Timing of exhumation of a high-pressure mafic granulite terrane of the Paleoproterozoic Ubende belt (West Tanzania)

The Ubendian belt, west of the Archean Tanzanian craton, is known as a prom inent crustal structural weakness. The belt is composed of different terran es set in a NW-SE oriented lateral shear pattern sharply outlined by steepl y inclined inter-terrane shear fault zones. Its predominant mafic terranes (Ubende and Mbozi) display different and more complex evolutionary characte ristics than their adjacent felsic terranes (Ufipa). Metamorphic studies re veal that mafic and ultramafic high-pressure granulites and rare eclogites subsist as rigid blocks and lenses within the highly sheared amphibolite fa cies country rock. P-T studies suggest two succeeding decompressional stage s for the uplift of the high-pressure mafic granulite terranes, the second stage being identified on high-pressure blastomylonites. Included in the la tter are amphibolites (mafic tectonites), for which barroisite constitutes a P-T marker. Near the southwestern boundary of the NW-SE oriented mafic te rranes, these barroisite-bearing tectonites display a pronounced extensiona l shear banding. Considering that this tectonite has been formed during the second stage of uplift of the Ubende high-pressure mafic granulites, timin g of that stage is pursued. Ar-Ar stepwise heating analyses on different barroisite separates from this mafic tectonite have yielded a weighted average cooling age of 1848 +/- 6 Ma for the argon fractions released at intermediate temperatures. This age falls in the time range (1950-1850 Ma) associated with the second phase of the evolution of the Ubende belt characterized by amphibolite facies metamo rphism. The isotopic composition of argon released at higher temperature st eps from compositionally different phases in the separates, yields apparent ages tentatively considered only as indicative for a possible older Paleop roterozoic event. Based on geochronological and structural data, the uplift of the high-press ure granulite Ubende terrane is interpreted as related to a late Paleoprote rozoic dextral lateral shear deformation in ductile amphibolite facies cond itions. This appears to be associated with the emplacement of granites. Sub sequent Proterozoic strike-slip reactivation of the Paleoproterozoic Ubende shear belt did not fundamentally contribute to the near surface exhumation of the high-pressure mafic granulites. (C) 1999 Elsevier Science B.V. All rights reserved.