On the Age of Rhodesian Greenstone Belts
The three mineral isochron ages (about 2.4 b.y.) are tentatively suggested to represent the time of an important yet not widely recognized low-grade metamorphic event. The whole rock age of Midlands-Bulawayo belt is not well defined, but the eruption of the Bulawayan volcanic group might have taken place as early as 3.08 b.y. ago. The ages of Shabani and Salisbury belts are practically identical and are indistinguishable from the ages of Gwenoro gneisses and Sesombi Tonalite. They are preferably interpreted as metamoraphic ages, but primary ages interpretation cannot be ruled out. If the concept of “contemporaneity” of greenstone-granite terrain holds in the Rhodesian craton, then the “average” age (2.75 ± 0.09 b.y.) may represent the primary age for the Midlands-Bulawayo belt. The I value (0.7011) for the Midlands-Bulawayo belt and the lowest Sr87/Sr86 ratio (0.7012) measured for a tholeitic basalt (RH-154) define an upper limit for the Sr isotopic composition of the upper mantle from which the volcanic rocks were derived about 3 b.y. ago.
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This work was done while the senior author (B.J.) held a visiting postdoctoral fellowship at the Lunar Institute. Suggestions and comments by Drs. M.H. Dodson, S. Moorbath and C.J. Hawkesworth are appreciated. The Lunar Science Institute is operated by the Universities Space Research Association under Contract No. NSR 09-051-001 with the National Aeronautics and Space Administration. This paper constitues the Lunar Science Institute Contributions No. 247.
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