RB-SR ISOTOPE SYSTEMATICS OF MUSCOVITE FROM PAN-AFRICAN GRANITIC PEGMATITES OF WESTERN AND NORTHEASTERN AFRICA

Rb-Sr investigations have been carried out on early-formed muscovite f rom three pegmatite fields of the late Proterozoic to early Phanerozoi c Pan-African Belt. The individual mineral ages obtained are highly di scordant for each pegmatite field. Using Best Isochron Diagrams, isoch ron construction of selected muscovite samples yielded geologically re alistic ages of pegmatite formation: around 670 Ma for the Bayuda Dese rt pegmatites of northern Sudan, around 550 Ma for the Wamba pegmatite s of central Nigeria, and around 465 Ma for the Majayahan pegmatites o f northeastern Somalia. Initial Sr ratios obtained from isochron calcu lations have unrealistic values and cannot be used for petrogenetic in terpretations. The geologically unrealistic young model ages of some o f the muscovite samples are most probably attributed to open-system be haviour and post-crystallization loss of Sr-87 from the respective mi nerals. The amounts of Sr-87 losses have been approximated from the d iscrepancies between isotopically measured and theoretically calculate d (from decay of Rb) Sr-87 concentrations. The loss of Sr-87* from th e micas is variable in each pegmatite field. In none of the three case s can this unsystematic, post-emplacement, open-system behaviour be di rectly related to a particular, temporally confined, geologic event.