Dehydration Partial Melting Experiments On UHP Eclogites And Its Dynamic Significance, Dabie-Sulu Terrane

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-â… .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-â…¡+Ky-â… +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-â…¡+Gt-â… +Ky-â… +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-â… +Kfs+Ky-â… +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.