Sankar Bose

India-Antarctica-Australia-Laurentia connection in the Paleoproterozoic–Mesoproterozoic revisited: Evidence from new zircon U-Pb and monazite chemical age data from the Eastern Ghats Belt, India

We present zircon and monazite U-Pb data from ultrahigh-temperature (UHT) metamorphosed orthogneisses and paragneisses collected from key areas of the Eastern Ghats Belt, India. The results show contrasting tectonothermal histories in different isotopic domains of the Eastern Ghats Belt that were identified by previous workers. Of particular importance is the discovery of a ca. 1760 Ma event (concordia age) in the southern domain 1A, which is interpreted to be the age of an early UHT metamorphism event. This was followed by a second granulite-facies metamorphism event and partial melting at ca. 1600 Ma. This domain was presumably cratonized with India at around 1600 Ma. The record of the ca. 1760–1600 Ma events in domain 1A of the Eastern Ghats Belt allows us to speculate on modeling the Paleoproterozoic–Mesoproterozoic transcontinental correlation. The accretionary orogenic processes in the supercontinent Columbia encompassed Australia, Antarctica, Laurentia, and parts of India. The central part of Eastern Ghats Belt (isotopic domain 2), on the other hand, contains zircons showing inherited ages of ca. 1880–1700 Ma, with a concordant age group of ca. 1760 Ma. Moderately to strongly discordant ages in the time span of ca. 1600–1100 Ma in domain 2 are interpreted to be mixing ages as a result of strong overprint of a ca. 1030–900 Ma tectonothermal event(s) that affected this domain. An early UHT metamorphism event in this domain is inferred to have occurred at ca. 1030–990 Ma (chemical dating of included monazite grains). Zircon records the most pervasive tectonothermal event in this domain at ca. 980–900 Ma, which is correlative with the Rayner orogeny in East Antarctica as a part of the formation of Rodinia.

Oxygenation of the Archean atmosphere: New paleosol constraints from eastern India

It is widely believed that atmospheric oxygen saturation rose from −5 present atmospheric level (PAL) in the Archean to >10 −2 PAL at the Great Oxidation Event (GOE) at ca. 2.4 Ga, but it is unclear if any earlier oxygenation events occurred. Here we report U-Pb zircon data indicating that a pyrophyllite-bearing paleosol, from Keonjhar in the Precambrian Singhbhum Craton of eastern India, formed between 3.29 and 3.02 Ga, making it one of very few known Archean paleosols globally. Field and geochemical evidence suggests that the upper part of the paleosol was eroded prior to unconformable deposition of an overlying sequence of shallow-marine siliciclastic sediments. A negative cerium anomaly within the currently preserved level of the paleosol indicates that ancient oxidative weathering occurred in the original upper soil profile. The presence of redox-sensitive detrital uraninite and pyrite together with a complete absence of pyrophyllite in the overlying sediments indicate that the mineralogical and geochemical features of the paleosol were established prior to the unconformable deposition of the sediments and are not related to subsequent diagenetic or hydrothermal effects. We suggest that a transient atmospheric oxygenation event occurred at least 600 m.y. prior to the GOE and ∼60 m.y. prior to a previously documented Archean oxygenation event. We propose that several pulsed and short-lived oxygenation events are likely to have occurred prior to the GOE, and that these changes to atmospheric composition arose due to the presence of organisms capable of oxygenic photosynthesis.

Garnet-spinel intergrowths in ultrahigh-temperature granulite, Eastern Ghats, India: Possible evidence of an early Tschermak-rich orthopyroxene during prograde metamorphism

Abstract UHT metamorphosed aluminous granulites from the Eastern Ghats Belt, India, contain contrasting mineral assemblages in closely spaced domains. Reaction textures and mineral chemistry in one domain are consistent with prograde metamorphism. A unique intergrowth texture is found in garnet porphyroblasts from the other domain. Lamellar to vermicular intergrowth of spinel solid-solution occurs in garnet, particularly in the central part of the porphyroblast. Locally, sapphirine, overgrowing spinel, is also intergrown with garnet. Three possible mechanisms are discussed to account for the garnet-spinel intergrowth: (1) from early stabilized orthopyroxene + sillimanite; (2) spinel forming pseudomorphs after early sillimanite inclusions in garnet; and (3) from an early stabilized hypothetical, highly aluminous orthopyroxene involving a complex reaction between the (Fe,Mg)-Tschermak and Fe3+-Tschermak components. Sapphirine is produced by a reaction involving spinel and aluminous pyroxene. Regardless of the exact mechanism, available thermodynamic data suggest that all such reactions presumably occurred due to loading and/or heating during prograde metamorphism consistent with the conclusion from the other domain. Such an intergrowth texture has not been reported previously in aluminous granulite parageneses, but these could provide crucial information regarding the prograde segment of the metamorphic evolution of deep crustal rocks.

Fluorine content of biotite in granulite-grade metapelitic assemblages and its implications for the Eastern Ghats Granulites

Biotite inclusions showing high fluorine content (up to 3.3 wt.%) within porphyroblastic aluminous orthopyroxene (>10 wt.% Al 2 O 3 ) and cordierite suggest high temperature of melting for the prograde assemblages. Textural data indicate that biotite dehydration melting produces peak assemblages orthopyroxene + spinel + quartz + melt and orthopyroxene + sapphirine + quartz ± cordierite + melt in different microdomains depending on the bulk composition. A comparative study of the biotite composition with those of experimental data and present one indicates a minimum temperature of 950°C for the terminal stability of biotite-bearing assemblages. This also explains the appearance of observed peak assemblage sapphirine + orthopyroxene + quartz + cordierite + spinel directly from F-rich biotite as a consequence of topological changes in the KFMASH grids. Enrichment of fluorine in retrograde biotite can be explained by interaction of solid phases with the in situ melt fraction during subsequent retrogressive stage of the granulites. This is the first report of its kind from the northern part of the Eastern Ghats Belt, India.

Zircon U–Pb SHRIMP and monazite EPMA U–Th–total Pb geochronology of granulites of the western boundary, Eastern Ghats Belt, India: a new possibility for Neoproterozoic exhumation history

Abstract We present detailed and high-precision geochronological data on granulites occurring along the western boundary of the Eastern Ghats Belt, India. Age data on systematically sampled rocks coupled with geochemical observation have a potential to unravel the overprinted tectonothermal events operated during Precambrian time. Zircon U–Pb SHRIMP and monazite electron probe microanalysis (EPMA) U–Th–total Pb analyses, chemical zoning, microtextural investigation, and pressure–temperature calculations were carried out on samples of four different rock types. Inherited zircons from migmatitic quartzofeldspathic gneiss and mafic granulite yielded ages of approximately 2900–2350 Ma, representing an older crustal component. The age of granulite metamorphism recorded from charnockite and pelitic granulite ranges between approximately 950 and 930 Ma (from zircon and monazite). A possible decompression event from this area that occurred during Rodinia break-up is recorded from the Y-rich zones of monazite closely associated with porphyroblastic garnet in pelitic granulite and dates from approximately 800 to 750 Ma. Zircon grains of charnockite also yield a similar age. The youngest age of approximately 525–510 Ma documented from the monazite grains of migmatitic quartzofeldspathic gneiss and pelitic granulite, along with a spot age from zircon of migmatitic quartzofeldspathic gneiss, testifies to the final assembly of East Antarctica with cratonic India as a part of East Gondwana.

The Eastern Ghats Belt, India, in the context of supercontinent assembly

Abstract The Eastern Ghats Belt (India) bears testimony to the assembly and dispersal of both the Columbia and Rodinia supercontinents, and possibly the formation of East Gondwana. The belt itself is a collage of different lithotectonic and isotopic domains, and therefore the petrological evolution of each domain is to be considered separately prior to the formation of the belt. In this paper, we present an updated review on the petrological and tectonic evolution of the different domains along with geochronological constraints. We develop tectonic models to show how different lithotectonic domains fit into supercontinent cycles in the Proterozoic period.

Petrotectonic framework of granulites from northern part of Chilka Lake area, Eastern Ghats Belt, India: Compressional vis-à-vis transpressional tectonics

Granulite-facies rocks occurring north-east of the Chilka Lake anothosite (Balugan Massif) show a complex metamorphic and deformation history. The M1–D1 stage is identified only through microscopic study by the presence of S1 internal foliation shown by the M1 assemblage sillimanite–quartz–plagioclase–biotite within garnet porphyroblasts of the aluminous granulites and this fabric is obliterated in outcrop to map-scale by subsequent deformations. S2 fabric was developed at peak metamorphic condition (M2–D2) and is shown by gneissic banding present in all lithological units. S3 fabric was developed due to D3 deformation and it is tectonically transposed parallel to S2 regionally except at the hinge zone of the F3 folds. The transposed S2/S3 fabric is the regional characteristic structure of the area. The D4 event produced open upright F4 folds, but was weak enough to develop any penetrative foliation in the rocks except few spaced cleavages that developed in the quartzite/garnet–sillimanite gneiss. Petrological data suggest that the M4–D4 stage actually witnessed reactivation of the lower crust by late distinct tectonothermal event. Presence of transposed S2/S3 fabric within the anorthosite arguably suggests that the pluton was emplaced before or during the M3–D3 event. Field-based large-scale structural analyses and microfabric analyses of the granulites reveal that this terrain has been evolved through superposed folding events with two broadly perpendicular compression directions without any conclusive evidence for transpressional tectonics as argued by earlier workers. Tectonothermal history of these granulites spanning in Neoproterozoic time period is dominated by compressional tectonics with associated metamorphism at deep crust.

Petrology and Geochemistry of Metamorphosed Basic Intrusives from Chilka Lake Granulites, Eastern Ghats Belt, India: Implications for Rodinia Breakup

High-grade granulites of Chilka Lake area, northern part of Eastern Ghats Belt, India record multiple stages of metamorphism and deformation. The major mapable foliation is designated as transposed S2/S3 which shows folding due to later deformation. Dark-colored massive two pyroxene-bearing mafic granulites occur along the transposed S2/S3 fabric in metapelitic granulite, quartzofeldspathic granulite as well as anorthosite. The contact of this rock with the host metapelitic granulite is marked by a zone of garnet that presumably developed during subsolidus cooling and associated metasomatism. The two-pyroxene granulites occur as folded lenses, pinched-and-swelled bodies due to post-emplacement deformations. All these rocks contain orthopyroxene, clinopyroxene, plagioclase, ilmenite and magnetite as major minerals. Presence of exsolution textures in pyroxene grains is notable feature which possibly developed during subsolidus cooling. Trace element discrimination plot of six samples reveals a MORB-type signature, whereas selective trace element ratios and enriched LREE, flat HREE pattern indicate either Rift-related Basalt or Oceanic Island Basalt (OIB) type characters. Field relations and available geochronological data imply that the mafic magma was intruded within the time span of c. 980–800 Ma. As these mafic bodies and the host rocks were metamorphosed and deformed by the tectonothermal events at c. 800–500 Ma, the mafic magmatism could be correlated with globally recognized igneous activities related to the breakup of Rodinia of which the Eastern Ghats Belt remained an integral part.

Origin of Spinel + Quartz Assemblage in a Si-undersaturated Ultrahigh-temperature Aluminous Granulite and its Implication for the P–T–fluid History of the Phulbani Domain, Eastern Ghats Belt, India

A suite of high-grade rocks including felsic gneiss, aluminous granulite, charnockite and calcsilicate granulite is exposed at Phulbani, which belongs to a petrologically little understood crustal domain (Phulbani domain) of the Eastern Ghats Belt. The aluminous granulite is constituted of corundum þ spinel þ ilmenite þ garnet þ sillimanite þ quartz 6 K-feldspar 6 plagioclase 6 biotite. Textural analysis indicates that corundum, spinel, garnet and/or K-feldspar were formed as a result of biotite dehydration melting of a Si-poor protolith during prograde metamorphism. Although corundum and quartz coexist in micro-scale domains, phase diagram modelling suggests that garnetþ corundum þ spinel þ ilmenite þ sillimanite (up to 800 C at 8 kbar) and garnet þ spinel þ sillimanite þ ilmenite þ quartz (above 950 C at 8 kbar) assemblages were stabilized in two different temperature intervals while attaining the ultrahigh-temperature metamorphic peak. The transformation from corundumto quartz-bearing assemblages was principally governed by chemical reactions. Quartz, formed at the peak stage, produced complex reaction textures involving spinel, corundum, garnet and sillimanite during near-isobaric cooling. Intersection of the same mineral reactions during the prograde and the retrograde paths implies the near-closed-system behaviour of the lower crust, at least at microdomain-scale, possibly achieved after large-scale melt loss along the prograde-to-peak stage of evolution. The pressure–temperature path remained near-isobaric during the prograde and the retrograde evolution of the assemblages. High-density (up to 1 03 g cm) CO2-rich fluid inclusions in aluminous granulite, coarse-grained charnockite and felsic gneiss indicate that peak metamorphism and subsequent evolution occurred under a CO2-dominated fluid regime. The pressure–temperature–fluid evolutionary history of the Phulbani domain shows similarity to that of the adjacent Visakhapatnam domain of the Eastern Ghats Belt and poses questions on the status of the boundary separating these two domains.

Meso-Neoproterozoic mid-crustal metamorphic record from the Ajmer–Shrinagar section, Rajasthan, India and its implication to the assembly of the Greater Indian Landmass during the Grenvillian-age orogenesis

Abstract The Shrinagar–Ajmer section of the South Delhi Fold Belt exposes a package of medium-grade metasedimentary rocks intruded by synkinematic granite, and the entire package was thrust on top of the basement gneisses occurring further east. The metamorphic history is best developed in the staurolite schist that shows an overall increase in modal abundance of staurolite towards the east. Textural analyses, garnet zoning profiles, thermobarometric data and phase equilibria analyses show an increase in metamorphic pressure and temperature, reaching peak conditions of 592±12°C and 7.7±0.11 kbar. In situ monazite dating of a staurolite schist sample yields a pooled age of 980±22 Ma, which is assumed to be close to the age of the peak metamorphism. The Shrinagar granite was possibly emplaced close to the orogeny occurring at approximately 980 Ma and deformed by later events. The style and timing of metamorphism in the Shrinagar–Ajmer section match with the granulite-facies reworking of the basement rocks of the Aravalli–Delhi Mobile Belt. We envisage that the Grenvillian-age orogeny with its characteristic collisional style involved deep- to mid-crustal sections of the Aravalli–Delhi Mobile Belt. Our results further indicate that the Greater Indian Landmass was assembled during the formation of the supercontinent Rodinia. Supplementary material: Electron microprobe data of the garnet used for chemical zoning in Figure 5 are available at https://doi.org/10.6084/m9.figshare.c.3738335