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The Archaean granitoid pluton of the Singhbhum craton in E India is overlain by Archaean to Palaeoproterozoic metasediments. These sediments are still poorly known and their stratigraphy is under debate. Several scattered, most probably Meso- to Neoarchaean, conglomerates are present in the state of Jharkhand that differ so much in characteristics that they are probably not related to each other. The sedimentology of a series of conglomerate patches and layers near Bhurkuli has been investigated, including the characteristics of the clasts. It is deduced on the basis of these characteristics and the sedimentological context that the Bhurkuli conglomerates represent the channel facies of a river system that differed from the types of fluvial systems that exist nowadays. [ABSTRACT FROM AUTHOR]

It is widely believed that atmospheric oxygen saturation rose from <10-5 present atmospheric level (PAL) in the Archean to >10-2 PAL at the Great Oxidation Event (GOE) at ca. 2.4 Ga, but it is unclear if any earlier oxygenation events occurred. Here we report U-Pb zircon data indicating that a pyrophyllite-bearing paleosol, from Keonjhar in the Precambrian Singhbhum Craton of eastern India, formed between 3.29 and 3.02 Ga, making it one of very few known Archean paleosols globally. Field and geochemical evidence suggests that the upper part of the paleosol was eroded prior to unconformable deposition of an overlying sequence of shallow-marine siliciclastic sediments. A negative cerium anomaly within the currently preserved level of the paleosol indicates that ancient oxidative weathering occurred in the original upper soil profile. The presence of redox-sensitive detrital uraninite and pyrite together with a complete absence of pyrophyllite in the overlying sediments indicate that the mineralogical and geochemical features of the paleosol were established prior to the unconformable deposition of the sediments and are not related to subsequent diagenetic or hydrothermal effects. We suggest that a transient atmospheric oxygenation event occurred at least 600 m.y. prior to the GOE and ~60 m.y. prior to a previously documented Archean oxygenation event. We propose that several pulsed and short-lived oxygenation events are likely to have occurred prior to the GOE, and that these changes to atmospheric composition arose due to the presence of organisms capable of oxygenic photosynthesis. [ABSTRACT FROM AUTHOR]

Reappraisal of field relationships between the different lithological ensembles supported by available geochronological data, and taking due note of the tectono-metamorphic, magmatic and sedimentation history helped to build up a coherent crustal evolutionary history of the Singhbhum Archaean craton, eastern India. The evolution of the earliest sialic crust, as the isotope ages suggest, was around 3700 Ma or even earlier. Deposition of the oldest, dominantly metasedimentary supracrustals, the Older Metamorphic Group (OMG), was initiated at around 3380 Ma, i.e. after a gap of about 320 million years. The closing of OMG basins synchronously with the emplacement of a granitoid phase was at ca.3285 Ma. No other fabric-forming ductile deformation and metamorphism associated with the development of foliation and mineral lineation is known in the rocks of the Singhbhum Archaean craton subsequent to this event. Formation of the succeeding geological ensembles including the deposition of BIF-bearing Iron Ore Group (IOG) and the emplacement of the post-IOG granitoids at ca.3100 Ma can be described as 'lsnon-orogenic' event taking place during the phase of tectonic quiescence. Supracrustals like the Dhanjori and Simlipal mafic volcanics with intercalated beds of arenite evolved later during the phase of Plume outburst at around 2800 Ma. The end-Archaean intrusion of Newer Dolerite dykes in conjugate sets and the deposition of Kolhan Group in an N-S oriented basin during an E-W stress system mark the culmination of the Archaean crust-building activity in the Singhbhum Archaean craton. [ABSTRACT FROM AUTHOR]

Two-dimensional geoelectric section derived along the Malkapur-Mandhata magnetotelluric (MT) profile, cutting across the Narmada-Son lineament (NSL) zone, brought out a conductive (< 50 Ohm m) middle and lower crust under the NSL region. This region underlying the resistive (300-3000 Ohm m) Archaean basement shows shallow depth underneath the weak tectonic zones identified in the area. The constrained density modelling of the Bouguer gravity values suggests high density (2.8 g/cm3) material corresponding to the conductive middle-lower crust zone. As the region has experienced widespread volcanism during the Cretaceous-Tertiary period, the crystallized mafic magmas that were poured into the crust during the volcanism and the fluids entrapped (expelled during magma cooling process) could be the possible explanation for the conductive and high-density characteristics beneath the NSL zone. [ABSTRACT FROM AUTHOR]

Abstract: Late Neoproterozoic to Cambrian ages have been widely recorded from the Southern Granulite Terrain (SGT) of India and were correlated with global tectono-thermal (Pan-African) events associated with amalgamation of the Gondwana supercontinent. We report results of single zircon dating from a charnockite and a granitic augen–gneiss from the SGT using SHRIMP and evaporation techniques, complemented by zircon Hf isotopic systematics. The SHRIMP data for zircons in the charnockite are spread along a well-defined discordia line suggesting variably strong lead–loss during the late Neoproterozoic Pan-African event. A concordant grain yielded a 207Pb/206Pb age of 1893±13Ma, and the upper intercept of the discordia line for all analyses with Concordia is at 1881±14Ma. The concordant grain age is considered to most closely reflect the time of emplacement of the charnockite protolith. The imprecise lower Concordia intercept at 572±25Ma is likely to reflect Pb-loss during granulite-facies metamorphism. Evaporation of four zircons yielded consistent 207Pb/206Pb isotopic ratios at high evaporation temperatures of ca. 1598°C, suggesting an age of ca. 1450Ma. This date is clearly much younger than the SHRIMP result and suggests that the evaporated grains no longer contained their original, magmatic 207Pb/206Pb ratios but record strong Pb-loss as also shown by the SHRIMP data. Lu–Hf isotopic analysis of ten spots on zircon grains previously analyzed by SHRIMP II yielded strongly negative εHf(t)-values of −6.1 to −9.2 and mean Hf crustal residence ages of 2.95 to 3.14Ga. Eight grains from the augen–gneiss analyzed on SHRIMP II also show variable but considerable Pb-loss. A concordant grain yielded an age of 1557±16Ma, and the other grains define a discordia line with an upper Concordia intercept at 1553±21Ma. Two unzoned grains produced a combined mean 206Pb/238U age of 583±6Ma and reflect the high-grade late Neoproterozoic metamorphic event. A concordant 207Pb/206Pb age of 2043±13Ma was obtained for a xenocryst. Evaporation of five large zircons from this sample yielded a precise mean 207Pb/206Pb age of 1556.0±0.5Ma, identical to the result for the SHRIMP-dated concordant grain. Lu–Hf isotopic analysis of ten spots on zircon grains from where SHRIMP data were gathered yielded strongly negative εHf(t)-values of −6.0 to −17.0 and mean Hf crustal residence ages of 2.68 to 3.37Ga. Our results suggest that not all rocks in the so-called Pan-African domain of southern India are Neoproterozoic in age, and that there could be more Mesoproterozoic to Archaean crust hidden in the “sea” of Neoproterozoic granulites, granitoids and migmatites than so far recognized. The Hf isotopic data reveal both granitoid samples to be derived from crustal melts generated from Archaean continental crust, and the chemistry of these rocks is inherited from their ancient source. We compare the zircon ages with those of similar rocks in the Highland Complex of Sri Lanka and discuss their relevance as vestiges of an older continental terrain incorporated within the late Neoproterozoic–Cambrian collision belt of East Gondwana. In view of rock and age similarities we tentatively suggest links between the Indian Trivandrum and Madurai Blocks and the Sri Lankan Highland and Wanni Complexes. [Copyright &y& Elsevier]

Abstract: Geochemical and Nd isotope data on granitoids of the NW part of the late Archaean eastern Dharwar craton are presented to elucidate their petrogenesis and role in crust formation. The granitoids are divided into three suites viz. trondhjemite–granodiorite gneisses, biotite monzogranites and porphyritic biotite granodiorites. The gneisses are pre- to syn-kinematic (with respect to deformation in the adjacent Hungund–Kushtagi schist belt), which show variable SiO2 and Al2O3, enriched LREE and depleted HREE with slightly negative to no Eu anomalies. They display unusual chemistry in having higher FeO(T), K2O, Ba, Cr and Ni compared to the typical Archaean tonalite–trondhjemite–granodiorite (TTG). The biotite monzogranites are mostly syn- to late-kinematic and exhibit evolved calc-alkaline compositions with high SiO2, K2O, LILE and LREE, depleted to undepleted HREE and strongly negative to no Eu anomalies. The porphyritic granodiorites show syn- to late-kinematic calc-alkaline, sanukitoid-like character with a wide range of SiO2, higher TiO2, P2O5, Sr, Ba, Cr and Ni, and lower Rb. They, however, uniquely display higher K2O, ΣREE and Th than typical sanukitoids. The trondhjemite–granodiorite gneisses are interpreted as product of melting of a subducted basaltic slab followed by slight contamination from the overlying metasomatized mantle wedge. Subsequent melting of the extremely metasomatized mantle wedge resulted in formation of the parental magma of the porphyritic granodiorites. Intrusion of the latter triggered melting of the TTG crust accreted earlier and generated the evolved monzogranites. The monzogranites occurring east of the Hungund–Kushtagi schist belt show higher εNd but lower T DM ages than those occurring to the west, indicating that terranes with different histories were juxtaposed by lateral accretion. [Copyright &y& Elsevier]

Abstract: The ca. 2.9Ga old Sittampundi layered complex (SLC) of South India belongs to a special group of deformed and metamorphosed Archaean rocks where chromitites are interlayered with anorthosite. Detailed field studies have revealed that the SLC comprises of a sequence of magmatic rocks consisting of clinopyroxenite, anorthosite and mafic rocks of tholeiitic composition. Minor BIF (Banded Iron Formations) intercalated with the mafic rocks are the only sedimentary rock within the SLC. Chromitite occurs as continuous to discontinuous bands (a few mm up to 6m thick) within clinopyroxenite and highly calcic anorthosite (An99–100) but never in the enclosing mafic rocks. Chromite occurs broadly in three textural types: (a) cumulus grains in chromitite layers; (b) disseminated grains in clinopyroxenite and anorthosite; and (c) small grains included in clinopyroxene and plagioclase. A suite of felsic magma intruded the SLC at ca. 2.51Ga. Subsequently the entire rock sequence underwent polyphase deformation and metamorphism at ca. 2.50–2.45Ga (granulite facies) and at 0.72–0.50Ga (amphibolite facies). During the latter event, extensive alteration of clinopyroxene to amphibole and chlorite, of plagioclase to clinozoisite and of chromite to Cr-poor green spinel took place. Chromite compositions show large variations (Cr#=26–60, Mg#=4–45, Fe3+#=2–24) depending upon textural type and nature of the associated phases. The most conspicuous compositional variation took place when chromite was metasomatically replaced by green spinel (Cr#=3–21; Mg# 45–61) during amphibolite facies metamorphism. On a Cr# vs. Mg# plot, pristine magmatic compositions of chromite define an array that suggest increasing Cr# with concomitant decrease in Mg#, presumably during magmatic fractionation. Highly calcic plagioclase in anorthosite and Fe–Al rich chromite in chromitite in the SLC are consistent with a hydrous parental magma of tholeiitic composition. This hydrous magma was generated in an oceanic arc environment and crystallized close to the surface of the earth. In terms of mineralogy and field relations, chromitite-bearing rocks of the SLC show remarkable similarity with the Archaean anorthosite-hosted chromitite deposits in layered magmatic complexes at Fiskenaesset (Greenland) and Messina (South Africa) but contrasts sharply with the chromitite deposits of Proterozoic and Phanerozoic age. A change in the style of arc magmatism appears to be the controlling factor for marked change in chromite compositions across the Archaean–Proterozoic boundary. [Copyright &y& Elsevier]

Abstract: The Sandmata Complex in Rajasthan within the NW Indian shield represents a metamorphosed volcano-sedimentary sequence incorporated within a late Paleoproterozoic suture zone. Here we report laser ablation ICP-MS age data and REE geochemistry of zircons from four rock types in the Sandmata Complex – a foliated biotite–quartz gneiss, a leucocratic gneiss, a biotite gneiss and a porphyritic granite. The zircons in the biotite gneiss and porphyritic granite show magmatic textures with a single Paleoproterozoic age population, whereas the leucocratic gneiss and biotite–quartz gneiss samples carry zircons with an Archean provenance history and metamorphic characteristics. U–Pb dating of zircon domains in these samples shows two dominant age populations: Paleoproterozoic and Neoarchean, and indicate that (a) granulite facies metamorphism (biotite–quartz gneiss: ca. 1708Ma) and the granitic magmatism (porphyritic granite: ca. 1733Ma) were synchronous; and (b) the igneous protoliths of the leucocratic gneiss (ca. 2750Ma) and biotite gneiss (ca. 2698Ma) were formed in the Archean. The zircons from both gniesses and granite show high REE contents, prominent HREE enrichment and a conspicuous negative Eu anomaly suggesting a common melt source. We correlate the Paleoproterozoic magmatic ages from Sandmata Complex with a subduction–accretion history prior to the final collisional assembly of the continental fragments in the Neoproterozoic, comparable with a similar history all along the Central Indian Tectonic Zone, as well as from the Eastern Indian Suture, probably indicating the continuity of the suture all along Peninsular India. Our zircon data also reveal the vestiges of Archean rocks within the Paleoproterozoic suture. [Copyright &y& Elsevier]

Abstract: Well-preserved alkaline basalts, bearing relict aegirine, leucite and nepheline mineralogy, are stratigraphically associated with high-Mg basalts in the Neoarchean Penakacherla greenstone belt, eastern Dharwar craton, India. Alkaline basalts (Mg #∼0.70–0.58) are enriched in alkalies (K2O+Na2O∼7wt.%), and TiO2 (2.3–2.1wt.%), and exhibit fractionated REE patterns with (La/Yb)N ranging from 23 to 29. On primitive mantle normalized diagrams they record a downturn from Ce to Th, small negative Nb anomalies relative to La, and Zr/Hf ratios higher than the primitive mantle value, in common with compositional characteristics of Phanerozoic alkaline ocean island basalts (OIB). Some interelement ratios are intermediate between EM1- and HIMU–OIB. Associated high-Mg basalts (MgO 17.2–9.2wt.%) have comparatively lower TiO2 (1.2–0.50wt.%), and flat HREE patterns with slight depletion in LREE, and small positive Nb anomalies. These basalts are compositionally similar to tholeiitic basalts associated with komatiites in many Neoarchean greenstone terranes. Two samples have the conjunction of Nb/Th<8 and fractionated LREE [(La/Yb)N 2.60–2.63] consistent with crustal contamination. On the global array of Phanerozoic to Recent ocean island basalts, in SiO2 versus Nb/Y coordinates, alkaline basalts plot with counterparts from Aitutaki and Heard, whereas high-Mg basalts plot near Iceland tholeiites. Alkaline basalts plot with OIB, and high-Mg basalts near N-MORB, on the Th/Yb versus Nb/Yb MORB-OIB array of Phanerozoic intraplate basalts; accordingly the mantle components of that array were established in the 2.7Ga mantle asthenosphere. Alkaline basalts are rare in Archean volcanic sequences. This occurrence of alkaline basalts indicates subduction, recycling, and incubation of Mesoarchaean oceanic and continental crust in the mantle, and generation of high-Mg and alkaline basalts from a mantle plume at 2.7Ga, possibly analogous to counterparts of Iceland. The mantle plume likely erupted at a thin craton margin, given flat HREE of most high-Mg basalts. [Copyright &y& Elsevier]

Abstract: In situ U–Pb–Hf isotope analyses of detrital zircons from the Alwar quartzite, the oldest member of the North Delhi fold belt in the Aravalli mountain range, indicate that the basement of NW India shares a common Archaean-Palaeoproterozoic evolutionary history, and is a consequence of several crust-forming and crust-reworking events between 3.70 and 1.71 Ga. This is suggested by a zircon age spectrum with most ages at 1.87-1.71 Ga (75%), minor age peaks at 2.5-2.2 and 2.9-2.7 Ga, and Palaeoarchaean ages of 3.27-3.23 Ga. Lu–Hf isotope data reveal that the oldest crust, either in the Aravalli mountain range or in its hinterland, was formed from a depleted mantle source at around 3.7 Ga. Furthermore, the data also indicate that the hinterland of the Alwar sediments was affected by several periods of new “juvenile” crust formation at 3.25, 2.89, 2.67, 2.51, and 1.87-1.80 Ga, while crustal reworking was predominant at 2.62-2.45, 2.2 and 1.87-1.77 Ga. Zircons with xenocrystic cores and metamorphic overgrowths, additionally suggest that the source region was affected by high-grade (?) metamorphic events at 2.5 and 2.2 Ga. Based on our new data, in conjunction with field relationships and existing zircon ages in the Archaean basement of NW India, it can be concluded that the Palaeo- to Neoarchaean detritus (3.3-2.5 Ga) in the Alwar quartzite stems from proximal sources. The predominant late Palaeoproterozoic zircon detritus (1.87-1.71 Ga) was supplied from a nearby basement, which was affected by subduction-accretion tectonics at around 1.85 Ga, and by a first, perhaps post-collisional rift-related phase at 1.77 Ga. The latter event is unrecognized so far, and predates a second phase of rift-related A-type magmatism at 1.72-1.70 Ga. The youngest detrital zircon age of ca. 1.71 Ga suggests that the sedimentation and metamorphism in the North Delhi fold belt postdate the second phase of rift-related A-type magmatic activity. [Copyright &y& Elsevier]

In quartz-free Fe, Na-poor and high-Mg, Al schists from the Bundelkhand craton in north central India, corundum porphyroblasts in finely interleaved phlogopite-chlorite aggregates with rare clinozoisite are mantled by fine-grained (<3 μm) intergrowths of outer collars (>200 μm wide) of phengitic-muscovite and chlorite (phlogopite + corundum + HO → phengitic-muscovite + chlorite), and <100 μm wide inner collars of margarite-muscovite (corundum + clinozoisite + phengitic muscovite → margarite + muscovite + HO). Wide-beam electron probe microanalyses indicate Mg in the bi-layered corona increases from corundum outwards, with a complementary decrease in Al and K. Si and Ca increase and then decrease to matrix values. The sharp chemical gradients across the highly structured bi-layered corona are inferred to suggest that the corona-forming reactions were promoted by local grain-boundary-controlled thermodynamic instability as opposed to element transport by advective diffusion. The P-T convergence of KMASH reactions and NCKMASH pseudosection phase relations computed using micro-domain compositions indicate the chlorite-phengitic muscovite outer collar formed at 18-20 kbar and ca. 630°C. The NCKMASH margarite-muscovite inner collar yielded lower metamorphic P-T conditions of 11 ± 3 kbar, ca. 630°C. U-Th-Pb chemical dating of metamorphic monazite and LA-ICPMS U-Pb isotope dating of re-equilibrated zircon yield ca. 2.78 Ga ages, which are interpreted to date corona formation and Neoarchean high-P metamorphism in the Bundelkhand craton, hitherto unknown in the Indian Precambrian. (220) [ABSTRACT FROM AUTHOR]

Abstract: The field setting, mineralogical, geochemical, genetic and depositional features of Late Archean metasedimentary manganese, as well as Cenozoic supergene manganese and ferromanganese ores, along the ~75km long linear tract of the Anmod–Bisgod region in the Western Dharwar Craton, southern India, are described. The 2700Ma metasedimentary manganiferous formation is underlain by stromatolitic limestone/dolomite and quartzite/arenite and also occurs within the former. The older quartzite/arenite, which contains redox-sensitive detrital pyrite and uraninite as well as thucolite, indicates sedimentation under anoxic to oxic transitional palaeoenvironmental condition, whereas the younger stromatolitic limestone/dolomite indicates development of oxygen in the depositional environment. The manganese ore types have been classified into three categories, one of which is a Mn-rich duricrust once capping the Late Archean manganiferous formation. It formed during a Cenozoic lateritic weathering event and was eroded in response to crustal uplift. Perhaps uniquely, clastic deposits of the eroded Mn-duricrust occur in mineable quantities in the form of fluvial palaeochannel fills. These cryptomelane-rich supergene ores are also suitable for radiometric dating. Knowing their age would help to constrain the timing of Mn-duricrust formation and, indirectly, the regional weathering event responsible for the extensive laterite occurrences of southern India. [Copyright &y& Elsevier]

The study incorporates a documentary evidence of vertical/subvertical shear zones in the central part of the undelkhand massif, central India. The shear zones are aligned E-W and are characterized by the occurrence of ylonites that show vertical/subvertical foliation. The shear zones involve a variety of metamorphic and metavolcanic lock types. These shear zones appear to be the oldest and pre-tectonic in the context of early Proterozoic undelkhand granitoids. It is possible that the thin nature of the Archaean crust was mainly responsible for the formation of these vertical shear zones. [ABSTRACT FROM AUTHOR]

Abstract: The Hutti greenstone belt, hosting major gold mines, is made up of tholeiitic basalts, felsic volcanic rocks and sediments metamorphosed to amphibolite facies and surrounded by granitoid rocks, predominantly of granodiorite in composition. The granitoid rocks show intrusive contact relationship at places along the eastern and northern margins of the belt while such relationships could not be observed along the western margin as it is covered by soil. The tholeiitic basalts have flat to moderately LREE-enriched chondrite normalized REE patterns and show depletion in Nb relative to MORB. The Hutti metabasalts define a collinear array in the Rb–Sr isotope evolution diagram with a slope corresponding to an age of 2706±130Ma. The higher uncertainty is due to scatter caused by mobility of Rb and Sr during metamorphism and low temperature hydrothermal alteration. In the 206Pb/204Pb vs. 207Pb/204Pb diagram the metabasalts define a collinear array corresponding to an age of 2637±150Ma with significant scatter indicating that Pb was also mobilized by the post-crystallization alteration processes. The Sm–Nd isotope system in the Hutti metabasalts has been least disturbed giving an isochron age of 2662±81Ma which is suggested as the age of formation of their precursors. Combined with the published U–Pb SHRIMP ages on zircons from the felsic volcanic rocks it is inferred that the Hutti greenstone belt formed over a period of ⩾86Ma. The major greenstone belts of the eastern Dharwar craton have evolved coevally at ca. 2700Ma ago and are older than the surrounding granitoid rocks. The metabasalts have ε Nd values of +1.4 to +3.0 and ε Sr values of −23.4 to +2.0 and plot close to the mantle array. They were derived from long-term LREE depleted mantle source. The felsic volcanic rocks have ε Nd values of −1.3 and −1.4 and ε Sr values of −30.6 and +89.3, respectively. Their magmas were derived from older continental crustal sources. [Copyright &y& Elsevier]

Abstract: We report on an inferred Paleoproterozoic soil horizon from the Singhbhum-Orissa craton near Keonjhargarh, Orissa, eastern India, which preserves vertic (Vertisol-like) features. The ∼2–10m thick, white shale consists of quartz–muscovite–pyrophyllite interpreted as a paleosol, overlies ∼3.32Ga granite and is covered by pre-2.0Ga sandstones (Kolhan Group) deposited in marginal marine environments. The paleosol passes downwards through a partly altered granite (saprolith) zone to fresh granite. Abundant pedogenic slickensides, ped structures, gilgai topography, reddish stain on ped surfaces, and sepic-plasmic microfabric in the paleosol were probably related to shrink and swell processes driven by marked seasonal precipitation in a well-drained landscape. An observed decrease of alkali elements and an increase of silica, alumina and CIA values from granite to paleosol, together with relatively constant Ti/Al ratios suggest in situ weathering of granite protolith. However, a paleo-Vertisol necessitates a mafic parent rock, either directly (igneous) or reworked through alluvium. The c. 2.25Ga Malangtoli-Jagannathpur basaltic rocks are relatively widespread and close to the inferred paleosol occurrences, and provide a logical solution. Geochemistry of the highly depleted paleosol, in contrast, particularly when applying the isochon method, strongly supports a granitic parent. The apparent conundrum can possibly be solved through repeated alluvial reworking of both granitic and mafic detritus and accumulation of weathered material with varying proportions of these two components, yet enough smectitic clays (∼30%) to form a paleo-Vertisol. This pre-2.0Ga weathering event in the Singhbhum craton is consistent with a global scenario of Paleoproterozoic weathering which followed an extended global glaciation, and thus has the potential to contribute to understanding of pre-2.0Ga amalgamation of regional and global cratonic blocks. [Copyright &y& Elsevier]

Abstract: Late Archaean metagreywackes of the Ranibennur Formation, Dharwar Supergroup, in the Dharwar–Shimoga schist belt of the Western Dharwar Craton (WDC) are texturally and mineralogically immature of the quartz-intermediate type. The SiO2 content in them ranges from 60.58 to 65.26 wt.%. Chemical Index of weathering (CIW) values varies between 50 and 65. 4 indicating a low degree of chemical alteration of the provenance rocks. A high degree of correlation between K2O and Al2O3 (r =−0.73) and low Rb/Sr ratios also suggest a low degree of alteration of provenance rocks. Abundances of transition group elements (Cr=118–221; N=89–154; V=89–192 and Sc=11–16 ppm) as well Zr (132–191 ppm) suggest a mixed mafic–felsic provenance for the metagreywackes. Low HREE and Y content, and low Tb/Yb ratios (0.23–0.41) suggest the presence of tonalite as an important component in the provenance areas. Values of Eu/Eu⁎(0.78) and Th/Sc (0.55) suggest that the granodioritic upper crust had evolved prior to serving as the provenance. Mixing calculations suggest 50–55 vol.% tonalite, 20–25 vol.% granite, 18–20 vol.% basalt and ~5 vol.% komatiite composition for the provenance. Geochemical characteristics of the Ranibennur metagreywackes suggest that sedimentary basin formed in the vicinity of a magmatic arc in a continental island arc setting, and the detritus were shed from the arc rock. [Copyright &y& Elsevier]

High-grade hematite ores of the Iron Ore Group in the Noamundi area, Jharkhand state, India, are hosted by a laterally extensive, 220-m-thick banded iron formation (BIF) in a folded greenstone belt succession of Paleoarchean age. Single orebodies, which are up to 3 km long along strike and several hundred meters wide, depending on dip of the beds, are strata bound and composed of two major ore types--namely, hard hematite ore that is of ancient geologic origin and ores related to recent weathering along a lateritized Cretaceous- Cenozoic land surface. The ancient hard hematite orebodies comprise laminated hematite ores in which microplaty hematite is dominant, and massive ores composed almost entirely of martite. Supergene ores, in contrast, are comprised of goethite-rich duricrust and soft saprolitic hematite ores representing the leached zone of the Cretaceous-Cenozoic laterite profile. Hard hematite-martite ores formed by hydrothermal replacement of BIF protolith, not only through leaching of silica but possibly 'also through introduction of iron by hydrothermal fluids of meteoric origin. Hematite iron ore pebbles in a conglomerate at the base of the Meso- to Neoproterozoic Kolhan Group near Noamundi attest to the antiquity of the hard hematite orebodies. Soft saprolitic iron ores formed as a product of lateritic weathering processes in Cretaceous-Cenozoic times. Supergene alteration took place under the influence of reducing and acidic meteoric water, which is typical for lateritic soil profiles developed under humid tropical climatic conditions with lush plant cover. Goethitic duricrusts capping the laterite profile suggest alternating wet and dry seasons in this environment. [ABSTRACT FROM AUTHOR]

Banded iron-formations (BIF) form an important part of the Archean supracrustal belts of the Jharkhand-Orissa region, India. Major, trace and REE chemistry of the banded iron-formation of the Gandhamardan, Deo Nala, Gorumahisani and Noamundi sections of the Jharkhand-Orissa region are utilized to explore the source of metals and to address the thermal regime of the basin floor and the redox conditions of the archean sea. Hydrothermal fluids of variable temperatures might have contributed the major part of the Fe and other trace elements to the studied banded iron-formations. Diagenetic fluids from the sea floor sediments and river water might have played a subdued role in supplying the Fe and other elements for the banded iron-formations. [ABSTRACT FROM AUTHOR]

The importance of study of heavy metal distribution in river sediments is a component in understanding the exogenic cycling as well as in assessing the effect of anthropogenic influences of the elements. In India, the river Subernarekha flows over Precambrian terrain of Singhbhum craton in the eastern India. The geological succession in this part of India is through (1) iron ore series, (2) ultrabasic igneous rocks, (3) diorite, (4) granite, (5) newer dolerites, (6) newer tertiary and (7) alluvium. The first four groups belong to the Archaean era, and the representative is the iron ore series consisting of iron ore, manganese and chromite which are abundantly present. The primary rock types are schist and quartzite layers. One main tributary, the Kharkhai flow through granite and schist and quartzite layers. Two important creeks are Gurma and Garra, respectively. The former after originating in basic igneous area travels through schist quartzite, while the later one originates in granite area where some functional but old gold mines are located. Freshly deposited sediments of river were collected upstream and downstream the industrial zone (East Singhbhum district). Samples were collected from four locations and analysed in <63 μm sediment fraction for heavy metals like Ni, Co and Cr by adsorption stripping voltammetry on hanging mercury drop electrode and Hg by anodic stripping voltammetry using polished rotating gold disk electrode. Enrichment of these metals over and above the local natural concentration level (NCL) has been calculated and applied to determine metal-pollution index (MPI) proposed by Goncalves et al. and also geo-accumulation index ( I geo) by Muller. Based on Muller’s classification, Ni, Cr and Hg have been classified from unpolluted to moderately polluted range in pre-monsoon period but for metals Ni and Cr, during the post-monsoon period the values have reached moderately polluted level while Hg has been classified under unpolluted to moderately polluted level except at the monitoring station situated upstream to Gurma Creek where it was found at unpolluted level. Presence of natural resources of the minerals is primarily the reason for their detection in river sediments (lithogenic) but some anthropogenic sources are also contributing for their presence at some sampling stations. Hg is considered to be chalcophilic in nature and is detected wherever known sulphidic ores of copper or nickel are present. [ABSTRACT FROM AUTHOR]

Silica in bedded cherts interstratified with manganese carbonates in a deep-water carbonate ramp succession of the Neoproterozoic Penganga Group, India, displays film-like, spherical to rod-shaped, and tubular branching microstructures. The microstructures resemble mineralized extracellular polysaccharides, biofilms and bacterial morphologies. The microstructures suggest silicification by nucleation of silica on organic-templates or indirectly as sorbed species accumulating on organic templates. Given that similar microstructures have also been documented in Archean cherts it is proposed that organotemplates might have been an important sink for the deposition of silica in Precambrian deep-water marine environments.Terra Nova, 16, 338–343, 2004 [ABSTRACT FROM AUTHOR]

The Ajjanahalli gold mine is spatially associated with a Late Archean craton-scale shear zone in the eastern Chitradurga greenstone belt of the Dharwar craton, India. Gold mineralization is hosted by an ∼100-m-wide antiform in a banded iron formation. Original magnetite and siderite are replaced by a peak metamorphic alteration assemblage of chlorite, stilpnomelane, minnesotaite, sericite, ankerite, arsenopyrite, pyrite, pyrrhotite, and gold at ca. 300° to 350°C. Elements enriched in the banded iron formation include Ca, Mg, C, S, Au, As, Bi, Cu, Sb, Zn, Pb, Se, Ag, and Te, whereas in the wall rocks As, Cu, Zn, Bi, Ag, and Au are only slightly enriched. Strontium correlates with CaO, MgO, CO2, and As, which indicates cogenetic formation of arsenopyrite and Mg-Ca carbonates. The greater extent of alteration in the Fe-rich banded iron formation layers than in the wall rock reflects the greater reactivity of the banded iron formation layers. The ore fluids, as interpreted from their isotopic composition (δ18O = 6.5-8.5‰; initial 87Sr/86Sr = 0.7068-0.7078), formed by metamorphic devolatilization of deeper levels of the Chitradurga greenstone belt. Arsenopyrite, chalcopyrite, and pyrrhotite have δ34S values within a narrow range between 2.1 and 2.7 per mil, consistent with a sulfur source in Chitradurga greenstone belt lithologies. Based on spatial and temporal relationships between mineralization, local structure development, and sinistral strike-slip deformation in the shear zone at the eastern contact of the Chitradurga greenstone belt, we suggest that the Ajjanahalli gold mineralization formed by fluid infiltration into a low strain area within the first-order structure. The ore fluids were transported along this shear zone into relatively shallow crustal levels during lateral terrane accretion and a change from thrust to transcurrent tectonics. Based on this model of fluid flow... [ABSTRACT FROM AUTHOR]

The article discusses the dyke map of Indian cratons, which focuses on presenting the status of Geochronology study on the Dykes of IndianCratons. It mentions that India has four four Archean Cratons namely Bundelkhand, Bastar, Dharwar and Singhbhum with a dense network of Archean to Proterozoic dykes cutting across the Archean crust. It also mentions the approach of this work: selecting the most reliable published map of the area and using this as the base map to trace the dykes.