The mafic granulites in Dinggye,as various scale lense-shaped enclaves within the high Himalayan crystalline rock series,occur along mylonitic foliations at the junction between the Southern Tibetan Detachment System(...The mafic granulites in Dinggye,as various scale lense-shaped enclaves within the high Himalayan crystalline rock series,occur along mylonitic foliations at the junction between the Southern Tibetan Detachment System(STDS)and the Xainza-Dinggye normal fault system.The main lithological assemblage comprises garnet plagioclase pyroxenite,garnet two-pyroxene granulite,pyroxene garnet amphibolite and so on.The detailed petrological analyses show that these mafic granulites underwent at least four-stage metamorphic evolution.The first metamor-phic stage,the garnet+clinopyroxene+quart mineral assemblage(M1)was probably formed un-der eclogite facies,the second stage,the plagioclase+clinopyroxene symplectite mineral as-semblage(M2)was produced under high-pressure granulite facies by the early decompressive breakdown of M1 mineral assemblage,the third stage,the plagioclase+clinopyroxene+hypersthene symplectite mineral assemblage(M3)was formed at granulite facies by the late period decompressive breakdown of M1 and M2 mineral assemblages and the final stage,pla-gioclase+hornblende mineral assemblage(M4)was formed by hydrolysis of earlier mineral as-semblages during late uplifting.The detailed mineral composition analyses suggest that garnets and clinopyroxenes within M1 and M2 mineral assemblages display similar compositions to the equivalents in the B and C types of eclogites,whereas the M3 clinopyroxenes are akin to these of the same kind of minerals in the granulite.These mineral chemistry features and P-T estimates calculated by mineral thermometers and barometers indicate that the early stage relic porphyro-blasts(M1)could be formed at the eclogite facies,the early decompressive breakdown(M2)occurred at the high-pressures granulite facies of 1.35―1.48 GPa and 625―675℃,the M3 mineral assemblage recorded the granulite facies of 0.7―0.95 GPa and 775―900℃and M4 plagioglase+hornblende retrograde mineral assemblage was produced under the amphibolite facies metamorphism with pressure of 0.4 to 0.75 GPa and temper展开更多
The Rauer Group is located on the eastern margin of the early Paleozoic Prydz Belt in East Antarctica,and the typical ultrahigh-temperature(UHT,>900℃)granulites outcrop on Mather Peninsula.However,the timing of UH...The Rauer Group is located on the eastern margin of the early Paleozoic Prydz Belt in East Antarctica,and the typical ultrahigh-temperature(UHT,>900℃)granulites outcrop on Mather Peninsula.However,the timing of UHT metamorphism and P–T path of the UHT granulites have long been debated,which is critical to understanding the tectonic nature and evolution history of the Prydz Belt.Thus,both a sapphirine-bearing UHT metapelitic granulite and a garnet-bearing UHT mafic granulite are selected for zircon SHRIMP U-Pb age dating.The results show that metamorphic zircon mantles yield weighted mean^(206)Pb/^(238)U ages of 918±29 Ma and 901±29 Ma for the metapelitic and mafic granulites,respectively,while zircon rims and newly grown zircons yield weighted mean^(206)Pb/^(238)U ages of 523±9 Ma and 532±11 Ma,respectively.These new zircon age data suggest that the UHT granulites may have experienced polymetamorphism,in which pre-peak prograde stage occurred in the early Neoproterozoic Grenvillian orogenesis(1000–900 Ma),whereas the UHT metamorphism occurred in the late Neoproterozoic to early Paleozoic Pan-African orogenesis(580–460 Ma).This implies that P–T path of the UHT granulites should consist of two separate high-grade metamorphic events including the Grenvillian and Pan-African events,which are supposed to be related to assembly of Rodinia and Gondwana supercontinents respectively,and hence the overprinting UHT metamorphic event may actually reflect an important intracontinental reworking.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
We report here rare evidence for the early prograde P-Tevolution of garnet-sillimanite-graphite gneiss(khondalite)from the central Highland Complex,Sri Lanka.Four types of garnet porphyroblasts(Grt_1,Grt_2,Grt_3 and G...We report here rare evidence for the early prograde P-Tevolution of garnet-sillimanite-graphite gneiss(khondalite)from the central Highland Complex,Sri Lanka.Four types of garnet porphyroblasts(Grt_1,Grt_2,Grt_3 and Grt_4)are observed in the rock with specific types of inclusion features.Only Grt_3 shows evidence for non-coaxial strain.Combining the information shows a sequence of main inclusion phases,from old to young:oriented quartz inclusions at core,staurolite and prismatic sillimanite at mantle,kyanite and kyanite pseudomorph,and biotite at rim in Grt_1;fibrolitic sillimanite pseudomorphing kyanite±corundum,kyanite,and spinel+sillimanite after garnet+corundum in Grt_2;biotite,sillimanite,quartz±spinel in Grt_3;and ilmenite,rulite,quartz and sillimanite in Grt_4.The pre-melting,original rock composition was calculated through stepwise re-integration of melt into the residual,XRF based composition,allowing the early prograde metamorphic evolution to be deduced from petrographical observations and pseudosections.The earliest recognizable stage occurred in the sillimanite field at around 575℃ at 4.5 kbar.Subsequent collision associated with Gondwana amalgamation led to crustal thickening along a P-T trajectory with an average dP/dT of ~30 bar/℃ in the kyanite field,up to ~660℃ at 6.5 kbar,before crossing the wet-solidus at around 675 ℃ at 7.5 kbar.The highest pressure occurred at P > 10 kbar and T around 780℃ before prograde decompression associated with further heating.At 825℃ and 10.5 kbar,the rock re-entered into the sillimanite field.The temperature peaked at 900℃ at ca.9-9.5 kbar.Subsequent near-isobaric cooling led to the growth of Grt_4 and rutile at T ~880℃.Local pyrophyllite rims around sillimanite suggest a late stage of rehydration at T<400℃,which probably occurred after uplift to upper crustal levels.U-Pb dating of zircons by LAICPMS of the khondalite yielded two concordant ^(206)Pb/^(238)U age groups with mean values of 542±2 Ma(MSWD=0.24,Th/U=0.01-0.03)and 514±3 Ma(MSWD=0.50,展开更多
As a window of insight into the lower crust, high pressure granulite has received much attention since last decade. Yushugou high pressure granulite-peridotite Complex was located in the northeast margin of Southern T...As a window of insight into the lower crust, high pressure granulite has received much attention since last decade. Yushugou high pressure granulite-peridotite Complex was located in the northeast margin of Southern Tianshan, NW China. Previous ideas agreed that the peridotite unit in Yushugou, combined with the ultramafic rocks in Tonghuashan and Liuhuangshan, represent an ophiolite belt. However, the metamorphic evolution and tectonic mechanism of the Yushugou high pressure （HP） granulite remain controversial. Petrological investigations and phase equilibrium modelling for two representative fclsic granulitc samples suggest two stages metamorphism of the rocks in Yushugou Complex. Granulite facies metamorphism （Stage I） with P-T conditions of 9.8-10.4 kbar at 895-920℃ was recorded by the porphyroblastic garnet core; HP granulite facies metamorphism （Stage II） shows P-T conditions of 13.2-13.5 kbar at 845-860℃, based on the increasing grossular and decreasing pyrope contents of garnet rims. The Yushugou HP felsic granulites have recorded an anti- clockwise P-T path, characterized by the temperature decreasing and pressure increasing simultaneously. The LA-ICP-MS isotopic investigations on zircons from the felsic granulite show that the protolith ages of the granlulites are -430 Ma, with two age groups of-390 Ma and 340-350 Ma from the metamorphic rims of zircon, indicating the Stage I and II metamorphic events, respectively. A tectonic model was proposed to interpret the processes. The investigated felsic granulite was derived from deep rooted hanging wall, with Stage I granulite facies metamorphism of -390 Ma, which may he related to the Devonian arc magmatic intrusion; Stage II HP granulite facies metamorphism （340-350 Ma） may due to the involvement of being captured into the subducting slab and experienced the high pressure metamorphism.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
Ultrahigh-temperature(UHT)metamorphism is critical for understanding the most extreme thermal evolution of continental crust.However,UHT metamorphism predominantly occurred in the Precambrian and is rarely observed in...Ultrahigh-temperature(UHT)metamorphism is critical for understanding the most extreme thermal evolution of continental crust.However,UHT metamorphism predominantly occurred in the Precambrian and is rarely observed in the modern Earth.Here,we report the discovery of~25 Ma UHT granulites from the Mogok metamorphic belt(MMB)in Myanmar via a combined study of petrology and geochronology.The studied pelitic granulites well preserve a peak mineral assemblage of garnet+sillimanite+plagioclase(antiperthite)+K-feldspar+quartz+Ti-rich biotite+rutile+ilmenite.Pressure(P)-temperature(T)pseudosections and conventional geothermobarometry data only constrain the P-T conditions of the peak stage to<12 kbar and 780–890°C.However,high Zr contents in the matrix rutile(3005–4308 ppm)and high Ti contents(up to 9.2 wt% TiO_(2))in the biotite demonstrate that the Mogok granulites may have experienced UHT metamorphism.The Zr-in-rutile thermometer and X_(Grs) isopleth in the pseudosections yield peak P-T conditions of~12 kbar and>900°C.In situ SIMS and LAICP-MS U-Pb dating and trace element analyses show that both metamorphic zircon cores and rims have flat heavy rare earth element(HREE)patterns with negative Eu anomalies.The metamorphic zircon rims show the lowest HREE contents and yield ^(206)Pb/^(238)U ages of 24.9±0.5 and 25.4±0.6 Ma,respectively,representing the timing of UHT metamorphism.Our results indicate that the central MMB underwent~25 Ma UHT metamorphism,which is possibly induced by continental rifting along the thinned orogenic lithosphere.Our data,as well as reported Cenozoic UHT events,further suggest that UHT metamorphism can be produced in the modern plate tectonic regime by lithospheric extension.展开更多
基金This work was suppotted by the Ministry of Saance and Techmology of the People's Republic of China(Gant No 2002CB412608)the Specific Project for the Aunthos of Best Dissertations of Chimese Univarsities and Colleges(Grant No.200022).
文摘The mafic granulites in Dinggye,as various scale lense-shaped enclaves within the high Himalayan crystalline rock series,occur along mylonitic foliations at the junction between the Southern Tibetan Detachment System(STDS)and the Xainza-Dinggye normal fault system.The main lithological assemblage comprises garnet plagioclase pyroxenite,garnet two-pyroxene granulite,pyroxene garnet amphibolite and so on.The detailed petrological analyses show that these mafic granulites underwent at least four-stage metamorphic evolution.The first metamor-phic stage,the garnet+clinopyroxene+quart mineral assemblage(M1)was probably formed un-der eclogite facies,the second stage,the plagioclase+clinopyroxene symplectite mineral as-semblage(M2)was produced under high-pressure granulite facies by the early decompressive breakdown of M1 mineral assemblage,the third stage,the plagioclase+clinopyroxene+hypersthene symplectite mineral assemblage(M3)was formed at granulite facies by the late period decompressive breakdown of M1 and M2 mineral assemblages and the final stage,pla-gioclase+hornblende mineral assemblage(M4)was formed by hydrolysis of earlier mineral as-semblages during late uplifting.The detailed mineral composition analyses suggest that garnets and clinopyroxenes within M1 and M2 mineral assemblages display similar compositions to the equivalents in the B and C types of eclogites,whereas the M3 clinopyroxenes are akin to these of the same kind of minerals in the granulite.These mineral chemistry features and P-T estimates calculated by mineral thermometers and barometers indicate that the early stage relic porphyro-blasts(M1)could be formed at the eclogite facies,the early decompressive breakdown(M2)occurred at the high-pressures granulite facies of 1.35―1.48 GPa and 625―675℃,the M3 mineral assemblage recorded the granulite facies of 0.7―0.95 GPa and 775―900℃and M4 plagioglase+hornblende retrograde mineral assemblage was produced under the amphibolite facies metamorphism with pressure of 0.4 to 0.75 GPa and temper
基金supported by the National Nature Science Foundation of China(Grant no.41972050).
文摘The Rauer Group is located on the eastern margin of the early Paleozoic Prydz Belt in East Antarctica,and the typical ultrahigh-temperature(UHT,>900℃)granulites outcrop on Mather Peninsula.However,the timing of UHT metamorphism and P–T path of the UHT granulites have long been debated,which is critical to understanding the tectonic nature and evolution history of the Prydz Belt.Thus,both a sapphirine-bearing UHT metapelitic granulite and a garnet-bearing UHT mafic granulite are selected for zircon SHRIMP U-Pb age dating.The results show that metamorphic zircon mantles yield weighted mean^(206)Pb/^(238)U ages of 918±29 Ma and 901±29 Ma for the metapelitic and mafic granulites,respectively,while zircon rims and newly grown zircons yield weighted mean^(206)Pb/^(238)U ages of 523±9 Ma and 532±11 Ma,respectively.These new zircon age data suggest that the UHT granulites may have experienced polymetamorphism,in which pre-peak prograde stage occurred in the early Neoproterozoic Grenvillian orogenesis(1000–900 Ma),whereas the UHT metamorphism occurred in the late Neoproterozoic to early Paleozoic Pan-African orogenesis(580–460 Ma).This implies that P–T path of the UHT granulites should consist of two separate high-grade metamorphic events including the Grenvillian and Pan-African events,which are supposed to be related to assembly of Rodinia and Gondwana supercontinents respectively,and hence the overprinting UHT metamorphic event may actually reflect an important intracontinental reworking.
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.
基金the National Research Council(NRC)of Sri Lanka(grant NO 15-089)and the Ministry of Technology and Research(MTR/TRD/AGR/3/1/04)Department of Science and Technology,India(Grant No.DST/INT/SL/P-004)L.M.K.acknowledges support by the Stichting Dr.Schurmannfonds(Grants Nos.88/2012,94/2013 and 101/2014)
文摘We report here rare evidence for the early prograde P-Tevolution of garnet-sillimanite-graphite gneiss(khondalite)from the central Highland Complex,Sri Lanka.Four types of garnet porphyroblasts(Grt_1,Grt_2,Grt_3 and Grt_4)are observed in the rock with specific types of inclusion features.Only Grt_3 shows evidence for non-coaxial strain.Combining the information shows a sequence of main inclusion phases,from old to young:oriented quartz inclusions at core,staurolite and prismatic sillimanite at mantle,kyanite and kyanite pseudomorph,and biotite at rim in Grt_1;fibrolitic sillimanite pseudomorphing kyanite±corundum,kyanite,and spinel+sillimanite after garnet+corundum in Grt_2;biotite,sillimanite,quartz±spinel in Grt_3;and ilmenite,rulite,quartz and sillimanite in Grt_4.The pre-melting,original rock composition was calculated through stepwise re-integration of melt into the residual,XRF based composition,allowing the early prograde metamorphic evolution to be deduced from petrographical observations and pseudosections.The earliest recognizable stage occurred in the sillimanite field at around 575℃ at 4.5 kbar.Subsequent collision associated with Gondwana amalgamation led to crustal thickening along a P-T trajectory with an average dP/dT of ~30 bar/℃ in the kyanite field,up to ~660℃ at 6.5 kbar,before crossing the wet-solidus at around 675 ℃ at 7.5 kbar.The highest pressure occurred at P > 10 kbar and T around 780℃ before prograde decompression associated with further heating.At 825℃ and 10.5 kbar,the rock re-entered into the sillimanite field.The temperature peaked at 900℃ at ca.9-9.5 kbar.Subsequent near-isobaric cooling led to the growth of Grt_4 and rutile at T ~880℃.Local pyrophyllite rims around sillimanite suggest a late stage of rehydration at T<400℃,which probably occurred after uplift to upper crustal levels.U-Pb dating of zircons by LAICPMS of the khondalite yielded two concordant ^(206)Pb/^(238)U age groups with mean values of 542±2 Ma(MSWD=0.24,Th/U=0.01-0.03)and 514±3 Ma(MSWD=0.50,
基金financially supported by the National Natural Science Foundation of China(Grants 41330210,41520104004)
文摘As a window of insight into the lower crust, high pressure granulite has received much attention since last decade. Yushugou high pressure granulite-peridotite Complex was located in the northeast margin of Southern Tianshan, NW China. Previous ideas agreed that the peridotite unit in Yushugou, combined with the ultramafic rocks in Tonghuashan and Liuhuangshan, represent an ophiolite belt. However, the metamorphic evolution and tectonic mechanism of the Yushugou high pressure （HP） granulite remain controversial. Petrological investigations and phase equilibrium modelling for two representative fclsic granulitc samples suggest two stages metamorphism of the rocks in Yushugou Complex. Granulite facies metamorphism （Stage I） with P-T conditions of 9.8-10.4 kbar at 895-920℃ was recorded by the porphyroblastic garnet core; HP granulite facies metamorphism （Stage II） shows P-T conditions of 13.2-13.5 kbar at 845-860℃, based on the increasing grossular and decreasing pyrope contents of garnet rims. The Yushugou HP felsic granulites have recorded an anti- clockwise P-T path, characterized by the temperature decreasing and pressure increasing simultaneously. The LA-ICP-MS isotopic investigations on zircons from the felsic granulite show that the protolith ages of the granlulites are -430 Ma, with two age groups of-390 Ma and 340-350 Ma from the metamorphic rims of zircon, indicating the Stage I and II metamorphic events, respectively. A tectonic model was proposed to interpret the processes. The investigated felsic granulite was derived from deep rooted hanging wall, with Stage I granulite facies metamorphism of -390 Ma, which may he related to the Devonian arc magmatic intrusion; Stage II HP granulite facies metamorphism （340-350 Ma） may due to the involvement of being captured into the subducting slab and experienced the high pressure metamorphism.
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.
基金This work was supported by the National Natural Science Foundation of China(Grant Nos.41822202,41490614).
文摘Ultrahigh-temperature(UHT)metamorphism is critical for understanding the most extreme thermal evolution of continental crust.However,UHT metamorphism predominantly occurred in the Precambrian and is rarely observed in the modern Earth.Here,we report the discovery of~25 Ma UHT granulites from the Mogok metamorphic belt(MMB)in Myanmar via a combined study of petrology and geochronology.The studied pelitic granulites well preserve a peak mineral assemblage of garnet+sillimanite+plagioclase(antiperthite)+K-feldspar+quartz+Ti-rich biotite+rutile+ilmenite.Pressure(P)-temperature(T)pseudosections and conventional geothermobarometry data only constrain the P-T conditions of the peak stage to<12 kbar and 780–890°C.However,high Zr contents in the matrix rutile(3005–4308 ppm)and high Ti contents(up to 9.2 wt% TiO_(2))in the biotite demonstrate that the Mogok granulites may have experienced UHT metamorphism.The Zr-in-rutile thermometer and X_(Grs) isopleth in the pseudosections yield peak P-T conditions of~12 kbar and>900°C.In situ SIMS and LAICP-MS U-Pb dating and trace element analyses show that both metamorphic zircon cores and rims have flat heavy rare earth element(HREE)patterns with negative Eu anomalies.The metamorphic zircon rims show the lowest HREE contents and yield ^(206)Pb/^(238)U ages of 24.9±0.5 and 25.4±0.6 Ma,respectively,representing the timing of UHT metamorphism.Our results indicate that the central MMB underwent~25 Ma UHT metamorphism,which is possibly induced by continental rifting along the thinned orogenic lithosphere.Our data,as well as reported Cenozoic UHT events,further suggest that UHT metamorphism can be produced in the modern plate tectonic regime by lithospheric extension.
