Systematical studies of post-collisional igneous rocks in the Dabie orogen suggest that the thickened mafic lower crust of the orogen was partially melted to form low-Mg#adakitic rocks at 143–131 Ma.Delamination and ...Systematical studies of post-collisional igneous rocks in the Dabie orogen suggest that the thickened mafic lower crust of the orogen was partially melted to form low-Mg#adakitic rocks at 143–131 Ma.Delamination and foundering of the thickened mafic lower crust occurred at 130 Ma,which caused the mantle upwelling and following mafic and granitic magmatic intrusions.Migmatite in the North Dabie zone,coeval with the formation of low-Mg#adakitic intrusions in the Dabie orogen,was formed by partial melting of exhumed ultrahigh-pressure metamorphic rocks at middle crustal level.This paper argues that the partial melting of thickened lower and middle crust before mountain-root collapse needs lithospheric thinning.Based on the geothermal gradient of6.6°C/km for lithospheric mantle and initial partial melting temperature of^1000°C for the lower mafic crust,it can be estimated that the thickness of lithospheric mantle beneath thickened lower crust has been thinned to<45 km when the thickened lower crust was melting.Thus,a two-stage model for mountain-root removal is proposed.First,the lithospheric mantle keel was partially removal by mantle convection at 145 Ma.Loss of the lower lithosphere would increase heat flow into the base of the crust and would cause middle-lower crustal melting.Second,partial melting of the thickened lower crust has weakened the lower crust and increased its gravity instability,thus triggering delamination and foundering of the thickened mafic lower crust or mountain-root collapse.Therefore,convective removal and delamination of the thickened lower crust as two mechanisms of lithospheric thinning are related to causality.展开更多
In the northern West Qinling orogen(WQO), granulite-facies metamorphic rocks are recognized within the Qinling Complex. These rocks are composed of amphibole-bearing two-pyroxene granulite and garnet-sillimanite gneis...In the northern West Qinling orogen(WQO), granulite-facies metamorphic rocks are recognized within the Qinling Complex. These rocks are composed of amphibole-bearing two-pyroxene granulite and garnet-sillimanite gneiss with widespread migmatitization. We investigate three granulite-facies samples and one leucosome sample from the Qinling Complex, which are suitable for U-Pb analyses of zircon and monazite. SHRIMP and LA-ICPMS U-Pb age dating of zircon and monazite from two pelitic granulites provides weighted mean ages of 430±4 Ma(MSWD=0.88) and 433±4 Ma(MSWD=0.27), respectively.Based on the petrographic characteristics and zircon CL imagery, we postulated a ca. 430 Ma metamorphic timing for the pelitic granulites. LA-ICPMS zircon U-Pb data from an amphibole two-pyroxene granulite sample reports two weighted mean age groups:424±3 Ma(MSWD=0.45) and 402±3 Ma(MSWD=1.4), which were interpreted as granulite-facies metamorphic and retrograde ages, respectively. LA-ICPMS U-Pb dating of zircons from the leucosome sample yields a weighted mean age of 426±2 Ma(MSWD=0.3), which is interpreted as the crystallization age of the leucosome. These data indicate that the West QOB experienced early Paleozoic granulite-facies metamorphism and anatexis similar to the East QOB. However, it remains unclear whether the early Paleozoic granulite facies metamorphism resulted from an arc setting created by the northward subduction of the Shangdan ocean or from a continental collisional orogenic event.展开更多
Data from a migmatised metapelite raft enclosed within charnockite provide quantitative constraints on the pressure-temperature-time （P-T-t） evolution of the Nagercoil Block at the southernmost tip of peninsular Ind...Data from a migmatised metapelite raft enclosed within charnockite provide quantitative constraints on the pressure-temperature-time （P-T-t） evolution of the Nagercoil Block at the southernmost tip of peninsular India. An inferred peak metamorphic assemblage of garnet, K-feldspar, sillimanite, plagioclase, magnetite, ilmenite, spinel and melt is consistent with peak metamorphic pressures of 6-8 kbar and temperatures in excess of 900 ℃. Subsequent growth of cordierite and biotite record high-temperature retrograde decompression to around 5 kbar and 800 ℃. SHRIMP U-Pb dating of magmatic zircon cores suggests that the sedimentary protoliths were in part derived from felsic igneous rocks with Palae- oproterozoic crystallisation ages. New growth of metamorphic zircon on the rims of detrital grains constrains the onset of melt crystallisation, and the minimum age of the metamorphic peak, to around 560 Ma. The data suggest two stages of monazite growth. The first generation of REE-enriched monazite grew during partial melting along the prograde path at around 570 Ma via the incongruent breakdown of apatite. Relatively REE-depleted rims, which have a pronounced negative europium anomaly, grew during melt crystallisation along the retrograde path at around 535 Ma. Our data show the rocks remained at suprasolidus temperatures for at least 35 million years and probably much longer, supporting a long-lived high-grade metamorphic history. The metamorphic conditions, timing and duration of the implied clockwise P-T-t path are similar to that previously established for other regions in peninsular India during the Ediacaran to Cambrian assembly of that part of the Gondwanan supercontinent.展开更多
The plate affiliation of the North Dabie terrane （NDT） has been controversial. To address this fundamental question, an integrated study of internal structure, in-situ U-Pb dating and trace element analysis in zirco...The plate affiliation of the North Dabie terrane （NDT） has been controversial. To address this fundamental question, an integrated study of internal structure, in-situ U-Pb dating and trace element analysis in zircons and field investigation for migmatite in the NDT was carried out, which reveals par- ticipation of crustal rocks of the North China Craton （NCC） in the protolith in addition to the more common crustal rocks of the Yangtze Craton （YC）. The evidence of an NCC affinity for protolith of migmatite in the NDT is the -2.5 Ga （2 486±14 and 2 406±26 Ma） magamtic age and -1.8 Ga （1 717±79 Ma） metamorphic age in the relict zircon domains because these two age groups are characteristic for the evolution of the NCC. The evidence of a YC affinity for protolith of migmatite in the NDT is the more common 0.7-0.8 Ga （e.g., 787±12 Ma） magamtic zircon age. Mid-Neoproterozoic magmatic age （0.7-0.8 Ga） is a symbol of YC basement rocks. In view of the widely exposed YC crustal components in the NDT, we suggest that the protolith of the NDT is mainly crustal rocks from the YC with minor crustal components from the NCC. The zircon rim domains and new growth grains from all the migmatite sam- ples are characterized by anatectic zircons and have a widely concordant ages ranging from 112.2±2.8 to 159.6±4.3 Ma with several peak values, suggesting a long lasting multistage anatexis. In conclusion, the NDT has a mixed protolith origin of both the YC and the NCC crustal rocks were strongly remoulded by anatexis during orogenic collapse.展开更多
Migmatization in Higher Himalayan Crystallines (HHC) results from anatexis. The widely distributed migmatites in HHC are an important clue to investigate the relationship be- tween anatexis and the origins of Higher H...Migmatization in Higher Himalayan Crystallines (HHC) results from anatexis. The widely distributed migmatites in HHC are an important clue to investigate the relationship be- tween anatexis and the origins of Higher Himalayan leucogranites (HHL), and to understand the effect of anatexis on crustal evolution during the post-collision period. We studied in detail the chemical features of three basic constituent parts of the migmatites, i.e. leucosome, mesosome and melanosome, and determined the K-Ar ages of leucosomes. Our studies indicate that type-I leucosome is the product of crystallization of melt generated by partial melting of mesosome at source region, but type-II leucosome and HHL probably underwent crystallization differentiation of plagioclase during melt aggregation and migration. The age of 22.67 Ma of Type-I leucosome, which is a little older than the beginning of MCT movement, indicates that anatexis may have played an important role in the formation of MCT. That the ages of type-II leucosome (ranging from 14.82 to 18.37 Ma) are consistent with that of HHL provides new chronological evidence for the relationship between migmatization and HHL. We obtained a very young age of 6.23 Ma of Type-II leucosome that provides new time constraint on magma activity in the central segment of Higher Himalayas.展开更多
基金supported by the National Basic Research Program of China(2009CB825002)the National Natural Science Foundation of China(40773013 and 91014007)
文摘Systematical studies of post-collisional igneous rocks in the Dabie orogen suggest that the thickened mafic lower crust of the orogen was partially melted to form low-Mg#adakitic rocks at 143–131 Ma.Delamination and foundering of the thickened mafic lower crust occurred at 130 Ma,which caused the mantle upwelling and following mafic and granitic magmatic intrusions.Migmatite in the North Dabie zone,coeval with the formation of low-Mg#adakitic intrusions in the Dabie orogen,was formed by partial melting of exhumed ultrahigh-pressure metamorphic rocks at middle crustal level.This paper argues that the partial melting of thickened lower and middle crust before mountain-root collapse needs lithospheric thinning.Based on the geothermal gradient of6.6°C/km for lithospheric mantle and initial partial melting temperature of^1000°C for the lower mafic crust,it can be estimated that the thickness of lithospheric mantle beneath thickened lower crust has been thinned to<45 km when the thickened lower crust was melting.Thus,a two-stage model for mountain-root removal is proposed.First,the lithospheric mantle keel was partially removal by mantle convection at 145 Ma.Loss of the lower lithosphere would increase heat flow into the base of the crust and would cause middle-lower crustal melting.Second,partial melting of the thickened lower crust has weakened the lower crust and increased its gravity instability,thus triggering delamination and foundering of the thickened mafic lower crust or mountain-root collapse.Therefore,convective removal and delamination of the thickened lower crust as two mechanisms of lithospheric thinning are related to causality.
基金supported by the National Natural Science Foundation of China (Grant Nos. 41630207, 41572180)the China Geological Survey Project (Grant No. 12120115027001)
文摘In the northern West Qinling orogen(WQO), granulite-facies metamorphic rocks are recognized within the Qinling Complex. These rocks are composed of amphibole-bearing two-pyroxene granulite and garnet-sillimanite gneiss with widespread migmatitization. We investigate three granulite-facies samples and one leucosome sample from the Qinling Complex, which are suitable for U-Pb analyses of zircon and monazite. SHRIMP and LA-ICPMS U-Pb age dating of zircon and monazite from two pelitic granulites provides weighted mean ages of 430±4 Ma(MSWD=0.88) and 433±4 Ma(MSWD=0.27), respectively.Based on the petrographic characteristics and zircon CL imagery, we postulated a ca. 430 Ma metamorphic timing for the pelitic granulites. LA-ICPMS zircon U-Pb data from an amphibole two-pyroxene granulite sample reports two weighted mean age groups:424±3 Ma(MSWD=0.45) and 402±3 Ma(MSWD=1.4), which were interpreted as granulite-facies metamorphic and retrograde ages, respectively. LA-ICPMS U-Pb dating of zircons from the leucosome sample yields a weighted mean age of 426±2 Ma(MSWD=0.3), which is interpreted as the crystallization age of the leucosome. These data indicate that the West QOB experienced early Paleozoic granulite-facies metamorphism and anatexis similar to the East QOB. However, it remains unclear whether the early Paleozoic granulite facies metamorphism resulted from an arc setting created by the northward subduction of the Shangdan ocean or from a continental collisional orogenic event.
基金Funding for analyses and fieldwork was provided through Australian Research Council(ARC)Discovery and DECRA projects DP0879330 and DE1201030(to CC)Future Fellowship Scheme#FT120100340(to ASC)+1 种基金the Australia-India Strategic Research Fund project#ST030046(to CC and ASC)support from Curtin University Strategic Research Funding
文摘Data from a migmatised metapelite raft enclosed within charnockite provide quantitative constraints on the pressure-temperature-time （P-T-t） evolution of the Nagercoil Block at the southernmost tip of peninsular India. An inferred peak metamorphic assemblage of garnet, K-feldspar, sillimanite, plagioclase, magnetite, ilmenite, spinel and melt is consistent with peak metamorphic pressures of 6-8 kbar and temperatures in excess of 900 ℃. Subsequent growth of cordierite and biotite record high-temperature retrograde decompression to around 5 kbar and 800 ℃. SHRIMP U-Pb dating of magmatic zircon cores suggests that the sedimentary protoliths were in part derived from felsic igneous rocks with Palae- oproterozoic crystallisation ages. New growth of metamorphic zircon on the rims of detrital grains constrains the onset of melt crystallisation, and the minimum age of the metamorphic peak, to around 560 Ma. The data suggest two stages of monazite growth. The first generation of REE-enriched monazite grew during partial melting along the prograde path at around 570 Ma via the incongruent breakdown of apatite. Relatively REE-depleted rims, which have a pronounced negative europium anomaly, grew during melt crystallisation along the retrograde path at around 535 Ma. Our data show the rocks remained at suprasolidus temperatures for at least 35 million years and probably much longer, supporting a long-lived high-grade metamorphic history. The metamorphic conditions, timing and duration of the implied clockwise P-T-t path are similar to that previously established for other regions in peninsular India during the Ediacaran to Cambrian assembly of that part of the Gondwanan supercontinent.
基金supported by the National Basic Research Program of China (No. 2015CB856101)the National Natural Science Foundation of China (Nos. 41372076 and 41572039)the Natural Science Foundation of Hubei Province (No. 2015CFB190)
文摘The plate affiliation of the North Dabie terrane （NDT） has been controversial. To address this fundamental question, an integrated study of internal structure, in-situ U-Pb dating and trace element analysis in zircons and field investigation for migmatite in the NDT was carried out, which reveals par- ticipation of crustal rocks of the North China Craton （NCC） in the protolith in addition to the more common crustal rocks of the Yangtze Craton （YC）. The evidence of an NCC affinity for protolith of migmatite in the NDT is the -2.5 Ga （2 486±14 and 2 406±26 Ma） magamtic age and -1.8 Ga （1 717±79 Ma） metamorphic age in the relict zircon domains because these two age groups are characteristic for the evolution of the NCC. The evidence of a YC affinity for protolith of migmatite in the NDT is the more common 0.7-0.8 Ga （e.g., 787±12 Ma） magamtic zircon age. Mid-Neoproterozoic magmatic age （0.7-0.8 Ga） is a symbol of YC basement rocks. In view of the widely exposed YC crustal components in the NDT, we suggest that the protolith of the NDT is mainly crustal rocks from the YC with minor crustal components from the NCC. The zircon rim domains and new growth grains from all the migmatite sam- ples are characterized by anatectic zircons and have a widely concordant ages ranging from 112.2±2.8 to 159.6±4.3 Ma with several peak values, suggesting a long lasting multistage anatexis. In conclusion, the NDT has a mixed protolith origin of both the YC and the NCC crustal rocks were strongly remoulded by anatexis during orogenic collapse.
基金supports from the National Natural Science Foundation of China(Grant No.40472113)the National Key Basic Research and Development Program of China(Grant No.2004CB418405)the Foundation of Earthquake Sciences of China Earthquake Administration.
文摘Migmatization in Higher Himalayan Crystallines (HHC) results from anatexis. The widely distributed migmatites in HHC are an important clue to investigate the relationship be- tween anatexis and the origins of Higher Himalayan leucogranites (HHL), and to understand the effect of anatexis on crustal evolution during the post-collision period. We studied in detail the chemical features of three basic constituent parts of the migmatites, i.e. leucosome, mesosome and melanosome, and determined the K-Ar ages of leucosomes. Our studies indicate that type-I leucosome is the product of crystallization of melt generated by partial melting of mesosome at source region, but type-II leucosome and HHL probably underwent crystallization differentiation of plagioclase during melt aggregation and migration. The age of 22.67 Ma of Type-I leucosome, which is a little older than the beginning of MCT movement, indicates that anatexis may have played an important role in the formation of MCT. That the ages of type-II leucosome (ranging from 14.82 to 18.37 Ma) are consistent with that of HHL provides new chronological evidence for the relationship between migmatization and HHL. We obtained a very young age of 6.23 Ma of Type-II leucosome that provides new time constraint on magma activity in the central segment of Higher Himalayas.
