Browsing by Author "Spencer, Christopher"

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    Geochronology of the Central Tanzania Craton and its Southern and Eastern Orogenic Margins
    (Elsevier, 2016) Thomas, Robert J.; Spencer, Christopher; Bushi, Alphonce M.; Baglow, Nick; Boniface, Nelson; de Kock, Gerrit; Horstwood, Matthew S. A.; Hollick, Louise; Jacobs, Joachim; Kajara, Sperartus; Kamihanda, Godson; Key, Roger M.; Maganga, Zortosy; Mbawala, Fabian; McCourt, W.; Momburi, Philip; Moses, Fadile; Mruma, Abdulkarim; Myambilwa, Yokbeth; Roberts, Nick M. W.; Saidi, Hamisi; Nyanda, Petro; Nyoka, Khalid; Millar, Ian
    Geological mapping and zircon U–Pb/Hf isotope data from 35 samples from the central Tanzania Craton and surrounding orogenic belts to the south and east allow a revised model of Precambrian crustal evolution of this part of East Africa. The geochronology of two studied segments of the craton shows them to be essentially the same, suggesting that they form a contiguous crustal section dominated by granitoid plutons. The oldest orthogneisses are dated at ca. 2820 Ma (Dodoma Suite) and the youngest alkaline syenite plutons at ca. 2610 Ma (Singida Suite). Plutonism was interrupted by a period of deposition of volcanosedimentary rocks metamorphosed to greenschist facies, directly dated by a pyroclastic metavolcanic rock which gave an age of ca. 2725 Ma. This is supported by detrital zircons from psammitic metasedimentary rocks, which indicate a maximum depositional age of ca. 2740 Ma, with additional detrital sources 2820 and 2940 Ma. Thus, 200 Ma of episodic magmatism in this part of the Tanzania Craton was punctuated by a period of uplift, exhumation, erosion and clastic sedimentation/volcanism, followed by burial and renewed granitic to syenitic magmatism. In eastern Tanzania (Handeni block), in the heart of the East African Orogen, all the dated orthogneisses and charnockites (apart from those of the overthrust Neoproterozoic granulite nappes), have Neoarchaean protolith ages within a narrow range between 2710 and 2630 Ma, identical to (but more restricted than) the ages of the Singida Suite. They show evidence of Ediacaran “Pan-African” isotopic disturbance, but this is poorly defined. In contrast, granulite samples from the Wami Complex nappe were dated at ca. 605 and ca. 675 Ma, coeval with previous dates of the “Eastern Granulites” of eastern Tanzania and granulite nappes of adjacent NE Mozambique. To the south of the Tanzania Craton, samples of orthogneiss from the northern part of the Lupa area were dated at ca. 2730 Ma and clearly belong to the Tanzania Craton. However, granitoid samples from the southern part of the Lupa “block” have Palaeoproterozoic (Ubendian) intrusive ages of ca. 1920 Ma. Outcrops further south, at the northern tip of Lake Malawi, mark the SE continuation of the Ubendian belt, albeit with slightly younger ages of igneous rocks (ca. 1870–1900 Ma) which provide a link with the Ponte Messuli Complex, along strike to the SE in northern Mozambique.