ORIGIN AND EVOLUTION OF THE KILIMAFEDHA GREENSTONE BELT, EASTERN TANZANIA CRATON: EVIDENCE FROM FIELD GEOLOGY AND WHOLE ROCK GEOCHEMISTRY

The Tanzania Craton underlies the Lake Victoria region and consists of numerous greenstone belts (Nyanzian Supergroup) with intervening granitic terranes. The relations among the greenstone belts are not well understood due to separation by cover and granitic intrusives. Although some western belts (e.g., Sukumaland) have recently received attention because of their economic potential, many of the eastern belts remain largely unstudied. The geochronology of the Kilimafedha greenstone belt is described by Wirth et al. (this volume).

Field observations indicate volcanic rock types from basalt to rhyolite. Minor ferruginous chert, banded iron formation, volcanic breccia, and volcaniclastic rock are also present. The rocks are regionally metamorphosed to low grade and have locally been sheared and metamorphosed to amphibolite facies in the northern part of the belt. Pillow structures are visible in the less deformed southwestern portions of the belt. Ultramafic flows are apparently absent.

Petrographic studies, major element, and trace element analyses have been completed on 55 samples from the Kilimafedha greenstone belt. Most samples of mafic volcanic rock are aphanitic and rarely include phenocrysts of plagioclase. Felsic volcanic rocks consist of abundant euhedral quartz phenocrysts and minor plagioclase. On major element discrimination diagrams the volcanic rocks plot along trends that range continuously from basalt through rhyolite. The rocks are largely sub-alkaline and are transitional between tholeiitic and calc-alkaline. On normalized trace element diagrams the mafic volcanic rocks are characterized by enriched large-ion lithophile and light rare earth elements (e.g., Th, U, La, Ce) and relatively depleted high field strength elements (e.g., Nb, Hf, Ti). The volcanic rocks of the Kilimafedha greenstone belt differ from those in Kenya (Migori belt) in that they lack distinct bimodal compositional trends. Compared with those of the Sukumaland greenstone belt to the west, Kilimafedha volcanic rocks exhibit greater enrichment of the highly incompatible trace elements and consistent negative HFSE anomalies. The combined field and geochemical data suggest that the Kilimafedha volcanics formed in a submarine environment that may be analogous to modern-day supra-subduction zone settings.