Thermal history and origin of the Tanzanian Craton from Pb isotope thermochronology of feldspars from lower crustal xenoliths

Common and radiogenic Pb isotopic compositions of plagioclase and anti-perthitic feldspars from granulite-facies lower crustal xenoliths from the Labait Volcano on the eastern margin of the Tanzanian Craton have been measured via laser ablation MC-ICP-MS. Common Pb in plagioclase and a single stage Pb evolution model indicate that the lower crust of the Tanzanian Craton was extracted from mantle having a U-238/Pb-204 of 8.1 +/- 0.3 and a Th-232/U-238 of 4.3 +/- 0.1 at 2.71 +/- 0.09 Ga (all uncertainties are 2 sigma). Since 2.4 Ga, some orthoclase domains within anti-perthites have evolved with a maximum U-238/Pb-204 of 6 and Th-232/U-238 of 4.3. The spread in Pb isotopic composition in the anti-perthitic feldspars yields single crystal Pb-Pb isochrons of similar to 2.4 Ga, within uncertainty of U-Pb zircon ages from the same sample suite. The Pb isotopic heterogeneities imply that these granulites resided at temperatures <600 degrees C in the lower crust of the Tanzanian Craton from ca. 2.4 Ga to the present. In concert with the chemistry of surface samples, mantle xenoliths, and lower crustal xenoliths, our data imply that the cratonic lithosphere in Tanzania formed ca. similar to 2.7 Ga, in a convergent margin setting, and has remained undisturbed since 2.7 Ga. (C) 2010 Elsevier B.V. All rights reserved.