The Central Qilian Block is a Precambrian block in the Qilian Orogen,which has long drawn international attention for the study of orogeny and continental dynamics.The Huangyuan Group in the Datong area is one of the ...The Central Qilian Block is a Precambrian block in the Qilian Orogen,which has long drawn international attention for the study of orogeny and continental dynamics.The Huangyuan Group in the Datong area is one of the Precambrian metamorphic basement units in the Central Qilian Block and reflects metamorphism in the Barrovian garnet zone and sillimanite zone from south to north.Based on detailed fieldwork,this study presents a systematic study of petrography,mineral chemistry and phase equilibria of schists and gneisses from the two metamorphic zones.The garnet metamorphic zone is composed of micaschist,garnet-bearing micaschist and felsic leptynite,with interlayered plagioclase amphibolite.The sillimanite metamorphic zone consists of garnet-bearing biotite micaschist,sillimanite-bearing biotite-plagioclase gneiss and felsic leptynite.Garnet from the garnet metamorphic zone shows growth zoning with increasing almandine and pyrope and decreasing spessartine from core to rim.Garnet from the sillimanite metamorphic zone is almost homogeneous.Towards the outer rim,the contents of almandine and pyrope slightly decrease and grossular slightly increase.Biotite in both metamorphic zones is ferro-biotite.Plagioclase is oligoclase in garnet metamorphic zone and andesine in sillimanite metamorphic zone.Phase equilibrium modeling of a sample from garnet metamorphic zone resulted in a clockwise P-T path with a prograde stage(4.5–5.0 kbar,520–530℃),a peak P stage(9.8–10.2 kbar,560–570℃),a stage of thermal relaxation(8.0–8.5 kbar,580–590℃)and finally a retrograde stage(6.8–7.0 kbar,560–580℃).Thermodynamic modeling of a sample from the sillimanite metamorphic zone indicates a prograde stage(5.5–6.0 kbar,540–550℃)and a peak stage(7.8–8.5 kbar,660–690℃).The results indicate that the Huangyuan Group experienced medium-pressure amphibolite-facies metamorphism,which resulted from continental-continental collision between the Qaidam Block and the Central Qilian Block.展开更多
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, sta...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＋H20 and kyanite=sillimanite respectively. Based on mineral compositions, the geothermo- barometry gives an average P, T condition of （4.9±0.3）×10^8 Pa, 543±30 ℃ for the first growth stage of the garnet and （5.8±0.3）×10^8 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 thrust-decollement zones is presented according to the P-T paths, metamorphic grades and deformation styles for the staurolite-kyanite zone of the Barrovian type metamorphism, which will provide some constraints for the evolution of the nappe complex.展开更多
The clinopyroxene amphibolite from the Bailang terrane is located in the central section of the Yarlung Zangbo suture zone(YZSZ),southern Tibet.The study of it is expected to provide important clues for the subduction...The clinopyroxene amphibolite from the Bailang terrane is located in the central section of the Yarlung Zangbo suture zone(YZSZ),southern Tibet.The study of it is expected to provide important clues for the subduction of the Neo-Tethyan Ocean below the Asian Plate and thus for better understanding of the development of the India-Asia collision zone.Based on integrated textural,mineral compositional,metamorphic reaction history and geothermobarometric studies of the clinopyroxene amphibolite within a serpentinite mélange,four overprinted metamorphic stages are established.They are the first metamorphic record of M1 stage indicated by a relict assemblage of plagioclase+clinopyroxene+amphibole,an early M2 stage characterized by an assemblage of medium-grained clinopyroxene+amphibole+plagioclase+quartz as well as rutile inclusion in titanite,which is formed during burial process,an isobaric cooling M3 stage which is characterized by an assemblage of clinopyroxene+amphibole+plagioclase+titanite,and a decomposing retrograde stage M4,which is represented by the amphibolite+plagioclase symplectite+titanite+rutile+quartz.By applying the THERMOCALC(versions 6.2 and 6.3)technique in the NCFMASHTO system,the P-T conditions estimated from M1 to M4 stages are ca.8.6 kbar/880℃,10.8–13.4 kbar/800–840℃,12.7–13.2 kbar/650–660℃ and<11.2 kbar/640℃,respectively.The mineral assemblages and their P-T conditions define a counterclockwise P-T path for the clinopyroxene amphibolite of the Xigaze ophiolite,suggesting that the rocks underwent a cooling process during burial from magmatic protolith,and a decompressing stage after the pressure peak metamorphic conditions,which implies that the Bailang terrane of the Xigaze ophiolite may have experienced subduction/collision-related tectonic processes.The peak metamorphism reaches to the transitional P-T conditions among amphibolite facies,granulite facies and eclogite facies with a burial depth of 30–40 km.After exhumation of the ophiolitic unit to the shallow crustal levels,the clinopyroxene amphibolite exposes to a high fO2 condition on the basis of the stable epidotebearing assemblage in the T-MO2 diagrams.A late subgreenschist facies overprinting subsequently occurs,the relevant mineral assemblage is prehnite+albite+chlorite+epidote+quartz.展开更多
Coesite inclusions are found in kyanite from the Lanshantou eclogite in the Sulu ultrahigh-pressure (UHP) metamorphic belt. This discovery extends the stable region of kyanite to over 2.4 GPa. As an important UHP meta...Coesite inclusions are found in kyanite from the Lanshantou eclogite in the Sulu ultrahigh-pressure (UHP) metamorphic belt. This discovery extends the stable region of kyanite to over 2.4 GPa. As an important UHP metamorphic belt in China, the Sulu eclogite belt is the product of A-subduction induced by strong compression of the Yellow Sea terrane to the Jiaodong-northereastern Jiangsu terrane during the interaction of the Eurasian plate and Palaeo-Pacific plate in the Indosinian. It stretches about 350 km and contains over 1000 eclogite bodies. Most eclogites in this belt belong to Groups B and C in the classification of Coleman et al., and commonly contain kyanite, while the Lanshantou eclogite belongs to Group A and contains coesite. The MgO, CaO and FeO contents in garnet and pyroxene show regular variation from the core to the rim, which reveals the PTt paths of progressive metamorphism during the Early Mesozoic (240-200 Ma) and retrogressive metamorphism during the Late Mesozoic and Cenozoic exhumation.展开更多
The first data on P-T metamorphic conditions coupled with U-Pb monazite and zircon age obtained for the Neoarchean Kitoy granulite-gneiss terrane(SW Siberian Craton).Alumina gneisses of the Kitoy terrane indicate two-...The first data on P-T metamorphic conditions coupled with U-Pb monazite and zircon age obtained for the Neoarchean Kitoy granulite-gneiss terrane(SW Siberian Craton).Alumina gneisses of the Kitoy terrane indicate two-staged metamorphic evolution.The first stage of regional metamorphism(M1)occurred at high-amphibolite facies conditions at T=780-800℃ and P=8-9 kbar.The second stage(M2)belongs to MT-HT/LP type of metamorphism with the wide temperature interval 600-750℃ and pressure 2-4 kbar.Two age peaks were established on the basis of U-Pb monazite and zircon dating in garnet-anthophyllite gneisses.Both of them correspond to the Neoarchean age:the age of M1 falls into the interval of ca.2489-2496 Ma,the age of M2-ca.2446-2456 Ma.The high-temperature metamorphism of the Kitoy block and nearly coeval granitoid magmatism can be an evidence for the Neoarchean collision in SW Siberian craton.展开更多
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 ...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.展开更多
The Qilian Orogen marks the junction of the North China, South China and Tarim cratons. The mechanism of continental growth during the formation of the orogen remains unclear. Based on detailed fieldwork, we present a...The Qilian Orogen marks the junction of the North China, South China and Tarim cratons. The mechanism of continental growth during the formation of the orogen remains unclear. Based on detailed fieldwork, we present a systematic study of petrography, mineral chemistry and phase equilibria of garnet amphibolites from the Hualong Group, which represents the Precambrian basement in the southern accretionary belt of the Qilian Orogen. The garnet amphibolites mainly consist of amphibole, plagioclase, garnet and quartz, with minor pyroxene, biotite and ilmenite. A peak stage of upper amphibolite facies to low-temperature granulite facies metamorphism and retrograde metamorphism in the amphibolite facies affected the samples. Garnet has a homogeneous composition of Alm66-71Grs14-17Prp9_12Sps3-s, amphibole is ferro-hornblende, biotite belongs to the ferro-biotite species and pyroxene is dominated by orthopyroxene with few clinopyroxene. Pseudosection modeling of the garnet amphibolite samples indicates clockwise P-T paths. The samples witness peak metamorphism at conditions of -4.9-6.3 kbar and -755-820 ℃ in the upper amphibolite facies to low- temperature granulite facies, and retrograde cooling and decompression at conditions of-2.5-3.1 kbar and -325-545 ℃. It is inferred that peak metamorphism with high temperature and low pressure occurred at ca. 450 Ma during northward subduction of the South Qilian oceanic crust beneath the central Qilian Block. When continental collision occurred between the central Qilian and the Qaidam blocks, the Hualong Block was aecreted onto the South Qilian accretionary complex and experienced amphibolite facies retrograde metamorphism at ca. 440 Ma.展开更多
: The kinematic vorticity number and strain of the mylonitic zone related to the detachment fault increase from ESE to WNW along the moving direction of the upper plate of the Xiaoqinling metamorphic core complex (XMC...: The kinematic vorticity number and strain of the mylonitic zone related to the detachment fault increase from ESE to WNW along the moving direction of the upper plate of the Xiaoqinling metamorphic core complex (XMCC) and the geometry of quartz c-axis fabrics changes progressively from crossed girdles to single girdles in the same direction. Therefore, pure shear is dominant in the ESE part of the XMCC while simple shear becomes increasingly important towards WNW. However, the shear type does not change with the strain across the shear zone, thus the variation of shear type is of significance in indicating the formation mechanism. The granitic plutons within the XMCC came from the deep source and their emplacement was an active and forceful upwelling prior to the detachment faulting. The PTt path demonstrates that magmatism is an important cause for the formation of the XMCC. The formation mechanism of the XMCC is supposed to be active plutonism and passive detachment. Crustal thickening and magmatic doming caused necking extension with pure shear, and magmatic heating and doming resulted in detachment extension with simple shear and formed the XMCC.展开更多
The Kadui blueschist is located in the central section of Yarlung Zangbo suture zone （YZSZ）, southern Tibet, and has been subjected to the subduction of the Neo-Tethyan Ocean below the Asian Plate and provides impor...The Kadui blueschist is located in the central section of Yarlung Zangbo suture zone （YZSZ）, southern Tibet, and has been subjected to the subduction of the Neo-Tethyan Ocean below the Asian Plate and provides important clues for better understanding the evolution of the India-Asia convergence zone. In this paper, the systematical petrographic and mineral chemical studies were carried out on the Kadui blueschist, which reveal a mineral assemblage of sodic amphibole, chlorite, epidote, albite and quartz with accessory minerals of titanite, calcite and zircon. Electron microprobe analyses demonstrate that amphibole shows zoned from actinolite core to ferrowinchite/riebeckite rim composition indicating that the sodic amphibole has formed during a prograde metamorphic event. The protolith of the blueschist is an intermediate-basic pyroclastic rock. The calculated pseudosection indicates a clockwise P-T path and constrains peak metamorphic conditions of about 5.9 kbar at 345 ℃. This condition is transitional between pumpellyite-actinolite, greenschist and blueschist facies with a burial depth of 20-22 km and a thermal gradient of 15-16 ℃/km. This thermal gradient belongs to high pressure intermediate P/T facies series and is possibly related to a warm subduction setting of young oceanic slabs. Our new findings indicate that the Kadui blueschist in the central part of YZSZ experienced a rapid subduction and exhumation process as a response to a northward subduction of the Neo-Tethyan oceanic lithosphere during the initial India-Asia collision stage.展开更多
The Khondalite Belt within the Inner Mongolia Suture Zone （IMSZ） in the North China Craton is a classic example for Paleoproterozoic ultrahigh-temperature （UHT） metamorphism. Here we report new spinel-bearing meta...The Khondalite Belt within the Inner Mongolia Suture Zone （IMSZ） in the North China Craton is a classic example for Paleoproterozoic ultrahigh-temperature （UHT） metamorphism. Here we report new spinel-bearing metapelitic granulites from a new locality at Xumayao within the southern domain of the IMSZ. Petrological studies and thermodynamic modeling of the spinel -- quartz-bearing assemblage shows that these rocks experienced extreme metamorphism at UHT conditions. Spinel occurs in two textural settings：（1） high Xzn（Zn/（Mg ＋ FeH -- Zn） = 0.071--0.232） spinel with perthitic K-feld- spar, sillimanite and quartz in the rock matrix; and （2）low Xzn （0.045-0.070） spinel as inclusions within garnet porphyroblasts in association with quartz and sillimanite. Our phase equilibria modeling indicates two main stages during the metamorphic evolution of these rocks：（l） near-isobaric cooling from 975 ℃ to 875 ℃ around 8 kbar, represented by the formation of garnet porphyroblasts from spinel and quartz; and（2）cooling and decompression from 850 ℃, 8 kbar to below 750 ℃- 6.5 kbar, represented by the break-down of garnet. The spinel ＋ quartz assemblage is considered to have been stable at peak metamorphism, formed through the break-down of cordierite, indi- caring a near isothermal compression process. Our study confirms the regional extent of UHT metamor- phism within the IMSZ associated with the Paleoproterozoic subduction-collision process.展开更多
The development of metamorphic petrology to metamorphic geology in China has a long history.Ancient basement metamorphic rocks are distributed primarily in the North China Craton,the Yangtze Block and Tarim Craton.The...The development of metamorphic petrology to metamorphic geology in China has a long history.Ancient basement metamorphic rocks are distributed primarily in the North China Craton,the Yangtze Block and Tarim Craton.They are mainly made up of plutonic gneiss and metamorphosed supercrust rock,transformed to granulite facies through Archean Paleoproterozoic.Many of the Paleoproterozoic metamorphic rocks have undergone high-pressure granulite facies metamorphism with a clockwise metamorphic evolution path.The ultrahigh temperature (UHT)granulites from the Late Paleoproterozoic are found in North China Craton.Many high-precision chronological data have allowed preliminary construction of the formation and evolutionary framework of different metamorphic basements.Primarily there are low-temperature and high-pressure blue schist,high-temperature and high-pressure granulite and ultrahigh-pressure (UHP)eclogite facies metamorphic rocks in the Phanerozoic orogenic belt.The discovery of eclogite in the Sulu orogen and a large quantity of coesite in its country rocks show that there was a deep subduction of voluminous continental materials during the collision process between the Yangtze block and the North China Craton in the Early Mesozoic phase.From the studies of,for instance,organic matter vitrinite reflectance,illite crystallinity,illite (muscovite)polytype and illite (muscovite)b dimension,the Late Paleozoic strata in the eastern region of Inner Mongolia and the north-central region of NE China have only experienced diagenesis to an extremely low-grade metamorphism.The discovery of impact-metamorphosed rocks in Xiuyan area of Liaoning province has enriched the type and category of metamorphic rocks in China.The phase equilibrium method has been widely used in the study of metamorphism of middle and high-grade metamorphic rocks.On the basis of existing geologic surveys and monographic study results, different scholars have respectively compiled 1:1500000 Metamorphic Geological Map and Specifications of Qinghai Tibet Plateau and its Adjacent Areas,1:2500000 Metamorphic Tectonic Map of China, and the 1:5000000 Metamorphic Geological Map and Specifications of China,among others repectively, which have systematically summarized the research results of metamorphic petrology and metamorphic geology in China.展开更多
Omphacite microstructures are important to decipher the metamorphic pressuretemperature(P-T)conditions of eclogite during subduction and exhumation processes.Here we present a systematic microstructural investigation ...Omphacite microstructures are important to decipher the metamorphic pressuretemperature(P-T)conditions of eclogite during subduction and exhumation processes.Here we present a systematic microstructural investigation of omphacite using transmission electron microscopy(TEM)in an ultrahigh-pressure(UHP)eclogite sample from Shuanghe,Dabie orogen,China.The omphacite can be divided into two phases:(1)Omp-1,exhibits as the main part of omphacite with P2/n space group containing small amount of dislocations and numerous antiphase domains(APDs).(2)Omp-2,shows as banded subgrains with C2/c space group containing large amount of dislocations.These two phases of omphacite have almost the same crystallography orientation and bounded by dislocation walls.Along these dislocation walls,we found some barroisite and albite microcrystals.The barroisite microcrystals contain large amount of dislocations and show a topological relation with host omphacite.While the albite microcrystals contain small amount of dislocations and does not show topological relation with host.These microstructures could give us the following metamorphic information to constrain the P-T path of eclogite:Firstly,this two omphacite space groups should be evolved from the same precursor C2/c omphacite,which had been underwent strongly plastic deformation during the syn-to late-peak metamorphic stage.Secondly,the precursor C2/c omphacite began to be retrograded and altered by small amount of barroisite microcrystals along its dislocation walls under the P-T condition of 2.2–2.6 GPa and 650–700℃ at the early amphibole eclogite stage.Thirdly,large amount of Na and other elements were exsolved from some precursor C2/c omphacite subgrains and crystallized numerous albite microcrystals at their boundaries,which is the necessary for the space group of C2/c surviving under the lower P-T conditions(1.7–1.9 GPa and 630–690 C)during the middle amphibole eclogite stage.Lastly,the rest of precursor C2/c omphacite subgrains were transformed into P2/n polymorph and formed the APDs structures under the P-T condition of^1.5 GPa and 650–680℃ during the late amphibole eclogite stage.展开更多
New field observations and petrological data from Early Cretaceous metamorphic rocks in the Central Cordillera of the Colombian Andes allowed the recognition of thermally overprinted high-pressure rocks derived from o...New field observations and petrological data from Early Cretaceous metamorphic rocks in the Central Cordillera of the Colombian Andes allowed the recognition of thermally overprinted high-pressure rocks derived from oceanic crust protoliths.The obtained metamorphic path suggests that the rocks evolved from blueschist to eclogite facies towards upper amphibolite to high-pressure granulite facies transitional conditions.Eclogite facies conditions,better recorded in mafic protoliths,are revealed by relic lawsonite and phengite,bleb-to worm-like diopside-albite symplectites,as well as garnet core composition.Upper amphibolite to high pressure granulite facies overprinting is supported by coarse-grained brown-colored Ti-rich amphibole,augite,and oligoclase recrystallization,as well as the record of partial melting leucosomes.Phase equilibria and pressure-temperature(P-T)path modeling suggest initial high-pressure metamorphic conditions M1 yielding 18.2-24.5 kbar and 465-580℃,followed by upper amphibolite to high pressure granulite facies overprinting stage M2 yielding 6.5-14.2 kbar and 580-720℃.Retrograde conditions M3 obtained through chlorite thermometry yield temperatures ranging around 286-400℃at pressures below 6.5-11 kbar.The obtained clockwise P-T path,the garnet zonation pattern revealing a decrease in X_(grs)/X_(prp)related to Mg#increment from core to rim,the presence of partial melting veins,as well as regional constraints,document the modification of the thermal structure of the active subduction zone in Northern Andes during the Early Cretaceous.Such increment of the metamorphic gradient within the subduction interface is associated with slab roll-back geodynamics where hot mantle inflow was triggered.This scenario is also argued by the reported trench-ward magmatic arc migration and multiple extensional basin formation during this period.The presented example constitutes the first report of Cretaceous roll-back-related metamorphism in the Caribbean and Andean realms,representing an additional piece of evidence for a margin-scale extensional event that modified the northwestern border of South America during the Early Cretaceous.展开更多
A chloritoid-garnet-bearing micaschist from the southern part of the Elstergebirge was studied to better understand the Variscan orogenic evolution in the Saxothuringian zone of the northwestern Bohemian Massif.Based ...A chloritoid-garnet-bearing micaschist from the southern part of the Elstergebirge was studied to better understand the Variscan orogenic evolution in the Saxothuringian zone of the northwestern Bohemian Massif.Based on the textural relations and compositions of minerals,especially of zoned garnet and potassic white mica,a P-T path was reconstructed using contoured P-T pseudosections.The U-Th-Pb dating of monazite in the micaschist was undertaken with the electron microprobe.The micaschist experienced P-T conditions along a clockwise path between 16 kbar at 510℃and 5 kbar at 555℃followed by isobaric heating to about 600℃.Monazite ages range between 315 and 480 Ma with the most prominent maxima and side maxima at 346.0±1.1(2σ),357.3±2.3,and 368.3±1.7 Ma.Ages older than 380 Ma were related to detrital monazite pointing to a Devonian sedimentary protolith.Other ages around 325 Ma were assigned to the isobaric heating by nearby post-tectonic granites.The high-pressure event,being the result of the collision of Laurussia and Gondwana after closure of the Rheic Ocean,occurred in the Late Devonian.The exhumation to 15-20 km(5 kbar)ended probably in the Early Carboniferous.The high-pressure micaschists from the Fichtelgebirge to the Erzgebirge crystalline complexes are suggested to represent a single nappe within a metamorphic nappe pile.This nappe is composed of metasedimentary slices,which experienced different peak pressures rather than representing a coherent crustal section.展开更多
In this study, we investigate the possible record of a Late Mesoproterozoic paired metamorphic belt in the Aravalli-Delhi Mobile Belt(ADMB), NW India using a suite of supracrustal and metaigneous granulites from the P...In this study, we investigate the possible record of a Late Mesoproterozoic paired metamorphic belt in the Aravalli-Delhi Mobile Belt(ADMB), NW India using a suite of supracrustal and metaigneous granulites from the Pilwa-Chinwali granulite terrain at the north-western margin of the ADMB. Using metamorphic reaction textures, mineral chemistry, metamorphic reaction history, geothermobarometric computations and electron microprobe dating of monazite in 5 samples of pelitic granulite, leptynite gneiss, enderbite and charnockite, we have deduced a medium-pressure granulite facies metamorphism(P between 4.9 and 6.8 kbar, T> 760-815℃) along a heating-cooling, counterclockwise P-T path between 1.09 and 1.01 Ga. When collated with published metamorphic and chronological constraints and geological settings of the adjoining crustal domains of the ADMB, these findings provide new insights into the developments of two tectonic domains of contrasting thermal gradients at ca. 1.0 Ga, consistent with metamorphic transformations in tectonically thickened middle-lower crustal sections during continental collision to continental subduction and in the root zones of spatially adjacent island arc, as part of the Rodinia supercontinent assembly event.展开更多
The Qinling orogenic belt is a collision zone between the North China and Yangtze cratons. The Qinling Complex is a Precambrian metamorphic complex, developed in the inner zone of the orogenic belt , which records the...The Qinling orogenic belt is a collision zone between the North China and Yangtze cratons. The Qinling Complex is a Precambrian metamorphic complex, developed in the inner zone of the orogenic belt , which records the metamorphic and deformational history and PTt path of the regional meta-morphism of the collision zone . The present paper studies the metamorphic and deformational history and the PTt path of various tectono-metamorphic cycles in order to describe the geodynamic processes prevailing in that part of the Qinling orogenic belt since Proterozoic.The tectonometamorphic history and evolution of the Qinling Complex is divided into two stages: the stage of formation and the stage of modification During the stage of formation dated as Proterozoic,three deformational sequences are recognized. The amphibolite facies regional metamorphism is earlier than or synchronous with the first or second phase of folding. Three metamorphic zones, i.e. And-Ms ,Sil- Ms, Sil Kfs are delimited. During the stage of modification , the emplacement of Caledonian granite , superimposed thermal aureole (garnet K feldspar zone )and the development of ductile shear zones are major events.Metamorphic reactions are reconstructed in terms of analysis of re-equilibrium textures. Geothermobarometric calculations are conducted to calibrate the PT conditions of metamorphism .The metamorphic PTt paths have been estabh'shed. The PTt path of the Proterozoic tectonometamorphic cycle shows a clockwise pattern and is characterized by prominent decompression. The PTt path of the Caledonian tectonometamorphic cycle is characterized first by PT rising ,then isothermal decompression (rapid uplift), and finally by isobaric cooling The PTt path of the two tectonometamorphic cycles evidence the geodynamic processes: two major stages of collision and uplift in the Proterozoic and Caledonian Hercynian periods, respectively, during the evolution of Qinling orogenic belt.展开更多
After the integration of petrographic study, geothermobarometry and Gibbs method, the synthetic P-T paths for the rocks from different geological profiles in the North Qilian, China, have been derived. The composite P...After the integration of petrographic study, geothermobarometry and Gibbs method, the synthetic P-T paths for the rocks from different geological profiles in the North Qilian, China, have been derived. The composite P-T paths from different methods indicate that all the high-pressure rocks in the Qilian area recorded P-T paths with clockwise loops starting at the blueschist facies, later reaching peak metamorphism at the blueschist facies, eclogite fades or epidote-amphibolite facies and ending up with the greenschist facies. The incremental Ar-Ar dating shows that the plateau ages for the high-pressure rocks range from 410 to 443 Ma. The plateau ages could be used as a minimum age constraint for the subduction that resulted in the formation of these high-pressure rocks in the Qilian area. It is proposed that the late-stage decompressional and cooling P-T paths with ends at the greenschist facies for these high-pressure rocks probably reflect the uplift process which could occur after shifting the arc-trench tectonic system to the system of continental orogenic belts. The retrograde paths for the high-pressure rocks in the North Qilian tectonic belt are characterized by dramatic decompression with slight cooling, which suggests very rapid exhumation. Petrography supports that the mountain-building for the Qilian mountain range could undergo a very fast process which caused rapid uplift and denudation.展开更多
Considering the mineral evolution and tectonic deformational history of the early Proterozoic metamorphic sequences in the Jiao Liao Massif, very similar CCW PTt D paths were established in east Liaoning and east Shan...Considering the mineral evolution and tectonic deformational history of the early Proterozoic metamorphic sequences in the Jiao Liao Massif, very similar CCW PTt D paths were established in east Liaoning and east Shandong district within this massif. Their style displays the process of developing and closing of some rifted depression belts within the rather stable continental crust of the Jiao Liao Massif during early Proterozoic, and they may be treated as a common kind of metamorphic geodynamic model.展开更多
The Yushugou HP granulite-peridotite complex is located at east of northern margin of southern Tianshan mountains,China,which consists of granulite unit and peridotite unit mainly.Because of the rare association of
基金funded by the National Natural Science Foundation of China (No. 41520104003)the National Key R & D Program of China (No. 2016YFC0600403)+1 种基金the China Geological Survey (No. DD20160201)the Fundamental Research Funds for the Central Universities,China University of Geosciences (Wuhan) (Nos. CUGL170404,CUG160232)
文摘The Central Qilian Block is a Precambrian block in the Qilian Orogen,which has long drawn international attention for the study of orogeny and continental dynamics.The Huangyuan Group in the Datong area is one of the Precambrian metamorphic basement units in the Central Qilian Block and reflects metamorphism in the Barrovian garnet zone and sillimanite zone from south to north.Based on detailed fieldwork,this study presents a systematic study of petrography,mineral chemistry and phase equilibria of schists and gneisses from the two metamorphic zones.The garnet metamorphic zone is composed of micaschist,garnet-bearing micaschist and felsic leptynite,with interlayered plagioclase amphibolite.The sillimanite metamorphic zone consists of garnet-bearing biotite micaschist,sillimanite-bearing biotite-plagioclase gneiss and felsic leptynite.Garnet from the garnet metamorphic zone shows growth zoning with increasing almandine and pyrope and decreasing spessartine from core to rim.Garnet from the sillimanite metamorphic zone is almost homogeneous.Towards the outer rim,the contents of almandine and pyrope slightly decrease and grossular slightly increase.Biotite in both metamorphic zones is ferro-biotite.Plagioclase is oligoclase in garnet metamorphic zone and andesine in sillimanite metamorphic zone.Phase equilibrium modeling of a sample from garnet metamorphic zone resulted in a clockwise P-T path with a prograde stage(4.5–5.0 kbar,520–530℃),a peak P stage(9.8–10.2 kbar,560–570℃),a stage of thermal relaxation(8.0–8.5 kbar,580–590℃)and finally a retrograde stage(6.8–7.0 kbar,560–580℃).Thermodynamic modeling of a sample from the sillimanite metamorphic zone indicates a prograde stage(5.5–6.0 kbar,540–550℃)and a peak stage(7.8–8.5 kbar,660–690℃).The results indicate that the Huangyuan Group experienced medium-pressure amphibolite-facies metamorphism,which resulted from continental-continental collision between the Qaidam Block and the Central Qilian Block.
基金supported by the National Natural Science Foundation of China (Nos. 40472097, 90814006)Re-search Fund for the Doctoral Program of Higher Education of China (No. 20070491516)
文摘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＋H20 and kyanite=sillimanite respectively. Based on mineral compositions, the geothermo- barometry gives an average P, T condition of （4.9±0.3）×10^8 Pa, 543±30 ℃ for the first growth stage of the garnet and （5.8±0.3）×10^8 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 thrust-decollement zones is presented according to the P-T paths, metamorphic grades and deformation styles for the staurolite-kyanite zone of the Barrovian type metamorphism, which will provide some constraints for the evolution of the nappe complex.
基金financially supported by the National Natural Science Foundation of China (No. 41572044)the SDUST Research Fund (No. 2015TDJH101)
文摘The clinopyroxene amphibolite from the Bailang terrane is located in the central section of the Yarlung Zangbo suture zone(YZSZ),southern Tibet.The study of it is expected to provide important clues for the subduction of the Neo-Tethyan Ocean below the Asian Plate and thus for better understanding of the development of the India-Asia collision zone.Based on integrated textural,mineral compositional,metamorphic reaction history and geothermobarometric studies of the clinopyroxene amphibolite within a serpentinite mélange,four overprinted metamorphic stages are established.They are the first metamorphic record of M1 stage indicated by a relict assemblage of plagioclase+clinopyroxene+amphibole,an early M2 stage characterized by an assemblage of medium-grained clinopyroxene+amphibole+plagioclase+quartz as well as rutile inclusion in titanite,which is formed during burial process,an isobaric cooling M3 stage which is characterized by an assemblage of clinopyroxene+amphibole+plagioclase+titanite,and a decomposing retrograde stage M4,which is represented by the amphibolite+plagioclase symplectite+titanite+rutile+quartz.By applying the THERMOCALC(versions 6.2 and 6.3)technique in the NCFMASHTO system,the P-T conditions estimated from M1 to M4 stages are ca.8.6 kbar/880℃,10.8–13.4 kbar/800–840℃,12.7–13.2 kbar/650–660℃ and<11.2 kbar/640℃,respectively.The mineral assemblages and their P-T conditions define a counterclockwise P-T path for the clinopyroxene amphibolite of the Xigaze ophiolite,suggesting that the rocks underwent a cooling process during burial from magmatic protolith,and a decompressing stage after the pressure peak metamorphic conditions,which implies that the Bailang terrane of the Xigaze ophiolite may have experienced subduction/collision-related tectonic processes.The peak metamorphism reaches to the transitional P-T conditions among amphibolite facies,granulite facies and eclogite facies with a burial depth of 30–40 km.After exhumation of the ophiolitic unit to the shallow crustal levels,the clinopyroxene amphibolite exposes to a high fO2 condition on the basis of the stable epidotebearing assemblage in the T-MO2 diagrams.A late subgreenschist facies overprinting subsequently occurs,the relevant mineral assemblage is prehnite+albite+chlorite+epidote+quartz.
文摘Coesite inclusions are found in kyanite from the Lanshantou eclogite in the Sulu ultrahigh-pressure (UHP) metamorphic belt. This discovery extends the stable region of kyanite to over 2.4 GPa. As an important UHP metamorphic belt in China, the Sulu eclogite belt is the product of A-subduction induced by strong compression of the Yellow Sea terrane to the Jiaodong-northereastern Jiangsu terrane during the interaction of the Eurasian plate and Palaeo-Pacific plate in the Indosinian. It stretches about 350 km and contains over 1000 eclogite bodies. Most eclogites in this belt belong to Groups B and C in the classification of Coleman et al., and commonly contain kyanite, while the Lanshantou eclogite belongs to Group A and contains coesite. The MgO, CaO and FeO contents in garnet and pyroxene show regular variation from the core to the rim, which reveals the PTt paths of progressive metamorphism during the Early Mesozoic (240-200 Ma) and retrogressive metamorphism during the Late Mesozoic and Cenozoic exhumation.
基金financial support of the Ministry of Science and Higher Education of the Russian Federationthe International Partnership Program of Chinese Academy of Sciences,Grant No.132744KYSB20190039。
文摘The first data on P-T metamorphic conditions coupled with U-Pb monazite and zircon age obtained for the Neoarchean Kitoy granulite-gneiss terrane(SW Siberian Craton).Alumina gneisses of the Kitoy terrane indicate two-staged metamorphic evolution.The first stage of regional metamorphism(M1)occurred at high-amphibolite facies conditions at T=780-800℃ and P=8-9 kbar.The second stage(M2)belongs to MT-HT/LP type of metamorphism with the wide temperature interval 600-750℃ and pressure 2-4 kbar.Two age peaks were established on the basis of U-Pb monazite and zircon dating in garnet-anthophyllite gneisses.Both of them correspond to the Neoarchean age:the age of M1 falls into the interval of ca.2489-2496 Ma,the age of M2-ca.2446-2456 Ma.The high-temperature metamorphism of the Kitoy block and nearly coeval granitoid magmatism can be an evidence for the Neoarchean collision in SW Siberian craton.
基金the National Natural Science Foundation of China grants 40172019 ,40272083 the Natural Science Foundation of HubeiProvince grant 2002AB020.
文摘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.
基金funded by the National Natural Science Foundation of China (No. 41520104003)the National Key R & D Program of China (No. 2016YFC0600403)+1 种基金the China Geological Survey (No. DD20160201)the Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) (Nos. CUGL170404, CUG160232)
文摘The Qilian Orogen marks the junction of the North China, South China and Tarim cratons. The mechanism of continental growth during the formation of the orogen remains unclear. Based on detailed fieldwork, we present a systematic study of petrography, mineral chemistry and phase equilibria of garnet amphibolites from the Hualong Group, which represents the Precambrian basement in the southern accretionary belt of the Qilian Orogen. The garnet amphibolites mainly consist of amphibole, plagioclase, garnet and quartz, with minor pyroxene, biotite and ilmenite. A peak stage of upper amphibolite facies to low-temperature granulite facies metamorphism and retrograde metamorphism in the amphibolite facies affected the samples. Garnet has a homogeneous composition of Alm66-71Grs14-17Prp9_12Sps3-s, amphibole is ferro-hornblende, biotite belongs to the ferro-biotite species and pyroxene is dominated by orthopyroxene with few clinopyroxene. Pseudosection modeling of the garnet amphibolite samples indicates clockwise P-T paths. The samples witness peak metamorphism at conditions of -4.9-6.3 kbar and -755-820 ℃ in the upper amphibolite facies to low- temperature granulite facies, and retrograde cooling and decompression at conditions of-2.5-3.1 kbar and -325-545 ℃. It is inferred that peak metamorphism with high temperature and low pressure occurred at ca. 450 Ma during northward subduction of the South Qilian oceanic crust beneath the central Qilian Block. When continental collision occurred between the central Qilian and the Qaidam blocks, the Hualong Block was aecreted onto the South Qilian accretionary complex and experienced amphibolite facies retrograde metamorphism at ca. 440 Ma.
文摘: The kinematic vorticity number and strain of the mylonitic zone related to the detachment fault increase from ESE to WNW along the moving direction of the upper plate of the Xiaoqinling metamorphic core complex (XMCC) and the geometry of quartz c-axis fabrics changes progressively from crossed girdles to single girdles in the same direction. Therefore, pure shear is dominant in the ESE part of the XMCC while simple shear becomes increasingly important towards WNW. However, the shear type does not change with the strain across the shear zone, thus the variation of shear type is of significance in indicating the formation mechanism. The granitic plutons within the XMCC came from the deep source and their emplacement was an active and forceful upwelling prior to the detachment faulting. The PTt path demonstrates that magmatism is an important cause for the formation of the XMCC. The formation mechanism of the XMCC is supposed to be active plutonism and passive detachment. Crustal thickening and magmatic doming caused necking extension with pure shear, and magmatic heating and doming resulted in detachment extension with simple shear and formed the XMCC.
基金financially supported by the National Natural Science Foundation of China(No.41572044)the SDUST Research Fund(No.2015TDJH101)
文摘The Kadui blueschist is located in the central section of Yarlung Zangbo suture zone （YZSZ）, southern Tibet, and has been subjected to the subduction of the Neo-Tethyan Ocean below the Asian Plate and provides important clues for better understanding the evolution of the India-Asia convergence zone. In this paper, the systematical petrographic and mineral chemical studies were carried out on the Kadui blueschist, which reveal a mineral assemblage of sodic amphibole, chlorite, epidote, albite and quartz with accessory minerals of titanite, calcite and zircon. Electron microprobe analyses demonstrate that amphibole shows zoned from actinolite core to ferrowinchite/riebeckite rim composition indicating that the sodic amphibole has formed during a prograde metamorphic event. The protolith of the blueschist is an intermediate-basic pyroclastic rock. The calculated pseudosection indicates a clockwise P-T path and constrains peak metamorphic conditions of about 5.9 kbar at 345 ℃. This condition is transitional between pumpellyite-actinolite, greenschist and blueschist facies with a burial depth of 20-22 km and a thermal gradient of 15-16 ℃/km. This thermal gradient belongs to high pressure intermediate P/T facies series and is possibly related to a warm subduction setting of young oceanic slabs. Our new findings indicate that the Kadui blueschist in the central part of YZSZ experienced a rapid subduction and exhumation process as a response to a northward subduction of the Neo-Tethyan oceanic lithosphere during the initial India-Asia collision stage.
文摘The Khondalite Belt within the Inner Mongolia Suture Zone （IMSZ） in the North China Craton is a classic example for Paleoproterozoic ultrahigh-temperature （UHT） metamorphism. Here we report new spinel-bearing metapelitic granulites from a new locality at Xumayao within the southern domain of the IMSZ. Petrological studies and thermodynamic modeling of the spinel -- quartz-bearing assemblage shows that these rocks experienced extreme metamorphism at UHT conditions. Spinel occurs in two textural settings：（1） high Xzn（Zn/（Mg ＋ FeH -- Zn） = 0.071--0.232） spinel with perthitic K-feld- spar, sillimanite and quartz in the rock matrix; and （2）low Xzn （0.045-0.070） spinel as inclusions within garnet porphyroblasts in association with quartz and sillimanite. Our phase equilibria modeling indicates two main stages during the metamorphic evolution of these rocks：（l） near-isobaric cooling from 975 ℃ to 875 ℃ around 8 kbar, represented by the formation of garnet porphyroblasts from spinel and quartz; and（2）cooling and decompression from 850 ℃, 8 kbar to below 750 ℃- 6.5 kbar, represented by the break-down of garnet. The spinel ＋ quartz assemblage is considered to have been stable at peak metamorphism, formed through the break-down of cordierite, indi- caring a near isothermal compression process. Our study confirms the regional extent of UHT metamor- phism within the IMSZ associated with the Paleoproterozoic subduction-collision process.
基金the National Natural Science Foundation of China (41203025)the Work Project of the China Geological Survey (1212010811048)the Fundamental Research Fund of the Institute of Geology of the Chinese Academy of Geological Sciences (J1615).
文摘The development of metamorphic petrology to metamorphic geology in China has a long history.Ancient basement metamorphic rocks are distributed primarily in the North China Craton,the Yangtze Block and Tarim Craton.They are mainly made up of plutonic gneiss and metamorphosed supercrust rock,transformed to granulite facies through Archean Paleoproterozoic.Many of the Paleoproterozoic metamorphic rocks have undergone high-pressure granulite facies metamorphism with a clockwise metamorphic evolution path.The ultrahigh temperature (UHT)granulites from the Late Paleoproterozoic are found in North China Craton.Many high-precision chronological data have allowed preliminary construction of the formation and evolutionary framework of different metamorphic basements.Primarily there are low-temperature and high-pressure blue schist,high-temperature and high-pressure granulite and ultrahigh-pressure (UHP)eclogite facies metamorphic rocks in the Phanerozoic orogenic belt.The discovery of eclogite in the Sulu orogen and a large quantity of coesite in its country rocks show that there was a deep subduction of voluminous continental materials during the collision process between the Yangtze block and the North China Craton in the Early Mesozoic phase.From the studies of,for instance,organic matter vitrinite reflectance,illite crystallinity,illite (muscovite)polytype and illite (muscovite)b dimension,the Late Paleozoic strata in the eastern region of Inner Mongolia and the north-central region of NE China have only experienced diagenesis to an extremely low-grade metamorphism.The discovery of impact-metamorphosed rocks in Xiuyan area of Liaoning province has enriched the type and category of metamorphic rocks in China.The phase equilibrium method has been widely used in the study of metamorphism of middle and high-grade metamorphic rocks.On the basis of existing geologic surveys and monographic study results, different scholars have respectively compiled 1:1500000 Metamorphic Geological Map and Specifications of Qinghai Tibet Plateau and its Adjacent Areas,1:2500000 Metamorphic Tectonic Map of China, and the 1:5000000 Metamorphic Geological Map and Specifications of China,among others repectively, which have systematically summarized the research results of metamorphic petrology and metamorphic geology in China.
基金This work was supported by the National Natural Science Foundation of China(Nos.41272224,41972231).
文摘Omphacite microstructures are important to decipher the metamorphic pressuretemperature(P-T)conditions of eclogite during subduction and exhumation processes.Here we present a systematic microstructural investigation of omphacite using transmission electron microscopy(TEM)in an ultrahigh-pressure(UHP)eclogite sample from Shuanghe,Dabie orogen,China.The omphacite can be divided into two phases:(1)Omp-1,exhibits as the main part of omphacite with P2/n space group containing small amount of dislocations and numerous antiphase domains(APDs).(2)Omp-2,shows as banded subgrains with C2/c space group containing large amount of dislocations.These two phases of omphacite have almost the same crystallography orientation and bounded by dislocation walls.Along these dislocation walls,we found some barroisite and albite microcrystals.The barroisite microcrystals contain large amount of dislocations and show a topological relation with host omphacite.While the albite microcrystals contain small amount of dislocations and does not show topological relation with host.These microstructures could give us the following metamorphic information to constrain the P-T path of eclogite:Firstly,this two omphacite space groups should be evolved from the same precursor C2/c omphacite,which had been underwent strongly plastic deformation during the syn-to late-peak metamorphic stage.Secondly,the precursor C2/c omphacite began to be retrograded and altered by small amount of barroisite microcrystals along its dislocation walls under the P-T condition of 2.2–2.6 GPa and 650–700℃ at the early amphibole eclogite stage.Thirdly,large amount of Na and other elements were exsolved from some precursor C2/c omphacite subgrains and crystallized numerous albite microcrystals at their boundaries,which is the necessary for the space group of C2/c surviving under the lower P-T conditions(1.7–1.9 GPa and 630–690 C)during the middle amphibole eclogite stage.Lastly,the rest of precursor C2/c omphacite subgrains were transformed into P2/n polymorph and formed the APDs structures under the P-T condition of^1.5 GPa and 650–680℃ during the late amphibole eclogite stage.
基金The National University of Colombia is acknowledged for its financial support。
文摘New field observations and petrological data from Early Cretaceous metamorphic rocks in the Central Cordillera of the Colombian Andes allowed the recognition of thermally overprinted high-pressure rocks derived from oceanic crust protoliths.The obtained metamorphic path suggests that the rocks evolved from blueschist to eclogite facies towards upper amphibolite to high-pressure granulite facies transitional conditions.Eclogite facies conditions,better recorded in mafic protoliths,are revealed by relic lawsonite and phengite,bleb-to worm-like diopside-albite symplectites,as well as garnet core composition.Upper amphibolite to high pressure granulite facies overprinting is supported by coarse-grained brown-colored Ti-rich amphibole,augite,and oligoclase recrystallization,as well as the record of partial melting leucosomes.Phase equilibria and pressure-temperature(P-T)path modeling suggest initial high-pressure metamorphic conditions M1 yielding 18.2-24.5 kbar and 465-580℃,followed by upper amphibolite to high pressure granulite facies overprinting stage M2 yielding 6.5-14.2 kbar and 580-720℃.Retrograde conditions M3 obtained through chlorite thermometry yield temperatures ranging around 286-400℃at pressures below 6.5-11 kbar.The obtained clockwise P-T path,the garnet zonation pattern revealing a decrease in X_(grs)/X_(prp)related to Mg#increment from core to rim,the presence of partial melting veins,as well as regional constraints,document the modification of the thermal structure of the active subduction zone in Northern Andes during the Early Cretaceous.Such increment of the metamorphic gradient within the subduction interface is associated with slab roll-back geodynamics where hot mantle inflow was triggered.This scenario is also argued by the reported trench-ward magmatic arc migration and multiple extensional basin formation during this period.The presented example constitutes the first report of Cretaceous roll-back-related metamorphism in the Caribbean and Andean realms,representing an additional piece of evidence for a margin-scale extensional event that modified the northwestern border of South America during the Early Cretaceous.
文摘A chloritoid-garnet-bearing micaschist from the southern part of the Elstergebirge was studied to better understand the Variscan orogenic evolution in the Saxothuringian zone of the northwestern Bohemian Massif.Based on the textural relations and compositions of minerals,especially of zoned garnet and potassic white mica,a P-T path was reconstructed using contoured P-T pseudosections.The U-Th-Pb dating of monazite in the micaschist was undertaken with the electron microprobe.The micaschist experienced P-T conditions along a clockwise path between 16 kbar at 510℃and 5 kbar at 555℃followed by isobaric heating to about 600℃.Monazite ages range between 315 and 480 Ma with the most prominent maxima and side maxima at 346.0±1.1(2σ),357.3±2.3,and 368.3±1.7 Ma.Ages older than 380 Ma were related to detrital monazite pointing to a Devonian sedimentary protolith.Other ages around 325 Ma were assigned to the isobaric heating by nearby post-tectonic granites.The high-pressure event,being the result of the collision of Laurussia and Gondwana after closure of the Rheic Ocean,occurred in the Late Devonian.The exhumation to 15-20 km(5 kbar)ended probably in the Early Carboniferous.The high-pressure micaschists from the Fichtelgebirge to the Erzgebirge crystalline complexes are suggested to represent a single nappe within a metamorphic nappe pile.This nappe is composed of metasedimentary slices,which experienced different peak pressures rather than representing a coherent crustal section.
基金financial support from the Indian Institute of Technology(IIT)Kharagpur in the form of a Cumulative Professional Development Allowance
文摘In this study, we investigate the possible record of a Late Mesoproterozoic paired metamorphic belt in the Aravalli-Delhi Mobile Belt(ADMB), NW India using a suite of supracrustal and metaigneous granulites from the Pilwa-Chinwali granulite terrain at the north-western margin of the ADMB. Using metamorphic reaction textures, mineral chemistry, metamorphic reaction history, geothermobarometric computations and electron microprobe dating of monazite in 5 samples of pelitic granulite, leptynite gneiss, enderbite and charnockite, we have deduced a medium-pressure granulite facies metamorphism(P between 4.9 and 6.8 kbar, T> 760-815℃) along a heating-cooling, counterclockwise P-T path between 1.09 and 1.01 Ga. When collated with published metamorphic and chronological constraints and geological settings of the adjoining crustal domains of the ADMB, these findings provide new insights into the developments of two tectonic domains of contrasting thermal gradients at ca. 1.0 Ga, consistent with metamorphic transformations in tectonically thickened middle-lower crustal sections during continental collision to continental subduction and in the root zones of spatially adjacent island arc, as part of the Rodinia supercontinent assembly event.
文摘The Qinling orogenic belt is a collision zone between the North China and Yangtze cratons. The Qinling Complex is a Precambrian metamorphic complex, developed in the inner zone of the orogenic belt , which records the metamorphic and deformational history and PTt path of the regional meta-morphism of the collision zone . The present paper studies the metamorphic and deformational history and the PTt path of various tectono-metamorphic cycles in order to describe the geodynamic processes prevailing in that part of the Qinling orogenic belt since Proterozoic.The tectonometamorphic history and evolution of the Qinling Complex is divided into two stages: the stage of formation and the stage of modification During the stage of formation dated as Proterozoic,three deformational sequences are recognized. The amphibolite facies regional metamorphism is earlier than or synchronous with the first or second phase of folding. Three metamorphic zones, i.e. And-Ms ,Sil- Ms, Sil Kfs are delimited. During the stage of modification , the emplacement of Caledonian granite , superimposed thermal aureole (garnet K feldspar zone )and the development of ductile shear zones are major events.Metamorphic reactions are reconstructed in terms of analysis of re-equilibrium textures. Geothermobarometric calculations are conducted to calibrate the PT conditions of metamorphism .The metamorphic PTt paths have been estabh'shed. The PTt path of the Proterozoic tectonometamorphic cycle shows a clockwise pattern and is characterized by prominent decompression. The PTt path of the Caledonian tectonometamorphic cycle is characterized first by PT rising ,then isothermal decompression (rapid uplift), and finally by isobaric cooling The PTt path of the two tectonometamorphic cycles evidence the geodynamic processes: two major stages of collision and uplift in the Proterozoic and Caledonian Hercynian periods, respectively, during the evolution of Qinling orogenic belt.
文摘After the integration of petrographic study, geothermobarometry and Gibbs method, the synthetic P-T paths for the rocks from different geological profiles in the North Qilian, China, have been derived. The composite P-T paths from different methods indicate that all the high-pressure rocks in the Qilian area recorded P-T paths with clockwise loops starting at the blueschist facies, later reaching peak metamorphism at the blueschist facies, eclogite fades or epidote-amphibolite facies and ending up with the greenschist facies. The incremental Ar-Ar dating shows that the plateau ages for the high-pressure rocks range from 410 to 443 Ma. The plateau ages could be used as a minimum age constraint for the subduction that resulted in the formation of these high-pressure rocks in the Qilian area. It is proposed that the late-stage decompressional and cooling P-T paths with ends at the greenschist facies for these high-pressure rocks probably reflect the uplift process which could occur after shifting the arc-trench tectonic system to the system of continental orogenic belts. The retrograde paths for the high-pressure rocks in the North Qilian tectonic belt are characterized by dramatic decompression with slight cooling, which suggests very rapid exhumation. Petrography supports that the mountain-building for the Qilian mountain range could undergo a very fast process which caused rapid uplift and denudation.
文摘Considering the mineral evolution and tectonic deformational history of the early Proterozoic metamorphic sequences in the Jiao Liao Massif, very similar CCW PTt D paths were established in east Liaoning and east Shandong district within this massif. Their style displays the process of developing and closing of some rifted depression belts within the rather stable continental crust of the Jiao Liao Massif during early Proterozoic, and they may be treated as a common kind of metamorphic geodynamic model.
文摘The Yushugou HP granulite-peridotite complex is located at east of northern margin of southern Tianshan mountains,China,which consists of granulite unit and peridotite unit mainly.Because of the rare association of
