Department of Geology

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Browsing Department of Geology by Author "Bose, Sankar"

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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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    Tectono metamorphic and geochronological evolution of the Rengali Province in the Riamal Rengali Khamar sector Odisha India
    Bose, Sankar
    Rengali Province occupies a unique geographical position between the Archean Singhbhum Craton and the Proterozoic Eastern Ghats Belt of eastern India. This province was variably considered a part of the Singhbhum Craton and/or the Eastern Ghats Belt, but their metamorphic characters are grossly contrasting. This opens a possibility that the Rengali Province evolved as a separate orogenic belt that may be unrelated to the Eastern Ghats Belt. The most striking feature of this province is the occurrence of several linear WNW-ESE trending zones separated by major ductile faults or shear zones. The rocks of this province show varying degree of metamorphism from granulite to greenschist facies. Geological mapping of the central part of the Rengali Province reveals presence of a gneissic basement block intercalated with low-grade supracrustal sequences. The basement rocks are mostly of granitoid composition showing gneissic fabric and the rocks are metamorphosed to amphibolite facies. Enclaves of granulite facies rocks, represented by charnockite gneiss and mafic granulite, occur within the gneissic basement. A part of this gneiss basement, referred to here as the Central Gneissic Belt, is the prime focus of petrological, geochemical and geochronological study. Supracrustal rocks are represented by quartzite, mica schist and calc- silicate schist that belong to the Tikra Association. Detailed structural analyses of the rocks from the central part of Rengali Province suggest that deformation was regionally partitioned into fold-thrust dominated shortening zones alternating with zones of dominant transcurrent deformation bounded between the Barkot Shear Zone in the north and the dextral Kerajang Fault Zone in the south. The strain partitioned zones are further restricted between two regional transverse shear zones, the sinistral Riamol Shear Zone in the west and the dextral Akul Fault Zone in the east. The overall structural disposition can be interpreted as a positive flower structure bounded between the longitudinal and transverse faults with vertical extrusion and symmetric juxtaposition of mid-crustal amphibolite grade basement gneisses over low-grade upper crustal rocks emanating from the central axis of the transpressional belt. The Central Gneissic Belt is constituted of charnockite gneiss, migmatitic hornblende gneiss and felsic gneiss often showing gradational contacts. While mafic granulite occurs as enclave within the charnockite gneiss, amphibolite and calc-silicate granofels enclaves are present within the felsic gneiss. Petrological study shows that the charnockites and mafic granulites underwent granulite facies metamorphism, whereas the gneisses were subjected to amphibolite facies metamorphism. Mafic granulite shows peak metamorphic assemblage of garnet + clinopyroxene + plagioclase + quartz ± orthopyroxene which was stabilized at 10.6 ± 0.5 kbar and 860 ± 20 °C. Charnockite gneiss with the peak assemblage of orthopyroxene +quartz + plagioclase +K-feldspar was metamorphosed at 792 ± 48°C and 7.6 ± 0.4 kbar. Amphibolite and migmatitic hornblende gneiss contain hornblende along with plagioclase and garnet and these rocks were metamorphosed at 800 ± 20 °C, 8.5 ± 0.2 kbar and 695 °C, 8 kbar respectively. Later meta-dolerite dikes exhibit relic igneous textures which are slightly modified by greenschist facies metamorphism. Charnockite gneiss, migmatitic hornblende gneiss and felsic gneiss show similar trace and REE characteristics (moderate fractionation in terms of La and Yb, LREE enrichment and flat HREE pattern) implying the same protolith composition for these rock groups. Field, petrographic and geochemical data suggest that the protoliths for the charnockite gneiss, the migmatitic hornblende gneiss and the felsic gneiss crystallized as fractionated magma in within-plate syncollisional setting during a prominent phase of orogeny at the Rengali Province. Results of detailed zircon U–Pb (SHRIMP) geochronological study of the amphibolite to granulite facies rocks of the Central Gneissic Belt reveal a complex evolutionary history. Charnockitic gneiss has protolith age of 2861 ± 30 Ma and high-grade metamorphism occurred at 2818 ± 15 Ma. Migmatitic hornblende gneiss has a protolith age of 2828 ± 9 Ma. The leucogranite was emplaced at 2807 ± 13 Ma. The protolith of the felsic gneiss was emplaced at 2776 ± 24 Ma. Most of the zircon samples contain overgrowths of c. 2500 Ma, inferred to be the age of reworking of the Central Gneissic Belt. These data suggest that the Rengali Province evolved as an orogenic belt in the Neoarchean time (ca. 2800–2500 Ma) during southward growth of the Singhbhum Craton. These tectonothermal imprints at the margin of the Singhbhum Craton are possibly related to its inclusion within the supercontinent Ur. Interestingly, the rocks of the Central Gneissic Belt and the associated supracrustals do not record any age signatures of ca. 1000-900 Ma orogeny that evolved the Eastern Ghats Province, thus discarding any genetic link between the two adjacent orogenic belts. The later transpression, extrusion and juxtaposition of deep crustal section to shallower level was achieved due to reactivation of the fault-thrust system during ca. 530-500 Ma which can be linked to far field stresses of global Pan-African orogeny.
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    Tectonothermal History of the Granulites and gneisses around Phulbani Odisha India and its bearing on the Evolution of the Proterozoic Eastern Ghats Belt
    Bose, Sankar
    The Proterozoic Eastern Ghats Belt of eastern India is a key terrane to understand the evolution of the India-East Antarctica blocks of the supercontinent Rodinia. This belt is known for lower crustal rocks that experienced ultrahigh temperature (UHT) metamorphism. Because of its geological diversity, this belt has been subdivided into several crustal provinces and domains each having discrete tectonometamorphic characters. Most of the petrological history of this belt is reported from the domains located at the central (Visakhapatnam domain) and the southern (Ongole domain) parts and while the vast area covering the northern part of this high-grade terrane remains unexplored. The present work has been carried out on the Phulbani domain which occupies a major portion of the north-western part of the belt. Apart from some isolated geochronological data, no systematic petrological, geochronological, structural and fluid investigation has been carried out from this domain. This poses severe constraints on the overall metamorphic history of the northern part of the Eastern Ghats Belt (Eastern Ghats Province) and its connection with the supercontinent Rodinia. In the present study, an attempt has been made to overcome these issues by constraining the geological history of the Phulbani domain in a holistic manner. Phulbani domain is composed of migmatitic felsic gneiss, felsic augen gneiss, fine-grained charnockite gneiss, aluminous granulite, calc-silicate granulite and mafic granulite which was metamorphosed at UHT condition and subsequently intruded by coarse-grained charnockite. The mineral assemblage formed at the peak-UHT metamorphic condition is best documented in the aluminous granulite as spinel+quartz-bearing mineral assemblage. Using textural, thermobarometric and phase equilibria data, it is inferred that the latter mineral assemblage stabilized at 950°C (at approximately 8 kbar) from a corundum-bearing mineral assemblage by chemical reactions during prograde heating. Scapolite-clinopyroxene-wollastonite-plagioclase bearing mineral assemblage in the calc-silicate granulite also indicates temperature in excess of 800°C which is corroborated by high anorthite content of scapolite. After attainment of peak-UHT condition, all the rocks including coarse-grained charnockite experienced pronounced phase of near-isobaric cooling along an almost isobaric prograde and retrograde path and produced coronal garnet in aluminous granulite, calc-silicate granulite, fine-grained charnockite gneiss and coarse-grained charnockite. From the fluid inclusion analyses, it is inferred that the peak- to post-peak metamorphic evolution of the Phulbani domain dominantly occurred in a CO2-dominated fluid regime as high-density (up to 1.03 gm/cm3) CO2-rich fluid inclusions are documented in aluminous granulite, coarse-grained charnockite and migmatitic felsic gneiss. Textures like K-feldspar micro-veins in migmatitic felsic gneiss, myrmekite-like intergrowth, Th-rich veins in monazite and the presence of pegmatoidal metasomatic rock at the contact of calc-silicate granulite and coarse-grained charnockite further point towards the presence of an additional fluid phase which was capable to transfer elements in micro- to mesoscopic-scale. In the present study such fluid phase is interpreted to be brine which escaped from the rock record perhaps due to its greater mobility. Phulbani domain is characterized by five phases of deformations (D1-D5) and related fabric developments (S1-S5S). Of these, the S1 fabric is found only as inclusion within porphyroblastic phases of aluminous granulite and migmatitic felsic gneiss and produced during D1. Being common in the aluminous granulite, calc-silicate granulite, migmatitic felsic gneiss and fine-grained charnockite gneiss, the S2/S3 gneissic fabric is interpreted to be the earliest recognizable planar fabric of the study area and resulted by successive D2-D3 deformations during UHT metamorphism. The S2/S3 gneissic fabric was transposed during D4 to form S4 and S4S fabrics which dominantly occur parallel to the axial plane of the folded S2/S3 fabric. The S4 fabric later on folded during D5 to form mylonitic fabric (S5S) of the Ranipathar shear zone. Pseudotachylite veins are always associated with this S5S fabric and dominantly occur parallel to this fabric. Outcrop-scale sheath folds also developed during ductile shearing in the Ranipathar shear zone and played a critical role during exhumation of the high-grade rocks of the Phulbani domain. Microstructural investigation of quartz grains of the migmatitic felsic gneiss showing the S4S fabric suggest dominant deformation by prism and rhomb slips. Quartz ribbons in the S5S mylonitic foliation suggest deformation by prism slip in the quartz grains. Microstructures in pseudotachylite veins of the Ranipathar shear zone indicate its origin by melt crystallization following development of mylonititic foliation. Deformed pseudotachylite matrix suggests at least one stage of deformation after pseudotachylite formation during reactivation of the RSZ. Zircon U-Pb (SHRIMP) and monazite U-Th-total Pb (EPMA) analyses obtained in the present study additionally put precise time constraints on the tectonothermal evolution of the Phulbani domain. Zircon from the coarse-grained charnockite shows crystallization age of ca. 970 Ma. Aluminous granulite possibly suffered UHT metamorphism at ca. 987 Ma as revealed from monazite included in porphyroblastic garnet. Monazite in the aluminous granulite and the migmatitic felsic gneiss grew dominantly at 966 ± 4 Ma and 968 ± 4 Ma ages respectively, which are interpreted as the cooling ages subsequent to the peak metamorphism. Oscillatory-zoned zircon grains of the felsic augen gneiss yield ca. 1173 Ma age which is interpreted as the crystallization age of the granitic protolith. This ca. 1173 Ma age granite possibly composed a part of the Proterozoic basement of the Eastern Ghats Province. Monazite age of ca. 781 Ma from aluminous granulite exposed at the N-S trending ductile shear zone and dates within the range of ca. 558–535 Ma from felsic augen gneiss exposed at the RSZ indicate localized shear-induced thermal process in the Phulbani domain. The presently studied rock suite thus recorded four distinct events (ca. 1173 Ma, ca. 1000–900 Ma ca. 781 Ma and ca. 558–535 Ma) of magmatism, metamorphism and deformation of the Eastern Ghats Belt. The tectonometamorphic and tectonothermal histories of the Phulbani domain during the time-frame of 1000–900 Ma appear to be similar if compared with the adjacent Visakhapatnam domain which argues against the domain-based classification of the Eastern Ghats Belt. Based on this, it is inferred that the shear zones marking the boundaries of the domains possibly formed during latter time and cannot be considered as domain boundaries. The metamorphic and the magmatic histories during the time-frame mentioned above additionally matches well with the Rayner complex of the East Antarctica indicating the contiguous nature of these terranes during the inferred ca. 950-900 Ma Rayner-Eastern Ghats orogeny.