Tourmaline occurs as a minor but important mineral in the alteration zc,ne of the Archean orogenic gold deposit of Guddadarangavanahalli （G.R.Halli） in the Chitradurga greenst^ne belt of the western Dharwar craton, ...Tourmaline occurs as a minor but important mineral in the alteration zc,ne of the Archean orogenic gold deposit of Guddadarangavanahalli （G.R.Halli） in the Chitradurga greenst^ne belt of the western Dharwar craton, southern India. It occurs in the distal alteration halo of the G.R.Halli golcl deposit as （a） clusters of very fine grained aggregates which form a minor constituent in the natrix of the altered metabasalt （AMB tourmaline） and （b） in quartz-carbonate veins （vein tourmaline）. ~[＇he vein tourmaline, based upon the association of specific carbonate minerals, is further grouped as （i） albite-tourmaline-ankerite-quartz veins （vein-1 tourmaline） and （ii） albite-tourmaline-calcite-quartz veins （vein-2 tourmaline）. Both the AMB tourmaline and the vein tourmalines （vein-I and vein-2） belong to the alkali group and are clas- sified under schorl-dravite series. Tourmalines occurring in the veins are zoned while the AMB tour- malines are unzoned. Mineral chemistry and discrimination diagrams 1eveal that cores and rims of the vein tourmalines are distinctly different. Core composition of the ve：n tourmalines is similar to the composition of the AMB tourmaline. The formation of the AMB tourmaline and cores of the vein tour- malines are proposed to be related to the regional D1 deformational event associated with the emplacement of the adjoining ca. 2.61 Ga Chitradurga granite whilst rims of the vein tourmalines vis-a- vis gold mineralization is spatially linked to the juvenile magmatic accretion （2.56-2.50 Ga） east of the studied area in the western part of the eastern Dharwar craton.展开更多
We present results of field studies for magmatic processes of 2.57-2.52 Ga calc-alkaline plutonic bodies from three corridors in the eastern Dharwar craton(EDC)corresponding to different crustal levels.At deeper level...We present results of field studies for magmatic processes of 2.57-2.52 Ga calc-alkaline plutonic bodies from three corridors in the eastern Dharwar craton(EDC)corresponding to different crustal levels.At deeper levels plutons are bounded by thick zone of migmatites with numerous melt filled shear bands which often overprinted by incipient charnockite.On the other hand in the mid-to-upper crustal levels plutons show relatively sharp contacts and truncates the adjoining basement.The plutons are composite which comprises voluminous intrusive monzodiorite,quartz-monzonite and porphyritic monzogranite in the central part and minor anatectic granites or diatexite at periphery.Numerous xenoliths,Mafic Magmatic Enclaves(MME),disrupted trains of synplutonic mafic dykes are found in both intrusive and anatectic facies.The plutons show magmatic as well as solid-state plastic fabrics defined by magmatic flow banding and C-S fabrics respectively.Crustal scale shear zone network comprising early melt filled NE trending hot ductile dextral shear bands and slightly later colder NW trending sinistral shear bands defined by rotation of mafic boudins,phenocrysts and C-S fabrics.The internal architecture of plutons is attributed to the crustal scale magma chamber processes where voluminous intrusive magmas emplaced into the crust caused reworking of surrounding basement resulting in production of anatectic magmas.Crystallization of voluminous intrusive magmas in the deep crust probably caused development of fractures to mantle depth causing decompression melting of mantle and resultant mafic magmas penetrated the crystallizing host in magma chambers.Field evidences together with published ages and Nd isotope data reveal a spatial link between late Archaean magmatic accretion,reworking and cratonization.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
Earth sciences are gaining much importance during 21st century in view of societal challenges of changing global climate,water,food and energy security for the growing population,geological hazards,rapid urbanization ...Earth sciences are gaining much importance during 21st century in view of societal challenges of changing global climate,water,food and energy security for the growing population,geological hazards,rapid urbanization and waste disposal.Earth science community have a greater role to play now than ever before in tackling global challenges,guiding communities and policy makers in planning,managing land,water,mineral and energy resources for sustainable development and to provide awareness to cope up and how to live with changing global climates.展开更多
文摘Tourmaline occurs as a minor but important mineral in the alteration zc,ne of the Archean orogenic gold deposit of Guddadarangavanahalli （G.R.Halli） in the Chitradurga greenst^ne belt of the western Dharwar craton, southern India. It occurs in the distal alteration halo of the G.R.Halli golcl deposit as （a） clusters of very fine grained aggregates which form a minor constituent in the natrix of the altered metabasalt （AMB tourmaline） and （b） in quartz-carbonate veins （vein tourmaline）. ~[＇he vein tourmaline, based upon the association of specific carbonate minerals, is further grouped as （i） albite-tourmaline-ankerite-quartz veins （vein-1 tourmaline） and （ii） albite-tourmaline-calcite-quartz veins （vein-2 tourmaline）. Both the AMB tourmaline and the vein tourmalines （vein-I and vein-2） belong to the alkali group and are clas- sified under schorl-dravite series. Tourmalines occurring in the veins are zoned while the AMB tour- malines are unzoned. Mineral chemistry and discrimination diagrams 1eveal that cores and rims of the vein tourmalines are distinctly different. Core composition of the ve：n tourmalines is similar to the composition of the AMB tourmaline. The formation of the AMB tourmaline and cores of the vein tour- malines are proposed to be related to the regional D1 deformational event associated with the emplacement of the adjoining ca. 2.61 Ga Chitradurga granite whilst rims of the vein tourmalines vis-a- vis gold mineralization is spatially linked to the juvenile magmatic accretion （2.56-2.50 Ga） east of the studied area in the western part of the eastern Dharwar craton.
基金funded by Department of Science and Technology(DST),Govt.of India funded project ESS/16/297/2007.
文摘We present results of field studies for magmatic processes of 2.57-2.52 Ga calc-alkaline plutonic bodies from three corridors in the eastern Dharwar craton(EDC)corresponding to different crustal levels.At deeper levels plutons are bounded by thick zone of migmatites with numerous melt filled shear bands which often overprinted by incipient charnockite.On the other hand in the mid-to-upper crustal levels plutons show relatively sharp contacts and truncates the adjoining basement.The plutons are composite which comprises voluminous intrusive monzodiorite,quartz-monzonite and porphyritic monzogranite in the central part and minor anatectic granites or diatexite at periphery.Numerous xenoliths,Mafic Magmatic Enclaves(MME),disrupted trains of synplutonic mafic dykes are found in both intrusive and anatectic facies.The plutons show magmatic as well as solid-state plastic fabrics defined by magmatic flow banding and C-S fabrics respectively.Crustal scale shear zone network comprising early melt filled NE trending hot ductile dextral shear bands and slightly later colder NW trending sinistral shear bands defined by rotation of mafic boudins,phenocrysts and C-S fabrics.The internal architecture of plutons is attributed to the crustal scale magma chamber processes where voluminous intrusive magmas emplaced into the crust caused reworking of surrounding basement resulting in production of anatectic magmas.Crystallization of voluminous intrusive magmas in the deep crust probably caused development of fractures to mantle depth causing decompression melting of mantle and resultant mafic magmas penetrated the crystallizing host in magma chambers.Field evidences together with published ages and Nd isotope data reveal a spatial link between late Archaean magmatic accretion,reworking and cratonization.
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Earth sciences are gaining much importance during 21st century in view of societal challenges of changing global climate,water,food and energy security for the growing population,geological hazards,rapid urbanization and waste disposal.Earth science community have a greater role to play now than ever before in tackling global challenges,guiding communities and policy makers in planning,managing land,water,mineral and energy resources for sustainable development and to provide awareness to cope up and how to live with changing global climates.
