• HIREDYA CHAUHAN

      Articles written in Journal of Earth System Science

    • 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.

    • A new analytical protocol for high precision U–Th–Pb chemical dating of xenotime from the TTG gneisses of the Bundelkhand Craton, central India, using CAMECA SXFive Electron Probe Micro Analyzer

      HIREDYA CHAUHAN APARAJITA TRIPATHI DINESH PANDIT N V CHALAPATHI RAO TALAT AHMAD

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      Xenotime is a significant accessory mineral which is being extensively used for precise U–Th–Pb geochronology by Electron Microprobe Analysis (EPMA). This paper presents a protocol for high analytical precision (<3% uncertainties on the measured ages) developed for the accurate estimation of U–Th and Pb content in xenotime using SXFive EPMA at the Department of Geology, Banaras Hindu University, by deploying five spectrometers attached with TAP, LIF, LPET, LTAP and PET crystals. The protocol is applied to the xenotime grains of tonalite-trondhjemite-granodiorite-gneiss (TTG) rocks from the geochronologically well-constrained terrain of the Bundelkhand Craton, central India. The obtained xenotime age 2929$\pm$23 Ma of TTGs is in agreement with the earlier published Neoarchaen 2697$\pm$3 Ma Pb–Pb zircon ages from the same area which validates the authenticity of the analytical method developed at the BHU-EPMA facility.

      $\bf{Highlights}$

      $\bullet$ Analytical protocol for high precision U–Th–Pb chemical dating of xenotime by EPMA.

      $\bullet$ High precision ages from TTG gneiss of the Bundelkhand Craton, Central India.

      $\bullet$ Ages distinguishable from earlier reported ages from other techniques and samples.

      $\bullet$ Validates the authenticity of the analytical method developed at the BHU-EPMA facility.

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