The Neoarchaean volcanic rocks of the Kilimafedha greenstone belt consist of three petrological types that are closely associated in space and time: the predominant intermediate volcanic rocks with intermediate calc-alkaline to tholeiitic affinities, the volumetrically minor tholeiitic basalts, and rhyolites. The tholeiitic basalts are characterized by slightly depleted LREE to nearly flat REE patterns with no Eu anomalies but have negative anomalies of Nb. The intermediate volcanic rocks exhibit very coherent, fractionated REE patterns, slightly negative to absent Eu anomalies, depletion in Nb, Ta, and Ti in multielement spidergrams, and enrichment of HFSE relative to MORB. Compared to the other two suites, the rhyolites are characterized by low concentrations of TiO2 and overall low abundances of total REE, as well as large negative Ti, Sr, and Eu anomalies. The three suites have a εNd (2.7？Ga) values in the range of ？0.51 to +5.17. The geochemical features of the tholeiitic basalts are interpreted in terms of derivation from higher degrees of partial melting of a peridotite mantle wedge that has been variably metasomatized by aqueous fluids derived from dehydration of the subducting slab. The rocks showing intermediate affinities are interpreted to have been formed as differentiates of a primary magma formed later by lower degrees of partial melting of a garnet free mantle wedge that was strongly metasomatized by both fluid and melt derived from the subducting oceanic slab. The rhyolites are best interpreted as having been formed by shallow level fractional crystallization of the intermediate volcanic rocks involving plagioclase and Ti-rich phases like ilmenite and magnetite as well as REE-rich phases like apatite, zircon, monazite, and allanite. The close spatial association of the three petrological types in the Kilimafedha greenstone belt is interpreted as reflecting their formation in an evolving late Archaean island arc. 1. Introduction The Kilimafedha greenstone belt of northeast Tanzania is one of the six greenstone belts of the Tanzania Craton occurring in the northern part of the country in the area south and east of the Lake Victoria. Other greenstone belts include the Sukumaland, Shinyanga-Malita, Nzega, Musoma-Mara, and Iramba-Sekenke [1, Figure 1]. All of these greenstone belts are prospective for gold mineralization with several large-scale mines now in operation including the Bulyanhulu, Tulawaka, Geita, Buzwagi, North Mara, and Golden Pride (Figure 1). Because of their economic significance, the greenstone belts of the Tanzania Craton
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