• RAJNEESH BHUTANI

      Articles written in Journal of Earth System Science

    • Tectono-thermal evolution of the India-Asia collision zone based on40Ar-39Ar thermochronology in Ladakh, India

      Rajneesh Bhutani Kanchan Pande T R Venkatesan

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      New40Ar-39Ar thermochronological results from the Ladakh region in the India-Asia collision zone provide a tectono-thermal evolutionary scenario. The characteristic granodiorite of the Ladakh batholith near Leh yielded a plateau age of 46.3 ± 0.6 Ma (2σ). Biotite from the same rock yielded a plateau age of 44.6 ± 0.3 Ma (2σ). The youngest phase of the Ladakh batholith, the leucogranite near Himya, yielded a cooling pattern with a plateau-like age of ∼ 36 Ma. The plateau age of muscovite from the same rock is 29.8 ±0.2 Ma (2σ). These ages indicate post-collision tectono-thermal activity, which may have been responsible for partial melting within the Ladakh batholith. Two basalt samples from Sumdo Nala have also recorded the post-collision tectono-thermal event, which lasted at least for 8 MY in the suture zone since the collision, whereas in the western part of the Indus Suture, pillow lava of Chiktan showed no effect of this event and yielded an age of emplacement of 128.2 ±2.6 Ma (2σ). The available data indicate that post-collision deformation led to the crustal thickening causing an increase in temperature, which may have caused partial melting at the base of the thickened crust. The high thermal regime propagated away from the suture with time.

    • Rb–Sr and Sm–Nd study of granite–charnockite association in the Pudukkottai region and the link between metamorphism and magmatism in the Madurai Block

      M Chandra Sekaran Rajneesh Bhutani S Balakrishnan

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      Pudukkottai region in the northeastern part of the Madurai Block exposes the garnetiferous pink granite that intruded the biotite gneiss. Charnockite patches are associated with both the rock types. Rb–Sr biotite and Sm–Nd whole-rock isochron ages indicate a regional uplift and cooling at ∼550 Ma. The initialNd isotope ratios (ε^t_{Nd} = −20 to −22) and Nd depleted-mantle model ages (T_{DM} = 2.25 to 2.79 Ga) indicate a common crustal source for the pink-granite and associated charnockite, while the biotite gneiss and the charnockite within it represent an older crustal source (ε^t_{Nd} = −29 and T_{DM} =>3.2 Ga). TheRb–Sr whole-rock data and initial Sr–Nd isotope ratios also help demonstrate the partial but systematic equilibration of Sr isotope and Rb/Sr ratios during metamorphic mineral-reactions resulting in an ‘apparent whole-rock isochron’. The available geochronological results from the Madurai Block indicate four major periods of magmatism and metamorphism: Neoarchaean–Paleoproterozoic, Mesoproterozoic, mid-Neoproterozoic and late-Neoproterozoic. We suggest that the high-grade and ultrahigh-temperature metamorphism was preceded by magmatism which ‘prepared’ the residual crust to sustain the high P–T conditions. There also appears to be cyclicity in the tectono-magmatic events and an evolutionary model for the Madurai Block should account for the cyclicity in the preserved records.

    • Causal relationship between mafic magma underplating and migmatization of arc crust: Evidence from the Madras block of Southern Granulite terrane, India

      MOHD AZHAR UL HAQ S BALAKRISHNAN RAJNEESH BHUTANI JITENDRA K DASH

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      Neoarchean migmatized granodioritic gneisses and mafic enclaves from the Madras block of the Southern Granulite Terrain (SGT) were studied to understand their genetic relationship. The gneisses show calc-alkaline trend, more magnesian than tonalites, enrichment of LILE and LREE with HFSE depletion, and zero to slightly negative $\varepsilon$Nd values (t=2600 Ma) which indicate their precursors fractionated from sanukitoid magma generated by partial melting of hybridized mantle sources. Gabbroic magmas representing mafic enclaves with $\varepsilon$Nd values, –1.68 to +0.45, formed by partial melting of̄ fluid metasomatised mantle wedge and hybridized by interaction with granite magma. Underplating of these mafic magmas provided heat to trigger anatexis of the granodioritic arc-crust in the presence of H$_{2}$O and formation of granite melts (leucosomes). The leucosomes with peritectic amphiboles have higher REE with prominent negative Eu anomaly, while quartzo-feldspathic leucosomes have lower REE, concave upward HREE and positive Eu anomaly. Fractionation and/or entrainment of amphibole, apatite, allanite, titanite and zircon controlled REE and other trace element abundances of the leucosomes. Thus, underplating of mafic magma caused migmatization, magma mixing and differentiation and transformation of the arc crust in the NE part of the Madras block which represents deeper parts of the eastern Dharwar craton.

      $\bf{Highlights}$

      $\bullet$ Neoarchean migmatitic gneisses in the Madras block of southern granulite terrain represent granodioritic magmas derived from metasomatised mantle wedge in arc setting.

      $\bullet$ Fluid present melting of the granodiorite crust resulted in formation of leucosomes and small granite plutons.

      $\bullet$ Source–melt relationship between them is confirmed by overlapping $\varepsilon$Nd (t=2600 Ma) values (–1.42 to +1.25) for gneisses and leucosomes.

      $\bullet$ Gabbroic or dioritic mafic microgranular enclaves represent magmas hybridized with the crustal melts.

      $\bullet$ Underplating of mafic magmas triggered migmatization of the arc crust and mingling of mafic and granite magmas.

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