Dharwar Craton 3cc1r
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DHARWAR CRATON BYNIRANKAR MISHRA DEPT. OF EARTH SCIENCES, IIT ROORKEE.
Craton,Mobile belts and shield CRATON:-They are broad central areas of continents that have remained tectonically stable for prolonged periods and are affected only by younger epeirogenic movements.They have a Palaeoarchaean nucleus(33103560Ma). Eg-Dharwar Craton,Singhbhum Craton.
MOBILE BELTS:-The cratonic nucleus is surrounded by a younger zone which has suffered from later deformational episodes and granitic activity.They are characterized by medium to high grade metamorphism.They are of Early Proterozoic evolution. Eg-Eastern Ghat Mobile Belt.
SHIELD:-The cratons and the mobile belts together constitute the shield.They are the stable parts of the earth’s crust composed of basement gneiss. Eg-Peninsular Indian shield,Canadian Shield.
The indian shield
Indian shield consists of five major Archaean cratons viz.,Dharwar,Bastar,Singhbhum,Bundelk hand and Aravalli. There are three prominent Proterozoic mobile belts, viz. Eastern Ghats mobile belt,Pandyan mobile belt and Satpura mobile belt. The Himalayan orogenic belt bounding the Indian shield to the north is largely a Tertiary mountain belt that has a Proterozoic foundation.
Indian shield .
Dharwar craton INTRODUCTION The Archaean Dharwar craton covers an area of about 4.5 lakh(0.45 million) sq. km.It extends from coast to coast,covering a vast geographic expanse of Goa,Karnataka,northwestern Tamil Nadu and Andhra Pradesh. It is one of the best-studied terrains of Peninsular India,and is renowned for its greenstone/schist belts,grey gneisses,charnockites and younger granites. The temporal range of these rocks is ~3300 Ma to 2550 Ma. These rocks experienced regional deformation and metamorphism,whose intensity increases progressively from green schist-lower amphibolite grade in the north to
The craton derives its name from the township Dharwar in northwestern Karnataka,where it was first studied by R.B. Foote in 1882. BOUNDARIES The craton is bounded to the south by the Pandyan mobile belt(PMB); to the north by the end Cretaceous Deccan Trap; to the north-east by the Archaean Karimnagar granulite belt(KGB) adjacent to Godavari basin; to the east by the Neoproterozoic Eastern Ghats mobile belt(EGMB); and to the west by the Arabian Sea.
Wdc &edc The craton is divided into two tectonic blocks after Swami Nath et al. (1976), as the WESTERN DHARWAR CRATON(WDC) and EASTERN DHARWAR CRATON(EDC). The WDC and EDC are separated by the Chitradurga Shear Zone,towards the western margin of Closepet Granite. The between WDC and EDC is not sharp,and there is a transition zone between the Chitradurga Shear Zone and Closepet Granite.
Differences between wdc and edc
lithostratigraphy The main lithotectonic units that are recognizable in the Dharwar craton are given below in the table.All of them are highly metamorphosed and the grade of metamorphism increases from north to south.
Western dharwar craton
Western dharwar craton
GORUR GNEISS(3300-3400 Ma) The oldest gneiss in WDC is a tonalitetrondhjemite-gneiss known as Gorur gneiss.These migmatitic gneisses are believed to have originated by low degree of partial melting of basaltic rocks in the lower crust.Fresh outcrops of Gorur gneiss are exposed in the stone quarries on either side of Gorur-Hassan road.
SARGUR GROUP(3100-3300 Ma) This group is represented by minor linear belts and enclaves of basic-ultrabasic magmatic rocks and quartzite-carbonatepelite(Q) alongwith Banded Iron Formation(BIF). They are massively invaded by the granitic rocks of Peninsular Gneiss. The principal igneous components are peridotites,pyroxenites and komatites. They have undergone upper amphibolitegrade metamorphism at 700±30°C and 8.6±0.8 kbar,and converted to a shistose assemblage.
PENINSULAR GNEISS(>3000 Ma) West of the Chitradurga Boundary Fault,the larger part of the western block of the Dharwar Craton is made up of gneisses of TTG composition,called the Peninsular Gneiss. It is a complex assemblage of granitic rocks.Some workers consider the Gorur Gneiss as a part of the Peninsular Gneiss. It is characterized by elongate belts and enclaves of schistose rocks.These schists exhibit shallow to steeply dipping schistosity and lineation.
DHARWAR SUPERGROUP(2600-2800 Ma) Dharwar Supergroup is developed in force in WDC and exposed in two large schist belts (1)-Bababudan-Western Ghats-Shimoga, and (2)-Chitradurga-Gadag It is divided into the Bababudan Group and the Chitradurga Group.The two are separated by an unconformity.
BABABUDAN GROUP It comprises basal oligomictic conglomerate,overlain by cross bedded quartz arenites,stromatolitic carbonates and banded ferrugious chert(BIF). The clastic sediments of the Bababudan represent a shallow-water platform deposit. The group can be divided into four formations,namelyKalasapura,Allampura,Santaveri and Mulaingiri.
CHITRADURGA GROUP It is made up of 1000m thick succession of polymictic conglomerate at the base; limestones and dolomites and banded ferruginous chert with manganese ore formation in the middle; and greywackes,carbonaceous phyllite and alteration of chert and ferruginous silica in the upper part of the sequence. Associated with predominant volcanics with some ultrabasics,the Chitradurga succession represents deposit of deep water basin.
YOUNGER INTRUSIONS IN WDC Late to post-tectonic Dharwar granites(~2600 Ma) occur as isolated discrete intrusions cutting across the foliation and banding of Peninsular Gneiss. Some of them are Chitradurga granite,Jampalnaikankote(J.N. Kote) granite,Hosadurga granite,Chamundi granite.
Eastern dharwar craton
The diffuse zone divides the EDC and WDC,which is about 200 km wide,starting from northeast sheared margin of Shimoga belt to the Closepet granite. The gneisses of diffuse zone have TTG composition like those of theWDC,but are mainly 2500-2700 Ma old like those of EDC.
Eastern dharwar craton
GREENSTONE BELTS OF DHARWAR SUPERGROUP The EDC is characterized by volcanicsdominated,sediment-poor and gold-rich belts,in contrast to those of WDC. Unlike WDC,division of EDC into distinct groups is not possible due the volcanic ensembles forming the bulk. Only the Yashwantanagar formation of Sandur belt has similarities with Bababudan group.
The greenstone belts of EDC form linear arrays of what may be called as superbelts or composite belts. The linear arrays of superbelts in the EDC are the following:(1)-Ramagiri-Hungund superbelt (2)-Kolar-Kadiri-Jonnagiri-Hutti superbelt (3)-Veligallu-Raichur-Gadwal superbelt
CLOSEPET GRANITE(2400-2500 Ma) Typically exposed at Closepet,the Closepet Granite forms a 400 km long and 20-30 km wide linear hill range. It extends from Kabbaldurga in the south to beyond Bellary in the north.It shows no substantial effects of deformation and metamorphism. It is a potash-rich ademellite,quartz monzonite and granite. It is believed that the Closepet batholith was emplaced along a crustal-scale shear zone during the Late Archaean time.
REGIONAL METAMORPHISM AND CHARNOCKITES
Progressive regional metamorphism from greenschist in the north to granulite facies in the south is well documented in Dharwar Craton,and more spectacularly in WDC. The P-T conditions graduate from 500°C/4-5kb in the north to 600°C/5-7kb in the middle and 700°C/7-8kb in the south.The regional metamorphism of WDC belongs to the classical Barrovian type. The amphibolite-granulite facies transition is marked by the development of incipient or arrested charnockite patches and veins.
Charnockites are products of deep-seated metamorphism of gneisses and granites. They are intercalated with subordinate sediments and basic-ultrabasic intrusives.The metamorphism took place under strong influence of CO2 rich fluids,derived from the mantle or the sedimentary material thrusted beneath the crustal slabs,and removing H2O from the deep crust,leading to pervasive dehydration and formation of granulites.Metamorphism of these granulites gave rise to charnockites at around 2500±50 Ma.
The regional metamorphism of EDC ocurred on similar lines ,but it is of lowpressure facies series. P-T conditions of EDC range from 670°C/3kb in the north ,through 710°C/45kb in the middle to 750°C/6-7kb in the south.
Tectonic evolution
Formation of Gorur Gneiss,representing the earliest Palaeoarchaean sialic crust. The ultrabasic- basic magmatic rocks,with intercalations of peletic-silicious sediments formed the Sargur Group,which is a product of intracratonic rift basin or an oceanic trench. The Gorur Gneiss and the Sargur Group together formed the 3400-3200 m.y. old nucleus of the Dharwar Craton. Invasion of many phases of gneissic magmasPeninsular Gneiss around 3000 Ma. Sagging and rifting of the craton due to extension,formation of elongate basins of Dharwar volcanism and sedimentation-Dharwar Supergroup
Oblique,westward convergence of the crustal blocks towards the end of the Neoarchaean era,resulting in E-W to NE-SW shortening of the basin leading to thrusting and transpression.The transpression along the N-S line is represented by the Chitradurga Boundary Fault. Possible activation of mantle plumes towards the end of the Neoarchaean time,upwelling of mantle material,resulting in emplacement of the Closepet Granite and the Dharwar Batholith in the larger northern part of the Dharwar province.
The granitic activity induced hightemperature-pressure granulite metamorphism,leading to formation of charnockites in the southern part of the Dharwar domain. Later,subhorizontal displacement on the N-S and NNW-SSE trending faults and shear zones aggravated the structural complexity of the Dharwar Craton.
ECONOMIC ASPECT OF DHARWAR SUPERGROUP
The Dharwar system carries the principal ore deposits of the country, e.g., those of gold, manganese, iron, chromium, copper, tungsten, lead, etc.
These with their associated rocks are also rich in such industrially useful products as mica, corundum, lithium minerals, titanium and thorium minerals, etc.
Rare valuable minerals like pitchblende, monazite and columbite, etc are also found in them as well as a few gems and semi-precious stones like ruby, sapphire, emerald, aquamarine, beryl, zircon, spinels, garnets, tourmalines, amethyst, rock crystals and some others.
Iron ore Two types of iron ore mainly found (1)--Lateritoid concentrations, essentially of haematite with minor limonite and goethite. Ores are mainly massive and often show pisolitic and oolitic structure. (2)-Magnetite mainly occur in the Bababudan Group as lenses or as beds in the banded magnetite quartzite.
Gold deposits Two types of gold loads mainly occur (1)-gold-quartz lodes represented mainly by the Champion lode. (2)- gold-quartz sulphide lodes represented by the Oriental and McTaggert lodes. In type-1 lodes, sulphides, viz., arsenopyrite, pyrite, chalcopyrite, pyrrhotite, pentlandite and galena are subordinate varying from traces to 2% of volume of Rock. In type-2 lodes, sulphides viz., pyrrhotite, pyrite, arsenopyrite, chalcopyrite, sphalerite and rare galena are in the order of 10-12% of volume of rock. Eg-Kolar gold field, Hutti mines
references
Geology Of IndiaM. Ramakrishnan R. Vaidyanadhan The Making Of India(Geodynamic Evolution) K.S. Valdiya
Thank you!
Craton,Mobile belts and shield CRATON:-They are broad central areas of continents that have remained tectonically stable for prolonged periods and are affected only by younger epeirogenic movements.They have a Palaeoarchaean nucleus(33103560Ma). Eg-Dharwar Craton,Singhbhum Craton.
MOBILE BELTS:-The cratonic nucleus is surrounded by a younger zone which has suffered from later deformational episodes and granitic activity.They are characterized by medium to high grade metamorphism.They are of Early Proterozoic evolution. Eg-Eastern Ghat Mobile Belt.
SHIELD:-The cratons and the mobile belts together constitute the shield.They are the stable parts of the earth’s crust composed of basement gneiss. Eg-Peninsular Indian shield,Canadian Shield.
The indian shield
Indian shield consists of five major Archaean cratons viz.,Dharwar,Bastar,Singhbhum,Bundelk hand and Aravalli. There are three prominent Proterozoic mobile belts, viz. Eastern Ghats mobile belt,Pandyan mobile belt and Satpura mobile belt. The Himalayan orogenic belt bounding the Indian shield to the north is largely a Tertiary mountain belt that has a Proterozoic foundation.
Indian shield .
Dharwar craton INTRODUCTION The Archaean Dharwar craton covers an area of about 4.5 lakh(0.45 million) sq. km.It extends from coast to coast,covering a vast geographic expanse of Goa,Karnataka,northwestern Tamil Nadu and Andhra Pradesh. It is one of the best-studied terrains of Peninsular India,and is renowned for its greenstone/schist belts,grey gneisses,charnockites and younger granites. The temporal range of these rocks is ~3300 Ma to 2550 Ma. These rocks experienced regional deformation and metamorphism,whose intensity increases progressively from green schist-lower amphibolite grade in the north to
The craton derives its name from the township Dharwar in northwestern Karnataka,where it was first studied by R.B. Foote in 1882. BOUNDARIES The craton is bounded to the south by the Pandyan mobile belt(PMB); to the north by the end Cretaceous Deccan Trap; to the north-east by the Archaean Karimnagar granulite belt(KGB) adjacent to Godavari basin; to the east by the Neoproterozoic Eastern Ghats mobile belt(EGMB); and to the west by the Arabian Sea.
Wdc &edc The craton is divided into two tectonic blocks after Swami Nath et al. (1976), as the WESTERN DHARWAR CRATON(WDC) and EASTERN DHARWAR CRATON(EDC). The WDC and EDC are separated by the Chitradurga Shear Zone,towards the western margin of Closepet Granite. The between WDC and EDC is not sharp,and there is a transition zone between the Chitradurga Shear Zone and Closepet Granite.
Differences between wdc and edc
lithostratigraphy The main lithotectonic units that are recognizable in the Dharwar craton are given below in the table.All of them are highly metamorphosed and the grade of metamorphism increases from north to south.
Western dharwar craton
Western dharwar craton
GORUR GNEISS(3300-3400 Ma) The oldest gneiss in WDC is a tonalitetrondhjemite-gneiss known as Gorur gneiss.These migmatitic gneisses are believed to have originated by low degree of partial melting of basaltic rocks in the lower crust.Fresh outcrops of Gorur gneiss are exposed in the stone quarries on either side of Gorur-Hassan road.
SARGUR GROUP(3100-3300 Ma) This group is represented by minor linear belts and enclaves of basic-ultrabasic magmatic rocks and quartzite-carbonatepelite(Q) alongwith Banded Iron Formation(BIF). They are massively invaded by the granitic rocks of Peninsular Gneiss. The principal igneous components are peridotites,pyroxenites and komatites. They have undergone upper amphibolitegrade metamorphism at 700±30°C and 8.6±0.8 kbar,and converted to a shistose assemblage.
PENINSULAR GNEISS(>3000 Ma) West of the Chitradurga Boundary Fault,the larger part of the western block of the Dharwar Craton is made up of gneisses of TTG composition,called the Peninsular Gneiss. It is a complex assemblage of granitic rocks.Some workers consider the Gorur Gneiss as a part of the Peninsular Gneiss. It is characterized by elongate belts and enclaves of schistose rocks.These schists exhibit shallow to steeply dipping schistosity and lineation.
DHARWAR SUPERGROUP(2600-2800 Ma) Dharwar Supergroup is developed in force in WDC and exposed in two large schist belts (1)-Bababudan-Western Ghats-Shimoga, and (2)-Chitradurga-Gadag It is divided into the Bababudan Group and the Chitradurga Group.The two are separated by an unconformity.
BABABUDAN GROUP It comprises basal oligomictic conglomerate,overlain by cross bedded quartz arenites,stromatolitic carbonates and banded ferrugious chert(BIF). The clastic sediments of the Bababudan represent a shallow-water platform deposit. The group can be divided into four formations,namelyKalasapura,Allampura,Santaveri and Mulaingiri.
CHITRADURGA GROUP It is made up of 1000m thick succession of polymictic conglomerate at the base; limestones and dolomites and banded ferruginous chert with manganese ore formation in the middle; and greywackes,carbonaceous phyllite and alteration of chert and ferruginous silica in the upper part of the sequence. Associated with predominant volcanics with some ultrabasics,the Chitradurga succession represents deposit of deep water basin.
YOUNGER INTRUSIONS IN WDC Late to post-tectonic Dharwar granites(~2600 Ma) occur as isolated discrete intrusions cutting across the foliation and banding of Peninsular Gneiss. Some of them are Chitradurga granite,Jampalnaikankote(J.N. Kote) granite,Hosadurga granite,Chamundi granite.
Eastern dharwar craton
The diffuse zone divides the EDC and WDC,which is about 200 km wide,starting from northeast sheared margin of Shimoga belt to the Closepet granite. The gneisses of diffuse zone have TTG composition like those of theWDC,but are mainly 2500-2700 Ma old like those of EDC.
Eastern dharwar craton
GREENSTONE BELTS OF DHARWAR SUPERGROUP The EDC is characterized by volcanicsdominated,sediment-poor and gold-rich belts,in contrast to those of WDC. Unlike WDC,division of EDC into distinct groups is not possible due the volcanic ensembles forming the bulk. Only the Yashwantanagar formation of Sandur belt has similarities with Bababudan group.
The greenstone belts of EDC form linear arrays of what may be called as superbelts or composite belts. The linear arrays of superbelts in the EDC are the following:(1)-Ramagiri-Hungund superbelt (2)-Kolar-Kadiri-Jonnagiri-Hutti superbelt (3)-Veligallu-Raichur-Gadwal superbelt
CLOSEPET GRANITE(2400-2500 Ma) Typically exposed at Closepet,the Closepet Granite forms a 400 km long and 20-30 km wide linear hill range. It extends from Kabbaldurga in the south to beyond Bellary in the north.It shows no substantial effects of deformation and metamorphism. It is a potash-rich ademellite,quartz monzonite and granite. It is believed that the Closepet batholith was emplaced along a crustal-scale shear zone during the Late Archaean time.
REGIONAL METAMORPHISM AND CHARNOCKITES
Progressive regional metamorphism from greenschist in the north to granulite facies in the south is well documented in Dharwar Craton,and more spectacularly in WDC. The P-T conditions graduate from 500°C/4-5kb in the north to 600°C/5-7kb in the middle and 700°C/7-8kb in the south.The regional metamorphism of WDC belongs to the classical Barrovian type. The amphibolite-granulite facies transition is marked by the development of incipient or arrested charnockite patches and veins.
Charnockites are products of deep-seated metamorphism of gneisses and granites. They are intercalated with subordinate sediments and basic-ultrabasic intrusives.The metamorphism took place under strong influence of CO2 rich fluids,derived from the mantle or the sedimentary material thrusted beneath the crustal slabs,and removing H2O from the deep crust,leading to pervasive dehydration and formation of granulites.Metamorphism of these granulites gave rise to charnockites at around 2500±50 Ma.
The regional metamorphism of EDC ocurred on similar lines ,but it is of lowpressure facies series. P-T conditions of EDC range from 670°C/3kb in the north ,through 710°C/45kb in the middle to 750°C/6-7kb in the south.
Tectonic evolution
Formation of Gorur Gneiss,representing the earliest Palaeoarchaean sialic crust. The ultrabasic- basic magmatic rocks,with intercalations of peletic-silicious sediments formed the Sargur Group,which is a product of intracratonic rift basin or an oceanic trench. The Gorur Gneiss and the Sargur Group together formed the 3400-3200 m.y. old nucleus of the Dharwar Craton. Invasion of many phases of gneissic magmasPeninsular Gneiss around 3000 Ma. Sagging and rifting of the craton due to extension,formation of elongate basins of Dharwar volcanism and sedimentation-Dharwar Supergroup
Oblique,westward convergence of the crustal blocks towards the end of the Neoarchaean era,resulting in E-W to NE-SW shortening of the basin leading to thrusting and transpression.The transpression along the N-S line is represented by the Chitradurga Boundary Fault. Possible activation of mantle plumes towards the end of the Neoarchaean time,upwelling of mantle material,resulting in emplacement of the Closepet Granite and the Dharwar Batholith in the larger northern part of the Dharwar province.
The granitic activity induced hightemperature-pressure granulite metamorphism,leading to formation of charnockites in the southern part of the Dharwar domain. Later,subhorizontal displacement on the N-S and NNW-SSE trending faults and shear zones aggravated the structural complexity of the Dharwar Craton.
ECONOMIC ASPECT OF DHARWAR SUPERGROUP
The Dharwar system carries the principal ore deposits of the country, e.g., those of gold, manganese, iron, chromium, copper, tungsten, lead, etc.
These with their associated rocks are also rich in such industrially useful products as mica, corundum, lithium minerals, titanium and thorium minerals, etc.
Rare valuable minerals like pitchblende, monazite and columbite, etc are also found in them as well as a few gems and semi-precious stones like ruby, sapphire, emerald, aquamarine, beryl, zircon, spinels, garnets, tourmalines, amethyst, rock crystals and some others.
Iron ore Two types of iron ore mainly found (1)--Lateritoid concentrations, essentially of haematite with minor limonite and goethite. Ores are mainly massive and often show pisolitic and oolitic structure. (2)-Magnetite mainly occur in the Bababudan Group as lenses or as beds in the banded magnetite quartzite.
Gold deposits Two types of gold loads mainly occur (1)-gold-quartz lodes represented mainly by the Champion lode. (2)- gold-quartz sulphide lodes represented by the Oriental and McTaggert lodes. In type-1 lodes, sulphides, viz., arsenopyrite, pyrite, chalcopyrite, pyrrhotite, pentlandite and galena are subordinate varying from traces to 2% of volume of Rock. In type-2 lodes, sulphides viz., pyrrhotite, pyrite, arsenopyrite, chalcopyrite, sphalerite and rare galena are in the order of 10-12% of volume of rock. Eg-Kolar gold field, Hutti mines
references
Geology Of IndiaM. Ramakrishnan R. Vaidyanadhan The Making Of India(Geodynamic Evolution) K.S. Valdiya
Thank you!
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