区的富集程度高于大洋俯冲带地区，与俯冲物质富集这些元素相关。.橄榄石蚀变形成的俐蛇纹石(lizardite)(胡家林-娑罗树)显示高富集的Sb和As，但相对低含量的Pb，U，Cs和LREEs，表明其在浅部蛇纹石化作用中纳入Sb和As；叶蛇纹石(橄榄石和斜方辉石)则强烈富集上述元素，表明其在深部蚀变过程中吸收这些元素。<300°C(相当于苏鲁地区~60 km深；地温梯度为~5°C/km)，俐蛇纹石形成，并从交代流体中强烈富集Sb和As；在300°C ~400°C之间（60~80 km），俐蛇纹石转化为叶蛇纹石，并进一步吸收FME（如，Pb，U，Cs和LREEs）。在进阶变质过程中（深俯冲过程），叶蛇纹石的分解（600~700 °C；大约120~140 km深），释放大量水和FME进入上覆地幔，并可能诱发地幔楔的部分熔融和岛弧岩浆。