Browsing by Author "Bahame, Gagao"

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    Geochemistry and geochronology of the Precambrian Ndembera volcanic and plutonic rocks of Mufindi district, Tanzania: the case of Mufindi area
    (University of Dar es Salaam, 2012) Bahame, Gagao
    The Precambrian Ndembera volcanic rocks of southwestern Tanzania unconformably overlie high grade metamorphic rocks of the Usagaran Belt and are intruded by plutonic rocks of granitic composition. No modern geochemical investigation has been carried on both the Ndembera volcanic and plutonic rocks. This study, therefore, intended to investigate the geochemistry and geochronology of the Ndembera volcanic and plutonic rocks in order to constrain their age, petrogenesis and tectonic setting. The study involved collection of rock samples in the field for geochemical, isotopic and petrographic analyses. Geochemical classification indicates that the volcanic rocks are calc-alkaline trachyandesites to rhyolites whereas the plutonic rocks are I-type granites. Both the Ndembera volcanic rocks and granites are meta-aluminous and are characterized by a decrease in the abundances of Fe2O3, MgO, TiO2 and CaO with increasing SiO2 whereas Na2O, K2O, Al2O3, P2O5 and MnO show poor correlation with SiO2. The volcanic rocks exhibit very coherent REE patterns characterized by enrichment of the LREE relative to the HREE (La/Yb) CN =7.7–32.8, mean = 23. 6), slightly negative Eu anomalies (Eu/Eu* = 0.45 – 1.02, mean = 0.83) and depletion of Nb, Ta and Ti relative to adjacent elements in incompatible multi-element diagrams which characteristics are akin to those exhibited by rocks of the upper continental crust.The granitic rocks exhibit fractionated REE patterns characterized by LREE enrichment, nearly flat HREE ((La/Yb) CN = 10.8 - 38.2, mean = 22.1) and negative Eu anomalies (Eu/Eu* = 0.48 – 0.91, mean = 0.61). On primitive mantle-normalized extended trace element diagrams, the granitic samples display large negative Nb, Ta and Ti anomalies relative to adjacent elements ((Nb/La)pm = 0.01 - 0.31, mean = 0.15). Overall, the major and trace element characteristics of the granites are akin to those exhibited by I-type granites. SHRIMP zircon U-Pb age data show that the Ndembera Ndembera volcanic rocks were extruded at 1871 ± 15 Ma, an event that was largely coeval with the emplacement of the granites for which an age of 1892 ± 38 Ma has been obtained. Calculated ƐNdt values range between -4.88 to -2.75 for the volcanic rocks and -4.64 to -3.6 for the granites whilst their mean crustal residence ages vary from 2486 to 2713 Ma for the volcanics and between 2469 and 2646 Ma for the granites. The geological setting of the Ndembera rocks together with their geochemical characteristics indicates that the magmatism occurred in an intra-continental setting Simple modelling of the REE abundances in the Ndembera volcanic rocks reveals that the Ndembera trachyandesite can be formed by 20 % modal batch partial melting of the lower continental crustal source involving Clinopyroxene 94 %, Orthopyroxene 3 %, Olivine 2 % and Garnet 1 % whereas the more felsic volcanic rocks can be derived from the parental magmas of the trachyandesites by 7 % Rayleigh fractionation involving Hornblende 30 %, Plagioclase 50 %, Biotite 10 %, Zircon 1 %, Apatite 3 % and Magnetite 6 % . Geochemical and isotopic evidence moreover show that the Ndembera rocks formed by partial melting of evolved continental rocks some of which contain Archaean components similar to those found in the Tanzania Craton.