Thermal history and origin of the Tanzanian Craton from Pb isotope thermochronology of feldspars from lower crustal xenoliths |
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| Authors: | Jeremy J Bellucci William F McDonough Roberta L Rudnick |
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| Institution: | 1. Institut für Geowissenschaften, Universität Tübingen, Wilhelmstrasse 56, 72074 Tübingen, Germany;2. Department of Earth Science and Engineering, Imperial College of London, United Kingdom;3. Institut für Geowissenschaften, Universität Mainz, Becher-Weg 21, 55099 Mainz, Germany;4. Department of Earth Sciences, University of Gothenburg, P.O. Box 460, 40530 Gothenburg, Sweden;5. Department of Earth and Planetary Sciences, Macquarie University, North Ryde 2109, Australia;1. Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China;2. EGRU (Economic Geology Research Centre), College of Science, Technology and Engineering, James Cook University, Townsville, QLD 4811, Australia;3. CODES, ARC Centre of Excellence in Ore Deposits, University of Tasmania, Hobart, Tasmania 7001, Australia;1. Dipartimento di Scienze della Terra, Università di Pisa, Via S. Maria 53, Pisa I-56126, Italy;2. Dipartimento di Fisica e Geologia, Università di Perugia, Piazza dell''Università, Perugia I-06123, Italy;3. Sezione di Perugia, Istituto Nazionale di Fisica Nucleare, Via Alessandro Pascoli 23c, Perugia I-06123, Italy |
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| Abstract: | 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 238U/204Pb of 8.1 ± 0.3 and a 232Th/238U of 4.3 ± 0.1 at 2.71 ± 0.09 Ga (all uncertainties are 2σ). Since 2.4 Ga, some orthoclase domains within anti-perthites have evolved with a maximum 238U/204Pb of 6 and 232Th/238U of 4.3. The spread in Pb isotopic composition in the anti-perthitic feldspars yields single crystal Pb–Pb isochrons of ～ 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 °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. ～ 2.7 Ga, in a convergent margin setting, and has remained undisturbed since 2.7 Ga. |
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