Browsing by Author "Messo, Charles W."

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    Geochemistry of the Neoarchaean Volcanic Rocks of the Kilimafedha Greenstone Belt, Northeastern Tanzania
    (Hindawi Publishing Corporation, 2012-10-24) Messo, Charles W.; Manya, Shukrani; Maboko, Makenya A. H.
    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.
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    Geochemistry of Volcanic and Plutonic Rocks from the Southern Musoma-Mara Greenstone Belt: Implication for the Evolution of the Tanzania Craton
    (2000) Messo, Charles W.; Yamashita, K.; Kobayashi, Katsura; Makishima, Akio; Sakaguchi, C.; Nakamura, Eizo
    The Neoarchean Tanzania craton contains patches of greenstone belts set in granitoid masses that occupy a large percent of the craton. Because of limited geological, geochronological and geochemical information, very little is known about this craton and its constituent terrains. We report new U-Pb chronological, Nd-Hf isotopic and major and trace element data for volcanic and granitoid rocks from the southern Musoma-Mara greenstone belts of the Northeast Tanzania craton. The volcanic rocks are mainly basaltic with minor felsic volcanic rocks. The intrusive rocks are mostly granodiorites and granites with few trondhjemitic samples. Geochemically, the basalts are tholeiitic and MORB-like but with minor depletions of Nb and Ti relative to N-MORB. The felsic volcanic and granitoid rocks are calc-alkaline with normal arc to adakitic signature. The basalts yielded a Sm - Nd isochron age of 2842 ± 65 Ma (εNd = +2.1, MSWD = 1.6) and εHf2840 = +1.7 - +3.2. One granitoid yielded U-Pb zircon age of 2689 ± 12 Ma which is similar to those reported for the oldest rocks in the North Musoma-Mara greenstone belt [1], and is interpreted to represent the age of granitoids and felsic metavolcanic rocks. The felsic volcanic rocks have εNd2689 ~ +1.6 - +2.5 and εHf2689 ~ +0.1 - +1.6 that are within error of those for granitods and meta-basalts. Collectively, these data show that the basaltic rocks may not be related to the felsic volcanic and intrusive rocks by fractionation but were likely formed from equally depleted sources, possibly in arc-back arc environment. The spatial association of MORB-like tholeiites and arc-like volcanic and granitoid rocks is not uncommon in the Tanzania craton [2], as well as other late-Archean cratons worldwide [3, 4], and may imply that the tectono-magmatic processes that produced this association were widespread during this period.
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