Browsing by Author "Mtoro, Mwache"

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    Geochemical investigation of metavolcanic rocks of the musoma-mara greenstone belt in the Suguti Area, Northern Tanzania
    (Unversity of Dar es Salaam, 2007) Mtoro, Mwache
    The Suguti volcanic rocks comprise mainly of a bimodal suite of tholeiitic basalts and calc-alkaline rhyolites with a subordinate amount of intennediate rocks. Zircon U-Pb and whole rock Sm-Nd geochronology, however, reveals that the two suites are cogenetic and were emplaced at 2755+1 Ma with a common initial cNd value of 2. The tholeiitic basalts are characterised by low abundances of the LILE and HSFE as well as relatively flat chondrite-nomialised REE patterns. The rocks also exhibit negative Ti and Nb anomalies in primitive mantle-normalized multi-element diagrams. 'Die flat REE patterns, the presence of prominent negative Nb anomalies and the positive initial eNa value of 2 suggest that the basalts were formed in a convergent margin setting by low pressure melting of a mantle wedge which was slightly more enriched than the MORB source. The enrichment of their source mantle wedge was caused by a moderate to high degree of metasomatism by slab derived fluid as indicated by high Ba/Th ratios (25 - 1492). The calc-alkaline rhyolites are characterised by high abundances of the LILE, moderately high HFSE abundances and very low abundances of the tmnsitiona] elements. The rhyolites display strongly fractionated, slightly upward concave patterns that are characterized by a slight depletion of the MREE relative to the HREE and moderate to large negative Eu anomalies (Eu/Eu* = 0.3 - 0.9). The depletion of the MREE relative to the HREE is an indication of fractionation of clinopyroxene and homblende during petrogenesis while the negative Eu anomalies indicate plagioclase fractionation. As isotopic evidence indicates that the basalts and rhyolites fomi a cogenetic suite with a common initial cNd value, the rhyolites are interpreted to have formed from the parental magma of the basalts by fractional crystallization in an active continental margin setting.