ORIGIN AND EVOLUTION OF THE KILIMAFEDHA GREENSTONE BELT, EASTERN TANZANIA CRATON: EVIDENCE FROM ND AND PB ISOTOPES

The Kilimafedha greenstone belt (Tanzanian Craton) extends eastward from Lake Victoria and underlies much of Serengeti National Park. Although several significant strata-bound gold-sulphide deposits occur within this region (e.g., Geita Mine and Bulyanhulu), relatively few of the greenstone belts have been studied using modern isotopic methods. The geology, major element, and trace element chemistry of the belt are summarized by Weisberger et al. (this volume).

Zircons were separated from two rhyolites (Kilimafedha GSTB) and two intrusive granites (Seronera and Maasai Kopjes). Pb ages were determined by LA-ICP-MS and Sm-Nd isotopic compositions were determined by MC-ICP-MS at Washington State University. All errors are reported as 2SE and are based on in-run statistics of the sample and reproducibility of standards. Zircons from the rhyolites are transparent and relatively unzoned. Rhyolite KM-115 yields a ²⁰⁷Pb/²⁰⁶Pb age of 2712 ± 5 Ma (MSWD=0.35) and rhyolite KM-120 yields a similar age (2720 ± 5 Ma; MSWD=1.9). Zircons from the granite exposed at Seronera and the Maasai Kopjes are rich in inclusions and are complexly zoned, but apparently do not contain inherited components. Analyses of granite from Seronera Kopjes and from Maasai kopjes yield ages of 2692 ± 11 Ma (MSWD=1.00) and 2643 ± 11 Ma (MSWD=0.17), respectively. Initial ¹⁴³Nd/¹⁴⁴Nd compositions of the volcanic rocks range from 0.509203 to 0.509237 (ε_Nd=+1.7 to +2.4). Initial ¹⁴³Nd/¹⁴⁴Nd compositions of the granites exposed at Seronera and Maasai Kopjes are 0.509218 and 0.509247, respectively (ε_Nd=+1.4 and 0.7, respectively).

The new ages from the Kilimafedha greenstone belt are similar to those from the nearby upper Sukumaland greenstone belt, but are significantly younger than those from the lower part of that belt (2800 Ma). The ages of the granites associated with the Kilimafedha greenstone belt document the timing of post-Nyanzian granite emplacement (2700-2640 Ma) in the eastern part of the craton. Rocks from the volcanic and granitic terranes have similar Nd isotopic compositions that are comparable to upper mantle (ε_Nd=0-2) at 2700 Ma. These relations suggest an origin in an oceanic setting without significant involvement of older basement.