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Christoph Hauzenberger, University of Graz (Austria)
Veronika Tenczer, University of Graz (Austria)
Andreas Bauernhofer, University of Graz (Austria)
Harald Fritz, University of Graz (Austria)
Urs Kloetzli, University of Vienna (Austria)
Eckart Wallbrecher, University of Graz (Austria)
Sospeter Muhongo, International Council for Science (South Africa)
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Based on detailed geological, petrological and geochronological studies the tectonostratigraphy and geologic evolution of central and north-eastern parts of Tanzania is fairly good known. From structural bottom to top the major units are: (1) the Archaean Tanzania Craton; (2) the Usagaran Belt which consists of eclogites, some granulites, greenschist facies metapelites and associated quartzites as well as abundant granitoids and volcanic sequences; (3) the "Western Granulites", part of the Neoproterozoic Mozambique Belt (MB) including reworked parts of the craton and Usagaran Belt and composed of granulite facies metapsammites, migmatic orthogneisses, and mafic granulites; and (4) the "Eastern Granulites" which are clearly distinct from the "Western Granulites" by the occurrence of a basal unit consisting of migmatic enderbitic gneisses, mafic granulites and meta-anorthosites and a cover sequence consisting of marbles, calcsilicates, and metapelites.
However the geological units change in the S and SW of Tanzania. Here, to the east of Lake Malawi, distinctly different crustal pieces occur that may have derived from the Paleoproterozoic Usagaran Belt, the Mesoproterozoic Irumide Belt and from the MB in north Mozambique. Since there is only limited information on the age of metamorphism and formation of the igneous precursor rocks, thirteen samples were dated by U/Pb zircon LA-MC-ICP-MS. The samples were taken along a traverse from Mbamba Bay, Lake Malawi to Masasi, S-Tanzania. Samples are derived from deformed granitic gneisses and undeformed granitoids as well as from metapelitic samples and Grt-Am-richrich gneisses.
One sample from Mbamba Bay, Lake Malawi, gave an upper intercept age of around 2000 Ma, another one a concordant age of c. 720 Ma. All other samples taken from Songea, Tunduru and Masasi show mostly concordant ages around 1000 - 1100 Ma obtained from magmatic zircon cores. Zircon rims are commonly observed and show also mainly concordant ages of 560-600 Ma. Some magmatic zircons also indicate ages of around 700-800 Ma.
The westernmost part of the studied area is probably a continuation of the 1.8-2.0 Ga old Usagaran Belt. All other samples have different metamorphic and intrusion ages compared to the Western & Eastern Granulites in Tanzania. Similar ages as well as similar lithological units can be found in the Unango and Marrupa Complexes in N-Mozambique. Thus a major tectonic boundary separating the Eastern Granulites (Mahenge Mountains) and the Unango and Marrupa Complexes is proposed between the Mahenge Mountains and the towns of Songea and Tunduru.
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