• RASHMI CHANDAN

      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.

    • Petrogenesis of the Late Archean Kanara Batholith of the Western Dharwar Craton: Evidence for mid-deep crustal recycling of the Archean felsic crust

      JAYANT K PADHI CHANDAN K BORAIAHA RASHMI CHANDAN

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      Kanara Batholith is an intrusive granitoid pluton into the basement biotite-gneisses and the supracrustal rocks of the Western Ghats Belt (WGB). The pluton is situated in the western margin of the Western Dharwar Craton (WDC). These granitoids are classified as granodiorite and granite based on their field and petrographic characteristics. Based on the abundance and presence of alkali-feldspar phenocrysts, the granites are further classified as porphyritic and non-porphyritic granites. Biotite$\pm$ amphibole is a dominant mafic mineral phase in the granodiorites, whereas amphiboles are absent in the studied granites. Textural coarsening has played a significant role in the growth of the mineral crystals within the studied granitoids. Thermobarometric study suggests that the granites were emplaced and crystallized at pressures between 4.32 and 4.92 kbar and temperatures between 548$\pm$ 15$^{\circ}$ and 715$\pm$ 15$^{\circ}$C. Further, it is estimated that the granitoid magma intruded the gneissic country rocks and the supracrustal sequences of the WGB corresponding to a depth of ${\sim}$15 to 17 km. Geochemical evidence indicates that the source magmas of the studied granitoids were derived from the interaction between (i) a melt derived from the partial melting of the basement biotite gneisses and (ii) a melt derived from the partial melting of the early Archean metasedimentary rocks that form enclaves within the basement biotite gneisses. These two source magmas have interacted subsequently to yield a hybrid felsic magma, resulting in the Kanara Batholith formation. The interaction between the melts took place at shallow to mid-crustal levels at pressures $\lesssim$ 5 kbar before the crystallization began.

      $\bf{Highlights}$

      $\bullet$ Kanara Batholiths are the products of hybrid magmatism.

      $\bullet$ They were emplaced in the shallow to mid-crustal levels.

      $\bullet$ Mid-deep crustal recycling of the Paleoarchean crust in the Dharwar Foreland region resulted in the formation of the source magmas for these granitoids.

      $\bullet$ The studied granitoids contains multiple phases of magma injections in to the basement biotite-gneisses and the Western Ghats greenstone belt.

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