• CHANDAN KUMAR BORAIAHA

      Articles written in Journal of Earth System Science

    • Field, petrographic and geochemical characteristics of Sullya alkaline complex in the Cauvery Shear Zone (CSZ), southern India: Implications for petrogenesis

      CHANDAN KUMAR BORAIAHA KUMAR BATUK JOSHI ANDREW C KERR JAYANT KUMAR PADHI SARBARTHA SHANKAR MISHRA RASHMI CHANDAN

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      Significant, but volumetrically smaller, unmetamorphosed and largely undeformed alkaline magmatic suites have been reported from the Southern Granulite Terrain in southern India. These Neoproterozoic alkaline magmatic rocks occur as lenses, dykes and plugs that are mostly within, or proximal to, major shear zones or transcrustal faults. In this contribution, field, petrographic and whole-rock geochemicaldata of Sullya syenites and associated mafic granulites from the Mercara Shear Zone (MSZ), which separates low-grade (greenschist to upper amphibolite facies) Dharwar Craton and high-grade (granulite facies) Southern Granulite Terrain is presented. The isolated body of the Sullya syenite, similar to other alkaline plutons of the Southern Granulite Terrain, shows an intrusive relationship with the host hornblende-biotite gneisses and mafic granulites. The Sullya syenites lack macroscopic foliations and unlike, other plutons, they are not associated with carbonatites and ultrapotassic granites. Potash feldspar and plagioclase dominates the felsic phases in the Sullya syenite and there is negligible quartz. The studied syenites show evidence of melt supported deformation, but show no evidence of recrystallization. Geochemically,they most resemble the Angadimogar syenites (situated 3 km west of the Sullya syenites) with similar major oxide and trace element concentrations. The petrogenetic studies of the Sullya syenite have indicated that they were generated by mixing of two different sources derived from the partial melting of metasomatized continental mantle lithosphere and lower crustal mafic granulites. This melt source could have been emplaced in a rift-related tectonic setting. The emplacement is considered to becontrolled by shears.

    • Geochemistry of the Precambrian mafic dykes in and around the Western Ghats greenstone belt, Western Dharwar Craton

      CHANDAN KUMAR BORAIAHA

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      The preliminary field, petrographic and whole-rock geochemical data of the mafic dykes in and around the Western Ghats belt (WGB), Western Dharwar Craton (WDC) is presented in this study. The NNW–SSE trending tholeiitic mafic dykes are basalt to basaltic-andesite in composition. The studied dykes are generally LREE enriched and exhibit negative Nb–Ta and Ti anomalies indicating crustal contamination. Petrological modelling shows that the chemical variability of the dykes is primarily due to crystal fractionation and crustal contamination. Rare earth element (REE) modelling suggests that the WGB dykes were likely derived by low-degree partial melting (${\sim}$3.5%) of a garnet-spinel bearing mantle source. The close similarity in the Beld, petrography and geochemical characteristics between the studied dykes and the 2.21 Ga mafic dykes of WDC suggests that the WGB dykes may represent the 2.21-Ga mafic event in the WDC.

    • Origin of the Neoarchean granites from the southeastern margin of the Western Ghats greenstone belt, Dharwar Craton: Implications for crustal evolution in the Western Dharwar Craton

      CHANDAN KUMAR BORAIAHA

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      The field, crystal size distribution, mineral-chemistry and bulk-rock geochemical study on the felsic magmatic rocks (granite and associated rhyolites) of the Western Ghats belt, Dharwar Craton, has been presented for the first time in this contribution. These Neoarchean felsic magmatic rocks occur as massive plugs and lensoidal bands running for several meters to kilometres, with a general trend of NNW–SSE. These rocks are characterised by the ubiquitous presence of biotite and muscovite and are devoid of amphiboles. These rocks show evidence of textural coarsening but show no evidence of magma mixing or assimilation. The petrogenetic studies indicate that the partial melting of tonalite–trondhjemite–granodiorite (TTG) crust together with a considerable amount of Al-rich sediments has resulted in the peraluminous granitic melt source. This melt, subsequently, was mobilised to shallow crustal levels where it was emplaced along the axial planes of F1 folds before it was erupted on to the surface, giving rise to rhyolites. The similarities in the field, mineralogical and geochemical characteristics amongst ${\sim}$2.61 Ga old felsic volcanic rocks of the different greenstone belts in the Western Dharwar Craton (WDC) suggest that they were all derived from the partial melting of ${\ge}$3.1 Ga felsic crust with a minor input of metasedimentary units at mid-crustal levels. The felsic volcanic rocks show evidence of emplacement in the active plate boundary, the interior of WDC.

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