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B. Chadwick ¹, M. Ramakrishnan ², M. N. Viswanatha ³

Affiliations
1 Department of Geology, University, Exeter EX4 4QE, GB
2 Geological Survey of India, Hyderabad 500001, IN
3 Geological Survey of India, Jayanagar IV Block, Bangalore 560011, IN

Abstract

Late Archaean t·ectonic LS fabrics in quartz-pebble conglomerates and grits in the lower part of the Dharwar Supergroup are defined by micas and the preferred planar orientation of long (X) and intermediate (Y) axes of ellipsoidal clasts of vein quartz. Long axes of clasts define the linear component ofthe fabric. With increasing strain the grits become L tectonites. Pressure solution and intragranular processes were important at lower strains, but dislocation creep was dominant at higher strains related to the LS fabric development.

Dharwar phyllites and polymict conglomerates have a bedding-parallel S fabric of phyllosilicates and detrital grains of quartz. This fabric is attributed to neocrystallization and recrystallization of phyllosilicates coplanar with a primary bedding-parallel fabric during diagenesis and subsequent burial and regional heating deep within the Dharwar volcanosedimentary pile. Coeval, domainal and penetrative tectonic LS fabrics, with transitions from domainal to penetrative types, which formed contemporaneously with LS fabrics in the grits were superimposed on the bedding-parallel fabrics. Microbuckling and pressure solution dominated early stages of the cleavage development, but they were overtaken by coplanar growth of biotite in the later stages. Discordant prisms of tourmaline and blue-green amphibole, radiating intergrowths of chlorite and quartz and octahedra of magnetite and their pseudomorphs of quartz indicate that grain growth took place during the low-grade metamorphism that outlasted the late Archaean deformation.

Transitional crenulation to penetrative S fabrics, locally with a linear component coaxial with folds, were superimposed on an older tectonic S fabric broadly parallel to bedding in the Sargur quartzites which form part of the basement immediately below the Dharwar Supergroup. The younger fabrics are correlated with the late Archaean tectonic fabrics in the Dharwar grits and phyllites. The older fabric, which includes prisms of kyanite, is believed to be related to pre-Dharwar deformation and medium- to high-grade metamorphism. Whilst microbuckling, pressure solution and dislocation deformation processes were important in the development of the younger fabric, their role in the development of the older fabric is not clear because of the superimposed effects of the later processes. Discordant chloritoid grown across pre-Dharwar garnet. staurolite and kyanite and their low-grade alteration products in the Sargur quartzites suggests grain growth took place during low-grade metamorphism that outlasted the development of the younger fabric, a relation identical to that in the Dharwar grits and phyllites.

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B. Chadwick ¹, M. Ramakrishnan ², M. N. Viswanatha ³

Affiliations
1 Department of Geology, University, Exeter EX4 4QE, GB
2 Geological Survey of India, Mukhramjahi Road, Hyderabad 500 001, IN
3 Geological Survey of India, Jayanagar IV Block, Bangalore 560 011, IN

Abstract

The stratigraphy of the volcanosedimentary rocks of the late Archaean Dharwar Supergroup that make up the Bababudan basin is described in terms of six lithostratigraphic divisions, in ascending order, the Kalasapura, the Allampur, the Santaveri, the Mulaingiri, the Jagar and the Mundre Formations, the Jagar and the Mundre Formations being lateral equivalents. The three lower formations, each varying in thickness from a few metres to about 2000m, are dominated by metabasaItic rocks with local cross-bedded quartzites, including the basal quartz-pebble Kartikere Conglomerate which unconformably overlies the basement Peninsular Gneiss (c. 3100 Ma) in the south; the western, northern and eastern boundaries of the basin arc steep faults. The Mulaingiri Formation (200-1500m) is dominated by banded ferruginous cherts interbedded with phyllites, the Jagar Formation (? up to 2000m) comprises poorly exposed metabasaltic rocks and phyllites on the west of the basin, and the Mundre Formation, restricted to the northeast, contains the KaIdurga Conglomerate with a lower division dominated by clasts of Dharwar cover and an upper division dominated by clasts of Peninsular Gneiss basement: the conglomerate passes laterally into phyllites and crossbedded quartzites. The Mundre Formation is intruded by small bodies of pre- or syn-tectonic granite s.l.

The lithostratigraphic formations thicken from west to east indicating greater subsidence of the basin in the east, with the thickest accumulations marked by the Kaldurga Conglomerate (7 up to 5000 m) in the northeast. Sedimentary facies suggest that relatively shallow, intertidal or nearshore, marine environments persisted throughout most of the depositional and volcanic phases of basin development with banded ferruginous cherts and interbedded phyllites accumulating in areas (also relatively shallow ?) starved of terriginous detritus. The Kaldurga Conglomerate built out as a composite, shallow marine and alluvial cone into a rapidly subsiding part of the basin, detritus being provided by erosion of uplifted cover and basement to the north and northeast.

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B. Chadwick ¹, M. Ramakrishnan ², M. N. Viswanatha ³

Affiliations
1 Department of Geology, University, Exeter EX4 4QE, GB
2 Geological Survey of India, Mukhramjahi Road, Hyderabad 500 001, IN
3 Geological Survey of India, Jayanagar, IV Block. Bangalore 560 011, IN

Abstract

The boundary between the Dharwar volcanosedimentary rocks and their basement (Peninsular Gneiss, c. 3100 Ma, with tracts and enclaves of the Sargur supracrustal association) on the south of the Bababudan basin is an unconformity, faulted and steepened in the southeast by inclined folds verging to the south. These folds pass east into a complex area of neulral folds and faults. Boundaries between the Dharwar cover and its basement on the west, north and east of the basin are steep or vertical faults. Basement gneisses within 2-3 km of the eastern and western boundary faults are intensely schistose or mylonitic with L and LS fabrics identical to those in the cover rocks. The S fabrics in the cover and basement in the east of Bababudan are steep or vertical and associated L fabrics have variable, gentle to moderate plunges to the north or south. Displacement on the eastern and western boundary faults appears to have been dominated by uplift of the basement and shortening at a high angle to the S. fabrics. Movements on the northern boundary fault gave rise to mylonites and tectonic melanges. in the basement and cover and took place before movements on the eastern and western boundary faults and folding in the northeast (Kaldurga syncline and the Nandi arch) and northwest (Lakkavalli syncline) of the basin. Further faulting (Tarikere Fault) related to basement uplift in the northeast occurred during or after folding in the cover.

The structure within the basin is dominated by steep faults and upright open folds with strongly curved hinge lines within steep axial surfaces. The Bababudan syncline is an open east-west fold which curves gently westwards into a tight cusp trending north parallel to the western boundary fault. East of the syncline the structure is dominated by the upright open arch and complex saddle zone of the Santaveri anticline. LS fabrics associated with the Bababudan syncline, the Santaveri anticline and other major folds within the basin have widely variable trends but their continuity from one structure to another suggests the folds, fabrics and faults formed contemporaneously with synchronous refolding that gave rise to the curved axial surfaces such as that of the Bababudan syncline. Synchronous refolding and rapid variations in style of small-scale folds are especially common in the multilayer complexes of banded ferruginous cherts. Whilst fabrics are mostly of LS type, L tectonites indicative of intense constrictional strains are prominent in the east. Mineral lineations and elongated clasts and amygdales forming the L fabric arc commonly parallel to fold axes.

The geometrical relations between upright folds, steep faults and LS fabrics and the effects of synchronous refolding suggest the Dharwar volcanosedimentary rocks deformed in response to constriction by segments of basement rising on all sides of the Bababudan basin, the rise beginning in the norlh. Uplift of basement segments with widespread, pervasive retrogression related flushing of fluids, including CO₂, through myriad cracks and narrow shear zones, especially north of the Bababudan basin, may have taken place as part of a regional thermal event manifested at the present level of outcrop by the Arsikere and Banavara granites to the east. The case for tectonic thickening by thrusts and nappe emplacement reported in Archaean terranes elsewhere has not been proved in the Bababudan basin.

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B. Chadwick ¹, N. H. G. Garrioch ², M. Ramakrishnan ³, M. N. Viswanatha ⁴

Affiliations
1 Department of Geology, University of Exeter, Exeter EX4 4QE, GB
2 Department of Geology, University of Exeter, Exeter EX4 4QE, IN
3 Geological Survey of India, Hyderabad 500001, IN
4 Geological Survey of India, Bangalore 560011, IN

Abstract

Assemblages rich in magnesioriebeckite, aegirine and magnesian siderite (var. pistomesite) characterise parts of the Banded Iron-Formation low in the Mulaingiri Formation of Bababudan, Electron microprobe analyses show that aegirine contains up to 53.29 wt% Si0₂, 0.17 wt% A1₂0₃, 32.20 wt% TotFeO, 0.23 wt% MgO and 14.09 wt% Na₂0; and magnesioriebeckite contains up to 55.57 wt% Si0₂, 0.07 wt% Al103, 29.16 wt% TotFeO, 8.53 wt% MgO and 7.37 wt% Na₂0. These abundances are broadly in accord with previously published results from wet analytical techniques.

A diagenetic or very low-grade metamorphic texture of randomly oriented needles of magnesioriebeckite and granoblastic-polygonal quartz in chert mesobands is transitional into a texture dominated by long prisms of magnesioriebeckite intergrown with elongate grains of quartz which forms an LS tectonic fabric. Aegirine and magnesian siderite have locally replaced magnesioriebeckite in this fabric. The carbonate also forms small ring structures, interpreted as deformation phenomena, in parts of some mesobands.

The pre- and syntectonic textures of magnesioriebeckite are regarded as the products of percolating solutions rich in Na⁺ and Mg⁺⁺ interacting with the host during diagenesis, very low-grade burial metamorphism and progressive deformation. The tectonic LS fabric appears to have been initiated early during deformation with textural modifications taking place during buckle shortening and layer-parallel shearing.

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B. Chadwick ¹, V. N. Vasudev ², S. Jayaram ³

Affiliations
1 Department of Geology, University, Exeter EX4 4QE, GB
2 Department of Mines and Geology, Government of Karnataka, Gandhinagar, Bellary 583 101, IN
3 Department of Mines and Geology, Government of Karnataka, Bangalore 560 001, IN

Abstract

The Dharwar Supergroup in the Shimoga basin east of Bhadravathi has been subdivided into seven formations on the basis of sedimentary structures and facies. The lowest is dominated by metabasites including metalavas and tuffs, metagabbros with seams of titaniferous magnetite, serpentinites and talc schists. A range of sedimentary rocks including phyllitic greywackes, quartzites, dolomitic limestones, conglomerates and local banded ferruginous cherts characterise the formations overlying the metabasites. These formations include local metabasites, and the youngest formation contains a thick suite of metarhyolites. The Dharwar lithostratigraphy is characterised by rapid lateral variations in thickness and facies which are the consequence of its original unstable alluvial and relatively shallow marine depositional environments. The instability is marked by slump structures in the phyllitic greywackes and by debris flows at many levels in the stratigraphy. The Dharwar rocks were deposited on a basement of homogeneous granodiorite s.l. and foliated multiphase quartzo-feldspathic orthogneisses. Erosion of elevated parts of the basement and the Dharwar cover provided detritus for alluvial fans, shallow marine quartzites and more distal phyllitic greywackes. Correlation of the lowest formations in tbe Bababudan basin immediately to the south shows that the Dharwar Supergroup becomes progressively younger from south to north in this part of Karnataka.

The structure is characterised by large upright synclines in the Dharwar cover with intervening domal areas of basement. Many of the cover-basement contacts are faults, but some are interpreted as rotated and steepened unconformities. Flushing of water and CO₂ through much of the basement and cover took place during deformation. Buoyancy forces related to heating at deep crustal levels and concomitant compressional forces, generated perhaps in a transpressive oblique-slip regime, may account for the structure of the cover and its basement. Comparable tectonothermal instability, but with trans-tensile components, may have characterised the Dharwar volcanic and depositional phases.

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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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A. P. Nutman ¹, B. Chadwick ², M. Ramakrishnan ³, M. N. Viswanatha ⁴

Affiliations
1 Research School of Earth Sciences, Australian National University, GPO Box 4, Canberra ACT 2601, AU
2 Earth Resources Centre, University, Exeter EX4 4QE, GB
3 Geological Survey of India, 2 Church Street, Bangalore 560 001, IN
4 17 Rajamahal Vilas Extension, Bangalore 560080, IN

Abstract

New limits have been set on the age of the provenance and the depositional period of the oldest known Archaean supracrustal rocks (Sargur Group) in southern India. Detrital zircon grains from a pelitic schist and a quartzite within major tracts of supracrustal rocks older than their host regional grey orthogneisses (peninsular Gneiss, c. 3000-2900 Ma) have yielded U-Pb ages in the range 3580-2960 Ma. The data indicate that granitoid rocks in the age range 3580-3130 Ma were a significant component of the provenance of the sedimentary protoliths. Ages younger than 3130 Ma are attributed to effects of high-grade metamorphism during emplacement of the igneous precursors to the host orthogneisses. Exhumation of the granitoid provenance, deposition of the sedimentary protoliths, intrusion of major gabbroic and peridotitic complexes and possible basaltic volcanism represented by amphibolites in the tracts of supracrustal rocks took place in the period 3130-2960 Ma. This age range is at variance with previous suggestions that the Sargur Group represents early Archaean or primitive crust.

Keywords

Geochronology, Zircon-Dating. U-Pb Ages, Sargur Supracrustals, Karnataka.

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A. P. Nutman ¹, B. Chadwick ², M. Ramakrishnan ³, M. N. Viswanatha ⁴

Affiliations
1 Research School of Earth Sciences, Australian National University, Canberra, ACT 2601, AU
2 Earth Resources Centre, University Exeter EX4 4QE, GB
3 Geological Survey of India, Bandlaguda, Hyderabad 500 660, IN
4 17, Rajamahal Vilas Extension, Bangalore 560 080, IN

Abstract

No Abstract.

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