Greenstone basalts and komatiites provide a means to track both mantle composition and magma generation temperature with time. Four types of mantle are characterized from incompatible element distributions in basalts ...Greenstone basalts and komatiites provide a means to track both mantle composition and magma generation temperature with time. Four types of mantle are characterized from incompatible element distributions in basalts and komatiites： depleted, hydrated, enriched and mantle from which komatiites are derived. Our most important observation is the recognition for the first time of what we refer to as a Great Thermal Divergence within the mantle beginning near the end of the Archean, which we ascribe to thermal and convective evolution. Prior to 2.5 Ga, depleted and enriched mantle have indistinguishable thermal histories, whereas at 2.5-2.0 Ga a divergence in mantle magma generation temperature begins between these two types of mantle. Major and incompatible element distributions and calculated magma generation temperatures suggest that Archean enriched mantle did not come from mantle plumes, but was part of an undifferentiated or well-mixed mantle similar in composition to calculated primitive mantle. During this time, however, high-temperature mantle plumes from dominantly depleted sources gave rise to komatiites and associated basalts. Recycling of oceanic crust into the deep mantle after the Archean may have contributed to enrichment ofTi, A1, Ca and Na in basalts derived from enriched mantle sources. After 2.5 Ga, increases in Mg# in basalts from depleted mantle and decreases in Fe and Mn reflect some combination of growing depletion and cooling of depleted mantle with time. A delay in cooling of depleted mantle until after the Archean probably reflects a combination of greater radiogenic heat sources in the Archean mantle and the propagation of plate tectonics after 3 Ga.展开更多
Komatiites are presented as direct evidence for higher mantle temperatures during the Archean. In the North China Craton, komatiites with spinifex structure have been identified only at one locality, i.e. the Sujiagou...Komatiites are presented as direct evidence for higher mantle temperatures during the Archean. In the North China Craton, komatiites with spinifex structure have been identified only at one locality, i.e. the Sujiagou area, western Shandong. They were considered as formed during the early Neoarchean mainly based on their association with supracrustal rocks considered to be that age. This study carried out SHRIMP U-Pb zircon dating on metamorphosed trondhjemitic and monzogranitic dykes intruding the Sujiagou komatiites, and they have magmatic zircon ages of 2592 ± 12 Ma and 2586 ± 13 Ma respectively. This provides direct evidence that the komatiites formed during the early Neoarchean.展开更多
Spinifex-textured, magnesian （MgO 〉25 wt.%） komatiites from Mesoarchean Banasandra greenstone belt of the Sargur Group in the Dharwar craton, India were analysed for major and trace elements and 147,146Sm-143,142Nd...Spinifex-textured, magnesian （MgO 〉25 wt.%） komatiites from Mesoarchean Banasandra greenstone belt of the Sargur Group in the Dharwar craton, India were analysed for major and trace elements and 147,146Sm-143,142Nd systematics to constrain age, petrogenesis and to understand the evolution of Archean mantle. Major and trace element ratios such as CaOJAl203, Al2O3JTiO2, GdJYb, LaJNb and NbJY suggest aluminium undepleted to enriched compositional range for these komatiites. The depth of melting is estimated to be varying from 120 to 240 km and trace-element modelling indicates that the mantle source would have undergone multiple episodes of melting prior to the generation of magmas parental to these komatiites. Ten samples of these komatiites together with the published results of four samples from the same belt yield 147Sm-143Nd isochron age of ca. 3.14 Ga with an initial ENd（t） value of ＋3.5. High precision measurements of 142Nd/144Nd ratios were carried out for six komatiite samples along with standards AMES and La Jolla. All results are within uncertainties of the terrestrial samples. The absence of 142Nd/144Nd anomaly indicates that the source of these komatiites formed after the extinction of 146Sm, i.e. 4.3 Ga ago. In order to evolve to the high eNd（t） value of ＋3.5 by 3.14 Ga the time-integrated ratio of 147Sm/144Nd should be 0.2178 at the minimum. This is higher than the ratios estimated, so far, for mantle during that time. These results indicate at least two events of mantle differentiation starting with the chondritic composition of the mantle. The first event occurred very early at ~ 4.53 Ga to create a global early depleted reservoir with superchondritic Sm/Nd ratio, The source of Isua greenstone rocks with positive ~42Nd anomaly was depleted during a second differentiation within the life time of 146Sm, i.e. prior to 4.46 Ga. The source mantle of the Banasandra komatiite was a result of a differentiation event that occurred after the extinction of the 146Sm, i,e. at 4.3 Ga and prior to 3,14 Ga. Banasandra komatiites therefore provide evidence for preservation of heterogeneities generated during mantle differentiation at 4,3 Ga.展开更多
The petrogenetic age of Tunchang basaltic komatiites in Hainan Island has never been solved for a long time.Five samples of the basaltic komatiites are taken to be measured by Sm－ Nd radioactive isotope method in thi...The petrogenetic age of Tunchang basaltic komatiites in Hainan Island has never been solved for a long time.Five samples of the basaltic komatiites are taken to be measured by Sm－ Nd radioactive isotope method in this study.The Sm－ Nd whole rock isochron age of 1 687± 10 Ma with INd=0.510796± 0.000006 (2σ ) and ε Nd(T)=＋ 6.64 has been obtained.Meanwhile the basaltic komatiites give Nd model age of 1637～ 1 756 Ma which correspond basically to the isochron age of 1 687 Ma.These indicate that Tunchang basaltic komatiites were formed in the Palaeoproterozoic (about 1 687 Ma) and its primary magma would be derived from a depleted mantle.展开更多
At Shizuitang-Taohulun, south suburb of Yiyang City, Hunan Province, a set of metamorphic basic volcanic rocks, with more than 2000-m thickness and 20 km^2 in surficial area, occurs in lower part of Lengjiaxi Group of...At Shizuitang-Taohulun, south suburb of Yiyang City, Hunan Province, a set of metamorphic basic volcanic rocks, with more than 2000-m thickness and 20 km^2 in surficial area, occurs in lower part of Lengjiaxi Group of the Proterozoic era. In this set of volcanic rocks the lava flows came first and tuff, tuffite, volcanic展开更多
Ore forming processes involve the redistribution of heat, mass and momentum by a wide range of processes operating at different time and length scales. The fastest process at any given length scale tends to be the dom...Ore forming processes involve the redistribution of heat, mass and momentum by a wide range of processes operating at different time and length scales. The fastest process at any given length scale tends to be the dominant control. Applying this principle to the array of physical processes that operate within magma flow pathways leads to some key insights into the origins of magmatic Ni-Cu-PGE sulfide ore deposits. A high proportion of mineralised systems, including those in the super-giant Noril'sk-Talnakh camp, are formed in small conduit intrusions where assimilation of country rock has played a major role. Evidence of this process is reflected in the common association of sulfides with varitextured contaminated host rocks containing xenoliths in varying stages of assimilation. Direct incorporation of S-bearing country rock xenoliths is likely to be the dominant mechanism for generating sulfide liquids in this setting. However, the processes of melting or dissolving these xenoliths is relatively slow compared with magma flow rates and, depending on xenolith lithology and the composition of the carrier magma, slow compared with settling and accumulation rates. Chemical equilibration between sulfide droplets and silicate magma is slower still, as is the process of dissolving sulfide liquid into initially undersaturated silicate magmas. Much of the transport and deposition of sulfide in the carrier magmas may occur while sulfide is still incorporated in the xenoliths, accounting for the common association of magmatic sulfide-matrix ore breccias and contaminated "taxitic" host rocks. Effective upgrading of so-formed sulfide liquids would require repetitive recycling by processes such as reentrainment, back flow or gravity flow operating over the lifetime of the magma transport system as a whole. In contrast to mafic-hosted systems, komatiite-hosted ores only rarely show an association with externally-derived xenoliths, an observation which is partially due to the predominant formation of ores in lava flows rather than deep-seated intrusions, but also to the much shorter timescales of key component systems in hotter, less viscous magmas. Nonetheless, multiple cycles of deposition and entrainment are necessary to account for the metal contents of komatiite-hosted sulfides. More generally, the time and length scale approach introduced here may be of value in understanding other igneous processes as well as non-magmatic mineral systems.展开更多
The Campos Gerais Domain(CGD)in southeastern Brazil is an approximately 180 km×35 km area of Archean–Proterozoic rocks located southwest of the São Francisco Craton(SFC).The Archean–Paleopro terozoic evolu...The Campos Gerais Domain(CGD)in southeastern Brazil is an approximately 180 km×35 km area of Archean–Proterozoic rocks located southwest of the São Francisco Craton(SFC).The Archean–Paleopro terozoic evolution of the CGD—alongside its potential correlation with the SFC or other cratonic blocks in the region-is currently poorly-constrained.We present the results of systematic petrography,bulkrock geochemistry,mineral chemistry and geochronology for a suite of scarcely studied mafic–ultramafic rocks from the CGD.We also provide a compilation of previously reported bulk-rock geochemical and spinel group mineral chemical data for mafic–ultramafic rocks throughout the CGD,and geochronological information for various lithotypes in the region.The CGD records a protracted Mesoarchean to Statherian(3.1–1.7 Ga)crustal evolution,which we interpret to share a common history with the southern SFC and their related reworked segments,suggesting that it is a westward extension of this cratonic terrain.The metavolcano-sedimentary rocks of the Fortaleza de Minas and Alpinópolis segments represent a Mesoarchean greenstone belt that is stratigraphically and chemically comparable to Archean greenstone belts worldwide,and that is broadly coeval with a local suite of tonalite-trondhjemite-grano diorite(TTG)gneisses and migmatites.U-Pb SHRIMP zircon data from a subalkaline metagabbro yielded a concordia age of ca.2.96 Ga,revealing a previously unrecognized phase of Archean magmatism in the CGD that can be chrono-correlated with metakomatiite and TTG generation elsewhere in the São Francisco paleocontinent.Our data contradict a hypothesis whereby the metavolcano-sedimentary rocks of the Jacuí-Bom Jesus da Penha and Petúnia segments represent an ophiolite,as previously suggested,instead presenting features that point to formation in association with a continental arc.Coupled with a U-Pb(SHRIMP)crystallization age of ca.2.13 Ga recorded by zircon grains from a metaultramafic rock,these data highlight that a magmatic event was chrono-correlated with the main accretionary phase of the Minas Orogeny,and with the Pouso Alegre/Amparo and São Vicente complexes.Finally,a U-Pb(SHRIMP)concordia age of ca.590 Ma—obtained from metamorphic-textured zircon grains from a metaultramafic rock—points to a late metamorphic overprint related to upper amphibolite conditions,brittle fault activation and the juxtaposition of crustal blocks in association with the latest stages of western Gondwana’s assembly in the southern SFC,with later retrogression to greenschist-facies.展开更多
We present field, petrographic, major and trace element data for komatiites and komatiite basalts from Sargur Group Nagamangala greenstone belt, western Dharwar craton. Field evidences such as crude pillow structure i...We present field, petrographic, major and trace element data for komatiites and komatiite basalts from Sargur Group Nagamangala greenstone belt, western Dharwar craton. Field evidences such as crude pillow structure indicate their eruption in a marine environment whilst spinifex texture reveals their komatiite nature. Petrographic data suggest that the primary mineralogy has been completely altered during post-magmatic processes associated with metamorphism corresponding to greenschist to lower amphibolite facies conditions. The studied komatiites contain serpentine, talc, tremolite, actinolite and chlorite whilst tremolite, actinolite with minor plagioclase in komatiitic basalts. Based on the published Sm-Nd whole rock isochron ages of adjoining Banasandra komatiites （northern extension of Naga- mangala belt） and further northwest in Nuggihafli belt and Kalyadi belt we speculate ca. 3.2-3.15 Ga for komatiite eruption in Nagamangala belt. Trace element characteristics particularly HFSE and REE patterns suggest that most of the primary geochemical characteristics are preserved with minor influ- ence of post-magmatic alteration and[or contamination. About 1[3 of studied komatiites show AI- depletion whilst remaining komatiites and komatiite basalts are Al-undepleted. Several samples despite high MgO, （Gd]Yb）N ratios show low CaO/AI203 ratios. Such anomalous values could be related to removal of CaO from komatiites during fluid-driven hydrothermal alteration, thus lowering CaOJAI203 ratios. The elemental characteristics of Al-depleted komatiites such as higher （Gd/Yb）N （〉 1.0）, CaO/AI203 （〉1.0）, Al203frio2 （〈18） together with lower HREE, Y, Zr and Hf indicate their derivation from deeper upper mantle with minor garnet （majorite？） involvement in residue whereas lower （GdIYb）N （〈1.0）, CaO/AI203 （〈0.9）, higher A1203]TiO2 （〉18） together with higher HREE, Y, Zr suggest their derivation from shallower upper mantle without garnet involvement in residue. The observed chemical characteristics （CaO/AI203, AI203]TiO2, MgO, Ni, Cr, Nb, Zr, Y, Hf, and REE） indicate derivation of the komatiite and komatiite basalt magmas from heterogeneous mantle （depleted to primitive mantle） at different depths in hot spot environments possibly with a rising plume. The low content of incompatible elements in studied komatiites suggest existence of depleted mantle during ca. 3.2 Ga which in turn imply an earlier episode of mantle differentiation, greenstone volcanism and continental growth probably during ca. 3.6-3.3 Ga which is substantiated by Nd and Pb isotope data of gneisses and komatiites in western Dharwar craton （WDC）.展开更多
Boninitic rocks and associated high-magnesian basalt and high-iron tholeiite in the Xiangcheng area constitute the basal horizon of the arc volcanic sequence in the Triassic Yidun Island-Arc, southwestern China. The b...Boninitic rocks and associated high-magnesian basalt and high-iron tholeiite in the Xiangcheng area constitute the basal horizon of the arc volcanic sequence in the Triassic Yidun Island-Arc, southwestern China. The boninite occurs as pillow, massive and ocellar lavas; the last one possesses well-developed globular structure and alternates with the former two. The boninite is characterized by the absence of phenocrysts of olivine and low-Ca pyroxenes and by low CaO/Al2O3 ratios (<0.67) and high Cr (>1000 ppm) and Ni (>250 ppm). The normalized abundance patterns (NAP) of trace elements to primitive mantle are similar to the NAP of low-Ca modern boninites and SHMB in the Archaean and Proterozoic.展开更多
The Jayachamarajapura schist belt in western Dharwar craton, southern India, is predominantly an ultramafics dominant terrain. These rocks have been extensively metamorphosed and altered to serpentinite. The komatiite...The Jayachamarajapura schist belt in western Dharwar craton, southern India, is predominantly an ultramafics dominant terrain. These rocks have been extensively metamorphosed and altered to serpentinite. The komatiite nature of ultramafics is conspicuous. In most of the areas of the belt these ultramafics are massive in nature. However, some of the ultramafic units show layered nature. But, their outcrops are encompassed within the massive komatiitic bodies. These komatiitic ultramafics are predominantly Mg-rich in nature. The layered rocks are also Mg-rich, and their field setting and geochemistry suggest their intermittent occurrence as sills, during the differentiation of peridotitic magma. The layered rocks, which have been intensely serpentinisation show homogenous nature. They are almost wholly made of serpentine with occasional relics of pyroxene. Secondary carbonate mineral is often noticed. Their higher MgO content indicates Mg-rich ultramafic magmatism during Archaean orogeny.展开更多
基金funding from the European Research Council(ERC StG 279828)
文摘Greenstone basalts and komatiites provide a means to track both mantle composition and magma generation temperature with time. Four types of mantle are characterized from incompatible element distributions in basalts and komatiites： depleted, hydrated, enriched and mantle from which komatiites are derived. Our most important observation is the recognition for the first time of what we refer to as a Great Thermal Divergence within the mantle beginning near the end of the Archean, which we ascribe to thermal and convective evolution. Prior to 2.5 Ga, depleted and enriched mantle have indistinguishable thermal histories, whereas at 2.5-2.0 Ga a divergence in mantle magma generation temperature begins between these two types of mantle. Major and incompatible element distributions and calculated magma generation temperatures suggest that Archean enriched mantle did not come from mantle plumes, but was part of an undifferentiated or well-mixed mantle similar in composition to calculated primitive mantle. During this time, however, high-temperature mantle plumes from dominantly depleted sources gave rise to komatiites and associated basalts. Recycling of oceanic crust into the deep mantle after the Archean may have contributed to enrichment ofTi, A1, Ca and Na in basalts derived from enriched mantle sources. After 2.5 Ga, increases in Mg# in basalts from depleted mantle and decreases in Fe and Mn reflect some combination of growing depletion and cooling of depleted mantle with time. A delay in cooling of depleted mantle until after the Archean probably reflects a combination of greater radiogenic heat sources in the Archean mantle and the propagation of plate tectonics after 3 Ga.
基金financially supported by the Key Program of the Ministry of Land and Resources of China(DD20190370,DD20190009,DD20190358,DD20190003)。
文摘Komatiites are presented as direct evidence for higher mantle temperatures during the Archean. In the North China Craton, komatiites with spinifex structure have been identified only at one locality, i.e. the Sujiagou area, western Shandong. They were considered as formed during the early Neoarchean mainly based on their association with supracrustal rocks considered to be that age. This study carried out SHRIMP U-Pb zircon dating on metamorphosed trondhjemitic and monzogranitic dykes intruding the Sujiagou komatiites, and they have magmatic zircon ages of 2592 ± 12 Ma and 2586 ± 13 Ma respectively. This provides direct evidence that the komatiites formed during the early Neoarchean.
基金PLANEX(Planetary Exploration) program,Department of Space,India for funding this study(PLANEX Ref.No.5940)
文摘Spinifex-textured, magnesian （MgO 〉25 wt.%） komatiites from Mesoarchean Banasandra greenstone belt of the Sargur Group in the Dharwar craton, India were analysed for major and trace elements and 147,146Sm-143,142Nd systematics to constrain age, petrogenesis and to understand the evolution of Archean mantle. Major and trace element ratios such as CaOJAl203, Al2O3JTiO2, GdJYb, LaJNb and NbJY suggest aluminium undepleted to enriched compositional range for these komatiites. The depth of melting is estimated to be varying from 120 to 240 km and trace-element modelling indicates that the mantle source would have undergone multiple episodes of melting prior to the generation of magmas parental to these komatiites. Ten samples of these komatiites together with the published results of four samples from the same belt yield 147Sm-143Nd isochron age of ca. 3.14 Ga with an initial ENd（t） value of ＋3.5. High precision measurements of 142Nd/144Nd ratios were carried out for six komatiite samples along with standards AMES and La Jolla. All results are within uncertainties of the terrestrial samples. The absence of 142Nd/144Nd anomaly indicates that the source of these komatiites formed after the extinction of 146Sm, i.e. 4.3 Ga ago. In order to evolve to the high eNd（t） value of ＋3.5 by 3.14 Ga the time-integrated ratio of 147Sm/144Nd should be 0.2178 at the minimum. This is higher than the ratios estimated, so far, for mantle during that time. These results indicate at least two events of mantle differentiation starting with the chondritic composition of the mantle. The first event occurred very early at ~ 4.53 Ga to create a global early depleted reservoir with superchondritic Sm/Nd ratio, The source of Isua greenstone rocks with positive ~42Nd anomaly was depleted during a second differentiation within the life time of 146Sm, i.e. prior to 4.46 Ga. The source mantle of the Banasandra komatiite was a result of a differentiation event that occurred after the extinction of the 146Sm, i,e. at 4.3 Ga and prior to 3,14 Ga. Banasandra komatiites therefore provide evidence for preservation of heterogeneities generated during mantle differentiation at 4,3 Ga.
基金The paper is finicially supported by the key gold and resource environment project(KZ952- J1- 007) of " the Eighth Five Years
文摘The petrogenetic age of Tunchang basaltic komatiites in Hainan Island has never been solved for a long time.Five samples of the basaltic komatiites are taken to be measured by Sm－ Nd radioactive isotope method in this study.The Sm－ Nd whole rock isochron age of 1 687± 10 Ma with INd=0.510796± 0.000006 (2σ ) and ε Nd(T)=＋ 6.64 has been obtained.Meanwhile the basaltic komatiites give Nd model age of 1637～ 1 756 Ma which correspond basically to the isochron age of 1 687 Ma.These indicate that Tunchang basaltic komatiites were formed in the Palaeoproterozoic (about 1 687 Ma) and its primary magma would be derived from a depleted mantle.
文摘At Shizuitang-Taohulun, south suburb of Yiyang City, Hunan Province, a set of metamorphic basic volcanic rocks, with more than 2000-m thickness and 20 km^2 in surficial area, occurs in lower part of Lengjiaxi Group of the Proterozoic era. In this set of volcanic rocks the lava flows came first and tuff, tuffite, volcanic
基金The computational fluid dynamic simulations were supported by resources provided by the Pawsey Supercomputing Centre with funding from the Australian Government and the Government of Western AustraliaBoth authors were supported by CSIRO Research Plus (formerly Office of the Chief Executive) internal fellowship funds
文摘Ore forming processes involve the redistribution of heat, mass and momentum by a wide range of processes operating at different time and length scales. The fastest process at any given length scale tends to be the dominant control. Applying this principle to the array of physical processes that operate within magma flow pathways leads to some key insights into the origins of magmatic Ni-Cu-PGE sulfide ore deposits. A high proportion of mineralised systems, including those in the super-giant Noril'sk-Talnakh camp, are formed in small conduit intrusions where assimilation of country rock has played a major role. Evidence of this process is reflected in the common association of sulfides with varitextured contaminated host rocks containing xenoliths in varying stages of assimilation. Direct incorporation of S-bearing country rock xenoliths is likely to be the dominant mechanism for generating sulfide liquids in this setting. However, the processes of melting or dissolving these xenoliths is relatively slow compared with magma flow rates and, depending on xenolith lithology and the composition of the carrier magma, slow compared with settling and accumulation rates. Chemical equilibration between sulfide droplets and silicate magma is slower still, as is the process of dissolving sulfide liquid into initially undersaturated silicate magmas. Much of the transport and deposition of sulfide in the carrier magmas may occur while sulfide is still incorporated in the xenoliths, accounting for the common association of magmatic sulfide-matrix ore breccias and contaminated "taxitic" host rocks. Effective upgrading of so-formed sulfide liquids would require repetitive recycling by processes such as reentrainment, back flow or gravity flow operating over the lifetime of the magma transport system as a whole. In contrast to mafic-hosted systems, komatiite-hosted ores only rarely show an association with externally-derived xenoliths, an observation which is partially due to the predominant formation of ores in lava flows rather than deep-seated intrusions, but also to the much shorter timescales of key component systems in hotter, less viscous magmas. Nonetheless, multiple cycles of deposition and entrainment are necessary to account for the metal contents of komatiite-hosted sulfides. More generally, the time and length scale approach introduced here may be of value in understanding other igneous processes as well as non-magmatic mineral systems.
基金the Brazilian research institutions:National Council for Scientific and Technological Development(CNPqprocess n°206612/2014-1)+2 种基金the Fundação de AmparoàPesquisa de Minas Gerais(FAPEMIGprocess n°CRA 1.058/04)for the financial assistancethe Geological Survey of Brazil(SGB/CPRM)for the support。
文摘The Campos Gerais Domain(CGD)in southeastern Brazil is an approximately 180 km×35 km area of Archean–Proterozoic rocks located southwest of the São Francisco Craton(SFC).The Archean–Paleopro terozoic evolution of the CGD—alongside its potential correlation with the SFC or other cratonic blocks in the region-is currently poorly-constrained.We present the results of systematic petrography,bulkrock geochemistry,mineral chemistry and geochronology for a suite of scarcely studied mafic–ultramafic rocks from the CGD.We also provide a compilation of previously reported bulk-rock geochemical and spinel group mineral chemical data for mafic–ultramafic rocks throughout the CGD,and geochronological information for various lithotypes in the region.The CGD records a protracted Mesoarchean to Statherian(3.1–1.7 Ga)crustal evolution,which we interpret to share a common history with the southern SFC and their related reworked segments,suggesting that it is a westward extension of this cratonic terrain.The metavolcano-sedimentary rocks of the Fortaleza de Minas and Alpinópolis segments represent a Mesoarchean greenstone belt that is stratigraphically and chemically comparable to Archean greenstone belts worldwide,and that is broadly coeval with a local suite of tonalite-trondhjemite-grano diorite(TTG)gneisses and migmatites.U-Pb SHRIMP zircon data from a subalkaline metagabbro yielded a concordia age of ca.2.96 Ga,revealing a previously unrecognized phase of Archean magmatism in the CGD that can be chrono-correlated with metakomatiite and TTG generation elsewhere in the São Francisco paleocontinent.Our data contradict a hypothesis whereby the metavolcano-sedimentary rocks of the Jacuí-Bom Jesus da Penha and Petúnia segments represent an ophiolite,as previously suggested,instead presenting features that point to formation in association with a continental arc.Coupled with a U-Pb(SHRIMP)crystallization age of ca.2.13 Ga recorded by zircon grains from a metaultramafic rock,these data highlight that a magmatic event was chrono-correlated with the main accretionary phase of the Minas Orogeny,and with the Pouso Alegre/Amparo and São Vicente complexes.Finally,a U-Pb(SHRIMP)concordia age of ca.590 Ma—obtained from metamorphic-textured zircon grains from a metaultramafic rock—points to a late metamorphic overprint related to upper amphibolite conditions,brittle fault activation and the juxtaposition of crustal blocks in association with the latest stages of western Gondwana’s assembly in the southern SFC,with later retrogression to greenschist-facies.
基金funded by DST,Government of India in the form of Transect Project(ESS/16/334/2007/dated 14-10-2008) and DU R & D Programme
文摘We present field, petrographic, major and trace element data for komatiites and komatiite basalts from Sargur Group Nagamangala greenstone belt, western Dharwar craton. Field evidences such as crude pillow structure indicate their eruption in a marine environment whilst spinifex texture reveals their komatiite nature. Petrographic data suggest that the primary mineralogy has been completely altered during post-magmatic processes associated with metamorphism corresponding to greenschist to lower amphibolite facies conditions. The studied komatiites contain serpentine, talc, tremolite, actinolite and chlorite whilst tremolite, actinolite with minor plagioclase in komatiitic basalts. Based on the published Sm-Nd whole rock isochron ages of adjoining Banasandra komatiites （northern extension of Naga- mangala belt） and further northwest in Nuggihafli belt and Kalyadi belt we speculate ca. 3.2-3.15 Ga for komatiite eruption in Nagamangala belt. Trace element characteristics particularly HFSE and REE patterns suggest that most of the primary geochemical characteristics are preserved with minor influ- ence of post-magmatic alteration and[or contamination. About 1[3 of studied komatiites show AI- depletion whilst remaining komatiites and komatiite basalts are Al-undepleted. Several samples despite high MgO, （Gd]Yb）N ratios show low CaO/AI203 ratios. Such anomalous values could be related to removal of CaO from komatiites during fluid-driven hydrothermal alteration, thus lowering CaOJAI203 ratios. The elemental characteristics of Al-depleted komatiites such as higher （Gd/Yb）N （〉 1.0）, CaO/AI203 （〉1.0）, Al203frio2 （〈18） together with lower HREE, Y, Zr and Hf indicate their derivation from deeper upper mantle with minor garnet （majorite？） involvement in residue whereas lower （GdIYb）N （〈1.0）, CaO/AI203 （〈0.9）, higher A1203]TiO2 （〉18） together with higher HREE, Y, Zr suggest their derivation from shallower upper mantle without garnet involvement in residue. The observed chemical characteristics （CaO/AI203, AI203]TiO2, MgO, Ni, Cr, Nb, Zr, Y, Hf, and REE） indicate derivation of the komatiite and komatiite basalt magmas from heterogeneous mantle （depleted to primitive mantle） at different depths in hot spot environments possibly with a rising plume. The low content of incompatible elements in studied komatiites suggest existence of depleted mantle during ca. 3.2 Ga which in turn imply an earlier episode of mantle differentiation, greenstone volcanism and continental growth probably during ca. 3.6-3.3 Ga which is substantiated by Nd and Pb isotope data of gneisses and komatiites in western Dharwar craton （WDC）.
文摘Boninitic rocks and associated high-magnesian basalt and high-iron tholeiite in the Xiangcheng area constitute the basal horizon of the arc volcanic sequence in the Triassic Yidun Island-Arc, southwestern China. The boninite occurs as pillow, massive and ocellar lavas; the last one possesses well-developed globular structure and alternates with the former two. The boninite is characterized by the absence of phenocrysts of olivine and low-Ca pyroxenes and by low CaO/Al2O3 ratios (<0.67) and high Cr (>1000 ppm) and Ni (>250 ppm). The normalized abundance patterns (NAP) of trace elements to primitive mantle are similar to the NAP of low-Ca modern boninites and SHMB in the Archaean and Proterozoic.
文摘The Jayachamarajapura schist belt in western Dharwar craton, southern India, is predominantly an ultramafics dominant terrain. These rocks have been extensively metamorphosed and altered to serpentinite. The komatiite nature of ultramafics is conspicuous. In most of the areas of the belt these ultramafics are massive in nature. However, some of the ultramafic units show layered nature. But, their outcrops are encompassed within the massive komatiitic bodies. These komatiitic ultramafics are predominantly Mg-rich in nature. The layered rocks are also Mg-rich, and their field setting and geochemistry suggest their intermittent occurrence as sills, during the differentiation of peridotitic magma. The layered rocks, which have been intensely serpentinisation show homogenous nature. They are almost wholly made of serpentine with occasional relics of pyroxene. Secondary carbonate mineral is often noticed. Their higher MgO content indicates Mg-rich ultramafic magmatism during Archaean orogeny.
