Peninsular Gneiss Research Articles

Coupling geomorphology parameters with lithology for micro-level groundwater resource assessment: a case study from semi-arid hard rock terrain in Tamil Nadu, India

The standard procedure of groundwater resource estimation in India till date is based on the specific yield parameters of each rock type (lithology) derived through pumping test analysis. Using the change in groundwater level, specific yield, and area of influence, groundwater storage change could be estimated. However, terrain conditions in the form of geomorphological variations have an important bearing on the net groundwater recharge. In this study, an attempt was made to use both lithology and geomorphology as input variables to estimate the recharge from different sources in each lithology unit influenced by the geomorphic conditions (lith-geom), season wise separately. The study provided a methodological approach for an evaluation of groundwater in a semi-arid hard rock terrain in Tirunelveli, Tamil Nadu, India. While characterizing the gneissic rock, it was found that the geomorphologic variations in the gneissic rock due to weathering and deposition behaved differently with respect to aquifer recharge. The three different geomorphic units identified in gneissic rock (pediplain shallow weathered (PPS), pediplain moderate weathered (PPM), and buried pediplain moderate (BPM)) showed a significant variation in recharge conditions among themselves. It was found from the study that Peninsular gneiss gives a net recharge value of 0.13 m/year/unit area when considered as a single unit w.r.t. lithology, whereas the same area considered with lith-geom classes gives recharge values between 0.1 and 0.41 m/year presenting a different assessment. It is also found from this study that the stage of development (SOD) for each lith-geom unit in Peninsular gneiss varies from 168 to 230 %, whereas the SOD is 223 % for the lithology as a single unit.

Chapter 15 Evolution of c. 3.5–2.5 Ga basins of the Dharwar Craton

Abstract The geology and basin evolutionary history of the Dharwar Craton is discussed. The Dharwar Craton comprises western (WDC) and eastern (EDC) subdivisions (possibly separated by the Closepet granite), predicated on lithological contrasts and inferred metamorphic and magmatic evolution. A postulated genesis of the WDC comprises early, c. 3.5 Ga protocrust, which possibly formed as basement to the c. 3.35–3.2 Ga Sargur Group greenstone belts. The latter are thought to have formed through accretion of plume-related ocean plateaux. The approximately coeval Peninsular Gneiss Complex possibly originated from beneath plateau remnants, leading to metamorphism of Sargur Group belts at c. 3.13–2.96 Ga. At c. 2.9–2.6 Ga, the Dharwar Supergroup, comprising lower Bababudan (mainly braided fluvial, glaciomarine and subaerial volcanic strata) and upper Chitradurga (marine clastic, chemical sedimentary and subaqueous volcanic rocks) groups developed. This supergroup formed younger greenstone belts characterized by two distinct magmatic events, at 2.7–2.6 and 2.58–2.54 Ga; the latter was approximately coeval with c. 2.6–2.5 Ga granitic magmatism, which marked final cratonization of the WDC. The EDC consists of 2.7–2.55 Ga tonalite–trondhjemite–granodiorite gneisses and migmatites, essentially coeval greenstone belts (mainly volcanic lithologies), with minor inferred remnants of an older, c. 3.38–3.0 Ga crust, and voluminous 2.56–2.5 Ga granitoids (including the Closepet). An east–west accretion of EDC island arcs (or possibly of an assembled arc-granitic terrane) on to the WDC is postulated, and the Closepet granite perhaps accreted earlier on to the WDC to form a ‘central Dharwar’ terrane. A final voluminous granitic cratonization event affected the assembled Dharwar Craton at c. 2.5 Ga.

Occurrence of Ground Water in Hard Rock Under Distinct Geological Setup

Groundwater movement and its properties are unique with reference to its place of occurrence. Karnataka State can be considered as having three major hydrogeological provinces Viz. Archaean crystalline province, Metamorphosed sedimentary province, and Deccan Trap province. Ground water occurs in these provinces under unconfined to semi-confined conditions and under confined conditions in depth. The rock units in these provinces do not have the primary porosity, therefore, the occurrence and movement of ground water is through secondary porosity developed from the weathering and structural deformity undergone by the rocks. Hence, a study of ground water occurrence in two distinct geological setups i.e. Deccan Trap terrain of Eocene to Upper Cretaceous age (Afzalpur taluk) andPeninsular Gneissic terrain of pre-Cambrian age (Hassan district) has been attempted. Afzalpur taluk, Gulbarga district is underlain by basalts and followed downwards by Shale and Limestone. Ground water accounts 88% of the net area under irrigation and is the only dependable source for domestic, agriculture and industrial sectors. As per the Dynamic Ground Water Resources Estimation (2009), Stage of ground water development in the taluk is 94% and is categorized as ‘Critical’. Under Ground Water Exploration programme during the year 2010-2013, 21 bore wells were constructed up to a depth of 300 meters in the taluk at different locations.Hassan district is underlain by peninsular gneiss, granites and schists. Ground water accounts 27% of the net area under irrigation. As per the Dynamic Ground Water Resources Estimation (2009), Stage of ground water development of the district is 46% and is categorized as ‘Safe’. Under Ground Water Exploration programme during the year 2010-2011, 18 bore wells were constructed up to a depth of 200 meters in Channarayapatna, Hassan, Alur and Belur taluks of Hassan district at different locations.Data collected during the drilling and post drilling were analysed and compared to decipher the occurrence of ground water in different geological setups. The analysis shows that the BGC forms a better aquifer as compared to the Basalts.

Locating New Uranium Occurrence by Integrated Weighted Analysis in Kaladgi Basin, Karnataka

Interpretation of airborne geophysical data especially gamma ray spectrometric data is important for mineral exploration, specifically radioactive elements and to identify uranium targets. Exploration was carried out in MesoNeoproterozoic Kaladgi basin for five decades to target different types of uranium mineralisation. The first sizable sub-surface uranium mineralisation was intercepted at Deshnur within Badami arenites close to unconformity contact along the southern margin of the basin (Shobhita et al., 2011). Heliborne geophysical surveys were carried out in western part of the basin from Kadgaon to Ramdurg to acquire Time Domain Electromagnetic, Magnetic and Radiometric data. The radiometric data was collected using Exploranium GR820 airborne (256-channel recording) spectrometer system mounted in a helicopter with four 4.2L downward looking crystals and one 4.2L upward looking crystal for a total effective volume of 16.8L downwards and 4.2L upwards. The raw radiometric data was processed for corrections (background, radon, stripping, height attenuation etc) and counts were converted to ground concentration of %K, eU ppm, and eTh ppm, and their grid maps were prepared (Sridhar et al, 2012) . Kaladgi basin is an irregular E-W trending area in the northern part of Dharwar craton. In the study area, Kaladgi basin sediments are exposed as unconformably overlying the basement crystallines in the south and are covered by Deccan traps in the north. The crystalline basement of the Kaladgi Supergroup is represented by NNW trending Meso to Neoarchaean peninsular gneiss, greenstone belts and Closepet granite. The sedimentary sequence in the Kaladgi Supergroup has been divided along an angular unconformity surface into two groups: the lower Bagalkot Group, and the overlying Badami Group. The sediments of Bagalkot Group comprises of arenaceous, argillaceous and carbonate sequences which are folded into a series of anticlines and synclines. The sediments of Badami Group are undeformed and rest unconformably over the Archaean basement rocks as well as the Bagalkot Group. The Badami Group comprises of horizontal to gently dipping beds of arenite, shale and limestones (Jayaprakash et al., 1987). This study aims at identifying uranium potential zones by integrated analysis of thematic layer interpreted and derived from airborne radiometric and magnetic data, satellite data along with available ground geochemical data in western part of Kaladgi basin. Integrated weighted analysis of spatial datasets which included airborne radiometric data (eU, eTh & % K conc.), litho-structural map, hydrogeochemical U conc., and geomorphological data pertaining to study area, was attempted. The weightage analysis was done in GIS environment where different spatial dataset were brought on to a single platform and were analyzed by integration (Fig.1). The processed radiometric database was taken up for quantitative analysis to identify radiometric uranium anomalies. The database was clipped by using exposed and inferred lithological contacts of Badami Group to separate out data over sediments of Badami Group only. The analysis was carried out in two methods - 1 st the whole block and 2 nd Badami Group exclusive. Statistical analysis was applied to the airborne spectrometric data for separation of uranium anomalies from background. The statistics of the databases for uranium, thorium, ratio of uranium to thorium and uranium squared by thorium were calculated. The anomalies were extracted by applying cutoff threshold values for uranium, uranium by thorium and uranium squared by thorium channels in both methods. These anomalies were plotted spatially and they were classified as cluster anomalies and point anomalies depending upon their nature. Weightages were assigned based on nature of anomalies and cluster anomalies were given more weightage as they are more significant compared to point anomalies. The anomalies extracted from the

A review of the inferred geodynamic evolution of the Dharwar craton over the ca. 3.5–2.5 Ga period, and possible implications for global tectonics

The geological history and evolution of the Dharwar craton from ca. 3.5–2.5 Ga is reviewed and briefly compared with a second craton, Kaapvaal, to allow some speculation on the nature of global tectonic regimes in this period. The Dharwar craton is divided into western (WDC) and eastern (EDC) parts (separated possibly by the Closepet Granite Batholith), based on lithological differences and inferred metamorphic and magmatic genetic events. A tentative evolution of the WDC encompasses an early, ca. 3.5 Ga protocrust possibly forming the basement to the ca. 3.35–3.2 Ga Sargur Group greenstone belts. The latter are interpreted as having formed through accretion of plume-related ocean plateaux. The approximately coeval Peninsular Gneiss Complex (PGC) was possibly sourced from beneath plateau remnants, and resulted in high-grade metamorphism of Sargur Group belts at ca. 3.13–2.96 Ga. At about 2.9–2.6 Ga, the Dharwar Supergroup formed, comprising lower Bababudan (largely braided fluvial and subaerial volcanic deposits) and upper Chitradurga (marine mixed clastic and chemical sedimentary rocks and subaqueous volcanics) groups. This supergroup is preserved in younger greenstone belts with two distinct magmatic events, at 2.7–2.6 and 2.58–2.54 Ga, the latter approximately coincident with ca. 2.6–2.5 Ga granitic magmatism which essentially completed cratonization in the WDC. The EDC comprises 2.7–2.55 Ga tonalite–trondhjemite–granodiorite (TTG) gneisses and migmatites, approximately coeval greenstone belts (dominated by volcanic lithologies), with minor inferred remnants of ca. 3.38–3.0 Ga crust, and voluminous 2.56–2.5 Ga granitoid intrusions (including the Closepet Batholith). An east-to-west accretion of EDC island arcs (or of an assembled arc – granitic terrane) onto the WDC is debated, with a postulate that the Closepet Granite accreted earlier onto the WDC as part of a “central Dharwar” terrane. A final voluminous granitic cratonization event is envisaged to have affected the entire, assembled Dharwar craton at ca. 2.5 Ga. When Dharwar evolution is compared with that of Kaapvaal, while possibly global magmatic events and freeboard–eustatic changes at ca. 2.7–2.5 Ga may be identified on both, the much earlier cratonization (by ca. 3.1 Ga) of Kaapvaal contrasts strongly with the ca. 2.5 Ga stabilization of Dharwar. From comparing only two cratons, it appears that genetic and chronologic relationships between mantle thermal and plate tectonic processes were complex on the Archaean Earth. The sizes of the Kaapvaal and Dharwar cratons might have been too limited yet to support effective thermal blanketing and thus accommodate Wilson Cycle onset. However, tectonically driven accretion and amalgamation appear to have predominated on both evolving cratons.

Geochemical behaviour of dissolved trace elements in a monsoon-dominated tropical river basin, Southwestern India

The study presents a 3-year time series data on dissolved trace elements and rare earth elements (REEs) in a monsoon-dominated river basin, the Nethravati River in tropical Southwestern India. The river basin lies on the metamorphic transition boundary which separates the Peninsular Gneiss and Southern Granulitic province belonging to Archean and Tertiary-Quaternary period (Western Dharwar Craton). The basin lithology is mainly composed of granite gneiss, charnockite and metasediment. This study highlights the importance of time series data for better estimation of metal fluxes and to understand the geochemical behaviour of metals in a river basin. The dissolved trace elements show seasonality in the river water metal concentrations forming two distinct groups of metals. First group is composed of heavy metals and minor elements that show higher concentrations during dry season and lesser concentrations during the monsoon season. Second group is composed of metals belonging to lanthanides and actinides with higher concentration in the monsoon and lower concentrations during the dry season. Although the metal concentration of both the groups appears to be controlled by the discharge, there are important biogeochemical processes affecting their concentration. This includes redox reactions (for Fe, Mn, As, Mo, Ba and Ce) and pH-mediated adsorption/desorption reactions (for Ni, Co, Cr, Cu and REEs). The abundance of Fe and Mn oxyhydroxides as a result of redox processes could be driving the geochemical redistribution of metals in the river water. There is a Ce anomaly (Ce/Ce*) at different time periods, both negative and positive, in case of dissolved phase, whereas there is positive anomaly in the particulate and bed sediments. The Ce anomaly correlates with the variations in the dissolved oxygen indicating the redistribution of Ce between particulate and dissolved phase under acidic to neutral pH and lower concentrations of dissolved organic carbon. Unlike other tropical and major world rivers, the effect of organic complexation on metal variability is negligible in the Nethravati River water.

Geophysical investigations for groundwater in a hard rock terrain, Salem district, Tamil Nadu, India

The main aim of a geophysical investigation in a hard rock region is to solve different hydrogeological problems like depth of water table, basement rock, thickness of weathered zone/formation, structural and stratigraphic conditions relevant to groundwater conditions, and permeability of aquifers. A total of 54 vertical electrode soundings were carried out by Schlumberger electrode arrangement to signify complete lithology of the study area and interpretation was carried out by using curve matching techniques. From the study, 65 % of area is dominated by ‘A’ type curve indicating increasing resistivity with depth and 17 % by ‘K’ type representing a high-resistivity layer sandwiched between two low-resistivity layers. Four layer cases were also noted in 12 % of the study area indicating gradation between the weathered and fractured layers as a semi-weathered zone. Greater thickness has been identified in the weathered zone at the contact of peninsular gneiss and charnockite regions. The maximum, minimum, mean, and arithmetic mean were plotted and identified higher background resistivity values in charnockite. The overlay of maximum and minimum curves for different layers signifies variation of weathering intensity with depth or the presence of weaker zones. In general, lithology plays a significant control over weathering of rocks and groundwater potential.

Meso/Neoarchean crustal domains along the north Konkan coast, western India: The Western Dharwar Craton and the Antongil-Masora Block (NE Madagascar) connection

Synthesis of mesoscopic structures and microscopic analyses of fabric superposition and deformation microstructures are combined with Th–U–Pb (total) ages in monazites to constrain the Meso/Neoarchean crustal domains in the Western Dharwar Craton (WDC) along the western coast of India; the domains are correlated with those in NE Madagascar to configure the assembly of crustal domains in the East Gondwanaland prior to the Mesozoic break up. In the WDC, para-schists and phyllites of the Shimoga schist belt, SSB (unmixed age components, 3067±26 and 3158±110Ma) and the Peninsular gneisses (3138±35Ma) are intruded by granitoid plutons (2924±50Ma) deformed at low-T. Five generations of deformation events are identified in the SSB and Peninsular gneisses. But the two early tectonic fabrics in Peninsular gneisses formed at anatectic amphibolite facies conditions are in sharp contrast to fabrics in SSB formed at greenschist facies. The two lithodemic units are inferred to share a pre-3.2Ga tectonic contact.By contrast, in the Goa schist belt (GSB), phyllites/schists (unmixed ages – 2436±34, 2543±66 and 2625±36Ma) and polymict conglomerates (unmixed ages – 2458±34 and 2566±53Ma) possess two generations of tectonic fabrics and are the youngest dated metamorphic unit in the WDC. The deformed Quepem granitoid pluton located along the axial zone of the regional-scale N-trending gently plunging upright folds in GSB yields comparable chemical age (2565±33Ma). Angular to sub-rounded and boulder to granule sized polymict clasts of Peninsular gneisses (3128±60Ma), foliated granitoids, amphibolites, BIFs and low-grade quartzite/chlorite-muscovite schists in GSB para-conglomerates suggest the sediments deposited in shallow basin were derived from neighboring sources, e.g. the Peninsular gneisses and SSB supracrustals. The Neoarchean deformation-metamorphism marks the closure of the Goa basin synchronous with the 2.4–2.6Ga accretion of the West and the East Dharwar Cratons.Based on the results of this study, the following correlation among Mesoarchean–Neoarchean crystallites in the Western Dharwar Craton and the Antongil-Masora Block, NE Madagascar (in italics below) appears realistic, e.g. 2.4–2.6Ga Goa schist belt≈Mananara Group; 2.5Ga Quepem granitoid≈Masoala Suite; 3.1Ga Shimoga schist belt≈Fenerivo Group; 3.1Ga Peninsular gneiss suite≈Nosy Boraha suite.

An appraisal of Archaean supracrustal sequences in Chitradurga Schist Belt, Western Dharwar Craton, Southern India

Archaean metasedimentary sequences of the Chitradurga Schist Belt in Western Dharwar Craton were reassessed in terms of their protoliths, potential timing of deposition and metamorphic events along with metamorphic conditions. Existing geological and stratigraphic constraints and our field observations suggest that the oldest supracrustal sequence of the Sargur Group, which occurs as enclaves in Peninsular Gneiss, is overlain by the Dharwar Supergroup consisting of lower Bababudan and upper Chitradurga Groups. In the Chitradurga Schist Belt, the metamorphic conditions of each stratigraphic unit are distinct, with most prominent difference observed in the Chitradurga Group. The lower unit of this group are metamorphosed under biotite-muscovite grade, whereas the upper unit at chlorite-muscovite grade. Detrital zircon U–Pb dates obtained by ion microprobe analyses suggest 3.14Ga and 3.22–2.92Ga for the youngest protolith's magmatic ages, which constrain the oldest age limit of deposition of Bababudan Group and Lower Unit of Chitradurga Group, respectively. The depositional age of Upper Unit of Chitradurga Schist Belt can be bracketed by 2.68Ga magmatic zircon ages from meta-rhyolite and 2.63Ga magmatic detrital zircon age of the overlying sandstone that are significantly younger than the Lower Unit. Monazite U–Th–Pb ages are similar for both Lower and Upper Units of the Chitradurga Group (∼c. 2.4Ga) but are significantly younger than those of the underlying Bababudan and Sargur Groups (∼c. 3.1Ga). Combined zircon and monazite data imply that the continuous or composite Mesoarchaean-Neoarchaean supracrustal sequences of the Chitradurga Group are metamorphosed together at the latest Archaean-early Proterozoic. These regional metamorphic ages are not detected in the underlying Bababudan and Sargur Groups, as well as the basement Peninsular Gneiss. Contrasting depositional and metamorphic history in coherent Mesoarchaean and Neoarchaean strata in the Chitradurga Schist Belt have important implications for the comparison with the well-established sedimentary sequences of similar ages in the Pilbara and Kaapvaal cratons. The protolith and metamorphic ages of the Dharwar Supergroup, when compared with available similar information from the surrounding Archaean cratons in India, Madagascar, and East Antarctica, testify the probable Archaean connection, and give clues to the formation of deep to shallow Archaean crust.

Palaeoproterozoic sedimentation in the Cuddapah Basin, south India and regional tectonics: a review

Abstract The Proterozoic Cuddapah Basin of south India hosts a number of unconformity-bound sequences deposited over continental crust. The Palaeoproterozoic Papaghani Group with basal polymict conglomerate unconformably overlies the Archaean Peninsular Gneiss and set the stage for Palaeoproterozoic sedimentation following a long hiatus. Two cycles of fluvial–shallow-marine psammite–carbonate–pelite sedimentation in the western part of the basin (Papaghni sub-basin) is punctuated at different levels by mafic flows, sills and dykes, and less common acidic tuffs. Each of the sedimentation cycles fosters extensive carbonate platforms with dolomitic stromatolites and algal laminites. Emplacement of shallow crustal intrusives at the end of the first cycle was possibly linked to thermal anomalies in the mantle and associated widespread crustal extension in south India. Brittle fault-slip analysis from the western Cuddapah indicates repetitive extensional to strike-slip regimes. This paper proposes a likely late Palaeoproterozoic shallow-marine sedimentary sequence of the Nallamalai Fold Belt as allochthonous, thrusted over the successions of the Papaghni sub-basin and the Kurnool Group in the western part of the basin. Inversion of the Papaghni sub-basin and the development of regional erosional unconformities is discussed in the context of Palaeoproterozoic and early Mesoproterozoic orogenic events at the SE margin of India.

Geochemical systematics and precious metal content of the sedimentary horizons of Lower Gondwanas from the Sattupalli coal field, Godavari Valley, India

Abstract Major, trace, rare earth (REE) and platinum group elements (PGE) have been analysed from the carbonaceous horizons of Permian Barakar Formation of Sattupalli coal field of Godavari Valley to understand the provenance and depositional environment. Barakar Formation of coal field consists of sandstone, siltstone, carbonaceous shale and coal seams. Coal is enriched in trace elements compared with the Clarke's background values. Ferromagnesian trace elements (Ni, Cr and Co) are similar in range in both coal and carbonaceous shales along with Large ion lithophile elements (LILE; Rb, Sr, Cs and Ba), High field strength elements (HFSE; Nb, Ta, Zr and Ti), Light REE (LREE; La-Sm) and Ag. Archean Upper Crust (AUC) normalized REE and multielement variation patterns of the coal show a flat REE with a slight depletion in Ce and Eu contents in most of the samples. Their multielement patterns exhibit enrichment of Th and U; negative Ta, Zr, Hf and positive Ti anomalies. Pronounced positive anomaly occurs at Zr, Hf and Ti that are combined with both positive and negative Nb and Ta anomalies in carbonaceous shales and clay. Sandstones have a high SiO 2 (50–80 wt.%), low Fe 2 O 3 , Al 2 O 3 and moderately high K 2 O contents. AUC normalized REE patterns are almost flat with slight negative to no Eu anomalies followed by small negative Ce anomalies (Ce/Ce* = 0.87–0.95 with one exception). Their multielement patterns exhibit positive U, small negative Nb-Ta, a pronounced positive Zr-Hf anomaly followed by negative Ti and V anomalies. Enrichment of both Zr and TiO 2 in some samples reflects on the mafic and felsic sources at the source region. The fractionated REE patterns, HREE enrichment and negative Eu anomalies are consistent with contribution of zircon through felsic source. Nb/Ta troughs endorse their inheritance from schistose rocks of Eastern Ghats complex. The high Al 2 O 3 /Yb and Th/Yb ratios along with negative Eu anomalies reflect on the TTG/felsic rocks of provenance. Based on the geochemical, available petrological and paleocurrent analysis indicate the source rocks of the Barakar Formation were granite, granitic gneiss, high grade metamorphic rocks from Eastern Ghats, and basic rocks. The provenance is located west and south east (i.e. partly Peninsular gneiss and partly Eastern Ghat Complex). The Au-PGE contents of Sattupalli coalfield have been deposited through NE–SW trending post depositional transverse faults that have affected all the Gondwana Formations during the early Cretaceous. The mafic rocks present within the basement could have been the source of these elements.

Assessment of groundwater quality with special emphasis on fluoride contamination in crystalline bed rock aquifers of Mettur region, Tamilnadu, India

Groundwater samples were collected from Mettur taluk of Salem district, Tamilnadu, India for two different seasons (pre-monsoon and post-monsoon) and analyzed for fluoride ion along with other chemical parameters. The major litho units of the study area are Charnockites, peninsular gneiss, and calc gneiss of meta-sedimentary group. The fluoride concentration ranges from 0.1 to 2.8 mg/L and 0.4 to 4.0 mg/L during pre-monsoon (PRM) and post-monsoon (POM) seasons, respectively. Results showed that collected water samples were contaminated by the presence of fluoride ion. During PRM and POM, 21% and 56% of samples recorded higher fluoride when compared with Indian Drinking Water Standard (1 mg/L) and (9% and 35%) of samples recorded higher fluoride when compared with World Health Organization tolerance limit (1.5 mg/L). The ratio of Na/Ca indicates high sodium content in groundwater enhances the dissolution of fluoride at higher pH. Hydrogeochemical facies indicates water-rock interaction as main source for high fluoride in groundwater. A positive correlation between pH, Mg, and F indicates high alkaline nature of water promotes fluoride leaching from source rocks into ground water. Factor analysis indicates hydro-geochemical processes like weathering, ion exchange, and anthropogenic contributes to groundwater chemistry. The saturation index indicates dissolution and precipitation contributes fluoride dissolution along with mixing.

A subsurface criterion for predictive exploration of kimberlites from Bouguer gravity in the eastern Dharwar craton, India

The Archaean-Proterozoic Dharwar craton has many recorded occurrences of diamondiferous kimberlites. Reports of kimberlite emplacement in parts of the tectonically complex eastern Dharwar craton and a significant density contrast between kimberlites and the host peninsular gneisses motivated us to conduct gravity studies in the Narayanpet-Irladinne area of the eastern Dharwar craton. This region is contiguous with the Maddur-Narayanpet kimberlite that lies to its north, while the river Krishna lies to its south. From observed association of reported kimberlites in the Maddur-Narayanpet field with subsurface topography of the assumed three-layer earth section obtained by Bouguer gravity modelling, we developed a subsurface criterion for occurrence of kimberlites in the present study area. Using this criterion, five potential zones for kimberlite localization were identified in the Narayanpet-Irladinne region, eastern Dharwar craton.

REE-HFSE distribution/partitioning between garnetiferous restites and TTG from Nademavinapura area, Western Dharwar craton

The major part of the Peninsular Gneiss in Dharwar craton is made up of Trondjhemite-Tonalite-Granodiorite (TTG) emplaced at different periods ranging from 3.60 to 2.50 Ga. The sodic-silicic magma precursors of these rocks have geochemical features characteristic of partial melting of hydrated basalt. In these TTGs, enclaves of amphibolites (± garnet) are abundant. These restites are considered to be the residue of a basaltic crust after its partial melting. A detailed study of these (residue) enclaves reveals textures formed due to the process of partial melting. Major, trace and REE analysis of these residue enclaves and the melt TTGs and microprobe analysis of the coexisting minerals show partitioning of REE and HFSE between the precursor melt of TTGs and the upper amphibolite facies residues. Formation of garnetiferous amphibolites with biotite, Cpx and plagioclase consequent to melting, has squeezed the original MORB type of basaltic crust and given rise to the TTGs, depleted in Y, Yb, K2O, MgO, FeO, TiO2 and enriched in La, Th, U, Zr and Hf. Coevally during the process of melting, the hydrated basalt was depleted in Na2O, Al2O3, LREE, Th, U and enriched in K2O, MgO, Nb, Ti, Yb, Y, Sc, Ni, Cr and Co. Mineral chemistry of co-existing garnet-biotite and amphibole-plagioclase in these amphibolitic (restite) enclaves indicates an average temperature of 700 ± 50° C and pressure of 5 ± 1 Kbar. These data are inferred to indicate that during the garnet stability field metamorphism, effective fractionation of HREE and HFSE has taken place between the restites having Fe-Mg silicates, ilmenites and the extracted melt generated from the MORB type of hydrated basalt. These results are strongly substantiated by the reported melting experiments on hydrated basalts.

Precambrian mafic magmatism in the Western Dharwar Craton, southern India

Mafic rocks of Western Dharwar Craton (WDC) belong to two greenstone cycles of Sargur Group (3.1-3.3 Ga) and Dharwar Supergroup (2.6-2.8 Ga), belonging to different depositional environments. Proterozoic mafic dyke swarms (2.4, 2.0-2.2 and 1.6 Ga) constitute the third important cycle. Mafic rocks of Sargur Group mainly constitute a komatiitic-tholeiite suite, closely associated with layered basic-ultrabasic complexes. They form linear ultramaficmafic belts, and scattered enclaves associated with orthoquartzite-carbonate-pelite-BIF suite. Since the country rocks of Peninsular Gneiss intrude these rocks and dismember them, stratigraphy of Sargur Group is largely conceptual and its tectonic environment speculative. It is believed that the Sargur tholeiites are not fractionated from komatiites, but might have been generated and evolved from a similar mantle source at shallower depths. The layered basic-ultrabasic complexes are believed to be products of fractionation from tholeiitic parent magma. The Dharwar mafic rocks are essentially a bimodal basalt-rhyolite association that is dominated by Fe-rich and normal tholeiites. Calc-alkaline basalts and andesites are nearly absent, but reference to their presence in literature pertains mainly to carbonated, spilitized and altered tholeiitic suites. Geochemical discrimination diagrams of Dharwar lavas favour island arc settings that include fore-, intra- and back-arcs. The Dharwar mafic rocks are possibly derived by partial melting of a lherzolite mantle source and involved in fractionation of olivine and pyroxene followed by plagioclase. Distinctive differences in the petrography and geochemistry of mafic rocks across regional unconformities between Sargur Group and Dharwar Supergroup provide clinching evidences in favour of distinguishing two greenstone cycles in the craton. This has also negated the earlier preliminary attempts to lump together all mafic volcanics into a single contemporaneous suite, leading to erroneous interpretations. After giving allowances for differences in depositional and tectonic settings, the chemical distinction between Sargur and Dharwar mafic suites throws light on secular variations and crustal evolution. Proterozoic mafic dyke swarms of three major periods (2.4, 2.0-2.2 and 1.6 Ga) occur around Tiptur and Hunsur. The dykes also conform to the regional metamorphic gradient, with greenschist facies in the north and granulite facies in the south, resulting from the tilt of the craton towards north, exposing progressively deeper crustal levels towards the south. The low-grade terrain in the north does not have recognizable swarms, but the Tiptur swarm consists essentially of amphibolites and Hunsur swarm mainly of basic granulites, all of them preserving cross-cutting relations with host rocks, chilled margins and relict igneous textures. There are also younger dolerite dykes scattered throughout the craton that are unaffected by this metamorphic zonation. Large-scale geochemical, geochronological and palaeomagnetic data acquisition through state-of-the-art instrumentation is urgently needed in the Dharwar craton to catch up with contemporary advancements in the classical greenstone terrains of the world.

Petrography and geochemical characteristics of the sediments of the small River Hemavati, Southern India: Implications for provenance and weathering processes

Small rivers (≤ 100 km length) are likely to drain fewer rock types. Therefore, their solutes and sediments are good indicators of weathering environments typical of their basins and help constraining the nature of their source rocks. To understand this, the texture, mineralogy, major and trace element compositions of the sediments deposited by the River Hemavati, a northern upland tributary of the Cauvery River in southern India, are analyzed and discussed. The Hemavati sediments are overall of fine sand size (mean 2–3ϕ), and have high concentrations of FeO (≤ 7 wt.%), TiO 2 (≤ 1.2 wt.%), Cr (≤ 350 ppm) and Ni (≤ 125 ppm). Major and trace element distribution call for a binary source for the sediments, and particularly point to contrasting climatic conditions of their provenances. The source areas in the upstream and downstream parts are exposed to sub-humid high relief and sub-arid low relief conditions, respectively, with distinct weathering characteristics. The CIA values (85–48) decrease from near the source to downstream, suggesting that the downstream rain-shadow part of the catchment suffered only minor chemical weathering. On the other hand, the REE distribution in the Hemavati sediments indicates contrasting lithologies in their provenance, and is not controlled by chemical weathering. On the basis of REE patterns, the sediments are divided into two compositional groups. The Type 1 sediments have a REE chemistry similar to the upper continental crust, and have been derived from the > 3.2 Ga composite peninsular gneisses occurring in the low-lying, semi-arid Mysore Plateau. The Type 2 sediments, however, have dominantly intermediate to mafic granulite contributions from the tectonically uplifted Western Ghats, weathered under sub-humid conditions. High concentrations of FeO, TiO 2, Cr and Ni in the sediments suggest mafic-dominated source lithologies in the upper catchment, a feature also confirmed by field observations and petrographic study.

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.

Comments on: “Trace element and isotopic evidence for Archean basement in the Lonar crater impact breccia, Deccan volcanic province” by Ramananda Chakrabarti and Asish R. Basu in Earth and Planetary Science Letters 247 (2006) 197–211

The effect of meteorite impact on basement rocks remains widely debated at Lonar Crater, Central India. We investigate an impact melt-bearing rock from the ejecta layer of Lonar to assess the potential interaction of the projectile with the underlying Archean basement. The investigated sample is a melanocratic, aphanitic melt rock comprising glassy inclusions, and plagioclase and quartz clasts. Plagioclase and quartz show shock-induced features including diaplectic glass and well-developed planar deformation features (PDFs), respectively. The decrease in shock pressure is thus established with the plagioclase of overlying basalt showing shock features like diaplectic glass whereas the quartz from lower Archean basement showing PDF development. Laser Raman spectroscopy reveals the presence of cristobalite. The impact melt-bearing rock shows major element geochemistry similar to that of Deccan basalt with a tholeiitic trend, but the Harker variation plots reveal an independent field for the melt rock between the two end members: Deccan basalt and Archean basement represented by Peninsular gneiss. The REE geochemistry also reveals more fractionation for the melt rock. Zircon U-Pb data indicate an upper intercept age around 3.0 to 3.1 Ga. Chondrite normalized zircon REE data display a positive Eu anomaly indicating a reheating during the impact event. This indicates that the melt rock sample was generated by the incorporation of Archean basement gneiss components, which is compatible with the estimated depth of excavation for an impact structure of this size. This specifies a depth to basement in the range between 522 and 570 m.

Crustal configuration of the Dharwar craton, India, based on joint modeling of regional gravity and magnetic data

Regional gravity and magnetic investigations were conducted along a 600 km transect across the northern part of the Dharwar craton in the Precambrian Indian peninsular shield from Vasco-da-gama (latitude 15°23′40″N and longitude 73°48′55″E) to Jadcharla (latitude 16°41′40″N and longitude 78°08′18″E). Joint modeling of the two data sets was carried out utilizing GM-SYS (Geosoft) software assuming a four-layer model. A tectonic re-classification of the craton is proposed, prompted by the location of four inferred deep-seated faults, into the western and eastern Dharwar Blocks with an intervening upthrown block. This classification is at variance with the current perceptions of the configuration of the Dharwar craton, some of which hold that the craton consists of only two sub-cratons, an eastern and a western, divided by the Chitradurga thrust fault. However, in the present study, this thrust fault is found to extend no deeper than the basement peninsular gneisses. Furthermore, according to the proposed classification, the thrust fault falls within the intervening block that forms the divide between the sub-units of the craton rather than representing a tectonic boundary. In addition, twenty-six near surface geological members in the region (supracrustals) consisting mainly of schists and younger granites were identified along the transect and modeled for the representative near-surface crustal configuration of the craton.

Fluoride incidence in groundwater in an area of Peninsular India

Groundwater samples were collected from Anantapur District, Andhra Pradesh, India. The district is mainly underlain by Peninsular Gneisses of Archaean age. The samples were analysed for fluoride (F−) along with other chemical parameters. The results suggest that the main sources of F− in groundwater in the district are the country rocks, in which fluorine is strongly absorbed in soils consisting of clay minerals. A strong positive correlation between F− and lithogenic sodium reflects weathering activity. This is responsible for the leaching of F−, which is also caused by the semi-arid climate and intensive irrigation in the area. An alkaline environment of circulating water in the investigated area mainly facilitates leaching of F− from the soils, contributing to high F−-containing groundwater. A longer residence time of water in the aquifer zone, caused by a high rate of evapotranspiration and a weathered zone of low hydraulic conductivity, which promotes the dissolution of fluorine-bearing minerals, is another factor that further increases the F− content in groundwater. Suggestions are made to improve groundwater quality and, thus, the health status of the population.