In the Mangodara area within the Banfora greenstone belts (Baoulé-Mossi domain of the West African Craton), our study focused on geochemical assessment of the mobility of major and trace elements. Gold and base m...In the Mangodara area within the Banfora greenstone belts (Baoulé-Mossi domain of the West African Craton), our study focused on geochemical assessment of the mobility of major and trace elements. Gold and base metal occurrences are hosted in highly metamorphic felsic (metarhyolite) and intermediate (metadacite and metaandesite) formations. Common mineral assemblages made up of staurolite - kyanite - pyrophyllite are interpreted to represent the metamorphosed equivalent of aluminous hydrothermal alteration. Associated felsic and intermediate volcanic rocks are enriched in Fe<sub>2</sub>O<sub>3</sub>, K<sub>2</sub>O (metaandesite, metarhyolite) and depleted in MgO, Al<sub>2</sub>O<sub>3</sub>, CaO, P<sub>2</sub>O<sub>5</sub>, Na<sub>2</sub>O (metarhyolite) and Fe<sub>2</sub>O<sub>3</sub>, MgO, CaO (metaandesite). Al<sub>2</sub>O<sub>3</sub> depletion in mineralized kyanite-staurotide bearing metarhyolites suggests corroded minerals. Mineralized metarhyolites show enrichment in Au, Ag, Ba, Bi, Cr, Cu, Eu, La, Mo, Ni, Pb, S, Sc, V and depletion in As Sb Co, Sn, Zn while mineralized metaandesites show enrichment in Au, Ag, As, Mo, S, Sb and depletion in Co, Sn, Zn, Bi, Cr, Cu, Eu, Ni, Pb, Sc. Ba, La, V are immobile in metaandesites. Finally, Ag, As, Sn appear as geochemical vectors for gold exploration in the study area since gold mineralization is characterized by Au + Ba + Cu + Eu + La + Mo + Ni + S association in metarhyolites and Au + S + Sb + As + Ag + Bi in metaandesites.展开更多
Mount Fouimba and Mount Goma (Seguela) greenstone belts petro-structural studies combine remote sensing, geophysics, petrography and structural analysis. In view of establishing mapping details of paleoproterozoic geo...Mount Fouimba and Mount Goma (Seguela) greenstone belts petro-structural studies combine remote sensing, geophysics, petrography and structural analysis. In view of establishing mapping details of paleoproterozoic geological formations, geological setting rocks observed are essentially magmatic formations, such as two-mica granite, granodiorites, and porphyritic basalts;and a few metamorphics which are metatonalite, amphibolites and amphibo-lopyroxenites. Remote sensing, such as Landsat 8 OLI satellite imagery and geophysical data, has been combined to show regional NNE-SW shear zone. Tectonic structures and microstructures have enabled to identify two main deformation phases: D1 phase corresponding to compression, and D2 is a transpression phase. Mechanisms responsible for deformations are respectively flattening and transpression. Geological formations derived from mantle origin but contain crustal components, and their tectonic setting occurred during subduction.展开更多
Chinese Achaean greenstone belts are mainly distributed along the northern and southwestern margins of the North China platform. In terms of their geological characteristics, the greenstone belts in China are comparab...Chinese Achaean greenstone belts are mainly distributed along the northern and southwestern margins of the North China platform. In terms of their geological characteristics, the greenstone belts in China are comparable to those in other countries but at the same time have unique features of their own. In view of their geochemistry, the Chinese greenstone belts may be grouped into three types: the Jiapigou type, Qingyuan type and Xiaoqinling type. The greenstone belts were formed possibly in a rift-type palaeo-tectonic setting, similar to that of the modern island are-continental margin mobile belts.展开更多
A comparative study of the Precambrian Sonakhan(SGB)and Mahakoshal(MGB)greenstones belts of Central India has been undertaken to decipher their provenance,paleoweathering,paleogeography,and tectonics.As compared to th...A comparative study of the Precambrian Sonakhan(SGB)and Mahakoshal(MGB)greenstones belts of Central India has been undertaken to decipher their provenance,paleoweathering,paleogeography,and tectonics.As compared to the Upper Continental Crust(UCC),the MGB samples are enriched while the SGB samples are depleted in mafic elements indicating the presence of mafic rocks in the source of the MGB.This is complemented by the Ni–Cr diagram.The REE concentrations,LREE fractionated patterns and negative Eu anomalies of the MGB and SGB samples indicate derivation of sediments from a highly fractionated granitic source.Since MGB samples also contain the geochemical signature of mafic rocks,it is,therefore proposed that the MGB clastic load were derived from two sources(mafic+felsic)with arc character.This is attested by Cr and Zr relationships,and LILE enrichment,and HFSE depletion.These features suggest that the SGB developed as autochthonous while the MGB developed as an allochthonous belt.The chemical alteration indices such as chemical index of alteration(CIA),plagioclase index of alteration(PIA),and index of compositional variability for MGB samples indicate that they were dominantly derived as the first cycle(with minor recycled)sediments from bimodal sources(dominantly continental arcs)by intense chemical weathering as compared to the SGB samples,which were derived from felsic sources(dominantly cratonic rocks),and partly by recycling through a low chemical weathering.The CIA and PIA values of the samples reveal a change in the climatic conditions from Late Archean to Late Paleoproterozoic.Such change is interpreted in terms of migration of the Indian plate from high latitudes in the Late Archean to lower latitudes during the Late Paleoproterozoic.This is consistent with the paleomagnetic data that placed India in the configuration of 2.45 Ga Ur and 1.78 Ga Columbia supercontinents.展开更多
The Maevatanana greenstone belt in north-central Madagascar contains widespread exposures of tonalite-trondhjemite-granodiorite （TTG） gneisses, and is important for its concentrations of various metal deposits （e.g...The Maevatanana greenstone belt in north-central Madagascar contains widespread exposures of tonalite-trondhjemite-granodiorite （TTG） gneisses, and is important for its concentrations of various metal deposits （e.g., chromium, niekle, iron, gold）. In this paper we report on the petrography, and major and trace element compositions of the TTG gneisses within the Berere Complex of the Maevatanana area, as well as LA-ICP-MS U-Pb ages and Lu-Hf isotopic compositions of zircons from the gneisses. The gneisses consist mainly of granitoid gneiss and biotite （± hornblende） plagiogneiss, and analysis of thin sections provides evidence of crushing, recrystallization, and metasomatism related to dynamic metamorphism. Samples have large variations in their major and trace element contents, with SiO2 = 55.87-68.06 wt%, Al2O3 = 13.9-17.8 wt%, and Na2O/K2O= 0.97-2.13. Geochemically, the granitoid gneisses and biotite plagiogneisses fall on a low-Al trondhjemite to granodiorite trend, while the biotite-hornblende plagiogneisses represent a high-Al tonalite TTG assemblage. Zircon U-Pb dating shows that the Berere Complex TTG gneisses formed at 2.5-2.4 Ga. Most εHf（t） values of zircons from the biotite （q- hornblende） plagiogneisses are positive, while most εHf（t） values from the granitoid gneisses are negative, suggesting a degree of crustal contamination. Two-stage Hf model ages suggest that the age of the protolith of the TTG gneisses was ca. 3.4-2.6 Ga, representing a period of paleocontinent formation in the Mesoarchean. Geothermometries indicate the temperature of metamorphism of the TTG gneisses was 522-612℃. Based on these data, the protolith of the TTG gneisses is inferred to have formed during the development of a Mesoarchean paleocontinent that is now widely exposed as a TTG gneiss belt （mostly lower amphibolite facies） in the Maevatanana area, and which records a geological evolution related to the subduction of an ancient oceanic crust and the collision of microcontinents during the formation of the Rodinia supercontinent. The lithological similarity of Precambrian basement, the close ages of metamorphism within greenstone belts and the comparable distribution of metamorphic grade all show a pronounced Precambrian geology similarity between Madagascar and India, which can provide significative clues in understanding the possible Precambrian Supercontinent tectonics, and also important constraints on the correlation of the two continental fragments.展开更多
Greenstone belts contain several clues about the evolutionary history of primitive Earth.Here,we describe the volcano-sedimentary rock association exposed along the eastern margin of the Gavião Block,named the No...Greenstone belts contain several clues about the evolutionary history of primitive Earth.Here,we describe the volcano-sedimentary rock association exposed along the eastern margin of the Gavião Block,named the Northern Mundo Novo Greenstone Belt(N-MNGB),and present data collected with different techniques,including U–Pb–Hf–O isotopes of zircon and multiple sulfur isotopes(^(32)S,^(33)S,^(34)S,and ^(36)S)of pyrite from this supracrustal sequence.A pillowed metabasalt situated in the upper section of the N-MNGB is 3337±25 Ma old and has zircon with ε_(Hf)(t)=2.47 to1.40,Hf model ages between 3.75 Ga and 3.82 Ga,and δ^(18)O=+3.6‰to+7.3‰.These isotopic data,together with compiled whole-rock trace element data,suggest that the mafic metavolcanic rocks formed in a subduction-related setting,likely a back-arc basin juxtaposed to a continental arc.In this context,the magma interacted with older Eoarchean crustal components from the Gavião Block.Detrital zircons from the overlying quartzites of the Jacobina Group are sourced from Paleoarchean rocks,in accordance with previous studies,yielding a maximum depositional age of 3353±22 Ma.These detrital zircons have ε_(Hf)(t)=5.40 to0.84,Hf model ages between 3.66 Ga and 4.30 Ga,and δ^(18)O=+4.8‰to+6.4‰.The pyrite multiple sulfur isotope investigation of the 3.3 Ga supracrustal rocks from the N-MNGB enabled a further understanding of Paleoarchean sulfur cycling.The samples have diverse isotopic compositions that indicate sulfur sourced from distinct reservoirs.Significantly,they preserve the signal of the anoxic Archean atmosphere,expressed by MIF-S signatures(Δ^(33)S between1.3‰to+1.4‰)and a Δ^(36)S/Δ^(33)S slope of0.81 that is indistinguishable from the so-called Archean array.A BIF sample has a magmatic origin of sulfur,as indicated by the limited Δ^(34)S range(0 to+2‰),Δ^(33)S~0‰,and Δ^(36)S~0‰.A carbonaceous schist shows positive Δ^(34)S(2.1‰–3.5‰)and elevated Δ^(33)S(1.2‰–1.4‰)values,with corresponding negative Δ^(36)S between1.2‰to0.2‰,which resemble the isotopic composition of Archean black shales and suggest a source from the photolytic reduction of elemental sulfur.The pillowed metabasalt displays heterogeneous Δ^(34)S,Δ^(33)S,and Δ^(36)S signatures that reflect assimilation of both magmatic sulfur and photolytic sulfate during hydrothermal seafloor alteration.Lastly,pyrite in a massive sulfide lens is isotopically similar to barite of several Paleoarchean deposits worldwide,which might indicate mass dependent sulfur processing from a global and well-mixed sulfate reservoir at this time.展开更多
Studies of accreted oceanic plateau sections provide crucial information on their structures,compositions,and origins.We investigate the petrogenesis of ultramafic–mafic rocks in the Tangjia–Sumdo greenstone belt of...Studies of accreted oceanic plateau sections provide crucial information on their structures,compositions,and origins.We investigate the petrogenesis of ultramafic–mafic rocks in the Tangjia–Sumdo greenstone belt of southeast Tibet using petrography,whole-rock geochemistry,and U-Pb zircon geochronology.These rocks are divided into four groups based on geochemical characteristics that include depleted and tholeiitic mafic rocks,transitional mafic rocks,enriched and alkaline mafic rocks,and picritic ultramafic rocks.Depleted and tholeiitic mafic rocks have the oldest crystallization ages(-272 Ma),followed by picritic ultramafic rocks(-270 Ma),transitional mafic rocks(267–254 Ma),and enriched and alkaline mafic rocks(252–250 Ma).Hafnium and neodymium isotope ratios of depleted and tholeiitic mafic rocks(ε_(Hf)(t)=+13.1–+16.9;ε_(Nd)(t)=+6.9–+7.1),transitional mafic rocks(ε_(Hf)(t)=+1.8–+16.9;ε_(Nd)(t)=+0.8–+5.5),enriched and alkaline mafic rocks(ε_(Hf)(t)=+0.5–+5.4;ε_(Nd)(t)=1.5 to+1.9)and picritic ultramafic rocks(ε_(Hf)(t)=+14.9–+17.2;ε_(Nd)(t)=+7.8–+9.0)are similar to those of N-MORB,E-MORB,OIB and depleted-type picritic mafic rocks in other oceanic plateaus,respectively.The geochemical characteristics of the depleted and tholeiitic mafic rocks suggest that they formed by partial melting of depleted spinel lherzolite in a mid-ocean ridge setting,whereas the picritic ultramafic rocks suggest a high degree of partial melting of depleted lherzolite in a hot mantle plume head.The transitional mafic rocks formed by partial melting of moderately enriched garnet lherzolite.The youngest rocks(enriched and alkaline mafic rocks)formed by partial melting of a more enriched garnet lherzolite(compared to transitional mafic rocks)at relatively low temperatures.We propose that the depleted and tholeiitic mafic rocks represent normal oceanic crust of the Sumdo Paleo-Tethys Ocean and the transitional mafic rocks,enriched and alkaline mafic rocks and picritic ultramafic rocks are the fragments of the oceanic plateau,which were related to middle–late Permian mantle plume activity in the Sumdo Paleo-Tethys Ocean.We further suggest that the majority of the Tangjia–Sumdo greenstone belt represents a middle–late Permian oceanic plateau that reflects a previously unrecognized middle–late Permian mantle plume.展开更多
The Banfora’s birimian greenstones belt is located in the western part of Burkina Faso (west Africa). Recent petrographic and lithogeochemical studies have highlighted plutons intruding the metasedimentary and metavo...The Banfora’s birimian greenstones belt is located in the western part of Burkina Faso (west Africa). Recent petrographic and lithogeochemical studies have highlighted plutons intruding the metasedimentary and metavolcanic series. These plutonic rocks are composed of leucogranites belonging to the so-called Ferkessedougou’s or Ferké’s batholith, granites, granodiorites, monzodiorites and quartz monzonites. From the lithogeochemical studies, these plutonic rocks have a calc-alkaline and peraluminous character. The rare earth elements spectra of the Ferké’s leucogranites let distinguished two sub-facies. One of the sub-facies is composed of quartz monzonite to granite, while the other is granitic sensu stricto. However, all these plutonic rocks were emplaced in a geodynamic context of subduction followed by collision.展开更多
Plate tectonic activity has played a critical role in the development of petrotectonic associations in the Kadiri schist belt. The calc alkaline association of basalt, andesite, dacite and rhyolite （BADR） is the sig...Plate tectonic activity has played a critical role in the development of petrotectonic associations in the Kadiri schist belt. The calc alkaline association of basalt, andesite, dacite and rhyolite （BADR） is the signature volcanic rock suite of the convergent margin. The N-S belt has gone below the unconformity plane of Cuddapah sediments. In the northern part geochemical and structural attributes of the Kadiri greenstone belt is studied along with microscopic observations of selected samples. Harker diagram plots of major elements generally indicate a liquid line of descent from a common source, such that BADR rocks are derived from a common parent magma of basaltic to andesitic composition. These calc-alkaline volcanic rocks are formed at convergent margins where more silicic rocks represent more highly fractionated melt. All the litho-units of this greenstone belt indicate crush and strain effects. The stretched pebbles in the deformed volcanic matrix with tectonite development along with associated greenschist facies metamorphism, alteration and hydration is remarkable. Flow foliation plane with N-S strike and very low angle （5~ to 10~） easterly dip and N-S axial planar schistosity formed due to later phase isoclinal folding can be clearly identified in the field. Basic intrusives are quite common in the surrounding area. All the observations including the field setting and geochemistry clearly demonstrate ocean-continent subduction as the tectonic environment of the study area.展开更多
Archean greenstone belts and their possible inclusion of fragments of ophiolites is an im-portant research subject, since it is correlated with the nature of early oceanic crust, and can yield in-formation on the natu...Archean greenstone belts and their possible inclusion of fragments of ophiolites is an im-portant research subject, since it is correlated with the nature of early oceanic crust, and can yield in-formation on the nature of early planetary lithospheres, the origin of TTG （tonalite-trondhjemite-granodiorite） continental crust, the formation of early cratons and continents, and is related to when plate tectonics started in the Earth＇s evolutionary history. This article briefly reviews the North China craton＇s Archean ophiolite argument and proposes further studies aimed at understanding the genera- tion of greenstone belts and Archean ophiolites, and suggests some key scientific questions that remain to be answered.展开更多
Much has been learned in the past 40 years about the great diversity of the internal structure and geochemical compositions of Phanerozoic ophiolites, indicating that these on-land fragments of ancient oceanic lithosp...Much has been learned in the past 40 years about the great diversity of the internal structure and geochemical compositions of Phanerozoic ophiolites, indicating that these on-land fragments of ancient oceanic lithosphere formed in distinctly different tectonic settings during their igneous evolution. Recent studies in Archean and Proterozoic greenstone belts have shown that the Precambrian rock record may also include exposures of a diverse suite of ophiolite complexes as part of craton development in the early history of the Earth. We review the salient features of the Precambrian ophiolite record to highlight what has been learned about Precambrian oceanic spreading systems since the original Penrose definition of ophiolites in 1972. Some of the diagnostic, characteristic, typical, and rare aspects of ophiolites of all ages are presented in a table in order to help determine if tectonically deformed and metamophosed sequences in Precambrian shield areas may be considered as ophiolites. The results of this comparative study are important in that they enable researchers to more realistically characterize allochthonous mafic/ultramafic rock sequences as ophiolitic or non-ophiolitic. This approach is more deterministic in contrast to some other arbitrary classification schemes requiring three or four of the Penrose-style ophiolitic units to be present in the Precambrian record for a specific rock sequence to be considered ophiolitic. Once these tectonic fragments are recognized as remnants of ancient oceanic lithosphere, great progress shall be made in understanding early Earth history. We discuss the significance and implications of the Precambrian ophiolite record to constrain the mode and nature of the plate tectonics that operated in deep time.展开更多
Archean greenstone belts are supracrustal sequences, the lower part of which is usually composed of voluminous ultramafic-mafic volcanics. Intermediate and acid volcanic rocks increase in abundance towards the upper d...Archean greenstone belts are supracrustal sequences, the lower part of which is usually composed of voluminous ultramafic-mafic volcanics. Intermediate and acid volcanic rocks increase in abundance towards the upper domains. Greenstone belts constitute ~30% of the total volume of Archean cratons, and preserve significant information on the surface environment and magmatism in the early earth, which are useful in unraveling the nature of crustal formation and evolution. The western Shandong Province(WSP) is located at the eastern part of the North China Craton(NCC), where greenstone sequences formed at ~2.7 and ~2.5 Ga were well preserved. The early Neoarchean supracrustal rocks include komatiite-basalt sequence, some meta-sediments of the lower part of the Taishan Group and the Mengjiatun Formation. The volcanism had been correlated to mantle plume, which resulted in vertical crustal accretion. The late Neoarchean supracrustal rocks were composed of metamorphosed felsic volcano-sedimentary sequences and BIFs of the upper part of the Taishan Group and the Jining Group. The geochemical features of the meta-volcanics show calc-alkaline affinities, similar to modern arc-related magmatism, suggesting that the continental crust in the western Shandong Province witnessed horizontal plate movements at ~2.5 Ga. The metasediments and leucosomes in the Qixingtai area display regional upper amphibolite facies metamorphism and anatexis at 2.53–2.50 Ga, coeval with formation of large volumes of crustally-derived granites. These tectono-thermal events suggest that a unified continental crust was formed in the western Shandong Province at the end of Neoarchean.展开更多
文摘In the Mangodara area within the Banfora greenstone belts (Baoulé-Mossi domain of the West African Craton), our study focused on geochemical assessment of the mobility of major and trace elements. Gold and base metal occurrences are hosted in highly metamorphic felsic (metarhyolite) and intermediate (metadacite and metaandesite) formations. Common mineral assemblages made up of staurolite - kyanite - pyrophyllite are interpreted to represent the metamorphosed equivalent of aluminous hydrothermal alteration. Associated felsic and intermediate volcanic rocks are enriched in Fe<sub>2</sub>O<sub>3</sub>, K<sub>2</sub>O (metaandesite, metarhyolite) and depleted in MgO, Al<sub>2</sub>O<sub>3</sub>, CaO, P<sub>2</sub>O<sub>5</sub>, Na<sub>2</sub>O (metarhyolite) and Fe<sub>2</sub>O<sub>3</sub>, MgO, CaO (metaandesite). Al<sub>2</sub>O<sub>3</sub> depletion in mineralized kyanite-staurotide bearing metarhyolites suggests corroded minerals. Mineralized metarhyolites show enrichment in Au, Ag, Ba, Bi, Cr, Cu, Eu, La, Mo, Ni, Pb, S, Sc, V and depletion in As Sb Co, Sn, Zn while mineralized metaandesites show enrichment in Au, Ag, As, Mo, S, Sb and depletion in Co, Sn, Zn, Bi, Cr, Cu, Eu, Ni, Pb, Sc. Ba, La, V are immobile in metaandesites. Finally, Ag, As, Sn appear as geochemical vectors for gold exploration in the study area since gold mineralization is characterized by Au + Ba + Cu + Eu + La + Mo + Ni + S association in metarhyolites and Au + S + Sb + As + Ag + Bi in metaandesites.
文摘Mount Fouimba and Mount Goma (Seguela) greenstone belts petro-structural studies combine remote sensing, geophysics, petrography and structural analysis. In view of establishing mapping details of paleoproterozoic geological formations, geological setting rocks observed are essentially magmatic formations, such as two-mica granite, granodiorites, and porphyritic basalts;and a few metamorphics which are metatonalite, amphibolites and amphibo-lopyroxenites. Remote sensing, such as Landsat 8 OLI satellite imagery and geophysical data, has been combined to show regional NNE-SW shear zone. Tectonic structures and microstructures have enabled to identify two main deformation phases: D1 phase corresponding to compression, and D2 is a transpression phase. Mechanisms responsible for deformations are respectively flattening and transpression. Geological formations derived from mantle origin but contain crustal components, and their tectonic setting occurred during subduction.
文摘Chinese Achaean greenstone belts are mainly distributed along the northern and southwestern margins of the North China platform. In terms of their geological characteristics, the greenstone belts in China are comparable to those in other countries but at the same time have unique features of their own. In view of their geochemistry, the Chinese greenstone belts may be grouped into three types: the Jiapigou type, Qingyuan type and Xiaoqinling type. The greenstone belts were formed possibly in a rift-type palaeo-tectonic setting, similar to that of the modern island are-continental margin mobile belts.
基金the financial support from UGC,Govt.of India in the form of a research project F.No.40-304/2011(SR)the Department of Science and Technology,Govt.of India for financial support in the form of a Research Project(SR/S4/ES-180/2005)。
文摘A comparative study of the Precambrian Sonakhan(SGB)and Mahakoshal(MGB)greenstones belts of Central India has been undertaken to decipher their provenance,paleoweathering,paleogeography,and tectonics.As compared to the Upper Continental Crust(UCC),the MGB samples are enriched while the SGB samples are depleted in mafic elements indicating the presence of mafic rocks in the source of the MGB.This is complemented by the Ni–Cr diagram.The REE concentrations,LREE fractionated patterns and negative Eu anomalies of the MGB and SGB samples indicate derivation of sediments from a highly fractionated granitic source.Since MGB samples also contain the geochemical signature of mafic rocks,it is,therefore proposed that the MGB clastic load were derived from two sources(mafic+felsic)with arc character.This is attested by Cr and Zr relationships,and LILE enrichment,and HFSE depletion.These features suggest that the SGB developed as autochthonous while the MGB developed as an allochthonous belt.The chemical alteration indices such as chemical index of alteration(CIA),plagioclase index of alteration(PIA),and index of compositional variability for MGB samples indicate that they were dominantly derived as the first cycle(with minor recycled)sediments from bimodal sources(dominantly continental arcs)by intense chemical weathering as compared to the SGB samples,which were derived from felsic sources(dominantly cratonic rocks),and partly by recycling through a low chemical weathering.The CIA and PIA values of the samples reveal a change in the climatic conditions from Late Archean to Late Paleoproterozoic.Such change is interpreted in terms of migration of the Indian plate from high latitudes in the Late Archean to lower latitudes during the Late Paleoproterozoic.This is consistent with the paleomagnetic data that placed India in the configuration of 2.45 Ga Ur and 1.78 Ga Columbia supercontinents.
基金funded by Geological Survey Project grants from the China Geological Survey(grant numbers DD20160056, 121201103000150002)
文摘The Maevatanana greenstone belt in north-central Madagascar contains widespread exposures of tonalite-trondhjemite-granodiorite （TTG） gneisses, and is important for its concentrations of various metal deposits （e.g., chromium, niekle, iron, gold）. In this paper we report on the petrography, and major and trace element compositions of the TTG gneisses within the Berere Complex of the Maevatanana area, as well as LA-ICP-MS U-Pb ages and Lu-Hf isotopic compositions of zircons from the gneisses. The gneisses consist mainly of granitoid gneiss and biotite （± hornblende） plagiogneiss, and analysis of thin sections provides evidence of crushing, recrystallization, and metasomatism related to dynamic metamorphism. Samples have large variations in their major and trace element contents, with SiO2 = 55.87-68.06 wt%, Al2O3 = 13.9-17.8 wt%, and Na2O/K2O= 0.97-2.13. Geochemically, the granitoid gneisses and biotite plagiogneisses fall on a low-Al trondhjemite to granodiorite trend, while the biotite-hornblende plagiogneisses represent a high-Al tonalite TTG assemblage. Zircon U-Pb dating shows that the Berere Complex TTG gneisses formed at 2.5-2.4 Ga. Most εHf（t） values of zircons from the biotite （q- hornblende） plagiogneisses are positive, while most εHf（t） values from the granitoid gneisses are negative, suggesting a degree of crustal contamination. Two-stage Hf model ages suggest that the age of the protolith of the TTG gneisses was ca. 3.4-2.6 Ga, representing a period of paleocontinent formation in the Mesoarchean. Geothermometries indicate the temperature of metamorphism of the TTG gneisses was 522-612℃. Based on these data, the protolith of the TTG gneisses is inferred to have formed during the development of a Mesoarchean paleocontinent that is now widely exposed as a TTG gneiss belt （mostly lower amphibolite facies） in the Maevatanana area, and which records a geological evolution related to the subduction of an ancient oceanic crust and the collision of microcontinents during the formation of the Rodinia supercontinent. The lithological similarity of Precambrian basement, the close ages of metamorphism within greenstone belts and the comparable distribution of metamorphic grade all show a pronounced Precambrian geology similarity between Madagascar and India, which can provide significative clues in understanding the possible Precambrian Supercontinent tectonics, and also important constraints on the correlation of the two continental fragments.
基金the Brazilian National Council for Scientific and Technological Development(CNPq)for financial support(grants 163459/2013-4 and 202267/2014-8 for G.S Teles and 305053/2014-0 for F.Chemale Jr.)the Australian Research Council(ARC)(grant DP140103393 for T.R.Ireland)。
文摘Greenstone belts contain several clues about the evolutionary history of primitive Earth.Here,we describe the volcano-sedimentary rock association exposed along the eastern margin of the Gavião Block,named the Northern Mundo Novo Greenstone Belt(N-MNGB),and present data collected with different techniques,including U–Pb–Hf–O isotopes of zircon and multiple sulfur isotopes(^(32)S,^(33)S,^(34)S,and ^(36)S)of pyrite from this supracrustal sequence.A pillowed metabasalt situated in the upper section of the N-MNGB is 3337±25 Ma old and has zircon with ε_(Hf)(t)=2.47 to1.40,Hf model ages between 3.75 Ga and 3.82 Ga,and δ^(18)O=+3.6‰to+7.3‰.These isotopic data,together with compiled whole-rock trace element data,suggest that the mafic metavolcanic rocks formed in a subduction-related setting,likely a back-arc basin juxtaposed to a continental arc.In this context,the magma interacted with older Eoarchean crustal components from the Gavião Block.Detrital zircons from the overlying quartzites of the Jacobina Group are sourced from Paleoarchean rocks,in accordance with previous studies,yielding a maximum depositional age of 3353±22 Ma.These detrital zircons have ε_(Hf)(t)=5.40 to0.84,Hf model ages between 3.66 Ga and 4.30 Ga,and δ^(18)O=+4.8‰to+6.4‰.The pyrite multiple sulfur isotope investigation of the 3.3 Ga supracrustal rocks from the N-MNGB enabled a further understanding of Paleoarchean sulfur cycling.The samples have diverse isotopic compositions that indicate sulfur sourced from distinct reservoirs.Significantly,they preserve the signal of the anoxic Archean atmosphere,expressed by MIF-S signatures(Δ^(33)S between1.3‰to+1.4‰)and a Δ^(36)S/Δ^(33)S slope of0.81 that is indistinguishable from the so-called Archean array.A BIF sample has a magmatic origin of sulfur,as indicated by the limited Δ^(34)S range(0 to+2‰),Δ^(33)S~0‰,and Δ^(36)S~0‰.A carbonaceous schist shows positive Δ^(34)S(2.1‰–3.5‰)and elevated Δ^(33)S(1.2‰–1.4‰)values,with corresponding negative Δ^(36)S between1.2‰to0.2‰,which resemble the isotopic composition of Archean black shales and suggest a source from the photolytic reduction of elemental sulfur.The pillowed metabasalt displays heterogeneous Δ^(34)S,Δ^(33)S,and Δ^(36)S signatures that reflect assimilation of both magmatic sulfur and photolytic sulfate during hydrothermal seafloor alteration.Lastly,pyrite in a massive sulfide lens is isotopically similar to barite of several Paleoarchean deposits worldwide,which might indicate mass dependent sulfur processing from a global and well-mixed sulfate reservoir at this time.
基金funded by the National Natural Science Foundation of China(Grant number 42172226)the Indepen-dent research fund of Key Laboratory of Mineral Resources Evaluation in Northeast Asia,Department of Natural Resources(DBYZZ-18-06).
文摘Studies of accreted oceanic plateau sections provide crucial information on their structures,compositions,and origins.We investigate the petrogenesis of ultramafic–mafic rocks in the Tangjia–Sumdo greenstone belt of southeast Tibet using petrography,whole-rock geochemistry,and U-Pb zircon geochronology.These rocks are divided into four groups based on geochemical characteristics that include depleted and tholeiitic mafic rocks,transitional mafic rocks,enriched and alkaline mafic rocks,and picritic ultramafic rocks.Depleted and tholeiitic mafic rocks have the oldest crystallization ages(-272 Ma),followed by picritic ultramafic rocks(-270 Ma),transitional mafic rocks(267–254 Ma),and enriched and alkaline mafic rocks(252–250 Ma).Hafnium and neodymium isotope ratios of depleted and tholeiitic mafic rocks(ε_(Hf)(t)=+13.1–+16.9;ε_(Nd)(t)=+6.9–+7.1),transitional mafic rocks(ε_(Hf)(t)=+1.8–+16.9;ε_(Nd)(t)=+0.8–+5.5),enriched and alkaline mafic rocks(ε_(Hf)(t)=+0.5–+5.4;ε_(Nd)(t)=1.5 to+1.9)and picritic ultramafic rocks(ε_(Hf)(t)=+14.9–+17.2;ε_(Nd)(t)=+7.8–+9.0)are similar to those of N-MORB,E-MORB,OIB and depleted-type picritic mafic rocks in other oceanic plateaus,respectively.The geochemical characteristics of the depleted and tholeiitic mafic rocks suggest that they formed by partial melting of depleted spinel lherzolite in a mid-ocean ridge setting,whereas the picritic ultramafic rocks suggest a high degree of partial melting of depleted lherzolite in a hot mantle plume head.The transitional mafic rocks formed by partial melting of moderately enriched garnet lherzolite.The youngest rocks(enriched and alkaline mafic rocks)formed by partial melting of a more enriched garnet lherzolite(compared to transitional mafic rocks)at relatively low temperatures.We propose that the depleted and tholeiitic mafic rocks represent normal oceanic crust of the Sumdo Paleo-Tethys Ocean and the transitional mafic rocks,enriched and alkaline mafic rocks and picritic ultramafic rocks are the fragments of the oceanic plateau,which were related to middle–late Permian mantle plume activity in the Sumdo Paleo-Tethys Ocean.We further suggest that the majority of the Tangjia–Sumdo greenstone belt represents a middle–late Permian oceanic plateau that reflects a previously unrecognized middle–late Permian mantle plume.
文摘The Banfora’s birimian greenstones belt is located in the western part of Burkina Faso (west Africa). Recent petrographic and lithogeochemical studies have highlighted plutons intruding the metasedimentary and metavolcanic series. These plutonic rocks are composed of leucogranites belonging to the so-called Ferkessedougou’s or Ferké’s batholith, granites, granodiorites, monzodiorites and quartz monzonites. From the lithogeochemical studies, these plutonic rocks have a calc-alkaline and peraluminous character. The rare earth elements spectra of the Ferké’s leucogranites let distinguished two sub-facies. One of the sub-facies is composed of quartz monzonite to granite, while the other is granitic sensu stricto. However, all these plutonic rocks were emplaced in a geodynamic context of subduction followed by collision.
文摘Plate tectonic activity has played a critical role in the development of petrotectonic associations in the Kadiri schist belt. The calc alkaline association of basalt, andesite, dacite and rhyolite （BADR） is the signature volcanic rock suite of the convergent margin. The N-S belt has gone below the unconformity plane of Cuddapah sediments. In the northern part geochemical and structural attributes of the Kadiri greenstone belt is studied along with microscopic observations of selected samples. Harker diagram plots of major elements generally indicate a liquid line of descent from a common source, such that BADR rocks are derived from a common parent magma of basaltic to andesitic composition. These calc-alkaline volcanic rocks are formed at convergent margins where more silicic rocks represent more highly fractionated melt. All the litho-units of this greenstone belt indicate crush and strain effects. The stretched pebbles in the deformed volcanic matrix with tectonite development along with associated greenschist facies metamorphism, alteration and hydration is remarkable. Flow foliation plane with N-S strike and very low angle （5~ to 10~） easterly dip and N-S axial planar schistosity formed due to later phase isoclinal folding can be clearly identified in the field. Basic intrusives are quite common in the surrounding area. All the observations including the field setting and geochemistry clearly demonstrate ocean-continent subduction as the tectonic environment of the study area.
基金supported by the National Natural Science Foundation of China (Nos. 91014002,40821061)Ministry of Education of China (No. B07039)
文摘Archean greenstone belts and their possible inclusion of fragments of ophiolites is an im-portant research subject, since it is correlated with the nature of early oceanic crust, and can yield in-formation on the nature of early planetary lithospheres, the origin of TTG （tonalite-trondhjemite-granodiorite） continental crust, the formation of early cratons and continents, and is related to when plate tectonics started in the Earth＇s evolutionary history. This article briefly reviews the North China craton＇s Archean ophiolite argument and proposes further studies aimed at understanding the genera- tion of greenstone belts and Archean ophiolites, and suggests some key scientific questions that remain to be answered.
基金supported by U.S. National Science Foundation Grants (Grant Nos. 02-07886, and 01-25925)National Natural Science Foundation of China (Grant Nos. 91014002, 40821061)Ministry of Education of China (Grant No. B07039)
文摘Much has been learned in the past 40 years about the great diversity of the internal structure and geochemical compositions of Phanerozoic ophiolites, indicating that these on-land fragments of ancient oceanic lithosphere formed in distinctly different tectonic settings during their igneous evolution. Recent studies in Archean and Proterozoic greenstone belts have shown that the Precambrian rock record may also include exposures of a diverse suite of ophiolite complexes as part of craton development in the early history of the Earth. We review the salient features of the Precambrian ophiolite record to highlight what has been learned about Precambrian oceanic spreading systems since the original Penrose definition of ophiolites in 1972. Some of the diagnostic, characteristic, typical, and rare aspects of ophiolites of all ages are presented in a table in order to help determine if tectonically deformed and metamophosed sequences in Precambrian shield areas may be considered as ophiolites. The results of this comparative study are important in that they enable researchers to more realistically characterize allochthonous mafic/ultramafic rock sequences as ophiolitic or non-ophiolitic. This approach is more deterministic in contrast to some other arbitrary classification schemes requiring three or four of the Penrose-style ophiolitic units to be present in the Precambrian record for a specific rock sequence to be considered ophiolitic. Once these tectonic fragments are recognized as remnants of ancient oceanic lithosphere, great progress shall be made in understanding early Earth history. We discuss the significance and implications of the Precambrian ophiolite record to constrain the mode and nature of the plate tectonics that operated in deep time.
基金supported by the National Natural Science Foundation (Grant Nos. 41430210 & 41302156)the National Basic Research Program of China (Grant No. 2012CB416606)the funding of the Institute of Geology, CAGS (Grant No. J1508)
文摘Archean greenstone belts are supracrustal sequences, the lower part of which is usually composed of voluminous ultramafic-mafic volcanics. Intermediate and acid volcanic rocks increase in abundance towards the upper domains. Greenstone belts constitute ~30% of the total volume of Archean cratons, and preserve significant information on the surface environment and magmatism in the early earth, which are useful in unraveling the nature of crustal formation and evolution. The western Shandong Province(WSP) is located at the eastern part of the North China Craton(NCC), where greenstone sequences formed at ~2.7 and ~2.5 Ga were well preserved. The early Neoarchean supracrustal rocks include komatiite-basalt sequence, some meta-sediments of the lower part of the Taishan Group and the Mengjiatun Formation. The volcanism had been correlated to mantle plume, which resulted in vertical crustal accretion. The late Neoarchean supracrustal rocks were composed of metamorphosed felsic volcano-sedimentary sequences and BIFs of the upper part of the Taishan Group and the Jining Group. The geochemical features of the meta-volcanics show calc-alkaline affinities, similar to modern arc-related magmatism, suggesting that the continental crust in the western Shandong Province witnessed horizontal plate movements at ~2.5 Ga. The metasediments and leucosomes in the Qixingtai area display regional upper amphibolite facies metamorphism and anatexis at 2.53–2.50 Ga, coeval with formation of large volumes of crustally-derived granites. These tectono-thermal events suggest that a unified continental crust was formed in the western Shandong Province at the end of Neoarchean.
