Joydip Mukhopadhyay

Dating the Oldest Greenstone in India: A 3.51-Ga Precise U-Pb SHRIMP Zircon Age for Dacitic Lava of the Southern Iron Ore Group, Singhbhum Craton

This article reports a precise 3506.8 ± 2.3-Ma U-Pb SHRIMP zircon age for dacitic lava in a well-preserved low-grade metamorphic and low-strained greenstone belt succession of the southern Iron Ore Group, Singhbhum craton, India. This age makes the succession the oldest-known greenstone belt succession in India and one of the oldest low-strain greenstone successions in the world after the 3.51-Ga Coonterunah Group of the Pilbara craton, Western Australia, and the moderately deformed 3.54-Ga Theespruit Formation of the Barberton Greenstone Belt, Kaapvaal craton, South Africa. The geochemical composition of the dacitic lava and related volcanic rocks suggests that they formed in a volcanic arc setting. The succession also contains a major ∼120-m-thick oxide facies banded iron formation that distinguishes it from the slightly older successions of the Pilbara and Kaapvaal cratons. This banded iron formation may well be one of the oldest and most well preserved, and together with associated volcanics, it may have immediate implications for understanding >3.5-Ga surface and tectonic processes on Earth.

Evidence for an early Archaean granite from Bastar craton, India

Abstract: Granitoids in the early Archaean are believed to be potassium-poor tonalite–trondhjemite–granodiorite rocks. Only after continental crust attained sufficient thickness did true (relatively potassium-rich) granites form. No record of true granite prior to 3.4 Ga is available. We report a 3.6 Ga true granite from the Archaean Bastar craton in India. In contrast to the typical early Archaean granitoids, which are commonly deformed into gneisses, this granite is relatively undeformed. The age and composition of the granite implies that continental crust of the Bastar craton attained sufficient thickness to permit intracrustal melting at 3.6 Ga. Supplementary material: Representative major element, trace element and REE composition of the Dalli-Rajhara granite samples and a summary of SHRIMP U-Pb zircon data for the granite sample D-9 are available at http://www.geolsoc.org.uk/SUP18337.

The geology and genesis of high-grade hematite iron ore deposits

Abstract This paper presents a first summary of results of an ongoing study, started some two years ago, of high-grade iron ore deposits in South Africa, India and Brazil, including a comparison with the rather well-studied deposits of the Hamersley Province in Australia. Large, high-grade hematite iron ore bodies hosted by Precambrian banded iron formations are the worlds most important source of iron ore. Despite their great economic importance, the origin of these deposits has remained rather enigmatic. Results of preliminary investigations into the geological setting, petrography and geochemistry of BIF-hosted high-grade hematite iron ore deposits in South Africa, India, Western Australia and Brazil reveal distinct similarities, suggest ing a very similar mode of hydrothermal and supergene-modified hydrothermal origin for most BIF-hosted highgrade hematite ore deposits. Differences between the deposits of this type are largely the result of varying intensities of supergene modification in Late Cretaceous to Tertiary times. Notable exceptions are the deposits in the Griqualand West region of South Africa, including the giant Sishen deposit, that are regarded to be of ancient supergene origin.

The Neoproterozoic Cratonic Successions of Peninsular India

Abstract The Peninsular India hosts extensive record of Mesoproterozoic, and Neoproterozoic successions in several mobile belts, and cratonic basins. The successions provide excellent opportunities for chronostratigraphic classification, in tune with the chronometric classification adopted by IUGS for inter-regional correlation on a global scale. Major tectono-thermal events at 1000–950 Ma in the mobile belts, correlatable with the Grenville orogeny may be considered as the datum for Meso-Neoproterozoic classification in India. Principles of chronostratigraphic classification, however, can not be applied yet to the cratonic successions of India because of inadequate radiometric data, paucity of biostratigraphic studies, and lack of regionally correlatable stratigraphic or palaeoclimatic datum. The kimberlite magmatism which affected the Peninsular India on a continental scale at about 1100 Ma, holds the key to the identification of Neoproterozoic successions of the cratonic basins. Thus, the stratigraphically confined diamond-bearing conglomerates and/or the tuffs associated with kimberlites, may be considered as the datum to define the base of the Neoproterozoic, fixed at about 1000 Ma. Accordingly, the Rewa, and Bhander Groups in the Vindhyan basin, the Kurnool Group in the Cuddapah basin, the Jagdalpur Formation in the Indravati basin, and the Sullavai Group in the Pranhita-Godavari basin are taken to represent the Neoproterozoic successions in the Peninsular India. The Chattisgarh Group in the central India, the lower part of the Marwar Supergroup in western Rajasthan, the Badami Group in the Kaladgi basin, and the Bhima Group are the other “possible Neoproterozoics” in the Peninsula. The closing phase of the Mesoproterozoic in all these basins are characterised by stable shelf lithologic associations attesting to high crustal stability. The Neoproterozoic basins, by contrast, mark a new phase of rifting, and extension, and the basin fills exhibit signatures of initial instability which evolved with time into a more stable platformal condition. A major episode of sea level rise has been recorded in most of the basins. The riftogenic origin, and evolution of the basins are comparable with the history of Neoproterozoic basins of Australia though there is no unequivocal record of glaciation in the Indian formations.

Oxygenation of the Archean atmosphere: New paleosol constraints from eastern India

It is widely believed that atmospheric oxygen saturation rose from −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.

Possible juvenile Palaeoarchaean TTG magmatism in eastern India and its constraints for the evolution of the Singhbhum craton

High-precision SHRIMP U–Pb zircon dating yields a late Palaeoarchaean age (3290 ± 8.6 Ma) for a large, unmetamorphosed, weakly peraluminous TTG body (the Keonjhargarh–Bhaunra pluton) in the Singhbhum craton of eastern India. One inherited subhedral zircon grain gave a concordant age of 3495.9 ± 5.3 Ma and Nd isotope characteristics show a juvenile trend with eNd t ~ 0 and T DM 3395–3453 Ma. The data support a model of typical Archaean crustal evolution until late Palaeoarchaean times for the Singhbhum craton, which is in contrast to the more southerly Bastar craton where Palaeoarchaean non-TTG granites have been identified. These data demonstrate the diachronous development of continental crustal blocks now forming the basement of the eastern and central peninsular of India.

Constraints on the development of Proterozoic basins in central India from 40Ar/39Ar analysis of authigenic glauconitic minerals

Ages of some key stratigraphic sequences in central Indian Proterozoic basins are based predominantly on lithostratigraphic relationships that have been constrained by only a few radioisotopic dates. To help improve age constraints, single grains of glauconitic minerals taken from sandstone and limestone in two Proterozoic sequences in the Pranhita-Godavari Valley and the Chattisgarh basin were analyzed by the 40 Ar/ 39 Ar incremental heating method. Analysis of the age spectra distinguishes between ages that are interpreted to reflect the time of glauconite formation, and anomalous ages that result from inherited argon or postcrystallization heating. The analyses indicate an age of 1686 ± 6 Ma for the Pandikunta Limestone and 1566 ± 6 Ma for the Ramgundam Sandstone, two units in the western belt of Proterozoic sequences in Pranhita-Godavari Valley. Glauconite from the Chanda Limestone, in the upper part of this sequence, contains inherited 40 Ar but is interpreted to reflect an age of ca. 1200 Ma. Glauconite from the Somanpalli Group in the eastern belt of the Pranhita-Godavari Valley gives an age of 1620 ± 6 Ma. In the Chattisgarh basin, glauconite from two units gives disturbed ages that suggest a period of regional heating in the Chattisgarh basin at ca. 960–1000 Ma. These new ages indicate that these sequences are 200–400 m.y. older than previously recognized, which has important implications for geochemical studies of Mesoproterozoic ocean redox conditions in addition to providing important constraints on regional tectonics and lithostratigraphy.

Stratabound magnetite deposits from the eastern outcrop belt of the Archaean Iron Ore Group, Singhbhum craton, India

Abstract Most high-grade banded iron formation (BIF)-hosted iron ores are composed almost exclusively of microplaty haematite and martite-textured haematite, i.e. pseudomorphs of haematite after magnetite. Goethite abounds only where haematite-rich iron ores are affected by geologically recent lateritic weathering. Magnetite and kenomagnetite occur usually only as small, irregular remnants enclosed in martite. Although low-grade magnetite–carbonate ores have been discovered in spatial association with high-grade iron ore bodies, high-grade BIF-hosted iron ores composed predominantly of magnetite have as yet only been reported from the Iron Quadrangle in Minas Gerais (Brazil). In this contribution, the authors describe such magnetite-rich high-grade BIF-hosted iron ores from the Gorumahishani deposit in the state of Orissa, India. The high-grade magnetite ores occur as stratabound bodies hosted by a single prominent BIF unit of the Mesoarchaean Iron Ore Group (IOG). The ores are composed of an interlocking xenotopic mosaic of fine-grained magnetite, accompanied by trace amounts of Fe-rich talc, sulphides and carbonaceous matter. The occurrence of the magnetite ores is very similar to that of economically significant microplaty haematite–martite ore bodies hosted by the same BIF from other parts of the IOG; both are regarded as being of hydrothermal–metasomatic origin, formed at the expense of the iron formation protolith. The abundance of magnetite, combined with the presence of sulphides, carbonaceous matter, and a marked enrichment of Ni, Co and Zn are attributes that differentiate the magnetite-rich ores from haematite–martite counterparts. These differences are tentatively attributed to differences in hydrothermal fluid composition. Based on the available evidence, it is suggested that magmatic–hydrothermal fluids derived from closely associated mafic intrusives were instrumental in the formation of the magnetite orebodies of the Gorumahishani deposit. The abundance of magnetite and the presence of sulphides and carbonaceous matter indicate that the hydrothermal fluid was mildly reducing, rather than oxidising as proposed for haematite-rich assemblages. The abundance of martite in the coexisting haematite–martite orebodies of other parts of the IOG may be used to speculate that the formation of massive magnetite bodies may have been an essential intermediate stage during the hydrothermal formation of high-grade iron ore deposits of the IOG.

Recognition, Characterization and Implications of High-Grade Silicic Ignimbrite Facies from the Paleoproterozoic Bijli Rhyolites, Dongargarh Supergroup, Central India

Abstract A controversy regarding the distinction between the highly welded lava-like ignimbrites sometimes showing strongly rheomorphic characters, and the extensive silicic lava flow has been overwhelming in the recent literature. However, a rethinking, after Walker (1983), has brought into light the concept of ‘grade’ referring to the degree and extent of welding between the pyroclasts. Various parameters and characteristics were suggested for strengthening the idea of densely welded ignimbrites, which differentiate them from lava. Here, a comprehensive study on early Proterozoic acid magmatic rocks forming lower part of the Dongargarh Supergroup, central India, has been made to suggest extensive occurrence of high-grade welded rheomorphic tuffs. The possibility of their being welded ignimbrite rather than lava flow has been explored in the light of facies analysis as well as detailed microscopic evidences. Despite having overall monolithologic look various units bear distinction on account of their nature of welding, enrichment of phenocrysts and degree of stretching. The presence of vitroclastic texture, melt inclusions and radial fracturing of phenocrysts suggests pyroclastic nature of these deposits. Based on these characters four facies — A, B, C and D from bottom to the top respectively, have been identified from field studies around Salekasa. Facies-A and B represent clast-supported/matrix-supported welded pyroclastic flow deposits. Facies-C represents extremely welded thinly laminated rheomorphic tuffs while lava-like tuffs with an autobreccia carapace is represented by facies D. A complete gradation of facies A/B to D through C exists. High to extremely high-grade nature of welding in these deposits suggests a low column-height subaerial plinian to fissure eruption of a very high temperature silicic magma in a continental setting.

Deep-water manganese deposits in the mid- to late Proterozoic Penganga Group of the Pranhita-Godavari Valley, South India

Abstract Facies analysis of the unmetamorphosed sediments enclosing the stratiform manganese oxide deposits of the mid- to late Proterozoic Penganga Group identifies the base of slope of a distally steepened deep-water ramp as their site of accumulation. The interpretation is based on their close association with a variety of mass flow deposits ranging from limestone conglomerates to calcarenites and deep-water, plane-bedded micritic limestone devoid of current- or wave-generated structures as well as detritus coarser than fine silt. These deposits occur within a major transgressive succession. The base of slope origin of stratiform manganese deposits is uncommon in the rock record and their origin is to be constrained against the background of base of slope depositional setting.