Browsing by Author "Rohit Kumar Giri"

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Arc-related pyroxenites derived from a long-lived neoarchean subduction system at the southwestern margin of the cuddapah basin: Geodynamic implications for the evolution of the eastern dharwar craton, southern india
(University of Chicago Press, 2019) Abhinay Sharma; Rohit Kumar Giri; N.V. Chalapathi Rao; Waliur Rahaman; Dinesh Pandit; Samarendra Sahoo
Mineralogy and bulk-rock geochemistry (including Sr and Nd isotopes) of two ultramafic (pyroxenite) intrusions at Gurramkonda, situated adjacent to the southwestern margin of the Paleo-Mesoproterozoic Cuddapah Basin within the Eastern Dharwar Craton, southern India, are presented. The pyroxenites have a characteristic cumulate texture, with a predominance of pyroxene (augite) and amphibole (hornblende and actinolite), whereas chromite is present mostly as inclusions in pyroxene. Mineral chemistry of these ultramafics is akin to that of arc cumulate rocks, particularly those associated with the Alaskan-type complexes, formed in suprasubduction zones. High Mg#’s (72–74), coupled with high bulk-rock Ni (830–1130 ppm) and Cr (1110–1,360 ppm), strikingly correspond to those of continental arc cumulates. The enrichment of large ion lithophile elements over high field strength elements, conspicuous negative Nb-Ta, Zr-Hf, and Ti anomalies, and chondrite-normalized light rare earth element patterns highlight their arc affinity. Fluid-related modification of the mantle source is apparent from the various trace-element ratios. Negative ɛNd2.15Ga values (20.93 to 22.90) also support contribution of an enriched-mantle reservoir in their genesis. Partial melting of this fertile mantle source has been attributed to the Paleoproterozic lithospheric extension with heat contributions from a mantle plume or convective asthenosphere. Ultramafic intrusions of arc-related geochemical affinity thus constitute rare, but genetically important, occurrences within the Eastern Dharwar Craton. We propose that, together with the recently studied calc-alkaline lamprophyres occurring immediately toward the western margin of the Cuddapah Basin, the Gurramkonda pyroxenites are manifestations of a long-lived and major subduction system of Neoarchean age in the Eastern Dharwar Craton, southern India. © 2019 by The University of Chicago.
Cobaltoan pyrite in a lamprophyre from the Sidhi Gneissic complex, Mahakoshal belt, Central India
(Geological Society of India, 2018) Rohit Kumar Giri; Dinesh Pandit; N.V. Chalapathi Rao
Pyrite containing appreciable content of cobalt (up to 5.61 wt%) and minor amounts of nickel (up to 0.70 wt%) is being reported from electron microprobe studies in a lamprophyre dyke from the Sidhi complex, Mahakoshal belt, Central India. However, cobalt and nickel are conspicuously absent from the associated chalcopyrite reflecting that in a pyrite-chalcopyrite association these elements are preferentially sequestered in pyrite. The microprobe analysis of pyrite grains reveal that their Fe content is variable (40.62 to 46.02 wt%) and reciprocally related to cobalt and nickel concentrations implying the presence of latter as solid solution in the structure of pyrite. The cobalt contents of the pyrite are comparable to those (up to 3.21 wt%) reported from the cobaltbearing pyrites of the Kalyadi copper deposits of Dharwar craton of southern India. Co:Ni (16.05 average) in the studied pyrite appears to be more consistent with a magmatic, rather than a hydrothermal, origin. However, further studies are in progess. This study highlights that besides gold and diamond, lamprophyres are also important carriers of cobalt when available in the magmatic system. © 2018, Geological Society of India.
Formation of manganoan ilmenite in Archean tonalite-trondhjemite-granodiorite (TTG) gneisses inferred from re-equilibration of biotite and Fe-Ti oxide assemblage: a case study from the Aravalli Craton, northwest India
(Schweizerbart Science Publishers, 2023) Hiredya Chauhan; Dinesh Pandit; Talat Ahmad; Rohit Kumar Giri; Avinash C. Pandey
Ilmenite is a widespread accessory mineral in tonalite-trondhjemite-granodiorite (TTG) gneisses of the Banded Gneissic Complex (BGC-I) from the Aravalli Craton. Studied ilmenites are found as an inclusion in micas, magnetite and has low Mg (~0.00-0.21 wt%) and high Mn contents (3.94-10.29 wt%) in contrast to the ilmenites from igneous rocks. Studies that record the transformation of Fe-Ti oxides under low- to medium-grade metamorphic conditions are very restricted. The rocks of BGC-I of the Aravalli Craton have experienced amphibolite-facies metamorphism under medium grade metamorphic conditions and the effects of this metamorphism can be studied in the Fe-Ti oxides. Petrographic, bulk rock and mineral chemical data reveal that the primary magmatic composition of ilmenite has been diversely modified during metamorphism by diffusion re-equilibration with a co-existing silicate mineral. Biotite being the dominant mafic mineral in the TTG gneisses, has re-equilibrated during metamorphism and the Ti-in-biotite geothermometer indicates that re-equilibrated primary biotite crystallized at temperatures between 530° and 680 °C. Therefore, enhanced Mn diffusion out of biotite into ilmenite under oxidizing conditions provides an explanation for high Mn contents in ilmenite. This further infers short-range equilibration during medium-grade metamorphism of the BGC-I from the Aravalli Craton. © 2023. E. Schweizerbart'sche Verlagsbuchhandlung, Stuttgart, Germany
Lamprophyres from the Indian shield: A review of their occurrence, petrology, tectonomagmatic significance and relationship with the Kimberlites and related rocks
(International Union of Geological Sciences, 2020) N.V. Chalapathi Rao; Rohit Kumar Giri; Abhinay Sharma; Ashutosh Pandey
Lamprophyres are some of the oldest recognized alkaline rocks and have been studied for almost the last 150 years. Known for hosting economic minerals such as gold, diamond and base metals, they are also significant in our understanding of the deep-mantle processes (viz., mantle metasomatism and mantleplume- lithosphere interactions) as well as large-scale geodynamic processes (viz., subduction-tectonics, supercontinent amalgamation and break-up). The Indian shield is a collage of distinct cratonic blocks margined by the mobile belts and manifested by large igneous provinces (LIPs) such as the Deccan. A plethora of lamprophyres, varying in age from the Archaean to the Eocene, with diverse mineralogical and geochemical compositions, are recorded from the Indian shield and played a key role in clarifying the tectonic processes, especially during the Paleo- and Mesoproterozoic and the Late Cretaceous. A comprehensive review of the occurrence, petrology, geochemistry and origin of the Indian lamprophyres is provided here highlighting their tectonomagmatic significance. The relationship of the lamprophyres to the Kimberlite clan rocks (KCRs), focusing on the Indian examples, is also critically examined. © 2020 International Union of Geological Sciences. All rights reserved.
Mineralogy and petrology of shoshonitic lamprophyre dykes from the Sivarampeta area, diamondiferous Wajrakarur Kimberlite field, Eastern Dharwar craton, Southern India
(Tohoku University, 2020) Praveer Pankaj; Rohit Kumar Giri; N.V. Chalapathi Rao; Ramananda Chakrabarti; Sneha Raghuvanshi
Petrology and geochemistry (including Sr and Nd isotopes) of two lamprophyre dykes, intruding the Archaean granitic gneisses at Sivarampeta in the diamondiferous Wajrakarur kimberlite field (WKF), eastern Dharwar craton, southern India, are presented. The Sivarampeta lamprophyres display porphyritic-panidiomorphic texture comprising macrocrysts/phenocrysts of olivine, clinopyroxene (augite), and mica set in a groundmass dominated by feldspar and comprising minor amounts of ilmenite, chlorite, carbonates, epidote, and sulphides. Amphibole (actinolite-tremolite) is essentially secondary in nature and derived from the alteration of clinopyroxene. Mica is compositionally biotite and occurs as a scattered phase throughout. Mineralogy suggests that these lamprophyres belong to calc-alkaline variety whereas their bulk-rock geochemistry portrays mixed signals of both alkaline as well as calc-alkaline (shoshonitic) variety of lamprophyres and suggest their derivation from the recently identified Domain II (orogenic-anorogenic transitional type mantle source) from eastern Dharwar craton. Trace element ratios imply melt-derivation from an essentially the garnet bearing-enriched lithospheric mantle source region; this is further supported by their 87Sr/86Srinitial (0.708213 and 0.708507) and 'enriched' εNdinitial (-19.1 and-24.2) values. The calculated TDM ages (2.7-2.9 Ga) implies that such enrichment occurred prior to or during Neoarchean, contrary to that of the co-spatial and co-eval kimberlites which originated from an isotopically depleted mantle source which was metasomatized during Mesoproterozoic. The close association of calc-alkaline shoshonitic lamprophyres, sampling distinct mantle sources, viz., Domain I (e.g., Udiripikonda) and Domain II (Sivarampeta), and kimberlites in the WKF provide further evidence for highly heterogeneous nature of the sub-continental lithospheric mantle beneath the eastern Dharwar craton. © 2020 Tohoku University.
Paleoproterozoic calc-alkaline lamprophyres from the Sidhi Gneissic complex, India: Implications for plate tectonic evolution of the Central Indian Tectonic Zone
(Elsevier B.V., 2021) Rohit Kumar Giri; N.V. Chalapathi Rao; Waliur Rahaman; Alok Kumar; M. Satyanarayanan; A. Keshav Krishna
Calc-alkaline lamprophyres are widely regarded as important probes for unravelling continental-scale geodynamic processes. We present petrographic, mineral chemical, bulk-rock geochemical, and Sr-Nd isotope data on eleven unmetamorphosed lamprophyre dykes intruding the ca. 2.5 Ga Sidhi Gneissic Complex in the Central Indian Tectonic Zone (CITZ) – an intercontinental suture which separates the Northern Indian (Aravalli-Bundelkhand Craton) and the Southern Indian (Dharwar-Bastar-Singhbhum Craton) blocks. The Sidhi dykes have porphyritic-panidiomorphic texture typical of lamprophyres, with phlogopite/biotite as the dominant phenocryst phase, in a orthoclase dominated groundmass. Based on mineralogy and geochemistry, the Sidhi dykes can be classified as calc-alkaline lamprophyres in general and minettes in particular. All these dykes are characterised by negative Ti, Ta and Nb anomalies typical of subduction-related global calc-alkaline lamprophyres. The trace element ratios of the Sidhi lamprophyres suggest derivation from variable degrees of partial melting of a similar magma source which experienced crustal input. Bulk-rock Rb-Sr isochron of the Sidhi lamprophyres yielded an age of ca. 2278 ± 230 Ma which is consistent with the Paleoproterozoic emplacement ages reported for other rocks from this region of CITZ. Negative ɛNdT (−3.68) values of the Sidhi lamprophyres suggest their derivation from an isotopically enriched mantle source whilst their TDM Nd model ages (~2.6 Ga) imply source enrichment that took place during the Neoarchaean. Petrogenetic modelling indicate that the Sidhi lamprophyres are generated from low degree partial melting of a mantle source metasomatised by sediment-derived melt having < 4% crustal material as an input in the E-DMM. Our study support models proposing a northward subduction of the Southern Indian Block beneath the Northern Indian Block along the CITZ. © 2021 Elsevier B.V.
Petrogenesis of an alkaline lamprophyre (camptonite) with ocean island basalt (OIB)-affinity at the NW margin of the Cuddapah basin, eastern Dharwar craton, southern India
(E. Schweizerbart'sche Verlagsbuchhandlung, 2019) Rohit Kumar Giri; Praveer Pankaj; N.V. Chalapathi Rao; Ramananda Chakrabarti; Dinesh Pandit
We report petrology and geochemistry (including Sr and Nd isotopes) of a fresh lamprophyre at Ankiraopalli area at the north-western margin of Paleo-Mesoproterozoic Cuddapah basin, eastern Dharwar craton, southern India. Ankiraopalli samples possess a typical lamprophyre porphyritic-panidiomorphic texture with phenocrysts of kaersutite and diopside set in a plagioclase dominant groundmass. Combined mineralogy and geochemistry classify it as alkaline lamprophyre in general and camptonite in particular. Contrary to the calc-alkaline and/or shoshonitic orogenic nature portrayed by lamprophyres occurring towards the western margin of the Cuddapah basin, the Ankiraopalli samples display trace element composition revealing striking similarity with those of ocean island basalts, Italian alkaline lamprophyres and highlights an anorogenic character. However, the 87Sr/86Srinitial (0.710316 to 0.720016) and Ndinitial (-9.54 to -9.61) of the Ankiraopalli lamprophyre show derivation from an ‘enriched’ mantle source showing long term enrichment of incompatible trace elements and contrast from those of (i) OIB, and (ii) nearby Mahbubnagar alkaline mafic dykes of OIB affinity. Combining results of this study and recent advances made, multiple mantle domains are identified in the Eastern Dharwar craton which generated distinct Mesoproterozoic lamprophyre varieties. These include (i) Domain I, involving sub-continental lithospheric mantle source essentially metasomatized by subduction-derived melts/fluids (represented by orogenic calcalkaline and/or shoshonitic lamprophyres at the Mudigubba, the Udiripikonda and the Kadiri); (ii) Domain II, comprising a mixed sub-continental lithospheric and asthenospheric source (represented by orogenic-anorogenic, alkaline to calc-alkaline transitional lamprophyres at the Korakkodu), and (iii) Domain III, representing a sub-continental lithospheric source with a dominant overprint of an asthenospheric (plume) component (represented by essentially alkaline lamprophyres at the Ankiraopalli). Our study highlights the varied mantle source heterogeneities and complexity of geodynamic processes involved in the Neoarchean-Paleo/Mesoproterozoic evolution of the Eastern Dharwar craton. © 2019 E. Schweizerbart’sche Verlagsbuchhandlung, Stuttgart, Germany.
Pyroxenite dykes with petrological and geochemical affinities to the Alaskan-type ultramafics at the northwestern margin of the Cuddapah basin, Dharwar craton, southern India: Tectonomagmatic implications
(Springer, 2019) Rohit Kumar Giri; Praveer Pankaj; Dinesh Pandit; Samarendra Sahoo; Ramananda Chakrabarti; N.V. Chalapathi Rao
Two previously reported lamprophyre dykes from the Kalwakurthy area, at the northwestern margin of the Cuddapah basin, Dharwar craton, southern India, are reinvestigated. Petrography reveals that they have an overall cumulate texture and comprise clinopyroxene (dominant phase), amphibole (mostly secondary), magnetite, ilmenite and chromite and are reclassified as clinopyroxenites. The chemistry of clinopyroxene and chromite, bulk-rock major and trace element composition and the Sr–Nd isotopic systematics of the Kalwakurthy dykes strongly favour the involvement of subduction-related processes in their genesis and are strikingly similar to those of the continental arc-cumulates and Alaskan-type ultramafics reported from the supra-subduction type of tectonic settings. Incompatible trace element ratios, involving high field strength elements, of these clinopyroxenites are also suggestive of the fluid-related metasomatism influencing their source regions. Petrogenetic modelling reveals that 10–20% partial melting of the fertile lithospheric mantle source was involved in their genesis. The tectonomagmatic significance of the studied clinopyroxenites is evaluated in light of the existing models invoking a Neoarchaean subduction in the evolution of the Dharwar craton. © 2019, Indian Academy of Sciences.