Closepet Granite Research Articles

Structural fabric of the Southern Indian shield as defined by gravity trends

Southern Indian shield represents a mosaic comprised of several smaller structural domains separated by discrete shear zones. Here we present a horizontal Bouguer gravity gradient map of the Indian shield, south of 14 °N, to define a continental mosaic of gravity trends domains akin to structural domains. The gravity gradient image is based on 7862 newly collected observations merged with 6359 old gravity data. This combined dataset delineates structural boundaries of the five gravity domains related to the Eastern Dharwar Craton, the Eastern Ghats Mobile Belt, the extended Eastern Ghats Mobile Belt, the Southern Granulite Terrain, and the Western Dharwar Craton. Other belts of significant gravity gradients are found associated with the Eastern and the Western coasts. The loci of Closepet granite and Kolar schist belts do not manifest themselves as boundary zones between two distinct gravity domains of the Eastern Dharwar Craton. Lack of a gravity gradient across Karur–Oddanchatram–Kodaikanal and Karur–Kambam–Painavu–Trichur Shear Zones may be attributed to a lack of gravity measurements caused by difficulties in collecting data in topographically difficult terrain. The subdued gravity gradient across the Palghat–Cauvery Shear Zone and a weak gradient across the Achankovil Shear Zone indicates a lithological and/or morphological boundary rather than a terrane boundary. Alternatively, structural domains encompassing Palghat–Cauvery and Achankovil Shear Zones may have been in a neighbouring position during the Gondwana assembly, when Pan-African thermal perturbation reactivated the structures and reworked partly or totally obliterating earlier crustal fabric.

Geochemical systematics of komatiite–tholeiite and adakitic-arc basalt associations: The role of a mantle plume and convergent margin in formation of the Sandur Superterrane, Dharwar craton, India

The ∼ 2.7 Ga Sandur Superterrane is located within the central belt of the ∼ 2.6 Ga Closepet granite that divides the Dharwar craton into eastern and western sectors. The composite SST includes multiple terranes defined by distinct lithological associations, and metamorphic-deformational histories, demarked by accretionary structures. The Sultanpura volcanic terrane includes well preserved spinifex textured komatiites and komatiitic-basalts, with pillowed tholeiitic basalts. Komatiites and komatiitic-basalts have Mg# of 0.82–0.84 and 0.55–0.64 respectively, and plot near the olivine control line, whereas basalts have Mg# 0.53–0.69. All three volcanic types can be divided into two populations based on Nb/Th ratios: for rocks with Nb/Th < 8, there is covariation with Th, and (La/Sm) N interpreted to be the result of crustal assimilation fractional crystallization (AFC), whereas those rocks with Nb/Th > 8 plot along the Mid Oceanic Ridge Basalt–Oceanic Island Basalt array in Th/Yb vs. Nb/Yb coordinates. Collectively, the data are interpreted as signatures of a zoned mantle plume, having multiple sources that erupted through, or at the margin of, continental lithosphere. Felsic flows associated with arc basalts of the eastern felsic volcanic terrane, tectonically juxtaposed to the Sultanpura volcanic terrane, have adakitic compositional characteristics: elevated Al 2O 3 but low Yb (0.30–0.50 ppm) contents, coupled with high (La/Yb) N (43–71) and Zr/Sm (37–41) ratios, but low Nb/Ta (5–12). These features, in conjunction with mostly positive Eu anomalies, rule out detectable crustal contamination, such that adakitic flows and associated basalts and volcanogenic sedimentary rocks having normalized anomalies at Nb–Ta–P–Ti, represent an arc association. Consequently, the distinctive magmatic associations of the Sultanpura and eastern felsic volcanic terranes are consistent with the Sandur Superterrane being tectonic fragments of distinct continental and oceanic provenance tectonically juxtaposed in a Cordilleran type, accretionary orogen at ∼ 2.7 Ga.

Heat flow and crustal thermal structure in the Late Archaean Closepet Granite batholith, south India

The Late Archaean Closepet Granite batholith in south India is exposed at different crustal levels grading from greenschist facies in the north through amphibolite and granulite facies in the south along a ∼400 km long segment in the Dharwar craton. Two areas, Pavagada and Magadi, located in the Main Mass of the batholith, best represent the granitoid of the greenschist and amphibolite facies crustal levels respectively. Heat flow estimates of 38 mW m−2 from Pavagada and 25 mW m−2 from Magadi have been obtained through measurements in deep (430 and 445 m) and carefully sited boreholes. Measurements made in four boreholes of opportunity in Pavagada area yield a mean heat flow of 39 ± 4 (s.d.) mW m−2, which is in good agreement with the estimate from deep borehole. The study, therefore, demonstrates a clear-cut heat flow variation concomitant with the crustal levels exposed in the two areas. The mean heat production estimates for the greenschist facies and amphibolite facies layers constituting the Main Mass of the batholith are 2.9 and 1.8 μW m−3, respectively. The enhanced heat flow in the Pavagada area is consistent with the occurrence of a radioelement-enriched 2-km-thick greenschist facies layer granitoid overlying the granitoid of the amphibolite facies layer which is twice as thick as represented in the Magadi area. The crustal heat production models indicate similar mantle heat flow estimates in the range 12–14 mW m−2, consistent with the other parts of the greenstone-granite-gneiss terrain of the Dharwar craton.

Isotope hydrochemical approach to understand fluoride release into groundwaters of Ilkal area, Bagalkot District, Karnataka, India

Isotope and hydrochemical investigations have been carried out in the Ilkal area of Karnataka, India, in order to determine the source and mechanism of fluoride release into groundwaters and to understand groundwater hydrochemistry. Agriculture, granite quarrying and rock-polishing industries are the main occupations in this area. Closepet granite, Peninsular gneiss and Dharwar schists are the major geological formations. Results show that the fluoride concentration in groundwater is 0.3–6.5 mg/L and it is found to increase from recharge area to discharge area. Fluoride variability is found to be influenced by the geology of the area and depth wise correlation was not observed. Water samples are unsaturated with respect to fluorite, indicating the possibility of further increase in fluoride in groundwater. Positive correlations between fluoride with sodium and bicarbonate in groundwater show that high fluoride content and alkaline sodic characteristics are the result of dissolution of fluoride bearing minerals, possibly derived from weathered granite and gneiss. A positive correlation between fluoride and δ18O, and the presence of high tritium in fluoride-contaminated groundwater, point to contribution from surface waters, contaminated by anthropogenic activities. Dumping of rock wastes that are rich in fluoride into the streams by the rock-polishing industries plays a significant role in contaminating groundwater.

Radioelements and heat production of an exposed Archaean crustal cross-section, Dharwar craton, south India

The Dharwar craton (DC) in south India exposes a tilted ∼30-km-thick crustal section equivalent to upper, middle and lower Archaean continental crust. The DC is composed of poly-magmatic–metamorphic terrains such as the Western Dharwar Craton (WDC) and Eastern Dharwar Craton (EDC) whose contact is marked by the Closepet Granite batholith (CG). In-situ gamma-ray spectrometric analysis has been carried out to measure K, U and Th abundance at 1023 sites covering all major rock formations of the DC. The data show that the greenschist and amphibolite facies tonalite–trondhjemite–granodioritic (TTG) gneisses of the WDC are poorer in radioelements compared to granodioritic gneisses of the EDC. Similarly, the WDC granites of the amphibolite facies region have lower abundance indicating their derivation from a depleted lower crust, which had suffered an earlier crustal differentiation. The elemental data on the gneisses in the WDC and EDC do not show a difference between the greenschist and amphibolite facies, but there is a marked depletion in the granulite facies. The depletion reaches a maximum in the high- P granulites in the southernmost part of the DC. The elongate CG is an I-type, calc-alkaline, metaluminous granite, which exposes a ∼12-km-thick batholith, where K is remarkably uniform but U and Th show several fold increase with decreasing crustal levels. Heat production was calculated from the radioelemental data. For estimating the radiogenic heat contribution of crust to heat flow, we arrive at a present-day crustal configuration comprising four northerly dipping metamorphic facies layers: greenschist, amphibolite, metasomatized granulite and depleted granulite. Gross heat production of each layer is computed from heat production of constituent rocks and their abundance. The crustal contribution is found to decrease, from greenschist to granulite facies regions, from 23 to 18 mW m −2 in the WDC, 27 to 7 mW m −2 in the EDC. Surface heat flow and the crustal contribution models indicate that mantle heat flow of the WDC is lower (7–10 mW m −2) compared to the EDC (17–24 mW m −2) in the greenschist and amphibolite facies regions. High mantle heat flow of 29 mW m −2 beneath the depleted granulite facies region of the EDC appears anomalous in the Dharwar craton.

Late Archaean granites: a typology based on the Dharwar Craton (India)

Abstract Extensive field work in the Eastern Dharwar Craton, associated with petrographic and geochemical (major and trace elements) investigations, allows four main types of Late Archaean granitoids to be distinguished. (1) Na-rich granitoids of trondhjemitic, tonalitic and granodioritic composition (TTG) that are characterised by strongly fractionated REE patterns and low HREE contents and generally interpreted as “slab melts” generated by partial melting of metamorphosed hydrated basalt, most likely in a subduction environment. (2) Sanukitoids, which are K- and Mg-rich monzonites and granodiorites with TTG-like REE patterns associated with marked LILE-enrichment, and considered to result from the reaction of slab melts generated in a subduction environment with, and assimilation of, mantle wedge peridotite. (3) Uncommon high-HFSE, Mg and K granites with strongly REE and LILE-enrichment that, probably formed by partial melting of an enriched mantle source; unlike in the genesis of sanukitoids, in this case the slab melt is considered to be wholly consumed by reaction with mantle minerals, resulting in mantle-enrichment. Subsequent melting of this enriched mantle (probably in a post-subduction setting) gives rise to high-HFSE, Mg and K magmas. As demonstrated for the Closepet Granite (Dharwar Craton), the hot mantle-derived magma can induce melting of continental crust and then mix with the anatectic products. (4) K-rich, Mg-poor anatectic biotite-granites with REE patterns that are less fractionated and show negative Eu anomalies. These granites result from the remelting of old basement or recently accreted plutons, both with TTG compositions. Such anatexis can occur either in a subduction or in a post-subduction environment. This typology, based on the well-exposed Dharwar Craton, can be extended to Late Archaean granitoids from all over the world. About such 500 analysis were compiled, and a number of discriminant diagrams, based on both major and trace elements, are proposed. The most likely tectonic setting for the observed rock types is an accretionary orogen with accretion of continental blocks above a subduction system, followed by thermal reworking of the newly accreted continental material. However, it has been found that Archaean “subduction-related” granitoids are significantly different from their modern counterparts, implying progressive changes in the modes of magma generation at convergent margins from the Archaean to the present.

Crustal structure and domain tectonics of the Dharwar Craton (India): insight from new gravity data

The Precambrian Dharwar Craton of Peninsular India comprises three distinct crustal domains, namely the Western Dharwar Craton (WDC), the Eastern Dharwar Craton (EDC) and the Eastern Ghats Mobile Belt (EGMB). The domain boundaries are demarcated by bipolar (positive and negative) gravity anomalies, measured along a 600 km long Udipi–Kavali seismic line. The two and half dimensional modelling of the gravity anomalies, well constrained by available seismic depth-sections, identified the two contact zones. One occurs between the WDC (relatively low density) and the EDC (intermediate density), to the east of the Chitradurga Schist Belt and the second occurs between the EDC and the EGMB (high density), to the east of Cuddapah Basin as west verging thrust faults. The three-layer crustal structure is up to 41 km thick beneath the WDC, thinning to 37 km beneath the west coast. Under the EDC, the depth of the Moho varies from 38 km towards the west to 40 km beneath the Cuddapah Basin, and decreases to a depth of 35 km beneath the EGMB. A high-density steeply inclined ridge-like body is modelled between 5 and 20 km depth beneath the Closepet Granite, and a similar body is modelled under the EGMB. It is suggested that the three domains of the Dharwar Craton were brought and welded together at the suture zones in the period from Late Archaean to Late Proterozoic through collisional tectonics. The apparent offsets in the Moho depth may be the imprint of the relative domain movement that probably was initiated by the convergent plate motion. The steeply inclined ridge-like high-density bodies are interpreted as stacked lower crustal blocks from the convergent tectonic setting. The Cuddapah Basin, with a 10 km thick shelf-marine sedimentary sequence, is interpreted as a peripheral foreland basin abutting the EGMB. A lopolith of likely mafic composition is modelled at 10 km depth, and is interpreted as a product of juvenile magmatism beneath the EDC.

A Plausible Model for Evolution of Schist Belts and Granite Plutons of Dharwar Craton, India and Madagascar During 3.0-2.5 Ga: Insight from Gravity Modeling Constrained in Part from Seismic Studies

Gravity modeling of an E-W profile across Dharwar Craton, India and Madagascar, integrated with the results of Deep Seismic Sounding (DSS) across the Dharwar Craton suggest a thick crust of 40-42 km under the eastern part of Eastern Dharwar Craton (EDC), the Western Dharwar Craton (WDC) and the central part of the Madagascar. Towards east of these blocks, the crustal thickness is reduced to 36-38 km along the Eastern Ghat Fold Belt (EGFB), shear zone between the EDC and the WDC and the east coast of Madagascar, respectively. These zones of thin crust are also characterized by high density lower crustal rocks associated with thrusts. The seismic section across Dharwar Craton shows domal- shaped reflectors in the lower crust and upper mantle under the WDC which may be related to asthenopheric upwelling during an extension phase. The occurrences of large schist belts with volcano-sedimentary sequences of marine origin of late Archean period (3.0-2.7 Ga) as rift basins in the WDC and Madagascar also suggest an extensional phase in this region during that period. It is followed by a convergence between the WDC and the EDC giving rise to collision-related shear and thrust zones between the WDC and the EDC associated with high density lower crustal rocks. The seismic section shows upwarped reflectors in the upper crust which may be related to this convergence. Eastward dipping reflectors under WDC and EDC and west verging thrusts suggest convergence from the west to the east which resulted in easterly subduction giving rise to subduction-related K-granite plutons of the EDC of 2.6-2.5 Ga. In this regard, the Closepet granite in the EDC which extends almost parallel to the shear zone between the WDC and EDC and shows an I-type calk-alkaline composition may represent relict of an island arc and the linear schist belts with bimodal volcanics of the EDC east of it might have developed as back arc rift basins. Subsequent collision between India and Antarctica along the EGFB during Middle Proterozoic, indicated by eastward dipping reflectors in the crust and the upper mantle and west verging thrust gave rise to contemporary high-grade rocks of the EGFB (1.6-1.0 Ga) and associated mafic and felsic intrusives of this belt. The part of adjoining Cuddapah basin contemporary to the EGFB towards the west consisting of marine shelf type of sediments which are highly disturbed and thickest at its contact with the EGFB may represent a peripheral foreland basin. Gravity modeling provides thickest crust of 42 km in the southern part of the WDC and does not support sharp increase in crustal thickness of 50-60 km with high velocity upper mantle as suggested from receiver function analysis. It may represent some foreign material of high density trapped in this section such as part of oceanic crust during convergence and subduction that is referred to above. It is supported from eastward dipping reflectors in lower crust and upper mantle in adjoining region.

Ion Microprobe U‐Pb Age Determinations on Zircon from the Late Archean Granulite Facies Transition Zone of Southern India

Abstract The southern Dharwar Craton of Karnataka and Tamil Nadu states in south India exposes a depth profile of Late Archean crust. Amphibolite facies felsic to intermediate Peninsular Gneiss of protolith ages 3.4–2.9 Ga with subordinate infolded supracrustal relics and the large Closepet Granite body give way southward to granulite facies (orthopyroxene‐bearing) gneisses over a transition zone about 40 km wide, with mineralogic pressure indicators increasing from about 4 kbar (corresponding to about 15 km paleodepth) to about 7 kbar (corresponding to near 25 km paleodepth). Previous whole rock and zircon geochronology have established that amphibolite facies metamorphism, migmatization, and granite intrusion took place in the latest Archean at 2.53–2.51 Ga. However, some workers have considered the foliated granulites as products of an earlier metamorphism, distinct from the terminal Archean events. This work presents whole rock geochemical studies coupled with ion microprobe 207Pb/206Pb ages and Th/U ...

Syntectonic granite emplacement at different structural levels: the Closepet granite, South India

The Closepet granite, in South India, is a large (400 km long but only 30 km wide), elongate, Late Archaean granitic body. Structural levels from deep crust to upper levels crop out, as evidenced by a shallowing of paleo-depths from north to south all along the Closepet granite. This allows the study of the emplacement of the same granitic body at various crustal levels. Four zones have been identified: a root zone, where magmas are collected in active shear zones; a transfer zone, featuring large-scale magma ascent and crystal–liquid partitioning in the granitic ‘mush’; a ‘gap’, where the mush was filtered, allowing only the liquids to rise; shallow intrusions, filled with this liquid. The Closepet granite was emplaced syntectonically. Field work and anisotropy of magnetic susceptibility allowed documentation of steep foliations with subhorizontal lineations, both in the root and transfer zones and in the shallow intrusions. Remote sensing evidenced a network of shear zones bounding the Closepet granite. In the porphyritic root and transfer zones, magmas cooled slowly, thus developing strong fabrics during large-scale dextral shearing. Ascent of residual liquids amidst the crystallizing solid framework was not recorded in the fabrics. However, these liquids were channelised through the gap and infilled the homogeneous shallow intrusions, where rapid cooling only permitted the development of feint, although wholly consistent, fabrics.

Evaluation of Natural Defects in Commercial Decorative Rock Deposits in Karnataka, India

Karnataka state is a pioneer in the exploitation of commercial decorative rock resources and in establishing a firm base for the stone industry. The commercial stone industry in the state has grown in status contributing about 40% of the decorative/dimensional stones of both polished and rough blocks exported from the country. Karnataka state forms a part of the Indian Peninsular Shield and extends over an area of 1, 91, 792 km 2. Peninsular gneisses, Closepet granite and Younger dykes such as dolerite, felsite and pyroxenite of different ages are the main sources of commercial rock deposits. The state also produces premium varieties like ruby red and black (dolerite) granite and other popular varieties. The demand for the premium varieties is generally steady in international market. World famous commercial ornamental stone varieties are being exploited from Kanakapura, Bangalore, Malaikaval, Hassan, Sira, Tumkur and Illkal in Bijapur districts of Karnataka. Important commercial stones in the Karnataka stone industry are marketed with commercial names such as Imperial red, Himalayan blue, Ocean white, Sabal black, Ruby red, Hassan green, Sawan rose, Sira grey, English teak, Tiger's skin, Pink panther, Salt and pepper, Cat's eye, and Chilly red. The success of the commercial stone industry solely depends upon the availability of large reserves of defect free rock deposits. Detection of defects in commercial rock deposits plays an important role in quality assessment. Macro defects can be detected by visual examination of commercial rock deposits. Micro discontinuities such as altered or twisted minerals can be detected by systematic micropetrographic analysis of the rock specimens. In Karnataka, too many organizations, entrepreneurs and individuals are investing huge amounts without evaluating the quality of the deposit. This results in poor quality stone and subsequent quarry abandonment. Quarry owners and stone exporters should be quality conscious about export-oriented natural stones in order to remain in the business and maintain the reputation of the Karnataka stone industry.

Multi-element geochemical modelling of crust–mantle interactions during late-Archaean crustal growth: the Closepet granite (South India)

The Closepet granite, in the Dharwar craton of south India, is a large, late Archaean magmatic body. Its composition can be explained as a result of interactions between mantle-derived melts and pre-existing continental crust (TTG gneisses). Using geochemical modelling based upon major and trace element compositions the following petrogenetic model is proposed for the formation of the batholith: (i) an enriched mantle (garnet- and amphibole-bearing lherzolite) melts to produce a basaltic liquid. (ii) The basaltic liquid undergoes limited fractional crystallization of biotite and amphibole ( F>0.9). (iii) The differentiated liquid rises into the old continental crust, and induces water-saturated anatexis of the TTG gneisses (K-Feldspar+Qtz+Plagioclase→melt). (iv) Both mantle-derived and anatectic liquid mix and give rise to magma compositions ranging from quartz-monzonite to granite. The origin of the mantle enrichment is discussed. In the context of the regional geological setting the most likely possibility seems to be metasomatism by slab melts the metasomatism of a mantle wedge by slab melts. This suggests a two-stage evolution for the Dharwar craton during the late Archaean: (i) a subduction related event with the formation of TTGs, sanukitoids, and with associated mantle metasomatism; and (ii) re-melting of the metasomatized mantle. This evolutionary history implies that at least some of the K-rich, late-Archaean granites are juvenile, rather than products of intracrustal reworking, as frequently assumed.

Contrasted granite emplacement modes within an oblique crustal section: the Closepet Granite, South India

The Closepet Granite, in South India, is a large, syntectonic Archaean granitic complex. Differen- tial erosion has exposed it fl'om the lower (25 km) to upper crust (5 km). Four main parts are recognized from bottom to top: (i) A root zone, where magmas formed, collected and rose within active shear zones, leaving schlieren behind. The surrounding crust was highly ductile, leading to diffuse deformation. (ii) A transfer zone, where the magma was progressively en- riched in K-feldspar phenocrysts during its ascent. In this part, the granite rose as a mush moving as a whole within a less ductile crust. Slow cooling was responsi- ble for a long magma residence time under conditions favoring to fabric enhancement and strain partitioning, leading to horizontal and vertical melt migration. (iii) A gap (dyke complex that acted as a filter zone), were the ascent of the mush was stopped, probably due to high phenocryst load and high viscosity contrast with the wall rocks. Only crystal-poor melts could continue their ascent through the dykes. (iv) A zone of shallow intrusions, where the liquids extracted from the mush filled small, elliptical plutons, cooling quickly and de- veloping only very weak fabrics. © 2001 Elsevier Science Ltd. All rights reserved

Propagation of Crustal-Waveguide-Trapped Pg and Seismic Velocity Structure in the South Indian Shield

Short-period vertical-component digital recordings of the seismic wave field generated by mine tremors and explosions recorded at the Gauribidanur array (GBA) reveal relatively high-frequency secondary energy, with a group velocity of about 6 km/sec accompanied by a long, incoherent coda. We interpret this part of the wave field including the coda as resulting from a crustal-waveguide-trapped Pg , and it is observable from a distance of about 150 km behind the first arrival Pn phase. These observations and extensive modeling results reveal the fine structure of the crust beneath the Archaean eastern Dharwar craton in the south Indian shield. A waveguide with random successions of high- and low-velocity layers in the upper crust, possibly with considerable lateral extent beneath the Closepet granite, explains the main features of the waveguide Pg , such as coda length and group velocity. Reflectivity synthetic seismograms for 1D lamellae structures in the upper crust are found to reproduce the observed features of a record section assembled from GBA seismograms extending to a distance of 240 km. The preferred models of P - and S -velocity structure reveal lower than normal V P / V S and Poisson9s ratios: respectively, 1.63–1.65 and 0.20–0.21 in the upper crust, including the lamellae structure, and 1.70 and 0.235 in the lower crust. A Q P / Q S ratio nearly equal to, or even slightly less than, 1 in the upper crust is found to generate the synthetics consistent with the observed seismogram section. F-K spectral analysis of the observed P coda following the waveguide Pg , for travel paths mostly through the Closepet granite, convincingly reveals two dominant energy peaks, at near-on-azimuth as well as relatively off the source azimuth, both with an apparent velocity of 6.1 km/sec appropriate to the upper crustal depths. The inferred upper crustal structure modeled by alternating high and low velocity lamellae, together with the derived physical properties and the F-K analysis of the P coda, suggests a scattering waveguide at 5–15 km depth in this region. The scattering waveguide appears to be related to the large Closepet granite intrusion, which is believed to be a Precambrian suture between the western and eastern Dharwar cratons in the south Indian shield. Possible lateral extent of the modeled lamellae structure receives added support from a recently proposed hypothesis of a late Archaean mantle plume centered below the Closepet granite. The inferred crustal P -velocity model for the eastern Dharwar craton, revealing a transitional Moho at a shallow depth of 34–36 km, is consistent with the models for other Archaean cratons. The observed Pn velocity of 8.2 km/sec and its gradient 0.013 sec –1 compare fairly well with similar values found for other shield regions. V P / V S and Poisson9s ratios are found to increase respectively from 1.71 and 0.24 below the Moho to 1.78 and 0.27 at about 60 km depth in the upper mantle.

Origine du granite fini-Archéen de Closepet (Inde du Sud): apports de la modélisation géochimique du comportement des éléments en trace

The Closepet granite is the latest known Archaean magmatic event in the Dharwar craton (South India). It consists of a composite massif showing interactions between a mantle-derived magma and the surrounding continental crust. Modeling based on trace element behaviour indicates that its geochemical characteristics are mainly accounted for by mixing between a mafic magma, and products of the partial melting of the surrounding continental crust. To a small extent fractional cristallization took place prior to the mixing. The recalculated parental magma's chemical composition is very similar to that of typical late-Archaean rocks known as ‘sanukitoïds”.

Demarcation of geological and geomorphological features of parts of Dharwar Craton, Karnataka, using IRS LISS-II data

Abstract To demonstrate the capabilities of Indian Remote Sensing Satellite (IRS) Linear Imaging Self-scanning Sensor-II (LISS-II) data in preparation of regional level geological, structural and geomorphological maps of 1:500000 scale and to understand the interrelation between lithology, structures, landforms and drainage details in diverse geologic environs, a study has been carried out. The area chosen for the study is a strip of terrain in Karnataka between latitudes 13° N and 14° N. Different rock types such as peninsular gneisses, granites, schists and laterites were mapped based on their distinctive image characteristics like colour and textural variations, typical landforms, soil cover and drainage networks. The satellite images have exhibited more clearly the Bababudan ‘horse shoe’ fold, Doddaguni fold, Chikanayanakanahalli fold, Kolar schist belt. bounding lineaments of Closepet granite, regional scale NNW-SSE trending lineament located to the west of Bababudan schist belt, N-S trending lineame...

Response of zircon U?Pb isotopes and whole-rock geochemistry to CO2 fluid-induced granulite-facies metamorphism, Kabbaldurga, Karnataka, South India

The arrested, prograde amphibolite- to granulite-facies transition at Kabbaldurga, south India, overprints Archacan amphibolite-facies nebulitic gneisses and the late Archaean Closepet granite. Previous studies have shown that this facies transition was controlled by a channelled influx of a dehydrating fluid, assumed to be CO2, at ∼750°C and 5.5 kbar confining pressure. The effect of this type of prograde transition on zircon U−Pb isotopic systematics and whole-rock geochemistry has been studied using ∼1 kg amphibolite-facies, transitional and granulite-facies domains from a single block of gneiss. The zircon populations from all three domains have essentially similar morphology and U−Pb systematics. This similarity shows that at the conditions under which the prograde granulite-facies transition took place via fluid influx, the zircon U−Pb systematics were not disturbed by the process. Using the pooled data from all three domains, it is concluded that the protolith of the gneiss formed at 2965±4 Ma (2σ), and that zircons also grew during an anatectic event common to all domains at 2528±5 Ma. The granulite-facies metamorphism has not been dated directly due to the lack of response to the zircon U−Pb isotopic systematies to it. However, field and petrographic criteria dictate that its maximum age is 2528±5 Ma, the age of the anatectic event common to each domain in the gneiss block, which was overprinted during the granulite-facies event. For most major and trace elements, consistent enrichment or depletion trends associated with the transition to granulite facies cannot be identified with confidence. However, the granulite-facies portion is LREE (light-rare-earth-element)-enriched and H (heavy) REE-depleted compared with the amphibolite-facies domain, and the transitional domain is at intermediate values. The isotopic and geochemical evidence presented supports the conclusion that the granulite-facies charnockitic rocks at Kabbaldurga were not formed by removal of an anatectic melt, but that they formed later by simple metamorphic overprint of amphibolite-facies rocks.

Sedimentational, structural and migmatitic history of the Archaean Dharwar tectonic province, southern India

The earliest decipherable record of the Dharwar tectonic province is left in the 3.3 Ga old gneissic pebbles in some conglomerates of the Dharwar Group, in addition to the 3.3–3.4 Ga old gneisses in some areas. A sialic crust as the basement for Dharwar sedimentation is also indicated by the presence of quartz schists and quartzites throughout the Dharwar succession. Clean quartzites and orthoquartzite-carbonate association in the lower part of the Dharwar sequence point to relatively stable platform and shelf conditions. This is succeeded by sedimentation in a rapidly subsiding trough as indicated by the turbidite-volcanic rock association. Although conglomerates in some places point to an erosional surface at the contact between the gneisses and the Dharwar supracrustal rocks, extensive remobilization of the basement during the deformation of the cover rocks has largely blurred this interface. This has also resulted in accordant style and sequence of structures in the basement and cover rocks in a major part of the Dharwar tectonic province. Isoclinal folds with attendant axial planar schistosity, coaxial open folds, followed in turn by non-coaxial upright folds on axial planes striking nearly N-S, are decipherable both in the “basement” gneisses and the schistose cover rocks. The imprint of this sequence of superposed deformation is registered in some of the charnockitic terranes also, particularly in the Biligirirangan Hills, Shivasamudram and Arakalgud areas. The Closepet Granite, with alignment of feldspar megacrysts parallel to the axial planes of the latest folds in the adjacent schistose rocks, together with discrete veins of Closepet Granite affinity emplaced parallel to the axial planes of late folds in the Peninsular Gneiss enclaves, suggest that this granite is late-tectonic with reference to the last deformation in the Dharwar tectonic province.

Relationship between shear zones and igneous activity: The Closepet Granite of Southern India

The late Archaean Closepet granite of southern India is bounded by N-S trending shear zone. At the southern end of the granite both charnockite and granite veins are spatially associated with ductile shears. These shears continue further north and are confined to the contact zones in the central part of the granite outcrop. The main component of the shear zone are highly deformed granite sheets, augen gneisses and mylonites. Field observations and microstructural fabric of mylonites indicate a dextral sense of shear movement. Field evidence suggests that shear deformation was active throughout the evolution of the Closepet granite

Evidence for fluid pathways through Archaean crust and the generation of the Closepet granite, Karnataka, South India

In the Archaean amphibolite—granulite facies transition zone of southern Karnataka anatexis of amphibolite facies Peninsular gneiss occurred. Based on cross-cutting relationships this melting event, which produced the Closepet granite, was contemporaneous with the formation of charnockitic rocks. This formation of charnockite took place initially due to channelized CO2-rich volatiles moving along an anastamosing network of shears and micro-fissures. These fluid pathways are now represented by brown veins of charnockite. Later, the fluid became more pervasive and resulted in areas of total inversion to charnockite. A lateral change along the fluid pathways from charnockite through incipient charnockite to either a thin granitic vein or a vein of reddening, can be traced through the country rocks. These different effects along the same, laterally continuous, vein are interpreted to represent the lateral change of volatile composition which, in front of the CO2-rich fluids, were increasingly H2O-dominated.