Browsing by Author "Manya, Shukrani"

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    Geochemistry and geochronology of the volcanic and plutonic rocks of the Tarime segment of the Musoma-Mara greenstone belt, northern Tanzania
    (University of Dar es Salaam, 2005) Manya, Shukrani
    The magmatic rocks of the Musoma-Mara Greenstone Belt (MMGB) are grouped into five geochemically-coherent suites: the high-Mg basaltic andesites (HMA): the rhyodacites - Kavirondian dacites - Na-granitoids (RKN) group; the gabbros; the high silica rhyolites (HSR) and the K-granites. lon microprobe zircon U-Pb dating reveals that the HMA were erupted between 2691and 2685 Ma followed by the corral emplacement of the bulk of the felsic volcanics and Na-granitoids at 2668 Ma. He youngest volcanism represented by the kavirondian dacites occurred at 2653 Ma shortly before the emplacement of the post-orogenic K-granites at 2643 Ma. The HMA have high contents of MgO, Cr and Ni indicating equilibration of their parental magmas with mantle peridotite. The RKN group is characterized by high contents of MgO, Cr and Ni, fractionated REE patterns and high Sr/Y ratios, features also shared by modem adakites. The gabbros show high contents of Cr and Ni. moderately fractionated REE patterns and negative Nb and Ta anomalies, a feature also shared by rocks of the other two suites. The HSR are characterized by extremely low contents of CaO, Cr, Ni and Sr, highly variable HREE and HFSE contents and negative Eu anomalies suggestive of formation by partial melting of a heterogeneous crustal source. Relative to the HSR, the K-granites have higher contents of CaO and Sr, low AL2O3 contents and strongly negative Eu anomalies suggesting derivation by low pressure melting leaving plagioclase as a residual phase. Apart from the HSR, the MMGB rocks have a restricted range of εNd values (+0.47 to +3.38), indicating juvenile additions to the crust at -2670 Ma that was variably contaminated with a relatively minor amount of older crustal material. The HSR, however, show a wider range of εNd values (-10.27 to +2.47) indicative of a more extensive mixture of juvenile and older continental crustal material. The HMA are interpreted to have formed by partial melting of mantle peridotite that has been fluxed by slab-derived aqueous fluids. The melts were subsequently contaminated by continental crystal material and underwent fractional crystallization of ferromagnesian phases. in contrast, rocks of the RKN suite are interpreted as partial melts of mantle peridotite that had reacted with slab-derived melts. The melts subsequently underwent crustal contamination and fractional crystallization above a late Archaean Andean type subduction zone. Association of Kavirondian dacites with clastic rudimentary rocks suggests that the latest volcanic episode in the MMGB occurred in a continental back arc basin. The HSR are partial melts of a mixture of LIMA, RKN and older palates whereas the K-gravitas were generated by low pressure partial melting of a mixture of HMA and RKN. Emplacement of the gabbros, derived by partial melting of he mantle wedge above a subduction zone, into the lower crust provided the heat needed melting the crust to generate the K-rich granites and the HSR.