Geochemistry of neoarchean volcanic rocks of the ikoma area in the kilimafedha greenstone belt, east of the Lake Victoria, Tanzania

The Archean volcanic rocks of the Ikoma area can be divided into three petrological types that are closely associated in space and time. The first type consists of low-K tholeiitic basalts that can be subdivided into two suites on the basis of their chondrite normalized REE patterns. The first suite is characterized by flat REE patterns (mean La/YbcN =1.8) similar to those observed in modem arc basalts whereas the second suite is characterized by depletion in the Light (L)REE (La and Ce) relative to the middle and Heavy (H)REE (mean La/YbcN = 0.5). The second type consists of rocks of tholeiitic to calc-alkaline transitional affinity. These rocks exhibit REE patterns characterized by enrichment of the LREE relative to the HREE, slightly negative to non-existent negative Eu anomalies and depletion in Nb and Ti, features typical of modem convergent margin magmatism. The last type is composed of basaltic andesites, andesite, dacites and rhyolites of calc-alkaline affinity. These rocks (except rhyolites) show enrichment of the LREE and depletion of the HREE relative to MORB as well as prominent negative Nb and Ti anomalies. The rhyolites are characterized by strongly negative Ti anomalies, small negative Zr anomalies, lack of Nb anomalies and lower concentrations of LREE, Zr and P2O5 as well as lower Zr/Y compared to the other member of the group. These features can be explained by formation of the rhyolites from the intermediate members of the group by shallow level fractional crystallization of ilmenite, magnetite and LREE rich phases such as zircon, monazite and apatite. The close spatial association of different rock types is interpreted as reflecting their formation in an evolving late Archean island arc similar to that forming the Japanese islands. The tholeiitic basalts are interpreted to have formed by higher degree of partial melting of a spinel lherzolite mantle wedge that has been variably metasomatized by fluids derived from dehydration of the subducting slab. The rocks showing transitional affinities are interpreted to have formed as a differentiate of a primary magma formed later by low degree of partial melting of a garnet free mantle wedge that was strongly metasomatized by both fluid and melts derived from the subducting oceanic slab, whereas the calc-alkaline suite is interpreted to have formed as a differentiate of a primary magma derived by partial melting of a metasomatized mantle wedge with a garnet peridotite composition.