Browsing by Author "Kwelwa, Shimba"

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    Geochemistry and petrogenesis of intrusions at the Golden Pride gold deposit in the Nzega greenstone belt, Tanzania
    (Elsevier, 2013-10-31) Kwelwa, Shimba; Manya, Shukrani; Vos, Ivo M. A.
    The greenstone sequence at Golden Pride gold deposit in Nzega greenstone belt of northern Tanzania is cross-cut by several intrusions whose geochemistry and petrogenesis is unknown. We present major and trace elements geochemistry of the Golden Pride igneous intrusions with the aim of constraining their petrogenesis and their ancient tectonic setting. The Golden Pride intrusions are geochemically categorized into two main rock suites: the granodiorites (which include the porphyries) and the lamprophyres (formerly intermediate intrusions). The granodiorites are characterized by SiO2 contents of 54.5–69.9 wt%, elevated MgO (1.22–3.59 wt%) Cr (up to 54 ppm), Mg# (35–55) pointing to a mantle component in the source. Compared to the TTG and adakites, the granodiorites are characterized by higher K2O (1.52–4.30 wt%), medium HREE (Gd/ErCN = 2.13–3.77) and marked enrichment in Ba and Sr (Ba + Sr = 819–2922 ppm) and are in these respects similar to Archean high Ba–Sr sanukitoids. The rocks in this suite are interpreted to have formed by partial melting of an enriched mantle wedge through two metasomatic events: subduction-related fluids/melts and by metasomatism related to asthenospheric mantle upwelling caused by slab break-off. Compared to the granodiorites, the lamprophyres have higher MgO contents (2.37–3.81 wt%), Cr (60–298 ppm), Co (31–57 ppm) and Mg# (32–40). They also show slight enrichment of the LREE relative to HREE (La/YbCN = 3.3–7.1), moderate Nb–Ti depletion and sub-chondritic Zr/Hf ratios (34–41). These geochemical features are attributed to derivation of the Golden Pride lamprophyres by partial melting of the amphibole-rich metasomatized mantle by slab derived hydrous fluids. Both of the Golden Pride intrusion suites show strong affinity to subduction related magmas and we interpret that the entire greenstone sequence and the associated intrusions at Golden Pride gold deposit formed in a late Archaean convergent margin.
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    Holocene Explosive Eruptions in the Rungwe Volcanic Province, Tanzania
    (2010-07) Fontijn, Karen; Ernst, Gerald G. J.; Elburg, Marlina A.; Williamson, David; Abdallah, Edista; Kwelwa, Shimba; Mbede, Evelyne I.; Jacobs, Patric
    The Holocene explosive eruptive record of Rungwe and Ngozi volcanoes of the Rungwe Volcanic Province in Tanzania was reconstructed based on detailed stratigraphic field evidence combined with whole-rock major and trace element analyses of tephra samples. This reconstruction is supported by 25 new radiocarbon dates on palaeosols that provide additional constraints on the Holocene tephro-chronostratigraphy. We show evidence of two catastrophic Ngozi eruptions and five Rungwe pumice fallout deposits, and also identify several more intercalated poorly preserved pumice and ash deposits. The Ngozi eruptions probably played a role in shaping the present-day caldera. The Rungwe record includes a ca. 2.2 km3 deposit of a Plinian-style eruption dated at ca. 4 ka, a sub-Plinian one at ca. 2 ka and at least three additional smaller-scale fallout deposits. The Rungwe explosive eruptive record shows that the volcano has been frequently active in its late Holocene past. This study highlights the need for volcanic monitoring in the region and presents herewith the first basis of future volcanic hazard assessment.