Dharwar Craton Research Articles

Insights into the evolution of the Dharwar craton, South India through three dimensional P-wave velocity imaging of upper mantle

Insights into the evolution of the Dharwar craton, South India through three dimensional P-wave velocity imaging of upper mantle

Paleogeography of India at ∼1.8 Ga: New constraints from baddeleyite geochronology and paleomagnetism of mafic dykes from the Dharwar craton

The paleogeographic position of Dharwar craton/India within Paleoproterozoic supercontinent configurations has been well demonstrated using highly reliable key paleopoles from ∼2.37 Ga to ∼1.86 Ga mafic events in recent studies. Here, we report a paleomagnetic pole at ∼1.8 Ga along with a precise Pb-Pb baddeleyite age. The emplacement age of an NW-SE trending dolerite dyke yields a weighted mean age of 1799 ± 0.8 Ma (MSWD = 0.7), which coincides with earlier reported U-Pb baddeleyite age (1794 ± 7 Ma) and confirms the existence of significant emplacement of Pebbair dyke swarm at ∼1.8 Ga. Paleomagnetic results from eleven sites shows mean D = 306°, I = 31°, α95 = 10° (k = 23), and corresponding paleopole is calculated at 38°N, 357°E (dp = 6.1° and dm = 11°). These remanent directions are carried by the pseudo-single domain (PSD) magnetite. This pole passes the paleosecular variation test of the geomagnetic field and fulfills the reliability criteria (R = 5) of Meert et al. (2020), so, suggested to be a key pole for Dharwar craton/ India (∼1.8 Ga). The paleopositon of Dharwar craton is occupied at shallow paleolatitude and drifted towards the north with a minimum velocity (∼5.7 cm/yr) from ∼1.86 Ga to ∼1.80 Ga. The paleogeographic reconstruction model at ∼1.8 Ga has been attempted with available coeval paleopoles from other global cratons. The Dharwar craton (South India block), the Amazonia craton, and the Baltica craton have assembled at ∼1.86 Ga, and North Australia, Sao Francisco, Rio de la Plata cratons came in close to the South India block, Amazonia, and Baltica assembly at ∼1.80 Ga.

Granitoid characteristics of basement to the U-mineralized Bhima basin: Implications for crustal epizone processes

The Dharwar Craton (DC) is an archive containing a good proportion of ancient records of crustal evolution in its rocks. The greenstone-gneiss-migmatite and granitoid complex show significant variation in terms of time, space and mechanism of emplacement according to tectonics and geodynamic set up. The plutons of the northernmost part of Eastern Dharwar Craton (EDC) are especially recognizable from typical litho-structural attributes. The intensity of regional/burial metamorphism is negligible in these granitoids with versatile nature. Simple and composite plutons are demarcated on the basis of field association with distinguishable petro-mineralogy and geochemical characters. K-metasomatism in pluton is established from petrography with supportive geochemistry. Simple pluton is syenitic in nature unlike the composite plutons with lithological variation from Tonalite-Trondhjemite-Granodiorite (TTG) to alkali feldspar granite. The granitoids are developed from crustal melting during the plate convergence. Post-orogenic nature of syenite is indicated from geochemical data. The sharp contact between the plutons and country rocks indicates significant temperature difference between country rocks and emplaced magma. The characteristic features like brittle deformation dominated structures, sharp contact between emplaced plutons and country rocks, very low grade of regional metamorphism, intensely differentiated nature and absence of diatexitic migmatite indicate a shallow crustal depth of plutonic emplacement. Higher concentration of large ion incompatible elements (e.g., U, Th, K) and associated high heat production imply shallow crustal segment with rocks derived from evolved differentiated magma. The study indicates a depth less than 8 km and temperature condition less than 250°C which correspond to epizone, where uranium remobilization is facilitated.

Lithosphere structure beneath the eastern Dharwar craton kimberlite field, India, inferred from joint inversion of surface wave dispersion and receiver function data

Cratons are the oldest parts of continents with cold and thick, high velocity, depleted lithosphere, which remained stable for billions of years. However, many of them are susceptible to deformation and erosion under various geological settings, such as near subduction zones and above mantle plumes. The role of mantle plumes in eroding the overlying lithosphere remains elusive, particularly for the fast-moving cratons. In this study, we examine the plume-lithosphere interaction in the Dharwar Craton located in South India by comparing the lithospheric property derived from the diamondiferous kimberlite xenoliths with its present-day shear velocity structure computed from collocated seismological data. The lithosphere-asthenosphere system beneath a 200 km long profile is explored at 13 locations through joint inversion of receiver function and surface wave phase dispersion data. The velocity model suggests 165 ± 15 km thick present-day lithosphere, marginally lower than 180 ± 20 km derived from the kimberlite xenoliths which erupted around 1100 Myr. The velocity increases from the Moho to ∼100 km depth, possibly due to the spinel-garnet peridotite transformation. The cold cratonic lithosphere of Eastern Dharwar craton (EDC) is manifested in very high shear velocities, up to >4.7 km/s indicating harzburgite-dunite composition, correlating well with the composition inferred from analysis of xenolith data. Our result suggests no significant thermal alteration to the lithosphere of the eastern Dharwar Craton despite being affected by three mantle plumes in the last 90 Myr.

Early Mesoproterozoic (1.72–1.68 Ga) convergence between Bastar and Eastern Dharwar cratons along the Bhopalpatnam orogen, southcentral India and implication for paleogeography of supercontinent Columbia

Temporal constraints on orogen initiation and duration are critical for the Paleoproterozoic reconstruction of cratons presently constituting India. Recent late Paleoproterozoic paleomagnetic pole data indicate proximity of Eastern Dharwar, Bastar and Singhbhum cratons and in view of global Mesoproterozoic-aged final Columbia assembly, reconstruction sequence of Indic cratons is critical to assess whether they constituted Columbia. The initiation time and duration of the Bhopalpatnam orogen, developed between the Eastern Dharwar and Bastar cratons, range from earliest Mesoproterozoic to Neoarchean. Therefore, in order to constrain the earliest stages of cratonic convergence and crustal thickening, the time of peak metamorphism was established on tectonites, sampled from > 300 km along newly classified longitudinal domains representative of the entire Bhopalpatnam orogen, by precise combined garnet Lu-Hf and Sm-Nd chronology of amphibolite-and granulite-facies rocks. Interpreting the Lu-Hf and Sm-Nd isochron weighted mean ages, an orogenic duration of 38 ± 19 Myr is inferred between the high-pressure amphibolite-facies through to granulite-facies event at 1721 ± 14 Ma and a post-peak cooling to ≤ 650 °C at 1683 ± 6 Ma, being the Sm-Nd garnet weighted mean age. Archean two-stage depleted mantle model ages (at 1.7 Ga) of tectonites and rotated dyke swarm orientations support the reactivation and reworking of the Paleo- to Neoarchean Bastar craton marginal rocks in response to the early Mesoproterozoic orogeny. Based on the clockwise P-T path involving an early high-pressure amphibolite-facies stage (Pmax ≥ 12 kbar at ∼ 750 °C) followed by a granulite-facies stage (Tmax ≤ 850 °C at lower pressures), we interpret the orogen to represent deeply exhumed thrust slices of the reworked Bastar craton leading edge underthrusted beneath the Eastern Dharwar craton. As paleomagnetic data indicates proximity or contiguity of Bastar and Eastern Dharwar cratons by 1.79 Ga, the Bhopalapatnam orogen could represent site of early Mesoproterozoic cratonic collision or locus of post amalgamation reactivation due to convergence.

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

Precise Pb-Pb baddeleyite geochronology, geochemistry, and Sr-Nd isotopic constraints on the 2.36 & 1.88 Ga mafic dykes from the Bastar craton, India: Implications for their petrogenesis in conjunction with the Dharwar mafic dykes

This research focuses on the geochronology and geochemistry of WNW-NW-NNW trending Paleoproterozoic mafic dykes from the Bastar craton in India. The weighted mean 207Pb/206Pb ages of baddeleyite grains obtained from the Tuskal, Pakhanjore, and Gariaband regions were 2365.25 ± 0.54 Ma, 2365.70 ± 0.48 Ma, and 2363.85 ± 0.79 Ma, respectively. Baddeleyite grains from a mafic dyke near Narayanpur-Kondagaon yielded a weighted mean 207Pb/206Pb age of 1886.5 ± 1.8 Ma. In the TAS (Total Alkali Silica) diagram, the 2.36 Ga and 1.88 Ga mafic dykes have basalt to basaltic andesite composition. Dykes from both age groups are represented by the tholeiitic magma series in the AFM diagram. In the spidergrams, the dykes show LREE and LILE enrichment, with the 2.36 Ga dykes showing flat HREE and the 1.88 Ga dykes showing slightly inclined HREE patterns. The REE non-modal batch melting model indicates derivation of the 2.36 Ga mafic dykes from a shallower spinel lherzolitic source, whereas the 1.88 Ga mafic dykes were derived from a deeper spinel-garnet lherzolitic mantle source. The 2.36 Ga dykes have near chondritic to slight super chondritic initial Nd isotopic values (εNd(2.36 Ga) = −0.95 to 2.36), whereas the 1.88 Ga dykes have sub chondritic initial Nd isotopic values (εNd(1.88 Ga) = −2.37 to −9.67). Present-day orientations and similar precise ages of the mafic dykes reported from the Bastar and Dharwar cratons indicate the counter-clockwise rotation of the Bastar craton during the Paleoproterozoic.

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.

U-Pb Neoproterozoic age and petrogenesis of a calc-alkaline shoshonitic lamprophyre from Simdega area, Chhotanagpur Gneissic Complex (Eastern India): Implication for the evolution of the Central Indian Tectonic Zone and Rodinia tectonics

The age of emplacement and geochemistry of the lamprophyres are of tectonic significance as they have potential to unravel global scale geodynamic processes. We present petrology, U-Pb SHRIMP apatite and titanite ages and bulk-rock Sr-Nd isotope data for an unmetamorphosed and undeformed lamprophyre dyke from the Simdega area of the Chhotanagpur Gneissic Complex (CGC) which is a component of the E-W trending Central Indian Tectonic Zone (CITZ), India. The CITZ is a major intercontinental suture which separates the northern Indian and the southern Indian blocks whose polarity of their subduction is a contentious issue. The lamprophyre exhibits a strong porphyritic-panidiomorphic texture imparted by the megacrysts/phenocrysts of mica and amphibole with feldspar, apatite, titanite, zircon and opaques confined to the groundmass. Based on combined mineralogy and geochemistry, the lamprophyre is classified to be of calc-alkaline variety (minette) with shoshonitic affinities. Mg# (70.7–78.2) contents highlight the primitive melt character whereas incompatible trace element ratios exclude crustal contamination and are indistinguishable from those of the subduction-related global as well as Eastern Dharwar craton (southern India) calc-alkaline lamprophyres. U-Pb dating of apatite gave an emplacement age of 944 ± 82 Ma which is indistinguishable, within the error limits, from the U-Pb titanite age of 942.1 ± 5 Ma demonstrating a Neoproterozoic magmatic emplacement age of the lamprophyre synchronous with the Rodinia assembly. Bulk-rock 87Sr/86Srinitial (0.707239 and 0.708910) and ɛNdinitial (-8.9 to -8.3) highlights the involvement of an enriched mantle source. Calculated Paleoproterozoic model ages (Nd depleted mantle) of 2.1 Ga of the lamprophyre are indistinguishable from those of the co-spatial amphibolite dykes. Petrogenetic modeling involving rare earth elements reveals that the derivation of the lamprophyre magma from 2 to 3% partial melting of a mixed garnet (70%) and spinel (30%) lherzolitic mantle source with minor phlogopite. Our study highlights that the western part of the CGC was less affected, relative to the eastern part, by the M3 regional amphibolite grade metamorphic event (ca. 920-880 Ma) and also supports the geodynamic models involving northward-directed subduction of the Southern Indian block under the Northern Indian block.-

Origin of clinopyroxene megacrysts from the 1.1 Ga Chigicherla-4 kimberlite (CC4), Dharwar craton, southern India: Implications for multi-stage metasomatism of the sub-continental lithospheric mantle

Origin of clinopyroxene megacrysts from the 1.1 Ga Chigicherla-4 kimberlite (CC4), Dharwar craton, southern India: Implications for multi-stage metasomatism of the sub-continental lithospheric mantle

Assessment of Groundwater Potential Zone using Multi-criteria Decision-making Technique: A Case Study in the South-western Part of Yadadri Bhuvanagiri District, Telangana, India

Abstract Groundwater which occurs beneath the earth’s surface is an important natural resource. The study area, part of Yadadri Bhuvanagiri district, Telangana, which is a part of Musi river basin, is a hard rock terrain covering an area of 1107 sq km, part of the eastern Dharwar craton (EDC) of the southern India peninsular shield. Exploration of groundwater in this terrain is difficult because groundwater is confined to weathered and fractured layers. The present study aims to delineate groundwater potential zones using the multi-criteria decision analysis (MCDA) technique. An integrated application of remote sensing and GIS is very effective in mapping features that provide information about groundwater potential zones using several thematic layers such as geology, soil, geomorphology, slope, lineament density, drainage density, rainfall, and land use/land cover maps. The groundwater potential zones of the study area are classified into five categories, namely very high, high, medium, low, and very low and they occupied an area of 116.8, 381.29, 345.15, 149.70, and 115.42 km2 respectively. The results were validated using discharge data from existing wells in the study area, and the majority of the high-productivity wells are located in very high and high groundwater potential zones. The groundwater potential zone mapping using this model will be a very useful improving the groundwater resources of the study area.

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.

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.

Clay mineralogy and provenance modeling of the Paleoproterozoic Kaladgi shales, Dharwar Craton, Southern India: Implications on paleoweathering and source rock compositions

Proterozoic clastic sediments reveal vast information regarding provenance, depositional conditions, and environmental evolutions. Peninsular India comprising Archean cratons, also have numerous intracratonic Proterozoic sedimentary basins along their margins. The Archean Dharwar craton in southern India has many Proterozoic successions, namely Cuddapah, Kurnool towards the East, and Kaladgi, Badami, and Bhima towards the northern margin. The Paleoproterozoic Kaladgi Basin (∼1.85 Ga) consists of siliciclastic sedimentary rocks with stromatolitic carbonate formations. XRD analysis of shale layers of the Lower Lokapur and Upper Simikere Subgroups have been carried out to understand the primary clay mineral assemblages, weathering history, and provenance. The Lower and Upper shale layers of Lokapur and Simikere Subgroups show a dominance of montmorillonite and kaolinite, respectively. The geochemical affinities and the clay mineral assemblages indicate a more mafic source to the lower shales (Manoli and Hebbal Formations) and increased felsic contribution to the upper shales (Govindakoppa and Daddanhatti Formations). Illite is ubiquitous in all the shales of the Kaladgi Supergroup possibly representing the diagenetic transformation of montmorillonite and kaolinite to illite. Geochemical modeling of provenance has been carried out using (Eu/Eu*)N, (La/Yb)N, (Gd/Yb)N, and (La/Sm)N of all the plausible source rocks and the average compositions of lower and upper shales. The modeling results suggest that the lower shales are derived from a source of mafic rocks – 45 %, K-rich granite – 35 %, and TTG – 20 %. While the upper shales are derived from source characterized by K-rich granites – 61 % and intermediate volcanic rocks – 39 %. These results signify the classical unroofing of TTG-greenstone belts exposing K-rich granites with the progression of sedimentation. Further, a good correlation between K-enrichment (a measure of % difference between CIA and pre-metasomatic CIA) and ƩLREE is attributed to the abundance of kaolinites that fractionate more LREE.

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.

Constraints on the lithospheric structure and tectonics of Archean Dharwar Craton, southern India from geophysical and geological data: Evidence for modified lithosphere

• Mantle plume activities eroded the lithospheric keel of the Archean Dharwar Craton. • Small-scale convection destabilized and promoted erosion of lithospheric mantle. • SCLM is metasomatized as indicated by low S-wave velocities (Vs = 4.1-4.4 km/s). • Effective distance of mantle plume activity is ∼1500 km from the source region. Cratons are the stable portions of the continents for at least the past two billion years. Their longevity is generally attributed to their thick, cold, rheologically strong and chemically depleted subcrustal mantle lithosphere. The lithosphere achieved both buoyancy and strength by relatively low thermal regime and compositional differences due to dehydration, presence of melt-depleted peridotites and low iron content. Low temperature of the mantle lithosphere leads to high viscosity, which prevents deformation and thus preserves the cratonic root. However, significant modifications/erosion of the lithospheric root is observed for some cratons from different parts of the globe. The present paper discusses some aspects of lithospheric modification of the Archean Dharwar Craton, southern India. Southern India is a mosaic of several Archean cratonic blocks, the Western and Eastern Dharwar cratons, and Southern Granulite Terrain. It has experienced several tectonic and magmatic events, including mafic dykes of various periods since Paleoproterozoic. The Dharwar Craton was accreted with the Karwar and Coorg blocks on the west and Eastern Ghats and Nallamalai fold belts on the east during the Proterozoic. The Cuddapah, Bhima and Kaladgi basins were formed during the Proterozoic period. A 1.1 Ga kimberlite magmatism and 550 Ma East-African orogen are other important geodynamic events of the region. The western and eastern continental margins, together with the Deccan Large Igneous Province, one of the world's largest igneous provinces, were formed due to mantle plume activities during 130-65 Ma. 2500 km long Himalayan Mountain chain was formed during ∼55 Ma. Active and passive seismic, magnetotelluric, gravity, xenolith and elevation data complimented with geological data were used to understand the lithospheric structure. Subduction, collision and mantle plume activities, which are main processes for crustal evolution are also found responsible for lithospheric destruction in the region. The detached subducted slabs from various geodynamic activities of the region might have led to rheological weakening of the subcontinental lithospheric mantle and thereby craton modification/erosion in the southern India. Mantle plume activities are responsible for epeirogenic uplift and lithospheric thinning in the southern India.

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.

Multiple Sulfur Isotope Geochemistry of the Precambrian Mafic Dykes and Komatiites in the Dharwar Craton, Southern India: Evidence for Crustal Recycling and Enrichment in the Subcontinental Lithospheric Mantle

Abstract Komatiites, greenstone volcanic rocks, and mafic dyke swarms are constituents of early earth magmatic activity, crucial for understanding the chemical evolution of the Archean mantle. The composition of the subcontinental lithospheric mantle (SCLM) is systematically modified throughout the Earth’s history by the addition of geochemically diverse oceanic and continental crustal materials through subduction and can be sampled through intraplate mafic/ultramafic volcanic activities. Here, we present a first report on the multiple sulfur isotope characteristics of the mafic dyke swarms and komatiites from the Dharwar craton in southern India and discuss the geochemical modifications of SCLM through crustal recycling. δ34SV-CDT values of the samples are all negative ranging from -0.15 to -2.91‰. Δ33S values for all the samples are close to 0 with the lowest value of -0.060‰ and highest of 0.146‰. Δ36S values are mostly negative with very few exceptions, ranging from -1.184 to 1.111‰. Near zero values of Δ33S and negative values for δ34S indicate an early formed mantle reservoir with a possible mixture of sulfur from subducting oceanic sediments. Together with trace element geochemistry, we suggest a depleted MORB source mantle (DMM) modified by oceanic crustal components and a depleted mantle (DM) modified by recycled continental crustal sediments as the two end members of the mantle source that produced mafic dyke swarms in the Late Archean to Proterozoic Dharwar craton.