• JITENDRA K DASH

      Articles written in Journal of Earth System Science

    • Geochemistry of ultramafic–mafic rocks of Mesoarchean Sargur Group, western Dharwar craton, India: Implications for their petrogenesis and tectonic setting

      KIRANMALA PATRA R ANAND S BALAKRISHNAN JITENDRA K DASH

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      The Nuggihalli and Holenarsipur greenstone belts of the western Dharwar craton expose ultramafic–mafic rocks of the Mesoarchean. The rocks in these belts are geochemically considered as komatiites and komatiitic basalts with minor occurrences of tholeiitic and calc-alkaline basalts. The dominant ultramaficrocks of the Nuggihalli greenstone belt are layered and indicate fractionation processes at relatively shallower crustal levels. The Al-undepleted and Al-depleted signatures obtained could be attributed to magmatic differentiation processes and might be due to fractional crystallization of minerals such as hornblende and plagioclase, in addition to cumulus olivine and pyroxene. The chemical heterogeneity in the rocks of these greenstone belts might have therefore developed during the intrusion of the parental melts and their differentiation into a layered igneous complex. The differences in the lithological characteristics of the Holenarsipur and Nuggihalli greenstone belts can be explained by their different crustal levels of exposure. Presence of spinifex-textured komatiites need not necessarily imply that the sources have to be ultramafic and therefore of a deeper origin. This study indicates that the parental melts for unambiguous layered intrusive ultramafic–mafic complexes could be high-Mg basalts originating from relatively shallower levels. The probable geodynamic setting for the emplacement of the rocks of the two greenstone belts could be in a plume-modified mid-ocean ridge that was too thick and buoyant to be subducted, and the decompression-melted magma chamber developed igneous layering as the magma stalled in the lithosphere.

    • 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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