New metamorphic petrology and geochronology from the Loe Sar dome in the Swat region of northern Pakistan place refined constraints on the pressure, temperature and timing of metamorphism and deformation in that part ...New metamorphic petrology and geochronology from the Loe Sar dome in the Swat region of northern Pakistan place refined constraints on the pressure, temperature and timing of metamorphism and deformation in that part of the Himalayan orogen. Thermodynamic modelling and monazite petrochronology indicate that metamorphism in the area followed a prograde evolution from ～525 ± 25 ℃and 6 士 0.5 kbar to ～610 ± 25 ℃ and 9 士 0.5 kbar, between ca. 39 Ma and 28 Ma. Partitioning of heavy rare earth elements between garnet rims and 30-28 Ma monazite are interpreted to indicate coeval crystallization at peak conditions. Microtextural relationships indicate that garnet rim growth post-dated the development of the main foliation in the area. The regional foliation is folded about large-scale N-S trending fold axes and overprinting E-W trending folds to form km-scale domal culminations. The textural relationships observed indicate that final dome development must be younger than the 30-28 Ma monazite that grew with garnet rims post-regional foliation development, but pre-doming-related deformation. This new timing constraint helps resolve discrepancy between previous interpretations,which have alternately suggested that N-S trending regional folds must be either pre-or post-early Oligocene. Finally, when combined with existing hornblende and white mica cooling ages, these new data indicate that the study area was exhumed rapidly following peak metamorphism.展开更多
This study presents a re-examination of historical specimens(DG136 and DG167)from the Monashee complex in the southeastern Canadian Cordillera that are critical to the current understanding of rare earth element(REE)d...This study presents a re-examination of historical specimens(DG136 and DG167)from the Monashee complex in the southeastern Canadian Cordillera that are critical to the current understanding of rare earth element(REE)distribution between garnet and monazite(and other accessory minerals)during metamorphism.Nine-hundred and fifty-one new monazite petrochronology spot analyses on 29 different grains across two specimens outline detailed(re)crystallization histories.Trace element data collected from the same ablated volume,interpreted in the context of new phase equilibria modelling that includes monazite,xenotime and apatite,link ages to specific portions of the pressure-temperature(P-T)paths followed by the specimens.These linkages are further informed by garnet Lu-Hf geochronology and xenotime petrochronology.The clockwise P-T paths indicate prograde metamorphism was ongoing by ca.80 Ma in both specimens.The structurally deeper specimen,DG136,records peak P-T conditions of~755-770℃and 8.8-10.4 kbar,interpreted to coincide with(re-)crystallization of low Y monazite at~75-70 Ma.Near-rim garnet isopleths from DG167 cross in the observed peak assemblage field at~680℃ and 9.3 kbar.These conditions are interpreted to correspond with low Y monazite(re-)crystallisation at~65 Ma.Both specimens record decompression along their retrograde path coincident with high Y 70-55 Ma and 65-55 Ma monazite populations in DG136 and DG167,respectively.These findings broadly agree with those initially reported~20 years ago and confirm early interpretations using trace elements in monazite as generally reliable markers of metamorphic reactions.Modern phase equilibria modelling and in situ petrochronological analysis,however,provide additional insight into monazite behaviour during anatexis and the effects of potential trace element buffering by REE-bearing phases such as apatite.展开更多
A garnet-bearing schist from the southernmost such exposure along the Himalaya in east central Nepal records prograde metamorphism at 32.4±0.3 Ma.Phase equilibria modelling,combined with Ti-in-biotite and quartz ...A garnet-bearing schist from the southernmost such exposure along the Himalaya in east central Nepal records prograde metamorphism at 32.4±0.3 Ma.Phase equilibria modelling,combined with Ti-in-biotite and quartz caxis thermometry,outline a tight-to-hairpin pressure-temperature(P-T)path extending from~515℃ and 5.5 kbar to peak conditions at~575℃ and 7 kbar followed by deformation during the retrograde phase at 480-515℃ and 6-7 kbar.The new geochronology data place an upper bound on the evolution of metamorphism and deformation in the frontal-most part of the Himalaya,which lasted until 17.5 Ma,as indicated by previously published ^(40)Ar/^(39)Ar data.The P-T-time data from this part of the Himalaya,as well as that from more hinterlandward portions of the orogen,outline a progressive,stepwise,commonly out-of-sequence evolution.Further data from along the orogen indicates that this evolution is not a local phenomenon,but instead characterizes the tectonics of this system as a whole.展开更多
The putative Jambil meta-carbonatites of Swat,northern Pakistan,occur as discrete intrusions into the Proterozoic Manglaur Formation,which are difficult to be distinguished from nearby calc-silicate marble because bot...The putative Jambil meta-carbonatites of Swat,northern Pakistan,occur as discrete intrusions into the Proterozoic Manglaur Formation,which are difficult to be distinguished from nearby calc-silicate marble because both rock types experienced regional metamorphism during Himalayan orogenesis that resulted in similar mosaic textures and mineral assemblages.Carbonatites are often significant repositories of economic mineral resources and,therefore,are important to be distinguished from calc-silicate marble.We present new geochemical and geochronology data to distinguish between the two rock types and interpret the petrogenesis and tectonic evolution of the Jambil metacarbonatites.Whole rock chemical data from the Jambil meta-carbonatites show characteristically high rare earth element(REE),Sr contents and lack of negative Eu anomaly,consistent with average calcio-carbonatite values worldwide and an igneous origin.More than 0.5 wt.% SrO in the metacarbonatites and SrO> 0.15 wt.% in constituent rock forming calcite are discriminating signatures of the Jambil meta-carbonatites.Chemically,the Jambil meta-carbonatites are relatively depleted in Rb,Nb,Ta,Ti,Zr and Hf,relatively enriched in Ba,Th,Sr,and have a high LREE/HREE ratio when normalized to primitive mantle.Their carbon and oxygen isotope compositions vary from-3.5‰ to-4.3‰and from 9.7‰ to 12.3‰,respectively.These geochemical characteristics indicate generation of the carbonatites through small degree of partial melting from a carbonated eclogitic source.In-situ,U/Pb analysis of titanite indicates that the Jambil meta-carbonatites were emplacement at 438 ±3 Ma.When combined with regional geological observations,we interpret the emplacement of the Jambil metacarbonatites to have taken place during the Silurian back arc extension within greater Gondwana and mark a transition from a compressional tectonic regime,brought about by collision of microcontinental blocks along the northern margin of Gondwana,to post-orogenic extension in the waning stages of the pre-Himalayan Ordovician orogeny.Finally,in-situ ^(208)Pb/^(232)Th monazite dates(40.3-27.6Ma) extracted from the meta-carbonatites are consistent with the Cenozoic metamorphism of the area.展开更多
Re-examination of three specimens from the Kanchenjunga Himal of Nepal via in situ Lu-Hf garnet geochronology yields evidence of multiple garnet growth events.Spot analyses from grain cores in two specimens define Pal...Re-examination of three specimens from the Kanchenjunga Himal of Nepal via in situ Lu-Hf garnet geochronology yields evidence of multiple garnet growth events.Spot analyses from grain cores in two specimens define Paleozoic regressions whereas analyses from grain rims in the same specimens define low-precision regressions consistent with the timing of Himalayan orogenesis.These dates contrast with previously published low dispersion,ca.290 Ma isotope dissolution(ID)Lu-Hf garnet dates for the same rocks.Modelling of Lu and spot age distribution in representative grains from the specimens examined yields calculated dates that approximate the Permian-age regressions through the original ID data.These findings demonstrate that it is possible to generate low dispersion ID Lu-Hf data from multigenerational garnet with significantly different-age growth events when approximately equal proportions of the different age reservoirs are included in multi-component aliquots.展开更多
基金supported by a Higher Education Commission of Pakistan Post-Doctoral Scholarship to A. Ali, NSERC Discovery Grant and Canada Foundation for Innovation grants to K. Larson, and NSF grant NSF-EAR-1119380 to J. Cottle. D. Arkinstall is thanked for his assistance in the FiLTER
文摘New metamorphic petrology and geochronology from the Loe Sar dome in the Swat region of northern Pakistan place refined constraints on the pressure, temperature and timing of metamorphism and deformation in that part of the Himalayan orogen. Thermodynamic modelling and monazite petrochronology indicate that metamorphism in the area followed a prograde evolution from ～525 ± 25 ℃and 6 士 0.5 kbar to ～610 ± 25 ℃ and 9 士 0.5 kbar, between ca. 39 Ma and 28 Ma. Partitioning of heavy rare earth elements between garnet rims and 30-28 Ma monazite are interpreted to indicate coeval crystallization at peak conditions. Microtextural relationships indicate that garnet rim growth post-dated the development of the main foliation in the area. The regional foliation is folded about large-scale N-S trending fold axes and overprinting E-W trending folds to form km-scale domal culminations. The textural relationships observed indicate that final dome development must be younger than the 30-28 Ma monazite that grew with garnet rims post-regional foliation development, but pre-doming-related deformation. This new timing constraint helps resolve discrepancy between previous interpretations,which have alternately suggested that N-S trending regional folds must be either pre-or post-early Oligocene. Finally, when combined with existing hornblende and white mica cooling ages, these new data indicate that the study area was exhumed rapidly following peak metamorphism.
文摘This study presents a re-examination of historical specimens(DG136 and DG167)from the Monashee complex in the southeastern Canadian Cordillera that are critical to the current understanding of rare earth element(REE)distribution between garnet and monazite(and other accessory minerals)during metamorphism.Nine-hundred and fifty-one new monazite petrochronology spot analyses on 29 different grains across two specimens outline detailed(re)crystallization histories.Trace element data collected from the same ablated volume,interpreted in the context of new phase equilibria modelling that includes monazite,xenotime and apatite,link ages to specific portions of the pressure-temperature(P-T)paths followed by the specimens.These linkages are further informed by garnet Lu-Hf geochronology and xenotime petrochronology.The clockwise P-T paths indicate prograde metamorphism was ongoing by ca.80 Ma in both specimens.The structurally deeper specimen,DG136,records peak P-T conditions of~755-770℃and 8.8-10.4 kbar,interpreted to coincide with(re-)crystallization of low Y monazite at~75-70 Ma.Near-rim garnet isopleths from DG167 cross in the observed peak assemblage field at~680℃ and 9.3 kbar.These conditions are interpreted to correspond with low Y monazite(re-)crystallisation at~65 Ma.Both specimens record decompression along their retrograde path coincident with high Y 70-55 Ma and 65-55 Ma monazite populations in DG136 and DG167,respectively.These findings broadly agree with those initially reported~20 years ago and confirm early interpretations using trace elements in monazite as generally reliable markers of metamorphic reactions.Modern phase equilibria modelling and in situ petrochronological analysis,however,provide additional insight into monazite behaviour during anatexis and the effects of potential trace element buffering by REE-bearing phases such as apatite.
基金supported by an NSERC Discovery Grant and Accelerator SupplementCanadian Foundation for Innovation John Evans Leadership Fund award to K.Larson+1 种基金Stipend and research support for S.Shrestha was provided by an NSERC Postgraduate Scholarshipa UBC Okanagan Fipke Scholar Award。
文摘A garnet-bearing schist from the southernmost such exposure along the Himalaya in east central Nepal records prograde metamorphism at 32.4±0.3 Ma.Phase equilibria modelling,combined with Ti-in-biotite and quartz caxis thermometry,outline a tight-to-hairpin pressure-temperature(P-T)path extending from~515℃ and 5.5 kbar to peak conditions at~575℃ and 7 kbar followed by deformation during the retrograde phase at 480-515℃ and 6-7 kbar.The new geochronology data place an upper bound on the evolution of metamorphism and deformation in the frontal-most part of the Himalaya,which lasted until 17.5 Ma,as indicated by previously published ^(40)Ar/^(39)Ar data.The P-T-time data from this part of the Himalaya,as well as that from more hinterlandward portions of the orogen,outline a progressive,stepwise,commonly out-of-sequence evolution.Further data from along the orogen indicates that this evolution is not a local phenomenon,but instead characterizes the tectonics of this system as a whole.
基金financial support from the National Centre of Excellence in Geology, University of Peshawar, Pakistan。
文摘The putative Jambil meta-carbonatites of Swat,northern Pakistan,occur as discrete intrusions into the Proterozoic Manglaur Formation,which are difficult to be distinguished from nearby calc-silicate marble because both rock types experienced regional metamorphism during Himalayan orogenesis that resulted in similar mosaic textures and mineral assemblages.Carbonatites are often significant repositories of economic mineral resources and,therefore,are important to be distinguished from calc-silicate marble.We present new geochemical and geochronology data to distinguish between the two rock types and interpret the petrogenesis and tectonic evolution of the Jambil metacarbonatites.Whole rock chemical data from the Jambil meta-carbonatites show characteristically high rare earth element(REE),Sr contents and lack of negative Eu anomaly,consistent with average calcio-carbonatite values worldwide and an igneous origin.More than 0.5 wt.% SrO in the metacarbonatites and SrO> 0.15 wt.% in constituent rock forming calcite are discriminating signatures of the Jambil meta-carbonatites.Chemically,the Jambil meta-carbonatites are relatively depleted in Rb,Nb,Ta,Ti,Zr and Hf,relatively enriched in Ba,Th,Sr,and have a high LREE/HREE ratio when normalized to primitive mantle.Their carbon and oxygen isotope compositions vary from-3.5‰ to-4.3‰and from 9.7‰ to 12.3‰,respectively.These geochemical characteristics indicate generation of the carbonatites through small degree of partial melting from a carbonated eclogitic source.In-situ,U/Pb analysis of titanite indicates that the Jambil meta-carbonatites were emplacement at 438 ±3 Ma.When combined with regional geological observations,we interpret the emplacement of the Jambil metacarbonatites to have taken place during the Silurian back arc extension within greater Gondwana and mark a transition from a compressional tectonic regime,brought about by collision of microcontinental blocks along the northern margin of Gondwana,to post-orogenic extension in the waning stages of the pre-Himalayan Ordovician orogeny.Finally,in-situ ^(208)Pb/^(232)Th monazite dates(40.3-27.6Ma) extracted from the meta-carbonatites are consistent with the Cenozoic metamorphism of the area.
文摘Re-examination of three specimens from the Kanchenjunga Himal of Nepal via in situ Lu-Hf garnet geochronology yields evidence of multiple garnet growth events.Spot analyses from grain cores in two specimens define Paleozoic regressions whereas analyses from grain rims in the same specimens define low-precision regressions consistent with the timing of Himalayan orogenesis.These dates contrast with previously published low dispersion,ca.290 Ma isotope dissolution(ID)Lu-Hf garnet dates for the same rocks.Modelling of Lu and spot age distribution in representative grains from the specimens examined yields calculated dates that approximate the Permian-age regressions through the original ID data.These findings demonstrate that it is possible to generate low dispersion ID Lu-Hf data from multigenerational garnet with significantly different-age growth events when approximately equal proportions of the different age reservoirs are included in multi-component aliquots.
