• Granite series assessment, nature and crystallization condition of Paleoproterozoic granite gneisses from Askot and Chiplakot klippe, Kumaun Lesser Himalaya, India

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    • Keywords


      Granite series; biotite; muscovite; granite gneisses; Askot–Chiplakot; Kumaun Lesser Himalaya; India.

    • Abstract


      Felsic magmatic rocks of Askot and Chiplakot regions in the inner segment of the Kumaun Lesser Himalaya are represented by the Paleoproterozoic two-mica (biotite–muscovite) granite gneisses (ca. 1850 Ma), which are referred herein as Askot (AGGn) and Chiplakot (CGGn) granite gneisses, respectively. They are invariably metamorphosed, giving rise to augen bearing or augen free gneissose texture exhibited mainly by micaceous minerals. They bear a common bt-ms-pl-qz-Kf-zrn-ap-ttn±mag assemblage. The magnetic susceptibility (MS) mapping, phase petrology of biotite and muscovite from AGGn and CGGn are employed to assess the granite series, nature and crystallization condition of their respective granitic magmas. Although the observed average MS values of the AGGn (0.019–0.028${\times}$10$^{–3}$ SI) and CGGn (0.051–0.098${\times}$10$^{–3}$ SI) are slightly distinct, both typically belong to ilmenite series (reduced) granites. The AGGn and CGGn biotites are primary, siderophyllite, belonging to ferri-siderophyllite transition and ferri biotites, respectively, which crystallized with muscovite unaccompanied with other mafic minerals. The AGGn and CGGn muscovites are primary in nature, belonging to celadonite and paragonite solid solution series, which evolved in peraluminous (S-type) granite melt under reduced conditions.The AGGn and CGGn biotite and muscovite thus represent primary liquidus phases, not the secondary or restite or metamorphic products except a few tiny muscovites. The observed FeO$^t$/MgO ratio of AGGn (5.47–10.72; av. 7.78) and CGGn (2.44–3.69; av. 2.90) biotites dictate anorogenic alkaline (A-type) and syn-collisional peraluminous (S-type) host granite magmas, respectively. However, siderophyllite (aluminous) nature and vital 3Fe ${\Leftrightarrow}$2Al substitution of AGGn and CGGn biotites strongly propound their evolution in peraluminous (S-type) granite magmas. The CGGn biotites are enriched in phlogopite (Mg$_{apfu}$ = 1.75) as compared to AGGn(Mg$_{apfu}$=0.88) biotites,which probably reflect derivation of CGGn melt from crustal source with slightly more mafic as compared to the crustal source of AGGn melt. The estimated physico-chemical conditions of AGGn (P=3.6–4.21 kbar,T=690–780°C, f O$_2$=10$^{–16.29}$ to 10$^{–15.72}$ bars, Fe$^{3+}$/Fe$^{2+}$=0.12–0.16, H$_2$O${\thickapprox}$4 wt.%) and CGGn (P = 3.03–6.63 kbar, T = 750–840°C, f O$_2$ = 10$^{–16.54}$ to 10$^{–14.22}$ bars, F$^{3+}$/Fe$^{2+}$ = 0.04–0.11, H$_2$O${\thickapprox}$3 wt.%) point to strongly reduced and moderately reduced nature of respective host magma, that prevailed at mid-crustal depths. The differential reducing conditions of host magma evolution are unequivocally demonstrated by the stability of AGGn and CGGn biotites from the Fayalite–Magnetite–Quartz (FMQ) to the above Nickel-Nickel Oxide (NNO) buffers and observed MS values.

    • Author Affiliations



      1. Department of Geology, Centre of Advanced Study, Kumaun University, Nainital 263 001, India.
    • Dates

  • Journal of Earth System Science | News

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