【作者】 刘强；

【导师】 金振民；

【作者基本信息】 中国地质大学 ， 构造地质学， 2009， 博士

【摘要】 在过去二十年中，超高压变质作用及超高压岩石研究是国际地学研究中的热点课题。中国学者通过岩石学、矿物学、地球化学、年代学等多学科综合研究并结合大陆科学钻探工程（CCSD）实施，使我国超高压地质研究水平得到极大提升，尤其是大别—苏鲁变质带的研究成果为世界超高压地质研究做出很大贡献。当前，超高压地体折返过程及控制机制仍是该领域研究中亟待回答的重大科学问题之一。地质观察、实验模拟和年代学分析均表明，超高压岩石在深俯冲大陆折返过程中存在明显的部分熔融作用。该过程对超高压岩石的物质交换、流变学性质改变等产生重要影响，这对于认识大陆深俯冲过程和超高压变质岩折返机制具有重要启示意义。在大别—苏鲁超高压变质带中，以威海和碧溪岭为代表的地区出露有典型的超高压岩石部分熔融现象。野外实际观测表明，威海超高压榴辉岩岩块边缘部位分布有长英质脉体，脉体产出与剪切面理一致。在面理化榴辉岩中，发育有由长石和石英矿物组成的细微斑点，长英质矿物同石榴石、绿辉石等一同发生塑性变形。这显示出不同产状产出的超高压榴辉岩在剪切变形下的部分熔融特征。经过部分熔融作用的超高压片麻岩主要表现出较为强烈的混合岩化，出现黑云母和角闪石的深色成分带、长石和石英的浅色成分带以及钾长石伟晶岩脉等三种特征明显不同的成分层。室内岩相学观察显示，威海超高压榴辉岩和碧溪岭超高压片麻岩中存在岩石部分熔融的结构证据。二者内部发育有毫米级的长英质脉体，脉体由细粒、自形斜长石+石英构成。榴辉岩中多硅白云母和黝帘石具有后成合晶环边结构。以上观察结果表明，超高压岩石可以在含水矿物脱水分解下发生部分熔融。该现象的观察和解释对应于目前对超高压岩石部分熔融作用机制的认识，即在超高压岩石折返的特定阶段中、岩石中含水矿物通过脱水析出流体来诱发岩石部分熔融，并相应地导致岩石化学成分变化和流变学性质的改变。由于多硅白云母、黝帘石／斜黝帘石和硬柱石等是榴辉岩中普遍存在的超高压含水矿物，认识这些含水矿物在超高压变质演化P-T轨迹内的稳定性对于了解岩石部分熔融特征具有重要意义。作为超高压岩石中常见的富钾含水矿物，多硅白云母在2.3～3.2GPa的脱水分解温度最为接近榴辉岩初始部分熔融温度。因此，非常有必要查明多硅白云母在不同温压条件下的脱水熔融特征及对寄主岩石起始熔融条件的控制与影响。本论文以大别山东部碧溪岭超高压变质榴辉岩为天然的实验样品，使用活塞圆筒式高温高压装置，模拟超高压岩石折返过程的温压条件，在1.5、2.0、2.4、3.0GPa，800～1000℃下进行18个封闭体系条件下的脱水熔融实验，系统研究多硅白云母在榴辉岩中的脱水熔融特征，来认识超高压榴辉岩中脱水熔融记录所表达的地质信息和动力学意义。实验研究取得以下三个方面的主要认识：1．温度和压力条件对多硅白云母脱水熔融反应具有明显控制作用通过对1.5～3.0 GPa和800～1000℃下实验产物和熔融结构分析，表明榴辉岩中多硅白云母脱水熔融反应随温度和压力改变而变化。在1.5～2.0 GPa和800～850℃下，多硅白云母和黝帘石在亚固相下析出流体弥散到体系中。在流体助熔作用下，体系中易熔组分优先熔融，原生蓝晶石形成由更长石组成的反应边，反应体系初始熔融反应表示为Ky+Q+Omp+H₂O→Melt和Ky+Melt→P1-Ⅰ。随着温度升高，多硅白云母和黝帘石直接熔融，熔体由含水矿物边部逐渐扩展至反应体系内，较大比例的熔体结晶形成更长石。该阶段熔融反应表示为：Phe+Omp+Q→Pl-Ⅱ+Ky-Ⅰ+Melt，更长石是多硅白云母在榴辉岩中主要的熔融反应产物。新生矿物相是含水矿物通过直接熔融结晶（如蓝晶石）和与不同矿物相发生熔融反应（如更长石）来形成。随温度升高，体系内熔体比例逐渐增加，多硅白云母完全熔融形成新生石榴石：Phe+Omp+Q→Pl-Ⅱ+Gt-Ⅰ+Ky-Ⅰ+Melt。多硅白云母由亚固相脱水至完全熔融是一个逐步过程，在1.5～2.0GPa为100℃、2.4～3.0GPa下则＜50℃。随着压力升高，反应体系中熔体比例有所减少，在脱水熔融产物中形成钾长石。在该压力范围内，硬玉出现在熔融反应产物中。多硅白云母在2.4～3.0 GPa和900～950℃熔融反应表示为：Phe+Omp+Q→Jd+Gt-Ⅰ+Kfs+Ky-Ⅰ+Melt。单斜辉石（硬玉分子）对多硅白云母在基性岩中脱水熔融反应具有重要意义。2．榴辉岩脱水熔融的残余矿物相随温压条件变化而改变石榴石和绿辉石是榴辉岩部分熔融实验中的主要残留矿物。随着温度和压力变化，石榴石中铁铝榴石和钙铝榴石分子的变化趋势不同，不同压力区间内具有一定差异。同—压力条件下的升温过程中，铁铝榴石分子呈现升高—降低—升高的变化规律，钙铝榴石分子则相应地呈反向的变化规律。镁铝榴石和锰铝榴石分子受温压条件变化影响较小。在更高温度下（950～1000℃），多硅白云母脱水熔融形成新生石榴石。新生石榴石镁铝榴石端元分子数升高，钙铝榴石端元分子数降低，表明多硅白云母熔融为其提供MgO来源。相同压力条件下，随着温度升高，绿辉石中硬玉分子含量降低。随着熔融比例升高（由7％升高到30％），绿辉石中硬玉分子含量明显下降。相同温度下，随着压力升高，绿辉石中硬玉分子总体上呈现升高趋势，不同温度区间内升高值不同。在≤2.0GPa下，绿辉石中硬玉分子随着温度升高明显出溶，矿物中Ca-Tschermaks分子和顽辉石—铁辉石组分随之升高。在2.4～3.0GPa下，绿辉石中硬玉分子变化平缓。压力升高则对绿辉石矿物中硬玉分子的出溶具有明显地抑制作用。长石成分对比表明，蓝晶石边部形成的斜长石和多硅白云母熔融形成的斜长石没有明显区别，均属于更长石。随着温度升高，斜长石中钠长石端元组分明显升高而钙长石端元组分明显降低；随着压力升高，斜长石中钠长石端元组分变化不明显，钙长石端元组分降低，钾长石端元组分有所升高。黝帘石具有典型的逐步分解熔融特征，可在较宽的温度范围（如750～950℃）发生熔融并形成反应边，反应边由斜长石和熔体构成。随着温度升高，黝帘石熔融明显，形成长石和蓝晶石矿物等。相同压力下，黝帘石的初始脱水熔融温度较多硅白云母更低，但是本次约束的岩石脱水熔融固相线低于黝帘石脱水熔融固相线近75℃。黝帘石脱水熔融主要反应与多硅白云母有明显不同，压力是黝帘石反应变化的主要控制因素。蓝晶石是在反应物和生成物中均出现的特殊矿物。矿物成分和分布特征可确定新生蓝晶石从多硅白云母熔融后的熔体中直接结晶形成。实验所获得原生蓝晶石的反应边结构可用来指示反应体系中流体活动的强弱。金红石矿物具有较好稳定性，没有参与熔融反应的特征。3．压力和温度变化对榴辉岩部分熔融的熔体成分具有控制作用在1.5GPa、800℃，2.0GPa、850℃，2.4 GPa、850℃，3.0 GPa、950℃下，实验体系中形成熔融比例在～3％的初始熔体，熔体环绕含水矿物边缘或呈团块状分布在不同矿物相接触区，表明熔体形成与含水矿物脱水分解及与相邻矿物（如绿辉石、石英等）熔融反应相关。在1.5～3.0GPa和850～950℃，岩石中初始熔体SiO₂在67.02％～74.76％，TiO₂+FeO^*+MgO=0.56％～2.22％，CAO=0.22％～3.44％，Na₂O+K₂O=4.04％～8.27％。熔体总体呈花岗质特点，分布在奥长花岗岩（1.5～2.0GPa）区域。在1.5～2.0GPa、1000℃下，实验获得熔融比例较高（～30％）的熔体，2.4GPa下熔体比例较低。熔体中SiO₂在68.48％～71.08％，其它主要氧化物组分为：TiO₂+FeO^*+MgO=0.62％（2.4 GPa）～3.99％（1.5GPa）、CaO=0.25％（2.4GPa）～2.37％（1.5GPa）、Na₂O+K₂O=0.70％（1.5GPa）～2.03％（2.4GPa）。压力和温度变化对熔体中SiO₂、TiO₂+FeO^*+MgO、CaO、Na₂O+K₂O具有较强控制作用。总的变化规律是：相同温度下，随着压力增加，SiO₂、Na₂O+K₂O含量有所增加，TiO₂+FeO^*+MgO、CaO含量降低；压力≥2.4 GPa，熔体中Na₂O明显递减，而K₂O明显增加；相同压力下，随温度增加，熔体中SiO₂、TiO₂+FeO^*+MgO、CaO含量增加，Al₂O₃、Na₂O+K₂O明显降低，表明石英、金红石、绿辉石、多硅白云母及黝帘石等矿物不同程度上发生熔融为熔体贡献组分。1.5～2.0GPa、1000℃下熔体微量元素特征显示，熔体具有高Sr、低Y和Yb，高Sr／Y和La／Yb比、负Nb-Ta异常特征。熔体的LREE富集、HREE亏损，具有高La／Yb比、正Eu异常。这表明实验形成的熔体和残余矿物组合与埃达克质岩石特征具有较好一致性，榴辉岩部分熔融可以直接形成低镁的埃达克质岩浆。通过本次榴辉岩部分熔融实验研究，所获得的主要动力学意义表现在：1．实验结果对大别-苏鲁榴辉岩的部分熔融条件和性质具有明确约束意义本实验获得多硅白云母在1.5～2.0GPa下脱水熔融温度≤800～850℃，熔融温度随压力增加而升高，表明多硅白云母1.5～3.0GPa的脱水熔融曲线为正斜率。本次多硅白云母脱水熔融温度值高于含水合成体系约50℃，低于中酸性岩石体系约50℃。将本实验结果与超高压榴辉岩的“热”折返P-T轨迹相结合，表明超高压榴辉岩脱水部分熔融的最合理压力和温度区间为1.5～2.0GPa、800～850℃，即石英榴辉岩相向角闪岩相转变过程中。这表明，在没有外界流体参与下，依靠超高压变质岩中含水矿物的自身脱水熔融可以导致岩石局部范围内的部分熔融或混合岩化。2．实验结果对大别-苏鲁榴辉岩的部分熔融过程具有明确指示意义实验结果表明，在≥2.4 GPa、≥850℃下，熔体可更多地溶解体系中的自由流体而使体系处于流体不饱和状态，这使得多硅白云母可以通过脱水熔融形成钾长石。超高压榴辉岩中钾长石可视为多硅白云母脱水熔融的产物，指示此条件下反应体系处于流体不饱和状态，暗示榴辉岩由柯石英榴辉岩相向石英榴辉岩相转变过程中可能经历一次局部熔融过程。蓝晶石具有斜长石反应边则指示了另一次反应条件有明显差异的部分熔融作用。结合1.5～2.0GPa、800～850℃实验结果，多硅白云母和黝帘石在亚固相下脱水，流体在体系中运移和活动导致部分熔融，富钾钠的熔体与蓝晶石发生反应形成斜长石反应边。蓝晶石的斜长石反应边及长英质脉体用以指示榴辉岩在1.5～2.0GPa和含自由流体条件下的部分熔融过程，表明超高压榴辉岩在石英榴辉岩相向角闪岩相转变中经历了流体明显活动的部分熔融作用。3．实验结果具有重要的物质交换和构造物理意义将榴辉岩部分熔融的熔体成分与造山带碰撞后“滞后”侵入的埃达克质岩性（如东大别天堂寨岩体）对比，熔体成分与大别山早白垩世高钾低镁埃达克质岩相类似，证明这类高钾低镁的埃达克质岩浆形成深度应当＞50Km。超高压榴辉岩部分熔融可形成低密度岩浆，这将弱化岩石力学强度并改变岩石变形机制。在岩石较高部分熔融度下，残余物（石榴石+绿辉石）密度值超过正常榴辉岩密度，熔体与高密度残余物将有效分离，使该条件下重力趋于不稳定并促进造山带加厚下地壳拆沉作用的发生。更多还原

【Abstract】 Ultrahigh-pressure（UHP） metamorphism has been one of the most rapidly moving fields ingeology over the last two decades. Our understanding of the UHP metamorphism has been greatlyimproved due to the implementation of the Chinese Continental Scientific Drilling（CCSD） projectand a multidisciplinary investigation, including petrology, mineralogy, geochemistry and chrono-logy. Chinese scientists have made significant contributions to the development of the UHPmetamorphism by their outstanding achievements on studying the Dabie-Sulu UHP terrane.However, the exhumation mechanism of the UHP terrane still remains enigmatic and an importantscientific issue that has to be addressed.It is widely accepted that the UHP rocks experienced partial melting during exhumation ofdeeply subducted continental crust based on geological observations, results of experimentalpetrology at high-T and high-P and chronOlogical analyses. This process has signifycant influenceon chemical exchange and rheological properties of the UHP rocks. There is not doubt thatstudying partial melting of the UHP rocks will improve our understanding of the deep subductionof continental crust and the exhumation mechanism of UHP metamorphic rocks. Field observa-tions show evident partial melting of UHP rocks from Weihai and Bixiling, Dabie-Sulu UHPmetamorphic belt. In UHP eclogites from Weihai, partial melting is manifested by quartzofeld-spathic veins distributed along the margin of UHP rock body. These veins have the same occur-ence as shear foliation does, suggesting an intrinsic relationship between partial melting of UHPeclogites and shear deformation. Small clasts of feldspar and quartz can be seen in foliated eclogite,and were deformed plastically together with garnet and omphacite. The UHP gneiss from Weihaialso shows strong migmatization, with muscovite and amphibole comprising melanosome andfeldspar and quartz comprising leucosome. Petrographic investigation shows that the UHP eclogitefrom Weihai and UHP gneiss from Bixiling both had experienced partial melting. Phengite andzoisite from eclogite both developed symplectitic texture composed of fine-grained, anhedralplagioclase and biotite. In gneiss, however, the grain boundary of zoisite was embayed, indicativeof metasomatism by plagioclase. These features indicate that the UHP rocks had experiencedpartial melting assisted by dehydration of hydrous minerals, which in turn leads to modification ofchemical composition and rheological properties of these rocks. Currently, it is known that partial melting of UHP rocks was triggered by fluid released fromdehydration of hydrous minerals and nominally anhydrous minerals during peak metamorphism orexhumation of UHP rocks. Since phengite, zoisite/clinozoisite and lawsonite are common hydrousminerals in UHP eclogites, the stability of these minerals along P-T path of UHP metabasites is ofgreat signi-ficance for partial melting. As a common potassium-rich hydrous mineral in UHProcks, phengite breaks down at a temperature quite close to the initial partial melting temperatureof eclogites under pressures of 2.3-3.2 GPa. Thus, it is important to investigate the features ofdehydration melting of phengite under different P-T conditions and their implications for initialpartial melting of host rocks. We have chosen as starting materials a phengite-bearing UHP eclogitecollected from Bixiling in the eastern Dabie orogen. Dehydration melting experiments were usednon-end-loaded piston-cylinder high temperature and pressure apparatus to directly simulate P-Tconditions for the hot exhumation of UHP rocks. This research aims to systematically studydehydration melting of the phengite-bearing eclogite at 1.5-3.0 GPa and 800-1000℃in order tofurther unravel the information and implications in association with dehydration melting of UHPeclogites. The results of this research are as follows:1. Dehydration melting texture and melting reactionsThe textural features and mineral assemblages of reaction products of dehydration-melting ofphengite-bearing eclogites at 1.5-3.0 GPa and 800-1000℃vary as a function of temperatureand pressure. Phengite and zoisite release fluid under subsolidus conditions at 1.5-2.0 GPa and800-850℃. Eclogites subsequently begin to melt with the assistance of fluid and the lessrefractory comp-onents enter the melt with priority. The melts further react with kyanite and formreaction rim. Thus, the initial melting reactions could be expressed by the following two relations:（1） Ky+Q+ Omp+H₂O→Melt; （2） Ky+Melt→Pl-Ⅰ.At elevated temperatures, phengite and zoisite start to melt. The induced melt occurredinitially adjacent to hydrous minerals and then are irregularly distributed to grain boundaries ofdifferent minerals. Based on phase relation and textural analyses, the oligoclase forms in the meltpools and becomes more abundant, the following melting reaction could be inferred: Phe+Omp+Q→Pl-Ⅱ+Ky-Ⅰ+Melt. Oligoclase is the primary dehydration-melting reaction product of phengite innatural eclogites. It suggests that some neoformed phases such as kyanites be crystallized frommelt pools, while others such as oligoclase formed by the dehydration-melting reaction. Phengitesare dissolved completely through reaction Phe+Omp+Q→Pl-Ⅱ+Gt-Ⅰ+Ky-Ⅰ+Melt to producegarnets. The melt fraction decreased slightly with increasing pressure. Potassium feldsparsproduced by dehydration melting of phengite is xenomorphic and usually occurred at the edge ofphengite, while the relict phengite is substituted by jadeite. Our experimental results also show thatthe temperature interval from initial dehydration to complete breakdown of phengite changes as afunction of pressure. The temperature interval is 100℃at pressures of 1.5-2.0 GPa, and＜50℃at pressures of 2.4-3.0 GPa. On the basis of phase relation and textural features, the dehydrationmelting reaction of phengite under pressures of 2.4-3.0GPa and 900-950℃can be showed asPhe+Omp+Q→Jd+Gt-Ⅰ+Kfs+Ky-Ⅰ+Melt. It was suggested that jadeite component be of greatsignificance for dehydration melting of phengite from basic rocks. 2. Relict products of melting reaction of dehydration meltingGarnets and omphacites are the main relict minerals for partial melting of eclogites. Thecontents of almandine and grossular component in garnets will vary with temperature and pressure.Under the same pressure, the contents of almandine component will firstly increase, then decreaserapidly, and then increase again with increasing temperature, while those of grossular componentwill firstly decrease, then increase, and then decrease again with increasing temperature. Incontrast, temperature and pressure has little, if any, effect on pyrope and spessartine component. Atan even higher temperature, newly-born garnet with higher pyrope component and lower grossularcomponent was formed by dehydration melting of phengite.The jadeite component in omphacite decreases with increasing temperature under the samepressure. In particular, the jadeite component of omphacite decreases even remarkably withincreasing melt fraction from 7% to 30%. In contrast, the jadeite component of omphacitegenerally increases with increasing pressure at the same temperature, though the increasing rangemay vary under different temperatures. Jadeite exsolution from omphacite with increasingtemperature is evident under≤2.0 GPa, accompanied by increasing of Ca-Tschermaks componentand enstatite-ferrosilite component in omphacite. On the other hand, the jadeite component inomphacite only slightly varies with increasing temperature under the pressures of 2.4-3.0 GPa. Itis the increasing pressure that significantly restrains the exsolution of jadeite from omphacite.It is shown that plagioclases from the margin of kyanite and those formed by dehydrationmelting of phengite are both oligoclase, and there are no significant differences between them. Thecontents of albite end-member increase significantly with increasing temperature, while those ofanorthite end-member decrease. In comparison, the contents of albite end-member didn’t varysignificantly with increasing pressure, while those of anorthite end-member and K-feldsparend-member decrease and increase with pressure, respectively.Zoisite can develop reaction rim consist of plagioclase and melt under a wide range oftemperature（750-950℃） by means of stepwise decomposition melting. With increasingtemperature, the melting of zoisite becomes significant, which produces feldspar and kyanite, etc.The incipient dehydration melting temperature for zoisite is much lower than that for phengiteunder the same pressure. In addition, the main reaction involved in dehydration melting of zoisiteis much different from that of phengite. Pressure is the main factor that controls the meltingreaction of zoisite.Kyanites are special for their presence both in the starting material and in the reactionproducts. However, one can tell the difference between newly-born kyanites directly recrystallizedfrom melt generated by dehydration melting of phengite and those from the starting material on thebasis of mineral composition and distribution features. The reaction rim of kyanites can be used toindicate the fluid mobility in the system. Rutiles are relatively stable and show no signs of partialmelting.3. Variation of melt composition of partial meltingUnder 1.5 GPa and 800℃, 2.0 GPa and 850℃, 2.4 GPa and 850℃, and 3.0 GPa and 950 ℃, about～3% melt was formed in the system, which occurred surrounding the hydrous minerals,or distributed as patch at the contact zone among different minerals, suggesting that the formationof melt is related with dehydration decomposition of hydrous minerals and melting reaction ofadjacent minerals, such as omphacite and quartz, etc.At pressures of 1.5-3.0 GPa and temperatures of 850-950℃, the initial melt contain67.02 %-74.76% SiO₂, 0.56 %-2.22% TiO₂+FeO^*+MgO, 0.22%-3.44% CaO, and 4.04%-8.27% Na₂O+K₂O. In the standard Ab-An-Or diagram, melt was projected into trondhjemite field.A high melt fraction（30%） of melt was obtained under 1.5-2.0 GPa and 1000℃, however, themelt fraction is relatively low at a pressure of 2.4 GPa. The melt has SiO₂ of 68.48 %-71.08%,TiO₂+FeO^*+MgO ranging from 0.62% at 2.4 GPa to 3.99 % at 1.5 GPa, CaO varying between0.25% at 2.4 GPa and 2.37% at 1.5 GPa, and Na₂O+K₂O ranging from 0.70% at 1.5 GPa to 2.03%at 2.4 GPa.The pressure and temperature both has a strong influence on the contents of SiO₂, TiO₂+FeO^*+MgO, CaO, and Na₂O+K₂O in the melt. That is, the contents of SiO₂ and Na₂O+K₂O will increase,while those of TiO₂+FeO^*+MgO and CaO will decrease with increasing pressure underthe same temperature. At pressures≥2.4 GPa, the contents of Na₂O significantly decrease withincreasing pressure, while K₂O increase with increasing pressure. On the other hand, the contentsof SiO₂, TiO₂+FeO^*+ MgO, and CaO increase, while those of Al₂O₃ and Na₂O+K₂O decrease withincreasing temperature under the same pressure, suggesting melting of different minerals,including quartz, rutile, omphacite, phengite and zoisite, all contributes to the variation of meltcomposition but to a different degree.The trace elements of melt at pressures of 1.5-2.0 GPa and temperature of 1000℃showhigh Sr, low Y and Yb, high Sr/Y and La/Yb ratio, and negative Nb-Ta anomaly. The melt is rich inlight rare earth elements（LREE）, depleted in high rare earth elements（HREE）, and has highLa/Yb ratio, and positive Eu anomaly. These features suggest that the melt and residual mineralassociation has characteristics consistent with those of adakitic rocks, and low-Mg adakitic magmacan be formed by partial melting of eclogites.4. Implications for partial melting of eclogites during continental collisionOur experiments show that phengite will dehydrate at T≤800-850℃under pressures of1.5-2.0 GPa. The dehydration temperature of phengite will increase gradually with increasingpressure, suggesting that phengite phase boundary has a positive dP/dT slope over the pressurerange of 1.5-3.0 GPa. The dehydration breakdown temperature of phengite is about 50℃higherthan that of hydrous synthetic system and 50℃lower than that of intermediate to acid rocks.Considering the P-T path for the hot exhumation of UHP eclogites, it is concluded that the mostfavorable P-T conditions at which UHP eclogites would experience dehydration melting is 800-850℃and 1.5-2.0 GPa. This coincides with the P-T conditions for transformation from quartzeclogite phase to amphibolite phase, suggesting that local partial melting or migmatization of UHProcks could occur by dehydration breakdown of hydrous minerals even without presence ofexternal fluid. This study shows that dehydration breakdown of phengite will result in melt with high potassiumcontent under pressures≥2.4 GPa and temperatures≥850℃. In addition, aqueous fluid isprone to enter melt so that the whole system will become fluid-undersaturated. These conditionswill all promote the formation of potassium feldspar in eclogites. Therefore, the occurrence ofpotassium feldspar in UHP eclogites is a manifestation of dehydration melting of phengite, indicatingthat the reaction system is under fluid-undersaturated condition. It was implied that the UHPeclogites experienced local melting under conditions of coesite eclogite phase to quartz eclogitephase. Combining experimental results under pressures of 1.5-2.0 GPa and 800-850℃, theplagioclase reaction rim around kyanite and feldspathic vein may be a manifestation of partialmelting of eclogites with the presence of aqueous fluid.In comparison with adakites emplaced during post-collisional orogen（e. g., Tiantangzhaibody in eastern Dabie）, the melt is analogous in composition to the early Cretaceous high-K,low-Mg adakitic rocks from Dabie, suggesting that these adakitic magma was formed at a depth＞50 km. The low density melt formed by partial melting of UHP eclogites will lead to mechanicalweakening of rocks and modification of dominant deformation mechanism. Under the conditionthat eclogites were melted to a high degree, the density of reaction residue（garnet and omphacite）will be larger than that of normal eclogites. Thus, the high density residue tends to separate frommelt, which will promote the delamination of thickened lower crust in orogens.更多还原