Rajib Kar

Bio: Rajib Kar is an academic researcher from J. K. College. The author has contributed to research in topics: Charnockite & Granulite. The author has an hindex of 2, co-authored 5 publications receiving 52 citations. Previous affiliations of Rajib Kar include Indian Statistical Institute.

Early Archaean continental crust in the Eastern Ghats granulite belt, India: isotopic evidence from a charnockite suite

Abstract: The Eastern Ghats granulite belt of India has traditionally been described as a Proterozoic mobile belt, with probable Archaean protoliths. However, recent findings suggest that synkinematic development of granulites took place in a compressional tectonic regime and that granulite facies metamorphism resulted from crustal thickening. The field, petrological and geochemical studies of a charnockite massif of tonalitic to trondhjemitic composition, and associated rocks, document granulite facies metamorphism and dehydration partial melting of basic rocks at lower crustal depths, with garnet granulite residues exposed as cognate xenoliths within the charnockite massif. The melting and generation of the charnockite suite under granulite facies conditions have been dated c. 3.0 Ga by Sm–Nd and Rb–Sr whole rock systematics and Pb–Pb zircon dating. Sm–Nd model dates between 3.4 and 3.5 Ga and negative epsilon values provide evidence of early Archaean continental crust in this high-grade terrain.

Patchy charnockites from Jenapore, Eastern Ghats granulite belt, India: Structural and petrochemical evidences attesting to their relict nature

Abstract: The charnockite patches that occur within leptynite host, in and around Jenapore, northern sector of the Eastern Ghats granulite belt, are disposed in a linear fashion and generally have sharp lithological contact with the host leptynite. Sometimes the patches and foliations of the host are cofolded. Also, the patches sometimes have the internalS 1 foliation, while the host leptynite records onlyS2 foliation. Mineralogically and chemically patchy charnockites and host leptynites are distinct entities, and cannot be related by any prograde and retrograde reactions. Particularly important is the peraluminous granitic composition and high Rb/Sr ratios of the leptynites, presumably resulting from biotite-dehydration melting; as against metaluminous granodioritic to tonalitic composition and low Rb/Sr ratios of the patchy charnockites, presumably resulting from hornblende-dehydration melting. The charnockite patches here can be interpreted as caught up patches or xenolith within granitic melt (leptynite). Mg-rich rims of garnet in the charnockite patch were probably caused by heat from the crystallising melt or decompression during ascent of melt.

New experimental constraints: implications for pe-trogenesis of charnockite of dioritic composition

Abstract: Hornblende-dehydration melting experiments at high temperatures (> 950oC) indicate change of melt composition from tonalite/granodiorite to quartz-diorite; clinopyroxene instead of hornbl- ende as the residual phase and change in melting reaction from peritectic hornblende-dehydr- ation to eutectic clinopyroxene-orthopyroxene- plagioclase. In the light of these experimental results, petrogenesis of a charnockite pluton of homogeneous dioritic composition in the Eastern Ghats Belt, India, can be explained as melting at high-temperatures (> 950oC). Negative Sr and Eu anomalies further indicate plagioclase as a major residual phase, consistent with melt- ing at high-temperatures (> 950oC).

Relative Chronology in High-Grade Crystalline Terrain of the Eastern Ghats, India: New Insights

Abstract: The two major lithology or gneiss components in the polycyclic granulite terrain of the Eastern Ghats, India, are the supracrustal rocks, commonly described as khondalites, and the charnockite-gneiss. Northern Eastern Ghats belt, north of the Godavari rift has been defined as the Eastern Ghats Province, while that to the south has been defined as the Ongole domain; and although, these distinct crustal domains also record different ages of granulite metamorphism, both of these domains are dominated by the two lithologies. Many of the workers considered the khondalites as the oldest component with unknown basement and the charnockite- protoliths as intrusive into the khondalites. However, published geochronological data do not corroborate the aforesaid relations. Onset of khondalite sedimentation in the Proterozoic Eastern Ghats Province, constrained by detrital zircon data, as around 1.3 Ga and the charnockite-protolith emplacement between 1.9 and 2.9 Ga, argue against intrusion of felsic magma (tonalite, now enderbite!) in to the khondalites. The field relations of the hornblende-mafic granulite with the two gneiss components together with Sm-Nd isotopic data of the hornblende-mafic granulites (both the xenoliths within charnockites and those interbanded with the khondalites) indicate that khondalite sediments were deposited on older mafic crustal rocks. Mafic basement and supracrustal rocks were subsequently deformed and metamorphosed together during collisional orogeny at high to ultra-high temperatures – partial melting of mafic rocks producing the charnockitic melt; and partial melting of pelitic sediments producing the peraluminous granitoids. This is compatible with all the geochronological data as well as the petrogenetic model of partial melting for the charnockitic rocks in the Eastern Ghats Belt. The Ongole domain, south of the Godavari rift, though, is distinct in terms of the age of first/ earliest UHT metamorphism, but here too the charnockite-protoliths are older mafic rocks evidently not intrusive in to the khondalites.

Relative chronology in high-grade crystalline terrain of the Eastern Ghats, India: new insights

Abstract: . The two major lithology or gneiss components in the polycyclic granulite terrain of the Eastern Ghats, India, are the supracrustal rocks, commonly described as khondalites, and the charnockite-gneiss. Many of the workers considered the khondalites as the oldest component with unknown basement and the charnockite-protoliths as intrusive into the khondalites. However, geochronological data do not corroborate the aforesaid relations. The field relations of the hornblende- mafic granulite with the two gneiss components together with geocronological data indicate that khondalite sediments were deposited on older mafic crustal rocks. We propose a different scenario: Mafic basement and supracrustal rocks were subsequently deformed and metamorphosed together at high to ultra-high temperatures – partial melting of mafic rocks producing the charnockitic melt; and partial melting of pelitic sediments producing the peraluminous granitoids. This is compatible with all the geochronological data as well as the petrogenetic model of partial melting for the charnockitic rocks in the Eastern Ghats Belt.

Late Cretaceous charnockite with adakitic affinities from the Gangdese batholith, southeastern Tibet: Evidence for Neo-Tethyan mid-ocean ridge subduction?

Abstract: The Gangdese batholith emplaced during the time span of Cretaceous to Neogene in the southern Lhasa terrane of Tibet has been considered as a major constituent of an Andean-type convergent margin derived from the northward subduction of the Neo-Tethyan oceanic lithosphere under Asia. Whereas previous studies assigned the Gangdese granitoids to be comprised predominantly of calc-alkaline rocks, here we report a suite of charnockites from the eastern part of the belt and characterize their petrology, geochemistry and age. These rocks possess an assemblage of andesine, enstatite, diopside, calcic amphibole, Ti-rich biotite, quartz and minor K-feldspar. Geochemically, they are characterized by intermediate SiO2 (54–63 wt.%), relatively high Al2O3 (15.9–18.9 wt.%), REE (55.7–89.4 ppm) and Sr (419.6–619.4 ppm), and low Y (11.3–17.2 ppm) and Yb (1.2–1.8 ppm) concentrations. The rocks display geochemical affinities similar to those of adakites derived from the partial melting of a subducted slab, and also can be compared to magnesian charnockites formed within a continental magmatic arc. The crystallization conditions of the charnockites were estimated at 900 °C and 1.0 GPa. LA-ICP-MS zircon U–Pb analyses of eleven samples yield consistent 206Pb/238U weighted mean ages of 86 to 90 Ma, indicating that the charnockites were emplaced in the Late Cretaceous. Considering the coeval calc-alkaline magmatism and high-temperature granulite-facies metamorphism, we propose that such high-temperature and low-H2O activity charnockites were derived through Neo-Tethyan mid-ocean ridge subduction before the collision of India with the Asian continent.

Tectonic evolution of the Eastern Ghats Belt, India

Abstract: We present possible tectonic models for two crustal domains of the Proterozoic Eastern Ghats Belt, India based on recent petrological, geophysical and geochronological data. Although both the domains presently expose deep crustal sections, they evolved in two distinct time segments of the Precambrian through accretion–collision processes. This is why we argue that no unique model can explain the complexities of the belt. The southern part of the belt evolved through subduction-dominated accretionary processes encompassing India, east Antarctica, Australia and Laurentia as part of supercontinent Columbia during the span of ca. 1.90–1.60 Ga. To the contrary, the central domain witnessed a prolonged accretion–collision history initiated at ca. 1.50 Ga and culminated at ca. 0.90 Ga with the formation of supercontinent Rodinia. The latter united cratonic India with east Antarctica as a separate continent Enderbia that existed until about ca. 0.50 Ga. The pre-1.50 Ga history of this domain is ambiguous at the present state of knowledge. The northern domain has a much younger (ca. 0.90–0.50 Ga) tectonothermal history which is unrelated to either of the studied domains. The present models explain the reported petrotectonic processes including the ultra high temperature metamorphism in both the domains. The episodic growth of the Eastern Ghats Belt matches with Proterozoic supercontinent cycles.

A 2.5 Ga porphyry Cu–Mo–Au deposit at Malanjkhand, central India: implications for Late Archean continental assembly

Abstract: Eight Re–Os ages from six molybdenite samples representative of the Cu–Mo–Au mineralization at the giant Malanjkhand deposit in Madhya Pradesh were obtained using ID-NTIMS. These data provide a clear Late Archean–Early Paleoproterozoic age for this deposit and by implication for its calc-alkaline granitoid host. Among other diverse models, the origin of Malanjkhand has been debated as orogenic lode-style associated with development of a quartz reef in a shear zone or as porphyry-style associated with a pink granitoid intrusion that is conspicuously present in the vicinity of the brittle–ductile deformed quartz reef. Previous models included Middle Proterozoic ages for the Cu–Mo–Au mineralization based on poorly constrained Rb–Sr and K–Ar data. Direct dating of molybdenite shows that stringer mineralization in the quartz reef and disseminated and vein mineralization in the granite were contemporaneous at 2490 ± 8 Ma (2489.5 ± 1.4 Ma based on regression without uncertainty in the 187Re decay constant, MSWD = 0.5, n = 5). Additional molybdenite from the quartz reef dated at ∼2475 and ∼2450 Ma represents at least two pulses of reworking that we suggest configured the elongate and arcuate quartz reef into its present position. We suggest that the quartz reef is a high-silica stockwork cap embedded in its primary potassic alteration halo and subsequently tilted so that it lies convex to the east with deeper levels exposed to the north. Petrographic evidence coupled with high Re concentrations for molybdenites (400–650 ppm) support this scenario and suggest that Malanjkhand is a subduction-related stockwork-porphyry-style Cu–Mo–Au deposit involving mantle. Malanjkhand geology shares little affinity with three surrounding and tectonically juxtaposed terranes. We suggest that the Malanjkhand Cu–Mo–Au deposit and the surrounding batholith containing multiple Cu–Mo–Au prospects may be the vestiges of a microplate captured along the north margin of the nearly amalgamated southern Indian craton at 2.5 Ga, as the southern craton began to converge with the northern Indian craton or other protocontinents now embedded or consumed within the central Indian Tectonic Zone (CITZ). This scenario could imply an Early Paleoproterozoic age for the earliest development of the CITZ, at least in the region of the Central Indian Shear/Suture (CIS), whose marked ENE trend imprints the northwestern part of the Malanjkhand batholith and the Cu–Mo–Au deposit. Based on a compilation of ∼2.5 Ga events in both the southern and northern Indian cratons, we suggest that some units in the CITZ linked an extensive and continuous Late Archean–Early Paleoproterozoic orogenic belt that may extend to the Moyar–Bavali, Bhavani, Attur, Palghat, and Cauvery shear zones in southern India, and include inboard units of the Eastern Ghats and the Aravalli–Delhi orogenic belts. Robust geochronology that can be tied to the early history within these orogenic belts is scarce, and extensive reworking during Grenvillian and Pan-African time creates a pervasive overprint at some localities. This Late Archean–Early Paleoproterozoic belt preserves sutures within a coalesced Late Archean continent that included East Antarctica (Napier and Vestfold Hills complexes) at ∼2.5 Ga.

The Eastern Ghats Belt … A polycyclic granulite terrain

Abstract: The Eastern Ghats Belt is a polycyclic granulite terrain along the east coast of India whose western boundary is marked by a shear zone along which the granulites are thrusted over the cratonic units of the Indian shield, and its northern margin is marked by the presence of a number of fault-bounded blocks. Recent work has convincingly brought out that there are domains within the belt having different evolutionary histories. The segment south of the Godavari Rift went through a high grade thermo-tectonic event at ∼1.6-1.7 Ga. North of the Godavari Rift in a narrow zone along the western boundary the last high-grade metamorphic event is of late Archaean age. A series of alkaline plutons along the western boundary zone testifies to a rifting episode at ∼1.3-1.5 Ga. In the major part of the EGB the metamorphism is broadly of Grenvillian age, with two major thermo-tectonic pulses at ∼1.1-1.2 Ga and ∼0.95-1.0 Ga. But high grade conditions persisted for a long period and younger thermal events of ∼0.65 Ga to ∼0.80 Ga are locally recorded. There are differences in the tectonometamorphic histories of different domains, but the tectonic significance of these differences remains uncertain. Pan-African (0.50-0.55) thermal overprints are common and become conspicuous along the western boundary zone. The thrusting of the Eastern Ghats granulites in a hot state over the cratons to the west is of Pan-African age. In the Rodinia assembly (∼0.9 Ga) the Eastern Ghats and the Rayner-Napier Complexes of Antarctica were contiguous, but the pre-Rodinia configuration of these terrains remains unclear. At ∼0.8 Ga during the Rodinia break up Greater India rifted apart from East Antarctica, and only later it docked with Australia-East Antarctica at 530-550 Ma. The continuation of the East Antarctic Pan-African orogenic belts into the Eastern Ghats is yet to be ascertained.