Geological and petrological studies indicate that three phases of metabasic dykes are present in theTaipingzhai-Jinchangyu area within the high-grade metamorphic terrane of eastern Hebei. Garnet andhornblende in metab...Geological and petrological studies indicate that three phases of metabasic dykes are present in theTaipingzhai-Jinchangyu area within the high-grade metamorphic terrane of eastern Hebei. Garnet andhornblende in metabasic dykes of the second and third phases occur separately, forming two mineral aggregateareas gl+cpx+pl±hy and hb+cpx+pl-hy. P_(H_2O) in the rocks appears to be the main factor controlling theformation of the two aggregate areas. Both were formed simultaneously at the same metamorphic temperature. The second-phase basic dykes underwent metamorphism of pyroxene-granulite facies at a temperature ofsome 825C: later the dykes, together with the third-phase basic dykes experienced metamorphism ofamphibole-granulite facies at a temperature of about 750C under pressure of 0.9GPa.展开更多
The Dabie metamorphic terrane is tectonically located within the intersection area of the WNW trending east Qinling orogenic belt with the NNE trending west Pacific domain. The Punctuated multistage convergent movemen...The Dabie metamorphic terrane is tectonically located within the intersection area of the WNW trending east Qinling orogenic belt with the NNE trending west Pacific domain. The Punctuated multistage convergent movement and the emplacemeat of YanShanian granitoid intrusions account for the complication of its tectonic framework. The Dabie metamorphic complex is composed of 3 units: supracrustals of early Precambrian age, granite gneiss and metamorphic magic rocks of lats Precambrian age. Three major metsmorphic Stages can be traced, i. e. granulite factes, amphibolite facies of Precambrian and the HP/ LT epidote amphibolite and greenschist facies of Phanerozoic. The Hong'an Group underlying the Dabie complex shows epidote amphibolite facies which is probably synchronous with the amphibolite facies of the Dabie complex. The Protolith of the HP and UHP eclogiter are diversided. The amphibole, epidote and other mineral inclusions within the eclogite minerals testify to the earlier history of Pre-eclogitic metamorphism. The peak metamorphic assemblage coesite-omphacite-garnet is superimposed by amphibolite, epidote amphibolite and greenschist facies of retrograde metamorphism. In many regions the eclogites appear as pods, lenses and layers within ductile shear zones having synkinematic greenschist or epidote amphibolite facies of retrograde metamorphism. The Sm-Nd isochron of whole rock and garnet combined with Ph-Ph dating of zircon crystals from Mifengjian eclogite block in Hubei suggest that the UHI, metamorphism probably took Place nearly 480 Ma Which evideoces one of the most important tertono-thermal events of the punctuated evolution in the Dabie terrane.展开更多
Asthemaintectoniccomponentofthe Himalayan–Tibetan orogen,the Lhasa terrane has received much attention as it records the entire history of the orogeny.The occurrence of high pressure eclogite in the Sumdo complex in ...Asthemaintectoniccomponentofthe Himalayan–Tibetan orogen,the Lhasa terrane has received much attention as it records the entire history of the orogeny.The occurrence of high pressure eclogite in the Sumdo complex in central Lhasa terrane has a significant importance on the understanding of the Paleo-Tethys subduction and plate itineration processes in this area.The petrological,geochemical and geochronological data of eclogite and associated blueschist and garnet-bearing mica schist from Sumdo,Jilang and Bailang area have been briefly reviewed to explore the origin and metamorphic evolution of this suture.Eclogites from the Sumdo complex have experienced low temperature,high pressure to ultrahigh pressure metamorphism,revealing a fastsubduction and exhumation process in a typical oceanic subduction zone.The large P-T range between different eclogites in the literature may be affected by the big error of unappropriated using geothermobarometry and may also because of slices of subducted blocks derived from different depths juxtapose together during exhumation.By summarizing the U-Pb,Lu-Hf and Sm-Nd ages of eclogites,the eclogite facies metamorphism is likely to occur in early Triassic during 245-225 Ma,but not the previously accepted late Permian at ca.260 Ma by the reinterpretation of the former geochronological data from literature.The opening of Paleo-Tethys Ocean between the Lhasa terrane initiate prior to ca.280 Ma and ultimate closure to integrate the Lhasa terrane was no earlier than225 Ma and may triggered by the initial subduction of Bangong-Nujiang Tethys Ocean in the north.展开更多
A suit of metamorphic rocks experienced amphibolite and partly granulite facies metamorphism exposed on the Lhasa block,which are recognized as the basement of the Lhasa block named as Nyainqentanglha Group in the
The first data on P-T metamorphic conditions coupled with U-Pb monazite and zircon age obtained for the Neoarchean Kitoy granulite-gneiss terrane(SW Siberian Craton).Alumina gneisses of the Kitoy terrane indicate two-...The first data on P-T metamorphic conditions coupled with U-Pb monazite and zircon age obtained for the Neoarchean Kitoy granulite-gneiss terrane(SW Siberian Craton).Alumina gneisses of the Kitoy terrane indicate two-staged metamorphic evolution.The first stage of regional metamorphism(M1)occurred at high-amphibolite facies conditions at T=780-800℃ and P=8-9 kbar.The second stage(M2)belongs to MT-HT/LP type of metamorphism with the wide temperature interval 600-750℃ and pressure 2-4 kbar.Two age peaks were established on the basis of U-Pb monazite and zircon dating in garnet-anthophyllite gneisses.Both of them correspond to the Neoarchean age:the age of M1 falls into the interval of ca.2489-2496 Ma,the age of M2-ca.2446-2456 Ma.The high-temperature metamorphism of the Kitoy block and nearly coeval granitoid magmatism can be an evidence for the Neoarchean collision in SW Siberian craton.展开更多
Single zircons from two trondhjemitic gneisses and two clastic metasedimentary rocks without Eu anomaly of the Kongling high-grade metamorphic terrain are dated by the in situ SHRIMP U-Pb method. The results show that...Single zircons from two trondhjemitic gneisses and two clastic metasedimentary rocks without Eu anomaly of the Kongling high-grade metamorphic terrain are dated by the in situ SHRIMP U-Pb method. The results show that the trondhjemitic magma emplaced at 2947-2903 Ma. Concordant age of as old as 3.3 Ga is present in the detrital zircons from the clastic metasedimentary rocks. Together with the depleted mantle Nd model age (TDM =3.2-3.3 Ga) of the clastic metasedimentary rocks, this documents the presence of Paleoarchean continental crust in the Yangtze craton.展开更多
The early Precambrian high-grade metamorphosed basement in the Xi Ulanbulang area, central Inner Mongolia of China, is composed mainly of intermediate granulites and charnockitic gneisses. Both types of the rocks are ...The early Precambrian high-grade metamorphosed basement in the Xi Ulanbulang area, central Inner Mongolia of China, is composed mainly of intermediate granulites and charnockitic gneisses. Both types of the rocks are closely associated spatially and temporally, with a gradual variation between them. In order to understand timing of the high-grade metamorphism, we carried out SHRIMP U-Pb dating of zircons of the rocks. Zircons from the granulites and charnockitic gneisses are similar in structure and age. Zircon cores show magmatic zoning and have ages of 2507-2545 Ma. The ages are interpreted as the forming time of protolith of the granulites and charnockitic gneisses, indicating that a strong magmatism existed at that time in the Yinshan Block. The zircon mantles and rims show homogeneous structures and record a strong granulite facies metamorphism event around 2500 Ma, with a time interval between the metamorphism and magamatism being less than 50 Ma. These suggest that the Western Block was similar to the Eastern Block in tectono-thermal timing at the end of the Neoarchean.展开更多
The investigated area around Sarvapuram represents a part of the Karimnagar granulite terrane of the Eastern Dharwar Craton, India. Garnet–bearing gneiss is hosted as enclaves, pods within granite gneiss and charnock...The investigated area around Sarvapuram represents a part of the Karimnagar granulite terrane of the Eastern Dharwar Craton, India. Garnet–bearing gneiss is hosted as enclaves, pods within granite gneiss and charnockite. It is largely made up of garnet, orthopyroxene, cordierite, biotite, plagioclase, K–feldspar, sillimanite and quartz. The peak metamorphic stage is represented by the equilibrium mineral assemblage i.e. garnet, orthopyroxene, cordierite, biotite, plagioclase, sillimanite and quartz. Breakdown of the garnet as well as preservation of the orthopyroxene–cordierite symplectite, formation of cordierite with the consumption of the garnet + sillimanite + quartz represents the decompressional event. The thermobarometric calculations suggest a retrograde P–T path with a substantial decompression of c. 3.0 kbar. The water activity(XH2 O) conditions obtained with the win TWQ program for core and symplectite compositions from garnet–bearing gneiss are 0.07–0.14 and 0.11–0.16 respectively. The quantitative estimation of oxygen fugacity in garnet–bearing gneiss reveal log f O2 values ranging from-11.38 to-14.05. This high oxidation state could be one of the reasons that account for the absence of graphite in these rocks.展开更多
The Madurai Block in the Southern Granulite Terrane(SGT)of Peninsular India is one of the largest crustal blocks within the Neoproterozoic Gondwana assembly.This block is composed of three sub-blocks:the Neoarchean No...The Madurai Block in the Southern Granulite Terrane(SGT)of Peninsular India is one of the largest crustal blocks within the Neoproterozoic Gondwana assembly.This block is composed of three sub-blocks:the Neoarchean Northern Madurai block,Paleoproterozoic Central Madurai block and the dominantly Neoproterozoic Southern Madurai Block.The margins of these blocks are well-known for the occurrence of ultrahigh-temperature(UHT)granulite facies rocks mostly represented by Mg-Al metasediments.Here we report a dismembered layered mafic–ultramafic intrusion occurring in association with Mg-Al granulites from the classic locality of Ganguvarpatti in the Central Madurai Block.The major rock types of the layered intrusion include spinel orthopyroxenite,garnet-bearing gabbro,gabbro and gabbroic anorthosite showing rhythmic stratification and cumulate texture.The orthopyroxene-cordierite granulite from the associated Mg-Al layer is composed of spinel,cordierite and orthopyroxene.The pyroxene in both rock units is high-Al orthopyroxene formed under UHT metamorphic conditions.Conventional thermobarometry yields near-peak metamorphic conditions of 9.5–10 kbar pressure and a minimum temperature of 980℃.We computed P–T pseudosections and contoured for the compositional as well as modal isopleths of the major mineral phases,which yield temperature above 1000℃.FMAS petrogenetic grid,Al-in-orthopyroxene isopleth,conventional thermobarometry and calculated pseudosection reveal a clockwise pressure–temperature(P–T)path and near isothermal decompression.The U–Pb data on zircon grains from the layered magmatic suite indicate emplacement of the protolith at ca.2.0 Ga and the metamorphic overgrowths yield weighted ^(206)Pb/^(238)U mean ages ca.520 Ma.Monazite from the garnet-bearing gabbro and Opx-Crd granulite yielded ^(206)Pb/^(238)U weighted mean ages of ca.532 Ma and 523 Ma marking the timing of metamorphism.We correlate the layered intrusion to a Paleoproterozoic suprasubduction zone setting,defining the Ganguvarpatti area as part of a collisional suture assembling the Northern and Central Madurai Blocks.The Paleoproterozoic magmatism and late Neoproterozoic-Cambrian UHT metamorphism can be linked to the tectonics of the Columbia and Gondwana supercontinents.展开更多
Ordovician diorite-quartz diorite mylonite(Saganoseki quartz diorite) was discovered in Sambagawa metamorphic terrane at the northern margin of Saganoseki Peninsula,Kyushu Island,Japan.The LA-ICP-MS zircon U-Pb geochr...Ordovician diorite-quartz diorite mylonite(Saganoseki quartz diorite) was discovered in Sambagawa metamorphic terrane at the northern margin of Saganoseki Peninsula,Kyushu Island,Japan.The LA-ICP-MS zircon U-Pb geochronology revealed that the intrusion age of Saganoseki quartz diorite was 473.3±3.6 Ma.These rocks show the volcanic arc affinity based on the trace element composition.On the basis of geochronological and geochemical results,Saganoseki quartz diorite is considered to be a member of the Early Paleozoic igneous rocks of the Kurosegawa tectonic zone.Saganoseki quartz diorite is located just south of the Median Tectonic Line(MTL)and is in close contact with the pelitic and psammitic schist without any brittle shear zone.U-Pb ages of detrital zircon grains from two psammitic schists show the estimated sedimentation age of early Late Cretaceous,indicate that these psammitic schists are the member of Sambagawa metamorphic rocks.Together with these results and the mode of occurrence in the field,we argue that the Early Paleozoic igneous rocks of the Kurosegawa tectonic zone existed as an upper structural unit of the Sambagawa terrane.This relationship is the key to reconstruct the Mesozoic tectonics of Japan as a part of East Asia,and its evolution through time.展开更多
Petrological analysis and LA-ICP-MS zircon U-Pb dating were conducted on high- pressure marie granulites, which occured as xenolith within TTG gneisses, from the Nanshankou Village of the Jiaobei terrane, Shandong Pen...Petrological analysis and LA-ICP-MS zircon U-Pb dating were conducted on high- pressure marie granulites, which occured as xenolith within TTG gneisses, from the Nanshankou Village of the Jiaobei terrane, Shandong Peninsula in the north-eastern part of the North China Craton (NCC). The mafic HP grannlite is composed of garnet, clinopyroxene, orthopyroxene, amphibole and symplectitic clinopyroxene, orthopyroxene, plagioclase, ilmente and magnetite which were formed after the decomposition of porphyroblastic garnet and clinopyroxene. Four stages of metamorphic mineral assemblages for the mafic HP granulites were constrained by detail petrological and mineralogical in- vestigations. The early prograde assemblage is represented by the mineral inclusions within garnet and clinopyroxene porphyroblasts (Opx1+Pl1+Qtz1), recording the metamorphic conditions at -754-757 ℃, 0.63-0.71 GPa; peak metamorphic conditions were determined at -874-891 ℃, 1.32-1.35 GPa with the mineral assemblage of Grt2+Cpx2+Amp2+Pl2+Qtz2. Retrograde minerals derived from symplectitic assemblage Opx3+Cpx3+Amp3+Pl3+Qtz3+Ilm3±Mag3 were formed at 693-796℃, 0.60-0.84 GPa. A final greensehist to sub-greenschist facies event was recorded by the exsolution of actinolite and albite within a retrograded clinopyroxene, as well as the occurrence of prehnite, chlorite and calcite minerals. Accordingly, a clockwise P-T path was concluded on the basis of the different stages of mineral asseblage. Cathodoluminescence imaging, trace element and U-Pb dating of zircons from the mafic HP granulites recorded similar charactistics for three episodes of Paleo-Meso Proterozoic metamorphic events. These are the metamorphic events preserved in mafic and pelitic granulites in the Jiao-Liao-Ji belt (JLJB) with 207 pb/206pb ages of 2.0-1.9 Ga for peak metamorphism and of 1.86-1.84 Ga for decomposing process, followed by a retrograde amphibolite facies metamorphic event related to the post-orogenic extension at the age of 1.76-1.74 Ga, resulting the exhumation of the granulite to the upper crust level.展开更多
Some geologists reported their discovery of sandwiched low-grade metamorphic slabs within UHP metamorphic Complexes in Changpu, Yuexi County, Anhui Province. They also suggested that some coesite-bearing eclogites are...Some geologists reported their discovery of sandwiched low-grade metamorphic slabs within UHP metamorphic Complexes in Changpu, Yuexi County, Anhui Province. They also suggested that some coesite-bearing eclogites are igneous veins, which intruded low-grade metamorphic slabs and other related rocks. Moreover they further called in question to UHP matamorphic process and continental collision tectonism in the Dabieshan terrane. Based on our recent study so-called low-grade metamorphic slabs are strongly deformed fabric, fine-grained and tectonic recrystallized mylonites and tectonites. Their protolith rocks are garnet-bearing orthogneiss, eclogite and marble, as well as a few amounts of strongly deformed acid and basic veins. Their metamorphic, geochemical and geochronological characteristics are also identical with UHPM rocks and regional country orthogneisses. Therefore we conclude that there are not low-grade metamorphic slabs sand wiched with UHP metamorphic rocks in the Dabieshan terrane.展开更多
The present study reports and discusses the genesis of zincian chromite in the ultramafic xenoliths from the Dongripali area,Bastar craton,Central India.The zincian chromite is in the ultramafic xenoliths of Bengpal s...The present study reports and discusses the genesis of zincian chromite in the ultramafic xenoliths from the Dongripali area,Bastar craton,Central India.The zincian chromite is in the ultramafic xenoliths of Bengpal supracrustal rock hosted by Neoarchaean Bundeli gneisses.Compositionally zincian chromite shows a range of Cr_(2)O_(3)(39.69 to 51.66 wt%),Al_(2)O_(3)(05.30 wt%to 08.71 wt%),FeO(21.74 wt%to 27.51 wt%),Fe_(2)O_(3)(10.19 wt%to 19.36wt%)with higher ZnO content ranging from 1.73 wt%to 4.08 wt%.Accordingly,their Cr#[Cr/(Cr+Al)]varies in a narrow range from 0.83 to 0.85.Its calculated melt composition supports metamorphic or post-magmatic nature rather than common occurrences such as inclusion in diamonds,meteorites,and association with any sulfide-rich mineralised belt.This reveals that the post-magmatic processes play a vital role in transforming chromite to zincian chromite.The empirical thermometric calculation from chromite,amphibole,and pyroxene support their metamorphic origin and formed during low-P and high-T amphibolite grade facies of metamorphism(~700℃).The Neoarchaean granitic magmatism has a significant role in generating and transferring the heat during contact metamorphism with hydration of ultramafic xenoliths and further alteration,i.e.,serpentinisation.The olivine is a major repository for Mn,Zn,and Co in peridotite/ultramafic;these elements get mobilised during the metamorphism and serpentinisation.This is a possible reason for the mobilisation of zinc and incorporation in the chromite within altered ultramafic.As a result,chromiterich ultramafic xenolith subjected to metamorphic process gets enrichment of Zn and Fe due to elemental exchange.It converts common chromite into zincian chromite,as reported in altered ultramafics elsewhere.展开更多
High-pressure(HP)metamorphic terrane in the Tongbai orogen comprises two HP slices(I and II)and a tectonic mélange zone in the northeast and a blueschist-greenschist zone in the southwest.HP slice I is represente...High-pressure(HP)metamorphic terrane in the Tongbai orogen comprises two HP slices(I and II)and a tectonic mélange zone in the northeast and a blueschist-greenschist zone in the southwest.HP slice I is represented by the northern and southern eclogite zones on the two sides of the Tongbaishan antiform.HP slice II is represented by retrograded eclogite-bearing metamorphic enclaves in Cretaceous gneissic granites in the Tongbai Complex.U-Pb,Lu-Hf,Rb-Sr and40Ar/39Ar multichronometric data indicate that the peak metamorphism of HP slice I took place at^255 Ma,whereas the metamorphic ages of HP slice II are as young as232–220 Ma.By contrast,the tectonic mélange zone near the suture was metamorphosed at^256 Ma.Such a diachroneity of different slices across the direction of the orogen in the Hong’an-Dabie-Sulu HP/UHP terrane is ubiquitous,and it can be interpreted by a syn-subduction detachment/exhumation model.Furthermore,the metamorphic age of HP slice I in the Tongbai orogen is older than that of the equivalent HP slice in the Hong’an orogen by^15 Ma,suggesting that the diachroneity may have also existed along the direction of the orogen.A seesaw-type subduction/exhumation model is proposed to explain this age disparity and the subduction of the South China Block becomimg shallower towards the west.展开更多
文摘Geological and petrological studies indicate that three phases of metabasic dykes are present in theTaipingzhai-Jinchangyu area within the high-grade metamorphic terrane of eastern Hebei. Garnet andhornblende in metabasic dykes of the second and third phases occur separately, forming two mineral aggregateareas gl+cpx+pl±hy and hb+cpx+pl-hy. P_(H_2O) in the rocks appears to be the main factor controlling theformation of the two aggregate areas. Both were formed simultaneously at the same metamorphic temperature. The second-phase basic dykes underwent metamorphism of pyroxene-granulite facies at a temperature ofsome 825C: later the dykes, together with the third-phase basic dykes experienced metamorphism ofamphibole-granulite facies at a temperature of about 750C under pressure of 0.9GPa.
文摘The Dabie metamorphic terrane is tectonically located within the intersection area of the WNW trending east Qinling orogenic belt with the NNE trending west Pacific domain. The Punctuated multistage convergent movement and the emplacemeat of YanShanian granitoid intrusions account for the complication of its tectonic framework. The Dabie metamorphic complex is composed of 3 units: supracrustals of early Precambrian age, granite gneiss and metamorphic magic rocks of lats Precambrian age. Three major metsmorphic Stages can be traced, i. e. granulite factes, amphibolite facies of Precambrian and the HP/ LT epidote amphibolite and greenschist facies of Phanerozoic. The Hong'an Group underlying the Dabie complex shows epidote amphibolite facies which is probably synchronous with the amphibolite facies of the Dabie complex. The Protolith of the HP and UHP eclogiter are diversided. The amphibole, epidote and other mineral inclusions within the eclogite minerals testify to the earlier history of Pre-eclogitic metamorphism. The peak metamorphic assemblage coesite-omphacite-garnet is superimposed by amphibolite, epidote amphibolite and greenschist facies of retrograde metamorphism. In many regions the eclogites appear as pods, lenses and layers within ductile shear zones having synkinematic greenschist or epidote amphibolite facies of retrograde metamorphism. The Sm-Nd isochron of whole rock and garnet combined with Ph-Ph dating of zircon crystals from Mifengjian eclogite block in Hubei suggest that the UHI, metamorphism probably took Place nearly 480 Ma Which evideoces one of the most important tertono-thermal events of the punctuated evolution in the Dabie terrane.
文摘Asthemaintectoniccomponentofthe Himalayan–Tibetan orogen,the Lhasa terrane has received much attention as it records the entire history of the orogeny.The occurrence of high pressure eclogite in the Sumdo complex in central Lhasa terrane has a significant importance on the understanding of the Paleo-Tethys subduction and plate itineration processes in this area.The petrological,geochemical and geochronological data of eclogite and associated blueschist and garnet-bearing mica schist from Sumdo,Jilang and Bailang area have been briefly reviewed to explore the origin and metamorphic evolution of this suture.Eclogites from the Sumdo complex have experienced low temperature,high pressure to ultrahigh pressure metamorphism,revealing a fastsubduction and exhumation process in a typical oceanic subduction zone.The large P-T range between different eclogites in the literature may be affected by the big error of unappropriated using geothermobarometry and may also because of slices of subducted blocks derived from different depths juxtapose together during exhumation.By summarizing the U-Pb,Lu-Hf and Sm-Nd ages of eclogites,the eclogite facies metamorphism is likely to occur in early Triassic during 245-225 Ma,but not the previously accepted late Permian at ca.260 Ma by the reinterpretation of the former geochronological data from literature.The opening of Paleo-Tethys Ocean between the Lhasa terrane initiate prior to ca.280 Ma and ultimate closure to integrate the Lhasa terrane was no earlier than225 Ma and may triggered by the initial subduction of Bangong-Nujiang Tethys Ocean in the north.
基金funded by grants from the NSF China(No.41572051)the China Geological Survey(No.DD20160022-01)project from Institute of Geology,Chinese Academy of Geological Sciences(J1518)
文摘A suit of metamorphic rocks experienced amphibolite and partly granulite facies metamorphism exposed on the Lhasa block,which are recognized as the basement of the Lhasa block named as Nyainqentanglha Group in the
基金financial support of the Ministry of Science and Higher Education of the Russian Federationthe International Partnership Program of Chinese Academy of Sciences,Grant No.132744KYSB20190039。
文摘The first data on P-T metamorphic conditions coupled with U-Pb monazite and zircon age obtained for the Neoarchean Kitoy granulite-gneiss terrane(SW Siberian Craton).Alumina gneisses of the Kitoy terrane indicate two-staged metamorphic evolution.The first stage of regional metamorphism(M1)occurred at high-amphibolite facies conditions at T=780-800℃ and P=8-9 kbar.The second stage(M2)belongs to MT-HT/LP type of metamorphism with the wide temperature interval 600-750℃ and pressure 2-4 kbar.Two age peaks were established on the basis of U-Pb monazite and zircon dating in garnet-anthophyllite gneisses.Both of them correspond to the Neoarchean age:the age of M1 falls into the interval of ca.2489-2496 Ma,the age of M2-ca.2446-2456 Ma.The high-temperature metamorphism of the Kitoy block and nearly coeval granitoid magmatism can be an evidence for the Neoarchean collision in SW Siberian craton.
基金Ministry of Science and Technology of China (Grant No.1999043202), the National Natural Science Foundation of China (Grant Nos. 49625305, 49573183, 49673184, 49794043), the Northwest University, the Ministry of Education of China and the Australian Res
文摘Single zircons from two trondhjemitic gneisses and two clastic metasedimentary rocks without Eu anomaly of the Kongling high-grade metamorphic terrain are dated by the in situ SHRIMP U-Pb method. The results show that the trondhjemitic magma emplaced at 2947-2903 Ma. Concordant age of as old as 3.3 Ga is present in the detrital zircons from the clastic metasedimentary rocks. Together with the depleted mantle Nd model age (TDM =3.2-3.3 Ga) of the clastic metasedimentary rocks, this documents the presence of Paleoarchean continental crust in the Yangtze craton.
基金supported by National Natural Science Foundation of China (Grant Nos. 90814006, 40972135)Geological Survey of China (Grant No. 1212010510515)
文摘The early Precambrian high-grade metamorphosed basement in the Xi Ulanbulang area, central Inner Mongolia of China, is composed mainly of intermediate granulites and charnockitic gneisses. Both types of the rocks are closely associated spatially and temporally, with a gradual variation between them. In order to understand timing of the high-grade metamorphism, we carried out SHRIMP U-Pb dating of zircons of the rocks. Zircons from the granulites and charnockitic gneisses are similar in structure and age. Zircon cores show magmatic zoning and have ages of 2507-2545 Ma. The ages are interpreted as the forming time of protolith of the granulites and charnockitic gneisses, indicating that a strong magmatism existed at that time in the Yinshan Block. The zircon mantles and rims show homogeneous structures and record a strong granulite facies metamorphism event around 2500 Ma, with a time interval between the metamorphism and magamatism being less than 50 Ma. These suggest that the Western Block was similar to the Eastern Block in tectono-thermal timing at the end of the Neoarchean.
文摘The investigated area around Sarvapuram represents a part of the Karimnagar granulite terrane of the Eastern Dharwar Craton, India. Garnet–bearing gneiss is hosted as enclaves, pods within granite gneiss and charnockite. It is largely made up of garnet, orthopyroxene, cordierite, biotite, plagioclase, K–feldspar, sillimanite and quartz. The peak metamorphic stage is represented by the equilibrium mineral assemblage i.e. garnet, orthopyroxene, cordierite, biotite, plagioclase, sillimanite and quartz. Breakdown of the garnet as well as preservation of the orthopyroxene–cordierite symplectite, formation of cordierite with the consumption of the garnet + sillimanite + quartz represents the decompressional event. The thermobarometric calculations suggest a retrograde P–T path with a substantial decompression of c. 3.0 kbar. The water activity(XH2 O) conditions obtained with the win TWQ program for core and symplectite compositions from garnet–bearing gneiss are 0.07–0.14 and 0.11–0.16 respectively. The quantitative estimation of oxygen fugacity in garnet–bearing gneiss reveal log f O2 values ranging from-11.38 to-14.05. This high oxidation state could be one of the reasons that account for the absence of graphite in these rocks.
基金funded by Foreign Expert grants to M.Santosh from the China University of Geosciences(Beijing)。
文摘The Madurai Block in the Southern Granulite Terrane(SGT)of Peninsular India is one of the largest crustal blocks within the Neoproterozoic Gondwana assembly.This block is composed of three sub-blocks:the Neoarchean Northern Madurai block,Paleoproterozoic Central Madurai block and the dominantly Neoproterozoic Southern Madurai Block.The margins of these blocks are well-known for the occurrence of ultrahigh-temperature(UHT)granulite facies rocks mostly represented by Mg-Al metasediments.Here we report a dismembered layered mafic–ultramafic intrusion occurring in association with Mg-Al granulites from the classic locality of Ganguvarpatti in the Central Madurai Block.The major rock types of the layered intrusion include spinel orthopyroxenite,garnet-bearing gabbro,gabbro and gabbroic anorthosite showing rhythmic stratification and cumulate texture.The orthopyroxene-cordierite granulite from the associated Mg-Al layer is composed of spinel,cordierite and orthopyroxene.The pyroxene in both rock units is high-Al orthopyroxene formed under UHT metamorphic conditions.Conventional thermobarometry yields near-peak metamorphic conditions of 9.5–10 kbar pressure and a minimum temperature of 980℃.We computed P–T pseudosections and contoured for the compositional as well as modal isopleths of the major mineral phases,which yield temperature above 1000℃.FMAS petrogenetic grid,Al-in-orthopyroxene isopleth,conventional thermobarometry and calculated pseudosection reveal a clockwise pressure–temperature(P–T)path and near isothermal decompression.The U–Pb data on zircon grains from the layered magmatic suite indicate emplacement of the protolith at ca.2.0 Ga and the metamorphic overgrowths yield weighted ^(206)Pb/^(238)U mean ages ca.520 Ma.Monazite from the garnet-bearing gabbro and Opx-Crd granulite yielded ^(206)Pb/^(238)U weighted mean ages of ca.532 Ma and 523 Ma marking the timing of metamorphism.We correlate the layered intrusion to a Paleoproterozoic suprasubduction zone setting,defining the Ganguvarpatti area as part of a collisional suture assembling the Northern and Central Madurai Blocks.The Paleoproterozoic magmatism and late Neoproterozoic-Cambrian UHT metamorphism can be linked to the tectonics of the Columbia and Gondwana supercontinents.
基金This study was supported by the Fukada Geological Institute[Fukada Grant-in-Aid].
文摘Ordovician diorite-quartz diorite mylonite(Saganoseki quartz diorite) was discovered in Sambagawa metamorphic terrane at the northern margin of Saganoseki Peninsula,Kyushu Island,Japan.The LA-ICP-MS zircon U-Pb geochronology revealed that the intrusion age of Saganoseki quartz diorite was 473.3±3.6 Ma.These rocks show the volcanic arc affinity based on the trace element composition.On the basis of geochronological and geochemical results,Saganoseki quartz diorite is considered to be a member of the Early Paleozoic igneous rocks of the Kurosegawa tectonic zone.Saganoseki quartz diorite is located just south of the Median Tectonic Line(MTL)and is in close contact with the pelitic and psammitic schist without any brittle shear zone.U-Pb ages of detrital zircon grains from two psammitic schists show the estimated sedimentation age of early Late Cretaceous,indicate that these psammitic schists are the member of Sambagawa metamorphic rocks.Together with these results and the mode of occurrence in the field,we argue that the Early Paleozoic igneous rocks of the Kurosegawa tectonic zone existed as an upper structural unit of the Sambagawa terrane.This relationship is the key to reconstruct the Mesozoic tectonics of Japan as a part of East Asia,and its evolution through time.
基金supported by the National Natural Science Foundation of China (No. 41272072)the NSFC/NRF Research Cooperation Programm (No. 41761144061)the SDUST Research Fund (No. 2015TDJH101)
文摘Petrological analysis and LA-ICP-MS zircon U-Pb dating were conducted on high- pressure marie granulites, which occured as xenolith within TTG gneisses, from the Nanshankou Village of the Jiaobei terrane, Shandong Peninsula in the north-eastern part of the North China Craton (NCC). The mafic HP grannlite is composed of garnet, clinopyroxene, orthopyroxene, amphibole and symplectitic clinopyroxene, orthopyroxene, plagioclase, ilmente and magnetite which were formed after the decomposition of porphyroblastic garnet and clinopyroxene. Four stages of metamorphic mineral assemblages for the mafic HP granulites were constrained by detail petrological and mineralogical in- vestigations. The early prograde assemblage is represented by the mineral inclusions within garnet and clinopyroxene porphyroblasts (Opx1+Pl1+Qtz1), recording the metamorphic conditions at -754-757 ℃, 0.63-0.71 GPa; peak metamorphic conditions were determined at -874-891 ℃, 1.32-1.35 GPa with the mineral assemblage of Grt2+Cpx2+Amp2+Pl2+Qtz2. Retrograde minerals derived from symplectitic assemblage Opx3+Cpx3+Amp3+Pl3+Qtz3+Ilm3±Mag3 were formed at 693-796℃, 0.60-0.84 GPa. A final greensehist to sub-greenschist facies event was recorded by the exsolution of actinolite and albite within a retrograded clinopyroxene, as well as the occurrence of prehnite, chlorite and calcite minerals. Accordingly, a clockwise P-T path was concluded on the basis of the different stages of mineral asseblage. Cathodoluminescence imaging, trace element and U-Pb dating of zircons from the mafic HP granulites recorded similar charactistics for three episodes of Paleo-Meso Proterozoic metamorphic events. These are the metamorphic events preserved in mafic and pelitic granulites in the Jiao-Liao-Ji belt (JLJB) with 207 pb/206pb ages of 2.0-1.9 Ga for peak metamorphism and of 1.86-1.84 Ga for decomposing process, followed by a retrograde amphibolite facies metamorphic event related to the post-orogenic extension at the age of 1.76-1.74 Ga, resulting the exhumation of the granulite to the upper crust level.
文摘Some geologists reported their discovery of sandwiched low-grade metamorphic slabs within UHP metamorphic Complexes in Changpu, Yuexi County, Anhui Province. They also suggested that some coesite-bearing eclogites are igneous veins, which intruded low-grade metamorphic slabs and other related rocks. Moreover they further called in question to UHP matamorphic process and continental collision tectonism in the Dabieshan terrane. Based on our recent study so-called low-grade metamorphic slabs are strongly deformed fabric, fine-grained and tectonic recrystallized mylonites and tectonites. Their protolith rocks are garnet-bearing orthogneiss, eclogite and marble, as well as a few amounts of strongly deformed acid and basic veins. Their metamorphic, geochemical and geochronological characteristics are also identical with UHPM rocks and regional country orthogneisses. Therefore we conclude that there are not low-grade metamorphic slabs sand wiched with UHP metamorphic rocks in the Dabieshan terrane.
基金Geological Survey of India,Ministry of Mines,Government of India for funding the opportunity to work in this projects。
文摘The present study reports and discusses the genesis of zincian chromite in the ultramafic xenoliths from the Dongripali area,Bastar craton,Central India.The zincian chromite is in the ultramafic xenoliths of Bengpal supracrustal rock hosted by Neoarchaean Bundeli gneisses.Compositionally zincian chromite shows a range of Cr_(2)O_(3)(39.69 to 51.66 wt%),Al_(2)O_(3)(05.30 wt%to 08.71 wt%),FeO(21.74 wt%to 27.51 wt%),Fe_(2)O_(3)(10.19 wt%to 19.36wt%)with higher ZnO content ranging from 1.73 wt%to 4.08 wt%.Accordingly,their Cr#[Cr/(Cr+Al)]varies in a narrow range from 0.83 to 0.85.Its calculated melt composition supports metamorphic or post-magmatic nature rather than common occurrences such as inclusion in diamonds,meteorites,and association with any sulfide-rich mineralised belt.This reveals that the post-magmatic processes play a vital role in transforming chromite to zincian chromite.The empirical thermometric calculation from chromite,amphibole,and pyroxene support their metamorphic origin and formed during low-P and high-T amphibolite grade facies of metamorphism(~700℃).The Neoarchaean granitic magmatism has a significant role in generating and transferring the heat during contact metamorphism with hydration of ultramafic xenoliths and further alteration,i.e.,serpentinisation.The olivine is a major repository for Mn,Zn,and Co in peridotite/ultramafic;these elements get mobilised during the metamorphism and serpentinisation.This is a possible reason for the mobilisation of zinc and incorporation in the chromite within altered ultramafic.As a result,chromiterich ultramafic xenolith subjected to metamorphic process gets enrichment of Zn and Fe due to elemental exchange.It converts common chromite into zincian chromite,as reported in altered ultramafics elsewhere.
基金supportedby the National Basic Research Program of China(2009CB825006)
文摘High-pressure(HP)metamorphic terrane in the Tongbai orogen comprises two HP slices(I and II)and a tectonic mélange zone in the northeast and a blueschist-greenschist zone in the southwest.HP slice I is represented by the northern and southern eclogite zones on the two sides of the Tongbaishan antiform.HP slice II is represented by retrograded eclogite-bearing metamorphic enclaves in Cretaceous gneissic granites in the Tongbai Complex.U-Pb,Lu-Hf,Rb-Sr and40Ar/39Ar multichronometric data indicate that the peak metamorphism of HP slice I took place at^255 Ma,whereas the metamorphic ages of HP slice II are as young as232–220 Ma.By contrast,the tectonic mélange zone near the suture was metamorphosed at^256 Ma.Such a diachroneity of different slices across the direction of the orogen in the Hong’an-Dabie-Sulu HP/UHP terrane is ubiquitous,and it can be interpreted by a syn-subduction detachment/exhumation model.Furthermore,the metamorphic age of HP slice I in the Tongbai orogen is older than that of the equivalent HP slice in the Hong’an orogen by^15 Ma,suggesting that the diachroneity may have also existed along the direction of the orogen.A seesaw-type subduction/exhumation model is proposed to explain this age disparity and the subduction of the South China Block becomimg shallower towards the west.