Rb-Sr isotope systematics of muscovite from Pan-African granitic pegmatites of Western and Northeastern Africa

摘要: Rb-Sr investigations have been carried out on early-formed muscovite from three pegmatite fields of the late Proterozoic to early Phanerozoic Pan-African Belt. The individual mineral ages obtained are highly discordant for each field. Using Best Isochron Diagrams, isochron construction selected samples yielded geologically realistic formation: around 670 Ma Bayuda Desert pegmatites northern Sudan, 550 Wamba central Nigeria, and 465 Majayahan northeastern Somalia. Initial Sr ratios calculations unrealistic values cannot be used petrogenetic interpretations. young model some most probably attributed open-system behaviour post-crystallization loss of87Sr* respective minerals. amounts losses approximated discrepancies between isotopically measured theoretically calculated (from decay Rb)87Sr* concentrations. micas is variable in In none cases can this unsystematic, post-emplacement, directly related a particular, temporally confined, geologic event.