Granitic gneiss(orthogneiss)and Himalayan leucogranite are widely distributed in the Himalayan orogen,but whether or not the granitic gneiss made a contribution to the Himalayan leucogranite remains unclear.In this st...Granitic gneiss(orthogneiss)and Himalayan leucogranite are widely distributed in the Himalayan orogen,but whether or not the granitic gneiss made a contribution to the Himalayan leucogranite remains unclear.In this study,we present the petrological,geochronological and geochemical results for orthogneisses and leucogranites from the Zhada area,Western Himalayas.Zhada orthogneiss is composed mainly of quartz,plagioclase,K-feldspar,biotite and muscovite,with accessory zircon and apatite.Orthogneiss zircon cathodoluminescence(CL)images show that most grains contain a core with oscillatory zoning,which indicates an igneous origin.Sensitive high-resolution ion microprobe(SHRIMP)U-Pb dating of the zircon cores in the orthogneiss shows a weighted ^(206)Pb/^(238)U age of 515±4 Ma(early Paleozoic),with sponge-like zircon rims of 17.9±0.5 Ma(Miocene).Zhada leucogranite shows^(206)Pb/^(238)U ages ranging from 19.0±0.4 Ma to 12.4±0.2 Ma,the weighted average age being 16.2±0.4 Ma.The leucogranites have a low Ca content(<1 wt%),FeOt content(<1 wt%),Rb content(67.0-402 ppm),Sr content(<56.6 ppm),Ba content(3.35-238 ppm)and Rb/Sr ratio(0.5-14.7),which are similar to the geochemical characteristics of the Himalayan leucogranite derived from muscovite dehydration partial melting of metasediments and representative of most Himalayan leucogranites.The highly variable Na_(2)O+K_(2)O(4.33 wt%-9.13 wt%),Al_(2)O_(3)(8.44 wt%-13.51 wt%),ΣREE(40.2-191.0 ppm),Rb(67.0-402 ppm)and Nb(8.23-26.4 ppm)contents,^(87)Sr/^(86)Sr(t)ratios(0.7445-0.8605)andεNd(t)values(−3.6 to−8.2)indicate that the leucogranite is derived from a heterogenetic source.The nonradiogenic Nd isotope values of the studied Zhada leucogranite and orthogneiss range from−8.2 to−3.6 and from−8.7 to−4.1,respectively.Therefore,the general mixing equation was used to perform the Sr and Nd isotope mixing calculations.The results indicate that the heterogenetic source was the Tethyan Himalayan Sequence(THS)/Higher Himalayan Crystalline(HHC)metasediments and Zhada orthogneiss.The Zhada area experienced crustal anatexis during the Miocene and the heterogenetic source of the orthogneiss and metasediment may have experienced crustal anatexis controlled by muscovite dehydration.The Zhada leucogranite inherited not only the geochemical characteristics of the Himalayan metasediment(muscovite dehydration melting),but also the trace elements and Sr-Nd isotopic characteristics of the Zhada orthogneiss.These results indicate that the Paleozoic Zhada orthogneiss was involved in crustal anatexis at 17.9±0.5 Ma(Miocene)and that the muscovite dehydration of the metasediments in the heterogenetic source produced fluid,which may have caused the orthogneiss solidus lines to decline,triggering a partial melting of the Zhada orthogneiss.It is therefore proposed that Himalayan leucogranite is a crust-derived granite rather than a S-type granite,as previously hypothesized.展开更多
Granitoids and orthogneisses(1.7 Ga)from the northern part of the Banded Gneissic Complex(BGC-II domain),Aravalli Craton(NW India)are geochemically analyzed to understand the geodynamic condition and crustal evolution...Granitoids and orthogneisses(1.7 Ga)from the northern part of the Banded Gneissic Complex(BGC-II domain),Aravalli Craton(NW India)are geochemically analyzed to understand the geodynamic condition and crustal evolution processes.The samples are metaluminous to peraluminous(molar A/CNK:0.74±2.12),and characterized by Eu-anomaly ranging from 0.17 to 1.06,Fe_(2)O_(3)^(T)/(Fe_(2)O_(3)^(T)+MgO)from 0.8 to 0.9 and high magmatic zircon saturation temperature(>830°C)which are the features suggestive of A-type granite affinity.Tectonic discrimination diagrams classify the samples as post-collisional extensional A2-type granites.Geochemical characteristics along with trace element ratios[(Y/Nb)_(N)=0.15 to 4.33(avg.0.76),(Th/Nb)_(N)=4.63 to 255.47(avg.63.13),(Th/Ta)_(N)=1.37 to 9.84(avg.8.86),(Ce/Pb)_(N)=0.05 to 3.05(avg.1.43)]indicate that the rocks were derived from a plagioclase-rich and garnet-free crustal source under lowpressure conditions.Further,it is also proposed that tonalite-trondhjemite-granodiorite(TTG)rocks which occur dominantly in southern Rajasthan(BGC-I)are precursors and their partial melting led to the generation of the studied A-type granite samples.The studied samples also bear close geochemical similarity with A-type granites of similar age(1.7 Ga)near the Sakhun-Ladera region of northern BGC-II.The studied A-type granites are believed to be coeval to similar aged A-type granites of the Khetri and Alwar sub-basins of the North Delhi Fold Belt(NDFB).They are comparable in age and magmatic history to recorded A-type magmatism in North America and parts of the Chinese craton.The large geographical extents of synchronous A-type granites are proposed to be related to the Columbia Supercontinent assembly(ca.1.7 Ga;postcollisional granites).Thus,based on the studied extensional granites,we surmise that BGC-II was part of the Columbia Supercontinent.展开更多
Coesite inclusions, together with omphacite, jadeite, garnet and phengite inclusions, were identified in zircons separates from almost all gneissic core samples of pre-pilot drillhole CCSD-PP2 by the Laser Raman spect...Coesite inclusions, together with omphacite, jadeite, garnet and phengite inclusions, were identified in zircons separates from almost all gneissic core samples of pre-pilot drillhole CCSD-PP2 by the Laser Raman spectros-copy and the cathodoluminescence method. These data indicate that gneissic rocks consisting of paragneisses and or-thogneisses ubiquitously experienced UHP metamorphism. This research may be of great significance for an in-depth study of the subduction-exhumation mechanism of the Sulu UHP metamorphic belt and selecting the drilling site for the Chinese Continental Scientific Drilling Project.展开更多
By using the SHRIMP U-Pb and single zircon stepwise evaporation methods, the authors have obtained some results for granitoids from eastern Hebei Province. The Yuhuzhai hyperthene tonalitic granite was formed 2550 Ma ...By using the SHRIMP U-Pb and single zircon stepwise evaporation methods, the authors have obtained some results for granitoids from eastern Hebei Province. The Yuhuzhai hyperthene tonalitic granite was formed 2550 Ma ago, the Qingyangshu gabbroic gneiss 2536 Ma, the Yinmahe granodioritic gneiss near Lücao, Lulong County, 2533 Ma, the gabbro-dioritic gneiss near Longwan, Qianxi County, 2518-2515 Ma, the Qiuhuayu trondjemitic gneiss at Zunhua 2515 Ma, the Xiaoguanzhuang tonalitic gneiss at Zunhua 2495 Ma, and the Cuizhangzi gneiss in Qianxi County 2492 Ma. These geochronilogical data demonstrate that, though diverse in composition, type and origin, the granitic gneisses in eastern Hebei Province were emplaced and crystallized in a rather short period of magmatic activity. The formation of such a great amount of gneisses in this small time gap suggests that it was a critical crust accretion stage at the end of Neoarchean. The fact that granitoids of various types occurred at the same time implies a large-scale underplating （mantle plume） activity, which was then responsible for the crust accretion.展开更多
Two Neoarchean alkaline feldspar-rich granites sourced from partially melted granulite-facies granodioritic orthogneiss have been here recognised in the eastern part of the North China Block(NCB).These poorly foliated...Two Neoarchean alkaline feldspar-rich granites sourced from partially melted granulite-facies granodioritic orthogneiss have been here recognised in the eastern part of the North China Block(NCB).These poorly foliated granites have previously been assumed to be Mesozoic in age and never dated,and so their significance has not been recognised until now.The first granite(AG1)is a porphyritic syenogranite with megacrystic K-feldspar,and the second(AG2)is a quartz syenite with perthitic megacryst.Zircons from the granites yield LA-ICP-MS U-Pb ages of 2499±10 Ma(AG1),and 2492±28 Ma(AG2),which are slightly younger than the granodioritic orthogneiss that they intrude with a crystallisation U-Pb age of 2537±34 Ma.The younger granites have higher assays for SiO_(2)(71.91%for AG1 and 73.22%for AG2)and K_(2)O(7.52%for AG1 and 8.37%for AG2),and much lower assays for their other major element than the granodioritic orthogneiss.All of the granodioritic orthogneiss and granite samples have similar trace element patterns,with depletion in Th,U,Nb,and Ti and enrichment in Rb,Ba,K,La,Ce,and P.This indicates that the granites are derived from the orthogneiss as partial melts.Although they exhibit a similar REE pattern,the granites have much lower total REE contents(30.97×10^(−6) for AG1,and 25.93×10^(−6) for AG2),but pronounced positive Eu anomalies(Eu/Eu^(*)=8.57 for AG1 and 27.04 for AG2).The granodioritic orthogneiss has an initial ^(87)Sr/^(86)Sr ratio of 0.70144,εNd(t)value of 3.5,and εHf(t)values ranging from−3.2 to+2.9.The orthogneiss is a product of fractional crystallisation from a dioritic magma,which was derived from a mantle source contaminated by melts derived from a felsic slab.By contrast,the AG1 sample has an initial ^(87)Sr/^(86)Sr ratio of 0.6926 that is considered too low in value,εNd(t)value of 0.3,andεHf(t)values between+0.57 and+3.82;whereas the AG2 sample has an initial ^(87)Sr/^(86)Sr ratio of 0.70152,εNd(t)value of 1.3,andεHf(t)values between+0.5 and+14.08.These assays indicate that a Sr-Nd-Hf isotopic disequilibrium exists between the granite and granodioritic orthogneiss.The elevatedεHf(t)values of the granites can be explained by the involvement of Hf-bearing minerals,such as orthopyroxene,amphibole,and biotite,in anatectic reactions in the granodioritic orthogneiss.Based on the transitional relationship between the granites and granodioritic orthogneiss and the geochemical characteristics mentioned above,it is concluded that the granites are the product of rapid partial-melting of the granodioritic orthogneiss after granulite-facies metamorphism,and their crystallisation age of about 2500 Ma provides the minimum age of the metamorphism.This about 2500 Ma tectonic-metamorphic event in NCB is similar to the other cratons in India,Antarctica,northern and southern Australia,indicating a possible connection between these cratons during the Neoarchean.展开更多
In continental subduction complexes minor volumes of high-pressure mafic rocks(eclogites)often co-exist with much more abundant felsic(granitic)and metasedimentary rocks,which are vital for resolving the origin and me...In continental subduction complexes minor volumes of high-pressure mafic rocks(eclogites)often co-exist with much more abundant felsic(granitic)and metasedimentary rocks,which are vital for resolving the origin and metamorphic evolution of subducted continental crust.In SW Mongolia,the Alag Khadny eclogite-bearing accretionary complex(AKC)is assumed to represent either a remnant of oceanic slab,or a continental margin,subducted in the Early Cambrian.Here we present geochronological,geochemical and petrological evidence of subduction records for the three major types of lithologies that host mafic eclogites,including Mesoproterozoic and Neoproterozoic granitic basement and overlying Neoproterozoic continental-margin sediments.Variably deformed,ferroan and peraluminous metagranitoids compose a major part of AKC and are interlayered with eclogites in its southern and eastern margins.They have geochemical features of post-collisional/intraplate high-K calc-alkaline granites.LA-ICP-MS U-Pb zircon geochronology of three distinct metagranite samples show uniform protolith crystallization ages of ca.0.96 Ga and uncertain re-crystallization in the Late Neoproterozoic or Early Paleozoic metamorphic event,whereas abundant zircon inheritance indicates older,Mesoproterozoic to Paleoproterozoic crustal substrate during granite generation.The existence of Mesoproterozoic crust is highlighted by finding of distinct metagranitoids with the U-Pb zircon crystallization age of ca.1.6 Ga.Hafnium isotope signatures(T_(DM)^(C)2.88-1.85 Ga)of zircons from all lithologies preserved the evidence of reworked Neoarchean to Paleoproterozoic crust,similar to that of the Baidrag block(southern Mongolia),for both Mesoproterozoic and Neoproterozoic rocks.Regardless of the specific lithology,the rocks display indicators of high-pressure metamorphic re-equilibration,including garnet(X_(Ca)up to 0.65)+epidote+phengite(Si p.f.u.up to 3.56)±rutile assemblage in metagranitoids,garnet+phengite(Si p.f.u.up to 3.42)in quartz-rich semi-pelites and garnet+phengite(Si p.f.u.up to 3.39)+medium-Mg chloritoid(X_(Mg)up to 0.25)+kyanite+rutile in metapelites.Corresponding P-T conditions recovered from different lithologies reveal incoherent subduction of rocks,which could be shallow for granitic basement(1.1-1.4 GPa and 600-670℃)and clastic metasediments(1.4-1.6 GPa,570-620℃),but deeper for metapelites(2.1-2.3 GPa,500-570℃).consistent with that of eclogites,The combined data show that the Alag Khadny complex represents a remnant of a rifted Mesoproterozoic to Neoproterozoic(ca.1.6-0.96 Ga)continental margin consequently metamorphosed under HP conditions during Late Neoproterozoic-Early Cambrian evolution of the southern Central Asian Orogenic Belt.Acquired P-T estimates imply that high-pressure metagranitoids and metasedimentary rocks equilibrated at different depths,but most likely shared a common subduction-related metamorphic evolution.展开更多
The Beshta-Kamenistaya intrusions are located in the Main Range structural zone of the Greater Caucasus.They are composed of tonalitic gneisses that genetically resemble granites of the tholeiitic series of the ophiol...The Beshta-Kamenistaya intrusions are located in the Main Range structural zone of the Greater Caucasus.They are composed of tonalitic gneisses that genetically resemble granites of the tholeiitic series of the ophiolitic complexes.The Beshta-Kamenistaya orthogneisses and associated rocks of the nappes differ markedly from those of the Main Range zone.All of them were overthrust to the Main Range zone during the Bretonian orogeny.The age of their protolith and the metamorphism are still not defined.Using the zircon U-Pb LA-ICP-MS dating two age populations of zircons have been distinguished in the rocks of the Beshta-Kamenistaya intrusions.The age of the main population of zircons from orthogneisses is 426-300 Ma.Several age groups can be distinguished in this population.The main group yielded a Concordia age of386.9±1.4 Ma.There are also smaller peaks at 409-405,375-373,and 351 Ma.The oldest ages(426-395 Ma)were detected in the core parts of the complex crystals.We assume that the crystallization of the parental for orthogneisses rocks(tonalities)took place at 410-395 Ma,whereas the Concordia age of 386.9±1.4 Ma and a peak at 375-373 Ma correspond to the metamorphic event.The whole metamorphic cycle,including progressive and regressive stages,occurred between 395 and 370 Ma.Zircons,dated in the Beshta-Kamenistaya intrusion at350 Ma and younger,correspond to the Late Variscan orogeny.Zircons dated 3102-2769 Ma represent xenocrysts captured by the melts during their formation from the ancient rocks in the crystalline basement.展开更多
The Bastar craton largely consists of Mesoarchaean orthogneisses with vestiges of supracrustal rocks that have been intruded by Proterozoic granites and mafic dykes.Many regions in the Bastar craton have been subjecte...The Bastar craton largely consists of Mesoarchaean orthogneisses with vestiges of supracrustal rocks that have been intruded by Proterozoic granites and mafic dykes.Many regions in the Bastar craton have been subjected to medium or high grade metamorphic conditions but the nature and timing of regional metamorphism is poorly understood.In this study,metamorphosed mafic igneous rocks collected from two different dyke swarms from the southern part of the Central Indian Bastar craton have been studied to evaluate the timing and conditions of metamorphism.In this region,two distinct metamorphosed dyke swarms are recognized based on differences in petrology and geochemistry:the Meso-Neoarchaean sub-alkaline BD1 swarm and the Neoarchaean boninite-norite(BN)swarm.展开更多
Continental growth is episodic,and arc-continentalmargin has importance for the formation of continentalcrust in the Shyok Suture Zone(SSZ).To shed light onthe petrogenesis of mantle derived orthogneiss and hybridsour...Continental growth is episodic,and arc-continentalmargin has importance for the formation of continentalcrust in the Shyok Suture Zone(SSZ).To shed light onthe petrogenesis of mantle derived orthogneiss and hybridsource derived leucogranites,a detailed geochemicalstudy has been conducted on granitoids which areexposed between the Shyok-Darbuk of the Pangongtranspressional zone.The Shyok-Darbuk is a complexzone,marked by migmatization,crustal anatexis,in-situemplacement of various granitoids within metapelitesand orthogneiss.Major and trace element data have beenused in this paper to classify the granitoids to estimatethe possible source regions and tectonic environment ofmagma generation and emplacement.展开更多
基金This work was supported by the National Natural Science Foundation of China(Grant Nos.42072114,41503006)the Deep Resources Exploration and Mining,the National Key R&D Program of China(Grant No.2018YFC0604106)+1 种基金the Chinese Geological Survey(Grant No.DD20190167)the China Scholarship Council(Grant No.201808110080).
文摘Granitic gneiss(orthogneiss)and Himalayan leucogranite are widely distributed in the Himalayan orogen,but whether or not the granitic gneiss made a contribution to the Himalayan leucogranite remains unclear.In this study,we present the petrological,geochronological and geochemical results for orthogneisses and leucogranites from the Zhada area,Western Himalayas.Zhada orthogneiss is composed mainly of quartz,plagioclase,K-feldspar,biotite and muscovite,with accessory zircon and apatite.Orthogneiss zircon cathodoluminescence(CL)images show that most grains contain a core with oscillatory zoning,which indicates an igneous origin.Sensitive high-resolution ion microprobe(SHRIMP)U-Pb dating of the zircon cores in the orthogneiss shows a weighted ^(206)Pb/^(238)U age of 515±4 Ma(early Paleozoic),with sponge-like zircon rims of 17.9±0.5 Ma(Miocene).Zhada leucogranite shows^(206)Pb/^(238)U ages ranging from 19.0±0.4 Ma to 12.4±0.2 Ma,the weighted average age being 16.2±0.4 Ma.The leucogranites have a low Ca content(<1 wt%),FeOt content(<1 wt%),Rb content(67.0-402 ppm),Sr content(<56.6 ppm),Ba content(3.35-238 ppm)and Rb/Sr ratio(0.5-14.7),which are similar to the geochemical characteristics of the Himalayan leucogranite derived from muscovite dehydration partial melting of metasediments and representative of most Himalayan leucogranites.The highly variable Na_(2)O+K_(2)O(4.33 wt%-9.13 wt%),Al_(2)O_(3)(8.44 wt%-13.51 wt%),ΣREE(40.2-191.0 ppm),Rb(67.0-402 ppm)and Nb(8.23-26.4 ppm)contents,^(87)Sr/^(86)Sr(t)ratios(0.7445-0.8605)andεNd(t)values(−3.6 to−8.2)indicate that the leucogranite is derived from a heterogenetic source.The nonradiogenic Nd isotope values of the studied Zhada leucogranite and orthogneiss range from−8.2 to−3.6 and from−8.7 to−4.1,respectively.Therefore,the general mixing equation was used to perform the Sr and Nd isotope mixing calculations.The results indicate that the heterogenetic source was the Tethyan Himalayan Sequence(THS)/Higher Himalayan Crystalline(HHC)metasediments and Zhada orthogneiss.The Zhada area experienced crustal anatexis during the Miocene and the heterogenetic source of the orthogneiss and metasediment may have experienced crustal anatexis controlled by muscovite dehydration.The Zhada leucogranite inherited not only the geochemical characteristics of the Himalayan metasediment(muscovite dehydration melting),but also the trace elements and Sr-Nd isotopic characteristics of the Zhada orthogneiss.These results indicate that the Paleozoic Zhada orthogneiss was involved in crustal anatexis at 17.9±0.5 Ma(Miocene)and that the muscovite dehydration of the metasediments in the heterogenetic source produced fluid,which may have caused the orthogneiss solidus lines to decline,triggering a partial melting of the Zhada orthogneiss.It is therefore proposed that Himalayan leucogranite is a crust-derived granite rather than a S-type granite,as previously hypothesized.
基金supported by the Science and Engineering Research Board(SERB),Government of India,New Delhi,under a Major Research Project(File No.CRG/2019/000088)sanctioned to MEAM.
文摘Granitoids and orthogneisses(1.7 Ga)from the northern part of the Banded Gneissic Complex(BGC-II domain),Aravalli Craton(NW India)are geochemically analyzed to understand the geodynamic condition and crustal evolution processes.The samples are metaluminous to peraluminous(molar A/CNK:0.74±2.12),and characterized by Eu-anomaly ranging from 0.17 to 1.06,Fe_(2)O_(3)^(T)/(Fe_(2)O_(3)^(T)+MgO)from 0.8 to 0.9 and high magmatic zircon saturation temperature(>830°C)which are the features suggestive of A-type granite affinity.Tectonic discrimination diagrams classify the samples as post-collisional extensional A2-type granites.Geochemical characteristics along with trace element ratios[(Y/Nb)_(N)=0.15 to 4.33(avg.0.76),(Th/Nb)_(N)=4.63 to 255.47(avg.63.13),(Th/Ta)_(N)=1.37 to 9.84(avg.8.86),(Ce/Pb)_(N)=0.05 to 3.05(avg.1.43)]indicate that the rocks were derived from a plagioclase-rich and garnet-free crustal source under lowpressure conditions.Further,it is also proposed that tonalite-trondhjemite-granodiorite(TTG)rocks which occur dominantly in southern Rajasthan(BGC-I)are precursors and their partial melting led to the generation of the studied A-type granite samples.The studied samples also bear close geochemical similarity with A-type granites of similar age(1.7 Ga)near the Sakhun-Ladera region of northern BGC-II.The studied A-type granites are believed to be coeval to similar aged A-type granites of the Khetri and Alwar sub-basins of the North Delhi Fold Belt(NDFB).They are comparable in age and magmatic history to recorded A-type magmatism in North America and parts of the Chinese craton.The large geographical extents of synchronous A-type granites are proposed to be related to the Columbia Supercontinent assembly(ca.1.7 Ga;postcollisional granites).Thus,based on the studied extensional granites,we surmise that BGC-II was part of the Columbia Supercontinent.
基金This work was supported by the Project "1 :250000 Regional Geological Survey of the Lianyungang Sheet (I50C002004) of China" and "Geological Research for Selecting the Drillhole in the Donghai Area of the Chinese Continental Scientific Drilling Project"
文摘Coesite inclusions, together with omphacite, jadeite, garnet and phengite inclusions, were identified in zircons separates from almost all gneissic core samples of pre-pilot drillhole CCSD-PP2 by the Laser Raman spectros-copy and the cathodoluminescence method. These data indicate that gneissic rocks consisting of paragneisses and or-thogneisses ubiquitously experienced UHP metamorphism. This research may be of great significance for an in-depth study of the subduction-exhumation mechanism of the Sulu UHP metamorphic belt and selecting the drilling site for the Chinese Continental Scientific Drilling Project.
基金The paper is supported by the National Natural Science Foundation of China （Grant Nos. 40420120135 and 40072060）.
文摘By using the SHRIMP U-Pb and single zircon stepwise evaporation methods, the authors have obtained some results for granitoids from eastern Hebei Province. The Yuhuzhai hyperthene tonalitic granite was formed 2550 Ma ago, the Qingyangshu gabbroic gneiss 2536 Ma, the Yinmahe granodioritic gneiss near Lücao, Lulong County, 2533 Ma, the gabbro-dioritic gneiss near Longwan, Qianxi County, 2518-2515 Ma, the Qiuhuayu trondjemitic gneiss at Zunhua 2515 Ma, the Xiaoguanzhuang tonalitic gneiss at Zunhua 2495 Ma, and the Cuizhangzi gneiss in Qianxi County 2492 Ma. These geochronilogical data demonstrate that, though diverse in composition, type and origin, the granitic gneisses in eastern Hebei Province were emplaced and crystallized in a rather short period of magmatic activity. The formation of such a great amount of gneisses in this small time gap suggests that it was a critical crust accretion stage at the end of Neoarchean. The fact that granitoids of various types occurred at the same time implies a large-scale underplating （mantle plume） activity, which was then responsible for the crust accretion.
基金The CAGS Research Fund(YYWF201715,YK1608)the project of China Geological Survey(DD20190437)financially support this study and contribution.
文摘Two Neoarchean alkaline feldspar-rich granites sourced from partially melted granulite-facies granodioritic orthogneiss have been here recognised in the eastern part of the North China Block(NCB).These poorly foliated granites have previously been assumed to be Mesozoic in age and never dated,and so their significance has not been recognised until now.The first granite(AG1)is a porphyritic syenogranite with megacrystic K-feldspar,and the second(AG2)is a quartz syenite with perthitic megacryst.Zircons from the granites yield LA-ICP-MS U-Pb ages of 2499±10 Ma(AG1),and 2492±28 Ma(AG2),which are slightly younger than the granodioritic orthogneiss that they intrude with a crystallisation U-Pb age of 2537±34 Ma.The younger granites have higher assays for SiO_(2)(71.91%for AG1 and 73.22%for AG2)and K_(2)O(7.52%for AG1 and 8.37%for AG2),and much lower assays for their other major element than the granodioritic orthogneiss.All of the granodioritic orthogneiss and granite samples have similar trace element patterns,with depletion in Th,U,Nb,and Ti and enrichment in Rb,Ba,K,La,Ce,and P.This indicates that the granites are derived from the orthogneiss as partial melts.Although they exhibit a similar REE pattern,the granites have much lower total REE contents(30.97×10^(−6) for AG1,and 25.93×10^(−6) for AG2),but pronounced positive Eu anomalies(Eu/Eu^(*)=8.57 for AG1 and 27.04 for AG2).The granodioritic orthogneiss has an initial ^(87)Sr/^(86)Sr ratio of 0.70144,εNd(t)value of 3.5,and εHf(t)values ranging from−3.2 to+2.9.The orthogneiss is a product of fractional crystallisation from a dioritic magma,which was derived from a mantle source contaminated by melts derived from a felsic slab.By contrast,the AG1 sample has an initial ^(87)Sr/^(86)Sr ratio of 0.6926 that is considered too low in value,εNd(t)value of 0.3,andεHf(t)values between+0.57 and+3.82;whereas the AG2 sample has an initial ^(87)Sr/^(86)Sr ratio of 0.70152,εNd(t)value of 1.3,andεHf(t)values between+0.5 and+14.08.These assays indicate that a Sr-Nd-Hf isotopic disequilibrium exists between the granite and granodioritic orthogneiss.The elevatedεHf(t)values of the granites can be explained by the involvement of Hf-bearing minerals,such as orthopyroxene,amphibole,and biotite,in anatectic reactions in the granodioritic orthogneiss.Based on the transitional relationship between the granites and granodioritic orthogneiss and the geochemical characteristics mentioned above,it is concluded that the granites are the product of rapid partial-melting of the granodioritic orthogneiss after granulite-facies metamorphism,and their crystallisation age of about 2500 Ma provides the minimum age of the metamorphism.This about 2500 Ma tectonic-metamorphic event in NCB is similar to the other cratons in India,Antarctica,northern and southern Australia,indicating a possible connection between these cratons during the Neoarchean.
基金performed within the framework of the IGC State Assignment Project No.0350-2016-0028Ministry of Science and Technology of Taiwan(MOST 106-2811-M-001-035)during postdoctoral research of SYuS at Institute of Earth Science(Academia Sinica,Taipei,Taiwan)further supported by the Foundation of Russian Federation President(Grant No.MK-67.2020.5)。
文摘In continental subduction complexes minor volumes of high-pressure mafic rocks(eclogites)often co-exist with much more abundant felsic(granitic)and metasedimentary rocks,which are vital for resolving the origin and metamorphic evolution of subducted continental crust.In SW Mongolia,the Alag Khadny eclogite-bearing accretionary complex(AKC)is assumed to represent either a remnant of oceanic slab,or a continental margin,subducted in the Early Cambrian.Here we present geochronological,geochemical and petrological evidence of subduction records for the three major types of lithologies that host mafic eclogites,including Mesoproterozoic and Neoproterozoic granitic basement and overlying Neoproterozoic continental-margin sediments.Variably deformed,ferroan and peraluminous metagranitoids compose a major part of AKC and are interlayered with eclogites in its southern and eastern margins.They have geochemical features of post-collisional/intraplate high-K calc-alkaline granites.LA-ICP-MS U-Pb zircon geochronology of three distinct metagranite samples show uniform protolith crystallization ages of ca.0.96 Ga and uncertain re-crystallization in the Late Neoproterozoic or Early Paleozoic metamorphic event,whereas abundant zircon inheritance indicates older,Mesoproterozoic to Paleoproterozoic crustal substrate during granite generation.The existence of Mesoproterozoic crust is highlighted by finding of distinct metagranitoids with the U-Pb zircon crystallization age of ca.1.6 Ga.Hafnium isotope signatures(T_(DM)^(C)2.88-1.85 Ga)of zircons from all lithologies preserved the evidence of reworked Neoarchean to Paleoproterozoic crust,similar to that of the Baidrag block(southern Mongolia),for both Mesoproterozoic and Neoproterozoic rocks.Regardless of the specific lithology,the rocks display indicators of high-pressure metamorphic re-equilibration,including garnet(X_(Ca)up to 0.65)+epidote+phengite(Si p.f.u.up to 3.56)±rutile assemblage in metagranitoids,garnet+phengite(Si p.f.u.up to 3.42)in quartz-rich semi-pelites and garnet+phengite(Si p.f.u.up to 3.39)+medium-Mg chloritoid(X_(Mg)up to 0.25)+kyanite+rutile in metapelites.Corresponding P-T conditions recovered from different lithologies reveal incoherent subduction of rocks,which could be shallow for granitic basement(1.1-1.4 GPa and 600-670℃)and clastic metasediments(1.4-1.6 GPa,570-620℃),but deeper for metapelites(2.1-2.3 GPa,500-570℃).consistent with that of eclogites,The combined data show that the Alag Khadny complex represents a remnant of a rifted Mesoproterozoic to Neoproterozoic(ca.1.6-0.96 Ga)continental margin consequently metamorphosed under HP conditions during Late Neoproterozoic-Early Cambrian evolution of the southern Central Asian Orogenic Belt.Acquired P-T estimates imply that high-pressure metagranitoids and metasedimentary rocks equilibrated at different depths,but most likely shared a common subduction-related metamorphic evolution.
基金the Curtin Research Office for providing support to LS。
文摘The Beshta-Kamenistaya intrusions are located in the Main Range structural zone of the Greater Caucasus.They are composed of tonalitic gneisses that genetically resemble granites of the tholeiitic series of the ophiolitic complexes.The Beshta-Kamenistaya orthogneisses and associated rocks of the nappes differ markedly from those of the Main Range zone.All of them were overthrust to the Main Range zone during the Bretonian orogeny.The age of their protolith and the metamorphism are still not defined.Using the zircon U-Pb LA-ICP-MS dating two age populations of zircons have been distinguished in the rocks of the Beshta-Kamenistaya intrusions.The age of the main population of zircons from orthogneisses is 426-300 Ma.Several age groups can be distinguished in this population.The main group yielded a Concordia age of386.9±1.4 Ma.There are also smaller peaks at 409-405,375-373,and 351 Ma.The oldest ages(426-395 Ma)were detected in the core parts of the complex crystals.We assume that the crystallization of the parental for orthogneisses rocks(tonalities)took place at 410-395 Ma,whereas the Concordia age of 386.9±1.4 Ma and a peak at 375-373 Ma correspond to the metamorphic event.The whole metamorphic cycle,including progressive and regressive stages,occurred between 395 and 370 Ma.Zircons,dated in the Beshta-Kamenistaya intrusion at350 Ma and younger,correspond to the Late Variscan orogeny.Zircons dated 3102-2769 Ma represent xenocrysts captured by the melts during their formation from the ancient rocks in the crystalline basement.
基金the Department of Science and Technology,New Delhi for providing research grant for this work(ESS/16/185/2003)。
文摘The Bastar craton largely consists of Mesoarchaean orthogneisses with vestiges of supracrustal rocks that have been intruded by Proterozoic granites and mafic dykes.Many regions in the Bastar craton have been subjected to medium or high grade metamorphic conditions but the nature and timing of regional metamorphism is poorly understood.In this study,metamorphosed mafic igneous rocks collected from two different dyke swarms from the southern part of the Central Indian Bastar craton have been studied to evaluate the timing and conditions of metamorphism.In this region,two distinct metamorphosed dyke swarms are recognized based on differences in petrology and geochemistry:the Meso-Neoarchaean sub-alkaline BD1 swarm and the Neoarchaean boninite-norite(BN)swarm.
文摘Continental growth is episodic,and arc-continentalmargin has importance for the formation of continentalcrust in the Shyok Suture Zone(SSZ).To shed light onthe petrogenesis of mantle derived orthogneiss and hybridsource derived leucogranites,a detailed geochemicalstudy has been conducted on granitoids which areexposed between the Shyok-Darbuk of the Pangongtranspressional zone.The Shyok-Darbuk is a complexzone,marked by migmatization,crustal anatexis,in-situemplacement of various granitoids within metapelitesand orthogneiss.Major and trace element data have beenused in this paper to classify the granitoids to estimatethe possible source regions and tectonic environment ofmagma generation and emplacement.
