Browsing by Author "Bikash Mahanta"

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Geochemical and geochronological evolution of the UHT granulites from the Eastern Ghats Mobile Belt, India: implications from F-OH substitution in biotite, thermal events and cooling age
(Taylor and Francis Ltd., 2024) Saurabh Singh; Divya Prakash; Roopali Yadav; Manish Kumar; Bikash Mahanta; Jesús Solé Viñas
The thin lenses and bands of ultra-high temperature (UHT) granulites with diagnostic sapphirine and quartz in contact are occasionally found on the western flank of the Eastern Ghats Mobile Belt (EGMB), which inherits the suturing and over-thrusting signatures of the EGMB with the Greater Indian Landmass (GIL). The mineral biotite contains an abnormally high fluorine concentration, as revealed by the geochemical examination of these granulites from the research area (Kothuru), which was caused by the F-OH substitution to withstand extreme thermal metamorphic conditions. The geochemical results from the multi-element spider diagram and the Harker’s variation diagram with primitive-mantle normalization also show an enrichment of light rare earth elements and large-ion lithophile elements over depleted heavy rare earth elements and high-field strength elements. An evolutionary history has been established for the high-grade rocks that were subjected to ultra-high temperature metamorphism, based on the geochemical signatures of the examined granulites. The use of isotopic K-Ar cooling ages and electron microprobe monazite chemical ages of the thermal episodes, which act as useful tools to explain the time and span of the polycyclic metamorphic terrane, is another significant component of the current work. Numerous researchers have attempted to estimate the thermal episodes in various areas of the Eastern Ghats Mobile Belt, but their cooling ages are still poorly understood. Consequently, two thermal episodes that happened at 842 Ma and 637 Ma ago, as well as a cooling age of about 448 Ma that marked the quick exhumation of the high-grade rocks under study, have been recorded in the current investigation. The association between the Greater Indian Landmass and Eastern Antarctica during the existence of the Rodinian supercontinent has been established by recorded geochronological events. © 2023 Informa UK Limited, trading as Taylor & Francis Group.
Geochemistry, Monazite (U–Pb–Th) Geochronology, and P-T Pseudosection Modelling of Two-Pyroxene Mafic Granulite from Sonapahar, Shillong Meghalaya Gneissic Complex, India: Implication for Tectono-Metamorphic Evolution and Global Pan-African Correlation
(2024) Bikash Mahanta; Divya Prakash; Manish Kumar; Saurabh Singh; Rajeev Kumar Pandey; Chandra Kant Singh; Suparna Tewari
Abstract: Granulites exposed in high-grade regional metamorphic belts and exhumed as xenoliths in basaltic pipes are considered as window into the deep crust thus play a key role in constraining models of crustal processes and evolution. Here we present a detailed investigation of the tectono-metamorphic history of the two-pyroxene mafic granulite located in the southern region of the Sonapahar area. This involves conducting monazite chemical dating, analyzing petrological and geochemical characteristics, applying geothermobarometry, performing phase equilibria modeling, and tracing a pressure-temperature (P-T) path. Metamorphic P-T conditions estimated for the mafic granulite using conventional thermobarometer and winTWQ shows temperature in excess of 800°C and pressure of about 8.6 kbar, stand for high temperature granulite facies metamorphism. The metamorphic evolution path obtained from P-T pseudosection suggest a clockwise P-T evolution path, thus signify isothermal decompression and indicate rapid upliftment. Geochemical study of trace and rare earth elements (REE), suggest protolith is of tholeiite basalt in nature that is derived from back arc basin setting near to subduction zone. Additionally, the analyzed rock was examined using primitive mantle-normalized trace element spider diagram. The results indicate an enrichment in large-ion lithophile elements (Th, U, K, Pb) and a depletion in high field-strength elements (Nb, Ta, Ti). The presence of negative anomalies in Nb and Ti, coupled with elevated values of Th, K, and Pb, suggests the possibility of crustal contamination. Monazite chemical data from the studied rock reveals a peak metamorphism age of 521.3 ± 4.20 Ma, which corresponds to the Kuunga Orogeny in the later phase of global Pan-African collision. © Pleiades Publishing, Ltd. 2024. ISSN 0016-7029, Geochemistry International, 2024, Vol. 62, No. 6, pp. 574–608. Pleiades Publishing, Ltd., 2024. ISSN 0016-7029, Geochemistry International, 2024. Pleiades Publishing, Ltd., 2024.
Metamorphic evolution of sapphirine- and sodicgedrite-anorthite-bearing granulites, Rampur domain, Eastern Ghats Province, India
(Cambridge University Press, 2025) Rajeev Kumar Pandey; Divya Prakash; Saurabh Kumar Singh; Bikash Mahanta; Chandrakant Singh; Kamesh Sharma; Manish Kumar; Mahendra Kumar Singh
The Eastern Ghats Belt (EGB) has been extensively studied by the geoscientific community; however, this communication reports unique mineral assemblages that have not been documented previously. This study documents the occurrence of sapphirine, spinel, orthopyroxene, sodic-gedrite, calcic-amphibole, biotite and plagioclase assemblage indicating in ultrahigh temperature (UHT) metamorphic conditions. The significance of this study lies in the peculiarity of sapphirine being present within anorthite matrix which has been reported for the first time from the Indian subcontinent. The studied assemblage has been correlated with the more or less similar assemblage of rock called 'Sakenites' reported from southern Madagascar to correlate the most probable source rock 'anorthosites' that underwent metamorphic transformations and led to the unique UHT mineral assemblage. The Na-rich gedrite identified within the assemblage represents a relict mineral indicative of high-grade amphibolite-facies metamorphism. The derived pressure-temperature (P-T) trajectory reveals a decompression path with almost uniformly decreasing P-T conditions in contrast to the commonly reported isothermal decompression (ITD) path from various other domains and provinces of the EGB. The corresponding retrograde assemblage has been recalibrated by the sequential removal of sapphirine and corroborated with T-X (H2O) constraints. The analyzed EMP U-Th-Pb monazite chemical age constraints suggest mesoproterozoic to neoproterozoic episodes corresponding to a pair of ∼959 Ma and ∼846 Ma thermal events. These metamorphic events have been correlated to reconstructing the Rodinian supercontinent at ∼959 Ma and the initiation of its subsequent break-up at ∼846 Ma. © The Author(s), 2025. Published by Cambridge University Press.
Metamorphic P-T evolution and tectonic implications of UHT metamorphism from the Shillong Meghalaya Gneissic Complex, India: evidence from phase equilibria modelling, monazite U-Pb-Th geochronology, K-Ar dating and geochemistry
(Cambridge University Press, 2024) Bikash Mahanta; Divya Prakash; Saurabh Singh; C.K. Singh; Manish Kumar; Rajeev Pandey; Suparna Tewari
The study area Sonapahar is an integral part of Shillong Meghalaya Gneissic Complex (SMGC), which is located in the Northeastern part of India. This complex mainly comprises metamorphic formations spanning from Upper Amphibolite to Ultra-high temperature granulite, interspersed with various igneous intrusions. In this study, particular attention is directed towards unravelling the metamorphic history of Mg-Al granulite. For the very first time, we establish the pressure–temperature (P-T) trajectory of the Mg-Al granulite from Sonapahar, SMGC. Employing conventional thermobarometry along with winTWQ analysis, the inferred metamorphic conditions for this granulite reveal temperatures exceeding 900°C and pressures of approximately >8 kbar. These conditions firmly indicate the presence of ultra-high-temperature metamorphism. By utilizing the Perple_X software in the MnO-Na2OCaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-Fe2O3 compositional system, we construct a P-T pseudosection. This gives a clockwise P-T path, signifying an episode of cooling (þ minor decompression). Such a pattern also suggests rapid cooling of the tectonically-thickened crust. Concurrently, a geochemical exploration of trace and rare earth elements in the rocks offers further insights. These investigations give an idea about the protolith, having a clay-to-sandstone in nature. Additionally, chemical data from monazite within the studied rock provide a weighted mean age of 682 Ma for the peak metamorphic stage. This age aligns with the global Pan-African orogenic events. The biotite K-Ar isotopic geochronology from the symplectite position provides decompression history or cooling age of 442 Ma. This age corresponds to a period after the last peak metamorphic phase that occurred during the Pan-African thermal event. © The Author(s), 2024.
Phase equilibria modelling, fluid inclusion study, and U-Pb zircon dating of ultra-high temperature mafic granulites from Rampur domain, Eastern Ghats province: implications for the Indo-Antarctic correlation
(Springer Science and Business Media Deutschland GmbH, 2025) Divya Prakash; Rajeev Kumar Pandey; Saurabh Kumar Singh; Chandrakant Singh; Manish Kumar; Bikash Mahanta; Aditya Kharya; Himanshu Kumar Sachan; Kamesh Sharma
The study area (Rampur domain) is situated to the east of the Eastern Ghats Boundary Shear Zone (EGBSZ) and encompasses portions of the granulite facies rocks of the exhumed Proterozoic Eastern Ghats Province (EGP), India. The EGP is characterized by a diverse array of rock types, featuring a wide variety of mineral parageneses and chemical compositions, including charnockite, mafic granulite, Mg-Al granulite, felsic granulites, amphibolite, khondalite and anorthosite. In this study, we report for the first time evidence of ultra-high temperature (UHT) metamorphism within the mafic granulites of the relatively unexplored Rampur domain of the Eastern Ghats Province, using the two-pyroxene assemblage. The stable mineral assemblage present during peak metamorphism typically includes garnet, orthopyroxene1, clinopyroxene, hornblende1, quartz, and plagioclase1. The consumption of garnet observed in different reaction textures, alongside the formation of striking orthopyroxene2–plagioclase2 and hornblende2–plagioclase2 symplectites, represent the later phases of metamorphism. By applying TWQ calculation procedures to the mineral core compositions, we have determined peak metamorphic conditions of approximately 970 °C at a pressure of 10.5 kbar. Zircon dating results from LA-HR-ICP-MS indicate upper intercept ages of 2509.9 ± 21.7 Ma and 2479.9 ± 21.0 Ma for the protolith, while lower intercept ages of 965.7 ± 40.7 Ma and 979.8 ± 18.1 Ma correspond to the metamorphic age of the analyzed samples E-185 and E-186, respectively. Based on the textural relationship, derived zircon ages, fluid-P-T constraints, and P-T pseudosection model, we propose a decompressional evolutionary P-T-t path that supports the Neo-Proterozoic assembly of the Indo-Antarctic region. © The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature 2025.
Ultrahigh-temperature granulites from the Shillong-Meghalaya Gneissic Complex, NE India: Implications for the Indo-Antarctic Correlation
(Elsevier Ltd, 2025) Bikash Mahanta; Divya Prakash; Manish Kumar; Saurabh Kumar Singh; Rajeev Kumar Pandey; Chandrakant Singh; Roopali Yadav; Jesus Solé Vinas
The Shillong-Meghalaya Gneissic Complex (SMGC) in the Riangdo region, northeastern India, is mainly composed of metamorphic rocks from upper amphibolite to ultrahigh temperature (UHT) granulite facies with several igneous intrusions. The pelitic granulite comprises biotite, garnet, K-feldspar, sillimanite, spinel, quartz, and biotite. This study is the first to report ultrahigh-temperature (UHT) metamorphism in the Riangdo (Sonapahar) block of the SMGC. Metamorphic pressure–temperature conditions estimated from the Spinel + Quartz bearing pelitic granulite using conventional thermobarometer (THERMOCALC) and pseudosection modelling in the MnO-Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 system are more than 900 °C and pressure of about 8 kbars, representing UHT metamorphism. The sequence of reactions, constructed from the post-peak textural relationship, along with petrogenetic grid and pseudosection modelling, records a clockwise P–T evolution. This indicates an isothermal decompression path associated with rapid uplift before cooling of the tectonically thickened crust. Available geochronological ages suggest the presence of widespread Pan-African tectonothermal events in the SMGC. The K-Ar isotopic ages obtained from biotite suggest a cooling age of 407.20 ± 3.49 Ma. Given the general acceptance of UHT with clockwise isothermal decompression in Pan-African age metamorphism in the East-African-Antarctic Orogen (EAAO) and Eastern Ghat Mobile Belt (EGMB), the Sonapahar UHT metamorphic history is considered to be part of this record. © 2024 Elsevier Ltd