Low-Pressure Metamorphism of Granulite Facies in an Early Proterozoic Orogenic Event in Central Inner Mongolia

According to the kinds of feldspar and rock associations in the Ai-rich gneisses, the low-pressure metamorphic crust of the Early Proterozoic granulite facies in central Inner Mongolia can be divided into southern and northern belts which are composed of six rock associations. They represent the relevant rock sequences of the layered metamorphic rock series formed under specific metamorphic temperature and pressure conditions as well as tectonic environments. Mineral inclusions and reaction texture have recorded that the medium-temperature high-pressure mineral assemblages are replaced by the high-temperature low-pressure mineral assemblages, thus, giving rise to: garnet+quartz→ hypersthene+plagioclase; kyanite→sillimanite and garnet+kyanite/sillimanite+quartz→cordierite. The deformation fabrics of the rocks, the change of mineral assemblages and the PTt path of metamorphism indicate that the contempranceous high-temperature normal-slip ductile shearing is the main cause of the formation of the low-pressure metamorphic crust of granulite facies. In the orogenic event, the co-action of thrusting and extension resulted in the change of a medium-temperature high-pressure metamorphic environment into the high-temperature low-pressure metamorphic conditions.     

Genetic Mechanism of Mineral Inclusions in Zircons from the Khondalite Series, Southeastern Inner Mongolia

The early Precambrian khondalite series is widely distributed in the Jining-Zhuozi-Fengzhen-Liangcheng area, southeastern Inner Mongolia. The khondalite series mainly consists of sillimanite garnet potash feldspar (or two-feldspar) gneiss and garnet biotite plagioclase gneiss. These gneissic rocks have commonly experienced granulite-facies metamorphism. In zircons separated from sillimanite garnet potash feldspar gneisses, many mineral inclusions, including Sil, Grt, Ky, Kfs, Qtz and Ap, have been identified by the Laser Raman spectroscopy. Generally, prograde metamorphic mineral inclusion assemblages such as Ky + Kfs + Qtz + Ap and Ky + Grt + Kfs + Qtz are preserved in the core of zircon, while peak granulite-facies metamorphic minerals including Sil + Grt + Kfs + Qtz and Sil + Grt + Kfs + Qtz + Ap are identified in the mantle and rim of the same zircon. However, in some zircons are only preserved the peak metamorphic minerals such as Sil + Grt + Kfs + Qtz and Sil + Grt + Kfs + Qtz + Ap from core to ri     

Trace Elements of Multi-stage Minerals and Titanite U-Pb Dating for the Gneisses from Liansan Island, Sulu UHPM Belt

Gneisses with anatectic characteristics from the Liansan island in the Sulu UHPM (ultra-high pressure metamorphic) belt were studied for petrography, titanite U-Pb dating and mineral geochemistry. Three origins of garnets are distinguished: metamorphic garnet, peritectic garnet and anatectic garnet, which are formed in the stages of peak metamorphism, retrograde anatexis and melt crystallization, respectively. The euhedral titanite has a high content of REE and high Th/U ratios, which is interpreted as indicating that it was newly-formed from an anatectic melt. The LA-ICP-MS titanite U-Pb dating yields 214–217 Ma ages for the titanite (melt) crystallization. The distribution of trace elements varies in response to the different host minerals at different stages. At the peak metamorphic stage, Y and HREE are mainly hosted by garnet, Ba and Rb by phengite, Sr, Nb, Ta, Pb, Th, U and LREE by allanite and Y, U and HREE by zircon. During partial melting, Y, Pb, Th, U and REE are released into the melt, which causes a dramatic decline of these element contents in the retrograde minerals. Finally, titanite absorbs most of the Nb, U, LREE and HREE from the melt. Therefore, the different stages of metamorphism have different mineral assemblages, which host different trace elements.     

Pressure-temperature Evolution of the Metapelites in the Motuo Area,the Eastern Himalayan Syntaxis

The Motuo area is located in the east of the Eastern Himalayan Syntaxis. There outcrops a sequence of high-grade metamorphic rocks, such as metapelites. Petrology and mineralogy data suggest that these rocks have experienced three stages of metamorphism. The prograde metamorphic mineral assemblages(M1) are mineral inclusions(biotite + plagioclase + quartz ± sillimanite ± Fe-Ti oxides) preserved in garnet porphyroblasts, and the peak metamorphic assemblages(M2) are represented by garnet with the lowest XSps values and the lowest XFe# ratios and the matrix minerals(plagioclase + quartz ± Kfeldspar + biotite + muscovite + kyanite ± sillimanite), whereas the retrograde assemblages(M3) are composed of biotite + plagioclase + quartz symplectites rimming the garnet porphyroblasts. Thermobarometric computation shows that the metamorphic conditions are 562–714°C at 7.3–7.4 kbar for the M1 stage, 661–800°C at 9.4–11.6 kbar for the M2 stage, and 579–713°C at 5.5–6.6 kbar for the M3 stage. These rocks are deciphered to have undergone metamorphism characterized by clockwise P-T paths involving nearly isothermal decompression(ITD) segments, which is inferred to be related to the collision of the India and Eurasia plates.     

Petrology of the Non-mafic UHP Metamorphic Rocks from a Drillhole in the Southern Sulu Orogenic Belt, Eastern-Central China

The Drillhole ZK703 with a depth of 558 m is located in the Donghai area of the southern Sulu ultrahigh-pressure (UHP) metamorphic belt, eastern China, and penetrates typical UHP eclogites and various non-mafic rocks, including peridotite, gneiss, schist and quartzite. Their protoliths include ultramafic, mafic, intermediate, intermediate-acidic, acidic igneous rocks and sediments. These rocks are intimately interlayered, which are meters to millimeters thick with sharp and nontectonic contacts, suggesting in-situ metamorphism under UHP eclogite facies conditions. The following petrologic features indicate that the non-mafic rocks have experienced early-stage UHP metamorphism together with the eclogites: (1) phengite relics in gneisses and schists contain a high content of Si, up to 3.52 p.f.u. (per formula unit), while amphibolite-facies phengites have considerably low Si content (<3.26 p.f.u.); (2) jadeite relics are found in quartzite and jadeitite; (3) various types of symplectitic coronas and pseud     

Mineral Composition and Metamorphic P-T Conditions of the Eclogitic Rocks in the Eastern Dabie Mountains and Their Genetic Implication

There are obvious differences in the mineral assemblage and metamorphic P-T conditions between the eclogites from the northern and southern parts of the eastern Dabie Mountains. Those from the northern part of the mountains are developed in Alpine peridotite and gneiss. They have a mineral assemblage of garnet+diopside with no quartz, and were formed at temperatures of 600℃-740℃. Those from the southern part are developed in gneiss and marble. They consist of garnet+omphacite+less quartz and were metamorphosed at temperatures in the range of 650°-800℃. These differences suggest that the former may be formed during the metamorphism of the deep subducted oceanic crust, whereas the latter may be genetically related to the subduction of the continental crust in this area.     

P-T Paths Derived from Garnet Growth Zoning in Danba Domal Metamorphic Terrain, Sichuan Province, West China

Danba (丹巴) domal metamorphic terrain belongs to Songpan (松潘)-Ganze (甘孜) orogenic belt, where typical Barrovian and Buchan metamorphic zones are preserved. The former included chlorite, biotite, garnet, staurolite, kyanite and sillimanite zones, while the latter only developed silimanite+muscovite and sillimanite+K-feldspar zones. Integrated study has been carried on metamorphic reactions of garnet production and consumption, P-T paths and P-T-X-M phase relation and thermal tectonic model for Danba metamorphic zones. Petrological textures in thin sections show that garnet production and consumption in kyanite-sillimanite zone is mainly attributed to ChI+Ms+PI+Q=Grt+Bt+H2O and kyanite=sillimanite respectively. Based on mineral compositions, the geothermobarometry gives an average P, T condition of (4.9±0.3)×108 Pa, 543±30℃ for the first growth stage of the garnet and (5.8±0.3)±108 Pa, 534±29 ℃ for the second stage of garnet growth respectively. Anti-counter clockwise P-T paths were drawn using Gibbs method by NCMnKFMASH system for sample G98686 in the kyanite zone. The P-T-X-M modeling for the first mineral assemblages shows that the prediction is similar to the measured values in gossular, almandine and spessartine but mole fraction of pyrope and Fe/(Fe+Mg) deviated far from the contours; while that for the second mineral assemblages exhibits that the prediction is consistent with the measured value of pyrope, grossular content and Fe/(Fe+Mg) of garnet. A thermal tectonic model that there are at least three structure levels across the thrnst-decollement zones is presented according to the P-T paths, metamorphic grades and deformation styles for the staurolite-kyanite zone of the Barroviau type metamorphism, which will provide some constraints for the evolution of the nappe complex.     

Differential Evolution of High-Pressure and Ultrahigh-Pressure Metapelites from Habutengsu,Chinese Western Tianshan: Phase Equilibria Modelling and 40Ar/39Ar Geochronology

Metapelite is one of the predominant rock types in the high-pressure–ultrahigh-pressure(HP–UHP) metamorphic belt of western Tianshan, NW China; however, the spatial and temporal variations of this belt during metamorphism are poorly understood. In this study, we present comparative petrological studies and 40Ar/39 Ar geochronology of HP and UHP pelitic schist exposed along the Habutengsu valley. The schist mainly comprises quartz, white mica, garnet, albite and bluish amphibole. In the Mn O–Na2O–Ca O–K2O–Fe O–Mg O–Al2O3–Si O2–H2O(Mn NCKFMASH) system, P–T pseudosections were constructed using THERMOCALC 333 for two representative pelitic schists. The results demonstrate that there was a break in the peak metamorphic pressures in the Habutengsu area. The northern schist has experienced UHP metamorphism, consistent with the presence of coesite in the same section, while the southern one formed at lower pressures that stabilized the quartz. This result supports the previous finding of a metamorphic gradient through the HP–UHP metamorphic belt of the Chinese western Tianshan by the authors. Additionally, phengite in the northern schist was modelled as having a Si content of 3.55–3.70(a.p.f.u.) at the peak stage, a value much higher than that of oriented matrix phengite(Si content 3.32–3.38 a.p.f.u.). This indicates that the phengite flakes in the UHP schist were subjected to recrystallization during exhumation, which is consistent with the presence of phengite aggregates surrounding garnet porphyroblast. The 40Ar/39 Ar age spectra of white mica(dominantly phengite) from the two schists exhibit similar plateau ages of ca. 315 Ma, which is interpreted as the timing of a tectonometamorphic event that occurred during the exhumation of the HP–UHP metamorphic belt of the Chinese western Tianshan.     

Metamorphic Evolution and Tectonic Implications of Garnet–Bearing Mica Schist in Sumdo High Pressure Belt from Eastern Lhasa Block,Tibet

<正>The garnet–bearing mica–quartz schist of the Sumdo high pressure metamorphic beltfrom the Lhasa block is mainly composed of garnet,muscovite,albite,quartz and minor chlorite,rutile and sphene.Garnet displays obvious zonation where Xpy[=Mg/(Mg+Fe+Mn+Ca)]increases from the core to the mantle,and then decreases in the rim;     

A Petrographic and Mineralogical Study of Volcanic Rocks from the Mayaxueshan Area, North Qilian Fold Belt, NW China

The Ordovician volcanic rocks in the Mayaxueshan area have been pervasively altered or metamorphosed and contain abundant secondary minerals such as albite, chlorite, epidote, prehnite, pumpellyite, actinolite, titanite, quartz, and/or calcite. They were denoted as spilites or spilitic rocks in terms of their petrographic features and mineral assemblages. The metamorphic grades of the volcanic rocks are equivalent to that of the intercalated metaclastic rocks. This indicates that both the spilitic volcanic rocks and metaclastic rocks in the Mayaxueshan area have formed as a result of Caledonian regional metamorphism. We suggest that the previously denoted spilitic rocks or altered volcanic rocks should be re-denoted as metabasalts or metabasaltic rocks. The metamorphic grade of the volcanic rocks increases with their age: prehnite-pumpellyite facies for the upper part of the Middle Ordovician volcanic rocks, prehnite-pumpeilyite to lower greenschist facies for the lower part of the Middle Ordovician vol     

Calculation of the Model of Coesite Inclusions and Analysis of Their Retrometamorphic Paths

The process and path of retrometamorphism of coesite have great significance to our understanding of the P-T tracks of the exhumation of ultrahigh-pressure metamorphic rocks. Most of the coesites in the eclogite from Shima, Anhui Province, the Dabie Mountains, China, are found degraded to quartz partly or wholly, with ruptures occurring in the shells, outside which include the coesite and quartz. According to the microscopic observation, the sample of coesite inclusion is composed of garnet, quartz and coesite, based on which we have built a three-shelled composite sphere model to compute the transition of coesite. Based on the crystal growth formulas and pressure conditions of the ruptures in the garnet, we have calculated the radius of the quartz sphere, which depends on temperature, and eventually drawn the different retrometamorphic paths for different retrometamorphism rates.     

Phase Equilibria of Hornblende-Bearing Eclogite in the Western Dabie Mountain,Central China

The high-pressure (HP) eclogite in the western Dabie Mountain encloses numerous hornblendes,mostly barroisite.Opinions on the peak metamorphic P-T condition,PT path and mineral paragenesis of it are still in dispute.Generally,HP eclogite involves garnet,omphacite, hornblendes and quartz,with or without glaucophane,zoisite and phengite.The garnet has compositional zoning with X_(Mg) increase,X_(Ca) and X_(Mn) decrease from core to rim,which indicates a progressive metamorphism.The phase equilibria of the ...     

Petrochemical Comparison of 3.5 Ga Old Mafic Amphibolite Inclusions from Eastern Hebei Province with Archean Mafic—ultramafic Supracrustals of Uncertain Antiquity,Southern Jilin Eastern Liaoning Prov

New major element data for coexisting minerals and bulk-rock REE analyses,combined with previously obtained petrologic/geologic information,allow comparison of Archean mafic-ultramafic meta-igneous rocks from adjacent portions of northeastern Sino-Korean shield .Isotope data published by previous workers document an Early Archean age of formation for metabasaltic rocks now occurring as mafic amphibolite inclusions in tonalitic gneisses west of Bohai Bay(area).Mafic-ultramafic mineral assemblages developed in tonalitic gneisses west of Bohai Bay(areaA).Mafic-ultramafic mineral assemblages developed in amphibolite facies Archean supracrustals east of Bohai Bay(areaB)are chemically similar ,but pre-existing phase associations have been overprinted by the effects of several later metamorphic events.Non0peridotitic area B protoliths appear to be more magnesian ,averaging-12wt.%MgO(versus-7wt.%for area A),and are slightly less fractionated in REE concentrations relative to chondrites(La-Lu=30-8X versus 37-14X for area A),thus perhaps being more mantle-like than area A metabasaltic analogues.The basaltic-komatiitic series east of Bohai Bay probably represents the most ancient crust of that region,being invaded by Archean tonalities,which were subsequently converted to orthogneisses.These observations are compatible with the hypothesis that ,similar to area A metabasaltic rocks now found as amphibolitic inclusions in orthogneisses,are B mafic supracrustals were formed during Early of Middle Archean time ,prior to widespread isofacial amphibolite facies metamorphism(600-700℃,5-7kb,low αH2O).     

Study on the Physical and Chemical Conditions of Ore Formation of Hetai Ductile Shear Zone—Hosted Gold Deposit and Discovery of Melt Inclusions

The Hetai ductile shear zone-hosted gold deposit occurs in the deep-seated fault mylonite zone of the Sinian-Silurian metamorphic rock series. In this study there have been discovered melt inclusions, fluid-melt inclusions and organic inclusions in ore-bearing quartz veins of the ore deposit and mylonite for the first time. The homogenization temperatures of the various types of inclusions are 160℃, 180 - 350℃, 530℃ and 870℃ for organic inclusions, liquid inclusions, two-phase immiscible liquid inclusions and melt inclusions, respectively. Ore fluid is categorized as the neutral to basic K+ -Ca2+ -Mg2+ -Na+ - SO2- 4-HCO3-Cl- system. The contents of trace gases follow a descending order of H2O＞CO2＞CH4＞(or ＜ ) H2＞CO＞C2H2＞C2I-I6＞O2＞N2.The concentrations of K , Ca2 + ,SO2-4,HCO3-,Cl- H2O and C2H2 in fluid inclusions are related to the contents of gold and the Au/Ag ratios in ores from different levels of the gold deposit. This is significant for deep ore prospecting in the region. Daughter minerals in melt inclusions were analyzed using SEM. Quartz, orthoclase, wollastonite and other silicate minerals were identified. They were formed in different mineral assemblages.This analysis further proves the existence of melt inclusions in ore veins. Sedimentary metamorphic rocks could form silicate melts during metamorphic anatexis and dynamic metamorphism, which possess melt-solution characteristics. Ore formation is related to the multi-stage forming process of silicate melt and fluid.     

The Microarea Composition of Garnet and Biotite from the Ji‘an Group in Tonghua District and Its Significance as an Indicator of Metamorphic P—T—t Path

The garnet(Grt)and biotite(Bt) from gneisses of the Ji‘an Group are characterized by diffusion zoning at the rim,but equilibrium composition of metamorphic peak is usually remained in extensive interior area.Garnet with growth zoning is also found in the kyanite zone.In the light of microarea compositional variation of Grt and Bt,the temperature and pressure at the progressive,peak and post-peak metamorphic stages are determined by correctly using Grt Bt thermometer and GASP barometer.On this basis,a counterclockwise P-T-t path can be constructed,which reflects the closing process of an ensialic rift belt in this region during the Early Proterozoic.     

Zoned Zircon from Eclogite Lenses in Marbles from the Dabie-Sulu UHP Terrane, China： A Clear Record of Ultra-deep Subduction and Fast Exhumation

Eclogite lenses in marbles from the Dabie-Sulu ultrahigh-pressure (UHP) terrane are deeply subducted meta-sedimentary rocks. Zircons in these rocks have been used to constrain the ages of prograde and UHP metamorphism during subduction, and later retrograde metamorphism during exhumation. Inherited (detrital) and metamorphic zircons were distinguished on the basis of transmitted light microscopy, cathodoluminescence (CL) imaging, trace element contents and mineral inclusions. The distribution of mineral inclusions combined with CL imaging of the metamorphic zircon make it possible to relate zircon zones (domains) to different metamorphic stages. Domain 1 consists of rounded, oblong and spindly cores with dark-luminescent images, and contains quartz eclogite facies mineral inclusion assemblages, indicating formation under high-pressure (HP) metamorphic conditions of T = 571-668℃and P = 1.7-2.02 GPa. Domain 2 always surrounds domain 1 or occurs as rounded and spindly cores with white-luminescent images. It contains coesite edogite facies mineral inclusion assemblages, indicating formation under UHP metamorphic conditions of T = 782-849℃and P > 5.5 GPa. Domain 3, with gray-luminescent images, always surrounds domain 2 and occurs as the outermost zircon rim. It is characterized by low-pressure mineral inclusion assemblages, which are related to regional amphibolite facies retrograde metamorphism of T = 600-710℃and P = 0.7-1.2 GPa. The three metamorphic zircon domains have distinct ages; sample H1 from the Dabie terrane yielded SHRIMP ages of 245±4 Ma for domain 1, 235±3 Ma for domain 2 and 215±6 Ma for domain 3, whereas sample H2 from the Sulu terrane yielded similar ages of 244±4 Ma, 233±4 Ma and 214±5 Ma for Domains 1, 2 and 3, respectively. The mean ages of these zones suggest that subduction to UHP depths took place over 10-11 Ma and exhumation of the rocks occurred over a period of 19-20 Ma. Thus, subduction from～55 km to > 160 km deep mantle depth took place at rates of approximately 9.5-10.5 km/Ma and exhumation from depths >160 km to the base of the crust at～30 km occurred at approximately 6.5 km/Ma. We propose a model for these rocks involving deep subduction of continental margin lithosphere followed by ultrafast exhumation driven by buoyancy forces after break-off of the UHP slab deep within the mantle.     

Phase Equilibria of Hornblende‐Bearing Eclogite in the Western Dabie Mountain,Central China

The high-pressure (HP) eclogite in the western Dabie Mountain encloses numerous hornblendes, mostly barroisite. Opinions on the peak metamorphic P-T condition, PT path and mineral paragenesis of it are still in dispute. Generally, HP eclogite involves garnet, omphacite, hornblendes and quartz, with or without glaucophane, zoisite and phengite. The garnet has compositional zoning with XMg increase, XCa and XMn decrease from core to rim, which indicates a progressive metamorphism. The phase equilibria of the HP eclogite modeled by the P-T pseudosection method developed recently showed the following: (1) the growth zonation of garnet records a progressive metamorphic PT path from pre-peak condition of 1.9–2.1 GPa at 508°C–514°C to a peak one of 2.3–2.5 GPa at 528°C–531°C for the HP eclogite; (2) the peak mineral assemblage is garnet+omphacite+glaucophane+quartz±phengite, likely paragenetic with lawsonite; (3) the extensive hornblendes derive mainly from glaucophane, partial omphacite and even a little garnet due to the decompression with some heating during the post-peak stage, mostly representing the conditions of about 1.4–1.6 GPa and 580°C–640°C, and their growth is favored by the dehydration of lawsonite into zoisite or epidote, but most of the garnet, omphacite or phengite in the HP eclogite still preserve their compositions at peak condition, and they are not obviously equilibrious with the hornblendes.     

Relic Oceanic Crust at Sub-Arc Depth: An Example from UHP Eclogites Enclosed in Serpentinites from the Southwestern Tianshan, China

Ultra-high pressure(UHP)eclogites that derive from subducted oceanic crust are rarely found at the Earth’s surface because they need to be enclosed in a buoyant host rock such as serpentinites that facilitate exhumation(Hermann et al.,2000;Guillot et al.,2001).Under normal subduction geotherms,serpentinites break down just before UHP conditions are reached and therefore most of the exhumed eclogites representing subducted oceanic crust formed under fore-arc conditions.We investigated eclogite blocks enclosed into serpentinites that occur in the southwestern Tianshan oceanic subduction,China.A previous study proved that the serpentinites derive from altered oceanic crust and experienced UHP metamorphism at low temperatures of 510-530°C(Shen et al.,2015).Three relatively fresh eclogite samples were studied in detail.Sample 129-7 shows the retrograde mineral assemblage of amphibole+biotite+albite+chlorite+minor titanite and peak metamorphic relics of omphacite+garnet±chlorite.Sample C107-23 is mainly composed of amphibole+albite+chlorite+zoisite+muscovite+minor titanite as a retrograde assemblage and garnet+phengite as the peak metamorphic relics with omphacite only found as inclusions in garnet.Similar to sample C107-23,sample C11066 preserves large-grained euhedral to subhedral garnet relics with omphacite inclusions,and epidote,diopside,amphibole,muscovite,chlorite,albite and biotite are in the matrix belong to the retrograde assemblage.These three retrograde eclogite samples were modelled using thermodynamic calculations in the Mn NCKFMSHO(Mn O-Na_2O-Ca O-K_2O-FeO-Mg O-Al_2O_3-SiO_2-H_2O-Fe_2O_3)system.Based on the peak assemblage of omphacite+garnet and the crossing of the grossular and pyrope isopleths in garnet,peak P-T conditions of~460-470oC,28-29 kbar(129-7),450-500oC,28-35 kbar(C107-23),~475-505oC,26-29 kbar(C11066)were calculated.The retrograde assemblages indicate near isothermal decompression resulting in a clockwise P-T evolution of these eclogites.The peak metamorphic pressures at 500°C are well within UHP conditions(coesite stability field)and are within error the same as peak conditions of the host serpentinites(Shen et al.,2015).This provides evidence that eclogites and serpentinites shared the same evolution.We infer that the subducted low-density serpentinites were assembled with the high-density eclogites during subdution and helped the latter to exhume back to the surface.The studied eclogites thus represent rare examples of relics of oceanic crust that was subducted to sub-arc depth.     

P-T-t Path of Mafic Granulite Metamorphism in Northern Tibet and Its Geodynamical Implications

Mafic granulites have been found as structural lenses within the huge thrust system outcropping about 10 km west of Nam Co of the northern Lhasa Terrane, Tibetan Plateau. Petrological evidence from these rocks indicates four distinct metamorphic assemblages. The early metamorphic assemblage (M1) is preserved only in the granulites and represented by plagioclase+hornblende inclusions within the cores of garnet porphyroblasts. The peak assemblage (M2) consists of garnet+clinopyroxene+hornblende+plagioclase in the mafic granulites. The peak metamorphism was followed by near-isothermal decompression (M3), which resulted in the development of hornblende+plagioclase symplectites surrounding embayed garnet porphyroblasts, and decompression-cooling (M4) is represented by minerals of hornblende+plagioclase recrystallized during mylonization. The peak (M2) P-T conditions of garnet+ clinopyroxene+plagioclase+hornblende were estimated at 769-905℃ and 0.86-1.02 GPa based on the geothermometers and geobarometers. The     

Subduction channel fluid-rock interaction:Indications from rutile-quartz veins within eclogite from the Yuka terrane,North Qaidam orogen

High-pressure(HP)or ultrahigh-pressure(UHP)rutile-quartz veins that form at mantle depths due to fluid-rock interaction can be used to trace the properties and behavior of natural fluids in subduction zones.To explore the fluid flow and the associated element mobility during deep subduction and exhumation of the continental crust,we investigated the major and trace elements of Ti-rich minerals.Additionally,U–Pb dating,trace element contents,and Lu–Hf isotopic composition of zircon grains in the UHP eclogite and associated rutile-quartz veins were examined in the North Qaidam UHP metamorphic belt,Yuka terrane.The zircon grains in the rutile-quartz veins have unzoned or weak oscillatory zonings,and show low Th/U ratios,steep chondrite-normalized patterns of heavy rare earth elements(HREEs),and insignificant negative Eu anomalies,indicating their growth in metamorphic fluids.These zircon grains formed in 431
3 Ma,which is consistent with the 432
2 Ma age of the host eclogite.As for the zircons in the rutile-quartz veins,they showed steep HREE patterns on one hand,and were different from the zircons present in the host eclogite on the other.This demonstrates that their formation might have been related to the breakdown of the early stage of garnet,which corresponds to the abundance of fluids during the early exhumation stage.The core-rim profile analyses of rutile recorded a two-stage rutile growth across a large rutile grain;the rutile core has higher Nb,Ta,W,and Zr contents and lower Nb/Ta ratios than the rim,indicating that the rutile domains grew in different metamorphic fluids from the core towards the rim.The significant enrichment of high field strength elements(HFSEs)in the rutile core suggests that the peak fluids have high solubility and transportation capacity of these HFSEs.Furthermore,variations in the Nb vs.Cr trends in rutile indicate a connection of rutile to mafic protolith.The zircon grains from both the rutile-quartz veins and the host eclogite have similar Hf isotopic compositions,indicating that the vein-forming fluids are internally derived from the host eclogite.These fluids accumulated in the subduction channel and were triggered by local dehydration of the deeply subducted eclogite during the early exhumation conditions.