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
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
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
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 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
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
Recent progress in the study of the UHP metamorphic belt in southwestern Tianshan,China,is summarized in this paper.This about 80-km-long and over 10-km-wide UHP belt has been recognized by the discovery of coesite,co...Recent progress in the study of the UHP metamorphic belt in southwestern Tianshan,China,is summarized in this paper.This about 80-km-long and over 10-km-wide UHP belt has been recognized by the discovery of coesite,coesite pseudomorphs and other UHP minerals.It is the largest oceanic-type UHP metamorphic belt reported so far.It has formed due to northward subduction of the Tianshan Paleo-Ocean.U-Pb dating of metamorphic rims of zircons from a coesite-bearing garnet-phengite schist yields a peak UHP metamorphic ages of 320±3.7 Ma.Combined with ages of 233–226 Ma obtained from rims of zircons from retrograded eclogites,a long retrograde metamorphic evolution(>70 Ma)has been revealed.According to phase equilibria modeling,the P-T paths of both coesite-bearing eclogites and garnet-phengite schists are characterized by thermal relaxation,i.e.,the metamorphic temperature peak lags behind the pressure peak,indicating that the UHP rocks experienced slow and long heating and decompression during exhumation in the subduction channel.On the basis of the field observation that a small amount of eclogite lenses is wrapped in large volumes of metapelites,and the similar P-T paths of both rock types,we propose that the exhumation of the UHP eclogites from southwestern Tianshan,China,may have resulted from the exhumation of large volumes of low-density metapelites,which carried the denser eclogites to the Earth’s surface.展开更多
Mafic granulites have been found as structural lenses within the huge thrust system outcropping about 10 km west of Nam Co of the northern Lhasa Terrane, Tibetan Plateau. Petrological evidence from these rocks indicat...Mafic granulites have been found as structural lenses within the huge thrust system outcropping about 10 km west of Nam Co of the northern Lhasa Terrane, Tibetan Plateau. Petrological evidence from these rocks indicates four distinct metamorphic assemblages. The early metamorphic assemblage (M1) is preserved only in the granulites and represented by plagioclase+hornblende inclusions within the cores of garnet porphyroblasts. The peak assemblage (M2) consists of garnet+clinopyroxene+hornblende+plagioclase in the mafic granulites. The peak metamorphism was followed by near-isothermal decompression (M3), which resulted in the development of hornblende+plagioclase symplectites surrounding embayed garnet porphyroblasts, and decompression-cooling (M4) is represented by minerals of hornblende+plagioclase recrystallized during mylonization. The peak (M2) P-T conditions of garnet+ clinopyroxene+plagioclase+hornblende were estimated at 769-905℃ and 0.86-1.02 GPa based on the geothermometers and geobarometers. The P-T conditions of plagioclase+hornblende symplectites (M3) were estimated at 720-800℃ and 0.55-0.68 GPa, and recrystallized hornblende+plagioclase (M4) at 594-708℃ and 0.26-0.47 GPa. It is impossible to estimate the P-T conditions of the early metamorphic assemblage (M1) because of the absence of modal minerals. The combination of petrographic textures, metamorphic reaction history, thermobarometric data and corresponding isotopic ages defines a clockwise near-isothermal decompression metamorphic path, suggesting that the mafic granulites had undergone initial crustal thickening, subsequent exhumation, and cooling and retrogression. This tectonothermal path is considered to record two major phases of collision which resulted in both the assemblage of Gondwanaland during the Pan-African orogeny at 531 Ma and the collision of the Qiangtang and Lhasa Terranes at 174 Ma, respectively.展开更多
Although zircon U-Pb geochronometer has been widely used for dating metamorphism in moderate- to high-grade metamorphic rocks, it is still difficult to link the zircon U-Pb age to pressure and temperature （P-T） cond...Although zircon U-Pb geochronometer has been widely used for dating metamorphism in moderate- to high-grade metamorphic rocks, it is still difficult to link the zircon U-Pb age to pressure and temperature （P-T） conditions. In this study, zircon trace elements and Hf isotopes and REE partitioning between zircon and garnet are adopted to track the formation condition of zircon in the granu- lites from North Tongbai Orogen, Central China. Combined with previous metamorphic P-T path results, a quantitative integrated anticlockwise P-T-t path was established for Tongbai granulites. These grauulites recorded an early low-P heating followed by a dramatic pressure increase. Evidence for the prograde history （M1） is provided by cordierite, orthopyroxene and biotite inclusions in garnet. The prograde metamorphism occurred at around 443±3 Ma, with P-T conditions of ca. 730-820 ℃ and 〈6 kbar. The peak metamorphic （M2） condition is 〉920 ℃ and 8.5-10 kbar and the peak metamorphism age is ca. 432±4 Ma. At around 419 Ma, the granulites suffered an amphibolite-facies retrograde metamorphism （M3）, represented by the replacement of garnet by biotite and plagioclase, and clinopyroxene by amphibole, with metamorphic condition of ca. 700℃ and ca. 7 kbar. The last retrograde metamorphism （M4） is a greenschist-facies overprint with an age of ca. 404 Ma. It is concluded that the meta- morphism of Tongbai granulite lasted for more than 40 Ma, including a stage of more than 20 Ma granulitc-facies metamorphism. The prolonged granulite-facies metamorphism resulted from the continuous northward subduction of the Shangdan oceanic crust beneath the North Qinling terrane.展开更多
The garnet (Grt) and biotite (Bt) from gneisses of the Ji’ an Group are characterized by diffusion zoning at the rim, but equilibrium composition of metamorphic peak is usually remained in extensive interior area. Ga...The garnet (Grt) and biotite (Bt) from gneisses of the Ji’ an Group are characterized by diffusion zoning at the rim, but equilibrium composition of metamorphic peak is usually remained in extensive interior area. Garnet with growth zoning is also found in the kyanite zone.In the light of microarea compositional variation of Grt and Bt, the temperatuIe and pressuIe at the progressive, peak and post peak metamorphic stages are determined by correctly using GrtBt thermometer and GASP barometer. On this basis, a counterclockwise P-T-t path can be constructed, which reflects the closing process of an ensialic rift belt in this region during the Early Proterozoic.展开更多
The \%P_T\% paths of the Xiaoqinling metamorphic core complex (XMCC) have been investigated with the Gibbs method by researching the compositional changing of zoned epidotes which formed during syn_deformation metamor...The \%P_T\% paths of the Xiaoqinling metamorphic core complex (XMCC) have been investigated with the Gibbs method by researching the compositional changing of zoned epidotes which formed during syn_deformation metamorphism. These \%P_T\% paths indicate that the XMCC had experienced the following thermodynamics processes: firstly, near isobaric falling slightly in temperature in lower crust; secondly, fast decompression and rising in temperature during extensional uplifting to middle crust level; and finally, isobaric falling in temperature in middle crust. The upwelling and emplacement of the deep magma may be a major factor during the uplifting processes of the metamorphic core complex.展开更多
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.
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.
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.
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
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 Basic Research Program of China(2009CB825007)the National Natural Science Foundation of China(41121062 and41272069)
文摘Recent progress in the study of the UHP metamorphic belt in southwestern Tianshan,China,is summarized in this paper.This about 80-km-long and over 10-km-wide UHP belt has been recognized by the discovery of coesite,coesite pseudomorphs and other UHP minerals.It is the largest oceanic-type UHP metamorphic belt reported so far.It has formed due to northward subduction of the Tianshan Paleo-Ocean.U-Pb dating of metamorphic rims of zircons from a coesite-bearing garnet-phengite schist yields a peak UHP metamorphic ages of 320±3.7 Ma.Combined with ages of 233–226 Ma obtained from rims of zircons from retrograded eclogites,a long retrograde metamorphic evolution(>70 Ma)has been revealed.According to phase equilibria modeling,the P-T paths of both coesite-bearing eclogites and garnet-phengite schists are characterized by thermal relaxation,i.e.,the metamorphic temperature peak lags behind the pressure peak,indicating that the UHP rocks experienced slow and long heating and decompression during exhumation in the subduction channel.On the basis of the field observation that a small amount of eclogite lenses is wrapped in large volumes of metapelites,and the similar P-T paths of both rock types,we propose that the exhumation of the UHP eclogites from southwestern Tianshan,China,may have resulted from the exhumation of large volumes of low-density metapelites,which carried the denser eclogites to the Earth’s surface.
基金China Geological Survey(Grant No.20013000166) Natural Science Foundation of China(Grant No.49902006).
文摘Mafic granulites have been found as structural lenses within the huge thrust system outcropping about 10 km west of Nam Co of the northern Lhasa Terrane, Tibetan Plateau. Petrological evidence from these rocks indicates four distinct metamorphic assemblages. The early metamorphic assemblage (M1) is preserved only in the granulites and represented by plagioclase+hornblende inclusions within the cores of garnet porphyroblasts. The peak assemblage (M2) consists of garnet+clinopyroxene+hornblende+plagioclase in the mafic granulites. The peak metamorphism was followed by near-isothermal decompression (M3), which resulted in the development of hornblende+plagioclase symplectites surrounding embayed garnet porphyroblasts, and decompression-cooling (M4) is represented by minerals of hornblende+plagioclase recrystallized during mylonization. The peak (M2) P-T conditions of garnet+ clinopyroxene+plagioclase+hornblende were estimated at 769-905℃ and 0.86-1.02 GPa based on the geothermometers and geobarometers. The P-T conditions of plagioclase+hornblende symplectites (M3) were estimated at 720-800℃ and 0.55-0.68 GPa, and recrystallized hornblende+plagioclase (M4) at 594-708℃ and 0.26-0.47 GPa. It is impossible to estimate the P-T conditions of the early metamorphic assemblage (M1) because of the absence of modal minerals. The combination of petrographic textures, metamorphic reaction history, thermobarometric data and corresponding isotopic ages defines a clockwise near-isothermal decompression metamorphic path, suggesting that the mafic granulites had undergone initial crustal thickening, subsequent exhumation, and cooling and retrogression. This tectonothermal path is considered to record two major phases of collision which resulted in both the assemblage of Gondwanaland during the Pan-African orogeny at 531 Ma and the collision of the Qiangtang and Lhasa Terranes at 174 Ma, respectively.
基金supported by the National Natural Science Foundation of China (No. 41302040)the China Geological Survey (No. DD20160201)
文摘Although zircon U-Pb geochronometer has been widely used for dating metamorphism in moderate- to high-grade metamorphic rocks, it is still difficult to link the zircon U-Pb age to pressure and temperature （P-T） conditions. In this study, zircon trace elements and Hf isotopes and REE partitioning between zircon and garnet are adopted to track the formation condition of zircon in the granu- lites from North Tongbai Orogen, Central China. Combined with previous metamorphic P-T path results, a quantitative integrated anticlockwise P-T-t path was established for Tongbai granulites. These grauulites recorded an early low-P heating followed by a dramatic pressure increase. Evidence for the prograde history （M1） is provided by cordierite, orthopyroxene and biotite inclusions in garnet. The prograde metamorphism occurred at around 443±3 Ma, with P-T conditions of ca. 730-820 ℃ and 〈6 kbar. The peak metamorphic （M2） condition is 〉920 ℃ and 8.5-10 kbar and the peak metamorphism age is ca. 432±4 Ma. At around 419 Ma, the granulites suffered an amphibolite-facies retrograde metamorphism （M3）, represented by the replacement of garnet by biotite and plagioclase, and clinopyroxene by amphibole, with metamorphic condition of ca. 700℃ and ca. 7 kbar. The last retrograde metamorphism （M4） is a greenschist-facies overprint with an age of ca. 404 Ma. It is concluded that the meta- morphism of Tongbai granulite lasted for more than 40 Ma, including a stage of more than 20 Ma granulitc-facies metamorphism. The prolonged granulite-facies metamorphism resulted from the continuous northward subduction of the Shangdan oceanic crust beneath the North Qinling terrane.
文摘The garnet (Grt) and biotite (Bt) from gneisses of the Ji’ an Group are characterized by diffusion zoning at the rim, but equilibrium composition of metamorphic peak is usually remained in extensive interior area. Garnet with growth zoning is also found in the kyanite zone.In the light of microarea compositional variation of Grt and Bt, the temperatuIe and pressuIe at the progressive, peak and post peak metamorphic stages are determined by correctly using GrtBt thermometer and GASP barometer. On this basis, a counterclockwise P-T-t path can be constructed, which reflects the closing process of an ensialic rift belt in this region during the Early Proterozoic.
文摘The \%P_T\% paths of the Xiaoqinling metamorphic core complex (XMCC) have been investigated with the Gibbs method by researching the compositional changing of zoned epidotes which formed during syn_deformation metamorphism. These \%P_T\% paths indicate that the XMCC had experienced the following thermodynamics processes: firstly, near isobaric falling slightly in temperature in lower crust; secondly, fast decompression and rising in temperature during extensional uplifting to middle crust level; and finally, isobaric falling in temperature in middle crust. The upwelling and emplacement of the deep magma may be a major factor during the uplifting processes of the metamorphic core complex.
