Browsing by Author "Kumari Minu Singh"

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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.
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.
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.
Phase Chemistry and Formational Conditions of Mesoproterozoic Gabbro and Diabase Dykes from Prakasam Alkaline Province, SE India: Insights into Crystallisation of Fractionated-Oxidised Calc-Alkaline Magmas of Lower-Mid Crustal Magma Chambers
(Geological Society of India, 2025) Nalluri Sridhar; Ragi Mallikarjuna Reddy; Rohit K. Pandey; Vv V.Sesha Sai; Kumari Minu Singh; Pothuri Ramesh Chandra Phani
The texture and composition of rock-forming minerals help in understanding magma chamber dynamics because of their ability to preserve crystallisation conditions and latestage magmatic processes. This study investigates the mineral compositions of co-spatial gabbro and diabase dykes in the Prakasam Alkaline Province (PAP) to reveal their petrogenetic aspects. The studied rocks exhibit a similar mineral assemblage with distinct disequilibrium textures, and their corresponding chemistry indicates the fractionated nature of their parent magma. Crystallisation of clinopyroxene, amphibole, and biotite in the host gabbro and dykes occurred at 1141–682°C at 12.85–0.6 kbar and 1107–753°C at 10.2–0.6 kbar, respectively. The estimates of oxygen fugacity, crystallisation pressures, and temperatures indicate an increase in the oxidation state and PH2O of the magma during the final stages of crystallisation. Disequilibrium textures (skeletal, resorbed, and sieve) and a broad range of crystallisation pressures for individual minerals imply Polybaric/ Polythermal Open system Fractionation Reactions (POFR). Non-isochemical reactions between anhydrous phases and interstitial residual fluids led to the formation of amphibole and biotite coronas, emphasising the impact of hydration on early crystallised minerals at shallow depths. From a tectonomagmatic perspective, the synthesis of mineral chemistry supports the presence of a calc-alkaline magmatic source believed to originate from the Subcontinental Lithospheric Mantle (SCLM). The compositional coherence of minerals from both the host gabbro and dykes suggests a genetic relationship, although they originate from different batches. This study highlights the SCLM-related calc-alkaline mafic magmatism in the PAP, which is associated with Mesoproterozoic rifting. © 2025 Geological Society of India, Bengaluru, India.