• S Balakrishnan

      Articles written in Journal of Earth System Science

    • 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.

    • Fluid inclusion, geochemical, Rb–Sr and Sm–Nd isotope studies on tungsten mineralized Degana and Balda granites of the Aravalli craton, NW India

      Sundarrajan Vijay Anand M S Pandian S Balakrishnan R Sivasubramaniam

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      Granitic plutons occurring within and to the west of the Delhi Fold Belt in the Aravalli craton, northwestern India are the result of widespread felsic magmatism during Neoproterozoic, some of which are associated with greisen and skarn tungsten deposits. In this paper, we present the result of our study on fluid inclusions, geochemistry and geochronology of two such tungsten mineralized granite plutons at Degana and Balda, and interpret the nature of ore fluid, and petrogenesis and age of these mineralized granites. Fluid inclusion study reveals coexistence of moderate and hyper-saline aqueous fluid inclusions along with aqueous-carbonic inclusions, suggesting their origin due to liquid immiscibilityduring fluid–rock interaction. Geochemically, the granites are peraluminous, Rb enriched, Sr and Ba depleted and highly differentiated. The Rb–Sr isotopic systematics yielded 795±11 Ma for Balda granite and 827 ± 8 Ma for Degana granite. We show that major phase of widespread granitoid magmatismand mineralization during the Neoproterozoic (840–790 Ma) in NW India is coeval with breakup of the Rodinia supercontinent and infer a causal relationship between them.

    • Geochemistry and petrogenesis of acidic volcanics from Betul–Chhindwara Belt, Central Indian Tectonic Zone (CITZ), central India


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      Betul–Chhindwara belt is part of Central Indian Tectonic Zone (CITZ) that includes Proterozoic basalt, rhyolite, quartzite, mafic–ultramafic rocks, volcano sediments and banded iron formation (BIF). Studied rhyolites and leuco-micro granites are deformed due to shearing and includes quartz, K-feldspar (microcline), muscovite, biotite and epidote. In some samples, feldspar has been sericitized due to interaction with hydrothermal fluids. The major element geochemistry of volcanic rocks clearly indicates acidic nature and falls in the rhyolite field. Rhyolites show difference in the enrichment of REEs and major element composition which help us divide them into two groups and also indicate heterogenous source. The rhyolites show very strong negative Eu anomaly, which indicates fractionation of feldspar. Positive anomalies of U–Th–Zr for the rhyolites indicate crustal involvement. The $\varepsilon\rm{Ndt (t=1500)}$ for the Group I rhyolites vary from –1.42 to –0.19 and for the Group II rhyolites vary from –5.81 to +0.14 and DM model ages for Group I rhyolites vary from 2284 to 2464 Ma and for Group II vary from 2174 to 2863 Ma. It is suggested that contemporary mafic magma of the Betul–Chhindwara belt while ascending from mantle sources interacted with the continental crust at different levels, supplying heat and fluids which reduced the melting points of the crustal source rocks, producing felsic melt of varying compositions. Tectonic discriminant diagrams and geochemical data indicate subduction zone tectonic environment for the genesis of the Betul–Chhindwara acidic volcanism. The acidic volcanics of Betul–Chindwara, Sakoli and the Bijli rhyolites from the adjoining areas display similarity in terms of the total alkali vs. silica diagram and many of the major and trace elements, including rare earth element characteristics. Compared to Betul Rhyolite, Sakoli Rhyolites are derived from less enriched source with less involvement of crust and/or the latter represents high degree of partial melting of similar source. They are considered contemporaneous to Betul Rhyolite based on geochronological data. Contrastingly, Bijli Rhyolite show highly fractionated patterns with high LREE enrichment indicating considerable crustal involvement which is very obvious for within plate magmatism, assigned for the Bijli rhyolites.

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


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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.

    • Zircon SHRIMP U–Pb geochronology, geochemical and Nd isotope systematics of Neoarchean granitoids, Gadag Greenstone Belt, Dharwar Craton, southern India: Petrogenesis and tectonic significance


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      Coupling of the geological processes in the arc-magmatic and back-arc provinces of accretionary orogens in Neoarchean plate tectonic setting is a subject of current research all over the world. The Dharwar Craton of southern India is an example of such an accretionary orogen, with an arc-magmatic province in the east and a back-arc province in the west, referred as the Eastern Dharwar Craton and the Western Dharwar Craton, respectively. The boundary between the two provinces is considered to be marked by a 400-km long shear zone along the eastern margin of the Gadag–Chitradurga–Karighatta greenstone belt which is called as the Chitradurga Boundary Shear Zone. Potassic, metaluminous, I type, calc-alkalic to alkali-calcic, arc-magmatic granitoids are widespread in the EDC. But they are also found to occur along the western margin of the Chitradurga–Gadag greenstone belt. SHRIMP U–Pb zircon ages of the granitoids in the western back-arc province in the Gadag region occurring near Srimant Gudda, Mulgund and Chabbi have been determined as 2565  $\pm$   24 to 2591  $\pm$   64 Ma old. Within errors, the ages of these granitoids are the same as the Lakundi and Turchihal granitoids occurring to the east of the Gadag Greenstone Belt in the arc-magmatic province. Nd isotope systematics of the granitoids suggest that they were formed from magmatic melts that were produced by remelting of 3200–3500 Ma old heterogenous continental crust. Rare inherited zircons support this antiquity of the protoliths. Occurrence of granitoids of similar age and origin, in the western back-arc province and eastern arc-magmatic province in the Gadag area was attributed to thrust duplex structure in the Gadag region. However, elsewhere, along the western margin of the Chitradurga greenstone belt near Harpanhalli, Hosdurga–Nagamangala–Pandavapura sector, or away from it in the Arsikere–Banavara, where repetition by thrusting is not obvious, late potash arc-magmatic type granitoids of similar age as the Gadag arc-magmatic granitoids are observed. The arc-magmatic type granitoids appear to have overstepped the boundary shear into the back-arc province at several places. Arc-magmatic and back-arc boundary may be diffuse rather than sharp, as also suggested by some earlier workers.


      $\bullet$ The ages and petrogenesis of the granitoids around GGB of both the sides of EDC (i.e., eastern magmatic-arc province) and WDC (i.e., western back-arc provinces) are very much similar.

      $\bullet$ The geochemistry and isotope systematics (i.e., Nd TDM2 ages and εNdT values at 2.5–2.6 Ga) of the granitoids exposed one both the sides are also similar.

      $\bullet$ This shows not only a similar antiquity of both the provinces, but also gives evidence for a possible diffusive nature of boundary between the EDC and the WDC around GGB.

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


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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.


      $\bullet$ 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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