Iron Ore Group Research Articles

Vestiges of Earth’s earliest depleted mantle reservoir

Abstract There is a paucity of evidence preserved in the rock record regarding Earth’s earliest enriched crust and its complementary depleted mantle during the Hadean. In recent years, vestiges of these early reservoirs have been inferred by examination of Hf isotope systematics compiled from zircons. The Singhbhum craton of Eastern India, for example, preserves only the existence of an enriched (εHf <0) crustal reservoir during the Hadean–Eoarchean, with the notable absence of a depleted mantle reservoir signature (εHf >0) until ca. 3.5 Ga. Here we report a new Sm-Nd isochron for the Lower Lava greenstones of the western Iron Ore Group from the Singhbhum craton, confirming a 3.42 ± 0.14 Ga crystallization age with an initial εNd of +5.7 ± 2.5. This is the highest positive εNd value derived from an isochron of this age. We infer that this depleted mantle source is a vestige complementary to the primary crust following planetary differentiation. Furthermore, we present U-Pb zircon ages for a 3.39 ± 0.02 Ga tuff that lies stratigraphically above the Lower Lava and <30 cm below an extensive conformable banded iron formation (BIF). This age implies that the western Iron Ore Group’s BIF is the largest economic-grade iron formation of its Paleoarchean age, suggesting that free atmospheric oxygen existed as more than just whiffs at this time.

Origin of Alteration Patterns in Accessory Chromites from the Kudada Metaperidotites, East Singhbhum District (Jharkhand, India)

Abstract The metamorphosed ultramafic-mafic bodies of the Kudada area are located close to the Singhbhum Shear Zone (SSZ) in eastern India, where the major rock types are talc-magnesite schist and serpentinite with accessory chromite and magnetite veins. The ultramafic bodies and associated metavolcanic rocks are part of the northern extension of the Early Archean Gorumahisani greenstone belt and belong to the Iron Ore Group (IOG) supracrustal sequence. This study reveals intense compositional variability in accessory chromites of serpentinite with core composition of chromites are characterized by the variable Cr# [Cr/(Cr+Al)] = 0.53 - 0.82 and Mg# [Mg/(Mg+Fe2+)] = 0.01 - 0.17. Compositional variability on the scale of a single chromite grain occurs in the form of multi-stage zoning. To identify the patterns of compositional zoning, chromites of serpentinite are subdivided into four types depending on their grain size, reflectivity of different rims, intensity of fracture, and porosity, and supported by in-situ mineral chemistry. The type-I chromites are less fractured and non-porous variety showing the outermost chrome magnetite rim envelops the inner ferritchromit rim. Porosity is mainly developed in the type-II grains where the inner ferritchromit is formed surrounding the pore spaces. The type-III chromites are small clustered grains having ferritchromit core and chrome magnetite rim while the type-IV grains are completely altered to chrome magnetite. Textural relations and mineral chemistry indicate that metamorphism and activities of H2O and CO2-rich hydrothermal fluids during tectonic evolution of the Singhbhum Shear Zone (SSZ) might have caused these zoning patterns and compositional variabilities in accessory chromites of the Kudada area. Cation exchange between chromite and silicate minerals along with intra-grain cation diffusion within different Cr-spinel zones further intensified these processes.

Petrology of Palaeoarchaean mafic–ultramafic rock suites of the western Iron Ore Group, Singhbhum Craton, eastern India, using the chemistry of minerals

AbstractThe petrogenesis of Palaeoarchaean mafic–ultramafic rock suites from the western Iron Ore Group, Singhbhum Craton, eastern India, has been evaluated based on the chemistry of constituent mineral phases. The rock suites include basaltic rocks and mafic (gabbro) to ultramafic (serpentinized peridotite) intrusive rocks, which occur in host rocks covering phyllite, ferruginous shale, banded haematite quartzite and jasper. The constituent clinopyroxene shows dominant uralitization, whereas plagioclase grains are generally saussuritized, being marked by relatively tiny granular aggregates of albite, chlorite, epidote and K-feldspar. The ultramafic intrusive rocks are overwhelmingly serpentinized. Clinopyroxene compositions are augitic, whereas relict plagioclase is typically bytownite. Amphiboles of the investigated rock suites are divisible into the ‘uralite’ type (occurring peripherally to clinopyroxene) and the ‘completely changed-over amphibole’ type (with no traces of initial clinopyroxene). Both the amphibole types belong to the ‘calcic group’, showing a compositional spectrum from actinolite–magnesiohornblende–ferrohornblende–ferroactinolite. Opaque minerals include magnetite (both Cr-magnetite and Al-magnetite), chromite (Al-chromite) and ilmenite, whereas serpentine (belonging to the ultramafic intrusive rocks) corresponds to lizardite. Looking at the mineral compositions of the pyroxene, glass and amphibole, the studied rock suites show a wide equilibration temperature–pressure domain range (∼750 °C to ∼1400 °C at ∼0.26 kbar to ∼21 kbar), which corresponds to an ascending magma that underwent a ‘hydration event’ at a shallow level. The assessment of the clinopyroxene and spinel chemistry characteristically suggests an arc setting for the parent magma that has undergone both equilibrium and fractional crystallization in the course of magmatic evolution. During differentiation, the magmatic density remains almost constant, with variable oxygen fugacity.

Deformation Episodes in Iron Formation of Eastern Province of North Odisha Iron Ore Craton, Eastern India

Banded Iron Formation (BIF), a bi-component rock of the Precambrian period is exposed along with iron ore in distinct geographical entities encircling the North Odisha Iron Ore Craton (NOIOC) in Odisha – Jharkhand states, eastern India. Iron formation of the eastern province is confined to Badampahar-Gorumahisani-Sulaipat belt. The litho assemblages belonging to this Iron Ore Group comprise of banded cherty quartzite, banded magnetite quartzite, banded magnetite grunerite quartzite, tremolite-actinolite schist and fuchsite quartzite. Deformations in phases affected the BIF members and associated rocks of the area resulting in successive fold structures. The first generation folds (F1) are characteristically tight and isoclinal having NE plunging axes. The second phase fold (F2) structures in the area with reference to first fold are co-axial, upright and tight to open in nature profusely overprinting the F1 folds are parallel to the general trend (NE-SW) of the belt. Axial plane of the third phase folds (F3) are gentle and broad warps having NW-SE trending axial planes and are found to be the last traceable ones in the area. trending in NW-SE direction are found to be the last traceable ones in the area having gentle and broad warps. The co-axial F1 and F2 folds along the general trend (NE-SW) of the belt are superposed by NW-SE trending F3 fold, which exhibits a geometric configuration as F1 // F2 F3. Such type of multiphase deformed terrain has produced many interference fold patterns in minor scale out of superposition i.e. dome and basin structures, hook shaped patterns, eyed fold and S, Z, and M shaped folds. The paper discusses the episodes of structural events and their signatures in the interference fold patterns in the eastern province of the NOIOC. Keywords: Banded Iron Formation, Badampahar-Gorumahisani-Sulaipat Belt, Deformation Episodes, NOIOC

Detrital zircon ages from Archaean conglomerates in the Singhbhum Craton, eastern India: implications on economic Au-U potential

New U–Pb age and Hf isotope data obtained on detrital zircon grains from Au- and U-bearing Archaean quartz-pebble conglomerates in the Singhbhum Craton, eastern India, specifically the Upper Iron Ore Group in the Badampahar Greenstone Belt and the Phuljhari Formation below the Dhanjori Group provide insights into the zircon provenance and maximum age of sediment deposition. The most concordant, least disturbed 207Pb/206Pb ages cover the entire range of known magmatic and higher grade metamorphic events in the craton from 3.48 to 3.06 Ga and show a broad maximum between 3.38 and 3.18 Ga. This overlap is also mimicked by Lu–Hf isotope analyses, which returned a wide range in εHf(t) values from + 6 to − 5, in agreement with the range known from zircon grains in igneous and metamorphic rocks in the Singhbhum Craton. A smaller but distinct age peak centred at 3.06 Ga corresponds to the age of the last major magmatic intrusive event, the emplacement of the Mayurbhanj Granite and associated gabbro, picrite and anorthosite. Thus, these intrusive rocks must form a basement rather than being intrusive into the studied conglomerates as previously interpreted. The corresponding detrital zircon grains all have a subchondritic Hf isotopic composition. The youngest reliable zircon ages of 3.03 Ga in the case of the basal Upper Iron Ore Group in the east of the craton and 3.00 Ga for the Phuljhari Formation set an upper limit on the age of conglomerate sedimentation. Previously published detrital zircon age data from similarly Au-bearing conglomerates in the Mahagiri Quartzite in the Upper Iron Ore Group in the south of the craton gave a somewhat younger maximum age of sedimentation of 2.91 Ga. There, the lower limit on sedimentation is given by an intrusive relationship with a c. 2.8 Ga granite. The time window thus defined for conglomerate deposition on the Singhbhum Craton is almost identical to the age span established for the, in places, Au- and U-rich conglomerates in the Kaapvaal Craton of South Africa: the 2.98–2.78 Ga Dominion Group and Witwatersrand Supergroup in South Africa. Since the recognition of first major concentration of gold on Earth’s surface by microbial activity having taken place at around 2.9 Ga, independent of the nature of the hinterland, the above similarity in age substantially increases the potential for discovering Witwatersrand-type gold and/or uranium deposits on the Singhbhum Craton. Further age constraints are needed there, however, to distinguish between supposedly less fertile (with respect to Au) > 2.9 Ga and more fertile < 2.9 Ga successions.

Style of Fractional Crystallization in Basalts from the Paleoarchean Western Iron Ore Group of Singhbhum Craton, Eastern India: Implications from One Atmosphere Experimental Studies

Abstract One atmosphere experimental study on fresh basaltic rocks of Paleoarchean western Iron Ore Group (IOG) basin of Singhbhum Craton, Eastern India gives insight of the crystallization behavior of the parent magma, liquid lines of descent and appearance of the different constituent phases. The natural starting material is sufficiently ferroan (FeOt: 14.28 wt%) and corresponds to ‘ferro-basalt’. Regarding the experimental run products, clinopyroxene shows Al2O3 rich character (ranging from 2.23 – 6.94 wt%; formed due to fluctuating silica activities in the magma) while plagioclase reveals typical Fe-Mg enrichment (that corresponds to higher KdFe-Mg between plagioclase and the parent liquid caused due to immiscible silicate melt inclusion in plagioclase where Mg enters more readily in plagioclase than Fe). The run-product glass corresponds to both high and low iron contents suggesting liquid immiscibility in the parent magma at particular temperature-domains. This study suggests that the crystallization of the western Iron Ore Group basalts represents a broad ‘Skaergaard analogue’ involving depression/shift of eutectic point in the diopside-anorthite system during crystallization.

3D Phase tensor inversion and Joint inversion using impedance and phase tensor components aiming better subsurface mapping and interpretation in Dhanjori basin, Singhbhum craton, eastern India

Dhanjori formation, a significant member of Singhbhum Craton, Eastern India, is well-known for its complex mineralogical character and huge content of Au-mineralization. This 2.1 Ga old Paleoproterozoic formation is composed of siliciclastic metasedimentary rocks interlayered with mafic/ultramafic volcanic rocks. Traces of gold has been spotted by many researchers in the sulfide-hosted mafic-ultramafic volcanic rocks and quartz pebble conglomerate units by different studies. Regional Audio Magnetotelluric (AMT) survey was conducted in 10 kHz–10 Hz frequency range over various lithological units of Dhanjori and adjacent Iron Ore Group. The key challenge faced during the analysis of magnetotelluric (MT) data is due to the presence of near-surface heterogeneities. The amplitude of the MT response gets distorted by galvanic effects which further create difficulties in modelling and inversion. If the effect is not removed in advance, it might lead to erroneous subsurface modelling and interpretations. On the contrary, the phase tensor data does not show any change in values even in the presence of galvanic distortion. Though phase tensors are the computed values from impedance tensor, still the response is free from any shallow level ambiguities unlike impedance data. We have performed 3-D inversion of phase tensor data to generate a conductivity model of Dhanjori basin. The objective was to verify whether the inclusion of phase tensor components in the inversion routine was able to provide some extra information that might add value to the analysis. Model from phase tensor inversion indicates only relative variation in the subsurface conductivity structure due to lack of amplitude information in the data. Hence, we also performed 3D joint inversion including both impedance and phase tensor components for obtaining the final conductivity model of Dhanjori to recover the absolute subsurface conductivity structure unaffected by galvanic distortion. Correlation of the conductivity model with available geoscientific information in Dhanjori and adjacent geological units helped us to relate the conductors to the sulfide mineralization prospect.

Prospects of Daschkesan iron ore deposit development

The topicality the subject. To meet Azerbaijan's growing demand for ferrous metallurgical raw materials, there are sufficient reserves of iron ore deposits that can be put into industrial operation. These are related to the future development of the mining industry in the Daschkesan iron ore group, including the promising South-Daschkesan and Damirov fields. The re-exploitation of the Daschkesan iron ore group will have a positive impact on the socioeconomic development of the region, as well as strengthen the position of the mining industry and metallurgy in the country's non-oil sector. The resumption of Daschkesan iron ore production will also make an important contribution to the development of a number of by-products, improve employment in the regions, and create new and wider opportunities for local companies. The purpose of the study. In the rich areas of the Daschkesan iron ore deposit, due to the reduction of the amount of iron in the ores, the enrichment is the production of granules for the production of iron by the sponge method. Research methods. Along with the study, research and exploration of ferrous metal ore deposits in the territory of the republic, geophysical research and geological exploration work has been started for the re-commissioning of the Daschkesan iron ore deposit. The first 3-dimensional geological model of the Daschkesan iron ore deposit was developed using special software. The goal is to produce iron ore deposits in a short period of time and meet the demand for quality raw materials for the production of ferrous metallurgy products. Results. Considering that the use of wastes will increase the efficiency of field development, it is necessary to economically evaluate, sort and use metal-bearing wastes discharged into the Goshgarchay River during the enrichment of magnetite ore in the past as a valuable building material such as sand and gravel in Ganja region. It is planned to establish joint ventures with waste from mining enterprises and concentrators, slag from the pipe rolling plant, and waste from the Ganja aluminum plant. Conclusion. The future development of the mining industry in our country can be achieved through the complex use of iron ore deposits of the Daschkesan group, quarry waste, waste from the enrichment of magnetic ores.

Petrogenesis of the Mesoarchean (~3.05 Ga) mafic volcanics from the western Iron Ore Group volcano-sedimentary succession, Singhbhum craton, eastern India: Constraints from geochemical modelling and Sm-Nd geochronology

Cratonic Mesoarchean volcano-sedimentary successions preserve compositionally diverse volcanic rocks which give insight into Archean mantle characteristics and crustal evolution. The late Archean western Iron Ore Group (W-IOG) succession in the Singhbhum craton represents deposition during this time period. As the W-IOG depositional age has remained poorly constrained, to between Paleo- and Mesoarchean, petogenesis and chronology of the interbedded volcanics would be an important geological constraint. Mafic volcanics, interlayered with BIF and Fe-Mn-rich phyllite/shale, were sampled from Kalta and Ongarbira, and petrogenetic modelling of their mantle source and Sm-Nd geochronology were conducted. The Ongarbira basalts show tholeiitic affinity, are depleted in LILE, LREE, Zr and Th and an absence of Nb-Ta-Ti anomalies that imply a depleted MORB-type mantle source. Tholeiitic basalts yield an isochron age of 3050 ± 71 Ma (2SD) with Ndi = 0.50885 ± 0.00010, MSWD = 0.17 (n = 10) and εNd(T) = +3.3 ± 1.6. Geochemical modelling indicates the most primitive tholeiitic basalt (Mg# = 61) formed by ~14% partial melting of depleted mantle whereas the most enriched variant formed by its subsequent assimilation-fractional crystallization (AFC) (~15% with r = 0.2 being the ratio between assimilation rate and fractional crystallization rate). In contrast, the Kalta basaltic andesite have calc-alkaline affinity, are enriched in LILE, LREE, Zr and Th with pronounced negative Nb-Ta-Ti anomalies indicating incompatible trace element enrichment in their petrogenesis. The basaltic andesite yield an isochron age of 3041 ± 94 Ma (2SD) with Ndi = 0.50875 ± 0.00009, MSWD = 0.62 (n = 10) and εNd(T) = +1.1 ± 1.6. Geochemical modelling indicates that their compositions were generated by 20–40% AFC (r = 0.2) of ~14% partial melt of depleted mantle. The source magma to the Ongarbira basalt is interpreted to have formed during lithospheric extension of the Singhbhum craton, by partial melting of depleted mantle followed by minor AFC, whereas the Kalta basaltic andesite represents higher extents of AFC of the Ongarbira-type tholeiitic magma.

Geochemistry and tectonic setting for the deposition of IOG siliciclastics at the western margin of Bonai Granite, Singhbhum-Orissa Craton, India

Bagiyabahal and Birtola areas are located in the south-western extension of the Noamundi-Koira Iron Ore Group (IOG) basin. Rock types exposed in the area comprises of siliciclastics and volcanics which occurs unconformably over the basement tonalite-trondhjemite granite-gneiss (Bonai Granite Phase-I). The cover rocks show sheared contact with the porphyritic Bonai Granite Phase-II. The IOG basin margin is suggested to be a part of a ‘volcanic passive margin’ as indicated by the geochemical behaviour of the siliciclastics as well as massive emplacements of mafic intrusives (doleritic sill, dyke and gabbro) and extrusives (basaltic lava flow) along faulted continental blocks. The siliciclastics comprise of U and Au bearing quartz-pebble conglomerate (QPC) and quartzite succession. It was deposited along the western margin of the Bonai granite (phase I) in anoxic conditions as indicated by their low Th/U ratios and presence of detrital uraninite grains. Repeated cycles of sedimentation and volcanism led to the formation of alternate layers of siliciclastics and basic bodies in the area. Major, trace and rare earth elements (REE) geochemical data suggests a semi-humid to humid palaeo-climatic environment of during the deposition in the passive continental margin setting characterized by fault-controlled sedimentation over a rift related faulted continental crust and shelf. Geochemical data suggests chemically weathered provenance dominated by clay minerals. Higher content of U, Th, Au, Cr, REE, platinum group of elements (PGE) and other geochemical ratios suggest a mixed provenance for the deposition of the siliciclastics comprising a predominantly acidic/granitic source possibly from the Bonai Granitic Complex (BGC) along with granite derived reworked quartzose sediments, minor basic and ultrabasic sources of Older Metamorphic Group (OMG). This paper attempts to characterize the geochemical behaviour, tectonic setting and provenance of the siliciclastics of Birtola and Bagiyabahal areas by analyzing drill core and surface samples.

Development of Crustal-Scale Shear Zones at the Singhbhum Craton–Eastern Ghats Belt Boundary Region: A Critical Review of the Mesoarchean–Neoproterozoic Odyssey

Abstract A number of crustal-scale shear zones have developed along the southern margin of the Singhbhum Craton, in the boundary with the Neoarchean Rengali Province and the Meso-Neoproterozoic Eastern Ghats Belt. The cratonic part, evolved in a suprasubduction zone setting, bears imprints of late Mesoarchean orogenic episode (D1C) at ca. 3.1 Ga with folding and thrust imbrication of the cratonic rocks. The succeeding orogenic imprint is etched in the Neoarchean (~2.8 Ga) with development of the Sukinda thrust along the craton margin and thrust-related deformation of the rocks of the Rengali Province (D2C-D1R). The latter event remobilized cratonic fringe with development of a spectacular E-W trending transpressional belt in the Southern Iron Ore Group rocks cored by the Sukinda ultramafics. In the Eastern Ghats Belt, the major ultrahigh-temperature orogeny took place during the Grenvillian-age (~1.0-0.9 Ga) assembly of the supercontinent Rodinia. This belt eventually got juxtaposed against the expanded Singhbhum Craton in the end-Neoproterozoic time (~0.5 Ga) along the Kerajang Fault Zone. This latter event remobilized a large part of the Rengali Province (D2R) with development of an intraterrane transpressional belt bounded by the Barkot Shear Zone in the north. The northern fringe of the intruding Eastern Ghats Belt developed a complex network of strike-slip fault system under this impact, probably an outcome of tectonic activity along the Kuunga suture, which signifies the joining of greater India with East Antarctica. The present synthesis visualizes early development in the craton through formation of a typical orogenic sequence, imbricated in thrust piles, resulting from a ca. 3.1 Ga orogeny. Further cratonic expansion was achieved via repetitive accretion and remobilization, development of crustal-scale faults and transpressional belts at ca. 2.8 Ga and ca. 0.5 Ga, much in a similar fashion as documented along oblique convergent margins of all ages.

Paleoarchean evolution of the Singhbhum Craton, eastern India: New constraints from geochemistry and geochronology of granitoids of Bonai and Champua area

The Singhbhum Craton, one of five major Archean cratons in the Indian subcontinent, contains abundant well-preserved Paleoarchean supracrustals and granitoids. This study presents zircon U-Pb ages and whole rock geochemistry of tonalite-trondhjemite-granodiorites (TTGs) and granites from the Bonai Granite Complex (BGC) and Older Metamorphic Tonalite Gneiss (OMTG), which are separated from each other by the Jamda-Koira-Noamundi Iron Ore Group (IOG) supracrustals. Emplacement ages obtained in this study indicate that a major episode of TTG magmatism took place in BGC around 3368 ± 8 Ma (1σ), followed by granitic magmatism around 3331 ± 33 Ma (1σ). In contrast, a TTG from the Deo Nala area representing OMTG yielded crystallization age of 3312 ± 8 Ma (1σ). The emplacement and evolution of the BGC were coeval with granitoid magmatism from the central part of the Singhbhum Craton. Whole rock geochemical data identify both high- and low-HREE TTGs in both the BGC and OMTG to the west and east of the IOG basin, respectively. The trace element systematics of high-HREE Bonai TTG are similar to those of Icelandic dacites, suggestive of their derivation from a garnet free, plagioclase rich amphibolite. The low-HREE TTGs of the BGC and OMTG were derived from an amphibolite source with varying amounts of garnet. The potassic granites of the BGC were sourced from the older TTGs which had undergone partial melting at a shallow depth. The evolution of the BGC and OMTG can be attributed to the partial melting under a thickened mafic crust. Dome and keel structures and emplacement ages of granitoids from the west and east of the Jamda-Koira-Noamundi IOG basin, support the origin of these Paleoarchean granitoids in a stagnant lid regime. High geothermal gradients induced by heat supplied by mantle upwelling appear to have induced the melting of the thickened crust, to form the TTG. Delamination induced mafic–ultramafic underplating resulted in melting of early formed TTGs, to form the younger potassic granites of the BGC at ∼ 3.33 Ga.

Possible occurrence of Palaeoarchean ferropicrite cumulates and ferrobasalts in the Jojohatu area of North Singhbhum Craton, eastern India: Evidence for a mantle plume source

An occurrence of Palaeoarchaean ferropicrite cumulates and ferrobasalts is being reported from the Jojohatu area of the Iron Ore Group (IOG) greenstone belt, which is situated in the northern part of the Singhbhum Craton (eastern India). The ferropicrite cumulates contain serpentine and clinopyroxene as major constituents and spinel, magnetite, and ilmenite as accessories. Their bulk rock chemistry exhibits an extremely high amount of FeOT (19.0–20.8 wt%) and MgO (22.0–23.9 wt%) and quite low Al2O3 (4.5–5.2 wt%) and Na2O + K2O (&lt;1.0 wt%), thus making them as one of the most Fe‐rich ferropicrite cumulates in the world. They are also characterized by high contents of compatible trace elements (Ni: 986–1,470 ppm; Cr: 990–1,256 ppm) and moderate TiO2 (1.5–1.7 wt%), Nb (15–18 ppm), and Zr (82–90 ppm). These cumulates are possibly formed due to accumulation of olivine from the ferrobasaltic melts. In comparison, ferrobasalts consisting of plagioclase, pyroxene, and opaques are alkaline in nature. Some of the plagioclase and pyroxene grains have shown a signature of metasomatic alteration, which resulted into formation of secondary minerals such as epidote, chlorite, and hornblende. They exhibit relatively low contents of FeOT (14.6–18.4 wt%), MgO (8.4–10.8 wt%), Ni (32–122 ppm), and Cr (41–131 ppm) than the ferropicrite cumulates, while TiO2 (2.9–4.1 wt%), Nb (42–61 ppm), and Zr (216–318 ppm) contents are higher. Incompatible elemental patterns and their ratios, as well as Zr/Y versus Nb/Y plot, indicate their ocean island basalts (OIB) affinity and genesis from a hot mantle plume, having mantle potential temperature of about 1,600°C and pressure of 3.5–5 GPa. Considerably low CaO (3.00–7.16 wt%) and FeO/MgO and CaO/MgO systematics indicate that the primary source of the basaltic melt could be pyroxenite. These volcanic occurrences possibly represent remnants of the OIB formed during the Palaeoarchean in the eastern part of the Indian shield.

A plume - mantle interaction model for the petrogenesis of komatiite - komatiitic basalt - basalt - basaltic andesite volcanism from the Paleoarchean (3.57–3.31 Ga) Iron Ore Group greenstone belts, Singhbhum craton, India: Constraints from trace element geochemistry and Sm[sbnd]Nd geochronology

The Paleoarchean Singhbhum craton greenstone belts comprise basalt, interbedded minor komatiite (both alumina depleted and undepleted, ADK and AUK, respectively), komatiitc basalt, dacite and chert grouped as (~3.57 Ga) southern Iron Ore Group (S-IOG, Daitari samples) and ~ 3.31 Ga eastern Iron Ore Group (E-IOG, Dabanki samples). Considering the importance of metabasalt in formation of TTG batholiths, modeling their petrogenesis from mantle sources and SmNd geochronology was conducted. The rocks resemble Archean non-arc greenstone basalt (Nb/Th = 3.61–10.3) with tholeiitic affinity. The ADK of S-IOG was modeled by 37% partial melting of garnet lherzolite, komatiitic basalt was derived from ADK magma by ~25% assimilation fractional crystallization (AFC) and ~ 16% AFC of this magma formed tholeiitic basalt. Derived from depleted parental magmas they yield an isochron age of 3573 ± 63 Ma (2SD), Ndi = 0.50810 ± 0.00008, MSWD = 0.59 (n = 10) and εNd (3573 Ma) = +2.03 ± 0.03. Dacite formation, modeled by mixing of melts between Eoarchean felsic crust and metabasalt, yielded an isochron age of 3507 ± 180 Ma (2SD), Ndi = 0.50800 ± 0.00015, MSWD = 0.15 (n = 11), εNd (3507 Ma) = −1.46 ± 1.71; ca. 3.8–3.9 Ga Nd TDM model ages indicate vestiges of Eoarchean crust. The AUK of E-IOG was modeled by ~45% partial melting and ADK by ~33% melting of garnet lherzolite mantle. About 32% AFC of primary ADK magma formed the komatiitic basalt that evolved to tholeiitc basalt magma (~24%) which on ~15% AFC evolved to basaltic andesite - andesite magma. Derived from depleted parental magmas, the E-IOG rocks yielded an isochron age of 3313 ± 110 Ma (2SD), Ndi = 0.50847 ± 0.00013, MSWD = 0.06 (n = 10), εNd (3313 Ma) = +2.62 ± 0.28. Petrogenesis of Paleoarchean greenstone non-arc basaltic volcanism and associated komatiite can be best explained by plume-mantle interaction and partial melting of ambient mantle followed by assimilation fractional crystallization of parental magmas, and does not preclude recycling of pre-existing crust.

Variations in crustal and lithospheric structure across the Eastern Indian Shield from passive seismic source imaging: Implications to changes in the tectonic regimes and crustal accretion through the Precambrian

Changes in the dominant tectonic regimes and the modes of formation and maturation of the continental crust during the Precambrian are a subject of active debate. Here, we discuss these issues in terms of new information on the crustal and lithospheric architecture of the different terranes in the Eastern part of the Indian Shield (EIS). We present well-constrained estimates of the crustal and lithospheric thickness characterizing different terranes of the EIS through the joint inversion of P-receiver functions (PRF) and fundamental mode group velocity dispersion data of Rayleigh waves (10–100 s). The lithospheric thicknesses are also computed independently using the S-receiver function (SRF) modeling. The study involved 2167 PRFs and 100 SRFs from 15 broadband stations. Our modeling detects a marked crustal (~4–8 km) and lithospheric (~15–20 km) thinning beneath the Singhbhum-Odisha-Craton (SOC), spatially correlating with the central part of the craton comprising the gneissic terrain contrasted by a crustal thickening (~2–6 km) beneath the horseshoe-shaped Iron Ore Group. We propose that such a crustal configuration beneath the Paleoarchean craton may suggest the dominance of vertical tectonic processes such as thickening of an oceanic mafic plateau and recurrent melting producing the TTGs and the Singbhum Granite suite. Infra-crustal reworking involving Rayleigh-Taylor instabilities and gravitational processes could explain the crustal architecture of the SOC much similar to a few other Paleoarchean cratons such as Pilbara (Australia), Barberton and Kappvaal (South Africa). Our modeling has also revealed a relatively smaller degree of crustal (2–4 km) and lithospheric thinning (4–10 km) beneath the Eastern Ghat Mobile Belt, south of the SOC. We observe a relatively thicker crust (~42 km) characterized by a flat Moho over a large area (~40,000 sq. km) encompassing the Chota Nagpur Granite Gneiss Terrane (CGGT). Such thickening is consistent with the convergence and collision tectonics involving the SOC and CGGT. However, we note a 15–20 km lithospheric thinning associated with the CGGT. Such thinning may mainly relate to Phanerozoic rifting along the Damodar graben and Rajmahal magmatism. Broadly, our model of the crustal and lithospheric architecture of the EIS reflects the imprints of northward subduction of SOC beneath the CGGT.

Paleoarchean terrestrial to shallow marine sedimentation on Singhbhum Craton, eastern India (the Western Iron Ore Group)

Distinguishing shallow-marine and terrestrial (fluvial) sedimentary deposits is often difficult in the absence of fossils and challenging in deformed and metamorphosed early Precambrian terrains. Herein we report for the first time a > 3.4 Ga shallow marine to terrestrial deposit from the Western Iron Ore Group (WIOG), Singhbhum Craton, India. The WIOG sedimentary succession, lying below a ~ 3.4 Ga banded iron formation, has been divided into three facies associations: a lower alluvial fan-fluvial association, a middle shallow-marine association, and an upper fluvial association. The bottommost conglomerate–very coarse- to coarse-grained sandstone constitutes the alluvial fan-braided fluvial association. Sandstones of the facies association two bears mud drapes and herringbone cross-stratification and thus indicates tidal influence. These shallow-marine sandstones bear wave ripples, ladderback ripples, and adhesion structures and indicate an intertidal depositional environment. The overlying fluvial facies association is characterized by several fining-upward cycles of conglomerate–coarse- to medium-grained sandstones with large-scale trough cross beds, parallel lamination and penecontemporaneous deformation structures and minor shale, indicating the braided nature of the postulated river system. The WIOG succession indicates an overall shallowing-up trend because of sea level fall. The sedimentary structures and texture of the WIOG fluvial deposit are very similar to recent fluvial deposits and are likely to have formed by similar sediment transportation and depositional processes in a tectonically stable sedimentary basin.

Mantle heterogeneity and crust-mantle interaction in the Singhbhum craton, India: New evidence from 3340 Ma komatiites

The Singhbhum Craton in eastern India preserves the records of Paleoarchean - Mesoarchean oceanic crust dominated by ultramafic-mafic extrusive rocks (greenstones), and sialic rafts composed of tonalite-trondhjemite-granodiorite, together with younger granitoids constituting the composite Singhbhum granite batholith. The greenstone sequences, intruded by younger granitoids are entrained as synformal keels in the batholith. The lower greenstone sequence of eastern Iron Ore Group (IOG) is composed of peridotitic komatiite, and komatiitic basalt with minor chert and BIF, whereas the upper sequence has tholeiite to tholeiitic andesite. Here we report results from first direct dating of the Karanjharan spinifex-textured komatiite from the lower part of Badampahar greenstone sequence of IOG. LA-ICPMS U-Pb analysis of zircon grains in the komatiites yielded consistent weighted mean 207Pb/206Pb age of 3340 Ma, with closely similar Hf depleted mantle age (TDM) between 3357 Ma and 3432 Ma. The high MgO (25–35%), and low Al2O3/TiO2 (<10) and CaO/Al2O3 (2.3–3.7) ratios in the serpentinized Karanjharan komatiites suggest strong similarity with the Al-depleted komatiites from Barberton Mountainland, South Africa. The Badampahar komatiites have Gd/YbN in the range 2–3, and show depletion in incompatible elements including LREE, slight enrichment of MREE and near chondritic HREE, suggesting derivation from a depleted mantle source with garnet in the residue. Juvenile Paleoarchean mantle source is also suggested by positive εHf(t) (3.0–4.6) and broadly similar 207Pb/206Pb age and TDM age. Geochemical features of basalts from the upper greenstone sequence overlap with those of the lower greenstone sequence, but are more enriched in HFSE (Th, U) and LREE possibly due to contamination with continental crust or oceanic sediments. The variation in Al2O3/TiO2 ratios as well as Gd/YbN ratios from the Karanjharan komatiite and other coeval komatiites in the eastern IOG suggest Paleoarchean mantle heterogeneity and/or melt extraction from different depths.

Magmatic and Tectonic Setting of Archean Granitoids in the Southeastern Singhbhum Craton, India: Developing constraints with major and trace element geochemistry and geochronology

The Singhbhum craton is one of five Archean nuclei comprising Peninsular India. It is a composite Archean block that includes the Older Metamorphic Group, the Older Metamorphic Tonalite Gneisses, the Singhbhum Granite, and the Iron Ore Group as its major units. The ages of these components range from ~3.5 to ~3.1 Ga, although overlapping ages and similar rock types confound their genetic relationships. Plutonic felsic rocks from the southeastern Singhbhum craton (BK1: a foliated tonalite, KP1: a non-foliated granite, and SG14: a non-foliated granite) yield U-Pb (zircon) ages of 3321 ± 2 Ma (BK1), 3301 ± 1 Ma (KP1), and 3261 ± 1 Ma (SG14) that coincide with a pulse of Singhbhum Granite emplacement at 3.27 to 3.33 Ga. REE patterns and tectonic discrimination diagrams based on major and trace element ratios suggest a subduction zone setting for these rocks. We report major and trace element data for and compare them to previous works in order to characterize the Archean felsic plutonic history of the craton.

Enigma of Dangoaposi and Jagannathpur Lavas of Singhbhum Craton, Eastern India: Possible Solution

Abstract In the Precambrian Singhbhum stratigraphy, some enigmatic aspects exist as regards to Dangoaposi and Jagannathpur lavas. The existing enigma embodies locational aspects of these lavas, structural characteristics, stratigraphic position, and geochemical affinities. Based on recent field studies by the authors and knowledge acquired from existing literature, a possible solution has been proposed in the present study, which may be followed by Precambrian geologists in order to avoid all existing confusion and dilemma. The present study clearly indicates that in Jagannathpur, there is the presence of dyke rocks only (and there is no lava) and as such stratigraphic position of Jagannathpur lava in Singhbhum Geology should be discontinued. Dangoaposi lava (described as Jagannathpur lava by some previous workers) explicitly represents a portion of western Iron Ore Group (IOG) lava and therefore, may be considered to represent a sub-domain within western IOG.

Mafic volcanic rocks of western Iron Ore Group, Singhbhum Craton, eastern India: Geochemical evidence for ocean–continent convergence

Precambrian mafic magmatism is an important global episode which played a significant role in the crustal evolution. In India, Singhbhum Craton being the oldest craton, witnessed significant occurrences of Precambrian geological activity, marked by several episodes of volcanism, plutonism, sedimentation spanning from Palaeoarchean to Mesoproterozoic age. Here we present petrological and geochemical characteristics of Precambrian mafic volcanic rocks (occurring in western Iron Ore Group (IOG), Singhbhum Craton, eastern India) to evaluate their petrogenetic aspects, tectonic setting, and magma generation. The mafic volcanic rocks are porphyritic in nature with the phenocrysts of plagioclase and groundmass composed of clinopyroxene, plagioclase, ilmenite, and volcanic glass. These rocks are mostly tholeiitic, sometimes with a transitional behaviour towards calc‐alkaline nature and display basalt‐basaltic andesite affinity. These mafic volcanic rocks also preserve geochemical signatures (high Nb/U, Nb/La, [Nb/Th]pm ratios) in support of Nb‐enriched basalts and are classified as Nb‐enriched basalts (NEB; Nb &gt; 7 ppm) and high‐Nb basalts (HNB; Nb &gt; 20 ppm) on the basis of Nb concentrations and mantle normalized Nb/La ratios (&gt;0.5). The NEBs and HNBs are marked by lesser magnitude of negative Nb anomalies with high (Nb/Th)pm, (Nb/La)pm, and Nb/U ratios as compared to normal arc basalts. Several major element oxides, trace elements, and selected element ratios (like SiO2, CaO/Al2O3, Y, V/Cr, Zr/Nb, and ∑REE) show systematic variations with MgO which suggests role of magmatic fractionation. Chondrite‐normalized REE patterns for NEB and HNB rocks exhibit uniform LREE enrichment with distinct Eu negative anomalies while primitive mantle‐normalized incompatible trace element patterns reflect enrichment in LILE and LREE with prominent Nb‐Ta anomalies. Different HFSE ratios corroborate a subduction related setting for magma generation formed by ∼10%–20% melting in the domain of garnet lherzolite. Relative enrichment of LILE and LREE with depleted HFSE characteristics attest a garnet‐bearing mantle source and melt extraction with garnet in the residue. Geochemical signatures suggest that the genesis of NEB and HNB is attributable to slab‐melting and wedge hybridization processes during matured stages of subduction. Selected incompatible trace element ratios for the studied mafic volcanic rocks invoke an enriched (EM1‐EM2 type) mantle source and unequivocally suggest effects of continental crustal assimilation of the parent magma. Liquid immiscibility has played an important role as a differentiation mechanism leading to presence of high and low FeO types. The IOG mafic volcanic rocks preserve distinct geochemical signatures of matured stage of subduction, slab melting, crustal contamination and magma generation at an Archean ocean–continent convergent margin setting.