大别-苏鲁榴辉岩中长英质多晶包裹体的实验研究及部分熔融意义

Felsic polyphase inclusions within ultra-high pressure (UHP) primary minerals provide excellent evidence for dehydration melting events. Moreover these inclusions are also unique recorders of the composition of trapped hydrous felsic melts in these host minerals. Thus, the understanding of the formation mechanisms of polyphase inclusions has great significance for the research of partial melting in UHP rocks..In this application we propose to perform a series of homogenization experiments on felsic polyphase inclusions hosted by garnet and omphacite from the Dabie-Sulu UHP rocks. Experiments will be conducted at 1.5～3.5 GPa and 750～900 C using a piston cylinder apparatus. The characteristics of experimental run products will be determined using scanning electron microscope (SEM), electron microprobe and laser ablation inductive coupled plasma mass spectrometry (LA-ICP-MS). The textural observations together with chemical analyses will be combined to clarify the formation conditions and compositions of these felsic inclusions in the studied eclogites and to explore the effects of the composition of the starting materials and experimental run conditions on the experimental run products.Based on the stability fields and key elements (e.g. LILE, LREE) of major hydrous minerals (e.g. phengite, epidote and zoisite) in the host eclogites, the contributions of different hydrous minerals to the formation of felsic melts will be discussed. This data will show whether the partial melts are internally formed or whether they derive from the outside. We will carry out forward modeling experiments under selected experimental conditions to simulate the formation of felsic melt using eclogites with multiple hydrous minerals as the starting material. .The experimental results will be used to reveal the nature of fluid activities, melt segregation and accompanying elements exchange within continental subduction zones.

长英质多晶包裹体是确定超高压岩石部分熔融的重要证据，其总体成分特征和成因机制研究对于认识大陆俯冲带内熔/流体活动特征具有重要意义。 .本研究以大别-苏鲁榴辉岩石榴石/绿辉石内长英质多晶包裹体为研究对象，模拟超高压岩石主要变质条件，在1.5～3.5 GPa/750～900 ℃下进行成分均一化实验，使用扫描电镜、电子探针和LA-ICP-MS等观察分析实验结果，确定此类包裹体的形成条件、总体成分特征及寄主岩石/主矿物与实验条件的控制作用。通过榴辉岩中主要含水矿物（多硅白云母/绿帘石/黝帘石等）的关键性元素和稳定域对比，对比分析不同含水矿物对长英质包裹体的成分贡献，讨论熔体来源并建立此类包裹体成因判别标准。在关键温压条件下进行含多种含水矿物榴辉岩的脱水部分熔融实验，模拟与均一结果相匹配的熔体的形成过程，验证均一化实验结果的有效性，为确定超高压岩石内熔体成分特征、形成条件与物质交换提供实验证据。

超高压岩石快速折返过程中的脱水熔融作用对于认识碰撞造山带熔流体特征具有重要科学意义。本研究以大别-苏鲁超高压榴辉岩/片麻岩部分熔融矿物学记录为研究主线，将天然观测与高温高压实验结合，为认识大陆折返过程中超高压榴辉岩/片麻岩的脱水部分熔融特征提供重要观测证据和可靠分析资料。本基金的主要研究成果为：①在大别山双河超高压榴辉岩石榴石中观察到多类型多晶包裹体，超高压含水矿物包裹体主要为多硅白云母，Kfs+Qtz包裹体代表微量长英质熔体，包裹体矿物组合特征记录的熔融反应为Phe+Omp±Qtz→Amp±Grt+Melt (Kfs+Qtz+Pl)。该类包裹体富集LILE、LREE和低HREE，证明此类熔体的形成受到多硅白云母脱水熔融作用控制。②在大别山双河超高压副片麻岩识别出Phe+Kfs+Ky、Phe+Grt+Kfs、Kfs+Qtz等类型多晶包裹体，分别对应多硅白云母的脱水分解、初始熔融和完全熔融等不同阶段，分析获得片麻岩部分熔融条件为783～839℃和2.0～2.5 GPa，确认多硅白云母矿物对片麻岩部分熔融具有重要控制意义。③在大别山甘家岭超高压钙质片麻岩石榴石中确认出残留柯石英，石榴石中多硅白云母包裹体Si最高为3.65 pfu，表明寄主岩石曾俯冲到很深地幔中。显微结构特征表明，超高压钙质片麻岩经历了热折返过程的部分熔融，残留的方解石薄膜表明长英质熔体中溶解有少量CO2或者存在碳酸盐熔体。④在实验方法探索方面，在南迦巴瓦岩群石榴黑云斜长片麻岩的石榴石中识别出较多数量的显微花岗岩包裹体，在1.5GPa、800～900℃下进行均一化实验，显微花岗岩包裹体在850℃下达到成分均一化，形成过铝质的花岗质熔体，可以与喜马拉雅淡色花岗岩进行类比。⑤在大别-苏鲁超高压片麻岩的锆石中识别出典型的显微花岗岩包裹体，代表性矿物组合为Pl+Qtz+Kfs，其代表超高压片麻岩初始部分熔融作用产物。双河超高压副片麻岩的锆石中还保留有Pl+Qtz+Ky+Ep多晶包裹体，表明超高压片麻岩部分熔融具有多阶段特征，绿帘石在其中具有重要作用。