Browsing by Author "Kobayashi, Katsura"

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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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    Ion Microprobe Zircon U–Pb Dating of the Late Archaean Metavolcanics and Associated Granites of the Musoma-Mara Greenstone Belt, Northeast Tanzania: Implications for the Geological Evolution of the Tanzania Craton
    (Pergamon, 2006-07-31) Manya, Shukrani; Kobayashi, Katsura; Maboko, Makenya A. H.; Nakamura, Eizo
    Ion microprobe zircon U–Pb ages from metavolcanic and associated granitic rocks of the late Archaean Musoma-Mara Greenstone Belt (MMGB) of northeast Tanzania reveal that the oldest mafic volcanism in the belt occurred at 2676–2669 Ma followed by felsic volcanism at ∼2668 Ma. The felsic volcanism was coeval with the emplacement of the oldest pulse of massive granitoids that is dated at 2668 Ma. The youngest volcanic episode, represented by a volcanic horizon in the largely sedimentary Kavirondian Supergroup that overlies the greenstone sequence with a marked unconformity, occurred at ∼2667 Ma. A younger phase of post-orogenic granites concluded the magmatic evolution of the MMGB at ∼2649 Ma. Our age data suggests that the entire volcano-sedimentary sequence in MMGB was emplaced in a relatively short time interval between ∼2676 and ∼2667 Ma. It also shows that contrary to arguments based on the degree of deformation, the foliated granites and some amphibolite rafts enclosed in them do not constitute the basement to the greenstone sequence. The data further shows that volcanism in the MMGB was younger than the ∼2820 Ma age of volcanism in the Sukumaland Greenstone Belt (SGB) to the far southwest and the ∼2720 Ma age of volcanism in the nearby Kilimafedha Greenstone Belt (KGB) to the south. The age of granitic magmatism (ca. 2.69–2.55 Ga) in the three belts was, however, largely coeval. Granitic magmatism of this age has also been reported in different parts of the Tanzania Craton suggesting that it was responsible for the late Archaean crustal growth and marks the beginning of a period of stability (or of cratonization).