Articles written in Journal of Earth System Science

    • Geochemical characterization of the siliciclastic rocks of Chitravati Group, Cuddapah Supergroup: Implications for provenance and depositional environment

      V Somasekhar S Ramanaiah D Srinivasa Sarma

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      Petrological and geochemical studies have been carried out on Pulivendla and Gandikota Quartzite from Chitravati Group of Cuddapah Supergroup to decipher the provenance and depositional environment. Both the units are texturally mature with sub-rounded to well-rounded and moderately to well-sorted grains. Majority of the framework grains are quartz, in the form of monocrystalline quartz, followed byfeldspars (K-feldspar and plagioclase), mica, rock fragments, heavy minerals, with minor proportion of the matrix and cement. Based on major element geochemical classification diagram, Pulivendla Quartzite is considered as quartz-arenite and arkose to sub-arkose, whereas Gandikota Quartzite falls in the field of lith-arenite and arkose to sub-arkose. Weathering indices like CIA, PIA, CIW, ICV, Th/U ratio and A–CN–K ternary diagram suggest moderate to intense chemical weathering of the source rocks of these quartzites. Whole rock geochemistry of quartzites indicate that they are primarily from the first-cycle sediments, along with some minor recycled components. Also their sources were mostly intermediate-felsicigneous rocks of Archean age. The tectonic discrimination plots, Th–Sc–Zr/10 of both these formations reflect active to passive continental margin setting. Chondrite-normalized rare earth element (REE) patterns, and various trace element ratios like Cr/Th, Th/Co, La/Sc and Th/Cr indicate dominantlyfelsic source with minor contribution from mafic source. Th/Sc ratios of Pulivendla and Gandikota Quartzite are in close proximity with average values of 2.83, 3.45 respectively, which is higher than AUCC (Th/Sc = 0.97), demonstrating that the contributions from more alkali source rocks than those that contributed to AUCC.

    • Geochemical characteristics of the Late Cretaceous radiolarian cherts from North Andaman Island, Bay of Bengal, India


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      The radiolarian cherts of Late Cretaceous age are reported in the Andaman-Nicobar ophiolites of Bay of Bengal, India. They are of chocolate-coloured (Type-I) and light pink-coloured (Type-II) varieties, juxtaposed and tectonically associated with phyllites, claystones and basalts and are exposed in eastern part of North Andaman Island. These cherts are composed of radiolarian tests in a fine-grained ferruginous matrix.Type-I radiolarian cherts have low $\rm{SiO}_{2}$, and high $\rm{Al_{2}O_{3}}$, $\rm{Fe_{2}O_{3}^{(T)}}$, $\rm{MgO}$, $\rm{Na_{2}O}$ and $\rm{TiO_{2}}$ and trace elements as compared to Type-II cherts. $\rm{Al–Fe–(\Sigma REE–Ce)}$ diagram of the studied cherts indicates a mixed terrigenous and volcanogenic source. $\rm{La_{N}/Ce_{N}}$ ratios (0.76–0.89 for Type-I and 0.71–0.88 for Type-II) and Ce-anomalies $\rm{(Ce/Ce^{\ast} = 1.15–1.33}$ for Type-I and 1.07–1.38 for Type-II) and other elemental ratios in these cherts suggest that they were deposited in continental margin environments. It has been suggested that the studied samples of cherts were deposited at different places, were scrapped off the subducting plate, became tectonically juxtaposed. They were obducted onto the leading edge of the Eurasian continent during the Late Cretaceous prior to the currently active Andaman–Java subduction, that was probably initiated during the Late Miocene.

    • Geochemistry and petrogenesis of tholeiitic dykes from the Chotanagpur Gneissic Complex, eastern India


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      In this study, we present the geochemical analysis on 14 samples from seven distinct E–W trending mafic dykes from the Chotanagpur Gneissic Complex. These dykes have not been studied previously and highlight the importance of igneous and tectonic processes in the Chotanagpur Gneissic Complex. The dykes, in terms of modal mineralogy, do not show notable variations, but textural variations are well noticed. These dykes are characterised as basalt and basaltic andesite (SiO$_{2}$ = 45.48–54.03 wt.%; Mg# = 21–68.5), and comparable with E-MORB type tholeiitic magma series. The dykes are classified into two groups, low Mg# and high Mg# dykes, based on their Mg# and silica content. The major fractionating mineral phases are olivine, plagioclase, and pyroxene. The dykes are variably contaminated with crustal input, as shown by Nb/U vs. (Th/Nb)PM, Th/Nb vs. La/Nb, and Th/Yb vs. Nb/Yb. The dykes also underwent post-magmatic hydrothermal alteration after their emplacement. Semi-quantitative trace element modelling suggests that these dykes are derived by partial melting within spinel peridotite-rich mantle and spinel–garnet peridotite-rich mantle source in the transition zone. The low Mg# dykes are consistent with 3–15% partial melting curves, whereas high Mg# dykes are comparable with 15% melting curve. Finally, we present a conceptual and simplified model for the E–W trending mafic dykes of the CGC, based on the geochemical data of the present study and the available information.

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