Isotopic age data of single zircons from the Archæan Sukumaland Greenstone Belt, Tanzania

This study presents precise U/Pb single zircon age data for three rhyolitic metavolcanic units: one trachyandesitic porphyry, presumably of extrusive origin; one pal ae osome of a migmatitic gneiss; and one lamprophyric dyke cross-cutting banded iron formation (BIF) of the Upper Nyanzian Supergroup in the Arch ae an Sukumaland Greenstone Belt, Tanzania. The obtained ages partly contradict the expected field stratigraphical relationships. The isotopic ages of the felsic metavolcanics give a good age bracket for the deposition of the Nyanzian Greenstone Belt units. The older felsic metavolcanics yield ages of 2808 +/- 3 Ma and 2780 +/- 3 Ma, respectively, whereas the younger felsic metavolcanics provide a minimum age of 2654 +/- 15/-13 Ma for the greenstone belt. The porphyritic trachyandesite, intercalated with oxide BIF at Geita, yield an age of 2699 +/- 9 Ma and the lamprophyre dyke an age of 2644 +/- 3 Ma, respectively. The data for the lamprophyre dyke imposes a maximum age on the Au mineralisation at Geita Mine, since the dyke has been mineralised together with adjacent BIF. The pal ae osome of a migmatitic gneiss, possibly a basement flanking the greenstone belt to the south at Kahama, has been dated at 2680 +/- 3 Ma. Previous models advocating the pre-existing, significantly older, Dodoman basement gneiss within the Arch ae an Sukumaland Greenstone Belt of northern Tanzania cannot be supported. Emplacement or complete reworking of the granitic protolith coeval with greenstone belt volcanism appears more likely. (C) 1999 Elsevier Science Limited. All rights reserved.