Articles written in Journal of Earth System Science

    • Strain analysis and stratigraphic status of Nongkhya, Sumer and Mawmaram conglomerates of Shillong basin, Meghalaya, India

      Niva Rani Devi K P Sarma

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      Shillong basin, one of the Purana basins of the Indian peninsula is situated in the central and eastern parts of the Shillong plateau of NE India. Metasedimentary and metavolcanic rocks of the basin are of Mesoproterozoic age and lithostratigraphically belong to Shillong Group (erstwhile Shillong series) comprising Lower Metapelitic Formation (LMF) and Upper Quartzitic Formation (UQF). A long, persistent, faulted and tectonically attenuated conglomerate known as Nongkhya–Sumer– Mawmaram-conglomerate separates these two formations. In the present work, quantitative strain analyses of the pebbles of Sumer, Nongkhya and Mawmaram conglomerates of Ri-Bhoi and West Khasi Hills districts of Meghalaya are carried out using manual and computerized programmes. Eight different techniques for intrapebble, interpebble and bulk rock strain estimation are applied and results are compared systematically. Sumer and Mawmaram conglomerates bear the testimony of broadly flattening type of deformation (0 > 𝑘 > 1) while Nongkhya conglomerate shows constriction type (1 > 𝑘 > 𝛼). The change in strain ellipsoid shape from Nongkhya to Mawmaram area is accompanied by a change of fabric from 𝑆 > 𝐿 to 𝐿 > 𝑆 tectonites. Affinity of rotational strain is more in Nongkhya conglomerate as compared to Sumer and Mawmaram conglomerates. The compactness of pebbles is high in case of Nongkhya conglomerate and low to moderate in Sumer and Mawmaram conglomerates indicating high strain in Nongkhya conglomerate (northeastern part of Shillong basin) relative to Sumer and Mawmaram conglomerates (southwestern part of Shillong basin). Thus strain magnitude increases from SW to NE direction of the Shillong basin.

      The tectonostratigraphic status of these conglomerates suggest that the Sumer and Mawmaram conglomerates were initially a part of one conglomerate horizon of interformation type between LMF and UQF of the Shillong Group. With progressive deformation, the northeastern part of the Sumer conglomerate suffered tectonic attenuation and separation and eventually thrusted over the Basement Gneissic Group (BGG) as a tectonic mélange. This sector of the conglomerate is known as Nongkhya conglomerate. The tectonic configuration of the Nongkhya conglomerate is the effect of right and left lateral strike slip movement of Sumer conglomerate at Sumer and Adabasti points, respectively. This is a positive signature of post 𝐷_{3} deformation on the Sumer conglomerate. The regional sigmoidal pattern of the interformational conglomerate broadly correlate with the Tyrsad-Barapani Shear Zone (TBSZ) of sinistral nature.

    • Phase petrographic, thermobarometric and petrochemical significance of Cretaceous mafic dykes along Nongchram Fault Zone of Swangkre–Rongmil area of Shillong plateau, NE India: Implications for genetic link to Kerguelen mantle plume


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      The present study elucidates the phase petrographic and petrochemical signatures of a group of Cretaceous maBc dykes emplaced in the Precambrian gneissic basement complex in the western part of the Shillong plateau, NE India, in order to trace their petrological, geotectonic, geothermobarometric and oxybarometric status. The whole rock geochemistry discriminates the dykes as basaltic andesites and basaltic trachyandesites genetically related to each other and derived from common parent magma. The enrichment in LREE relative to HREE and HFSE, systematic Nb anomalies, moderate MREE to HREE fractionation suggests variable depths of melting of slightly enriched mantle source in the garnet stability field. As per geochemical modelling, the studied dykes are derived by 3–5% non-modal batch melting of garnet peridotite source at melting depth of $\sim 65–80\rm{km}$. The clinopyroxene thermobarometry reveals a temperature span of $1250–800^{0}\rm{C}$ and <2 kb pressure of crystallization for the dykes. The oxygen fugacity (16.82–18.25) indicates extremely reducing conditions at the time of cooling. The very good correlation of petrochemical and phase chemical data with Kerguelen plume derived Rajmahal Group II basalt, Sylhet volcanics and some ODP (ocean drilling project) sites from Kerguelen basalts implicate a genetic link of the studied dykes with Kerguelen mantle plume. Finally, the present study deciphers subalkaline nature of the studied dyke rocks that have been generated by tholeiitic magmas probably in an anorogenic extensional environment. But we need more geochemical data especially good isotope geochronologic data to get a clear picture of the studied dyke.

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