Isotopic evidence of middle proterozoic magmatism from bombay high field: Implications to crustal evolution of western offshore of India |
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Authors: | S S Rathore A R Vijan M P Singh B N Prabhu Anand Sahu |
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Institution: | (1) Forward Base Office, Assam & Assam Arakan Basin-Tripura, ONGC, 799 014 Agartala, India;(2) KDM Institute of Petroleum Exploration, ONGC, 248 195 Dehradun, India;(3) Regional Chemical Laboratory, ONGC, 785 640 Sibsagar, Assam, India;(4) Geochronology Laboratory, KDM Institute of Petroleum Exploration, ONGC, 248 195 Dehradun, India |
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Abstract: | 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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Keywords: | Rb-Sr/K-Ar dating basement rocks granite Bombay High Field Mumbai Offshore Basin |
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