Browsing by Author "Rohit Pandey"

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Alkaline rocks and their economic and geodynamic significance through geological time
(Geological Society of London, 2025) Rohit Pandey; Lukáš Krmíček; Daniel Müller; Ashutosh Pandey; Ciro Cucciniello
Alkaline igneous rocks have a relative excess of alkalis over silica. Most are silica undersaturated and contain normative nepheline and real feldspathoids (nepheline, leucite). Although alkaline rocks make up only about 1% of total igneous rocks by volume, their diverse mineralogy accounts for 90% of all igneous rock names proposed by the IUGS. They occur in all tectonic settings; in the ocean basins and on the continents, along mid-ocean ridges (rare), on oceanic islands, in subduction zones in the oceans and along continental margins, as well as along rift zones. Alkaline rocks commonly include alkali basalts and foidites, tephrites, phonolites, trachytes and their intrusive equivalents, including lamprophyres and carbonatites. In the literature, a plethora of local names for alkaline rocks from different localities have been created and many geoscientists still consider them as petrological curiosities. However, their study can significantly aid the interpretation of mantle evolution, ancient terranes and their geodynamic settings. Additionally, alkaline rocks may host world-class precious- and rare-metal mineralization. During recent years, the exploration interest in critical and rare metal deposits (Nb, rare earth elements (REEs) and Th) has increased dramatically as they represent vital resources for the so-called ‘green energy transition’. This Special Publication presents new comprehensive data, results and findings on alkaline rocks from different terranes worldwide and uses their mineralogy and geochemical fingerprints in order to elucidate their petrogenesis, tectonic settings and mineralization potential. This volume is not only of interest for igneous petrologists, but also for exploration geologists prospecting for precious- and rare-metal mineralization worldwide. © 2025 The Author(s). Published by The Geological Society of London. All rights, including for text and data mining (TDM), artificial intelligence (AI) training, and similar technologies, are reserved. For permissions: https://www.lyellcollection.org/publishing-hub/permissions-policy. Publishing disclaimer: https://www.lyellcollection.org/publishing-hub/publishing-ethics.
Alkaline rocks from the Deccan Large Igneous Province: Time–space distribution, petrology, geochemistry and economic aspects
(Springer, 2022) Rohit Pandey; N V Chalapathi Rao; Mahendra K Singh; Debojit Talukdar
We present a comprehensive review on the alkaline rocks from the Deccan Large Igneous Province (DLIP) and discuss their (i) temporal and spatial association with the Deccan Traps, (ii) petrography, mineral- and whole-rock geochemistry (including radiogenic and stable isotopes) and geophysical aspects, and (iii) P–T data available on their entrained xenoliths. The alkaline rocks occur in seven sub-provinces, viz., (i) the Kachchh, (ii) the Saurashtra, (iii) the Gujarat Central and Chhotaudepur, (iv) the Mumbai–Trombay, (v) the Central Deccan, (vi) the Aravalli, and (vii) the Tethyan Himalayan, with the first five in association with the Deccan Traps. A diverse variety of silica under-saturated to over-saturated alkaline rocks with varied mineralogical and geochemical compositions have been reported from these sub-provinces. These include alkali basalt, basanite, carbonatite, ijolite, lamprophyre, leucite, melteigite, mugearite, nephelinite, nepheline syenite, orangeites, alkali pyroxenite, phonolite, tinguaite, etc. Available geochronological data on the Deccan alkaline rocks reveal a wide duration of the related magmatic activity (124–55 My), and suggest the presence of pre-, syn- and post-emplacement ages of the DLIP units. Alkaline rocks of the DLIP are hosted by discrete aged lithotypes in a variety of stratigraphic horizons, such as the Deccan Traps, Cretaceous Bagh beds, Jurassic sandstones, Triassic Shrinab sediments, Proterozoic Godhra Granite and unclassified gneisses. In a majority of the sub-provinces, intrusions of alkaline rocks are controlled by fractures, rift or lineament systems such as the Kutch rift, the Son–Narmada Tapti rift, etc. Their major mineralogy is dominated by pyroxene, feldspar, amphibole, mica, olivine, nepheline, leucite, sodalite and carbonate minerals whereas accessory and minor minerals include titanite, apatite, spinel, rutile, pyrite, chalcopyrite, epidote, zircon, pyrochlore, garnet, perovskite and other REE-bearing phases. Geochemical studies reveal their sodic to potassic nature, with distinct shoshonitic character for some alkaline rocks. Combined geochemical and isotopic studies highlight the role of mixed mantle sources ranging from spinel to garnet stability depths and involvement of the lower degrees of partial melting. Source modification by subduction and crustal contamination is evaluated. Geodynamic implications for the orogenic and anorogenic signatures found in various occurrences, depth of the lithosphere–asthenosphere boundary, and economic resources are also examined and future research directions are identified. © 2022, Indian Academy of Sciences.
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.
Geochemistry and petrology of deep seated mantle magmas and their mantle xenoliths and xenocrysts: Applications to the structure and compositions of mantle lithosphere
(Elsevier B.V., 2025) Igor V. Ashchepkov; N. V.Chalapathi Rao; Rohit Pandey; Sergei V. Rasskazov
[No abstract available]
Geochronological and Sr-Pb-C-O isotope constraints on the genesis of carbonatites from the polychronous Mundwara alkaline complex (north western India)
(Elsevier B.V., 2025) Sudipa Bhunia; Nittala Venkata Chalapathi Rao; Andrea Giuliani; Lorenzo Tavazzani; Debojit Talukdar; Rohit Pandey; Alok Kumar; Sirajuddin Ansari; Bernd Lehmann
The genesis of carbonatites is commonly related to either direct melting of carbonate-bearing mantle rocks or derivation from CO2-rich silicate melts via either igneous fractionation or liquid immiscibility. Testing these hypotheses requires accurate geochronological and isotopic constraints on carbonatite and silicate rocks from the same igneous complex. The polychronous Mundwara alkaline complex offers an excellent setting to examine the origin of carbonatites because it contains a range of silicate rocks (including alkali-gabbros, syenites and other alkaline lithologies) and their ages (110–68.5 Ma) and radiogenic isotopic compositions have been previously constrained. However, the age and isotopic composition of the Mundwara carbonatites are currently unknown. To further our understanding of carbonatite magmatism at Mundwara, new apatite U-Pb ages are combined with petrographic observations, bulk‑carbonate C-O isotope analyses and in-situ determinations of trace element contents and Sr-Pb isotopic ratios for calcite and apatite. The Mundwara carbonatite dykes consist of calcite cumulates along with accessory apatite, pyrochlore, alkali feldspar, Fe-oxides and biotite. A range of REE-bearing phases including bastnäsite, parisite, and monazite is also present, although in accessory amounts. Cumulitic and seriate texture along with high Sr contents (>1 wt%), attest to the primary igneous nature of the calcites. The apatites are magmatic as demonstrated by their pill shape, low Sr content and strongly fractionated chondrite-normalized REE patterns, which clearly distinguishes them from typical hydrothermal apatite elsewhere. The apatite grains yield a weighted mean 87Sr/86Sr of 0.70447 ± 0.00003 (n = 24), indistinguishable from those of the carbonates analyzed in the same samples (87Sr/86Sr = 0.70446 ± 0.00001; n = 54). Lead (207Pb/206Pb = 0.820–0.829; 208Pb/206Pb = 2.065–2.088) and Sr isotopic compositions of the calcites are broadly intermediate between enriched mantle (EM) and HIMU (high 238U/204Pb) compositions. The bulk‑carbonate δ13C and δ18O data of the Mundwara carbonatites have a narrow range from −6.2 ‰ to −6.8 ‰ and from +6.3 ‰ to +7.3 ‰ respectively, showing typical mantle values and excluding significant contamination or post-magmatic alteration as well as contribution by subducted carbon. Apatite defines an accurate, although imprecise U-Pb age of 100 ± 20 Ma, which closely matches the 110–102 Ma Ar-Ar ages of nepheline syenites from this complex. This observation coupled with overlapping Sr-Pb isotope compositions of the carbonatites and alkaline mafic rocks from Mundwara implies a close genetic link between carbonatites and silicate melts, which is best explained by either liquid immiscibility or differentiation of CO2-bearing ultramafic melts. The age and isotopic features of the Mundwara carbonatites and related silicate rocks are best explained by melting of metasomatised lithosphere or upper convective mantle affected by deeply subducted material in response to the extensional stress regime associated with the early stages of India-Madagascar separation. © 2025 Elsevier B.V.
Imprints of modal metasomatism in the post-Deccan subcontinental lithospheric mantle: Petrological evidence from an ultramafic xenolith in an Eocene lamprophyre, NW India
(Geological Society of London, 2018) Rohit Pandey; N.V. Chalapathi Rao; Dinesh Pandit; Samarendra Sahoo; Prashant Dhote
We report here on the occurrence of an interesting mantle-derived ultramafic xenolith entrained in an Eocene (c. 55 Ma) lamprophyre dyke from the Dongargaon area of the Chhotaudepur alkaline subprovince located within the Narmada Rift Zone,NWIndia. The mineralogy of the xenolith comprises olivine, clinopyroxene and mica (phlogopite), with the latter occurring essentially as rims around the clinopyroxene. Inclusions of apatite, interstitial sulphide (pyrite) and micron-scale exsolved spinel are widespread. Olivine is forsteritic (Fo85.34), displays little compositional variation and overlaps with that reported from worldwide mantle peridotite xenoliths. Clinopyroxene is a diopside with a compositional range of Wo48.36 En43.83, Fs6.53 and Ac1.27, and is conspicuous by its high CaO (up to 24.4 wt%) and TiO2 (up to 1.6 wt%) content. Clinopyroxene is also compositionally similar to that reported from 'enriched' (metasomatized) peridotite xenoliths rather than those that occur in the 'normal' (depleted) peridotitic xenoliths. Phlogopites have a high concentration of fluorine (up to 1 wt%), whereas the apatites show an anomalous enrichment of F (up to 5 wt%), as well as enrichment in Sr (SrO up to 1.9 wt%). Our study provides the first direct petrographical evidence for the modal metasomatism in the post-Deccan subcontinental lithospheric mantle (SCLM) from this domain. From the textural and mineralogical assemblage of the xenolith, we infer that a possible olivine + garnet + orthopyroxene assemblage, in the presence of a metasomatic fluid, has given rise to clinopyroxene + phlogopite + spinel. The paragenesis of apatite essentially as inclusions suggests that it to be the earliest crystallized phase during the metasomatic event. Geothermobarometry of the clinopyroxene in the xenolith reveal temperatures of approximately 1200°C and pressures of approximately 12 kb, which are comparable with such data reported from other Deccan-related xenoliths. Preservation of phlogopite and apatite in the ultramafic xenolith imply that some of the readily fusible metasomatized portions in this domain escaped wholesale melting during the eruption of the Deccan Traps, possibly due to the variable thickness of the underlying SCLM. © 2018 The Author(s).
Indicator Mineral Chemistry of P-17 Kimberlite, Wajrakarur Field, Eastern Dharwar Craton, Southern India: Implications for Diamond Prospectivity
(Geological Society of India, 2025) Ashrumochan Sahoo; Kumari Minu Singh; Mahimaa Dash; Rohit Pandey; Nittala Venkata Chalapathi Rao
Indicator mineral chemistry is a well-known prospecting tool in diamond exploration due to the unique association between indicator minerals such as pyrope garnets, chrome diopsides and Mg-ilmenites with kimberlites as well as diamonds. The Penna Ahobilam Pipe-17 kimberlite in the well-known Wajrakarur kimberlite field, Eastern Dharwar Craton, southern India, is one of the latest discoveries by the Geological Survey of India and very little information is available as to its diamond prospectivity vis-à-vis indicator mineral composition. Indicator mineral chemistry (IMC) indicates that majority of the Pipe-17 pyrope garnets belong to the G-4 and G-5 clusters of the pyroxenitic group, with a small population displaying affinity to the G-10 cluster of the peridotitic group. Ilmenites exhibit high MgO values, consistent with those reported from the kimberlitic ilmenites, and their low Fe3+/Fe2+ ratios indicate reducing conditions favourable for diamond formation. In contrast, the ilmenites have high Fe2 O3 values with respect to MgO contents and imply an intermediate to marginal preservation with significant diamond resorption. The apple-green chrome diopsides are characterized by distinct high Cr2 O3 and Na2 O contents and are indistinguishable from those reported from worldwide diamondiferous kimberlites of deeper mantle origin. The indicator minerals recovered from the Pipe-17 kimberlite highlight that it has a lower diamond potential which is also confirmed by the poor recovery of diamonds by testing. © 2025 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.
Mantle transition zone-derived eclogite xenolith entrained in a diamondiferous Mesoproterozoic (∼1.1 Ga) kimberlite from the Eastern Dharwar Craton, India: evidence from a coesite, K-omphacite, and majoritic garnet assemblage
(Cambridge University Press, 2023) Amitava Chatterjee; N.V. Chalapathi Rao; Rohit Pandey; Ashutosh Pandey
Subduction-related kimberlite-borne eclogite xenoliths of the Precambrian age may provide significant information about the evolution and recycling of a subducting crust as exhumed/orogenic eclogites of the pre-Mesoproterozoic time-frame are globally rare. In this paper, we report a kimberlite-borne eclogite xenolith from the diamondiferous Kalyandurg kimberlite cluster of the Eastern Dharwar Craton, India, which contains a plethora of ultra-high-pressure minerals such as coesite, majoritic garnet, and supersilicic K-rich omphacite. The presence of these ultra-high-pressure minerals is confirmed by in situ X-ray diffractometry, laser Raman spectra and electron probe microanalysis. The presence of coesite undisputedly pinpoints a subduction origin for the eclogite at ∼2.8 GPa pressure, which corresponds to ∼100 km depth. The geothermobarometric estimations involving garnet-omphacite-kyanite-coesite reveal that such an eclogitic assemblage equilibrated at ∼5-8 GPa (∼175-280 km) pressure during ultra-deep subduction. The textural relationship between omphacite, coarse-grained garnet and majoritic garnet coupled with the laser Raman spectra and geobarometric estimations obtained from the majoritic garnet demonstrate that the majoritic garnet formed at ∼8-19 GPa (∼280-660 km) owing to disassociation of omphacite and coarse-grained garnet to majoritic garnet during increment of pressure up to the mantle transition zone. Thus, the mineralogical and geothermobarometric data suggest that the studied eclogite possibly travelled down to the mantle transition zone before it was rapidly carried up by a pre-Mesoproterozoic mantle plume, and subsequently entrained as a xenolith by the Mesoproterozoic (∼1.1 Ga) kimberlite. © The Author(s), 2023. Published by Cambridge University Press.
Mineral chemistry of biotite and hornblende from mesoproterozoic quartz syenite intrusions of the Cuddapah Intrusive Province, Eastern Dharwar Craton, India: implications for their source characterization
(Springer, 2023) Sridhar Nalluri; Mallikarjuna Reddy Ragi; Sesha Sai Valivetti; Rohit Pandey
Through petrological and mineral chemistry studies of the mafic hydrous phases of the Proterozoic quartz syenites along the NE margin of Cuddapah Intrusive Province (CIP) of the Eastern Dharwar Craton (EDC), we present the nature of the magmatic sources of the Mesoproterozoic Purimetla quartz syenite (PUQS) and the Vikurthi and Kottappakonda quartz syenites (VKQS). Earlier studies suggested that these rocks evolved from subalkaline magmas. However, the present study, based on amphibole and biotite mineral chemistry, demonstrates the contrasting magmatic sources of the PUQS and VKQS. Results of electron probe micro-analyzer (EPMA) chemical analyses indicate that the calcium amphiboles in the PUQS are ferro-pargasite-hastingsite and edenite in the VKQS. Biotite is Fe-rich (annite) in the PUQS and Mg-rich (phlogopite affinity) in the VKQS. Bulk rock, amphibole, and biotite geochemical ratios [e.g. MgO/(FeOT + MgO); with FeOT = total Fe oxide assuming all Fe to be Fe2+] demonstrate the ferroan (PUQS) and magnesian (VKQS) nature of the respective magmas. Though both the plutons show variations in SiO2, FeO, MgO, and CaO contents, they present similarities in total alkalies. The mineral chemistry of the mafic hydrous minerals indicates the alkaline nature of PUQS and the subalkaline nature of the VKQS magmas. Temperature estimates for the crystallization of the PUQS and VKQS, based on amphibole Al2O3 and TiO2 contents, are 840–800 °C and 780–650 °C respectively. Lower-temperature estimates were obtained for the crystallization of biotite in the PUQS (760–730 °C) and VKQS (730–650 °C). Overall pressure estimates, based on Al in hornblende barometry, suggest that the PUQS formed at ~ 6–9 kbar and the VKQS at ~ 3– 6 kbar. Furthermore, the oxygen fugacity (fO2) estimates derived from amphibole chemistry suggest reduced conditions of crystallization for the PUQS and oxidized conditions for the VKQS. Likewise, the fO2 estimates using biotite Fe2+, Fe3+, and Mg contents also indicate reduced conditions for the PUQS (close to the QFM – quartz-fayalite-magnetite buffer) and oxidized conditions for the VKQS (NNO – nickel-nickel oxide buffer). Based on bulk-rock and mineral chemistry data integration, we envisage that the PUQS evolved from anorogenic alkaline magmas, while the VKQS evolved from an orogenic, calc-alkaline to subalkaline magmas. Finally, the thermobarometric data (amphibole and biotite) for the two quartz syenites correspond to a lower crustal level crystallization for the PUQS (~ 30 km) and upper crustal level crystallization for the VKQS (10–15 km). © 2022, The Author(s), under exclusive licence to Springer-Verlag GmbH Austria, part of Springer Nature.
Mineral chemistry of chrome-diopside bearing lamprophyre from Mesoproterozoic Settupalle igneous complex of Prakasam alkaline province: insights on the magma dynamics at shallow lithospheric mantle beneath the NE margin of the Cuddapah basin, Southern India
(Geological Society of London, 2025) Nalluri Sridhar; Ragi Mallikarjuna Reddy; Rohit Pandey; Sourav Naskar; Kirtikumar R. Randive; Kumari Minu Singh
The Eastern Dharwar Craton shows considerable variations in the activity of kimberlite, lamproite and lamprophyre and their related lithospheric thickness, thermal structure and magmatic source characteristics from the innermost core to the margin. These variations have been revealed through seismic studies and the analysis of xenoliths, and xenocrysts carried in alkaline magmas, particularly kimberlites. This study reports the occurrence of chrome-diopside phenocrysts from a lamprophyre of the Settupalle pluton, located on the northeastern margin of the Cuddapah basin, Prakasam alkaline province in South India. Studied lamprophyre are characterized by phenocrystic clinopyroxene (Cpx) and biotite, embedded in a groundmass of amphibole, biotite, opaques, calcite, feldspar, nepheline and sodalite, with the latter minerals forming ocellar structures. Clinopyroxene displays normal zoning, with a high Mg–Cr rich diopside core (up to 1.92 wt%) and a Na2O-, Al2O3, FeO and TiO2-enriched salite rim. These chemical variations, along with elemental substitutions from core to rim, indicate the significant role of fractionation in the evolution of lamprophyre magma. The lack of chemical disparity among the studied Cpx suggests closed system fractionation, under high oxygen fugacity conditions. Thermobarometry results indicate polybaric crystallization for the Cpx (1206–1269°C, 11.82–18.04 kbar) and biotite (825–856°C, 3.49–6.94 kbar). The gradient inferred from the pressure of the Cpx suggests that its crystallization occurred at a depth of 43–66 km, implying shallow lithospheric magma contribution for the genesis of the lamprophyre magma. © 2024 The Author(s). Published by The Geological Society of London. All rights, including for text and data mining (TDM), artificial intelligence (AI) training, and similar technologies, are reserved. For permissions: https://www.lyellcollection.org/publishing-hub/permissions-policy. Publishing disclaimer: https://www.lyellcollection.org/publishing-hub/publishing-ethics.
Mineral chemistry-perspective of the Kadavur gabbro-anorthosite Complex, Southern Granulite Terrane of the Indian shield: Implications to its petrogenesis in a ridge-subduction setting
(Elsevier B.V., 2025) Debaleena Sarkar; Jyotisankar S. Ray; Rohit Pandey; Moumita Chowdhury; Christian Koeberl; Suresh Chandra Patel; Desikan Ramakrishnan; Aantarica Chakraborty; Simran Dutta
Mineral chemistry data of constituent silicate and oxide phases are often regarded as a useful tool for assessing the petrogenesis of mafic rocks. In view of this, the chemistry of minerals from the Kadavur gabbro-anorthosite Complex (10°35′N: 78°11′E), a magmatic intrusion in the Southern Granulite Terrane of the Indian shield, has been evaluated using several thermo-barometric methods and tectonic discrimination diagrams. The Complex represents a magmatic intrusion that consists of a highly deformed schistose gabbro-anorthosite type and an undeformed layered gabbro-anorthosite type, with local patches of pegmatoidal gabbro-anorthosite bodies. The constituent minerals in the Complex include clinopyroxene, orthopyroxene, plagioclase, amphibole, and subordinate amounts of ilmenite and magnetite. Pyroxene thermometry (clinopyroxene thermometry, orthopyroxene thermometry and two pyroxene thermometry) gives mean temperatures of ∼1060 °C for the layered gabbro- anorthosite bodies, and ∼ 1124 °C for the pegmatoidal bodies; and ∼ 1130 °C for the schistose gabbro-anorthosite. Hornblende-plagioclase and amphibole thermometry give temperatures of ∼1000–1190 °C. The co-existing oxide (magnetite-ilmenite) thermometry gives a lower temperature of ∼420 °C. The mean clinopyroxene pressure value is 17 kbar for the schistose gabbro-anorthosite, 9 kbar for the layered gabbro-anorthosite, and 11 kbar for the pegmatoid bodies. During the earlier phase of deformation associated with magmatic crystallization (responsible for schistose type), the ambient temperature and pressure of crystallization of the Complex were 955 to 1285°C at ∼17 kbar. The magma equilibrated at shallow to intermediate levels allowing for localized input of H2O. Consideration of whole-rock geochemical data (especially immobile trace elements) indicates that parent magma was of tholeiitic and alkaline composition and later underwent fractionation. The rare earth element (REE) distribution in the schistose gabbro-anorthosite is broadly similar to that of N-MORB, while the geochemical characteristics of the layered gabbro-anorthosite resemble island arc basalt (IAB). The pegmatoidal gabbro-anorthosite bodies have a transitional affinity between both the N-MORB and IAB. The mineral chemistry and whole-rock geochemistry data suggest that the schistose gabbro-anorthosite mostly corresponds to “non orogenic” and “MORB-type” while layered gabbro-anorthosite and pegmatoid bodies correspond to an ‘orogenic’ and ‘island-arc’ setting, suggesting a clear shift from a MORB setting to an arc-setting suggesting a ridge-subduction event. The Kadavur gabbro-anorthosite Complex is analogous to other well-known Archaean gabbro-anorthosite Complexes around the world. © 2024 Elsevier B.V.
Mineralogy and petrology of lamprophyre and dolerite dykes from the end-Cretaceous (~ 66 Ma) Phenaimata alkaline igneous complex, north-western India: evidence for open magma chamber fractionation, mafic recharge, and disaggregation of crystal mush zone in a large igneous province
(Springer, 2023) Abhinay Sharma; Rohit Pandey; Nittala V. Chalapathi Rao; Samarendra Sahoo; Boris V. Belyatsky; Prashant Dhote
The end-Cretaceous (ca. 66 Ma) Phenaimata alkaline igneous complex, associated in space and time with the Deccan large igneous province (LIP) in Western India, consists of bimodal (tholeiitic to alkaline) differentiated plutonic to volcanic igneous rocks. Mineralogy and petrology of variably fractionated alkaline lamprophyre and dolerite dykes of the complex are the focus of this study. The two lamprophyre dykes (termed as camptonite-I and camptonite-II) which intrude the host basalt/olivine dolerites and gabbro, differ in their liquidus minerals and crystal size distribution. Their respective rare earth element (REE) and trace element patterns suggest the lamprophyres to be genetically related, with the camptonite-II being relatively more evolved than that of the camptonite-I. Binary mixing model involving trace elements as well as the Sr˗Nd isotopic data in case of the camptonite-I, brings out involvement of crustal contamination in the generation of the lamprophyres. The two dolerites mostly consist of the liquidus phases, with the exception of olivine antecrysts created during early stages of the host gabbro formation but subsequently inherited into their magma. In terms of their trace element composition, the dolerites of this study show a strong similarity with that of the alkali basalts of the complex. The normal compositional zoning of pyroxene and amphibole from the lamprophyres shows that their parental magma initially experienced a closed system fractionation to form a large crystal mush zone and subsequently developed a smaller magmatic chamber where biotites of the camptonite-II first crystallised. Reverse zoning, resorption of crystals, disparity in crystal size distribution and their composition also reveals that a newer batch of magma was introduced from the feeding zone into the initially developed crystal laden magmatic chamber. This replenishment led to the disaggregation of crystals from the mush zone and the resulting magma subsequently evolved to form camptonite-I. Later on, the successive lateral spreading of the newly generated magma increased its buoyancy to rise through the smaller crustal chamber (with biotites) to generate camptonite-II. Our study demonstrates not only the operation of diverse open and closed system processes such as fractionation, replenishment and mush capturing that have occurred within inter-connected plumbing magmatic chambers beneath the alkaline complexes associated with the flood basalt volcanism in large igneous provinces but also their significant role in influencing the ultimate composition of the associated diverse rocks. © 2021, The Author(s), under exclusive licence to Springer-Verlag GmbH Austria, part of Springer Nature.
Multi-stage enrichment in the mantle source region of cratonic lamproites: Geochemical and Sr-Nd-Hf-Os isotopic insights from the Mesoproterozoic lamproites of Bastar Craton, India
(Elsevier B.V., 2025) Rohit Pandey; Mahendra Kumar Singh; Ashutosh Pandey; Nittala V. Chalapathi Rao; Boris V. Belyatsky
Deep cratonic mantle-derived magmatic rocks such as lamproites provide significant insights into the composition and evolution of the sub-continental and sub-lithospheric mantle. This study presents SHRIMP U-Pb geochronology of rutile and bulk-rock geochemistry including Sr-Nd-Hf-Os isotopes on lamproite dykes from the diamondiferous Nuapada Lamproite Field (NLF) at the tectonic contact between Bastar Craton and Eastern Ghat Mobile Belt, India. The lamproites sampled from the Darlimunda cluster of NLF exhibit inequigranular-porphyritic texture with phenocrysts of olivine pseudomorphs in a groundmass rich in phlogopite. The abundance of low-temperature secondary phases such as vermiculite, chlorite, and carbonates in these rocks indicate significant post-magmatic alteration. The presence of quartz crystals and micro-veins along with zircon megacrysts indicate crustal assimilation to some extent but a strong enrichment in high-field strength elements buffers against significant crustal contamination. Rutile U-Pb geochronology reveals an emplacement age of 1009 ± 121 Ma, which is consistent with the (i) previously reported whole-rock 40Ar/39Ar age of lamproites from the NLF, and (ii) widespread ∼1.05 Gyr lamproite, kimberlite, and ultramafic lamprophyre magmatism in the Eastern Dharwar and Bastar Cratons, India. Moderate initial 87Sr/86Sr (0.703–0.706) and unradiogenic initial Nd-Hf isotopic (ƐNd of −5.96 to −8.08 and ƐHf of −7.25 to −9.02) composition is similar to the global cratonic lamproites and enriched mantle (EM I type) mantle reservoir. Moderately radiogenic and restricted initial 187Os/188Os (0.189–0.257) with low Os content, unlike that of the kimberlites and sub-continental lithospheric peridotites, show similarity to the Carboniferous orogenic Variscan lamproites from Bohemian massif and Neogene to Quaternary orogenic ultrapotassic mafic rocks from Italian peninsula. Based on the Sr-Nd-Hf-Os isotopic results, we propose that the cratonic lithospheric mantle source of these rocks record the evidence of a long-term enrichment in lithophile elements as a result of an ancient recycled crustal component. Elevated high-field strength elements in the lamproites and kamafugites of the NLF, similar to those of the global cratonic lamproites and ocean island basalts, represent an overprinting of ancient subduction-related metasomatized deep sub-continental lithospheric mantle sources by small-scale carbonatitic melts derived from the convecting sub-lithospheric mantle prior to their melting. Vein-plus-wall-rock mantle as a result of multi-stage mantle enrichment appears to play an important role in the origin of cratonic lamproites. © 2025 Elsevier B.V.
Neoarchean crust–mantle interactions from the Eastern Dharwar Craton: Insights from mineral chemistry of the Nizamabad granites, southern India
(Springer, 2022) Balaboina Vikram Raju; Ajay Dev Asokan; Rohit Pandey; Arathi G Panicker; M Ram Mohan
Abstract: We present field, petrographic and mineral compositions of biotite, amphibole, and feldspars from Neoarchean Nizamabad granites from the northeastern part of the Eastern Dharwar Craton, southern India. These granites are classified as hornblende biotite granite (HBG), biotite granite (BTG), monzogranite (MG), and microgranular enclaves (ME) hosted in HBG and BTG. The temperature estimates using amphibole and biotite thermometry exhibit similar results, with higher temperatures for HBG (818 to 859 ±22°C) and ME (800 to 855 ±22°C), and slightly lower temperatures for BTG and MG (829 to 830 ±12°C and 820 to 829 ±12°C). Based on barometry, HBG amphiboles crystallized at pressures between 363 and 448 ±60 MPa (avg. PHBG = 398 MPa), whereas the MEs crystallized at pressures between 313 and 438 ±60 MPa (avg. PME = 386 MPa). The estimated pressures suggest that these granites crystallized at depths of 14–15 km, corresponding to the upper to mid-continental crust. The amphibole compositions reveal that these granites crystallized from a water-rich magma, with >5 wt.% H2O and evolved under high oxidizing conditions NNO + 2 (Nickel–Nickel–Oxide), corresponding to magnetite (oxidized) series granites. The amphibole and biotite compositions suggest a crust-mantle mixed source for HBG, ME, and BTG, while the MG is purely crustal derived. The water-rich and highly oxidizing conditions of the parental magmas rule out a lower crustal granulitic source for the Nizamabad granites. The amphibole and biotite compositions suggest their crystallization from calc-alkaline parental magma in a subduction setting at high oxygen fugacity (fO2) conditions. This study infers the role of convergent margin tectonics in the emplacement of these granites, and their compositional variability is attributed to crust–mantle interactions in this domain of the Eastern Dharwar Craton. Research Highlights: The granitoids from the NE part of the Eastern Dharwar Craton are characterized as hornblende biotite granites (HBG), biotite granites (BTG), monzogranite (MG), and mafic enclave (ME).Estimated pressures suggest that these granites crystallized at depths of 14 to 15 km, corresponding to the upper to mid-continental crust.The water-rich (> 5 wt.% H2O content) and high oxidizing conditions (NNO+2) of HBG, BTG, and ME corresponds to magnetite (oxidized) series granites.The amphibole and biotite compositions from the Nizamabad granites suggest the crust-mantle mixed source for HBG, ME, and BTG, while the MG are purely crustal derived.The compositions of these granites suggest their crystallization from calc-alkaline parental magma in subduction settings at high oxygen fugacity (fO2) conditions.The study infers the role of convergent margin tectonics in the emplacement of compositionally variable granitoids in the NE part of the Eastern Dharwar Craton. © 2022, Indian Academy of Sciences.
Origin and evolution of vanadium-rich oxide and titanite phases in lamprophyre and mafic dykes from the Nuapada diamondiferous Lamproite field, Bastar Craton-Eastern Ghats Mobile belt contact, India: Metallogenic and geodynamic implications
(John Wiley and Sons Ltd, 2023) Rohit Pandey; Mahendra Kumar Singh; N. V. Chalapathi Rao
Transitional boundaries of the craton and mobile belt mark diverse and long-term tectono-thermal events like hot and cold lithospheric contact, back-arc-type subduction, and also constitute a favourable locale for metallogeny. The exact nature and origin of such metallogenic enrichment in the lithospheric mantle source beneath contact zones are still uncertain. The contact of Bastar Craton and the Eastern Ghat Mobile Belt (EGMB), India, provides an ideal geological setup to study the metallogenic enrichment processes. In this study, we present a detailed petrographic and mineralogical investigation of various iron oxide phases and titanites in spatially and temporally distinct lamprophyre and mafic dykes from the diamondiferous Nuapada Lamproite Field (NLF) at the contact between Bastar Craton and EGMB. Diverse textural assemblages of oxides, namely, trellis, sandwich-type exsolutions and reaction–replacement, are widespread. Ilmenites, magnetites and titanites from the mafic dykes as well as lamprophyre exhibit an anomalous enrichment of vanadium (V2O3 up to 3.7 wt%), whereas ilmenites are also characterized by their elevated manganese content (MnO up to 3.8 wt%). Mineral chemistry-based plots and thermo-oximetric studies suggest the magmatic origin of iron oxides belonging to the Fe–Ti–V-type ore in which partitioning and sequestration of the elements were controlled by differential crystallization, exsolution and fractionation rather than any imprints of contamination. Multiphase sequestration of vanadium as well as manganese in lamprophyre and mafic dykes, separated in space as well in relative depths of origin, signifies the presence of a fertile mantle. The role of subduction-derived metasomatized fluids as a causative factor of vanadium metallogeny in this domain is evaluated. Low-Mg and high-Mn ilmenites of this study are consistent with the reported occurrences of diamonds from the NLF and adjoining areas of the Bastar Craton. © 2023 John Wiley & Sons Ltd.
Petrogenesis of end-Cretaceous/Early Eocene lamprophyres from the Deccan Large Igneous Province: Constraints on plume-lithosphere interaction and the post-Deccan lithosphere-asthenosphere boundary (LAB) beneath NW India
(Elsevier B.V., 2019) Rohit Pandey; Ashutosh Pandey; N.V. Chalapathi Rao; B. Belyatsky; A.K. Choudhary; B. Lehmann; Dinesh Pandit; Prashant Dhote
We present petrology, geochemistry and radiogenic isotope (Sr and Nd) data of thirteen post-Deccan lamprophyre dykes in the Narmada rift zone from the Chhotaudepur alkaline province of the Deccan Large Igneous Province (DLIP). Mineralogically, these dykes show affinity towards alkaline (sannaite and camptonite) as well as ultramafic (damtjernite) varieties of lamprophyres. Their major oxides and certain trace element ratios increase with increasing silica content highlighting the strong influence of fractionation processes. Their Nb/U and Ce/Pb ratios are similar to the mantle array defined by MORBs and OIBs and suggests an uncontaminated nature. Major oxide (K2O, Na2O, SiO2 and TiO2) contents show geochemical similarity towards shoshonitic volcanic series, whereas elevated Zr/Hf and Nb/La coupled with suppressed Rb/Nb and Zr/b display their affinity towards HIMU-type intraplate basalts. Their radiogenic initial 87Sr/86Sr (0.706034–0.710582) and sub-chondritic initial ɛNd (−8.6 to 2.1) are akin to those of the (i) ca. 65 Ma Ambadongar carbonatite, NW India, and (ii) ca. 65 Ma orangeites from Bastar Craton, central India, highlighting an enriched lithospheric mantle source. REE inversion modeling suggests ~3% enrichment of an undepleted mantle followed by small degrees of melting of this enriched mantle source are sufficient- as in the case of ocean island basalts (OIB)- to reproduce their observed REE concentrations. Their TDM Nd model ages (564–961 Ma) are consistent with widespread convergent margin-related magmatism during the amalgamation of the Rodinia supercontinent. We propose that enriched lithospheric mantle developed during the Neoproterozoic was metasomatized by small-volume CO2-rich melts imparting a HIMU-type geochemical character during Late Cretaceous, when the mantle plume (viz., Réunion) responsible for the flood basalt eruption, impinged at the base of the NW Indian lithosphere. From the presence of F-rich apatite and high K/Rb in mica, we infer the (i) presence of F-phlogopite in their source regions, and (ii) that the depth of post-Deccan lithosphere-asthenosphere boundary (LAB) beneath NW India was at least ~100 km at ca. 65 Ma. © 2019 Elsevier B.V.
Petrogenetic implications of mineral chemistry and mode-based statistical studies of Sholayar alkaline syenite complex, Southern Granulite Terrane, India
(Springer, 2022) Moumita Chowdhury; Jyotisankar Ray; Rohit Pandey; Debaleena Sarkar; Paulomee Guha
Abstract: A recent petrological study on the lesser-known Sholayar alkaline syenite complex (10°17′59.9′′N: 076°47′26.3′′E) accommodated in the high-grade rocks of the Southern Granulite Terrane reveals the presence of three distinct petrographic types namely leuco syenite, melanofelsic syenite and mela syenite. The syenites, in general, have the following constituent minerals, namely alkali feldspar (Or76.8Ab23.2–Or93.4Ab6.6), plagioclase (An4.1–An17.3), calcic amphibole (edenitic hornblende/ferropargasitic hornblende), quad pyroxene (diopside–hedenbergite), biotite and opaque minerals (magnetite and ilmenite). Mode-based statistical studies indicate that a highly significant correlation exists among certain mineralogical parameters including quartz%, total feldspar%, colour index, alteration index and hydration index. Isopleth plots of these parameters strongly suggest maintenance of a magmatic regime throughout, with increasing water content towards the end stage of crystallization. Several geothermobarometric methods point to shallow to moderate level emplacement (~9–23 km) of the alkaline/sub-alkaline syenite magma which was facilitated by a ‘hydration event’. The steep change in water gradients (in localized pockets) of the syenite intrusives has been corroborated by textural evidences. Research highlights: 1.First systematic petro-mineralogical report of Sholayar alkaline syenite complex from SGT.2.Involvement of single magmatic lineage with variable water content.3.Geothermobarometry suggests a moderate to shallow level of syenite emplacement. © 2022, Indian Academy of Sciences.
Petrology and Geochemistry of a Calc-alkaline Lamprophyre-Diorite Dyke from Dancherla Area, in the Vicinity of Wajrakarur Kimberlite Field, Eastern Dharwar Craton
(Geological Society of India, 2025) Ashutosh Pandey; Rohit Pandey; Aparajita Tripathi; Manavalan Satyanarayanan; Aradhi Keshav Krishna; Mahendra Kumar Singh; Deepak Kumar; Kumari Minu Singh
The Eastern Dharwar Craton hosts several Proterozoic volatile-rich mafic-ultramafic alkaline intrusive magmatic rocks. Geological and geochemical studies on such rocks provide significant insights into the nature of the continental lithospheric/sub-lithospheric mantle and Precambrian geodynamic processes. In this study, we present petrology and geochemistry of a newly found calc-alkaline lamprophyre-diorite dyke from Dancherla area in the vicinity of the Wajrakarur Kimberlite Field of the Eastern Dharwar Craton. The lamprophyre consists of a high modal abundance of amphibole (magnesio-hornblende) phenocrysts and microphenocrysts along with clinopyroxene (diopside) microphenocrysts set in a feldspathic groundmass. The inequigranular texture of the lamprophyre gradually transitions into the equigranular (salt and pepper-like) texture of diorite along the trend of the dyke, with amphiboles and feldspar in almost equal proportions in the latter. Changing mineral mode and texture within the dyke points to the derivation of these rocks by crystal fractionation from the same hydrous parental melt. The amphibole and clinopyroxene-dominated crystal fractionation are evident from clustered phenocrysts of these minerals in the lamprophyres, giving rise to the glomeroporphyritic texture. As CaO is preferentially incorporated in clinopyroxene and amphibole, the decrease in CaO contents and CaO/Al2 O3 ratios with decreasing MgO, an index of differentiation supports fractional crystallization of clinopyroxene and amphibole. The predominance of calcic amphiboles, elevated Th/Yb and Zr/Y ratios, and significant depletion of high field strength elements suggest a calc-alkaline nature of the Dancherla lamprophyre. High LILE/HFSE in these rocks, like that found in the global calc-alkaline lamprophyres and arc-related magmatic rocks, reveals a subduction-fluid-aided enriched continental lithospheric mantle source of the parental magma. The timing of enrichment of the mantle source is suggested to be Neoarchean, during which the hot oceanic lithospheres accreted to the Western Dharwar Craton leading to the amalgamation of micro-blocks and cratonization of the Dharwar Craton. © 2025 Geological Society of India, Bengaluru, India.
Petrology and geochemistry of the diamondiferous Jamnidih occurrence, Bastar Craton, Central India: Metabasalt and not a kimberlite
(Elsevier B.V., 2022) Mahendra Kumar Singh; Rohit Pandey; Abhinay Sharma; N.V. Chalapathi Rao
We have re-visited the ‘diamondiferous kimberlite’ from the Jamnidih area of Mahasamund, Bastar Craton, Central India. This study presents detailed field observations, petrographic and geochemical studies of Jamnidih rocks in view of the reported occurrence of diamonds from them. Megascopically, the Jamnidih rocks are fine grained, yellowish green colour and fragile in nature and lack any visible phenocrysts or xenocrysts. Petrography reveals that chloritization and sericitization has altered much of the primary mineralogy. Feldspar and pyroxene are the identifiable major phases with imprints of alteration and development of feeble foliations at places. Lack of inequigranular texture (two generations of olivine) and kimberlitic indicator minerals viz. chromium- and pyrope-rich garnets, chrome-bearing diopsides, and magnesian-ilmenites are characteristic of Jamnidih rocks. This aspect is also reflected in the geochemistry of these rocks which is entirely different from that of the well-established kimberlites (Wajrakarur field, Eastern Dharwar Craton, southern India), orangeites (Behradih orangeites, Bastar Craton, central India) and transitional kimberlites (Tokapal kimberlite, Bastar Craton, central India). Geochemically the Jamnidih rocks display silica-saturated, and alumina-rich nature and depletion of Ba, Co, Ni, V, Nb, Hf and other metasomatically mobilised elements. Petrography (lack of inequigranular macrocrystic texture), mineralogy (paucity of kimberlite typomorphic phases) and bulk-rock geochemistry (extreme impoverishment of high field strength elements and presence of negative Eu anomaly in chondrite-normalized plot) testify them to be meta-basalts and exclude their affinities to the bonafide Group I or II or even transitional varieties of kimberlites. In this context, the reported occurrence of microdiamonds in these rocks is clearly anomalous and assumes significance. © 2021 The Author(s)