AIRBORNE GEOPHYSICS AND GEOCHEMISTRY PROVIDE NEW INSIGHTS IN THE INTRACONTINENTAL EVOLUTION OF THE MESOPROTEROZOIC KIBARAN BELT (CENTRAL-AFRICA)

The Mesoproterozoic Kibaran belt in southwest Rwanda (Central Africa) consists of two contrasting metamorphic sequences. The first is essent ially composed of weakly deformed, low-grade pelitic rocks with many q uartzitic intercalations and some volcano-sedimentary sequences. The s econd consists of medium- to high-grade metamorphic metasediments and gneisses, intruded by sheared granitoids. Existing geological maps are of limited use in understanding the evolution of this part of the Kib aran belt. A combination of airborne gamma-spectrometry data with Land sat TM imagery readily distinguishes known lithologies, and in particu lar detects two distinct granite types. Trace element data for one gra nite type does not correspond with known petrochemical trends of Kibar an granites, and may belong to a pre-Kibaran basement. The combining o f these data with a recently published schematic geological map of the northeast Kibaran belt and re-interpreted field structural data, sugg ests a model for the Kibaran orogenic evolution in terms of extensiona l detachment tectonics and associated metamorphic core complexes.