Western Dharwar Craton Research Articles

Chromitites from an Archean layered intrusion in the Western Dharwar Craton, southern India

Chromite deposits hosted in a layered ultramafic–mafic intrusion in the Archean Chithradurga greenstone belt are part of a suture that divides the Western and Central Dharwar craton blocks in southern India. Serpentinised ultramafic rocks including dunite, peridotite, and phlogopite-rich clinopyroxenite occur with massive chromitite. The olivine compositions show two distinct groups, one with Fo ~91–92 olivines in the massive chromitite, and another group with olivine Fo values of ~95–96, in the serpentinised dunite. The chromite occurs as massive chromitite and as an accessory phase in dunite. The former is characterized by high Cr# (Cr/(Cr + Al) × 100 = 67–70) and moderate to high Mg# (Mg/(Mg + Fe) = 52–66); The latter shows a larger variation in Cr which is likely related to sub-solidus re-equilibration of the chromite during the serpentinization event. In the massive chromitites, sub-solidus re-equilibration of chromite is less significant or absent. Bulk-rock major and trace element analyses of the peridotites reveal a composition less magnesian than depleted mantle rocks, with low TiO2, Na2O, CaO and high Cr, Ni content, a general depletion in Sr and enrichment in Th, U. The chromitites contain variable PGE abundances (ΣPGE = 8-244 ppb) with enrichments of Os, Ir and Ru, relative to Pt and Pd . These features suggest that the chromitites are cumulates formed from a highly magnesian parental magma with komatiitic affinity, derived by large degrees of partial melting, , which underwent crustal contamination. The ReOs isotope data demonstrate a minimum Re-depletion model age (TRD) of 2895 ± 24 Ma for the serpentinized dunite (MY11/3), and a more robust age of 3120 ± 12 Ma for two chromitite samples, which we interpret as the best estimate of the magmatic age of this ultramafic intrusion. Abundant zircon grains also occur within these ultramafic rocks with UPb ages spanning from ca. 2.9 Ga to ca. 2.4 Ga, with a dominant age population in the range 2.8–2.6 Ga. We interpret the upper intercept age at ca. 2.9 Ga to represent the initial crystallization of zircon in the ultramafic rocks, potentially as a result of an early stage of metamorphism and hydrothermal alteration. The 2.5–2.4 Ga zircon ages reflect a late-stage high-grade metamorphic overprint.

Gravity of Dharwar Craton, Southern Indian Shield

Dharwar craton (DC), by far the largest geological domain in South Indian Shield, occupying about 0.5 million sq. km area, is well-studied terrain both for regional geoscientific aspects and as part of mineral exploration over several important blocks such as the greenstone belts, ultramafic complexes, granite-gneissic terrain and the Proterozoic sediments of Cuddapah basin. The re-look into regional gravity data offers several insights into nature of crust, sub-divisions within the craton, bedrock geology in the covered areas and mineral potentiality of this ancient and stable crust. The regional gravity profiles drawn across the south Indian region mainly suggest that the area can be divided into five domains as Western Dharwar craton (WDC), Central Dharwar craton (CDC), Eastern Dharwar craton (EDC 1), transitory zone of EDC (EDC 2) and Eastern Ghats mobile belt (EGMB) areas. The Bouguer gravity anomaly pattern also questions some of the earlier divisions like eastern margin of Chitradurga schist belt between the WDC and EDC and the boundary of DC with southern granulite terrain (SGT) as they do not restrict at these main boundaries. In this study, mainly four issues are addressed by qualitative and quantitative analysis of regional gravity data and those revealed significant inferences. (1) A distinct gravity character in central part of south Indian shield area occupying about 60, 000 sq. km, suggests that the transitory crustal block, faulted on both sides and uplifted. This area designated as central Dharwar craton (CDC) is characterized with schist belts having characters of both parts of western and eastern Dharwar craton. This inference also opens up the debate about the boundary between western and eastern parts of the craton. Another significant inference is the extension of major schist belts beneath both Deccan volcanic province (DVP) in northwestern part and Cuddapah basin (CB) in southeastern part. (2) Eastern Dharwar craton is reflected as two distinct domains of different gravity characters; one populated with number of circular gravity lows and a few linear gravity high closures indicative of plutonic and volcanic activity and another domain devoid of these intrusive younger granites or schist belts. (3) Large wave length gravity highs occupying thousands of sq.km area and those not relatable to surface geology in eastern Dharwar craton that may have significance for mineral exploration. (4) Gravity data was subjected to further processing like two dimensional modeling which have yielded insights into crustal architecture beneath the Dharwar craton, crustal scale lineaments, craton-mobile belt contact zone and younger intrusives.

Geochemical characterization and geodynamic implication of N-trending mafic dyke swarm from the western Dharwar craton and their possible link to the ca. 2.22 Ga large igneous province

Several N-trending mafic dykes are exposed in the western Dharwar craton (WDC) and they are thought to be coeval with the ca. 2.22 Ga N- to NNW-trending Kandlamadugu dyke swarm of the eastern Dharwar craton (EDC). Geo- chemical characterization of these dykes is presented here to understand their genetic aspects and likely correlation with their counterpart in the EDC. Petrographic examinations suggest mineralogical and textural variations from dolerite to metadolerite types. Geochemically they are classified either as sub-alkaline tholeiitic basalt or basaltic andesite. Geochemical variations suggest evolution of mantle melt and demonstrate prominent clinopyroxene fractionation, however, minor role of olivine, orthopyroxene and plagioclase fractionation cannot be discarded at initial stages of crystallization. Fractionation trends of trace elements suggest crystallization of accessory phases like ilmenite, apatite and zircon, at later stages. Although observed geochemical nature suggests a little effect of involvement of crust, however, its role in the genesis of the studied mafic dykes cannot be ignored. Conversely, it is suggested that they are derived from a melt generated in a sub-continental lithospheric mantle (SCLM), which was metasomatized during an ancient subduction event before its cratonization. Based on the petrogenetic models of incompatible trace elements, it is inferred that they were likely to be derived from a melt generated by a lower percentage of melting within the garnet or garnet-spinel transition zone. Their connection to the ca. 2.22 Ga large igneous province (LIP) indicates as an integral part of the ca. 2.22 Ga N- to NNW-trending Kandlamadugu dyke swarm of the EDC. The existence of a mantle plume, substantiated by mantle potential temperature (Tp) estimate, is well-supported by higher thermal regime in the upper mantle. Although there is no direct age data available for the studied mafic dykes, however, their geochemical similarities with the ca. 2.22 Ga Kandlamadugu swarm suggest that they are co-genetic and could be linked to the same event. The likely age correlation of the ca. 2.22 Ga Kandlamadugu swarm with mafic dykes of North Atlantic and Superior cratons, support their link with the Superia supercraton.

Dharwar stratigraphy revisited: new age constraints on the ‘oldest’ supracrustal rocks of western Dharwar craton, southern India

ABSTRACT The timing of emplacement of the oldest supracrustal rocks represented by Sargur Group is significant in understanding crustal evolution processes in the western Dharwar craton. The age of ultramafic-mafic rocks of the Sargur Group is not well-constrained as the komatiitic and komatiitic basaltic rocks have always given unreliable ages due to larger errors attributed to their alteration. New Sm-Nd isotope data from relatively less-altered layered intrusive ultramafic-mafic rocks of Nuggihalli and Holenarsipur greenstone belts of Sargur Group, western Dharwar craton give a much precise age of 2934 ± 88 Ma. This is the youngest age obtained for the ultramafic-mafic rock complexes of Sargur Group and is indistinguishable from the oldest ages reported for the overlying Dharwar Supergroup rocks. Sargur Group rocks might merely represent older ultramafic equivalents of the Dharwar Supergroup. A clear temporal distinction does not exist to support the stratigraphic classification of these two groups separately. However, the sub-contemporaneous to older ages obtained for the tonalite-trondhjemite-granodiorite gneisses that surround these greenstone belts would imply that the narrow belts tucked within the gneisses are younger, and therefore, the greenstone belts, whose dismembered roots and parts are exposed alongside the gneisses, could represent an Archaean analogue of an ophiolite suite.

Coupled U-Pb and Rb-Sr laser ablation geochronology trace Archean to Proterozoic crustal evolution in the Dharwar Craton, India

The Western Dharwar Craton (WDC) and the Karwar Block (KB) in southern Peninsular India provide important windows to Earth’s early crustal evolution and continental growth as these domains likely represent fragments of the first landmass or supercontinent on our planet. Here we employ, for the first time, a coupled laser ablation zircon U-Pb dating and in-situ Rb-Sr dating of micas (biotite, muscovite) and feldspars to evaluate the Meso- to Neoarchean crustal evolution and Proterozoic magmatism/metamorphism in the Karwar Block. This novel approach based on integrated geochronological results show that the WDC and KB experienced (i) a major Mesoarchean continental growth event (c. 3.2 to 2.9 Ga), (ii) younger Neoarchean and Paleoproterozoic magmatism/migmatization (syn- and post-tectonic granite formation events at c. 2.8–2.6 Ga and 2.2 Ga, respectively), and (iii) Mesoproterozoic metamorphism/thermal imprint (as young as 1.2 Ga). The zircon εHf(t) values exhibit a range from +0.4 to +9.8 in amphibolite, hornblende gneiss and trondhjemite (3.28–3.08 Ga), as well as the +6.1 to +9.7 range for 2.96 Ga old zircons in porphyritic granite, which suggests juvenile source for the magmatism that constructed this crustal block.Overall, this study illustrates the benefits of using coupled zircon U-Pb and in-situ Rb-Sr mica and feldspar dating approaches, as the former (U/Pb) provides robust constraints or a ‘baseline age’ for initial crystallization of crustal sequences whereas the latter (Rb/Sr) is sensitive to younger magmatic and metamorphic events. Thus, the integration of these two laser ablation ICP-MS based geochronometers applied to crustal evolution studies, has the potential to provide more comprehensive insights into continental growth in the early Earth, and later magmatic and metamorphic history of the region.

Paleostress field reconstruction in the western Dharwar craton, south India: Evidence from brittle faults and associated structures of younger granites

We present new fault slip data to decipher the tectonic stress regime in younger granites (2.61 Ga) of western Dharwar craton (WDC), south India. The study is based on paleostress reconstruction of the terrain using fault-slip data from the small scale oblique slip normal faults of Chitradurga granite. Paleostress analysis using Right Dihedron and Rotational Optimization methods reveal that these faults have developed under NNE-SSW directed extension. Similar results are also obtained when left-lateral and right-lateral oblique-slip normal faults are treated as separate entities. It is envisaged that all faults in Chitradurga granite have developed under a single tectonic event and are considered to be extensional shear fractures. We interpret that the oblique-slip normal faults in granite oriented along NNW-SSE to N-S (P shear), NW-SE (R shear), NE-SW (R′ shear), and WNW-ESE (T) are defined as the respective shear components of the regional riedel shear system. It developed due to sinistral movement along the Chitradurga Shear Zone (CSZ) during late D3 deformation at shallow crustal level. The fault planes act as primary or secondary shear planes, depending on their proximity to the CSZ and kinematically fit well with regional far-field compression. The present study addresses the formation of oblique slip normal faults in the Chitradurga granite and its implication in reconstructing the paleostress field in the Precambrian south Indian shield.

Bundelkhand, Aravalli and Dharwar cratons Indian Shield: comparison of Archean crustal evolution and location in the Kenorland Supercontinent structure

Comparison of the Meso-Neoarchean crustal evolution of the Bundelkhand, Aravalli, Western and Eastern Dharwar Cratons shows that geodynamic mechanisms, similar to modern plate-tectonic and mantle-plume mechanisms, were active in that period. The Mesoarchean crustal evolution of the Bundelkhand Craton is considerably different from that of other cratons discussed here in that it comprises subduction-accretion processes, while in other cratons plume processes were more active in this period. In the Neoarchean (2.7-2.5 Ga), all the cratons display subduction-accretion processes. Each of the cratons displays its own crust formation pattern. It seems that in Mesoarchean time the Bundelkhand, Aravalli and Western and Eastern Dharwar Cratons were parts of the Kenorland Supercontinent, rather than one block.

Pb-Pb baddeleyite ages of mafic dykes from the Western Dharwar Craton, Southern India: a window into 2.21–2.18 Ga global mafic magmatism

Paleoproterozoic mafic magmatism in the Dharwar Craton (DC) is evident by ubiquitous dykes of varied orientations and emplacement ages. Eight distinct mafic dyke events have been identified in the Eastern Dharwar Craton (EDC), while studies in the Western Dharwar Craton (WDC) were restricted to a previously dated Dandeli dyke. Precise baddeleyite TE-TIMS Pb-Pb ages for five mafic dykes across the WDC reveal emplacement ages of 2212.3 ± 0.8 Ma, 2212.9 ± 0.4 Ma, 2208.1 ± 0.8 Ma, 2205.2 ± 0.8 Ma for NW-SE to NNW-SSE trending dykes and 1957.3 ± 0.7 Ma for ENE-WSW dyke. The ca. 1.96 Ga Mundargi dyke establishes a new episode of mafic magmatism in the DC, whereas ca. 2.21 Ga mafic dykes suggests widespread mafic magmatism across the DC. The collective trend of ca. 2.21–2.20 Ga radiating dykes vary from N100°E in EDC to N165°E in WDC, converging towards the north-western margin of DC (fanning angle ≃65°), where a mantle plume appears to be located. It is postulated that ca. 2.18 Ga Mahbubnagar-Dandeli dyke swarm of the DC also originated from a similar plume source, but as a distinct pulse within a time span of 35–40 Myr. Present study depicts barcode of the WDC and represents an epitome of correlations with the EDC, Superior, Slave and Greenland Cratons. Widespread ca. 2.21–2.18 Ga LIP event indicates that these cratons were located at proximity and suggests breakup of supercraton (Sclavia/Superia). The single ≃1.96 Ga Mundargi dyke is coeval with the dykes and sills of Karelia, North China and North Atlantic Cratons.

Geochemistry of ultramafic–mafic rocks of Mesoarchean Sargur Group, western Dharwar craton, India: Implications for their petrogenesis and tectonic setting

The Nuggihalli and Holenarsipur greenstone belts of the western Dharwar craton expose ultramafic–mafic rocks of the Mesoarchean. The rocks in these belts are geochemically considered as komatiites and komatiitic basalts with minor occurrences of tholeiitic and calc-alkaline basalts. The dominant ultramafic rocks of the Nuggihalli greenstone belt are layered and indicate fractionation processes at relatively shallower crustal levels. The Al-undepleted and Al-depleted signatures obtained could be attributed to magmatic differentiation processes and might be due to fractional crystallization of minerals such as hornblende and plagioclase, in addition to cumulus olivine and pyroxene. The chemical heterogeneity in the rocks of these greenstone belts might have therefore developed during the intrusion of the parental melts and their differentiation into a layered igneous complex. The differences in the lithological characteristics of the Holenarsipur and Nuggihalli greenstone belts can be explained by their different crustal levels of exposure. Presence of spinifex-textured komatiites need not necessarily imply that the sources have to be ultramafic and therefore of a deeper origin. This study indicates that the parental melts for unambiguous layered intrusive ultramafic–mafic complexes could be high-Mg basalts originating from relatively shallower levels. The probable geodynamic setting for the emplacement of the rocks of the two greenstone belts could be in a plume-modified mid-ocean ridge that was too thick and buoyant to be subducted, and the decompression-melted magma chamber developed igneous layering as the magma stalled in the lithosphere.

Early growth of the Indian lithosphere: Implications from the assembly of the Dharwar Craton and adjacent granulite blocks, southern India

This study provides a new perspective into the Archean accretionary tectonic evolution of the Dharwar Craton and the adjacent high-grade granulite blocks of southern India. The necessary constraints for this tectonic assembly were derived using new petrochemical and zircon U-Pb age data from the granulite facies Biligiri Rangan Block (BRB), which occupies a central key position in the Archean collage of southern India. The BRB, composed largely of charnockites and mafic granulites, is a vestige of ca. 3400–3200 Ma cratonic fragment that was amalgamated with the Western Dharwar Craton (WDC) at ca. 2700–2500 Ma. The spatial variations in composition and evolution of the charnockites (i.e. low- and high-Sr/Y variants) are attributed to arc magmatic processes at different crust-mantle depths accompanied by a flat- or shallow-dipping subduction geometry. Different stages in the subduction processes such as exhumation and magma underplating (ca. 2800–2950 Ma), initial arc magmatism (ca. 2700–2650 Ma), and peak magmatism together with metamorphism (ca. 2700–2500 Ma) were identified from zircon age populations of different rock-types. Our new results integrated with all published geological and geophysical data lead to the conclusion that the Kollegal and Mettur Shear Zones on the western and eastern sides of the BRB respectively were sited on an east-dipping Archean suture and a closed back-arc basin. We propose a tectonic model for southern India according to which subduction-accretion of the Coorg Block in the Mesoarchean (ca. 3300–3100 Ma) and of the Nilgiri Block, Biligiri Rangan Block and the Eastern Dharwar Craton in the Neoarchean (ca. 2700–2500 Ma) led to closure of the Paleo-Dharwar Ocean and to terminal collision with the Western Dharwar Craton.

Geochemistry, petrogenesis, and geodynamic implications of NE–SW to ENE–WSW trending Palaeoproterozoic mafic dyke swarms from southern region of the western Dharwar Craton

A number of NE–SW to ENE–WSW trending Palaeoproterozoic mafic dykes, intruded within the Archean basement rocks and more conspicuous in the southern parts of the western Dharwar Craton (WDC), was studied for their whole‐rock geochemistry to understand their petrogenetic and geodynamic aspects. Observed mineralogical and textural characteristics classify them either as meta‐dolerites or dolerites/olivine‐dolerites. They show basaltic to basaltic–andesitic compositions and bear sub‐alkaline tholeiitic nature. Three geochemically distinct groups of mafic dykes have been identified. Group 1 samples show flat REE patterns (LaN/LuN = ~1), whereas the other two groups have LaN/LuN = ~2–3 (Group 2; enriched LREE and flat HREE patterns) and LaN/LuN = ~4 (Group 3; inclined REE patterns). Chemistry is not straightforward to support any significant role of crustal contamination and probably reflect their source characteristics. However, their derivation from melts originated from a previously modified metasomatized lithospheric mantle due to some ancient subduction event cannot be ignored. Most likely different mantle melts were responsible for derivation of these distinct sets of mafic dykes. The Group 2 dykes are derived from a melt generated within spinel stability field by ~10% batch melting of a lithospheric mantle source, whereas the Group 3 dykes have their derivation from a melt originated within the spinel–garnet transition zone and were fed from slightly higher (~12–15%) batch melting of a similar source. The Group 1 samples were also crystallized from a melt generated at the transition zone of spinel–garnet stability field by higher degrees (~20%) of melting of a primitive mantle source. Geochemistry of the studied samples is typical of Palaeoproterozoic mafic dykes emplaced within the intracratonic setting, reported elsewhere globally as well as neighbouring cratons. Geochemistry of the studied mafic dyke samples is also compared with the mafic dykes of the eastern Dharwar Craton (EDC). Except the Group 3 samples, which have good correlation with the 1.88–1.89 Ga Hampi swarm, no other group shows similarity with the EDC mafic dykes. There is an ample possibility to have some different mafic magmatic events in the WDC, which could be different from the EDC. However, it can only be confirmed after precise age determinations.

Deformation Fabric and Vein Emplacement in the Rocks of Gadag-Chitraudrga Schist Belt (Western Dharwar craton, Southern India) - Relationship with Regional Tectonics

Abstract of Lecture delivered on 9th January, 2019 at the Montly Meeting of the Geological Society of India, Bangalore

Crustal and lithospheric mantle conductivity structure in the Dharwar craton, India

The vertical extension and structure of the sub-continental lithospheric mantle beneath the Archean Dharwar craton is the main attraction of the work presented here. To delineate the electrical conductivity structure of the Dharwar craton, a magnetotelluric study is carried out. This study comprises magnetotelluric data at 22 stations along a west-east slanting profile. Inter-station spacing is approximately 15 km. This magnetotelluric study is initiated from Dandeli (in the west) to Sindhanur (in the east side). The preferable geoelectric strike directions for the crust and lithospheric mantle are N3°E and N16°E respectively. A 2-dimensional (2-D) resistivity model derived by using the crustal and lithospheric mantle strike azimuths, identified conductive features in the stable continental Dharwar craton. In the crust, prominent conductors are present in the eastern and western part of the profile. A conducting feature is present in the deeper crust associated with the Chitradurga shear zone (CSZ). The study infers a thick lithosphere beneath Dharwar craton as a preserved cratonic nucleus on the eastern and a few conductive anomalies in the western part of the Dharwar craton. The model shows two separate conductors in the depth range of 110–250 km. This study shows, the possibility of presence of kimberlite melt in the western Dharwar craton in the depth range of 110–150 km.

Neoarchean magmatism in Shimoga greenstone belt, India: Evidence for subduction-accretion processes in the evolution of the western Dharwar stratigraphy

New geochemical and geochronological data are presented for a suite of metavolcanic rocks from Shimoga greenstone belt to understand their petrogenesis and to constrain the timing of final mafic magmatic activity in western Dharwar craton. We report an Sm-Nd isochron age of 2638 ± 66 Ma for metabasaltic and metaandesitic rocks of the stratigraphically uppermost Medur Formation of Dharwar Supergroup exposed in the belt, which is the youngest age obtained yet for Neoarchean mafic magmatism in western Dharwar craton. The emplacement of the basalt-andesite-dacite-rhyolite suite of rocks took place in a magmatic arc culminating in the deposition of vast greywacke-argillites in a continental margin-type setting. The magmatic precursors of the metavolcanic rocks were generated by partial melting of depleted mantle by slab-derived fluids with variable input from the subducting slab. Assimilation-fractional crystallization processes may also have contributed to the magmas to a significant extent. The younger mafic lithologies of the Dharwar stratigraphy represent accretionary orogens and the date obtained in this study points towards the final stages for the assembly of western Dharwar craton by subduction processes during Neoarchean. The thickness and extent of the greywacke-argillite suite in SGB could be attributed to the protracted deposition during amalgamation of western Dharwar craton with a northern Archean continental crust that might underlie Deccan volcanic rocks.

Complex upper mantle deformation beneath the Dharwar craton inferred from high density splitting measurements: Distinct lateral variation from west to east

Upper mantle anisotropy investigated using 172 core-refracted (SKS, SKKS) seismic phases along a ~660 km long profile at 10 to 20 km intervals from the west to the east coast of South India reveals significant lateral variations in its magnitude and direction. This profile, with 38 broadband seismic stations, covers mid-Archean Western Dharwar craton (WDC), late-Archean Eastern Dharwar Craton (EDC), Proterozoic Cuddapah Basin (CB) and the passive continental margins along the west and east coast. The observed fast polarization directions (FPDs) show lateral variability: NW50o to NW5o beneath the WDC, NW40o to NE30o beneath the EDC and N5o to N85o beneath the CB and further east. The delay time varies between 0.4 and 2.0 s with an average of 1 s. However, we are unable to fit a two layers anisotropy model for the region due to sparse azimuth coverage. Beneath the WDC, the direction of the fast axis follows trends of shear zones and faults, suggesting “frozen-in” anisotropy in the lithosphere, possibly established during the lithospheric evolution in mid-late Archean. In the EDC, the fast axis does not only follow the plate motion direction but it deviates, manifesting late Archean to Proterozoic deformation may still be present as fossil mantle anisotropy. The splitting trend beneath the CB and Eastern Ghat (EG) follows the strike of the rift along with plate motion direction, indicating anisotropy is influenced by the combination of “frozen” anisotropy due to continental rifting along the eastern margin of Indian plate and active asthenospheric flow.

Evolution of a Mesoarchean suprasubduction zone mantle wedge in the Dharwar Craton, southern India: Evidence from petrology, geochemistry, zircon U–Pb geochronology, and Lu–Hf isotopes

Petrological, geochemical, and zircon U–Pb geochronological features of Archean ultramafic–mafic complexes formed in subduction‐related settings provide significant insights into mantle source and geodynamic processes associated with subduction–accretion‐collision events in the early Earth. Here, we investigate a suite of serpentinized dunite, dunite, pyroxenite, and clinopyroxenite from an ultramafic complex along the collisional suture between the Western Dharwar Craton (WDC) and the Central Dharwar Craton (CDC) in southern India. We present petrology, mineral chemistry, zircon U–Pb geochronology, rare earth element (REE), Lu–Hf isotopes, and whole‐rock geochemistry including major, trace element, and platinum‐group element (PGE) data with a view to investigate the magmatic and metasomatic processes in the subduction zone. Mineral chemistry data from chromite associated with the serpentinised ultramafic rocks show distinct characteristics of arc‐related melt. Zircon U–Pb data from the ultramafic suite define different age populations, with the oldest ages at 2.9 Ga, and the dominant age population showing a range of 2.8–2.6 Ga. The early Paleoproterozoic (ca. 2.4 Ga) metamorphic age is considered to mark the timing of collision of the two WDC and CDC. Zircon REE patterns suggest the involvement continental crust components in the magma source. Zircon Lu–Hf analysis yields both positive and negative εHf(t) values from −3.9 to 1.5 with Hf‐depleted model ages (TDM) of 3,041–3,366 Ma for serpentinised dunite and −0.2–2.0 and 2,833–2,995 Ma for pyroxenite, suggesting that the magma was sourced from depleted mantle and was contaminated with the ancient continental crust. Geochemical data show low MgO/SiO2 values and elevated Al2O3/TiO2 ratios, implying subduction‐related setting. The serpentinized dunites and dunites show mild LREE enrichment over HREE, with relatively higher abundance of LILE (Ba, Sr) and depletion in HFSE (Nb, Zr), suggesting fluid–rock interaction, melt impregnation, and refertilization processes. The PGE data suggest olivine, chromite, and sulphide fractionations associated with subduction processes. Our study on the Mesoarchean to Neoarchean ultramafic complex provides important insights to reconstruct the history of the crust–mantle interaction in an Archean suprasubduction zone mantle wedge.

Magnetotelluric investigation of lithospheric electrical structure beneath the Dharwar Craton in south India: Evidence for mantle suture and plume-continental interaction

Broad-band and long period magnetotelluric measurements made at 63 locations along ∼500 km long Chikmagalur-Kavali profile, that cut across the Dharwar craton (DC) and Eastern Ghat Mobile Belt (EGMB) in south India, is modelled to examine the lithosphere architecture of the cratonic domain and define tectonic boundaries. The 2-D resistivity model shows moderately conductive features that intersperse a highly resistive background of crystalline rocks and spatially connect to the exposed schist belts or granitic intrusions in the DC. These features are therefore interpreted as images of fossil pathways of the volcanic emplacements associated with the greenstone belt and granite suite formation exposed in the region. A near vertical conductive feature in the upper mantle under the Chitradurga Shear Zone represents the Archean suture between the western and eastern blocks of DC. Although thick (∼200 km) cratonic (highly resistive) lithosphere is preserved, significant part of the cratonic lithosphere below the western DC is modified due to plume-continental lithosphere interactions during the Cretaceous–Tertiary period. A west-verging moderately conductive feature imaged beneath EGMB lithosphere is interpreted as the remnant of the Proterozoic collision process between the Indian land mass and East Antarctica. Thin (∼120 km) lithosphere is seen below the EGMB, which form the exterior margin of the India shield subsequent to its separation from East Antarctica through rifting and opening of the Indian Ocean in the Cretaceous.

Geochronology and geochemistry of Meso- to Neoarchean magmatic epidote-bearing potassic granites, western Dharwar Craton (Bellur–Nagamangala–Pandavpura corridor), southern India: implications for the successive stages of crustal reworking and cratonization

Abstract We present field and petrographical characteristics, zircon U–Pb ages, Nd isotopes, and major and trace element data for the magmatic epidote-bearing granitic plutons in the Bellur–Nagamangala–Pandavpura corridor, and address successive reworking and cratonization events in the western Dharwar Craton (WDC). U–Pb zircon ages reveal three stages of plutonism including: (i) sparse 3.2 Ga granodiorite plutons intruding the TTG (tonalite–trondhjemite–granodiorite) basement away from the western boundary of the Nagamangala greenstone belt; (ii) 3.0 Ga monzogranite to quartz monzonite plutons adjoining the Nagamangala greenstone belt; and (iii) 2.6 Ga monzogranite plutons in the Pandavpura region. Elemental data of the 3.2 Ga granodiorite indicate their origin through the melting of mafic protoliths without any significant residual garnet. Moderate to poorly fractionated REE patterns of 3.0 Ga plutons with negative Eu anomalies and Nd isotope data with ε Nd (T) = 3.0 Ga ranging from −1.7 to +0.5 indicate the involvement of a major crustal source with minor mantle input. Melts derived from those two components interacted through mixing and mingling processes. Poorly fractionated REE patterns with negative Eu anomalies of 2.6 Ga plutons suggest plagioclase in residue. The presence of magmatic epidote in all of the plutons points to their rapid emplacement and crystallization at about 5 kbars. The 3.2 Ga intrusions could correspond to reworking associated with a major juvenile crust-forming episode, whilst 3.0 Ga potassic granites correspond to cratonization linked to melting of the deep crust. The 2.6 Ga Pandavpura granite could represent lower-crustal melting and final cratonization, as 2.5 Ga plutons are absent in the WDC.

Stochastic Modeling of the Thermal Structure to Decipher the Lithospheric Thickness: Application to Dharwar Craton

A wide range of speculation regarding lithospheric thermal thickness in a region calls for quantifying the uncertainties associated with model parameters. In this paper, we present an attempt to decipher it by solving the differential equations governing the heat transfer through the lithosphere. Adequately quantifying the temperature field is very important, since it is highly dependent on the controlling thermal parameters. Thermal conductivity is modeled as a random parameter with a known mean value and correlation structure. We compute the temperatures in the crust and lithospheric mantle, along with their error bounds, using an analytical solution to the steady-state heat equation and the Adomian method of decomposition. We apply these solutions in the Archean Dharwar craton, which is divided into western Dharwar craton (WDC) and eastern Dharwar craton (EDC). We obtain a Moho temperature estimate of 304 ± 58 °C in WDC, whilst in EDC it is 375 ± 74 °C for a 20% coefficient of variability in thermal conductivity. The range of lithospheric thickness obtained for WDC is 150–280 km, and 110–160 km for the EDC. These values are in agreement with other geophysical and geological findings in the region. The thermal lithosphere obtained from this study is based on the mantle solidus and the surface wave seismological studies considering radial anisotropy reveal that the lithosphere is characterized by higher shear wave velocities, and decrease at the lithosphere–asthenosphere boundary. Average shear velocity model for two different cratonic blocks (EDC and WDC) is used to show the LAB thickness for the eastern and western Dharwar craton. The two independent studies show a correlation in the lithospheric thickness.

Boninites in the ~3.3 Ga Holenarsipur Greenstone Belt, Western Dharwar Craton, India

In this contribution, we present detailed field, petrography, mineral chemistry, and geochemistry of newly identified high-Si high-Mg metavolcanic rocks from the southern part of the ~3.3 Ga Holenarsipur greenstone belt in the western Dharwar craton, India. The rocks occur as conformable bands that were interleaved with the mafic-ultramafic units. The entire volcanic package exhibits uniform foliation pattern, and metamorphosed under greenschist to low grade amphibolite facies conditions. The rocks are extremely fine grained and exhibit relict primary igneous textures. They are composed of orthopyroxene and clinopyroxene phenocrysts with serpentine, talc, and amphibole (altered clinopyroxene). Cr-spinel, rutile, ilmenite, and apatite occur as disseminated minute grains in the groundmass. The mineralogical composition and the geochemical signatures comprising of high SiO2 (~53 wt. %), Mg# (~83), low TiO2 (~0.18 wt. %), and higher than chondritic Al2O3/TiO2 ratio (~26), reversely fractionated heavy rare earth elements (REE) (GdN/YbN ~ 0.8), resulting in concave-up patterns, and positive Zr anomaly, typically resembled with the Phanerozoic boninites. Depletion in the high field strength elements Nb, and Ti relative to Th and the REE in a primitive mantle normalized trace element variation diagram, cannot account for contamination by pre-existing Mesoarchean continental crust present in the study area. The trace element attributes instead suggest an intraoceanic subduction-related tectonic setting for the genesis of these rocks. Accordingly, the Holenarsipur high-Si high-Mg metavolcanic rocks have been identified as boninites. It importantly indicates that the geodynamic process involved in the generation of Archean boninites, was perhaps not significantly different from the widely recognized two-stage melt generation process that produced the Phanerozoic boninites, and hence provides compelling evidence for the onset of Phanerozoic type plate tectonic processes by at least ~3.3 Ga, in the Earth’s evolutionary history.