The Southern Granulite Terrane(Dharwar Craton),South India,is a key unit for understanding the origin of charnockite.New U-Pb and Lu-Hf analyses in zircon crys-tals from 16 samples representing a wide variety of litho...The Southern Granulite Terrane(Dharwar Craton),South India,is a key unit for understanding the origin of charnockite.New U-Pb and Lu-Hf analyses in zircon crys-tals from 16 samples representing a wide variety of litho-types from the quarries in Kabbaldurga reveal a complex geological history in the Archean and early Paleoprotero-zoic.Magmatic protoliths predominantly record Paleoar-chean ages between 3.4 and 3.2 Ga.Combined U-Pb and Lu-Hf signatures indicate a history of recurrent crustal anatexis,juvenile magmatic input,and felsic injections.Mesoarchaean magmatic charnockites were generated mainly from hornblende-dehydration melting of Paleoar-chaean mafic rocks.In addition,Peninsular Gneissic Com-plex of the Dharwar Craton,commonly described as TTG suites,are likely generated by melting of hydrated basalt.The new data are consistent with the idea of a convecting magmatic cycle and also support the proposal that the southern Dharwar Craton comprises a tilted cross-sec-tion through the Archaean crust.Paleoproterozoic high-temperature event is documented here as a complex unit involving juvenile mafic magmatism,granulite facies imprints and crustal anatexis as well as felsic injections,occurring within a short time period around 2.5 Ga.展开更多
Meso-Neoarchean fuchsite quartzites are present in different stratigraphic positions of Dharwar Craton including the oldest(~3.3 Ga)Sargur Group of western Dharwar Craton.The present study deals with the petro-graphic...Meso-Neoarchean fuchsite quartzites are present in different stratigraphic positions of Dharwar Craton including the oldest(~3.3 Ga)Sargur Group of western Dharwar Craton.The present study deals with the petro-graphic and geochemical characteristics of the fuchsite quartzites from the Ghattihosahalli belt to evaluate their genesis,depositional setting and the enigma involved in the ancient sedimentation history.Their major mineral assemblages include quartz,fuchsite,and feldspars along with accessory kyanite and rutile.The geochemical com-positions are characterized by high SiO_(2),Al_(2)O_(3),low MgO,CaO,strongly enriched Cr(1326–6899 ppm),Ba(1165–3653 ppm),Sr(46–210 ppm),V(107–868 ppm)and Zn(11–158 ppm)contents compared to the upper continental crust(UCC).The UCC normalized rare earth element(REE)patterns are characterized by depleted light REE[(La/Sm)UCC=0.33–0.95]compared to heavy REE[(Gd/Yb)_(UCC)=0.42–1.65]with conspicuous positive Eu-anomalies(Eu/Eu^(*)=1.35–18.27)characteristic of hydrothermal solutions evidenced through the interlayered barites.The overall major and trace element systematics reflect a combined mafic-felsic provenance and suggest their deposition at a passive continental margin environ-ment.The comprehensivefield,petrographic,and geo-chemical studies indicate that these quartzites are infiltrated by Cr-richfluids released during high-grade metamorphism of associated ultramafic rocks.The Sargur and the subse-quent Dharwar orogeny amalgamated diverse lithounits from different tectonic settings,possibly leading to the release of Cr-richfluids and the formation of fuchsite quartzite during or after the orogeny.Thesefindings sug-gest a pre-existing stable crust prior to the Sargur Group and the link between orogenic events and various mineral deposits in the Dharwar Craton.展开更多
Abstract Greenstone belts of the eastern Dharwar Craton, India are reinterpreted as composite tecto- nostratigraphic terranes of accreted plume-derived and convergent margin-derived magmatic sequences based on new hig...Abstract Greenstone belts of the eastern Dharwar Craton, India are reinterpreted as composite tecto- nostratigraphic terranes of accreted plume-derived and convergent margin-derived magmatic sequences based on new high-precision elemental data. The former are dominated by a komatiite plus Mg-tholeiitic basalt volcanic association, with deep water siliciclastic and banded iron formation （BIF） sedimentary rocks. Plumes melted at 〈90 km under thin rifted continental lithosphere to preserve intrao- ceanic and continental margin aspects. Associated alkaline basalts record subduction-recycling of Me- soarchean oceanic crust, incubated in the asthenosphere, and erupted coevally with Mg basalts from a heterogeneous mantle plume. Together, komatiites-Mg basalts-alkaline basalts plot along the Phanero- zoic mantle array in Th/Yb versus Nb/Yb coordinate space, representing zoned plumes, establishing that these reservoirs were present in the Neoarchean mantle. Convergent margin magmatic associations are dominated by tholeiitic to calc-alkaline basalts compo- sitionally similar to recent intraoceanic arcs. As well, boninitic flows sourced in extremely depleted mantle are present, and the association of arc basalts with Mg-andesites-Nb enriched basalts-adakites documented from Cenozoic arcs characterized by subduction of young （〈20 Ma）, hot, oceanic litho- sphere. Consequently, Cenozoic style ＂hot＂ subduction was operating in the Neoarchean. These diverse volcanic associations were assembled to give composite terranes in a subduction-accretion orogen at -2.7 Ga, coevally with a global accretionary orogen at -2.7 Ga, and associated orogenic gold mineralization.展开更多
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
Broad-band and long period magnetotelluric measurements made at 63 locations along -500 km long Chikmagalur-Kavali profile,that cut across the Dharwar craton (DC) and Eastern Ghat Mobile Belt (EGMB) in south India,is ...Broad-band and long period magnetotelluric measurements made at 63 locations along -500 km long Chikmagalur-Kavali profile,that cut across the Dharwar craton (DC) and Eastern Ghat Mobile Belt (EGMB) in south India,is modelled to examine the lithosphere architecture of the cratonic domain and define tectonic boundaries.The 2-D resistivity model shows moderately conductive features that intersperse a highly resistive background of crystalline rocks and spatially connect to the exposed schist belts or granitic intrusions in the DC.These features are therefore interpreted as images of fossil pathways of the volcanic emplacements associated with the greenstone belt and granite suite formation exposed in the region.A near vertical conductive feature in the upper mantle under the Chitradurga Shear Zone represents the Archean suture between the western and eastern blocks of DC.Although thick (-200 km) cratonic (highly resistive) lithosphere is preserved,significant part of the cratonic lithosphere below the western DC is modified due to plume-continental lithosphere interactions during the CretaceouseTertiary period.A west-verging moderately conductive feature imaged beneath EGMB lithosphere is interpreted as the remnant of the Proterozoic collision process between the Indian land mass and East Antarctica.Thin (-120 km) lithosphere is seen below the EGMB,which form the exterior margin of the India shield subsequent to its separation from East Antarctica through rifting and opening of the Indian Ocean in the Cretaceous.展开更多
Oldest rocks are sparsely distributed within the Dharwar Craton and little is known about their involvement in the sedimentary sequences which are present in the Archean greenstone successions and the Proterozoic Cudd...Oldest rocks are sparsely distributed within the Dharwar Craton and little is known about their involvement in the sedimentary sequences which are present in the Archean greenstone successions and the Proterozoic Cuddapah basin.Stromatolitic carbonates are well preserved in the Neoarchean greenstone belts of Dharwar Craton and Cuddapah Basin of Peninsular India displaying varied morphological and geochemical characteristics.In this study,we report results from U-Pb geochronology and trace element composition of the detrital zircons from stromatolitic carbonates present within the Dharwar Craton and Cuddapah basin to understand the provenance and time of accretion and deposition.The UPb ages of the detrital zircons from the Bhimasamudra and Marikanve stromatolites of the Chitradurga greenstone belt of Dharwar Craton display ages of 3426±26 Ma to 2650±38 Ma whereas the Sandur stromatolites gave an age of 3508±29 Ma to 2926±36 Ma suggesting Paleo-to Neoarchean provenance.The U-Pb detrital zircons of the Tadpatri stromatolites gave an age of 2761±31 Ma to1672±38 Ma suggesting Neoarchean to Mesoproterozoic provenance.The Rare Earth Element(REE)patterns of the studied detrital zircons from Archean Dharwar Craton and Proterozoic Cuddapah basin display depletion in light rare earth elements(LREE)and enrichment in heavy rare earth elements(HREE)with pronounced positive Ce and negative Eu anomalies,typical of magmatic zircons.The trace element composition and their relationship collectively indicate a mixed granitoid and mafic source for both the Dharwar and Cuddapah stromatolites.The 3508±29 Ma age of the detrital zircons support the existence of 3.5 Ga crust in the Western Dharwar Craton.The overall detrital zircon ages(3.5-2.7 Ga)obtained from the stromatolitic carbonates of Archean greenstone belts and Proterozoic Cuddapah basin(2.7-1.6 Ga)collectively reflect on^800-900 Ma duration for the Precambrian stromatolite deposition in the Dharwar Craton.展开更多
Anisotropy of magnetic susceptibility (AMS) studies were carried out on a precisely dated (2216.0 ± 0.9 Ma),450 km long NeS striking dyke in the Dharwar Craton,to determine the magma flow direction along the dyke...Anisotropy of magnetic susceptibility (AMS) studies were carried out on a precisely dated (2216.0 ± 0.9 Ma),450 km long NeS striking dyke in the Dharwar Craton,to determine the magma flow direction along the dyke length.In order to use the imbrication of the magnetic foliation,forty eight samples were collected from 13 locations along the length of the dyke.Magnetogranulometry studies show that AMS fabric is dominated by medium grained interstitial Ti-poor multidomain magnetite.The corrected anisotropy degree (Pj) of the samples was found to be low to moderate,between 1.007 and 1.072,which indicates primary magnetic fabric.The magnetic ellipsoid is either triaxial,prolate or oblate and clearly defines normal,intermediate and inverse magnetic fabrics related to magma flow during the dyke emplacement.The maximum susceptibility axes (Kmax) of the AMS tensor of the dyke is predominantly inclined at low angles (<30°),with no systematic variation in depth along the NeS profile,indicating sub-horizontal flow even at mid crustal levels which could probably be governed by location of the focal region of the magma source (mantle plume?),flow dynamics together with the compressive stresses exerted by the overlying crust.展开更多
The occurrence of rhythmic layering of chromite and host serpentinites in the deformed layered igneous complexes has been noticed in the Nuggihalli schist belt （NSB） in the western Dharwar craton, Karnataka, South I...The occurrence of rhythmic layering of chromite and host serpentinites in the deformed layered igneous complexes has been noticed in the Nuggihalli schist belt （NSB） in the western Dharwar craton, Karnataka, South India. For this study, the chromitite rock samples were collected from Jambur, Tagadur, Bhakatarhalli, Ranganbetta and Byrapur in the NSB. Petrography and ore microscopic studies on chromite show intense cataclasis and alteration to ferritchromite. The ferritchromite compositions are characterized by higher Cr number （Cr/[Cr＋AI]） （0.68-0.98） and lower Mg number （Mg/[Mg＋Fe]） （0.33-0.82） ratios in ferritchromite compared to that of parent chromite. The formation process for the ferritchromite is thought to be related to the exchange of Mg, AI, Cr, and Fe between the chromite, surrounding silicates （serpentines, chlorites）, and fluid during serpentinization.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
REE composition of the carbonates of the auriferous quartz carbonate veins（QCVs） of the Neoarchean Ajjanahalli gold deposit.Chitradurga schist belt.Dharwar Craton,is characterized by U-shaped chondrite normalized RE...REE composition of the carbonates of the auriferous quartz carbonate veins（QCVs） of the Neoarchean Ajjanahalli gold deposit.Chitradurga schist belt.Dharwar Craton,is characterized by U-shaped chondrite normalized REE patterns with both LREE and HREE enrichment and a distinct positive Eu anomaly.As positive Eu anomaly is associated with low oxygen fugacity,we propose that the auriferous fluids responsible for gold mineralization at Ajjanahalli could be from an oxygen depleted fluid.The observed positive Eu anomaly is interpreted to suggest the derivation of the auriferous fluids from a mantle reservoir.The location of Ajjanahalli gold deposit in a crustal scale shear zone is consistent with this interpretation.展开更多
An emerging view is that Earth’s geodynamic regime witnessed a fundamental transition towards plate tectonics around 3.0 Ga(billion years).However,the manifestations of this change may have been diachronous and crato...An emerging view is that Earth’s geodynamic regime witnessed a fundamental transition towards plate tectonics around 3.0 Ga(billion years).However,the manifestations of this change may have been diachronous and craton-specific.展开更多
Detailed mineralogical, bulk-rock geochemical and Sr-Nd isotopic data for the recently discovered Ahobil kimberlite(Pipe-16) from the Wajrakarur kimberlite field(WKF), Eastern Dharwar craton(EDC),southern India, are p...Detailed mineralogical, bulk-rock geochemical and Sr-Nd isotopic data for the recently discovered Ahobil kimberlite(Pipe-16) from the Wajrakarur kimberlite field(WKF), Eastern Dharwar craton(EDC),southern India, are presented. Two generations of compositionally distinct olivine, Ti-poor phlogopite showing orangeitic evolutionary trends, spinel displaying magmatic trend-1, abundant perovskite, Tirich hydrogarnet, calcite and serpentine are the various mineral constituents. On the basis of(i) liquidus mineral composition,(ii) bulk-rock chemistry, and(iii) Sr-Nd isotopic composition, we show that Ahobil kimberlite shares several characteristic features of archetypal kimberlites than orangeites and lamproites. Geochemical modelling indicate Ahobil kimberlite magma derivation from small-degree melting of a carbonated peridotite source having higher Gd/Yb and lower La/Sm in contrast to those of orangeites from the Eastern Dharwar and Bastar cratons of Indian shield. The T_(Dm) Nd model age(～2.0 Ga) of the Ahobil kimberlite is(i) significantly older than those(1.5～1.3 Ga) reported for Wajrakarur and Narayanpet kimberlites of EDC,(ii) indistinguishable from those of the Mesoproterozoic EDC lamproites,and(iii) strikingly coincides with the timing of the amalgamation of the Columbia supercontinent. High bulk-rock Fe-Ti contents and wide variation in oxygen fugacity fO_2, as inferred from perovskite oxybarometry, suggest non-prospective nature of the Ahobil kimberlite for diamond.展开更多
Neoarchean orogenic gold deposits, associated with the greenstone-granite milieus in the Dharwar Craton include(1) the famous Kolar mine and the world class Hutti deposit;(2) small mines at HiraBuddini, Uti, Ajjanahal...Neoarchean orogenic gold deposits, associated with the greenstone-granite milieus in the Dharwar Craton include(1) the famous Kolar mine and the world class Hutti deposit;(2) small mines at HiraBuddini, Uti, Ajjanahalli, and Guddadarangavanahalli;(3) prospects at Jonnagiri; and(4) old mining camps in the Gadag and Ramagiri-Penakacherla belts. The existing diametric views on the source of ore fluid for formation of these deposits include fluids exsolved from granitic melts and extracted by metamorphic devolatilization of the greenstone sequences. Lode gold mineralization occurs in structurally controlled higher order splays in variety of host rocks such as mafic/felsic greenstones, banded iron formations, volcaniclastic rocks and granitoids. Estimated metamorphic conditions of the greenstones vary from lower greenschist facies to mid-amphibolite facies and mineralizations in all the camps are associated with distinct hydrothermal alterations. Fluid inclusion microthermometric and Raman spectroscopic studies document low salinity aqueous-gaseous(H_2O + CO_2 ± CH_4 + NaCl) ore fluids,which precipitated gold and altered the host rocks in a narrow P-T window of 0.7-2.5 kbar and 215-320℃. While the calculated fluid O-and C-isotopic values are ambiguous, S-isotopic compositions of pyrite-precipitating fluid show distinct craton-scale uniformity in terms of its reduced nature and a suggested crustal sulfur source.Available ages on greenstone metamorphism, granitoid plutonism and mineralization in the Hutti Belt are tantamount, making a geochronology-based resolution of the existing debate on the metamorphic vs.magmatic fluid source impossible. In contrast, tourmaline geochemistry suggests involvement of single fluid in formation of gold mineralization, primarily derived by metamorphic devolatilization of mafic greenstones and interlayered sedimentary rocks, with minor magmatic contributions. Similarly, compositions of scheelite, pyrite and arsenopyrite point toward operation of fault-valves that caused pressure fluctuation-induced fluid phase separation, which acted as the dominant process of gold precipitation,apart from fluid-rock sulfidation reactions. Therefore, results from geochemistry of hydrothermal minerals and those from fluid inclusion microthermometry corroborate in constraining source of ore fluid,nature of gold transport(by Au-bisulfide complex) and mechanism of gold ore formation in the Dharwar Craton.展开更多
Hogbomite,a rare exotic mineral,is found to be associated with the vanadiferous-titaniferous （V-Ti） bearing magnetite bands at Bhakatarhalli,Nuggihaifi greenstone belt,western Dharwar Craton,India.We report on a sec...Hogbomite,a rare exotic mineral,is found to be associated with the vanadiferous-titaniferous （V-Ti） bearing magnetite bands at Bhakatarhalli,Nuggihaifi greenstone belt,western Dharwar Craton,India.We report on a second occurrence of hogbomite from the Dharwar craton in Karnataka,which is the sixth documented occurrence of this mineral from India.We evaluate the chemical characteristics of hogbomite and associated Fe-Ti-minerals in an attempt to identify its formation as a primary hydrothermal mineral in a metamorphosed magnetite layer.We report here the presence of hogbomite as a complex oxide of Fe,Mg,Al and Ti with accessory of Zn,V and Sn.Petrographic studies suggest the （V-Ti） bearing magnetite （Mt） contain spinel,hogbomite,chlorite,martite,ilmenite （Ⅱ） and minor amounts of diaspore.The hogbomite displays euhedral to subhedral textures,and is up to 250 μm along the grain boundaries of magnetite and ilmenite.In the samples studied,hogbomite is prismatic,irregular and elongated in shape.The genesis of hogbomite in veins between magnetite and ilmenite implies its precipitation from fluids without involving complicated reactions.Several models were proposed for the formation of hogbomite; however,the subject is still debatable.展开更多
This study presents a comprehensive account of the petrogenetic and geodynamic evolution of the Bellara Trap volcanic rocks from the Ingaldhal Formation, Chitradurga Group, western Dharwar Craton(WDC). Geochemical att...This study presents a comprehensive account of the petrogenetic and geodynamic evolution of the Bellara Trap volcanic rocks from the Ingaldhal Formation, Chitradurga Group, western Dharwar Craton(WDC). Geochemical attributes of these rocks are consistent with two groups with distinct evolutionary trends: one comprising tholeiitic, MORB(mid-ocean ridge basalt) type basalts(BTB) and the other corresponding to calc-alkaline andesites(BTA). Basalts are essentially composed of clinopyroxene and plagioclase whereas the andesites are porphyritic with phenocrysts of plagioclase, clinopyroxene and polycrystalline quartz embedded in a groundmass of K-feldspar, quartz and opaques. Primary igneous mineralogy is overprinted by greenschist facies metamorphism resulting in chlorite-actinolite-plagioclase assemblage. The BTB samples reflect nearly flat REE patterns with weak LREE enrichment in contrast to pronounced LREE enhancement over HREE discernible for BTA. Tectonically, the BTB samples correspond to an active mid-oceanic ridge-rift setting with a MORB composition, whereas a back-arc basin(BAB) regime is corroborated for the BTA samples fractionating from back-arc basin basalts. Geochemical imprints of subduction input are more pronounced in BTA compared to BTB as mirrored by their elevated abundances of incompatible fluid mobile elements like Ba, Th, U and LREE. The BTB is endowed with an N-to E-MORB signature attributable to minor contributions from subduction-related components at the inception of a back-arc basin in the vicinity of an active subduction system. The BTA derived through differentiation of a basaltic magma with BABB(back-arc basin basalt) affinity compositionally akin to a heterogeneous source mantle carrying depleted MORB-type and enriched arc-type components inducted with progressive subduction. The BABB-type andesites and MORB-type basalts from Bellara Traps record a compositional heterogeneity of mantle in an intraoceanic arc-back arc system. Mantle processes invoke a BABB-MORB spectrum with a MORB-like endmember and an arc-like endmember associated with a juvenile back-arc basin. This study infers a Neoarchean analogue of Mariana-type back-arc rift setting proximal to the arc with a gradual transition from anhydrous to hydrous melting processes synchronized with MORB-mantle and arc-mantle interaction during initiation of a nascent back arc adjacent to the arc. The MORB-BABB compositional spectrum for the Bellara Traps conforms to a Neoarchean back-arc basin that evolved under an extensional tectonic regime associated with incipient stages of back-arc rifting and incorporation of subduction-derived components in the mantle output. This study complies with Neoarchean intraoceanic accretionary cycle plate tectonics in WDC.展开更多
The Dharwar Craton in Peninsular India was intruded by a series of mafic dykes during the Paleoproterozoic and these mafic magmatic events have important implications on continental rifting and LIPs.Here we report ten...The Dharwar Craton in Peninsular India was intruded by a series of mafic dykes during the Paleoproterozoic and these mafic magmatic events have important implications on continental rifting and LIPs.Here we report ten precise Pb-Pb TE-TIMS age determinations on baddeleyite grains separated from seven mafic dykes and three sills,intruding into Archean basement rocks and Proterozoic sedimentary formations of the Eastern Dharwar Craton respectively.The crystallization age of the baddeleyite shows 2366.3±1.1 Ma,and 2369.2±0.8 Ma for the NE-SW trending dykes,2368.1±0.6 Ma,2366.4±0.8 Ma,2207.2±0.7 Ma and 1887.3±1.0 Ma for the ENE-WNW to E-W striking dykes,1880.6±1.0 Ma,1864.3±0.6 Ma and 1863.6±0.9 Ma for Cuddapah sills,and 1861.8±1.4 Ma for the N-S trending dyke.Our results in conjunction with those from previous studies identify eight distinct stages of widespread Paleoproterozoic magmatism in the Dharwar craton.The mantle plume centres of the four radiating dyke swarms with ages of^2367 Ma,~2210 Ma,~2082 Ma,and^1886 Ma were traced to establish their proximity to the EDC kimberlite province.Though the^2367 Ma and^1886 Ma plume centres are inferred to be located to the west and east of the present day Dharwar craton respectively away from the kimberlite province,location of plume heads of the other two swarms with ages of^2207 Ma and^2082 Ma are in close proximity.In spite of the ubiquitous occurrence of dyke intrusions of all the seven generations in the kimberlite province,only few of these kimberlites are diamondiferous.Kimberlite occurrences elsewhere in the vicinity of older Large Igneous Provinces(LIPs)like the Mackenzie,Karoo,Parana-Etendeka and Yakutsk-Vilui are also non-diamondiferous.This has been attributed to the destruction of the lithospheric mantle keel(that hosts the diamonds)by the respective mantle plumes.The diamondiferous nature of the EDC kimberlites therefore suggests that plume activity does not always result in the destruction of the mantle keel.展开更多
Gold mineralization at G. R. Halli is located along the Central shear zone of Chitradurga schist belt, extending from west of Gonur, through east of G. R. Halli and C. K. Halli to east of Honnemardi, roughly parallel ...Gold mineralization at G. R. Halli is located along the Central shear zone of Chitradurga schist belt, extending from west of Gonur, through east of G. R. Halli and C. K. Halli to east of Honnemardi, roughly parallel to stratigraphic units. The NNW-SSE trending shear zone has a width of 0.5 to 1.5 km shows extensive carbonatization of metabasalts and the associated lithologies confined to NNW-WNW trending arcuate brittle-ductile zone. The sheared and silicified contact zones between carbonaceous argillite and schistose metabasalt form the potential sites for localization of mineralization. The gold is associated with sulphides mainly pyrite, arsenopyrite, galena, sphalerite, minor chalcopyrite. Textural relationship indicates two stage sulphide mineral assemblages co-relatable with two stage fluid ascents having temperature of homogenization between 125°C and 256°C. It is a typical epigenetic lode gold system, which got affected by later deformation.展开更多
The Peddavura greenstone Linear Belt, NW-SE trending, is formed in the Eastern part of the Dharwar Craton of south India, extended over 62.5 sq•km in Nalgonda and Guntur districts region. The entire belt is i...The Peddavura greenstone Linear Belt, NW-SE trending, is formed in the Eastern part of the Dharwar Craton of south India, extended over 62.5 sq•km in Nalgonda and Guntur districts region. The entire belt is illustrated as Peninsular Gneiss. The Belt Study has attracted geologists for conducting further research to evaluate the crust forming process at the time of early volcanic eruption of Earth’s history. The South Indian Dharwar Craton depicts and exposes the crustal segments where geological activities took place consistently during the Precambrian. The PSB (Peddavura Schist Belt) mostly consists of meta volcanic (meta basalts), amphibolites, granites, dolerites, basaltic andesites, pegmatite and Banded Magnetite Quartzite’s (BMQ) rock types. The 20 represented rock samples made for thin section studies. Based on the Petrological studies minerals are showing uralitization, saussuritization in the granite with mylonite structures, perthite and dolerite are showing heavy metal such as rutile and other opaque minerals (Magnetite, hematite, and typical pyrite crystal) are present in different represented rock samples. The minerals are showing different alteration zones along with microstructures. Using the Petrological studies the minerals and rock types are identified in the study region.展开更多
The late Archean Dharwar Craton is an important part of the Archean and Proterozoic terrains in Peninsular India.Dharwar Craton consists of Western and Eastern Blocks,separated by the Chitradurga Shear Zone.Eastern
The dykes intruding the Hyderabad Granitic Region(HGR)which forms the part of eastern Dharwar Craton extending between northern and northwestern margins of the Cuddapah Basin and western margin of the Pakhal
基金funded by the India-Brazil bilateral co-operation Project:INT/BRAZIL/P-02/2013by Indian Statistical Institute,Geoscience Institute of São Paulo University,Brazil and Department of Geology,University of Calcutta.M.Hueck thanks FAPESP for a post-doctoral fellowship(grant 2019/06838-2).
文摘The Southern Granulite Terrane(Dharwar Craton),South India,is a key unit for understanding the origin of charnockite.New U-Pb and Lu-Hf analyses in zircon crys-tals from 16 samples representing a wide variety of litho-types from the quarries in Kabbaldurga reveal a complex geological history in the Archean and early Paleoprotero-zoic.Magmatic protoliths predominantly record Paleoar-chean ages between 3.4 and 3.2 Ga.Combined U-Pb and Lu-Hf signatures indicate a history of recurrent crustal anatexis,juvenile magmatic input,and felsic injections.Mesoarchaean magmatic charnockites were generated mainly from hornblende-dehydration melting of Paleoar-chaean mafic rocks.In addition,Peninsular Gneissic Com-plex of the Dharwar Craton,commonly described as TTG suites,are likely generated by melting of hydrated basalt.The new data are consistent with the idea of a convecting magmatic cycle and also support the proposal that the southern Dharwar Craton comprises a tilted cross-sec-tion through the Archaean crust.Paleoproterozoic high-temperature event is documented here as a complex unit involving juvenile mafic magmatism,granulite facies imprints and crustal anatexis as well as felsic injections,occurring within a short time period around 2.5 Ga.
基金funds MLP 6406-28(MRM)of CSIR-NGRIGH acknowledges DSTINSPIRE for PhD Fellowship(No.DST/INSPIRE/03/2021/001295)+1 种基金CM acknowledges the CSIR Emeritus Project funds。
文摘Meso-Neoarchean fuchsite quartzites are present in different stratigraphic positions of Dharwar Craton including the oldest(~3.3 Ga)Sargur Group of western Dharwar Craton.The present study deals with the petro-graphic and geochemical characteristics of the fuchsite quartzites from the Ghattihosahalli belt to evaluate their genesis,depositional setting and the enigma involved in the ancient sedimentation history.Their major mineral assemblages include quartz,fuchsite,and feldspars along with accessory kyanite and rutile.The geochemical com-positions are characterized by high SiO_(2),Al_(2)O_(3),low MgO,CaO,strongly enriched Cr(1326–6899 ppm),Ba(1165–3653 ppm),Sr(46–210 ppm),V(107–868 ppm)and Zn(11–158 ppm)contents compared to the upper continental crust(UCC).The UCC normalized rare earth element(REE)patterns are characterized by depleted light REE[(La/Sm)UCC=0.33–0.95]compared to heavy REE[(Gd/Yb)_(UCC)=0.42–1.65]with conspicuous positive Eu-anomalies(Eu/Eu^(*)=1.35–18.27)characteristic of hydrothermal solutions evidenced through the interlayered barites.The overall major and trace element systematics reflect a combined mafic-felsic provenance and suggest their deposition at a passive continental margin environ-ment.The comprehensivefield,petrographic,and geo-chemical studies indicate that these quartzites are infiltrated by Cr-richfluids released during high-grade metamorphism of associated ultramafic rocks.The Sargur and the subse-quent Dharwar orogeny amalgamated diverse lithounits from different tectonic settings,possibly leading to the release of Cr-richfluids and the formation of fuchsite quartzite during or after the orogeny.Thesefindings sug-gest a pre-existing stable crust prior to the Sargur Group and the link between orogenic events and various mineral deposits in the Dharwar Craton.
基金Department of Science and Technology(DST) for funding the Projects on Dharwar Craton
文摘Abstract Greenstone belts of the eastern Dharwar Craton, India are reinterpreted as composite tecto- nostratigraphic terranes of accreted plume-derived and convergent margin-derived magmatic sequences based on new high-precision elemental data. The former are dominated by a komatiite plus Mg-tholeiitic basalt volcanic association, with deep water siliciclastic and banded iron formation （BIF） sedimentary rocks. Plumes melted at 〈90 km under thin rifted continental lithosphere to preserve intrao- ceanic and continental margin aspects. Associated alkaline basalts record subduction-recycling of Me- soarchean oceanic crust, incubated in the asthenosphere, and erupted coevally with Mg basalts from a heterogeneous mantle plume. Together, komatiites-Mg basalts-alkaline basalts plot along the Phanero- zoic mantle array in Th/Yb versus Nb/Yb coordinate space, representing zoned plumes, establishing that these reservoirs were present in the Neoarchean mantle. Convergent margin magmatic associations are dominated by tholeiitic to calc-alkaline basalts compo- sitionally similar to recent intraoceanic arcs. As well, boninitic flows sourced in extremely depleted mantle are present, and the association of arc basalts with Mg-andesites-Nb enriched basalts-adakites documented from Cenozoic arcs characterized by subduction of young （〈20 Ma）, hot, oceanic litho- sphere. Consequently, Cenozoic style ＂hot＂ subduction was operating in the Neoarchean. These diverse volcanic associations were assembled to give composite terranes in a subduction-accretion orogen at -2.7 Ga, coevally with a global accretionary orogen at -2.7 Ga, and associated orogenic gold mineralization.
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under the Supra Institutional Project (SIP-0012)carried under INDEX (PSC0204) project,funded by Council of Scientific and Industrial Research (CSIR), New Delhi, India
文摘Broad-band and long period magnetotelluric measurements made at 63 locations along -500 km long Chikmagalur-Kavali profile,that cut across the Dharwar craton (DC) and Eastern Ghat Mobile Belt (EGMB) in south India,is modelled to examine the lithosphere architecture of the cratonic domain and define tectonic boundaries.The 2-D resistivity model shows moderately conductive features that intersperse a highly resistive background of crystalline rocks and spatially connect to the exposed schist belts or granitic intrusions in the DC.These features are therefore interpreted as images of fossil pathways of the volcanic emplacements associated with the greenstone belt and granite suite formation exposed in the region.A near vertical conductive feature in the upper mantle under the Chitradurga Shear Zone represents the Archean suture between the western and eastern blocks of DC.Although thick (-200 km) cratonic (highly resistive) lithosphere is preserved,significant part of the cratonic lithosphere below the western DC is modified due to plume-continental lithosphere interactions during the CretaceouseTertiary period.A west-verging moderately conductive feature imaged beneath EGMB lithosphere is interpreted as the remnant of the Proterozoic collision process between the Indian land mass and East Antarctica.Thin (-120 km) lithosphere is seen below the EGMB,which form the exterior margin of the India shield subsequent to its separation from East Antarctica through rifting and opening of the Indian Ocean in the Cretaceous.
基金funds provided from Council of Scientific and Industrial Research (CSIR) to National Geophysical Research Institute,Hyderabad through the projects of Ministry of Earth Sciences (No:MoES/PO(Geosci)/8/ 2014) and MLP 6406-28 (CM)
文摘Oldest rocks are sparsely distributed within the Dharwar Craton and little is known about their involvement in the sedimentary sequences which are present in the Archean greenstone successions and the Proterozoic Cuddapah basin.Stromatolitic carbonates are well preserved in the Neoarchean greenstone belts of Dharwar Craton and Cuddapah Basin of Peninsular India displaying varied morphological and geochemical characteristics.In this study,we report results from U-Pb geochronology and trace element composition of the detrital zircons from stromatolitic carbonates present within the Dharwar Craton and Cuddapah basin to understand the provenance and time of accretion and deposition.The UPb ages of the detrital zircons from the Bhimasamudra and Marikanve stromatolites of the Chitradurga greenstone belt of Dharwar Craton display ages of 3426±26 Ma to 2650±38 Ma whereas the Sandur stromatolites gave an age of 3508±29 Ma to 2926±36 Ma suggesting Paleo-to Neoarchean provenance.The U-Pb detrital zircons of the Tadpatri stromatolites gave an age of 2761±31 Ma to1672±38 Ma suggesting Neoarchean to Mesoproterozoic provenance.The Rare Earth Element(REE)patterns of the studied detrital zircons from Archean Dharwar Craton and Proterozoic Cuddapah basin display depletion in light rare earth elements(LREE)and enrichment in heavy rare earth elements(HREE)with pronounced positive Ce and negative Eu anomalies,typical of magmatic zircons.The trace element composition and their relationship collectively indicate a mixed granitoid and mafic source for both the Dharwar and Cuddapah stromatolites.The 3508±29 Ma age of the detrital zircons support the existence of 3.5 Ga crust in the Western Dharwar Craton.The overall detrital zircon ages(3.5-2.7 Ga)obtained from the stromatolitic carbonates of Archean greenstone belts and Proterozoic Cuddapah basin(2.7-1.6 Ga)collectively reflect on^800-900 Ma duration for the Precambrian stromatolite deposition in the Dharwar Craton.
基金supported by the NGRI MLP 6513INDEX project funds
文摘Anisotropy of magnetic susceptibility (AMS) studies were carried out on a precisely dated (2216.0 ± 0.9 Ma),450 km long NeS striking dyke in the Dharwar Craton,to determine the magma flow direction along the dyke length.In order to use the imbrication of the magnetic foliation,forty eight samples were collected from 13 locations along the length of the dyke.Magnetogranulometry studies show that AMS fabric is dominated by medium grained interstitial Ti-poor multidomain magnetite.The corrected anisotropy degree (Pj) of the samples was found to be low to moderate,between 1.007 and 1.072,which indicates primary magnetic fabric.The magnetic ellipsoid is either triaxial,prolate or oblate and clearly defines normal,intermediate and inverse magnetic fabrics related to magma flow during the dyke emplacement.The maximum susceptibility axes (Kmax) of the AMS tensor of the dyke is predominantly inclined at low angles (<30°),with no systematic variation in depth along the NeS profile,indicating sub-horizontal flow even at mid crustal levels which could probably be governed by location of the focal region of the magma source (mantle plume?),flow dynamics together with the compressive stresses exerted by the overlying crust.
基金project by PVSR(no.GAP 538-28[PVSR]),funded by the Department of Science and Technology,Government of India.
文摘The occurrence of rhythmic layering of chromite and host serpentinites in the deformed layered igneous complexes has been noticed in the Nuggihalli schist belt （NSB） in the western Dharwar craton, Karnataka, South India. For this study, the chromitite rock samples were collected from Jambur, Tagadur, Bhakatarhalli, Ranganbetta and Byrapur in the NSB. Petrography and ore microscopic studies on chromite show intense cataclasis and alteration to ferritchromite. The ferritchromite compositions are characterized by higher Cr number （Cr/[Cr＋AI]） （0.68-0.98） and lower Mg number （Mg/[Mg＋Fe]） （0.33-0.82） ratios in ferritchromite compared to that of parent chromite. The formation process for the ferritchromite is thought to be related to the exchange of Mg, AI, Cr, and Fe between the chromite, surrounding silicates （serpentines, chlorites）, and fluid during serpentinization.
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funding through Minor Research Project from Indian School of Mines,Dhanbad and SERC,Department of Science & Technology,New Delhi,India
文摘REE composition of the carbonates of the auriferous quartz carbonate veins（QCVs） of the Neoarchean Ajjanahalli gold deposit.Chitradurga schist belt.Dharwar Craton,is characterized by U-shaped chondrite normalized REE patterns with both LREE and HREE enrichment and a distinct positive Eu anomaly.As positive Eu anomaly is associated with low oxygen fugacity,we propose that the auriferous fluids responsible for gold mineralization at Ajjanahalli could be from an oxygen depleted fluid.The observed positive Eu anomaly is interpreted to suggest the derivation of the auriferous fluids from a mantle reservoir.The location of Ajjanahalli gold deposit in a crustal scale shear zone is consistent with this interpretation.
基金support from projects:CSIR-GeoMet(No.MLP-0002-FBR-2-EVB)Ministry of Earth Sciences(No.MoES P.O.(Geo)/99(i)/2017).
文摘An emerging view is that Earth’s geodynamic regime witnessed a fundamental transition towards plate tectonics around 3.0 Ga(billion years).However,the manifestations of this change may have been diachronous and craton-specific.
基金New Delhi sanctioned a major research project(IR/S4/ESF-18/2011 dated 12.11.2013)to NVCR which made this research possibleDST-SERB for financial assistance in the form of a research scientist. AS acknowledges CSIR for awarding JRF(NET)
文摘Detailed mineralogical, bulk-rock geochemical and Sr-Nd isotopic data for the recently discovered Ahobil kimberlite(Pipe-16) from the Wajrakarur kimberlite field(WKF), Eastern Dharwar craton(EDC),southern India, are presented. Two generations of compositionally distinct olivine, Ti-poor phlogopite showing orangeitic evolutionary trends, spinel displaying magmatic trend-1, abundant perovskite, Tirich hydrogarnet, calcite and serpentine are the various mineral constituents. On the basis of(i) liquidus mineral composition,(ii) bulk-rock chemistry, and(iii) Sr-Nd isotopic composition, we show that Ahobil kimberlite shares several characteristic features of archetypal kimberlites than orangeites and lamproites. Geochemical modelling indicate Ahobil kimberlite magma derivation from small-degree melting of a carbonated peridotite source having higher Gd/Yb and lower La/Sm in contrast to those of orangeites from the Eastern Dharwar and Bastar cratons of Indian shield. The T_(Dm) Nd model age(～2.0 Ga) of the Ahobil kimberlite is(i) significantly older than those(1.5～1.3 Ga) reported for Wajrakarur and Narayanpet kimberlites of EDC,(ii) indistinguishable from those of the Mesoproterozoic EDC lamproites,and(iii) strikingly coincides with the timing of the amalgamation of the Columbia supercontinent. High bulk-rock Fe-Ti contents and wide variation in oxygen fugacity fO_2, as inferred from perovskite oxybarometry, suggest non-prospective nature of the Ahobil kimberlite for diamond.
基金financially supported by two grants to BM from the Department of Science and Technology(DST),Government of India, under the area of 'Deep Continental Studies'(ESS/16/116/98 and ESS/16/259/2005)The SEM and the EPMA units were procured through DST funding(IR/S4/ ESF-08/2005)to the Department of Geology & Geophysics, ⅡT Kharagpur
文摘Neoarchean orogenic gold deposits, associated with the greenstone-granite milieus in the Dharwar Craton include(1) the famous Kolar mine and the world class Hutti deposit;(2) small mines at HiraBuddini, Uti, Ajjanahalli, and Guddadarangavanahalli;(3) prospects at Jonnagiri; and(4) old mining camps in the Gadag and Ramagiri-Penakacherla belts. The existing diametric views on the source of ore fluid for formation of these deposits include fluids exsolved from granitic melts and extracted by metamorphic devolatilization of the greenstone sequences. Lode gold mineralization occurs in structurally controlled higher order splays in variety of host rocks such as mafic/felsic greenstones, banded iron formations, volcaniclastic rocks and granitoids. Estimated metamorphic conditions of the greenstones vary from lower greenschist facies to mid-amphibolite facies and mineralizations in all the camps are associated with distinct hydrothermal alterations. Fluid inclusion microthermometric and Raman spectroscopic studies document low salinity aqueous-gaseous(H_2O + CO_2 ± CH_4 + NaCl) ore fluids,which precipitated gold and altered the host rocks in a narrow P-T window of 0.7-2.5 kbar and 215-320℃. While the calculated fluid O-and C-isotopic values are ambiguous, S-isotopic compositions of pyrite-precipitating fluid show distinct craton-scale uniformity in terms of its reduced nature and a suggested crustal sulfur source.Available ages on greenstone metamorphism, granitoid plutonism and mineralization in the Hutti Belt are tantamount, making a geochronology-based resolution of the existing debate on the metamorphic vs.magmatic fluid source impossible. In contrast, tourmaline geochemistry suggests involvement of single fluid in formation of gold mineralization, primarily derived by metamorphic devolatilization of mafic greenstones and interlayered sedimentary rocks, with minor magmatic contributions. Similarly, compositions of scheelite, pyrite and arsenopyrite point toward operation of fault-valves that caused pressure fluctuation-induced fluid phase separation, which acted as the dominant process of gold precipitation,apart from fluid-rock sulfidation reactions. Therefore, results from geochemistry of hydrothermal minerals and those from fluid inclusion microthermometry corroborate in constraining source of ore fluid,nature of gold transport(by Au-bisulfide complex) and mechanism of gold ore formation in the Dharwar Craton.
基金Funding by the DST and SHORE (CSIR) are gratefully acknowledged
文摘Hogbomite,a rare exotic mineral,is found to be associated with the vanadiferous-titaniferous （V-Ti） bearing magnetite bands at Bhakatarhalli,Nuggihaifi greenstone belt,western Dharwar Craton,India.We report on a second occurrence of hogbomite from the Dharwar craton in Karnataka,which is the sixth documented occurrence of this mineral from India.We evaluate the chemical characteristics of hogbomite and associated Fe-Ti-minerals in an attempt to identify its formation as a primary hydrothermal mineral in a metamorphosed magnetite layer.We report here the presence of hogbomite as a complex oxide of Fe,Mg,Al and Ti with accessory of Zn,V and Sn.Petrographic studies suggest the （V-Ti） bearing magnetite （Mt） contain spinel,hogbomite,chlorite,martite,ilmenite （Ⅱ） and minor amounts of diaspore.The hogbomite displays euhedral to subhedral textures,and is up to 250 μm along the grain boundaries of magnetite and ilmenite.In the samples studied,hogbomite is prismatic,irregular and elongated in shape.The genesis of hogbomite in veins between magnetite and ilmenite implies its precipitation from fluids without involving complicated reactions.Several models were proposed for the formation of hogbomite; however,the subject is still debatable.
基金supported by the projects of the CSIR-Emeritus ScientistINDEX Project from the Council of Scientific and Industrial Research (CSIR) to the National Geophysical Research Institute to Chakravadhanula Manikyamba+1 种基金the SERB-Research Scientist Project (No. SB/SRS/2019-20/27/EA) to Sohini Gangulythe DST INSPIRE fellowship for pursuing the Ph.D. programm at NGRI to Arijit Pahari。
文摘This study presents a comprehensive account of the petrogenetic and geodynamic evolution of the Bellara Trap volcanic rocks from the Ingaldhal Formation, Chitradurga Group, western Dharwar Craton(WDC). Geochemical attributes of these rocks are consistent with two groups with distinct evolutionary trends: one comprising tholeiitic, MORB(mid-ocean ridge basalt) type basalts(BTB) and the other corresponding to calc-alkaline andesites(BTA). Basalts are essentially composed of clinopyroxene and plagioclase whereas the andesites are porphyritic with phenocrysts of plagioclase, clinopyroxene and polycrystalline quartz embedded in a groundmass of K-feldspar, quartz and opaques. Primary igneous mineralogy is overprinted by greenschist facies metamorphism resulting in chlorite-actinolite-plagioclase assemblage. The BTB samples reflect nearly flat REE patterns with weak LREE enrichment in contrast to pronounced LREE enhancement over HREE discernible for BTA. Tectonically, the BTB samples correspond to an active mid-oceanic ridge-rift setting with a MORB composition, whereas a back-arc basin(BAB) regime is corroborated for the BTA samples fractionating from back-arc basin basalts. Geochemical imprints of subduction input are more pronounced in BTA compared to BTB as mirrored by their elevated abundances of incompatible fluid mobile elements like Ba, Th, U and LREE. The BTB is endowed with an N-to E-MORB signature attributable to minor contributions from subduction-related components at the inception of a back-arc basin in the vicinity of an active subduction system. The BTA derived through differentiation of a basaltic magma with BABB(back-arc basin basalt) affinity compositionally akin to a heterogeneous source mantle carrying depleted MORB-type and enriched arc-type components inducted with progressive subduction. The BABB-type andesites and MORB-type basalts from Bellara Traps record a compositional heterogeneity of mantle in an intraoceanic arc-back arc system. Mantle processes invoke a BABB-MORB spectrum with a MORB-like endmember and an arc-like endmember associated with a juvenile back-arc basin. This study infers a Neoarchean analogue of Mariana-type back-arc rift setting proximal to the arc with a gradual transition from anhydrous to hydrous melting processes synchronized with MORB-mantle and arc-mantle interaction during initiation of a nascent back arc adjacent to the arc. The MORB-BABB compositional spectrum for the Bellara Traps conforms to a Neoarchean back-arc basin that evolved under an extensional tectonic regime associated with incipient stages of back-arc rifting and incorporation of subduction-derived components in the mantle output. This study complies with Neoarchean intraoceanic accretionary cycle plate tectonics in WDC.
文摘The Dharwar Craton in Peninsular India was intruded by a series of mafic dykes during the Paleoproterozoic and these mafic magmatic events have important implications on continental rifting and LIPs.Here we report ten precise Pb-Pb TE-TIMS age determinations on baddeleyite grains separated from seven mafic dykes and three sills,intruding into Archean basement rocks and Proterozoic sedimentary formations of the Eastern Dharwar Craton respectively.The crystallization age of the baddeleyite shows 2366.3±1.1 Ma,and 2369.2±0.8 Ma for the NE-SW trending dykes,2368.1±0.6 Ma,2366.4±0.8 Ma,2207.2±0.7 Ma and 1887.3±1.0 Ma for the ENE-WNW to E-W striking dykes,1880.6±1.0 Ma,1864.3±0.6 Ma and 1863.6±0.9 Ma for Cuddapah sills,and 1861.8±1.4 Ma for the N-S trending dyke.Our results in conjunction with those from previous studies identify eight distinct stages of widespread Paleoproterozoic magmatism in the Dharwar craton.The mantle plume centres of the four radiating dyke swarms with ages of^2367 Ma,~2210 Ma,~2082 Ma,and^1886 Ma were traced to establish their proximity to the EDC kimberlite province.Though the^2367 Ma and^1886 Ma plume centres are inferred to be located to the west and east of the present day Dharwar craton respectively away from the kimberlite province,location of plume heads of the other two swarms with ages of^2207 Ma and^2082 Ma are in close proximity.In spite of the ubiquitous occurrence of dyke intrusions of all the seven generations in the kimberlite province,only few of these kimberlites are diamondiferous.Kimberlite occurrences elsewhere in the vicinity of older Large Igneous Provinces(LIPs)like the Mackenzie,Karoo,Parana-Etendeka and Yakutsk-Vilui are also non-diamondiferous.This has been attributed to the destruction of the lithospheric mantle keel(that hosts the diamonds)by the respective mantle plumes.The diamondiferous nature of the EDC kimberlites therefore suggests that plume activity does not always result in the destruction of the mantle keel.
文摘Gold mineralization at G. R. Halli is located along the Central shear zone of Chitradurga schist belt, extending from west of Gonur, through east of G. R. Halli and C. K. Halli to east of Honnemardi, roughly parallel to stratigraphic units. The NNW-SSE trending shear zone has a width of 0.5 to 1.5 km shows extensive carbonatization of metabasalts and the associated lithologies confined to NNW-WNW trending arcuate brittle-ductile zone. The sheared and silicified contact zones between carbonaceous argillite and schistose metabasalt form the potential sites for localization of mineralization. The gold is associated with sulphides mainly pyrite, arsenopyrite, galena, sphalerite, minor chalcopyrite. Textural relationship indicates two stage sulphide mineral assemblages co-relatable with two stage fluid ascents having temperature of homogenization between 125°C and 256°C. It is a typical epigenetic lode gold system, which got affected by later deformation.
文摘The Peddavura greenstone Linear Belt, NW-SE trending, is formed in the Eastern part of the Dharwar Craton of south India, extended over 62.5 sq•km in Nalgonda and Guntur districts region. The entire belt is illustrated as Peninsular Gneiss. The Belt Study has attracted geologists for conducting further research to evaluate the crust forming process at the time of early volcanic eruption of Earth’s history. The South Indian Dharwar Craton depicts and exposes the crustal segments where geological activities took place consistently during the Precambrian. The PSB (Peddavura Schist Belt) mostly consists of meta volcanic (meta basalts), amphibolites, granites, dolerites, basaltic andesites, pegmatite and Banded Magnetite Quartzite’s (BMQ) rock types. The 20 represented rock samples made for thin section studies. Based on the Petrological studies minerals are showing uralitization, saussuritization in the granite with mylonite structures, perthite and dolerite are showing heavy metal such as rutile and other opaque minerals (Magnetite, hematite, and typical pyrite crystal) are present in different represented rock samples. The minerals are showing different alteration zones along with microstructures. Using the Petrological studies the minerals and rock types are identified in the study region.
文摘The late Archean Dharwar Craton is an important part of the Archean and Proterozoic terrains in Peninsular India.Dharwar Craton consists of Western and Eastern Blocks,separated by the Chitradurga Shear Zone.Eastern
文摘The dykes intruding the Hyderabad Granitic Region(HGR)which forms the part of eastern Dharwar Craton extending between northern and northwestern margins of the Cuddapah Basin and western margin of the Pakhal
