• A R Vijan

      Articles written in Journal of Earth System Science

    • K-Ar age of Ukra glauconites from the Kutch Basin, India

      S S Rathore B N Prabhu A R Vijan K C Vig K N Misra

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      The present paper reports the K-Ar ages determined on glauconitic samples collected from the Ukra Member of the Mesozoic Bhuj Formation in two different sections, one located on the Ghuneri-Ghaduli road near Katesar Mahadeo temple and the other at the base of the Ukra hill in the northwestern part of the Kutch Mainland area.

      Three glauconite samples viz., UkraKT-1, UkraKT-4 and UkraUH-3 have yielded K-Ar ages of 107.9 ± 3.4 Ma, 105.5 ± 3.3 Ma and 103.5 ± 3.4 Ma, respectively. The sample UkraKT-l treated with 0.5N HC1 and analysed in duplicate has yielded a mean age of 104 ± 2.3 Ma while the sample UkraKT-4 treated with 0.1N HCl has given an age of 106.5 ± 3.3 Ma. The ages of the treated and untreated glauconites are indistinguishable within 2σ uncertainty with a mean of 105.2 ± 1.3 Ma, which has been interpreted as the depositional age of the Ukra Member of the Mesozoic Bhuj Formation. The study has further indicated that mild acid treatment (up to 0.5N HCl) does not lead to any loss of radiogenic argon in the glauconites and can be helpful in purification of the samples.

    • Isotopic evidence of middle proterozoic magmatism from bombay high field: Implications to crustal evolution of western offshore of India

      S S Rathore A R Vijan M P Singh B N Prabhu Anand Sahu

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      Precambrian granitic basement rocks obtained from well BH-36 of Bombay High Field, western offshore of India has been studied both by Rb-Sr and K-Ar dating methods. Seven basement samples chosen from two cores have yielded whole rock Rb-Sr isochron age of 1446 ± 67 Ma with an initial87Sr/86Sr ratio of 0.7062 ± 0.0012. This age has been interpreted as the formation/emplacement time of the granite. Two biotite fractions of different grain size separated from a sample CC6B2T have yielded Rb-Sr mineral isochron age of 1385 ± 21 Ma. However, these fractions when studied by K-Ar dating method have yielded slightly higher but mutually consistent ages of 1458 ± 43 Ma and 1465 ± 43 Ma, respectively.

      Further, two biotites separated from additional samples CC5B9T and CC6B3B have yielded K-Ar ages of 1452 ± 42 Ma and 1425 ± 40 Ma with an overall mean age of 1438 ± 19 Ma. This mean K-Ar age is indistinguishable from whole rock Rb-Sr isochron as well as mineral isochron age within experimental error. The similarity in the whole rock and biotite ages obtained by different isotopic methods suggests that no thermal disturbance has occurred in these rocks after their emplacement/formation around 1450 Ma ago. The present study provides the evidence for the existence of an important Middle Proterozoic magmatic event around 1400-1450 Ma on the western offshore of India which, hitherto, was thought to be mainly confined to the eastern Ghats, Satpura and Delhi fold belt of India. This finding may have an important bearing on the reconstruction of Proterozoic crustal evolution of western Indian shield.

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