Department of Geology

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Browsing Department of Geology by Author "Ghosh, Gautam"

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    Geological evolution of the mafic and felsic granulites from the central and northwestern parts of the Eastern Ghats Province in the context of Grenvillian-age tectonics
    Bose, Sankar; Ghosh, Gautam
    The Proterozoic Eastern Ghats Belt of eastern India is characterized by a complex evolutionary history punctuated by recurring magmatic, deformation and metamorphic episodes. The terrane is also characterized by extreme thermal structure that resulted from Proterozoic accretionary processes involving continental blocks of India and East Antarctica within the supercontinents Rodinia and Columbia. It consists of rocks that had undergone extreme ultrahigh temperature (UHT) metamorphism at deep crustal level. Due to its geological complexity, the terrane has been classified in terms of various crustal provinces and domains, each displaying distinct tectono-metamorphic characteristics. The centrally occurring Eastern Ghats Province (EGP) evolved during the later part of Proterozoic era. The central (Visakhapatnam domain), northern (Chilka Lake domain) and the northwestern (Phulbani domain) parts of the EGP are composed of migmatitic felsic gneiss, aluminous granulite, fine grained charnockite, felsic augen gneiss, mafic granulite and calc-silicate granulites which are metamorphosed to UHT condition. Following the attainment of peak-UHT condition, all these rocks underwent cooling along a near-isobaric trajectory. The dominant lithological units of the EGP are the orthogneissic rocks, represented by felsic gneiss, charnockite and mafic granulite. Compared to the other rock types like the aluminous granulite and calc-silicate granulite, which are mostly paragneissic in character, geological history of the orthogneissic rocks of this province is poorly known. In a granulite terrane, orthogneissic rocks are essential components of the Earth's lower continental crust and they play crucial roles in the orogenic processes. While mafic granulites are associated with the deeper sections of regional granulite belts, granitic rocks typically occur at lower to mid-crustal levels. Charnockite is diagnostic of Precambrian orogenic belts indicating a connection to the subduction-collision tectonics. Despite their prevalence in the EGP, very few studies have been conducted on mafic granulite, charnockite and felsic gneisses, posing a significant gap in knowledge. This study aims to investigate the geological evolution of orthogneissic rocks, from the central and northern regions of the EGP. By examining their field relationships, mineral evolution, geochemistry, geochronology, and fluid character, this work aims at unravelling the tectonic history of the EGP which is pertinent to the shared geological history of India and East Antarctica. Ultimately, the findings will contribute to the understanding of magmatic and anatectic processes in orogenic crusts and the tectonic evolution of the EGP in the context of the India-East Antarctica correlation. Massif type charnockite was intruded into the lower crust of the EGP and is currently exposed across a vast geographical expanse. The present work is based on charnockite samples collected from different localities of the Visakhapatnam and Phulbani domains of the EGP. These two domains occupy the major part of the central and northwestern part of the province. The rock exhibits distinct signs of its magmatic intrusion into lower crustal metasedimentary rocks, which are preserved as enclaves within the charnockite. The rock displays a range of deformation signatures, spanning from nearly undeformed varieties to highly deformed mylonitized types. The rock comprises primary minerals like orthopyroxene, quartz, K-feldspar (perthitic), plagioclase, ilmenite, ± garnet ± clinopyroxene. Secondary phases in the rock consist of hornblende and biotite. In the relatively less-deformed charnockite samples, magmatic features such as subhedral plagioclase (with primary zoning) and orthopyroxene are preserved. In contrast, the deformed samples exhibit a dynamically recrystallized fabric that shows local transition to a mylonitic fabric characterized by the presence of quartz ribbons. Textural observations indicate that the charnockite magma underwent a process of sub-solidus cooling, followed by metamorphism to granulite facies, reaching temperatures of approximately 910°C at pressures of 9 kbar as estimated from the garnet-orthopyroxene-plagioclase-quartz assemblage. Additionally, this rock preserved some metamorphic texture indicative of potential reworking associated with the second (M2) cycle of metamorphism of the EGP, which has been dated as ca. 950-900 Ma. Geochemical data indicate that the charnockite magma show varying chemical compositions, that possibly resulted from differentiation processes and contamination with the supracrustal materials. This rock exhibits a range of SiO2 content, including both high- and low-SiO2 varieties, displaying characteristics that vary from weakly peraluminous to metaluminous. Notable features include LREE enrichment, HREE depletion, and distinctive trace element patterns. The rock shows enrichment in Ba and positive anomalies of Pb, La, Nd, and Gd, along with negative anomalies of Nb, Ta, Sr, and Ti. Slightly negative Ce and Y anomalies further support evolution of the magma in a post-collisional arc setting. Theoretical modelling indicates that melting of a hydrated basaltic slab in presence of a CO2-rich fluid could be a suitable process for the generation of the charnockite magma. Felsic (granite) gneisses are characterized by porphyritic textures with similar mineralogy like charnockite except absence of orthopyroxene. It has a similar geochemical composition to charnockite, with SiO2, FeOT, and MgO values spanning a broad range and higher K2O concentration. These rocks are predominantly peraluminous, featuring enrichment in LREE and depletion in HREE, with a negative Eu anomaly. Trace element behavior also signifies their evolution in an arc setting. Based on geochemical and intimate field relation of charnockite and felsic gneiss, it can be inferred that these two rocks are differentiated products of same felsic magma under contrasting fluid regimes. The relatively dry nature of the charnockite magma alongside the contemporaneous granite magmatism speaks for the fluctuations in the fluid regime, particularly concerning CO2-H2O components. On the other hand, mafic granulitic rock is less voluminous, occur as enclaves or xenolith within charnockite and felsic gneiss but it plays a pivotal role in the lithological diversity of the EGP. Two distinct types of mafic granulite obtained in this study display somewhat different mineral assemblages and compositions. The two-pyroxene granulite (massive type) contains minerals orthopyroxene, clinopyroxene, plagioclase, magnetite, ilmenite, pyrite, and pyrrhotite. In contrast, the garnet bearing mafic granulite (migmatitic variety) includes an additional phase, garnet. Textural characteristics of both oxide and sulfide minerals differ between these two varieties. Through textural analysis and inferred mineral reactions, it is apparent that the variations in oxide-silicate, oxide-sulfide, and sulfate relationships are associated with changes in oxygen fugacity (fO2) during the pre-peak, peak, and post-peak stages of metamorphism. The determined fO2 values span from a maximum of +4 log units in relation to the QFM (quartz-fayalite-magnetite) buffer for most of the samples, with the exception of a single sample that exhibits lower values, approximately -10 log units concerning the FMQ (fayalite-magnetite-quartz) buffer. The enduringly elevated fO2 state in the lower crust may be attributed to various factors, with the influence of an externally sourced fluid being a credible explanation. The oxidation and localized metasomatism of the mafic lower crust could potentially be explained by the presence of a hot brine solution containing CaCl2 species, despite not being directly confirmed through methods such as fluid inclusion analysis. Sulphide-sulphate relation is also used to understand the fluid regime in shallower crustal level. Zircon U-Pb (LA-ICPMS) obtained in this study from the massif type charnockite from extensive region to understand the precise timing of crystallization of the magma. Crystallization ages were obtained through U-Pb analysis on oscillatory zoned zircon domains from eight samples. Among the majority of the samples, crystallization ages were observed within the range of ca. 980-940 Ma, with specific ages of 978 ± 16 Ma, 968 ± 22 Ma, 951 ± 9 Ma, 954 ± 8 Ma, 951 ± 13 Ma, and 939 ± 27 Ma, respectively. Two samples of charnockite show crystallization age of 1002 ± 13 Ma and 1020 ± 16 Ma. This implies that the emplacement of the charnockite magma was characterized by two distinct phases, and these phases can be associated with the tectono-metamorphic evolution of the province. The earlier phase of charnockite magmatism is indicated to be broadly concurrent with the first cycle (M1) of metamorphism, while the later phase occurred when the lower crust was still experiencing elevated temperatures. The timing of the two charnockite magmatic events aligns closely with the Mawson charnockite in the Rayner Province of East Antarctica. This suggests that the extensive charnockite magmatism in the combined Rayner-Eastern Ghats Province can be attributed to the process of arc-continent accretion and the collision between India and East Antarctica that occurred approximately during ca. 1030-900 Ma.
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    Stratigraphy and Nature of uranium mineralization from Precambrian Basement Granitoid -Srisailam Formation contact around Chitrial area, Cuddapah Basin, Telangana
    Ghosh, Gautam; Bose, Sankar
    The biotite rich granitoid rocks exposed around Chitrial village varies in character from a porphyritic to massive granite to gneiss or mylonite with characteristic foliations defined by alternate quartzo-feldspathic and biotite rich layers in the latter units. It is intruded by ca. 1.9-1.8 Ga age mafic dyke sets and is overlain by Mesoproterozoic Srisailam Formation rocks of the Cuddapah Supergroup represented by an arenaceous gritty or pebbly sandstone interspersed with thin shale and siltstone horizons. The granitoid locally becomes uranium-rich near its contact with the overlying Srisailam Formation rocks. The present work encompasses stratigraphic, petrological, geochemical and geochronological analyses of the granitoids and accompanying supracrustals with special emphasis on nature and localization of uranium mineralization. Major element geochemical data characterize the granitoid rocks as monzogranites and alkali feldspar granite and the cover rocks as quartz arenites. The trace and rare earth element data were used to identify the protolith history of the granitoid rocks as well as about the nature of provenance of the cover sediments. The geochemical data further provide clues regarding probable tectonic and geodynamic setting of these rocks. A marked enrichment in U, Th and REE (particularly LREE) content of the granitoids has been noted. The overall REE pattern suggests a similar source for all the granitoid types. Several tectonic discrimination diagrams suggest a volcanic arc tectonic setting for these rocks. The recycled mature quartzose cover rocks show distinctly similar geochemical characteristics as the granitoids suggesting a granitoid/felsic source of mature continental provenance. REE patterns of the basement granite and the cover sandstone show similar variation which represents that the derivation of the sediments could be from the underlying basement granite. Recent exploration programme by Atomic Minerals Directorate for Exploration and Research (AMDER) has led to the discovery of a number of potential radioactive mineralized zones in the northwestern part of the Cuddapah basin such as around the Chitrial area. Uranium bearing minerals are intimately associated with sulphide rich minerals within the basement granitoids of the area. There is ample evidence of hydrothermal activity straddling across the unconformity surface which includes- (1) development of fracture filling veins of various dimensions comprising quartz, quartz-epidote, quartz-chlorite or pyrite, (2) hydrothermal alteration of granitoids adjacent to these veins resulting in chloritization and sericitization and (3) epigenetic uranium mineralization in micro-fractures and inter-granular spaces within granitoids. Evidence of uranium mineralization within the cover rocks is comparatively less. In the uraniferous zones in granitoid and overlying quartzite, pitchblende and coffinite are the main uranium phases occurring in micro-fractures and inter-granular spaces of host rock, often in association with pyrite. Depending upon micro-textural data, the paragenetic history of the mineralization has been divided into seven stages in the present study. U-Pb zircon radiometric dating of the basement granitoids reveals that the main tectonothermal event took place in Chitrial area between ca. 2535 Ma and 2519 Ma. Granitoid samples including the grey massive variety, pink granite, granite gneiss, foliated granite and alkali feldspar granite show emplacement ages of 2525±20 Ma, 2519±12 Ma, 2524±18 Ma, 2514±22 Ma and 2524±20 Ma respectively. Hence, it can be concluded that major tectonothermal event affected these rocks of the study area around 2535 to 2514 Ma. Probability density plot of weighted mean ages for the sample CT206 (granite gneiss) shows a strong peak at ca. 2465 Ma while the sample CT207 (foliated granite) shows another strong peak at ca. 2455 Ma which may be related to a second phase of tectonothermal event. U-Pb zircon detrital age of the cover rocks of the Chitrial area gives major cluster ages at ca. 2468 and 2488 Ma that may be correlated with this second tectonothermal event. From these rocks, diagnostic detrital zircons show age peaks at ca. 2520 Ma, 3000 Ma and 3200 Ma, which correspond to the established emplacement ages of the basement granitoid plutons in the Eastern Dharwar Craton. Younger dates are discordant with a lower intercept ages of near 200 Ma in the Wetherill concordia. Older zircon cores with spot data ranging from 2636±28 Ma to 3200±7 Ma are interpreted as grains inherited from the crustal source region or from the wall-rock of the granite intrusion. From the detrital zircon data, it can be inferred that source of the sediments is proximal. EPMA chemical dates of uraninites from the drill core sample 226B shows that the area underwent several episodes of hydrothermal activity, which have left their imprints on the isotope systematics of uraninite. Thus the younger ages furnished by U-Pb zircon radiometric dating of uranium rich in-situ zircon grains of the granite drill core sample (289Av) shows a group age of 172 Ma, possibly related to the much younger tectonothermal event. From this study, it is concluded that the Chitrial granitoids are ‘S’ type in character and formed by intracrustal melting of the deeply buried clastic sediments and subsequent incubational heating. It also implies crustal recycling could be the likely mechanism for granite magmatism during ca. 2535-2514 Ma. Uranium mineralization in the granite was influenced by increased fracture volume in the rocks and was controlled by oxygen fugacity in the ore-bearing hydrothermal fluid. This mineralization is related with the later stage fracture reactivation of the Eastern Dharwar Craton during a major younger tectonic activity.
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    Structural and AMS analyses of basement Granitoid rocks of the Chitrial Area and its bearing on unconformity proximal type Uranium mineralization Cuddapah Basin Telangana
    Ghosh, Gautam
    The 2.51-2.52 Ga age Paleoproterozoic (Mukherjee et al., 2018) basement granitoid rocks of the Chitrial area in the Eastern Dharwar Craton (EDC) are unconformably overlain by the Mesoproterozoic Srisailam Formation (Nagaraja Rao et al., 1987) of the Cuddapah basin. Pre-Srisailam dyke sets (1.9-1.8 Ga; French et al., 2008) of N-S, WNW-ESE and ENE-WSW orientations traverse the granites around the Chitrial outlier. The map scale WNW – ESE trending Dindi lineament marks the southern limit of the outlier. Atomic Minerals Directorate (AMD) in recent times has established several ‘Uranium deposits’ from the Cuddapah basin of which one such deposit has been reported from the Chitrial outlier. In this latter deposit, uranium is hosted in Arechaean to Paleoproterozoic porphyritic basement granite unconformably overlain by the Mesoproteozoic unmetamorphosed sediments of the Srisailam Formation. According to the concentration of uranium mineralization, AMD further divided the Chitrial outlier into five blocks namely the ‘Main block’, ‘Block 1’, ‘Block 2’, ‘Block 3’ and ‘Block 4’.