International Geologiical Congress - Oslo 2008

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HPP-07 Late Neoproterozoic orogenic belts and assembly of Gondwana

 

Geochronology of the Mozambique Belt in north-eastern Tanzania

 

Veronika Tenczer, Department of Earth Sciences (Austria)
Christoph Hauzenberger, Department of Earth Sciences (Austria)
Harald Fritz, Department of Earth Sciences (Austria)
Georg Hoinkes, Department of Earth Sciences (Austria)
Urs Klötzli, Center for Earth Sciences (Austria)
Sospeter Muhongo, ICSU Regional Committee for Africa (South Africa)
Eckart Wallbrecher, Department of Earth Sciences (Austria)
 

 

The Mozambique Belt in Tanzania formed during the assembly of Gondwana at Neoproterozoic times. The tectonic style is dominated by westward thrusting of deep crustal units onto the Archean Tanzania Craton (2.7 Ga) and the Paleoproterozoic Usagaran Belt (1.8-2.0 Ga). A metamorphic gradient developed from greenschist facies along the Craton boundary over upper amphibolite facies in the Western Granulites to high pressure granulite facies in the easternmost units - the Eastern Granulites. The Eastern Granulites comprise juvenile rocks formed by the Pan-African orogeny around 800-1000. All the other units are composed of older material reworked during Neoproterozoic metamorphism. Previous studies inferred this scenario mainly from Central Tanzania where the age of metamorphism is constrained at ca. 640 Ma. A second metamorphic event is dated around 550-580 Ma.
These ages are mainly found along shear zones transecting the orogen (e.g. Galana Shear Zone in S-Kenya). Towards north-eastern Tanzania, the structural style changes significantly and important Precambrian structures are partly blurred by the East African Rift System. West-East striking large scale shear zones of unknown age transsect the orogen and complex refold structures in 100 km scale formed a complicated structural style. It is not known, if the Usagaran Belt continues to the north or if the crustal blocks in Central Tanzania (e.g. Western and Eastern Granulites) can directly be correlated with units in the north. In this large area, we started a campaign of geochronology, structural geology and petrology in order to explain the geometry of the Mozambique Belt there. A selection of 20 metamagmatic rocks was chosen for a geochronological U-Pb zircon study using LA-ICP-MS. The majority of these samples stem from the N-S trending Tanzanian Craton boundary for an investigation, if Usagaran magmatites are incorporated. The results show, that the formation age of these rocks are mainly Archean (around 2.64-2.7 Ga) with ill-defined lower intercept ages from metamorphic rims pointing to a Neoproterozoic overprint. The overprint has been inferred in more detail from metamorphic monazites giving concordant ages of 580 Ma. Single zircon cores from these samples point to 2.0 Ga, which are possible inherited relics of the Usagaran orogeny. Nevertheless, no well-defined Usagaran magmatic activity could be found along the Craton boundary. In the northernmost sections of the Craton boundary, two samples were analyzed giving formation ages of ca. 1000 Ma. The juvenile high-grade parts of the Mozambique Belt seem to bend N-W wards being juxtaposed to the Tanzania Craton in the very north. Most of the samples, however, that stem from the area east of the Craton yield clear Archean formation ages with Neoproterozoic overprint. This indicates that large volumes of the rocks were reworked during the Pan-African orogeny, most likely during the second event around 550-580 Ma. (Supported by FWF T247-N10)

 

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