When plate tectonics began on the Earth has been long debated and here we argue this topic based on the records of Earth-Moon geology and asteroid belt to conclude that the onset of plate tectonics was during the midd...When plate tectonics began on the Earth has been long debated and here we argue this topic based on the records of Earth-Moon geology and asteroid belt to conclude that the onset of plate tectonics was during the middle Hadean（4.37-4.20 Ga）. The trigger of the initiation of plate tectonics is the ABEL Bombardment, which delivered oceanic and atmospheric components on a completely dry reductive Earth, originally comprised of enstatite chondrite-like materials. Through the accretion of volatiles, shock metamorphism processed with vaporization of both CI chondrite and supracrustal rocks at the bombarded location, and significant recrystallization went through under wet conditions, caused considerable eclogitization in the primordial continents composed of felsic upper crust of 21 km thick anorthosite, and 50 km or even thicker KREEP lower crust. Eclogitization must have yielded a powerful slab-pull force to initiate plate tectonics in the middle Hadean. Another important factor is the size of the bombardment. By creating Pacific Ocean class crater by 1000 km across impactor, rigid plate operating stagnant lid tectonics since the early Hadean was severely destroyed, and oceanic lithosphere was generated to have bi-modal lithosphere on the Earth to enable the operation of plate tectonics.Considering the importance of the ABEL Bombardment event which initiated plate tectonics including the appearance of ocean and atmosphere, we propose that the Hadean Eon can be subdivided into three periods：（1） early Hadean（4.57-4.37 Ga）,（2） middle Hadean（4.37-4.20 Ga）, and（3） late Hadean（4.20-4.00 Ga）.展开更多
2D thermo-mechanical models are constructed to investigate the effects of oceanic and continental crustal eclogitization on plate dynamics at three successive stages of oceanic subduction, slab breakoff, and continent...2D thermo-mechanical models are constructed to investigate the effects of oceanic and continental crustal eclogitization on plate dynamics at three successive stages of oceanic subduction, slab breakoff, and continental subduction. Crustal eclogitization directly increases the average slab density and accordingly the slab pull force, which makes the slab subduct deeply and steeply. Numerical results demonstrate that the duration time from initial continental collision to slab breakoff largely depends on the slab pull force. Specifically, eclogitization of subducted crust can greatly decrease the duration time, but increase the breakoff depth. The detachment of oceanic slab from the pro-continental lithosphere is accompanied with obvious exhumation of the subducted continental crust and a sharp uplift of the collision zone in response to the disappearance of downward drag force and the induced asthenospheric upwelling, especially under the condition of no or incomplete crustal eclogitization. During continental subduction, the slab dip angle is strongly correlated with eclogitization of subducted continental lower crust, which regulates the slab buoyancy nature. Our model results can provide several important implications for the Himalayan-Tibetan collision zone. For example, it is possible that the lateral variations in the degree of eclogitization of the subducted Indian crust might to some extent contribute to the lateral variations of subduction angle along the Himalayan orogenic belt. Moreover, the accumulation of highly radiogenic sediments and upper continental crustal materials at the active margin in combination with the strong shear heating due to continuous continental subduction together cause rising of isotherms in the accretionary wedge, which facilitate the development of crustal partial melting and metamorphism.展开更多
The characteristic textures of eclogitic veins infilling the interstitial spaces among zoisite-glaucophanite pillows, the fractures crosscutting massive zoisite-glaucophanites and the relic conduits within the eclogit...The characteristic textures of eclogitic veins infilling the interstitial spaces among zoisite-glaucophanite pillows, the fractures crosscutting massive zoisite-glaucophanites and the relic conduits within the eclogite boudins demonstrate that the eclogitization of glaucophanites was accompanied by the presence of a free fluid phase in the Western Tianshan high-pressure metamorphic belt. Non-planar populations of fluid inclusions, parallel to C-axis, have been preserved within the omphacites in eclogitic veins. The microthermometric results indicate that fluids are low-salinity (1-4 wt%NaCI) aqueous fluids. The chemical compositions of major, trace and rare earth elements of eclogitic veins and related host rocks indicate that the eclogitization of glaucophanites required the addition of external Si and Na elements, and was accompanied by the loss of large-ion lithophile elements, rare earth elements and water. The record of eclogitization of glaucophanites along fractures and zones of high permeability by fluid infiltration has been preserved in the Wetsern Tianshan high-pressure metamorphic belt.展开更多
Phase equilibrium modeling using internally consistent thermodynamic dataset and associated activity-composition(a-x)models are very helpful for quantifying P-T evolution for eclogite,which is the basis for decipherin...Phase equilibrium modeling using internally consistent thermodynamic dataset and associated activity-composition(a-x)models are very helpful for quantifying P-T evolution for eclogite,which is the basis for deciphering the geodynamic processes in subduction zones.In this study,we apply different versions of datasets(ds55 and ds62)and associated a-x relations to a wellestablished LT-HP eclogite at Huwan in the classic western Dabie orogen to constrain its P-T evolution.The eclogite comprises garnet+omphacite+amphibole+white mica+epidote+quartz+chlorite+rutile/ilmenite/sphene.Garnet porphyroblasts show mono-variation in the end members(spessartine from 17 mol%to 0,pyrope from 2 mol%to 18 mol%,almandine from 47 mol%to 64mol%and grossular from 35 mol%to 18 mol%)from core to rim.Phase diagrams combined with compositional isopleth thermobarometry show that dataset ds62 and associated a-x relations yield P_(max)of~33 kbar at~560℃,conflicting with our petrological observations and previous studies.On the other hand,phase equilibrium modeling using dataset ds62 and a revised symmetric garnet a-x model irrespective of Fe^(3+)(O)gives P_(max)of~27 kbar at~560℃,consistent with the results using dataset ds55 and associated a-x relations.Therefore,we recommend a symmetric model for garnet involving py,alm,gr and spss,without Fe~(3+)components,instead of the asymmetric garnet a-x relations involving py,alm,gr,spss and kho by White et al.(2014),for calculating phase diagrams for LT-(U)HP eclogite when using dataset ds62.In this study,the defined P-T path is characterized by a segment of the prograde evolution showing a first moderate slope,followed by gentle then steep slopes,representing the thermal structure evolution recorded by slab surface during continental subduction.Our work combined with previous studies conclude that in western Dabie,the Huwan HP eclogite belt to the north and the Hong'an HP eclogite belt to the south belong to the same HP slice overlying the Xinxian UHP slice.展开更多
In order to decipher element mobility in UHP meta-sedimentary rocks in the continental subduction zone,major and trace element compositions are investigated for a continuous profile from a representative UHP region in...In order to decipher element mobility in UHP meta-sedimentary rocks in the continental subduction zone,major and trace element compositions are investigated for a continuous profile from a representative UHP region in the Dabie Mountains.Among the lithologic contact zone,contents of K,Ca,LREE,and LILE exhibit varying degrees of downward trends in both marble and eclogite toward the contact zone,indicating that marble and their associated eclogite can release a large amount of K,Ca,and a small number of LILEs and LREEs.Titanite is the main Ti phase in both marble and eclogite.Titanite rims around rutile can occasionally be seen in eclogite.Contents of Ti and HFSE exhibit a well-coupled relation among marble and eclogite,indicating that substantial Ti and HFSEs were migrated from eclogite to marble,in accord with the capacity of a melt medium.Rutiles and titanites in marble exhibit a relatively limited variation in Nb/Ta ratios(12.9-16.2),similar to those of titanites in eclogite(14.2-16.7),which demonstrates that rutiles and titanites in marble were sourced from eclogite because of short-distance migrations of Ti and HFSEs.According to the P-T path and the temperature and pressure conditions of the peak metamorphism reported by previous studies,the eclogite associated with marble may not form supercritical fluids in the subduction zone because of the addition of carbonate minerals.展开更多
The Permo-Triassic amalgamation of Northeast Asia began to receive much attention after discovering HP and UHP eclogites in the Dabie and Sulu areas in China.However,the Permo-Triassic tectonic evolution of Northeast ...The Permo-Triassic amalgamation of Northeast Asia began to receive much attention after discovering HP and UHP eclogites in the Dabie and Sulu areas in China.However,the Permo-Triassic tectonic evolution of Northeast Asia is still under argument.Over the past 30 years,several critical geological discoveries were reported from the Korean Peninsula;1)The Permo-Triassic eclogite in the Hongseong area,2)the Triassic post-collisional igneous rocks in the northern Gyeonggi Massif,Imjingang Belt,and the southern Nangrim Massif,3)the Permo-Triassic intermediate-P/T metamorphism in the Imjingang and Okcheon Belts and the Gyeonggi Massif,4)the strongest Permo-Triassic metamorphism along the E-W trending Hongseong-Yangpyeong-Odaesan Belt,and 5)the Devonian back arc origin for the Imjingang Belt.Besides these,the Honam Shear Zone was confirmed to be initiated in the early Jurassic.These dates indicate that the Qinling-Dabie-Sulu Continental Collision Belt in China extends not into the Imjingang Belt but into the Hongseong-Odaesan Belt within the Korean Peninsula.The Hongseong-Odaesan Belt extends into the inner part of southwest Japan and then into the Dumangang Belt in the northeast Korean Peninsula.展开更多
Coesite was discovered as inclusions in zircon separates from pelitic gneissassociated with a large eclogite body in the North Qaidam ultrahigh-pressure (UHP) terrane. Somegraphite inclusions were also found. This fin...Coesite was discovered as inclusions in zircon separates from pelitic gneissassociated with a large eclogite body in the North Qaidam ultrahigh-pressure (UHP) terrane. Somegraphite inclusions were also found. This finding suggested the occurrence of in-situ UHPmetamorphism and that the terrane was most likely recrystallized at pressures below the diamondstability field. It supported other previous indirect UHP evidence, such as polycrystalline quartzinclusions in eclogitic garnet, quartz lamellae in omphacite and P-T estimates for both eclogite andgarnet peridotite. The U-Pb and Sm-Nd ages of the North Qaidam eclogite indicated that subductionof continental crust occurred in the Early Palaeozoic, which probably recorded a collision betweenthe Sino-Korean and Yangtze plates.展开更多
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 metamorphi...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 eclogite 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.展开更多
Recently, an over 100 km long MORB-type eclogite belt of Permian was discovered in the Sumdo （松多） region of the Lhasa block, Tibet. A critical question thus is： what is the tectonic setting of the eelogite belt a...Recently, an over 100 km long MORB-type eclogite belt of Permian was discovered in the Sumdo （松多） region of the Lhasa block, Tibet. A critical question thus is： what is the tectonic setting of the eelogite belt and is it related to an unrecognized suture in the region？ Further investigations show that there are some mafic and ultramafic rocks spacially associated with the eclogite belt in the region. Three ultramafic massifs were recognized in the Sumdo region, and called the Luomaling （罗马岭）, Gongbupala （贡布爬拉） and Qiazhasumdo （卡扎松多） massifs. All the massifs are fault-contacted with greenschist （Chasagang （岔萨岗） Formation） or muscovite-quartz schist （Mabuku （马布库） Formation）, and individuals are about 100 m×50 m in size extending in EW as the regional structure. All the ultramafic rocks have been entirely serpentinized, and the Gongbupala massif has been selected for study in geochemistry. Eleven chemical analyses of the rocks from the Gongbupala massif show a narrow range in contents： SiO2 （35.97-40.63） wt.%, MgO （37.02-38.60） wt.%, TiO2 （0.01-0.08） wt.%, Al2O3 （0.80-1.64） wt.%, （Na2O＋K2O） less than 0.1 wt.%, with high volatile contents （H2O＋CO2） （11.24-14.91） wt.%. After recalculation without H2O＋CO2, the mean values are SiO2 45.24 wt.%, MgO 43.54 wt.%, FeOT （7.45-9.97） wt.% （8.55 wt.% in average）, （MgO＋FeOT） 52.09 wt.%, Mg# （100×Mg/（Mg＋Fe＊）, where Fe＊ represents total Fe）=89.42-90.08, （m＋f）/Si （（atomicity Mg＋atomicity Fe） /atomicity Si）=1.53-1.75 （1.59 in average）, respectively. The mean M/F （atomicity Mg/atomicity Fe） ratio of the rocks is 9.05, which is classified as magnesium enriched-type of ultramafic rocks. The compositional features, depleted in K, Na, Ca, AI and Ti and enriched in Mg#, indicate the characteristics of peridotite originated from a depleted mantle. The rocks have low REE with ∑（1.60-2.68）×10^-6 similar to those of the primitive mantle. The chondrite-normalized REE patterns of all samples show slightly enrichment in LREE, with （Ce/Yb）N 1.03-2.46, but a little depleted in HREE. Most samples show a slight negative anomaly in Eu, a feature in REE from a relic mantle and common features in highly serpentinized ultramafic rocks in the Yarlung-Zangbo （雅鲁藏布） ophiolite and the Bangong （班公）-Nujiang （怒江） ophiolite in Tibet. The primitive mantle-normalized spiderdiagram of trace elements for Gongbupala ultramafic rocks yields uniform distributed pattern. They are relatively enriched in Rb, Ba, La, P element （LHSE） and depleted in Sm, Ti, Y, Yb element （HFSE）, a feature of metasomatic mantle peridotite. The geochemical features of the rocks suggest that the protolith of Gongbupala serpentinite in Sumdo region is harzburgite, a typical depleted mantle rock, and may represent a dismembered ophiolite unit in the region.展开更多
The Qinling-Dabie-Sulu high-pressure and ultra-high pressure metamorphic belt wasformed by subduction and collision between the North China and Yangtze plates. The study ofthe eclogite belt is very important in unders...The Qinling-Dabie-Sulu high-pressure and ultra-high pressure metamorphic belt wasformed by subduction and collision between the North China and Yangtze plates. The study ofthe eclogite belt is very important in understanding the evolution of the Qinling Dabie orogen. Inthe present paper the geology, petrology, minerology and chronology of the eclogites in the Dabieand Sulu areas are described. The principal conclusions of this work are as follows: (1) Based up-on the field occurrence and the P-T conditions of the eclogites, two types of eclogite can be dis-tinguished: Type 1—the low-temperature and high-pressure eclogite in the mid-late Proterozoicmetamorphic series, and Type 2—the ultra-high pressure eclogite in the late Archaean to earlyProterozoic metamorphic complex. In the Dabie area, the ultra-high-pressure eclogite,high-pressure eclogite and epidote-blueschist units are nearly parallel to each other and stretchintermittently from north to south. (2) The P-T conditions of the high-pressure eclogites and ul-tra-high pressure eclogites have been estimated. The former are formed at 450-550℃ and1.4-1.6 GPa; while the latter at 650-870℃ and >2.7-2.9 GPa in the Dabie area and at820-1000℃ and >2.8-3.1 GPa in the Sulu area. The metamorphic temperatures of the eclogitesincrease progressively from west to east. (3) The ultra-high pressure eclogites were subjected to 5stages of metamorphism: pre-eclogite epidote amphibolite facies, peak coesite eclogite facies,post-eclogite amphibolite facies, epidote-blueschist facies or epidote amphibolite facies andgreenschist facies. The general features of the PTt path of the ultra-high pressure eclogite are:clockwise pattern, progressive metamorphism being a process of slow increasing temperature andrapid increasing pressure, and the retrogressive section with nearly isothermal decompression atthe early stage, isobaric cooling at the middle stage and nearly isothermal decompression at thelate stage. (4) At least two stages of high-pressure metamorphism occurred in the orogenic belt:the high-pressure eclogite and ultra-high pressure eclogite were formed by the subduction of theoceanic crust northward beneath the North China plate or the Dabie block during theCaledonian; while the epidote-blueschist belt came into being by subdution and collision be-tween the two continental plates during the Indosinian. (5) Due to the continuous sequentialsubduction of the cold plate, the ultra high-presssure metamorphic rocks were uplifted to thecrust by the underplating processes. They can be preserved just because of the 'frozen effect' re-sulting from the continuous subduction of the cold plate. (6) The carbonates, such as magnesite,breunnerite, aragonite and dolomite, and the H_2O-bearing minerals, such as phengite, epidoteand zoisite, were stable during the high-pressure and/or ultra-high pressure metamorphism.展开更多
: The 3He/4He ratios of most eclogites from the Dabie-Sulu terrane range from 0.056 to 0.67 Ra; the data points fall into the mixing part of the crust and the mantle in the 3He-4He diagram. The 3He/4He ratios of eclog...: The 3He/4He ratios of most eclogites from the Dabie-Sulu terrane range from 0.056 to 0.67 Ra; the data points fall into the mixing part of the crust and the mantle in the 3He-4He diagram. The 3He/4He ratios of eclogites are obviously correlated with the types of their surrounding rocks. The helium isotope composition of the eclogites from the Bixiling complex possesses characters of mantle-derived rocks with the 3He/4He ratio being 5.6 Ra. The 4He concentration of the eclogites exhibits visible inverse correlation with the δ18O value of the quartz in the eclogites from the Sulu area. The δ18O values of the eclogites change synchronously with those of the country rocks. Those results suggest that protoliths of the eclogites were basic-ultrabasic rock bodies or veins intruding into the continental crust in the early stage; strong exchange and hybridization between the basic-ultrabasic rocks and continental rocks and the atmospheric water during the intrusion led to abrupt increase of the 3He/4He ratios, δ18O values and Nd(0) values of the intrusive bodies or veins, which show characters of continental rocks. This indicates that the eclogites are autochthonous.展开更多
The paper reports SHRIMP U-Pb zircon data of a dark eclogite and a post-eclogite garnet-bearing gneissic granitic rock from the Bixiling area, Yuexi County, Anhui Province, in the eastern Dabie Mountains. The eclogite...The paper reports SHRIMP U-Pb zircon data of a dark eclogite and a post-eclogite garnet-bearing gneissic granitic rock from the Bixiling area, Yuexi County, Anhui Province, in the eastern Dabie Mountains. The eclogite, which is metamorphosed basic tuff, contains very scarce zircons in omphacite or garnet, but more zircons in quartz. They usually exhibit a double-layered texture, as shown clearly in cathodoluminescence images. Their inner main parts give a 206Pb/238U age of 757±7 Ma, representing the approximate age of the high-pressure (HP)- ultrahigh-pressure (UHP) metamorphic event during which the eclogite was formed. The outer peripheral parts of the zircons, which have been modified by late-stage fluids, give an age of 223±3 Ma. The granitic rock contains more zircons of anatectic origin found mostly in feldspar and quartz and usually also showing a similar composite texture. The inner main parts of the anatectic zircons with oscillatory zoning give a 206Pb/238U age of 727±15 Ma for the approximate age of the emplacement of the granitic rock, and their outer parts, an age of 219±3 Ma for a similar or even the same fluid event. It is thus suggested that the HP-UHP metamorphism of the Bixiling eclogite facies rocks took place during the Neoproterozoic Jinningian, and the Indosinian age values may only represent a late event in the nature of fluid activity.展开更多
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 disp...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.展开更多
Detailed three-dimensional structural studies indicate that the Bixiling area, Dabie massif, central China shows the deepest exposed levels of the orogenic wedge formed during the Triassic Yangtze -Sino-Korean contine...Detailed three-dimensional structural studies indicate that the Bixiling area, Dabie massif, central China shows the deepest exposed levels of the orogenic wedge formed during the Triassic Yangtze -Sino-Korean continental collision. New 1 : 10 000 scale structural mapping, combined with detailed petrological analysis in this area, has enabled us to accurately distinguish structures related to the Triassic continental collision from those related to post-collisional deformation in the ultrahigh pressure (UHP) metamorphic unit. The collisional or compressional structures include the massive eclogite with a weak foliation, foliated eclogite or UHP ductile shear zones, as well as upper amphibolite facies shear zones, whereas the post-collisional deformation is characterized by a regionally, flat-lying foliation containing stretching lineations and common reclined folds. The former is present exclusively in the eclogite lenses and their margins, representing orogenic thickening or syn-collisional events, while the latter was best occurred on variable scales under amphibolite facies conditions, showing sub-vertical, extreme shortening and ductile thinning of the metamorphic rock stack. The eclogite facies tectonites that have a marked fabric discordance to the penetrative amphibolite facies extension flow fabric are common. It is emphasized that an extensional tectonic setting following the collision-orogenic thickening stage was, at least partly, responsible for exhumation of the UHP metamorphic rocks in the Dabie massif. A new tectonic evolution model is proposed for the UHP metamorphic belt on the scale of the Dabie massif. The Bixiling area thus provides a window, from which the dynamic processes concerning the formation and exhumation of the UHP rocks can be observed. Regional studies in the Dabie Mountains have confirmed this interpretation.展开更多
Omphacite grains from UHP eclogite of the Dabie Mountains in eastern China are elongated and show strong lattice preferred orientations (LPOs). Observations by the transmission electron microscopy (TEM) identified not...Omphacite grains from UHP eclogite of the Dabie Mountains in eastern China are elongated and show strong lattice preferred orientations (LPOs). Observations by the transmission electron microscopy (TEM) identified not only structures of plastic deformation occurring as free dislocation, dislocation loops and dislocation walls, but also bubbles of water present in the deformed omphacite. The bubbles attach to the dislocation loops which are often connected to one another via a common bubble. Using infrared spectroscopy (IR), two types of hydrous components are identified as hydroxyl and free-water in the omphacite. An analysis of deformation mechanism of microstructure in omphacite suggests that the mineral deformed plastically under UHP metamorphic conditions by dislocation creep through hydrolitic weakening.展开更多
The granulitized eclogites from the Luliangshan terrane of the North Qaidam UHP metamorphic belt occur as lenses within pelitic gneisses and orthogneisses. Combined petrologic data and phase equilibrium modeling indic...The granulitized eclogites from the Luliangshan terrane of the North Qaidam UHP metamorphic belt occur as lenses within pelitic gneisses and orthogneisses. Combined petrologic data and phase equilibrium modeling indicate a multi-stage metamorphic history of the granulitized eclogites:(1) an earlier eclogite facies metamorphism(P>18.5 kbar, T> 830℃) is deduced from omphacite relics in the matrix and rare omphacite inclusions within garnet. The possible assemblage is garnet+omphacite+rutile+ quartz;(2) the early stage of high pressure granulite facies assemblages(garnet+clinopyroxene+ plagioclase+rutile+quartz+liquid) developed in the early decompression process has a P-T regime of 17.5 kbar and 852–858 ℃, constrained by plagioclase and clinopyroxene inclusions in garnet. The late stage of high pressure granulite assemblages(garnet+clinopyroxene+amphibole+plagioclase+rutile+quartz+liquid) records an isothermal decompression process with the pressure successively declining from 17.5 to 14.7 kbar and to 11.3 kbar at 858 ℃;(3) the later medium pressure granulite facies assemblage(garnet+ orthopyroxene+clinopyroxene+amphibole+plagioclase+ilmenite+liquid+quartz) indicates a drop in pressure and rise in temperature at P-T conditions of 7.6–7.7 kbar and 878–883 ℃;(4) retrogressive amphibolite facies stage, which is represented by amphibole+plagioclase kelyphitic rims around garnet, formed under conditions of <5 kbar and <650 ℃. The preservation of medium pressure granulite facies assemblage and the garnet composition feature constrain a following isobaric cooling path during late exhumation. This process suggests a clockwise P-T path and indicates that the granulitized eclogites record a high grade "Barrovian" metamorphic overprint at the middle-lower crust during exhumation. The present data show that the Luliangshan terrane is a "hot" HP-UHP terrane.展开更多
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...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 4313 Ma,which is consistent with the 4322 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.展开更多
For the first time, we apply different geospeedometric models to garnet zoning patterns that were obtained in this study from detailed EMP analyses for garnets from eclogites and granulite in the Dabie-Sulu orogen. Va...For the first time, we apply different geospeedometric models to garnet zoning patterns that were obtained in this study from detailed EMP analyses for garnets from eclogites and granulite in the Dabie-Sulu orogen. Various zonings of cation diffusion were preserved in the garnets, enabling the acquirement of average cooling rates for the high-to ultrahigh-pressure rocks without using geochronological approaches. The coesite-bearing hot eclogites yield fast cooling rates of about 20 to 30℃/Ma subsequent to peak metamorphic temperatures, whereas the cold eclogite gives a relatively slow cooling rate of 8℃/Ma at its initial exhumation. A very slow cooling rate of <0.3℃/Ma is obtained for the granulite at Huangtuling, suggesting that the granulite may not be involved in the continental deep subduction.展开更多
A typical HP/MT (high pressure/medium temperature) eclogite from Xiongdian, northwestern Dabie Mountains, has been geochronologically studied using the single-zircon U-Pb, 40Ar-39Ar and Sm-Nd methods. Prismatic zircon...A typical HP/MT (high pressure/medium temperature) eclogite from Xiongdian, northwestern Dabie Mountains, has been geochronologically studied using the single-zircon U-Pb, 40Ar-39Ar and Sm-Nd methods. Prismatic zircons occurring as inclusions within garnets define a minimum crystallization age of 399.5±1.6 Ma. 40Ar-39Ar dating on amphibole gives a plateau age.of 399.2 ± 4 Ma, which is interpreted as a retrogression age of amphibolite facies. This integrated study enables us to conclude that the age of high-pressure metamorphism is older than 399.5 ± 1.6 Ma, suggesting Caledonian collision between the North China and Yangtze plates. Round zircon within the aggregate of quartz and muscovite gives a concordant age of 301± 2 Ma, reflecting a later retrogression event. An age profile of post-eclogite metamorphism is documented, including amphibolite facies metamorphism at 399.2 Ma shortly after eclogitization and later retrogressive metamorphism at 301 Ma. Sm-Nd mineral isochron of garnet+omphacite gives an unreasonable age of 544 ± 23 Ma. which is quite older than that of prismatic zircon and may be attributed to isotopic disturbance by retrogression.展开更多
The results of recent geothermobarometric and geochronological investigations of scarce eclogites of the NW Himalaya (Tso Morari (Ladakh), India and Kaghan Valley, Pakistan) have caused a major rethink of tectonometam...The results of recent geothermobarometric and geochronological investigations of scarce eclogites of the NW Himalaya (Tso Morari (Ladakh), India and Kaghan Valley, Pakistan) have caused a major rethink of tectonometamorphic models for India\|Asia collision. Numerous petrologic studies have been undertaken on the age and origin of metamorphism in the Higher Himalayan Crystallines (HHC) and Lesser Himalaya formations (LH) and their relationship to granite magmatism and movements along the Main Central Thrust (MCT) and South Tibetan Detachment Fault (STDF). However, all of these events are essentially Miocene (or younger) in age and can clearly be distinguished from subduction and exhumation processes undergone by the eclogites which are of Eocene age (Tonarini et al. 1993; Spencer & Gebauer; 1996; de Sigoyer et al. 1999) and relate to the very early stages of the collision. Eclogites of eastern Ladakh are mafic lenses found in granitic gneisses (Ordovician intrusive age: Girard & Bussy 1999) and their surrounding late Pre\|Cambrian to early Cambrian sedimentary units in the Tso Morari dome (see Steck et al. 1998). Detailed petrological and geochronological studies (Guillot et al. 1997; de Sigoyer et al. 1997, 1999) have identified an eclogite facies stage (2000±300)MPa, (580±60)℃ followed by isothermal decompression associated with glaucophane growth at around (1100±200)MPa. Dating of different phases by different methods yielded ages around 55Ma for this stage ((55±17) Ma, U\|Pb, Aln; (55±12) Ma, Lu\|Hf, Grt\|Cpx\|Rt; (55±7) Ma, Sm\|Nd, Grt\|Gln\|Rt). A subsequent amphibolite facies overprint at slightly higher temperature (610±70)℃ was dated at 45～48Ma (metabasite: (47±11) Ma, Sm\|Nd, Grt\|Hbl; metapelite: (45±4) Ma, Rb\|Sr, Mu\|Ap\|WR and (48±2) Ma, Ar\|Ar, Phe). By (30±1) Ma (Ar\|Ar, Bt\|Mu) retrogression into the greenschist facies had occurred (de Sigoyer et al. 1999). These data indicate a two stage history with early exhumation being much faster (>4mm/a) than the later evolution (1～2mm/a).展开更多
基金supported by Grant-in-Aid for Scientific Research on Innovative Areas Grant Number 26106002
文摘When plate tectonics began on the Earth has been long debated and here we argue this topic based on the records of Earth-Moon geology and asteroid belt to conclude that the onset of plate tectonics was during the middle Hadean（4.37-4.20 Ga）. The trigger of the initiation of plate tectonics is the ABEL Bombardment, which delivered oceanic and atmospheric components on a completely dry reductive Earth, originally comprised of enstatite chondrite-like materials. Through the accretion of volatiles, shock metamorphism processed with vaporization of both CI chondrite and supracrustal rocks at the bombarded location, and significant recrystallization went through under wet conditions, caused considerable eclogitization in the primordial continents composed of felsic upper crust of 21 km thick anorthosite, and 50 km or even thicker KREEP lower crust. Eclogitization must have yielded a powerful slab-pull force to initiate plate tectonics in the middle Hadean. Another important factor is the size of the bombardment. By creating Pacific Ocean class crater by 1000 km across impactor, rigid plate operating stagnant lid tectonics since the early Hadean was severely destroyed, and oceanic lithosphere was generated to have bi-modal lithosphere on the Earth to enable the operation of plate tectonics.Considering the importance of the ABEL Bombardment event which initiated plate tectonics including the appearance of ocean and atmosphere, we propose that the Hadean Eon can be subdivided into three periods：（1） early Hadean（4.57-4.37 Ga）,（2） middle Hadean（4.37-4.20 Ga）, and（3） late Hadean（4.20-4.00 Ga）.
基金financial supports by the National Natural Science Foundation of China(Nos.41490613,41190073 and 41304071)the National Basic Research Program of China(Nos.2014CB440901 and 2015CB856106)
文摘2D thermo-mechanical models are constructed to investigate the effects of oceanic and continental crustal eclogitization on plate dynamics at three successive stages of oceanic subduction, slab breakoff, and continental subduction. Crustal eclogitization directly increases the average slab density and accordingly the slab pull force, which makes the slab subduct deeply and steeply. Numerical results demonstrate that the duration time from initial continental collision to slab breakoff largely depends on the slab pull force. Specifically, eclogitization of subducted crust can greatly decrease the duration time, but increase the breakoff depth. The detachment of oceanic slab from the pro-continental lithosphere is accompanied with obvious exhumation of the subducted continental crust and a sharp uplift of the collision zone in response to the disappearance of downward drag force and the induced asthenospheric upwelling, especially under the condition of no or incomplete crustal eclogitization. During continental subduction, the slab dip angle is strongly correlated with eclogitization of subducted continental lower crust, which regulates the slab buoyancy nature. Our model results can provide several important implications for the Himalayan-Tibetan collision zone. For example, it is possible that the lateral variations in the degree of eclogitization of the subducted Indian crust might to some extent contribute to the lateral variations of subduction angle along the Himalayan orogenic belt. Moreover, the accumulation of highly radiogenic sediments and upper continental crustal materials at the active margin in combination with the strong shear heating due to continuous continental subduction together cause rising of isotherms in the accretionary wedge, which facilitate the development of crustal partial melting and metamorphism.
文摘The characteristic textures of eclogitic veins infilling the interstitial spaces among zoisite-glaucophanite pillows, the fractures crosscutting massive zoisite-glaucophanites and the relic conduits within the eclogite boudins demonstrate that the eclogitization of glaucophanites was accompanied by the presence of a free fluid phase in the Western Tianshan high-pressure metamorphic belt. Non-planar populations of fluid inclusions, parallel to C-axis, have been preserved within the omphacites in eclogitic veins. The microthermometric results indicate that fluids are low-salinity (1-4 wt%NaCI) aqueous fluids. The chemical compositions of major, trace and rare earth elements of eclogitic veins and related host rocks indicate that the eclogitization of glaucophanites required the addition of external Si and Na elements, and was accompanied by the loss of large-ion lithophile elements, rare earth elements and water. The record of eclogitization of glaucophanites along fractures and zones of high permeability by fluid infiltration has been preserved in the Wetsern Tianshan high-pressure metamorphic belt.
基金financially supported by the National Natural Science Foundation of China(No.41930215)the Fundamental Research Funds for the Central Universities,China University of Geosciences(Wuhan)。
文摘Phase equilibrium modeling using internally consistent thermodynamic dataset and associated activity-composition(a-x)models are very helpful for quantifying P-T evolution for eclogite,which is the basis for deciphering the geodynamic processes in subduction zones.In this study,we apply different versions of datasets(ds55 and ds62)and associated a-x relations to a wellestablished LT-HP eclogite at Huwan in the classic western Dabie orogen to constrain its P-T evolution.The eclogite comprises garnet+omphacite+amphibole+white mica+epidote+quartz+chlorite+rutile/ilmenite/sphene.Garnet porphyroblasts show mono-variation in the end members(spessartine from 17 mol%to 0,pyrope from 2 mol%to 18 mol%,almandine from 47 mol%to 64mol%and grossular from 35 mol%to 18 mol%)from core to rim.Phase diagrams combined with compositional isopleth thermobarometry show that dataset ds62 and associated a-x relations yield P_(max)of~33 kbar at~560℃,conflicting with our petrological observations and previous studies.On the other hand,phase equilibrium modeling using dataset ds62 and a revised symmetric garnet a-x model irrespective of Fe^(3+)(O)gives P_(max)of~27 kbar at~560℃,consistent with the results using dataset ds55 and associated a-x relations.Therefore,we recommend a symmetric model for garnet involving py,alm,gr and spss,without Fe~(3+)components,instead of the asymmetric garnet a-x relations involving py,alm,gr,spss and kho by White et al.(2014),for calculating phase diagrams for LT-(U)HP eclogite when using dataset ds62.In this study,the defined P-T path is characterized by a segment of the prograde evolution showing a first moderate slope,followed by gentle then steep slopes,representing the thermal structure evolution recorded by slab surface during continental subduction.Our work combined with previous studies conclude that in western Dabie,the Huwan HP eclogite belt to the north and the Hong'an HP eclogite belt to the south belong to the same HP slice overlying the Xinxian UHP slice.
基金financially supported by the National Natural Science Foundation of China(No.41972198)the Science and Technology Innovation Program of Hunan Province(No.2021RC4055)+2 种基金the Cooperation Foundation of Key Laboratory of Mineralogy and Metallogeny,Guangzhou Institute of GeochemistryCAS(No.KLMM20180201)the Scientific Research Starting Foundation of Central South University。
文摘In order to decipher element mobility in UHP meta-sedimentary rocks in the continental subduction zone,major and trace element compositions are investigated for a continuous profile from a representative UHP region in the Dabie Mountains.Among the lithologic contact zone,contents of K,Ca,LREE,and LILE exhibit varying degrees of downward trends in both marble and eclogite toward the contact zone,indicating that marble and their associated eclogite can release a large amount of K,Ca,and a small number of LILEs and LREEs.Titanite is the main Ti phase in both marble and eclogite.Titanite rims around rutile can occasionally be seen in eclogite.Contents of Ti and HFSE exhibit a well-coupled relation among marble and eclogite,indicating that substantial Ti and HFSEs were migrated from eclogite to marble,in accord with the capacity of a melt medium.Rutiles and titanites in marble exhibit a relatively limited variation in Nb/Ta ratios(12.9-16.2),similar to those of titanites in eclogite(14.2-16.7),which demonstrates that rutiles and titanites in marble were sourced from eclogite because of short-distance migrations of Ti and HFSEs.According to the P-T path and the temperature and pressure conditions of the peak metamorphism reported by previous studies,the eclogite associated with marble may not form supercritical fluids in the subduction zone because of the addition of carbonate minerals.
基金supported by National Research Foundation of Korea(NRF)Grant funded by the Korean Government(NRF–2017K1A1A2013180).
文摘The Permo-Triassic amalgamation of Northeast Asia began to receive much attention after discovering HP and UHP eclogites in the Dabie and Sulu areas in China.However,the Permo-Triassic tectonic evolution of Northeast Asia is still under argument.Over the past 30 years,several critical geological discoveries were reported from the Korean Peninsula;1)The Permo-Triassic eclogite in the Hongseong area,2)the Triassic post-collisional igneous rocks in the northern Gyeonggi Massif,Imjingang Belt,and the southern Nangrim Massif,3)the Permo-Triassic intermediate-P/T metamorphism in the Imjingang and Okcheon Belts and the Gyeonggi Massif,4)the strongest Permo-Triassic metamorphism along the E-W trending Hongseong-Yangpyeong-Odaesan Belt,and 5)the Devonian back arc origin for the Imjingang Belt.Besides these,the Honam Shear Zone was confirmed to be initiated in the early Jurassic.These dates indicate that the Qinling-Dabie-Sulu Continental Collision Belt in China extends not into the Imjingang Belt but into the Hongseong-Odaesan Belt within the Korean Peninsula.The Hongseong-Odaesan Belt extends into the inner part of southwest Japan and then into the Dumangang Belt in the northeast Korean Peninsula.
基金the National Natural Science Foundation of China grant 49732070 the Chinese National Key Project for Fundamental Research on the TibetanPlateau(G1998040805).
文摘Coesite was discovered as inclusions in zircon separates from pelitic gneissassociated with a large eclogite body in the North Qaidam ultrahigh-pressure (UHP) terrane. Somegraphite inclusions were also found. This finding suggested the occurrence of in-situ UHPmetamorphism and that the terrane was most likely recrystallized at pressures below the diamondstability field. It supported other previous indirect UHP evidence, such as polycrystalline quartzinclusions in eclogitic garnet, quartz lamellae in omphacite and P-T estimates for both eclogite andgarnet peridotite. The U-Pb and Sm-Nd ages of the North Qaidam eclogite indicated that subductionof continental crust occurred in the Early Palaeozoic, which probably recorded a collision betweenthe Sino-Korean and Yangtze plates.
基金the National 973 Project of Chinese Ministry of Science and Technology （Grant No. 2003CB716502） the Natural Science Foundation of China （Grant No. 40399143） +1 种基金 the German Science Foundation （DFG grant No. GE 1152/2-2 , WE2850/3- 1）.
文摘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 eclogite 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.
基金supported by China Geological Survey (Nos. 1212010610107 and 1212010610105)the Institute of Geology, Chinese Academy of Geological Sciences (No. J0701)
文摘Recently, an over 100 km long MORB-type eclogite belt of Permian was discovered in the Sumdo （松多） region of the Lhasa block, Tibet. A critical question thus is： what is the tectonic setting of the eelogite belt and is it related to an unrecognized suture in the region？ Further investigations show that there are some mafic and ultramafic rocks spacially associated with the eclogite belt in the region. Three ultramafic massifs were recognized in the Sumdo region, and called the Luomaling （罗马岭）, Gongbupala （贡布爬拉） and Qiazhasumdo （卡扎松多） massifs. All the massifs are fault-contacted with greenschist （Chasagang （岔萨岗） Formation） or muscovite-quartz schist （Mabuku （马布库） Formation）, and individuals are about 100 m×50 m in size extending in EW as the regional structure. All the ultramafic rocks have been entirely serpentinized, and the Gongbupala massif has been selected for study in geochemistry. Eleven chemical analyses of the rocks from the Gongbupala massif show a narrow range in contents： SiO2 （35.97-40.63） wt.%, MgO （37.02-38.60） wt.%, TiO2 （0.01-0.08） wt.%, Al2O3 （0.80-1.64） wt.%, （Na2O＋K2O） less than 0.1 wt.%, with high volatile contents （H2O＋CO2） （11.24-14.91） wt.%. After recalculation without H2O＋CO2, the mean values are SiO2 45.24 wt.%, MgO 43.54 wt.%, FeOT （7.45-9.97） wt.% （8.55 wt.% in average）, （MgO＋FeOT） 52.09 wt.%, Mg# （100×Mg/（Mg＋Fe＊）, where Fe＊ represents total Fe）=89.42-90.08, （m＋f）/Si （（atomicity Mg＋atomicity Fe） /atomicity Si）=1.53-1.75 （1.59 in average）, respectively. The mean M/F （atomicity Mg/atomicity Fe） ratio of the rocks is 9.05, which is classified as magnesium enriched-type of ultramafic rocks. The compositional features, depleted in K, Na, Ca, AI and Ti and enriched in Mg#, indicate the characteristics of peridotite originated from a depleted mantle. The rocks have low REE with ∑（1.60-2.68）×10^-6 similar to those of the primitive mantle. The chondrite-normalized REE patterns of all samples show slightly enrichment in LREE, with （Ce/Yb）N 1.03-2.46, but a little depleted in HREE. Most samples show a slight negative anomaly in Eu, a feature in REE from a relic mantle and common features in highly serpentinized ultramafic rocks in the Yarlung-Zangbo （雅鲁藏布） ophiolite and the Bangong （班公）-Nujiang （怒江） ophiolite in Tibet. The primitive mantle-normalized spiderdiagram of trace elements for Gongbupala ultramafic rocks yields uniform distributed pattern. They are relatively enriched in Rb, Ba, La, P element （LHSE） and depleted in Sm, Ti, Y, Yb element （HFSE）, a feature of metasomatic mantle peridotite. The geochemical features of the rocks suggest that the protolith of Gongbupala serpentinite in Sumdo region is harzburgite, a typical depleted mantle rock, and may represent a dismembered ophiolite unit in the region.
文摘The Qinling-Dabie-Sulu high-pressure and ultra-high pressure metamorphic belt wasformed by subduction and collision between the North China and Yangtze plates. The study ofthe eclogite belt is very important in understanding the evolution of the Qinling Dabie orogen. Inthe present paper the geology, petrology, minerology and chronology of the eclogites in the Dabieand Sulu areas are described. The principal conclusions of this work are as follows: (1) Based up-on the field occurrence and the P-T conditions of the eclogites, two types of eclogite can be dis-tinguished: Type 1—the low-temperature and high-pressure eclogite in the mid-late Proterozoicmetamorphic series, and Type 2—the ultra-high pressure eclogite in the late Archaean to earlyProterozoic metamorphic complex. In the Dabie area, the ultra-high-pressure eclogite,high-pressure eclogite and epidote-blueschist units are nearly parallel to each other and stretchintermittently from north to south. (2) The P-T conditions of the high-pressure eclogites and ul-tra-high pressure eclogites have been estimated. The former are formed at 450-550℃ and1.4-1.6 GPa; while the latter at 650-870℃ and >2.7-2.9 GPa in the Dabie area and at820-1000℃ and >2.8-3.1 GPa in the Sulu area. The metamorphic temperatures of the eclogitesincrease progressively from west to east. (3) The ultra-high pressure eclogites were subjected to 5stages of metamorphism: pre-eclogite epidote amphibolite facies, peak coesite eclogite facies,post-eclogite amphibolite facies, epidote-blueschist facies or epidote amphibolite facies andgreenschist facies. The general features of the PTt path of the ultra-high pressure eclogite are:clockwise pattern, progressive metamorphism being a process of slow increasing temperature andrapid increasing pressure, and the retrogressive section with nearly isothermal decompression atthe early stage, isobaric cooling at the middle stage and nearly isothermal decompression at thelate stage. (4) At least two stages of high-pressure metamorphism occurred in the orogenic belt:the high-pressure eclogite and ultra-high pressure eclogite were formed by the subduction of theoceanic crust northward beneath the North China plate or the Dabie block during theCaledonian; while the epidote-blueschist belt came into being by subdution and collision be-tween the two continental plates during the Indosinian. (5) Due to the continuous sequentialsubduction of the cold plate, the ultra high-presssure metamorphic rocks were uplifted to thecrust by the underplating processes. They can be preserved just because of the 'frozen effect' re-sulting from the continuous subduction of the cold plate. (6) The carbonates, such as magnesite,breunnerite, aragonite and dolomite, and the H_2O-bearing minerals, such as phengite, epidoteand zoisite, were stable during the high-pressure and/or ultra-high pressure metamorphism.
文摘: The 3He/4He ratios of most eclogites from the Dabie-Sulu terrane range from 0.056 to 0.67 Ra; the data points fall into the mixing part of the crust and the mantle in the 3He-4He diagram. The 3He/4He ratios of eclogites are obviously correlated with the types of their surrounding rocks. The helium isotope composition of the eclogites from the Bixiling complex possesses characters of mantle-derived rocks with the 3He/4He ratio being 5.6 Ra. The 4He concentration of the eclogites exhibits visible inverse correlation with the δ18O value of the quartz in the eclogites from the Sulu area. The δ18O values of the eclogites change synchronously with those of the country rocks. Those results suggest that protoliths of the eclogites were basic-ultrabasic rock bodies or veins intruding into the continental crust in the early stage; strong exchange and hybridization between the basic-ultrabasic rocks and continental rocks and the atmospheric water during the intrusion led to abrupt increase of the 3He/4He ratios, δ18O values and Nd(0) values of the intrusive bodies or veins, which show characters of continental rocks. This indicates that the eclogites are autochthonous.
文摘The paper reports SHRIMP U-Pb zircon data of a dark eclogite and a post-eclogite garnet-bearing gneissic granitic rock from the Bixiling area, Yuexi County, Anhui Province, in the eastern Dabie Mountains. The eclogite, which is metamorphosed basic tuff, contains very scarce zircons in omphacite or garnet, but more zircons in quartz. They usually exhibit a double-layered texture, as shown clearly in cathodoluminescence images. Their inner main parts give a 206Pb/238U age of 757±7 Ma, representing the approximate age of the high-pressure (HP)- ultrahigh-pressure (UHP) metamorphic event during which the eclogite was formed. The outer peripheral parts of the zircons, which have been modified by late-stage fluids, give an age of 223±3 Ma. The granitic rock contains more zircons of anatectic origin found mostly in feldspar and quartz and usually also showing a similar composite texture. The inner main parts of the anatectic zircons with oscillatory zoning give a 206Pb/238U age of 727±15 Ma for the approximate age of the emplacement of the granitic rock, and their outer parts, an age of 219±3 Ma for a similar or even the same fluid event. It is thus suggested that the HP-UHP metamorphism of the Bixiling eclogite facies rocks took place during the Neoproterozoic Jinningian, and the Indosinian age values may only represent a late event in the nature of fluid activity.
基金funded by the National Natural Science Foundation of China (No.40525006 and 40372032).
文摘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.
基金This study is supported by the National Natural Science Foundation of China (No. 49972067) the Major State Basic Research Development Program (No. G1999075506).
文摘Detailed three-dimensional structural studies indicate that the Bixiling area, Dabie massif, central China shows the deepest exposed levels of the orogenic wedge formed during the Triassic Yangtze -Sino-Korean continental collision. New 1 : 10 000 scale structural mapping, combined with detailed petrological analysis in this area, has enabled us to accurately distinguish structures related to the Triassic continental collision from those related to post-collisional deformation in the ultrahigh pressure (UHP) metamorphic unit. The collisional or compressional structures include the massive eclogite with a weak foliation, foliated eclogite or UHP ductile shear zones, as well as upper amphibolite facies shear zones, whereas the post-collisional deformation is characterized by a regionally, flat-lying foliation containing stretching lineations and common reclined folds. The former is present exclusively in the eclogite lenses and their margins, representing orogenic thickening or syn-collisional events, while the latter was best occurred on variable scales under amphibolite facies conditions, showing sub-vertical, extreme shortening and ductile thinning of the metamorphic rock stack. The eclogite facies tectonites that have a marked fabric discordance to the penetrative amphibolite facies extension flow fabric are common. It is emphasized that an extensional tectonic setting following the collision-orogenic thickening stage was, at least partly, responsible for exhumation of the UHP metamorphic rocks in the Dabie massif. A new tectonic evolution model is proposed for the UHP metamorphic belt on the scale of the Dabie massif. The Bixiling area thus provides a window, from which the dynamic processes concerning the formation and exhumation of the UHP rocks can be observed. Regional studies in the Dabie Mountains have confirmed this interpretation.
基金funded by the Ministry of Science and Technology of China(No.G19990755-01)the National Natural Science Foundation of China,Postdoctoral Science Foundation of China,the Chinese Academy of Sciences Wong K.C.Post-doctoral Research Award Fund and the State Key Laboratory of Mineral Deposits,Nanjing University
文摘Omphacite grains from UHP eclogite of the Dabie Mountains in eastern China are elongated and show strong lattice preferred orientations (LPOs). Observations by the transmission electron microscopy (TEM) identified not only structures of plastic deformation occurring as free dislocation, dislocation loops and dislocation walls, but also bubbles of water present in the deformed omphacite. The bubbles attach to the dislocation loops which are often connected to one another via a common bubble. Using infrared spectroscopy (IR), two types of hydrous components are identified as hydroxyl and free-water in the omphacite. An analysis of deformation mechanism of microstructure in omphacite suggests that the mineral deformed plastically under UHP metamorphic conditions by dislocation creep through hydrolitic weakening.
基金financially supported by the National Natural Science Foundation of China (Nos. 41630207, 41572180)the China Geological Survey (No. DD20160022)
文摘The granulitized eclogites from the Luliangshan terrane of the North Qaidam UHP metamorphic belt occur as lenses within pelitic gneisses and orthogneisses. Combined petrologic data and phase equilibrium modeling indicate a multi-stage metamorphic history of the granulitized eclogites:(1) an earlier eclogite facies metamorphism(P>18.5 kbar, T> 830℃) is deduced from omphacite relics in the matrix and rare omphacite inclusions within garnet. The possible assemblage is garnet+omphacite+rutile+ quartz;(2) the early stage of high pressure granulite facies assemblages(garnet+clinopyroxene+ plagioclase+rutile+quartz+liquid) developed in the early decompression process has a P-T regime of 17.5 kbar and 852–858 ℃, constrained by plagioclase and clinopyroxene inclusions in garnet. The late stage of high pressure granulite assemblages(garnet+clinopyroxene+amphibole+plagioclase+rutile+quartz+liquid) records an isothermal decompression process with the pressure successively declining from 17.5 to 14.7 kbar and to 11.3 kbar at 858 ℃;(3) the later medium pressure granulite facies assemblage(garnet+ orthopyroxene+clinopyroxene+amphibole+plagioclase+ilmenite+liquid+quartz) indicates a drop in pressure and rise in temperature at P-T conditions of 7.6–7.7 kbar and 878–883 ℃;(4) retrogressive amphibolite facies stage, which is represented by amphibole+plagioclase kelyphitic rims around garnet, formed under conditions of <5 kbar and <650 ℃. The preservation of medium pressure granulite facies assemblage and the garnet composition feature constrain a following isobaric cooling path during late exhumation. This process suggests a clockwise P-T path and indicates that the granulitized eclogites record a high grade "Barrovian" metamorphic overprint at the middle-lower crust during exhumation. The present data show that the Luliangshan terrane is a "hot" HP-UHP terrane.
基金funded by the Fundamental Research Funds for National Universities, China University of Geosciences (Wuhan) and China Geological Survey (Grant Nos. 12120113032800, 21201011000150004, DD20190069)
文摘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 4313 Ma,which is consistent with the 4322 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.
文摘For the first time, we apply different geospeedometric models to garnet zoning patterns that were obtained in this study from detailed EMP analyses for garnets from eclogites and granulite in the Dabie-Sulu orogen. Various zonings of cation diffusion were preserved in the garnets, enabling the acquirement of average cooling rates for the high-to ultrahigh-pressure rocks without using geochronological approaches. The coesite-bearing hot eclogites yield fast cooling rates of about 20 to 30℃/Ma subsequent to peak metamorphic temperatures, whereas the cold eclogite gives a relatively slow cooling rate of 8℃/Ma at its initial exhumation. A very slow cooling rate of <0.3℃/Ma is obtained for the granulite at Huangtuling, suggesting that the granulite may not be involved in the continental deep subduction.
基金This paper is a partial result of a major tundamental project (No. 8502206) of the Ministry of Geology and Mineral Resources and a project (No. 9514) of the Foundation for Development of Science and Technology in Geology, and financially supported by the
文摘A typical HP/MT (high pressure/medium temperature) eclogite from Xiongdian, northwestern Dabie Mountains, has been geochronologically studied using the single-zircon U-Pb, 40Ar-39Ar and Sm-Nd methods. Prismatic zircons occurring as inclusions within garnets define a minimum crystallization age of 399.5±1.6 Ma. 40Ar-39Ar dating on amphibole gives a plateau age.of 399.2 ± 4 Ma, which is interpreted as a retrogression age of amphibolite facies. This integrated study enables us to conclude that the age of high-pressure metamorphism is older than 399.5 ± 1.6 Ma, suggesting Caledonian collision between the North China and Yangtze plates. Round zircon within the aggregate of quartz and muscovite gives a concordant age of 301± 2 Ma, reflecting a later retrogression event. An age profile of post-eclogite metamorphism is documented, including amphibolite facies metamorphism at 399.2 Ma shortly after eclogitization and later retrogressive metamorphism at 301 Ma. Sm-Nd mineral isochron of garnet+omphacite gives an unreasonable age of 544 ± 23 Ma. which is quite older than that of prismatic zircon and may be attributed to isotopic disturbance by retrogression.
文摘The results of recent geothermobarometric and geochronological investigations of scarce eclogites of the NW Himalaya (Tso Morari (Ladakh), India and Kaghan Valley, Pakistan) have caused a major rethink of tectonometamorphic models for India\|Asia collision. Numerous petrologic studies have been undertaken on the age and origin of metamorphism in the Higher Himalayan Crystallines (HHC) and Lesser Himalaya formations (LH) and their relationship to granite magmatism and movements along the Main Central Thrust (MCT) and South Tibetan Detachment Fault (STDF). However, all of these events are essentially Miocene (or younger) in age and can clearly be distinguished from subduction and exhumation processes undergone by the eclogites which are of Eocene age (Tonarini et al. 1993; Spencer & Gebauer; 1996; de Sigoyer et al. 1999) and relate to the very early stages of the collision. Eclogites of eastern Ladakh are mafic lenses found in granitic gneisses (Ordovician intrusive age: Girard & Bussy 1999) and their surrounding late Pre\|Cambrian to early Cambrian sedimentary units in the Tso Morari dome (see Steck et al. 1998). Detailed petrological and geochronological studies (Guillot et al. 1997; de Sigoyer et al. 1997, 1999) have identified an eclogite facies stage (2000±300)MPa, (580±60)℃ followed by isothermal decompression associated with glaucophane growth at around (1100±200)MPa. Dating of different phases by different methods yielded ages around 55Ma for this stage ((55±17) Ma, U\|Pb, Aln; (55±12) Ma, Lu\|Hf, Grt\|Cpx\|Rt; (55±7) Ma, Sm\|Nd, Grt\|Gln\|Rt). A subsequent amphibolite facies overprint at slightly higher temperature (610±70)℃ was dated at 45～48Ma (metabasite: (47±11) Ma, Sm\|Nd, Grt\|Hbl; metapelite: (45±4) Ma, Rb\|Sr, Mu\|Ap\|WR and (48±2) Ma, Ar\|Ar, Phe). By (30±1) Ma (Ar\|Ar, Bt\|Mu) retrogression into the greenschist facies had occurred (de Sigoyer et al. 1999). These data indicate a two stage history with early exhumation being much faster (>4mm/a) than the later evolution (1～2mm/a).
