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

Abstract

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