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SHRIMP zircon ages for post-Usagaran granitoid and rhyolitic rocks from the Palaeoproterozoic terrain of southwestern Tanzania

Abstract:

We report SHRIMP zircon U-Pb ages for post-Usagaran granitic- to granodioritic intrusives and a rhyolitic agglomerate from the Palaeoproterozoic terrain of southwestern Tanzania. This terrain consists of strongly deformed and metamorphosed rocks ascribed to the ca. 2 Ga Usagaran mobile belt, voluminous post-Usagaran granitoids, and minor supracrustal successions. The southeastern part of this terrain is characterized by the occurrence of little deformed and virtually unmetamorphosed rhyolithic, dacitic and andesitic volcanic rocks which overlie the older Usagaran basement. These rocks extruded between 1820 and 1921 Ma, as documented by SHRIMP zircon ages, which are in good agreement with previous ages reported from this region, from the Ubendian belt farther west, and from Palaeoproterozoic crustal fragments in the Neoproterozoic Pan-African Mozambique belt to the east. A 2519Ò9 Ma Archaean zircon xenocryst was found in one volcanic rock sample, also in good agreement with Archaean ages of the Tanzania Craton to the northwest and Archaean rocks in the central part of the Mozambique belt. Our age data suggest that the post-Usagaran assemblage, including crustal domains farther east in the MB, was generated during a relatively short period of ∼100 Ma in the late Palaeoproterozoic, probably during a crustal accretion event connected with widespread magmatic activity and generally ascribed in Africa to the Eburnian orogeny. © 2005 Geological Society of South Africa.