Western Dharwar Craton Research Articles

Gold Mineralisation and their Lithological Controls at Nagavi Area, Gadag Schist Belt, in Karnataka, India

Dharwar craton is located in southern India is part of the Dharwar-Singhbhum proto-continent and is known for its complex tectonic and structural controls. It’s made up of two distinct parts, Western Dharwar Craton (WDC) and the Eastern Dharwar Craton (EDC) house several schist belts. Many of these schist belts, such as the Kolar schist belt, Hutti schist belt, Chitradurga schist belt and the Gadag schist belt, are auriferous. In such schist belts, gold occurs in association with sulphides such as pyrite, pyrrhotite, arsenopyrite and chalcopyrite. Gold mineralization in these belts is found to occur in different lithologies. The Gadag schist belt is composed dominantly of metabasalt in its western half and meta sediments in the eastern half. Quartz-carbonate veins are observed to cut across these lithologies as well as the Banded Iron Formations (BIFs). This paper aimed to study the gold mineralization in different lithologies of the Gadag schist belt and found that metabasalts and BIFs are the dominant rocks hosting gold. It is also observed that mineralization is strata-bound. Most of the places’ auriferous sheared zones were observed in the study area. In Nagavi area, BIF hosted gold mineralization is observed, and major gold concentration varies from <21 to <26 ppb (parts per billion). Pyrite occurs as characteristic cubic crystals and as clusters of small grains. In carbonated sheared anorthosite, the rock is fine to medium-grained and in-equigranular. It consists of simple to polysynthetically twinned subhedral (100-200 by 400-600 μm) laths (~40-45%) set in a groundmass of irregular anhedral grains of secondary carbonate (~35-40%), chlorite (~8-10%), opaque (~5-7%) and accessory epidote and quartz.

Crustal structure beneath the Western Dharwar Craton segment of Western Ghats: Insights from ambient noise correlation technique

The Western Ghats (WG) comprise three major tectonic blocks: the Deccan Volcanic Province (DVP), Western Dharwar Craton (WDC) and Southern Granulite Terrain (SGT) from north to south. Among these, the WDC region encompasses the oldest basement rocks, including greenstone belts and calc-alkaline plutons dating back to ∼3.0 to 3.4 Ga. While the crustal structure of the DVP and SGT segments of the WG has been extensively studied, knowledge regarding the crustal structure beneath the WDC segment of the WG is limited due to the scarcity of seismic stations. To address this gap, we utilized continuous waveform data from a newly established network of seismic stations by the National Centre for Earth Science Studies (NCESS), in addition to other available stations and deciphered the shallow crustal shear-wave velocity structure along a profile beneath the WDC segment of the WG using ambient noise cross-correlation and surface wave dispersion. The results reveal a continuous upper layer characterized by a high shear wave velocity of ∼3.6 km/s with notable variation in thickness along the profile. Furthermore, a thick middle layer with velocities reaching up to ∼3.9 km/s and a lower layer with velocities of ≥4.2 km/s are observed. Intriguingly, variations in the thickness of the upper layer, ranging from 5 km to 20 km, are noted beneath the study region. The thickness of this layer exhibits changes in relation to topographical variations, with this dependency diminishing for deeper layers. Notably, high shear wave velocity perturbations (∼10 %) are observed in the upper layer beneath low-elevation regions, possibly indicating the presence of Tonalite–Trondhjemite–Granodiorite (TTG)-type gneiss − a significant rock type in the WDC region, alongside greenstone belts and late calc-alkaline to potassic plutons. These findings suggest that the uneven topography of the WG may result from long-term erosion, exposing deeper and higher-density rocks and leading to higher shear wave velocities at shallow depths. This study serves as a foundation for further exploration into the influence of the upper crustal layer and its correlation with the evolution of the WG.

Zircon U[sbnd]Pb geochronology and geochemistry of hot subduction volcanic suite in the deep bore-hole well-core KBH-5: Evidence of Neoarchean Shimoga greenstone belt beneath the Cretaceous Deccan Traps, India

The continents across Earth have grown through accretion and amalgamation of plume-generated oceanic plateaux and subduction-originated magmatic arcs. The evidence of such evolutionary processes is well preserved in the greenstone belts of Canada, South Africa, China, Australia and India. Unlike accretion that leads to lateral growth of the continents, flood basalt volcanism results in the superposition of large volumes of dense mafic material above relatively less dense sialic upper continental crust. The resulting density inversion is liable to generate tectonic stresses resulting in faulting of under burden along weak planes. The nature of the crust obscured beneath flood basalt provinces that have accumulated over the past 500 My in Large Igneous Provinces (LIPs) has remained enigmatic, largely because of the voluminous lava pile that deters access to the concealed basement, unless otherwise retrieved through deep continental drilling. Here, we present in situ zircon UPb geochronology and bulk-rock geochemistry of a volcanic sequence discovered at −400 m m.s.l in the deep bore-hole well-core KBH-5 of the southwestern Deccan Traps, India. The ∼300 m thick metavolcanic succession includes a basalt – Nb-enriched basalt – andesite – Mg-andesite – dacite – adakite suite. The zircons yielded a 2.58 Ga U-Pb age for the suite, which also incorporates entrained zircon xenocrysts of 2.7 Ga age from a preexisting juvenile material during its emplacement. The bore-hole KBH-5 is located on an NNW-SSE trending normal fault, paralleling a series of major faults west of the Western Ghat Escarpment. Although the crustal block plummeted ∼1000 m downward along the fault plane, the geochemical attributes of the metavolcanic rocks are thoroughly comparable to those reported in the Medur Formation of the Shimoga greenstone belt, ∼300 km south of KBH-5 in the western Dharwar craton. Our study provides the first physical evidence of Dharwar greenstone belt(s) extending further beneath the Deccan Traps, which was previously only speculative based on short-wavelength gravity highs. The metavolcanic sequence evolved in a subduction-related back-arc tectonic regime and accreted to the active continental margin of India in the Neoarchean. Unveiling of potentially similar scenarios in cratons elsewhere entrapped beneath the vast canopy of continental flood basalts, would reveal new crustal growth events during the Archean -Proterozoic transition across the Earth.

Petrology, U-Pb titanite dating and Sr-Nd isotope geochemistry of a shoshonitic lamprophyre dyke near the Western Dharwar Craton-Southern Granulite Terrane contact, southern India: Implications for long-lived enriched mantle, widespread Tonian-Cryogenian rifting, and Rodinia configuration

We present petrology, U-Pb titanite geochronology, bulk-rock and Sr-Nd geochemistry of an undeformed and unmetamorphosed shoshonitic lamprophyre (spessartite) from the Mysuru area (Halaguru- Harohalli alkaline province) located approximately 80 km north of the tectonic contact between the Western Dharwar Craton and the Southern Granulite Terrane, southern India, to understand its origin and to evaluate its geodynamic significance. The lamprophyre shows a typical porphyritic-panidiomorphic texture with amphibole phenocrysts. Clinopyroxene occurs only as xenocrysts with feldspar, apatite, titanite and titano-magnetite confined to the groundmass. Amphibole mineral composition classifies them as pargasite and tremolite, and the groundmass is essentially constituted of alkali feldspar. High whole-rock Mg# (56–65), Ni + Cr (245 to 730 ppm), a lack of correlation between SiO2 and Rb/Sr as well as ɛNd(i) indicates that crustal contamination experienced by the lamprophyre magma is limited. U-Pb titanite (n = 18) SHRIMP-II dating yielded a Neoproterozoic Tera-Wasserburg concordia age of 820 ± 15 Ma (MSWD = 5.9). Whole-rock derived 87Sr/86Sr(i) (0.70425–0.70530) and ɛNd(i) (-6.3 to −4.3) suggest origin of the lamprophyre from an enriched lithospheric mantle source. Paleoproterozoic depleted-mantle model ages of ca. 1.8 Ga suggest the existence of a long-lived enriched mantle which correspond to the timing (ca.1.9–1.7 Ga) of widespread mafic dyke swarms’ emplacement in the Dharmapuri-Agali-Tiruvannamalai domain in the Salem block at the northernmost portion of the Southern Granulite Terrane. The Tonian emplacement age of the studied lamprophyre is synchronous with several co-spatial Neoproterozoic (i) alkaline complexes ± carbonatite ± lamprophyre from the northern part of the Southern Granulite Terrane, and (ii) other mafic alkaline dykes of the Halaguru-Harohalli alkaline province from the southern part of the Eastern Dharwar Craton. Our results highlight the importance of this lamprophyre dyke, present close to the’Fermor Line’ as well as near the boundary between Eastern and Western Dharwar Cratons, which provides a rare opportunity to understand its spatio-temporal link with alkaline dykes of the Southern Granulite Terrain. Our findings also help understand the role of repeated large-scale rifting, associated with the Rodinia break-up, leading to the generation of a wide spectrum of Tonian-Cryogenian alkaline magmas in southern Indian Shield. The new age data also have implications for the configuration of India’s paleoposition during the Neoproterozoic and support the idea that South China Block-India were located on the periphery of Rodinia or were already detached from the supercontinent.

P–T path reconstruction in a multi‐layered garnet‐bearing schist: Evidence for 2.2–2.1 Ga near‐isothermal loading in the northern margin of the Western Dharwar Craton (India), and regional implications

P–T paths were reconstructed using P–T pseudosection modelling from garnets in a finely laminated mylonitic hornblende‐biotite schist with four sub‐cm wide mineralogically distinct layers separated by poly‐crystalline quartz layers from the northern margin of the Western Dharwar Craton (WDC). The almandine‐grossular‐rich garnets, syn/post‐tectonic with respect to the mylonite foliation, exhibit considerable variations in the zoning patterns of divalent cations in the different layers. By contrast, the compositions of the matrix minerals dominated by biotite, chlorite, hornblende and plagioclase are tightly constrained within and across the layers. The varying abundances of major element oxides in the layers reflect variations in the abundances of the minerals, rather than the mineral compositions. P–T conditions estimated using mineral thermo‐barometry in the garnet‐muscovite‐biotite‐plagioclase assemblages are 520–575°C and 6–8 kbar; somewhat higher P–T values (615–665°C, 7–9 kbar) are obtained from hornblende‐garnet‐plagioclase‐quartz assemblages. The P–T paths obtained from the multi‐mineralic layers using P–T pseudosection modelling yield near‐isothermal loading (4–7 kbar) at 500–550°C in all layers. The near‐isothermal loading experienced by the multi‐layered rock suggests rapid crustal thickening involving overthrusting followed by nucleation of steeply dipping transpressional shear zones due to oblique convergence of the WDC with a crustal domain in the north. U‐Th‐total Pb chemical dating of metamorphic monazites indicates the crustal convergence occurred at 2.2–2.1 Ga. The event does not have known equivalents in the neighbouring crustal domains eastwards in India as well as in Madagascar in the west. This suggests the crustal domain may be an exotic block.

Trondhjemites from the Western Dharwar Craton, Southern India: Implications for Mesoarchean crustal growth

Trondhjemite, together with tonalite and granodiorite (TTG) are important building blocks of the Archean continental crust and their studies provide insights into the formation of the early continents. The Western Dharwar Craton (WDC) in southern India contains voluminous TTG rocks that record prolonged Archean crustal evolution history with evidence for multiple episodes of magmatic accretion and metamorphic events from the Paleoarchean (∼3600 Ma) to Archean-Proterozoic transition (∼2500 Ma ago). In this study, we investigate trondhjemites from different typical locations in the WDC. Petrological, geochemical and zircon UPb, REE and LuHf data are presented and evaluated to understand the timing of magma emplacement and petrogenetic history. The trondhjemites in our study were generated from partial melting of juvenile hydrous basalt with arc features. The melting depth is variable, mainly shallow, while some melts originated at greater depths with variable plagioclase, amphibole with or without garnet, ilmenite or rutile in source residue. The age data presented here suggest that the crust accretion of the Mesoarchean Western Dharwar Craton occurred dominantly at 3.13–3.27 Ma followed by reworking of lower crustal basement 3.0–2.9 Ga.

Paired metamorphism in peel back tectonics at the Archaean-Proterozoic boundary: New insights into Early Earth Plate tectonics from the Western Dharwar Craton, South India

In this study, we deduce a spatially and temporally linked paired metamorphism from the Western Dharwar Craton (WDC), South India, for which we use representative rocks from the medium-grade (MG) (e.g. pelitic schist) and high-grade (HG) (e.g. mafic and metasedimentary granulites) domains in the craton. Using metamorphic reconstructions (P-T-t paths of evolution and thermobaric, T/P ratios at the metamorphic peak), garnet-biotite diffusion chronometry, monazite chemical dating techniques of the investigated rocks and published metamorphic and geochronological dataset, we demonstrate that the two domains, despite having a record of uniform burial to lower crustal depths, differ in their thermal (T-t) history. The medium-grade domain records relatively cooler, intermediate T/P type of metamorphism in the stability field of kyanite with TMax at ∼650-630°C, multiple metamorphic cycles, all along clockwise P-T paths of evolution, and representing recurring burial-exhumation and heating-cooling cycles and finally, relatively short-lived nature of residence at the medium- to lower crustal levels (Δt∼10-20 Myrs) and faster metamorphic cooling (dT/dt ∼10-20°C/Myr) and exhumation rates (dz/dt∼0.6-1.1 to 1-2 mm/yr). The high-grade domain, in contrast records relatively warmer, high T/P metamorphism (T/P ratios at ∼800-870°C/GPa) in the stability field of sillimanite with TMax at ∼800–870°C, recurring burial-exhumation stages as part of one prograde heating event along a monocyclic CW P-T path of evolution and finally, extremely long-lived lower-mid-crustal residence (Δt∼238 Myrs) and ultra-slow cooling (∼2°C/Myr) and exhumation (∼0.1 mm/yr) history. Both domains show broadly coeval metamorphism as part of the same orogenesis between c.2.53 and 2.41 Ga. We use these findings from the WDC to suggest the development of a paired metamorphism as part of a peel-back convergence style of plate tectonics at the Archaean-Proterozoic boundary in the planet Earth.

Paleomagnetism and rock magnetism of ∼2.37–1.89 Ga dykes from the Western Dharwar craton, southern India: Implications for drift history through paleoproterozic

Paleomagnetism and rock magnetism of ∼2.37–1.89 Ga dykes from the Western Dharwar craton, southern India: Implications for drift history through paleoproterozic

Zircon U–Pb ages and Hf and O isotope systematics of crustal zircons from Mesoproterozoic kimberlites of the Dharwar craton, India: Implications for Neoarchean craton assembly

Plate tectonic models for the amalgamation of the dominantly Paleo-Mesoarchean Western Dharwar Craton (WDC) and Neoarchean Eastern Dharwar Craton (EDC) remain inconclusive on the polarity of Neoarchean subduction. Here we present insitu U-Pb age and Hf-O isotopic compositions of xenocrystic zircons in ca. 1.4-1.1 Ga kimberlites and lamproites emplaced within the ca. 2.8-2.5 Ga EDC as deep probes of the crustal architecture across the WDC-EDC boundary. The different zircon populations yield wide range of ages extending back to ca. 3.6 Ga, including distinctly ‘kimberlitic zircons’ with a mean 206Pb/238U age of 1112 ± 42 Ma, consistent with previous age estimates for kimberlite magmatism. The presence of zircons with ages >3.33 Ga is surprising as this age group of zircons is atypical of magmatic rocks in the EDC, but wide-spread in the WDC. Also, the highest concentration of zircons with ages >3.0 Ga is found in the kimberlites closest to the notional boundary shear zone between the WDC and EDC. The Hf isotope systematics of a majority of magmatic zircons with ages between 2.75-2.50 Ga require melting of older (3.6-3.3 Ga) source rocks. O-isotope data for these Neoarchean zircons show a wide scatter, lie both above (δ18O to 13.5 ‰) and well below mantle values (δ18O to <2 ‰). δ18O values outside the mantle range are closely associated with low εHf. The data are interpreted as evidence for transportation of older upper crustal rocks to lower crust and mantle depths consistent with tectonic models invoking collisional under thrusting of WDC beneath EDC.

The elusive crustal resistive boundary beneath the Deccan Volcanic Province and the western Dharwar craton, India

The electrical properties of the boundary beneath the Deccan Volcanic Province and the western Dharwar craton are imaged by using the magnetotelluric method. The magnetotelluric study was carried out along a 150km long WNW-ESE profile from Belgaum (in the Deccan Volcanic Province) to Haveri (in the western Dharwar craton).Data from 19 magnetotelluric stations spaced 10-15km apart were used. The dominant regional geo-electric strike direction obtained is N20ºE. Two-dimensional (2-D) inversion is done by using the non-linear conjugate gradient scheme for both apparent resistivity and phase. The 2-D resistivity model shows a high electrical resistivity character (&gt;10,000ohm-m) in the western Dharwar craton. Two conductive anomalies are mapped in the crustal region. In the WNW side of the profile, a conductive feature (~200ohm-m) is imaged in the mid-lower crust and, in the central part of the profile another conductive feature is mapped in the lower crust. The conductive features robustness is tested using linear and non-linear sensitivity analyses. The conductor mapped in the WNW part of the profile is considered as a deep seated fault representing a boundary or a rift related feature beneath the Deccan Volcanic Province and the western Dharwar craton. A zone of enhanced conductivity (&lt;50ohm-m) at an approximate depth of 10-30km may represent the presence of the rift in the region. This conducting feature on the Western side of the E-W trending Kaladgi Basin can be interpreted as the extension of the Kaladgi Basin further west. A well-correlated geological cross-section is also derived to interpret the resistive features mapped in this study. The electrical resistivity nature of the crust is compared with other regions of the world.

Pioneer Studies on Metagenomic Evaluation of Diversity of Microbial Community in Banded Iron Formation (BIF) from Iron Ore Mining Belt of Goa, India

Goa is situated in the northwestern part of the Western Dharwar Craton (WDC) which is Asia’s major metallogenic province. The iron ore of Goa is associated with greenstone and occurs as bands, reefs, and lenses of Banded Hematite Quartzite (BHQ) and Banded Magnetite Quartzite (BMQ) none of these are explored for geo-microbiological dimensions, such as detection of microbioform Gold. The presence of metalliferous WDC in Goa affords sampling of auriferous materials from mining area having hitherto BHQ at latitude 15°29′54″ N and longitude 74° 03′44″ E) and BMQ at latitude 15°24′12″ N and longitude 74° 09′31″ E were collected, surface sterilized and drilled with a sterile driller to get the endolithic material under sterile conditions and sent for metagenomic analysis using Oxford nanopore sequencing. Both the samples showed the presence of microorganisms with Archea, such as Haloferax, Nitrososphaera, and Eubacteria, i.e., Bacillus, Ralstonia, Cupriavidus, Burkholderia, and Acetobacter. The traditional model of Banded Iron Deposition assumes precipitation by the oxidation of hydrothermal Fe (II), either via abiotic oxidation or biotic oxidation through chemo lithotrophic Eubacteria, such as cyanobacteria which produce oxygen. This paper reports the diversity and the presumptive role of these microorganisms in the biogeochemical cycling of metals, such as Fe, Mn, and gold (Au).

Role of Depleted-MORB Mantle in the Genesis of Basalts from the Neoarchean Eastern Felsic Volcanic Terrane of the Sandur Greenstone belt, Dharwar Craton, India

Abstract The mafic volcanic rocks from the eastern felsic volcanic terrane (EFVT) of the Neoarchean Sandur greenstone belt of Western Dharwar Craton (WDC) are studied to evaluate their geochemical characteristics and tectonic implications. Characterised as sub-alkaline, tholeiitic basalts, they display moderate SiO2 (49.82-53.74 wt.%), Al2O3 (9.08-15.91 wt.%), TiO2 (0.25-0.66 wt.%) and total alkalies (Na2O+K2O= 0.60-3.97 wt.%) with Mg# ranging from 37-55 at low Cr (7-33 ppm), Ni (8-24 ppm) and V (252-612 ppm). Their rare earth and trace element patterns are coherent exhibiting moderate fractionation (La/Yb=0.16-1.09), slight negative Eu anomalies indicating limited plagioclase fractionation (Eu/Eu*= 0.69-0.96), negative Nb, Ta, Ti and Zr-Hf anomalies revealing their arc signatures. The Nb-Th and Zr-Y relationships of these basalts suggest their derivation from the partial melting of depleted mantle in an intra-oceanic arc setting. Trace element modelling of these basalts indicates their genesis by ~15% partial melting of depleted MORB-mantle and depletion of the source is attributed to previous melting events. These basalts are geochemically similar with the depleted arc and forearc basalts of the Phanerozoic Tonga and Izu-Bonin-Mariana arcs emphasising the operation of analogous tectonic processes during the Archean era.

Aspect ratio analysis of distinct Paleoproterozoic mafic dyke swarms and related fracture systems in the Eastern Dharwar Craton, India: Implications for emplacement mechanism and depth of origin

Understanding emplacement mechanism and depth of origin of continental mafic dyke swarms is pivotal in resolving outstanding questions regarding magma flow dynamics, plumbing system architecture, and amount of crustal dilation related to large igneous provinces. Paleoproterozoic mafic dyke swarms are often deemed as suitable candidates to establish the crustal evolution of cratonic regions. Here, we present the geometric and statistical analyses of five distinct Paleoproterozoic mafic dyke swarms and associated fracture systems in the Eastern Dharwar Craton (EDC) to constrain their magmatic overpressures and magma chamber depths. The geometric analysis is further validated with field evidence and measurements. It is suggested that mafic dykes of different swarms are formed primarily due to vertical magma injection from deep-seated magma reservoirs, followed by lateral magma flow at shallower depths. This study suggests a direct derivation of magma from deep-seated magma reservoirs, extending from lower crustal depths (∼17 km) to the crust-mantle boundary (∼36 km). It is also suggested that the emplacement of mafic dykes at ca. 2.37–2.36 Ga, ca. 2.26–2.25 Ga, ca. 2.22 Ga, and ca. 2.21 Ga can be traced back to the development of fracture system formed during and subsequent to the Neoarchean accretion event of Eastern and Western Dharwar Cratons (< 2.52 Ga). Therefore, the emplacement systems of the studied mafic dyke swarms have been largely controlled by the regional stress field and large-scale structural architecture of the Dharwar Craton.

Petrogenesis of Neoarchean granitoids beneath the Koyna-Warna region, Deccan Volcanic Province, India

• A new find of ca. 2.5 Ga granitoids in the western Dharwar craton. • These I-type clac-alkaline granitoids are bimodal in composition. • Trace element patterns are identical to Archean TTGs. • Precursor magmatic sources evolved in a Neoarchean subduction zone. The continental crust that is concealed beneath the voluminous flood basalt provinces of the Deccan Traps (India) is inaccessible except by few deep bore-holes. The well-core extracted from the deep borehole KBH-8 (> 1000 m) penetrating the massive flood basalt cover provides an exclusive opportunity to examine the Precambrian continental crust in this part of the Indian shield. Here, we present detailed petrology, geochemistry and zircon U-Pb geochronology of the KBH-8 granitoids, which are granodiorite and monzogranites by their normative composition showing a magnesian calc-alkaline metaluminous I-type geochemical affinity. The granitoids are categorically identical to low-and high-HREE TTGs. Their fractionated REE patterns exhibit positive Eu anomaly. Besides the negative Nb and Ti anomalies, they exhibit positive Sr and Zr anomalies. Such attributes are atypical of the granitoids formed from a reworked older (> 3.0 Ga) TTG crust. The geochemistry of these granitoids cannot be reconciled with assimilation and fractional crystallization of the felsic crust in the Dharwar craton. Alternatively, the Ba+Sr enrichments in these granitoids are ascribed to melting of a slab fluid/melt metasomatized mantle wedge. Further, the Sr-Y and La-Yb systematics are consistent with melting of a low-Mg amphibolite source with garnet in the restite. In addition to the above, the variable Gd N /Yb N =1-2.3 ratios and Nb-Zr systematics suggest that the melts were produced from variable depths involving both depleted and undepleted segments of the sub-arc mantle. Consequently, we propose that the KBH-8 granitoids were sourced from a heterogeneous thickened arc-like crust in a Neoarchean subduction zone.

Geochemistry of Layered Ultramafic Rocks in J.C. Pura Schist Belt, Dharwar Craton, Karnataka, India

The Jayachamarajapura schist belt in western Dharwar craton, southern India, is predominantly an ultramafics dominant terrain. These rocks have been extensively metamorphosed and altered to serpentinite. The komatiite nature of ultramafics is conspicuous. In most of the areas of the belt these ultramafics are massive in nature. However, some of the ultramafic units show layered nature. But, their outcrops are encompassed within the massive komatiitic bodies. These komatiitic ultramafics are predominantly Mg-rich in nature. The layered rocks are also Mg-rich, and their field setting and geochemistry suggest their intermittent occurrence as sills, during the differentiation of peridotitic magma. The layered rocks, which have been intensely serpentinisation show homogenous nature. They are almost wholly made of serpentine with occasional relics of pyroxene. Secondary carbonate mineral is often noticed. Their higher MgO content indicates Mg-rich ultramafic magmatism during Archaean orogeny.

Lithospheric structure of the Dharwar Craton (India) from joint analysis of gravity, topography, and teleseismic travel-time residuals

• Effective elastic thickness of the Dharwar Craton varies between 12 and 35 km. • Both crust and lithospheric mantle contribute in supporting topography load. • Western Dharwar craton is thicker than the eastern Dharwar craton. • A Jelly-Sandwich type lithospheric architecture is possibly applicable in the Dharwar craton. The evolution and persistence of an uplifted surface along a passive continental margin is a long-standing enigma. The long-term vertical strains generated by such surface topographic loads have to be supported and maintained by the lithosphere's elastic strength. We estimate the lithospheric strength indicated by its effective elastic thickness (Te) and associated regional lithospheric structure along an ENE-WSW profile across the Dharwar craton-western Ghats (elevation ∼1200 m). We employ a joint analysis of teleseismic travel time residuals, Bouguer gravity, and topographic data. Our results suggest the topographic load of the Western Ghats is supported by both the Archean Dharwar crust and the lithospheric mantle (∼40 %), and indicate the possibility of a Jelly Sandwich type of rheology operating in this region. However, towards the eastern section of the profile, where the terrain flattens out, the contribution of the lithospheric mantle in maintaining the topography is observed to be uncertain due to resolvability of the method. Further, we infer the presence of ∼20–30 km thicker lithosphere beneath the western Dharwar craton (WDC) than the eastern Dharwar craton (EDC). The obtained Te values varies from ∼12 km (in the west of the WDC) to ∼32 km (towards the east of the EDC). These Te estimates are in agreement with previous studies from the region and are much lower than those for the more recent geological terrains in the subcontinent, such as the Gangetic plains and the Himalaya orogen.

Relics of 2.6–2.5 Ga oceanic crust from the ultramafic-mafic complex of Goa, western India: Magmatic response to a progressive subduction system

Subduction initiation in the Early Earth and the onset of plate tectonics are topics of wide interest in understanding the formation and evolution of continents and supercontinents on our planet. Here we report results from integrated petrological, geochemical and geochronological (Zircon U Pb ages) studies on an ultramafic-mafic complex from the Bondla-Usgao-Durigini (BUD) sectors of Goa to address its petro-tectonic evolution and correlation with Neoarchean crustal growth processes and cratonization events. The ultramafic-mafic cumulate sequence from BUD sectors of Goa comprising dunite, peridotite, troctolite, olivine gabbro and gabbro represents a dismembered crustal section of oceanic lithosphere formed either in a MOR or SSZ fore-arc extensional regime. The geochemical attributes marked by boninitic-IAT trend, depleted to enriched mantle compositions, selective enrichment in fluid-mobile LILE and LREE over HFSE enunciate a multistage petrogenetic evolution associated with a SSZ fore-arc tectonic setting. The ocean-continent transition featured by the geochemical signatures can be translated in terms of oceanic crust formation in a juvenile extensional fore-arc in a SSZ setting, its maturation and accretion at active continental margin during Neoarchean ocean basic closure and arc-continent collision. The studied ultramafic-mafic suite fingerprints the transition of depleted mantle wedge to a LILE-LREE enriched metasomatized one through influx of subduction components and hydrous arc magmatism typical of SSZ oceanic lithosphere formed above a nascent subduction zone. The magmatic precursors of the studied lithologies were derived from a transitional depleted to enriched mantle regime marked by hydration and metasomatism of a depleted MOR-type mantle by the influx of subduction-derived fluids. Zircon grains in the peridotite sample yield positive εHf(t) of +3.2 to +7.6, together with T DM C of 2610–2877 Ma suggesting depleted mantle source. The gabbro sample shows positive εHf(t) values of +3.7 to +6.3 and T DM C age of 2731–2868 Ma also attesting to depleted mantle source. Zircon U Pb ages of 2602.42 ± 0.82 Ma for peridotite and 2479–2684 Ma for gabbro represent the magma emplacement time and Neoarchean-early Paleoproterozoic melt activity. The highly variable ages from zircon grains in the studied samples might suggest multiple stages of metasomatism and melt-rock interaction in a long-lived sub-continental lithospheric mantle. The tectonic evolution can be equated with Neoarchean global subduction-accretion-collision and arc-continent amalgamation events. This study deciphers a 2.6–2.5 Ga Neoarchean oceanic crust formation and magmatic response to subduction initiation, arc maturation and accretion processes correlatable with global crustal growth processes and amalgamation of western Dharwar Craton (WDC) and Antongil-Masora blocks into the Greater Dharwar Craton (GDC). • Ultramafic-mafic cumulate sequence from Goa represents dismembered SSZ oceanic crust. • Depleted to enriched melt composition reflects mantle heterogeneity. • Paleo-ocean closure and arc accretion culminated to GDC formation. • 2.6–2.5 Ga age coincides with Neoarchean global subduction-accretion event.

Tectono-Thermal History of the Neoarchean Balehonnur Shear Zone, Western Dharwar Craton (Southern India)

Abstract A widely spaced Neoarchean shear zone network traverses the granite-greenstone terrains of the Western Dharwar craton (WDC). The NNW-SSE trending Balehonnur shear zone traverses the largest part of the preserved tilted Archean crustal ensemble in the Western Dharwar craton (WDC) from the amphibolite-granulite transition in the south to greenschist facies in the north and eventually concealed under Deccan lava flows. Published tectonic fabrics data and kinematic analysis, with our data reveal a sinistral sense of shearing that effectuate greenstone sequences, Tonalite-Trondhjemite-Granodiorite Gneisses (TTG), and Koppa granite as reflected in variable deformation and strain localization. A profound increase of strain towards the core of the shear zone in the ca. 2610 Ma Koppa granite is marked by a transition from weak foliation outside the shear zone through the development of C-S structures and C-prime fabrics, mylonite to ultramylonite. The mineral assemblages in the Koppa granite and adjoining greenstone indicate near peak P-T conditions of 1.2 Gpa, 775-800°C following a slow cooling path of 1.0 GPa and 650°C. Field-based tectonic fabrics data together with U-Pb zircon ages reveal that the Koppa granite emplaced along the contact zone of Shimoga-Bababudan basin ca. 2610 Ma, coinciding with the emplacement of ca. 2600 Ma Arsikere-Banavara, Pandavpura, and Chitradurga granites further east which mark the stabilization of WDC. Significant variation in major element oxide (SiO2 = 56-69 wt.%) together with high content of incompatible elements (REE, Nb, Zr, and Y) and high zircon crystallization temperatures (~1000°C) of Koppa granite suggests derivation by partial melting of composite sources involving enriched uppermost mantle and lower crust. The development of widely spaced shear zones is probably linked to the assembly of eastern and western blocks through westward convergence of hot oceanic lithosphere against already cratonized thick colder western block leading to the development of strain heterogeneities between greenstone and TTGs due to their different mineral assemblages leading to rheological contrast in the cratonic lithologies.

Genesis of Cobalt Mineralisation Associated With Kalyadi Schist Belt, Dharwar Craton, South India: A Petrological and Fluid Inclusion Study

The Sargur Group of rocks belonging to the Kalyadi Schist Belt (KSB) in the Western Dharwar Craton (WDC) occur as large linear enclaves within younger peninsular gneisses. This belt is known for copper mineralization and exploited good amount of copper for many years. Recent studies have shown significant concentration of cobalt up to 2000 ppm associated with copper in this belt. Earlier studies have indicated pyrite containing cobalt in its crystal lattice. In the present study an attempt has been made to understand the genesis of cobalt mineralization in this belt through detailed petrography, fluid inclusion studies along with chlorite chemistry. Two distinct ore mineral assemblages and fluid sources are proposed based on ore petrography, mineral chemistry and fluid inclusion studies. In the initial stage chalcopyrite1 is associated with cobalt bearing pyrite and magnetite and in the later stage chalcopyrite2 is associated with arsenopyrite, pyrrhotite and safflorite [(Co, As) As2]. The initial stage fluids are characterized by low saline (~ 5 wt % NaCl equivalent) and slightly higher temperature (268° to 300° C with pressure ranging from 1.9 to 2.3 Kb), which are almost similar to temperatures obtained from the chlorite i.e. 202.8° to 322.43° C, this indicates that the fluids are related to regional low grade greenschist facies metamorphism. On the contrary, the late stage fluids are characterized by high density and high saline nature and show temperature range varying from162° to 224° C with pressure values ranging from 1.4 to 1.9 Kb and in addition native Bi association suggest their derivation from the granitic or felsic magmatic source associated with emplacement of the Desani Granite in the study area. Presence of safflorite mineral associated with the second stage fluids and mineral assemblage suggest that the remobilization Co from the crystal lattice of pyrite and chalcopyrite has occurred. Keywords: Cobalt mineralisation, Sargur Group of Rocks, Kalyadi Schist Belt (KSB), Western Dharwar Craton

Deep crustal structure and compositions for tectonic and geodynamic implications of the Dharwar Craton (Southern India) inferred from 3-C wide-angle seismic data

The 3-C deep seismic study with integrated interpretation of Bouguer gravity, heat flow, geological and geochronological studies along the Perur-Chikmagalur profile of the Dharwar Craton reveals important crustal elements with compositional distinctions of different geological structures such as horst and grabens , major shear zones like BSZ, CSZ, prominent batholiths as CG , granitic and gneissic complexes, presence of LVL, plutons , magmatic intrusives, thick underplating in WDC and significant Moho upwarping towards EDC due to plume head and komatiite magmatism . • Crustal V P and V S models are derived using 3-C seismic data of the Dharwar Craton. • Rock compositions are constrained by V P / V S , Poisson’s ratio, density and heat-flow. • Imaged a major suture zone CSZ dividing EDC and WDC blocks of the Dharwar Craton. • The LVL forms a detachment zone sandwiching the supracrustals and Archean gneisses. • Moho upwarping and underplating justify plume-arc settings for the Dharwar Craton. The Dharwar Craton of southern India is an important stable cratonic province of the world with complex geology and tectonic settings. Extensive studies provide insights of crustal velocity structure for the tectonic and geodynamic evolution of this Archean craton. This region has experienced several tectonically disturbed zones like Chitradurga Shear Zone (CSZ), Bababudan Shear Zone (BSZ) and Closepet Granites (CG). We have developed a comprehensive geologically plausible tectonic model using both P- and S-wave velocity structures to image major structural elements like shear zones and decipher the compositional distinctions of different rock assemblages of Western Dharwar Craton (WDC) and Eastern Dharwar Craton (EDC) part using 3-C wide-angle seismic data acquired along the 200-km long Perur-Chikmagalur deep seismic profile. The tectonic model show large compositional changes of subsurface rocks with anomalous high V P , V S , V P / V S , Poisson’s ratio ( σ ) and density ( ρ ) forming a major tectonic divide or suture zone called CSZ between EDC and WDC blocks. Significant crustal thinning (37–41 km) is observed due to Moho upwarping towards the Neo-Archean EDC block mainly composed of felsic granites and granodiorites. The WDC block show relatively thick crust (48–50 km) due to mafic underplating and mantle plume activity below CSZ forming Meso-Archean greenschist-facies-gneisses with dominant mafic/ultra-mafic compositions. Hence, crustal velocity, density, heat-flow, geology and geochronology studies support a plume-arc model with evidence of thick magmatic underplating of the lower-crust, complex subduction and development of highly strained shear zones like CSZ as suture juxtaposing EDC and WDC blocks.