• ALONO THORIE

      Articles written in Journal of Earth System Science

    • Geochemistry of Neoproterozoic Chhaosa shales, Simla Group, Lesser Himalaya: Its implications on provenance and tectonics

      PRIYANKA MAZUMDAR ANANYA MUKHOPADHYAY ALONO THORIE TITHI BANERJEE SANTOSH KUMAR RAI

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      Neoproterozoic Chhaosa shales of Simla Group, Lesser Himalaya, were considered for major and trace element analysis to delineate palaeo-weathering, palaeo-oxygenation, tectonic setting, sediment maturity, and provenance. The shales exhibit a significant proportion of SiO$_{2}$, Al$_{2}$O$_{3}$, K$_{2}$O, MgO, Fe$_{2}$O$_{3}$, Zr, Zn, Rb, V, Cr, Sr, Y, Co, Ga, Th, Nb, Sc, and U. Source rocks are primarily of granitic (acidic) origin, as indicated by Al$_{2}$O$_{3}$ wt.% vs. TiO$_{2}$ wt.% and Cr (ppm) vs. Ni (ppm) bivariate plots. Th/Cr, Cr/Th, Th/Sc, and Th/Co values of studied samples have been correlated with post-Archaean Australian average shale (PAAS) and upper continental crust (UCC) values, which indicate that the source rock is felsic. Palaeo-weathering condition of the source is delineated by average CIA, chemical index of weathering (CIW), and A–CN–K triangular plot values. The shales depict intermediate to strong chemical weathering and semi-arid to humid climatic conditions. V/Cr, Cu/Zn, Ni/Co, and U/Th values reveal sedimentation of Chhaosa Formation under oxic atmospheric conditions, which corroborates with the inferred oxic conditions of the Neoproterozoic. SiO$_{2}$ vs. K$_{2}$O/Na$_{2}$O discriminant diagram exhibits deposition along a passive margin field. ICV values suggest deposition of compositionally mature sediments under tectonically stable conditions. Deposition of Chhaosa delta system occurred during a relatively quiescent stage of tectonism between intermittent phases of rifting related to the Rodinia Supercontinent.

      $\bf{Highlights}$

      $\bullet$ This is the first geochemical work ever done for the shales of Neoproterozoic Chhaosa Formation of Simla Group, Lesser Himalaya.

      $\bullet$K$_{2}$O/Na$_{2}$O vs. SiO$_{2}$/Al$_{2}$O$_{3}$, TiO$_{2}$ vs. Al$_{2}$O$_{3}$, and Zr vs. TiO$_{2}$ binary diagrams of the studied shales indicate felsic source rock.

      $\bullet$ High CIA, CIW, A–CN–K and Al$_{2}$O$_{3}$ values of Chhaosa shales support intermediate to strong weathering of source rock.

      $\bullet$ K$_{2}$O/Na$_{2}$O vs. SiO$_{2}$ plot, K$_{2}$O vs. Na$_{2}$O, plot and CaO–K$_{2}$O–Na$_{2}$O ternary plot represent deposition of Chhaosa deposits in a passive margin setting.

      $\bullet$ Values of Ni/Co, Cu/Zn, V/Cr, authigenic U and U/Th, represent a well-oxygenated condition during the deposition of the Chhaosa delta system. This is consistent with the concept of an increase in oxygen levels during the Neoproterozoic era is favoured by many workers.

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