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

Brian Chadwick ¹

Affiliations
1 Room 220, Laver Building University, Exeter 4QE 6DJ, GB

Abstract

No Abstract.

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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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Brian Chadwick ¹, M. Ramakrishnan ², M. N. Viswanatha ², V. Srinivasa Murthy ²

Affiliations
1 Department of Geology, The University, Exeter EX44QE, GB
2 Geological Survey of India, Karnataka Circle (South), Bangalore 560041, IN

Abstract

Structural studies in the linear supracrustal belts and adjacent Peninsular Gneiss in the areas of Sargur, Holenarsipur and Chitradurga have revealed Land S tectonites and related structures, which are attributed to phases of deformation SgDl, SgD2 and SgD3 in the older Sargur Group and DhDI, DhD2 and DhD3 in the younger Dharwar Supergroup. These phases gave rise to comparable patterns of strain in each group, the deformation taking place with similar timing relative to metamorphism. Metamorphic grade is higher (middle to upper amphibolite facies) in the Sargur Group and lower (upper greenschist to lower amphibolite facies) in the Dharwar Supergroup: the higher grade assemblages in the Sargur Group show some degree of retrogression. The structures and fabrics in both groups may either be of approximately the same age (SgD1=DhDl, SgD2=DhD2, SgD3=DhD3) or more probably, the SgDI, SgD2 and SgD3 structures and fabrics in the Sargur Group may be older and were rotated into parallelism with younger structures formed in the Dharwar Supergroup during phases of deformation DhD1 and DhD2 that gave rise to the present form of the linear belts of supracrustal rocks extending from Sargur to Chitradurga. Some aspects of the Archaean tectonometamorphic evolution of the Karnataka Craton are discussed in the light of these alternatives.

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Brian Chadwick ¹, M. Ramakrishnan ², M. N. Viswanatha ²

Affiliations
1 Department of Geology, University, Exeter EX4 4QE, GB
2 Geological Survey of India, Hyderabad 500001, IN

Abstract

The Dharwar Supergroup and its basement of Peninsular Gneiss and Sargur supracrustal rocks in the areas of Ghatti Hosahalli and southeast Bababudan display certain textural, structural and unconformable relations which have important implications for the Archaean chronology of the Karnataka craton. In the first. instance these relations show that certain tonalitic-granitic parental rocks of the Peninsular Gneiss basement to the Dharwar supracrustal rocks were formed as a series beginning with polyphase gneisses and ending with discordant plutons such as the Chikmagalur granite s.l. The Sargur rocks were deformed and metamorphosed to medium-high grade during intrusion of the polyphase gneisses. After cooling, uplift and erosion of the Peninsular Gneiss and the tracts and enclaves of Sargur rocks, the Dharwar supracrustal association was deposited unconformably on the medium-high grade basement. The pre-Dharwar metamorphic minerals in the Sargur rocks were partly retrogressed and then overprinted by a second major metamorphism, mainly low grade, whose climax was attained after the main deformation of the belts and basins of the Dharwar supracrustal rocks. This major low grade metamorphism in central Karnataka is correlated with the later Archaean high grade terrane (ca. 2600 Ma) in southern Karnataka and elsewhere in Peninsular India.

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Brian Chadwick ¹

Affiliations
1 Department of Geology, The University, Exeter EX4 4QE, GB

Abstract

No Abstract.

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Brian Chadwick ¹, V. N. Vasudev ², Nazeer Ahmed ³

Affiliations
1 Earth Resources Centre, University of Exeter, Exeter EX4 4QE, GB
2 120/45(A), 3rd Block, Tyagarajanagar, Bangalore - 560 028, IN
3 Department of Mines and Geology, 16/3-5, S.P. Complex, Bangalore - 560 027, IN

Abstract

Six formations are defined in a new lithostratigraphy of the volcanic and sedimentary rocks of the schist belt. The formations (Yeshwantnagar, Deogiri, Raman Mala, Donimalai, Taluru, Vibhuti Gudda) young consistently northeast, except for a part of the Taluru Formation in the northeast of the belt. We propose the new term Sandur Group to include these formations. Their combined thickness of c.35 km is in part an effect of thrust thickening. The Sandur Group was deposited in mainly shallow marine environments in a setting comparable with that of unstable mixed-mode basins. An incomplete, upright, synclinal sheath fold dominates the structure in the east of the schist belt, and another incomplete synclinal sheath fold occurs in the west, its hinge areas and northeastern limb having been cut out by a steepened thrust (the Sandur valley discontinuity). The margins of the belt were intruded by syntectonic, multipulse granites with magmatic and crystal-plastic solid-state linear and planar fabrics and the granite emplacement outlasted deformation. HT/LP metamorphism of the schist belt was contemporaneous with deformation and granite emplacement.

The granites are an integral part of the Late Archaean polyphase granite complex in eastern Karnataka and contiguous parts of Andhra Pradesh and Tamil Nadu, for which we propose the term Dharwar batholith. Published isotopic age data show that its western part includes anatectic granites and relies of Peninsular Gneiss (>2900 Ma), but the eastern part is dominated by juvenile granites. The batholith accreted onto the Archaean foreland to the west comprising Late Archaean marginal basins of the Dharwar Supergroup (Kudremukh. Bababudan-Ranibennur, Chitradurga-Gadag) and their basement of Peninsular Gneiss (>3000 Ma) during Late Archaean plate convergence with NE-SW shortening and sinistral transcurrent displacements. The steep linear belts (Kolar, Ramagiri, Kushtagi. etc.) and irregular tracts (Sandur, Hutti) of volcanic and sedimentary rocks ineastem Karnataka formed as intraarc basins above the evolving Dharwar batholith.

Keywords

Crustal Evolution, Stratigraphy, Structure, Intrusive Granites, Sandur Belt, Karnataka.

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A. P. Nutman ¹, Brian Chadwick ², B. Krishna Rao ³, V. N. Vasudev ⁴

Affiliations
1 Research School of Earth Sciences, Australian National University, A.C.T. 0200, AU
2 Earth Resources Centre, University of Exeter, Exeter EX4 4QE, GB
3 Department of Geology, University of Mysore, Mysore - 570 006, IN
4 120/45(A), III Block, Thyagarajanagar, Bangalore - 560 028, IN

Abstract

SHRIMP U/Pb dating of zircon grains from a schistose acid volcanic rock from the Daginkatte Formation in the Chitradurga Group of the Late Archaean Dhwar Supergroup in western Karnataka has yielded a precise concordant age of 2614 ± 8 Ma which indicates the time of melt crystallisation. In contrast, zircons from similar schistose acid volcanic rocks in the Vibhuti Gudda Formation of the Sandur Group in the Sandur schist belt of eastern Karnataka yielded less precise concordia intercept ages of 2658±14 and 2691±18 Ma.

Zircons from high-strain grey gneisses which appear to be the oldest recognisable component of the Late Archaean granite complex adjacent to the Sandur schist belt have an imprecise age of 2719 ± 40 Ma. Zircons from the youngest recognisable granite adjacent to the belt have also yielded an imprecise, but younger, age of 2570 ± 62 Ma. The ages show that granite emplacement and deformation adjacent to the Sandur schist belt took pIace in a period of c.150 Ma.

The imprecise ages of zircon in the acid volcanic rocks in the Sandur Group and the adjacent granites are related to Neoproterozoic loss of lead which may have been an effect of either weathering or a regional thermal event. We favour the latter in the light of the record of Pan-African thermal effects in the east and south of southern Peninsular India.

Keywords

Geochronology, Zircon, Acid Volcanics, Sandur Schist Belt, Karnataka.

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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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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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Brian Chadwick ¹

Affiliations
1 Department of Geology, The University, Exeter, GB

Abstract

No Abstract.

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