• Ashima Saikia

      Articles written in Journal of Earth System Science

    • Insights into the P–T evolution path of TsoMorari eclogites of the north-western Himalayas: Constraints on the geodynamic evolution of the region

      Preeti Singh Ashima Saikia Naresh Chandra Pant Pramod Kumar Verma

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      The present study is on the Ultra High Pressure Metamorphic rocks of the Tso Morari Crystalline Complex of the northwestern Himalayas. Five different mineral associations representative of five stages of P–T (pressure–temperature) evolution of these rocks have been established based on metamorphic textures and mineral chemistry. The pre-UHP metamorphic association 1 of Na-Ca-amphibole + epidote ± paragonite ± rutile ± magnetite with T–P of ∼500° C and 10 kbar. This is followed by UHP metamorphic regime marked by association 2 and association 3. Association 2 (Fe< Mg< Ca-garnet + omphacite + coesite + phengite + rutile ± ilmenite) marks the peak metamorphic conditions of atleast 33 kbar and ∼750° C. Association 3 (Fe< Mg< Ca-garnet + Na-Ca amphibole + phengite ± paragonite ± calcite ± ilmenite ± titanite) yields a P–T condition of ∼28 kbar and 700°C. The post-UHP metamorphic regime is defined by associations 4 and 5. Association 4 (Fe< Ca< Mg-garnet + Ca-amphibole + plagioclase (An05) + biotite + epidote ± phengite yields a P–T estimate of ∼14 kbar and 800°C) and association 5 (Chlorite + plagioclase (An0.5) + quartz + phengite + Ca- amphibole ± epidote ± biotite ± rutile ± titanite ± ilmenite) yields a P–T value of ∼7 kbar and 350°C.

    • Geochemical constraints on the evolution of mafic and felsic rocks in the Bathani volcanic and volcano-sedimentary sequence of Chotanagpur Granite Gneiss Complex

      Ashima Saikia Bibhuti Gogoi Mansoor Ahmad Talat Ahmad

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      The Bathani volcanic and volcano-sedimentary (BVS) sequence is a volcanic and volcano-sedimentary sequence, best exposed near Bathani village in Gaya district of Bihar. It is located in the northern fringe of the Chotanagpur Granite Gneiss Complex (CGGC). The volcano-sedimentary unit comprises of garnet-mica schist, rhyolite, tuff, banded iron formation (BIF) and chert bands with carbonate rocks as enclaves within the rhyolite and the differentiated volcanic sequence comprises of rhyolite, andesite, pillow basalt, massive basalt, tuff and mafic pyroclasts. Emplacement of diverse felsic and mafic rocks together testifies for a multi-stage and multi-source magmatism for the area. The presence of pillow basalt marks the eruption of these rocks in a subaqueous environment. Intermittent eruption of mafic and felsic magmas resulted in the formation of rhyolite, mafic pyroclasts, and tuff. Mixing and mingling of the felsic and mafic magmas resulted in the hybrid rock andesite. Granites are emplaced later, crosscutting the volcanic sequence and are probably products of fractional crystallization of basaltic magma. The present work characterizes the geochemical characteristics of the magmatic rocks comprising of basalt, andesite, rhyolite, tuff, and granite of the area. Tholeiitic trend for basalt and calc-alkaline affinities of andesite, rhyolite and granite is consistent with their generation in an island arc, subduction related setting. The rocks of the BVS sequence probably mark the collision of the northern and southern Indian blocks during Proterozoic period. The explosive submarine volcanism may be related to culmination of the collision of the aforementioned blocks during the Neoproterozoic (1.0 Ga) as the Grenvillian metamorphism is well established in various parts of CGGC.

    • Episodic crustal growth in the Bundelkhand craton of central India shield: Constraints from petrogenesis of the tonalite–trondhjemite–granodiorite gneisses and K-rich granites of Bundelkhand tectonic zone

      Hiredya Chauhan Ashima Saikia Talat Ahmad

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      Tonalite–trondhjemite–granodiorite gneisses (TTG) and K-rich granites are extensively exposed in the Mesoarchean to Paleoproterozoic Bundelkhand craton of central India. The TTGs rocks are coarsegrained with biotite, plagioclase feldspar, K-feldspar and amphibole as major constituent phases. The major minerals constituting the K-rich granites are K-feldspar, plagioclase feldspar and biotite. They are also medium to coarse grained. Mineral chemical studies show that the amphiboles of TTG are calcic amphibole hastingsite, plagioclase feldspars are mostly of oligoclase composition, K-feldspars are near pure end members and biotites are solid solutions between annite and siderophyllite components. The K-rich granites have biotites of siderophyllite–annite composition similar to those of TTGs, plagioclase feldspars are oligoclase in composition, potassic feldspars have XK ranging from 0.97 to 0.99 and are devoid of any amphibole. The tonalite–trondhjemite–granodiorite gneiss samples have high SiO₂ (64.17– 74.52 wt%), Na₂O (3.11–5.90 wt%), low Mg# (30–47) and HREE contents, with moderate (La/Yb)CN values (14.7–33.50) and Sr/Y ratios (4.85–98.7). These geochemical characteristics suggest formation of the TTG by partial melting of the hydrous basaltic crust at pressures and depths where garnet and amphibole were stable phases in the Paleo-Mesoarchean. The K-rich granite samples show high SiO₂ (64.72–76.73 wt%), K₂O (4.31–5.42), low Na₂O (2.75–3.31 wt%), Mg# (24–40) and HREE contents, with moderate to high (La/Yb)CN values (9.26–29.75) and Sr/Y ratios (1.52–24). They differ from their TTG in having elevated concentrations of incompatible elements like K, Zr, Th, and REE. These geochemical features indicate formation of the K-granites by anhydrous partial melting of the Paleo-Mesoarchean TTG or mafic crustal materials in an extensional regime. Combined with previous studies it is interpreted that two stages of continental accretion (at 3.59–3.33 and 3.2–3.0 Ga) and reworking (at 2.5–1.9 Ga) occurred in the Bundel khand craton from Archaean to Paleoproterozoic.

    • P–Ͳ estimates for the fractionated and primary melt of tholeiitic dykes from Multai area of Deccan flood basalt, Madhya Pradesh (India)


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      Petrological, mineral chemical and geochemical results are reported for the Cenozoic en-echelon tholeiitic dykes of the Multai area of Deccan flood basalt province, Madhya Pradesh (India) to estimate the pressure and temperature of fractionated and primary melt for these dykes. The rocks are composed of plagioclase, augite and olivine phenocrysts set in a holocrystalline groundmass of plagioclase microlites, augite, Ti–magnetite, ilmenite and interstitial glass. Plagioclases are labradorite in composition (An$_{48–70}$), and pyroxenes are augite with a composition of Wo$_{24–40}$, En$_{24–52}$ and Fs$_{13–56}$. Forsterite (Fo) contents of olivine phenocrysts range from Fo$_{51}$ to Fo$_{73}$. These tholeiitic dykes are rich in Al$_2$O$_3$, Rb, Ba and Sr and show enrichment in light rare-earth element relative to heavy rare-earth element with respect to enriched mid-ocean ridge basalt and normal mid-ocean ridge basalt. Studied samples have lower Ni, Cr, Co and MgO contents than primary compositions suggesting evolved nature of these rocks. The Sr–Nd isotopic ratios of the studied dyke samples indicate a Dnity to the Mahabaleshwar and Poladpur formations of the southwestern Deccan stratigraphy and the positive εNd(t) values suggest depleted mantle source. The fractionated melt for these dykes last equilibrated at P = 0.2–4.4 kbar and Ͳ = 1128–1169°C before the eruption, based on olivine, clinopyroxene and plagioclase mineral-melt equilibria thermo-barometers. The estimated mantle source and primary melt compositions suggest melting in the spinel stability field (P${\le}$28 kbar). It was followed by melt equilibration with mantle olivine Fo$_{89.6}$ at P = 18–22 kbar and Ͳ = 1419–1463°C. The evaluation of whole-rock geochemistry and mineral chemistry supports the hypothesis of fractional crystallisation of plagioclase + clinopyroxene ± olivine ± Fe–Ti oxides for the evolution of these basaltic dykes

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