Browsing by Author "Sneha Raghuvanshi"

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Chrome-diopside Xenocrysts Entrained in a Neoproterozoic Lamprophyre Dyke from the Mysuru Area: Their Origin and Implications for Lithospheric Thickness Beneath the Western Dharwar Craton, Southern India
(Springer, 2022) Sneha Raghuvanshi; N.V. Chalapathi Rao; Debojit Talukdar; Abhinay Sharma; Rohit Pandey
In comparison to the eastern Dharwar Craton, the mantle-derived xenocrysts/xenoliths are extremely rare or even unreported from the western Dharwar Craton, southern India. A Neoproterozoic (ca. 800–900 Ma) lamprophyre cropping out in the Mysuru area of southern India contains chrome-diopside xenocrysts (Cr2O3 content varying from 0.2–1.23 wt%) which provide important evidence about the pressure-temperature conditions and lithospheric thickness beneath the western Dharwar Craton. Studied chrome-diopsides show compositional zoning which is lacking in the liquidus phases (amphiboles and feldspars) of the lamprophyre which additionally favors a non-cognate origin of the former. Based on the compositional zoning, all the chrome-diopside xenocrysts can be divided into three groups: (i) Group I- which are euhedral and show reverse zoning with increasing Cr-content from core to rim; (ii) Group II- which are characterized by fractures and resorption textures, show complex reverse zoning and display up to three distinct compositional layers, and (iii) Group III- which evidence the reaction of chrome-diopsides with lamprophyric melt and are marked by alteration phases, such as actinolite and chlorite, together with relicts of some unaltered xenocrysts. High Cr2O3, moderate MgO and low Al2O3 content of all the three varieties of chrome-diopside suggest them to represent disaggregated xenocrysts of mantle-derived garnet peridotite. Temperature-pressure estimates for chrome-diopside xenocrysts ranges from 895–1026 °C (± 30 °C) and 32–38 kbar respectively and correspond to depth range of 106–127 km. The study reveals that lithospheric thickness during the Neoproterozoic beneath the western Dharwar craton was at least ∼115 km and is similar in composition to that of the cratonic lithosphere found in the other cratonic domains. © 2022, Geological Society of India, Bengaluru, India.
Gold-Sulfide Mineralization in Banded Iron Formation at Girar, Bundelkhand Craton, India: Inferences from Field Observations and Petrography
(Springer, 2023) Ajit Kumar Sahoo; Rajagopal Krishnamurthi; Gautam Kumar Dinkar; N. V. Chalapathi Rao; Arun Kumar Ojha; Sneha Raghuvanshi; Sudipa Bhunia
Gold mineralization associated with quartz veins and sulfide minerals has been reported in Banded Iron Formation (BIF) at Girar, Bundelkhand Craton, India. The BIF is deformed with the development of tight- isoclinal folds in high strain zones and hosts four types of quartz veins viz., (i) folded veins, (ii) band parallel veins, (iii) low angle cross-cutting veins, and (iv) high angle cross-cutting veins. Folded and band parallel veins are continuous with each other and spatially restricted to zones of tight-isoclinal folds in BIF. Shape of quartz aggregates in the folded and band parallel veins is interlobate, and these veins contain pyrite and chalcopyrite as dominant sulfide minerals along with gold. Microstructure of quartz grains and presence of fragments of BIF in sulfide mineralgold bearing folded and band parallel veins indicate that the formations of these veins were related to a single deformation episode. Spatial association of folded and band parallel veins to intensely folded zones in BIF is attributed to epigenetic nature of sulfide and gold mineralization. © 2023, Geological Society of India, Bengaluru, India.
Lithosphere–asthenosphere interaction and carbonatite metasomatism in the genesis of Mesoproterozoic shoshonitic lamprophyres at Korakkodu, Wajrakarur kimberlite field, Eastern Dharwar Craton, southern India
(John Wiley and Sons Ltd, 2019) Sneha Raghuvanshi; Ashutosh Pandey; Praveer Pankaj; N.V. Chalapathi Rao; Ramananda Chakrabarti; Dinesh Pandit; Rohit Pandey
The spatial and temporal association between lamprophyres and kimberlites provides unique opportunities to explore their genetic relationships. This paper explores such a relationship by detailing mineralogical and geochemical aspects of Korakkodu lamprophyre dykes located within the well-known Mesoproterozoic diamondiferous Wajrakarur Kimberlite field (WKF), towards the south-western margin of Paleo–Mesoproterozoic Cuddapah Basin, Eastern Dharwar Craton, southern India. Mineralogy reveals that these dykes belong to calc-alkaline variety of lamprophyres, but their geochemistry display mixed signals of both alkaline and calc-alkaline lamprophyres. These lamprophyres are highly potassic, and their high Al2O3 and low-TiO2 content implies a shoshonitic character. Low Mg#, Ni, and Cr concentration highlight their evolved nature. High (La/Yb)N and (Gd/Yb)N content is consistent with their derivation from low degrees of partial melting, whereas highly fractionated nature suggests the presence of garnet in their source. Absence of prominent Nb-Ta anomaly implies to the dilution of lithospheric mantle source by melts rich in HFSEs and low La/Nb ratio compared to those of the calc-alkaline island arc volcanics and suggests an asthenospheric overprint on lithospheric mantle source. Carbonatite metasomatism in the source region of these lamprophyres is apparent from conspicuously high-Zr/Hf ratio, and the HFSE budget of these lamprophyres are principally controlled by the presence of phlogopite veins in their lithospheric source. An extremely heterogeneous and layered lithospheric mantle beneath Eastern Dharwar Craton has been inferred from the divergent genetic history of Mesoproterozoic lamprophyres and kimberlites in the Wajrakarur field. © 2019 John Wiley & Sons, Ltd.
Magmatic Ni–Cu–Fe sulphide mineralization from the Halaguru area, Eastern Dharwar Craton, southern India
(Indian Academy of Sciences, 2022) Sneha Raghuvanshi; N. V. Chalapathi Rao; Ajit K. Sahoo; Debojit Talukdar
Here we report sulphide mineralization in a spinel-bear-ing orthopyroxene hornblendite from the Halaguru area, Eastern Dharwar Craton (EDC), southern India. The hornblendite contains approximately 3% olivine, 38% orthopyroxene, 56% amphibole and 2% oxide/ sulphide phases. Petrographic and mineral chemistry studies show its formation as a result of the reaction between early formed olivine and hydrous silicate melt without any metamorphic imprint, despite its proximity to amphibolites–granulite isograd in the EDC. The sulphides comprise the disseminated form of pyrrho-tite–pentlandite–chalcopyrite assemblage, which is well crystallized and the primary magmatic features are preserved as contact and granular textures. Absence of crustal xenoliths and lower SiO2 (48 wt%), Sr (35 ppm), Pb (<5 ppm), U (<0.1 ppm) and Th (0.10 ppm) content coupled with higher Mg# (~82), Ni (700 ppm), Cr (2500 ppm) concentration in the bulk sample rules out the possibility of sulphide saturation by the addi-tion of crustal-derived sulphur. Based on the combined textural and compositional criteria, the pyrrhotite–pentlandite–chalcopyrite assemblage is a product of the cooling and crystallization of immiscible sulphide globules. The initial high monosulphide solution, upon cooling, exsolves to pyrrhotite and pentlandite and later, at lower temperature, intermediate sulphide solid solution gives rise to chalcopyrite which is dominantly present around the edges of the pyrrhotite–pentlandite assemblage. © 2022. Current Science. 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.
Petrology, U-Pb titanite dating and Sr-Nd isotope geochemistry of a shoshonitic lamprophyre dyke near the Western Dharwar Craton-Southern Granulite Terrane contact, southern India: Implications for long-lived enriched mantle, widespread Tonian-Cryogenian rifting, and Rodinia configuration
(Elsevier B.V., 2023) Sneha Raghuvanshi; N.V. Chalapathi Rao; D. Talukdar; B. Belyatsky; P. Prabhat; Waliur Rahaman; B. Lehmann; J.G. Meert
We present petrology, U-Pb titanite geochronology, bulk-rock and Sr-Nd geochemistry of an undeformed and unmetamorphosed shoshonitic lamprophyre (spessartite) from the Mysuru area (Halaguru- Harohalli alkaline province) located approximately 80 km north of the tectonic contact between the Western Dharwar Craton and the Southern Granulite Terrane, southern India, to understand its origin and to evaluate its geodynamic significance. The lamprophyre shows a typical porphyritic-panidiomorphic texture with amphibole phenocrysts. Clinopyroxene occurs only as xenocrysts with feldspar, apatite, titanite and titano-magnetite confined to the groundmass. Amphibole mineral composition classifies them as pargasite and tremolite, and the groundmass is essentially constituted of alkali feldspar. High whole-rock Mg# (56–65), Ni + Cr (245 to 730 ppm), a lack of correlation between SiO2 and Rb/Sr as well as ɛNd(i) indicates that crustal contamination experienced by the lamprophyre magma is limited. U-Pb titanite (n = 18) SHRIMP-II dating yielded a Neoproterozoic Tera-Wasserburg concordia age of 820 ± 15 Ma (MSWD = 5.9). Whole-rock derived 87Sr/86Sr(i) (0.70425–0.70530) and ɛNd(i) (-6.3 to −4.3) suggest origin of the lamprophyre from an enriched lithospheric mantle source. Paleoproterozoic depleted-mantle model ages of ca. 1.8 Ga suggest the existence of a long-lived enriched mantle which correspond to the timing (ca.1.9–1.7 Ga) of widespread mafic dyke swarms’ emplacement in the Dharmapuri-Agali-Tiruvannamalai domain in the Salem block at the northernmost portion of the Southern Granulite Terrane. The Tonian emplacement age of the studied lamprophyre is synchronous with several co-spatial Neoproterozoic (i) alkaline complexes ± carbonatite ± lamprophyre from the northern part of the Southern Granulite Terrane, and (ii) other mafic alkaline dykes of the Halaguru-Harohalli alkaline province from the southern part of the Eastern Dharwar Craton. Our results highlight the importance of this lamprophyre dyke, present close to the'Fermor Line’ as well as near the boundary between Eastern and Western Dharwar Cratons, which provides a rare opportunity to understand its spatio-temporal link with alkaline dykes of the Southern Granulite Terrain. Our findings also help understand the role of repeated large-scale rifting, associated with the Rodinia break-up, leading to the generation of a wide spectrum of Tonian-Cryogenian alkaline magmas in southern Indian Shield. The new age data also have implications for the configuration of India's paleoposition during the Neoproterozoic and support the idea that South China Block-India were located on the periphery of Rodinia or were already detached from the supercontinent. © 2023 Elsevier B.V.