Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

V. N. Vasudev ¹, Brian Chadwick ², Allen P. Nutman ³, G. V. Hegde ⁴

Affiliations
1 120/45(A) III Block, Thyagrajanagar, Bangalore 560 028, IN
2 Earth Resources Centre, University, Exeter EX44QE, GB
3 Research School of Earth Sciences, Australian National University, Canberra ACT0200, AU
4 Department of Mines and Geology, Government of Karnataka, 16/3-5 S. P. Complex, Lalbagh Road, Bangalore 560 027, IN

Abstract

The Palkanmardi conglomerate is one of many polymict conglomerates interbedded with greywackes in the NE of the Hutti schist belt. These conglomerates are up to a few metres thick, unsorted and include rounded to sub-angular clasts of granodiorite, granite, vein quartz and rnetabasalt in a matrix of coarse-to medium-grained greywacke. Cross-bedding, convolute bedding and grading are well preserved in the interbedded greywackes in spite of deformation and homfelsing during Late Archaean regional high temperature - low pressure metamorphism. These primary structures and lack of sorting in the conglomerates are consistent with deposition as turbidites and debrites in an unstable submarine environment. This new interpretation contrasts with previous views that the Palkanmardi conglomerate is autoclastic, pyroclastic-detrital or glacio-fluvial. The conglomerate-greywacke sequence occurs low in the lithostratigraphy and is overlain by metabsalts.

A clast of granodiorite in the conglomerate bed at Palkanmardi village has yielded a SHRIMP weighted mean ²⁰⁷Pb/²⁰⁶Pb zircon age of 2576±12 Ma which is interpreted as the magmatic age of the erosional provenance. Moreover, SHRIMP zircon geochronology using a sample from the steep elongate wedge of granodiorite that extends for at least 150 km SE of the schist belt has yielded a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 2561±24 Ma and a concordia upper intercept age of 2580±31 Ma. These ages are indistinguishable within their errors and are interpreted as the age of magmatic crystallisation. Combined with the low stratigraphic position of the Palkanmardi conglomerate, the age data imply that basin development (volcanism and sedimentation) in the Hutti belt was not only rapid, but began very late in the Archaean history of this part of the Dharwar craton.

Keywords

Late Archaean, Hutti Schist Belt, SHRIMP Ages, Intra-Arc Basin, Dharwar Craton.

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B. Chadwick ¹, V. N. Vasudev ², B. Krishna Rao ³, G. V. Hegde ²

Affiliations
1 Earth Resources Centre, University, Exeter EX44QE,, GB
2 Department of Mines and Geology, Government of Karnataka, 16/3-5 S.P. Complex, Lalbagh Road, Bangalore 560027, IN
3 Department of Geology, University of Mysore, Manasa Gangotri, Mysore 576 006, IN

Abstract

The Dharwar sedimentary and volcanic rocks adjacent to their basement granodiorites and gneisses in the Honnali dome can be interpreted in terms of three principal lithostratigraphic units. Each unit represents a distinct stage of basin development. The lowest unit is formed by the mainly shallow marine metabasite-orthoquartzite association of the Bababudan Group. It marks an early extensional phase of basin development. The group in the Honnali area has been subdivided into the Kudrekonda and Kalva Rangan Durga Formations. The second unit comprises mainly shallow marine sedimentary rocks, including polymict conglomerates and limestones, with intercalations of basic to acid volcanic rocks. The lithological association indicates unstable depositional and volcanic conditions with variable uplift and subsidence during the second stage of basin development. The unit is equivalent to the lower part of the Chitradurga Group and it has heen subdivided into the Musinhal, Adrihalli, Aleshpur, Medur and Daginkatte Formations. The acid volcanic rocks forming the Daginkatte Formation were formerly regarded as rhyolitic flows, but they are reinterpreted as ash flow deposits on the grounds of their textures. The third unit corresponds to the upper part of the Chitradurga Group. It begins with the thin, but persistent Basavapatna Formation of banded ferruginous cherts and interbedded carbonaceous phyllites. This formation is overlain by the re-defined Ranibennur Formation of greywackes and local volcanic intercalations. Whilst a marine setting with initial widespread quiescent conditions is indicated by the third unit, its significance is uncertain because of the lack of data from the large, but poorly exposed tract of greywackes.

The Dharwar rocks are dominated by ductile structures in the form of south or southwesterly verging inclined folds with superimposed, steep north-trending folds. In contrast, the basement rocks in the Honnali dome deformed cataclastically on myriad fractures and retrograde shear zones. The dome has a faulted southern boundary, whilst a steepened unconfonnity forms the remaining parts of the boundary. The contrasting structures in the cover and the basement may be modelled in terms of crustal shortening propagating from the north ornortheast on listric faults dipping northeast through the basement into a deep detachment. Later strike-slip displacements on steep NS faults and shear zones modified the south-verging structures. Alternatively, the structure may be interpreted in terms of jostling of basement blocks within a zone of transpression related to sub-horizontal displacements on NS faults and shear zones.

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Brian Chadwick ¹, V. N. Vasudev ², G. V. Hegde ³, A. P. Nutman ⁴

Affiliations
1 17 St Mary's Park, Ottery St Mary EX 11 IJA, GB
2 Australian Indian Resources Group Companies, 125/45, 3rd Block, 6th Cross, 6th Main, Tyagarajanagar, Bangalore-560 028, IN
3 Department of Mines and Geology, Government of Karnataka, "Khanija Bhavan", Race Course Road, Bangalore 560001, IN
4 Research School of Earth Sciences, Australian National University, Canberra ACT 0200, AU

Abstract

Neoarchaean granites adjacent to the Chitradurga schist belt were emplaced in the inner margin of the foreland in the context of the Neoarchaean oblique convergent setting of the Dharwar craton. Two previously unreported granites, one 50 km and the other 80 km NW of Chitradurga town, and a mylonitised granite in the hanging wall of a duplex in the NW of the schist belt yielded SHRIMP U/Pb zircon emplacement ages of 2648±40 Ma, 2598±19 Ma, and ca. 2600 Ma, respectively, the large errors being due to radiogenic Pb loss during an unidentified Neoproterozoic event. Some discrete zircon grains and xenocrystic cores yielded ≥3000 Ma ages that were derived from older rocks during anatexis or emplacement. The granites NW of Chitradurga town were emplaced as steep sheets trending NW·SE. The Chitradurga granite has a similar form, bifurcating N of Chitradurga town into two separate, steeply dipping, NW-SE sheets. Magmatic-and solid-state fabrics in these granites show that emplacement took place during, but was outlasted by, sinistral and dextral strike-Parallel shear. Emplacement of the granite above the hanging wall of the duplex in the NW of the schist belt was outlasted by top-SW displacement.

The shapes of the granites and their emplacement in relation to the structure of the Ranibennur and Chitradurga schist belts in the west of the craton are modelled as a mid-Crustal part of a craton-Wide imbricate fold-Thrust belt. The relationships show that whereas some Neoarchaean granites in the craton were emplaced prior to, or during, SW-vergent thrust thickening, most granites and related plutonic suites in the foreland and accretionary complex were emplaced later as multipulse injections in steep NW-SE sheets or wedges during orogen-parallel. sinistral and dextral shear. Steep highstrain zones in the foreland and accretionary complex are interpreted as listric structures that ischolar_main into an attachment at a depth of ca. 18-20km in accord with the depth of the boundary between upper and lower crust placed at ca. 23 km from seismic reflection data published in 1979 and in more recent studies. The new structural observations and zircon dating, combined with published isotopic age data, show that the inner margin of the foreland in the west of the craton and the outer margin of the accretionary complex in the east are linked in a diffuse, steeply dipping, Orogen-parallel boundary zone at least 200 km wide.

Keywords

Neoarchaean, Dharwar Craton, Fold-Thrust Belt, Transpressive Orogen, Oblique Convergence, Attachment, Chitradurga Schist Belt, Karnataka.

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G. V. Hegde ¹

Affiliations
1 Department of Mines and Geology, Groundwater Survey Unit, Dharwad - 580 001, IN

Abstract

The chemical quality of groundwater resource of Dharwad district has been evaluated by considering 129 pre-Monsoon water samples collected from dug wells and bore wells. The study area has been divided into the eastern and western parts on the basis of occurrence of black, and red-Loamy-Sandy soils respectively. The total dissolved salts (TDS) vary from 222 to 1624 mg/l in the western part, while it ranges between 310 and 11684 mg/l in the eastern part. While the groundwater of the western part is "potable and suitable for irrigation", it is "brackish and unsuitable for irrigation" in the eastern part, barring few isolated pockets Na-Ca and Cl-SO₄-HCO₃ facies predominates the groundwater of the western part, while Ca-Na and Cl-SO₄ facies predominates the eastern part. The TDS isogram map has disclosed that the black soils and the weathered granitic and gneissic rocks with mafic enclaves may be the causative factors for the increased brackish quality of groundwater of the eastern part. The study has also indicated that the role of base exchange phenomenon and the rock-Water interaction in the weathered zone due to longer residence time of groundwater causing increased brackishness of groundwater in the study area.

Keywords

Hydrochemistry, Groundwater, Locality-Wise Evaluation, Dharwad District, Karnataka.

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Brian Chadwick ¹, V. N. Vasudev ², G. V. Hegde ³

Affiliations
1 Pennsylvania Close, Exeter EX4 6DJ, GB
2 Geomysore Services (India) Pvt. Ltd., 120/45(A), III Block, 6^th Cross, 6^th Main, Thyagarajanagar, Bangalore - 560 028, IN
3 Department of Mines and Geology, Government of Karnataka, "Khanija Bhavana", Race Course Road, Bangalore - 560001, IN

Abstract

The Late Archaean Dharwar Supergroup in the Chitradurga schist belt NW of Tungabhadra river is dominated by low-grade metabasalts, polymict conglomerates, ferruginous cherts and greywackes with intercalations of dolomitic limestone and felsic volcanic rocks. The belt was thickened as the result of overturning on large-scale folds and stacking on reverse faults in a hinterland-dipping duplex, herein called the Gadag duplex. Gold mineralisation is confined to, and was controlled by, the duplex structure. In the Late Archaean oblique convergent setting of the Dharwar craton, the duplex dips NE away from the foreland formed by the western part of the craton and into the accretionary complex of granites (Dharwar batholith) and intra-arc schist belts comprising the eastern part.

The Gadag duplex comprises four thrust slices. Its foot wall block comprises metabasalts and greywackes of the Dharwar Supergroup and underlying, variably mylonitised granites s.l. with magmatic-state and mylonite fabrics indicating top-NW shear sense. The supergroup in the foot wall block is arched over domes of these granites to link with the lowest metabasalts and greywackes in the eastern margin of the Ranibennur schist belt. Lit-par-lit injections in the metabasalts suggest the plutonic rocks are younger than the Dharwar Supergroup, i.e. they are probably Late Archaean, not pre-2900 Ma basement to the schist belt. Arching of the foot wall block is attributed to NE-SW shortening broadly contemporaneous with, or soon after, thrust stacking in the Gadag duplex.

The hanging wall block comprises mylonitised granites s.l. with top-SW shear sense indicators. The roof thrust south of Gadag dips gently NE, but steepens further SE. Lit-par-Ut injections in metabasalts adjacent to the roof thrust indicate a tectoni sed intrusive contact like that in the foot wall block. Large-scale gentle folding of the shallow part of the roof thrust is linked to sinistral displacement along the steepened thrust. Plutonic rocks in the hanging and foot wall blocks are tentatively correlated with granites in the western margin of the Dharwar batholith.

Keywords

Late Archaean, Duplex Structure, Oblique Convergence, Gold Mineralization, Chitradurga Schist Belt, Dharwar Craton, Karnataka.

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Brian Chadwick ¹, G. V. Hegde ², Allen P. Nutman ³, V. N. Vasudev ⁴

Affiliations
1 12 Pennsylvania Closc, Exeter EX4 6DJ, GB
2 Department of Mines and Geology, Govt. of Karnataka, Khanija Bhavana, Race Course Road, Bangalore 560 001, IN
3 Research School of Earth Sciences, Australian National University, Canberra ACT 0200, AU
4 Australian-Indian Resources Pvt. Ltd., 120/45(A) 111 Block, Thyagarajanagar, Bangalore 560 028, IN

Abstract

The predominantly syenitic Koppal pluton was emplaccd in the western flank of the Dharwar batholith which forms the eastern half of the Dharwar craton. Previous work has shown that the batholith accreted against an older continental foreland reprcsented by the western half of the craton during Late Archacan oblique convergence. A sample of syenitc has yielded a SHRIMP U/Pb zircon age of 2528±9 Ma which is interpreted as the timc of emplacement at a late stage in the accretion of the batholith. Its age and composition make the Koppal pluton a unique feature of the Dharwar craton. Melt commingling of syenitc with other intcrrnediate to basic magmas is indicated by commmon mafic enclaves, disrupted mafic sheets and larger netveined bodies rich in hornblende and pyroxene. The arc setting of the Koppal pluton is consistent with syenitic plutons in other Late Archaean terrains.

The pluton has an elliptical outcrop trending NE-SW with an upright, asymmetrical funnel shape indicated by its magmatic-state LS fabrics and diffuse compositional banding. Banding is a function of variations in microclinc, hornblende and clinopyroxene abundances. Thc pluton was ernplaced broadly parallel to diffuse magmatic banding in the host granites which lies oblique to trends of the steep NW-SE belts of plutonic rocks that characterise the Dharwar batholith as a whole. This oblique orientation of magmatic banding has been interpreted previously as the result of emplacement when magmatic pressure exceeded regional compressive stress during sinistral displacement on steep shear zones trending NW-SE. Emplacement of the Koppal pluton is interpreted in a similar way.

Keywords

Late Archaean, Syenitc, SHRIMP U/Pb Age, Koppal, Dharwar Craton.

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Brian Chadwick ¹, G. V. Hegde ², V. N. Vasudev ³

Affiliations
1 Pennsylvania Close, Exeter EX4 6DJ, GB
2 Department of Mines and Geology, Government of Karnataka, 49 Khanija Bhavan, Race Course Road, Bangalore - 560 001, IN
3 120/45(A), 111 Block, Thyagarajanagar, Bangalore - 560 028, IN

Abstract

No Abstract.

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P. Krishnamurthy , G. V. Hegde

Abstract

No Abstract.

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