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

M. N. Viswanatha ¹, M. Ramakrishnan ², T. R. Narayanan Kutty ³

Affiliations
1 17, Raja Mahal Vilas Extension, Bangalore 560006, IN
2 63, Geetha Colony, Jayanagar, IV Block, Bangalore 560011, IN
3 Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, IN

Abstract

No Abstract.

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M. N. Viswanatha ¹

Affiliations
1 Geological Survey of India, Mysore Circle, Jayanagar, Bangalore 560011, IN

Abstract

No Abstract.

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M. Ramakrishnan ¹, M. N. Viswanatha ¹, N. Chayapathi ¹, T. R. Narayanan Kutty ²

Affiliations
1 17, Raja Mahal Vilas Extension, Bangalore-560006, IN
2 Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore-560012, IN

Abstract

Anorthosites of Karnataka occur in two distinct tectonic milieu, within a major arcuate zone, namely, (i) as minor differentiates of gabbro in the ultramafic-mafic complexes emplaced in Archaean high-grade supracrustal belts and the 'Dharwar type' greenstone belt of Shimoga; and (ii) as layered concordant sheets, and as cumulates in a layered basic pluton occurring within the gneissic complex of Archaean high grade terrain. Their absence in 'Keewatin type' greenstone belts and the other  'Dharwar type' belts of Bababudan and Chittadurga is probably due to their location away from the arcuate zone in Karnataka Craton. This arcuate zone reflects a progressive vertical zonation of the crust to deeper levels from northwest to southeast.

The anorthositic rocks of mafic-ultramafic complexes are nonlayered and extensively deformed, whereas those of the high grade terrain show layering and cumulus textures. In the transitional zone, the anorthositic rocks of mafic-ultramafic complexes show cumulus textures, thereby indicating a genetic connection between the anorthosites of two tectonic milieu.

Whole-rock chemistry of the anorthosites of Karnataka indicates differentiation trend of iron-poor environment comparable to other Archaean anorthosites. They are enriched in CaO and A1₂O₃ Niggli mg against Cr or Ni shows positive correlation, corresponding to an igneous parentage for the original rock. Similarly, the plot of Niggli c against mg, c against (al-alk) and 100 mg-c-(al-alk) diagrams exhibit a well defined igneous trend. A common differentiation trend is shown by anorthosites of Karnataka, even though they occur in two distinct tectonic milieu. The singularised trends support the view that the ultramafic-mafic complexes are possibly derived by the dismembering of the basal parts of the layered complexes. The Archaean anorthosites of Karnataka, although comparable broadly to lunar anorthosites, are not related to the early history of the Earth-Moon system.

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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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A. S. Ramiengar ¹, G. R. Devadu ¹, M. N. Viswanatha ¹, N. Chayapathi ¹, M. Ramakrishnan ¹

Affiliations
1 Geological Survey of India, Jayanagar, Bangalore-560011, IN

Abstract

A small enclave of ancient supracrustal rocks (Sargur Group) in Kamataka contains an interesting occurrence of fuchsite quartzite with layers of chromite and other heavy minerals like magnetite, tourmaline, rutile and zircon. The opaque and non-opaque heavy mineral suite is suggestive of a source region rich in sialic components with subordinate ultramafic rocks. This is consistent with the spatial association of these supracrustal rocks with a vast gneissic complex containing numerous enclaves of chromite-rich ultramafic rocks in the vicinity.

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A. S. Ramiengar ¹, M. Ramakrishnan ¹, M. N. Viswanatha ¹

Affiliations
1 Geological Survey of India, Karnataka (South) Circle, V Block, Jayanagar, Bangalore-560041, IN

Abstract

The charnockites of southern Karnataka occur at the fringes of the greenstonemigmatite terrain. The contacts of the individual pods and balls of charnockite with the associated migmatites are invariably diffuse. Replication of all types of migmatitic structures by the conformable charnockite masses and pods, and discordant 'tree-structure' of charnockites transecting the early rnigrnatitic fabric of the gneisses strongly suggest that the gneiss complex has been transformed in situ into charnockite without much attendant ductile deformation. The mineralogy of the transformation phenomenon reflects the transitional nature of the amphibolite-granulite facies boundary, rather than a craton-wide regional retrogression. These transformed charnockite buds probably represent nuclei of dehydration.

The major 2900-3000 m.y. thermal event in the gneiss complex, involving the high-grade supracrustal rocks, is correlatable with the progressive metamorphism of amphibolite to granulite facies in the region. This event probably culminated in the production of charnockite around 2600-2700 m.y.

The progressive metamorphism in Karnataka from green schist to granulite facies is shown to be the cumulative effect of polymetamorphism and regional synclinorial structure plunging northward. Post-Dharwar uptilting of the Peninsula has served to accentuate this vertical zonation. As a result deeper crustal levels are exposed southward, involving older supracrustal rocks. The high-grade terrain in Karnataka is, therefore, older than the greenstone terrain. The greenstones and granulites were not juxtaposed against each other because of their differing levels. Parts of the high-grade terrain later evolved into a mobile belt of charnockite-khondalite association. This mobile belt has been interpreted variously as geosynclinal orogenic belt, collision feature similar to the Circum-Pacific belts of the Himalayas, or as a transform zone. The continent-continent collision model is preferred.

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

Affiliations
1 Department of Geology, The University, Exeter EX4 4QE, GB
2 Geological Survey of India, Karnataka Circle (South), Bangalore 560041, IN

Abstract

No Abstract.

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M. N. Viswanatha ¹, M. Ramakrishnan ¹, J. Swami Nath ¹

Affiliations
1 Geological Survey of India, Hyderabad 500 001, IN

Abstract

The major angular unconformity between the Peninsular Gneiss and Dharwar Supergroup in Sigegudda belt, Karnataka, is now 'discovered' to separate the Sargur from Dharwar Supracrustals. It is the most critical area known so far in the craton to exemplify the Sargur-Dharwar relationship. In conformity with modern stratigraphic practice, the unconformity marked by supermature conglomerates incontrovertibly separates two major rock sequences in early Precambrian geological record of Karnataka Craton.

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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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P. Raase ¹, M. Raith ², D. Ackermand ¹, M. N. Viswanatha ³, R. K. Lal ⁴

Affiliations
1 Meneralogisch-Petrographisches Institut, der Universitat Olshausenstr, 40-60, D 2300 Kiel, DE
2 Mineralogisch-Petrologisches Institut, der Universitat Bonn, Poppelsdorfer Schloss, D 5300 Bonn, DE
3 Geological Survey of India, Southern Region, Hyderabad 500001, IN
4 Department of Geology, Banaras Hindu University, Varanasi 221005, IN

Abstract

The mineralogy of chromiferous quartzites from Archaean greenstone belts of southern Peninsular India has been studied. The majority of the quartzites are metamorphosed clastic sediments derived from a sialic source with enclaves of ultramafic chromite-bearing rocks. For the quartzites from the Ghatti Hosahalli belt, however, a volcanic-exhalative origin is most likely. The chromium in the sediments was fixed in the detrital chromite and/or the clay minerals.

The typical mineral assemblages developed in a wide range of metamorphic conditions from greenschist to amphibolite facies comprise fuchsite, quartz, kyanite and accessory rutile, tourmaline and Cr-spinel. Microprobe analyses show high chromium contents for fuchsite (2-4wt. % Cr₂O₃), kyanite (up to 6wt. % Cr₂O₃), rutile (1.0-2-6wt. % Cr₂O₃) and tourmaline (2.5-7.6 wt. % Cr₂O₃). Cr-spinels are essentially solid solutions between hercynite and chromite (Mg/(Mg+Fe²⁺)=0.060.15; Cr/(Cr+Al)=0.42-0.75). Comparison with igneous chromites indicates that compositional reequilibration during metamorphism was characterised by replacement of Mg by Fe²⁺ and, to a lesser extent, of Cr³⁺ by Al. The micas of chromiferous quartzites from the Chitradurga area have high Cr and Ba contents (1.4-3.9wt. % Cr₂O₃; 0.5-8.5 wt. % BaO) and represent solid solutions extending from muscovite to Cr-oellacherite, A baryte-bearing quartzite from the Ghatti Hosahalli belt contains the rare assemblage quartz, baryte, celsian (Cs₈₅Or₁₁Ab₄), fuchsite (15 wt. % Cr₂O₃; 8.5 wt. % BaO), uvarovite (Uv₇₁Gro₂₈) and tremolite.

The regular distribution of chromium between mica and associated Cr-spinel, rutile and tourmaline indicates equilibration during metamorphism.

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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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M. Ramakrishnan ¹, M. N. Viswanatha ²

Affiliations
1 Geological Survey of India, Precambrian Geology Division, Chandra Vihar, M. J. Road, Hyderabad 500001, IN
2 No. 17, Rajamahal Vilas Extension, Bangalore 560080, IN

Abstract

Recent mapping by the authors in southern Bababudan has confirmed tbe angular unconformity between the Dharwar cover and Sargur-Peninsular Gneiss basement. The original structural trend of Sargur Group is preserved in an unmodified form in the low strain zone near Kalasapura. In the structural domain of eastern Bababudan with its N-S trends, the underlying Sargur enclaves retain their old N-S orientation but with minor rotation and enhancement during Dharwar overprinting. The apparent structural unity between the basement and cover, therefore, is due to the repetition of a stress regime similar to that of Sargur during the later Dharwar orogeny, which is a common feature of most Archaean cratons. Even in such domains of superposed deformation, the Sargur Group is lithologically distinct from the Dharwar cover rocks and occurs as a train of migmatised enclaves in Peninsular Gneiss, whereas the Dharwar cover forms a large athwart-lying fold belt with mature basal conglomerate preserved in the unaffected parts. These are prima facie evidences for separation of the Sargur and Dharwar into two distinct orogenies. Structural unity per se, divorced from regional stratigraphic considerations, is apt to be grossly misleading in interpreting the geological history of multiple deformed terrains.

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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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M. Ramakrishnan ¹, S. Moorbath ², P. N. Taylor ², G. V. Anantha Iyer ³, M. N. Viswanatha ¹

Affiliations
1 Geological Survey of India, Gujarat Circle, Ashram Road, Ahmadabad - 380014, IN
2 Department of Geology and Mineralogy, University of Oxford, Oxford, GB
3 Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore-560012, IN

Abstract

Rb-Sr and Pb-Pb whole rock isotopic systematics and petrochemical data for two suites of Peninsular Gneiss immediately underlying the first order regional unconformity at the base of the main greenstone successions of Dharwar Supergroup in Bababudan and Chitradurga belts are presented.

The Chikmagalur granite (s.l.) and associated migmatite gneisses (CMG suite) underlying the Bababudan greenstones define an eleven point Rb-Sr isochron of 3080 ± 80 m.y. (MSWD4.5) with an initial ⁸⁷Sr/⁸⁶86 isotope ratio (I.R.) of 0.7016 ± 0004. Pb isotope data for the same samples yield a 3185 ± 60 m.y. isochron (MSWO 7.0) with single stage 238_U/204_Pb (μ₁) ratio of 7.99. The crust-forming event in this area spans a range of 3000-3200 m.y. within the analytical error of the isochrons. The mantle-type I.R. and μ₁ values preclude interaction with or reworking of earlier continental crust in their genesis. Petrological and chemical studies of the CMG suite show that the Chikmagalur 'granite' is a biotite granodiorite and the intruded gneisses are low alumina trondjhemites having a calc-alkaline trend. It is significant that the CMG suite is located within a low strain zone in Central Karnataka.

The gneisses and granitoids close to the western margin of the Chitradurga belt (CDG Suite) give a Rb-Sr age of 2970 ± 100 m.y. (MSWD 11) with I.R. of 0.7035 ± 0.0013. The corresponding Pb-Pb isochron defines a date of 3044 ± 150 m.y. with μ₁ = 7.69. The μ₁ and I.R. values suggest the possible involvement of previous sialic crust in their generation during the gneiss-forming event around 3000 m.y. ago. The CDG suite is heterogeneous, but all its components are regarded as a part of the Peninsular Gneiss. The composition of the CDG suite has a wide range from tonalite to granite, with a typical calcalkaline granitic trend.

Recent intensive geochronological studies in central Karnataka suggest a widespread crust-forming event around 3000 m.y. together with older gneissic components of Ca 3300 m.y. The present data bridge the time gap between 3000 and 3300 m.y. suggesting that continental crust was being generated in Karnataka craton from 3300 to 3000 m.y. semi-continuously. The unambiguous field relations in central Karnataka suggest that this event occurred prior to the deposition of main greenstone piles of Dharwar Supergroup.

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M. Ramakrishnan ¹, M. N. Viswanatha ¹, J. Swami Nath ¹

Affiliations
1 Geological Survey of India, Bangalore, IN

Abstract

The low grade Archaean terrain of southern Karnataka is composed of greenstone belts and high grade schists set in a sea of gneiss complex (Peninsular gneiss). Peninsular gneiss contains three major components which are inter-gradational, namely: (a) a trimodal macrolayered unit of amphibolites, ultramafites and granite gneiss (b) normal migmatites with characteristic megascopic structures and (c) nebulitic, schlieric and homophanous granitoids. On the whole it is a polymigmatite encompassing several episodes. The ubiquitous enclaves of mafic-ultramafic rocks within Peninsular gneiss probably represent residues of repeated reworking of the crustal rocks. The composition of the gneiss complex varies in the indestructible range of tonalite through granodiorite to granite. The high grade schists or supracrustals consist of shelf sediments, volcanics and ironstones with profuse emplacements of ultramafites. Their contacts with the gneiss complex are concordant due to intense deformation, high metamorphic grade and extensive migrnatisation. This high grade thermal event (Pantectogenesis) dated at 2900-3000 m.y. has acted as an effective smokescreen blurring earlier episodes. Subsequent to the pantectogenesis, greenstone belts were evolved on a basement of gneiss complex. Two types of greenstone belts of mutually exclusive geographic distribution are seen in the craton: (i) dominantly volcanic belts, lacking shelf facies rocks and ultramafics at the base, called the Keewatin type, which are broadly comparable to the Archaean greenstone belts and (ii) Platformal volcanic and protogeosynclinal belts of Dharwar type, which are similar to the early Proterozoic basins and geosynclines. The contacts of the Dharwar type greenstone belts with the gneiss complex are almost always unconformable, whereas in the case of Keewatin type of belts, the contacts are obscured either by soil cover or by massive invasion of later granites. This major granitic event possibly corresponds to the 2500-2600m.y. isochron in the gneiss complex. This evolutionary picture reconstructed mainly from newer field data is consistent with the available limited information on geochronology, geochemistry and mineralisation trends.

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