Constraints on the age of the Bulawayan group metavolcanic sequence, Harare Greenstone Belt, Zimbabwe
Abstract
Renewed attempts to date the bimodal felsic-mafic-felsic Archaean metavolcanic sequence of the Harare Greenstone Belt in Zimbabwe by Rb:Sr and Sm:Nd methods show that the Rb/Sr isotope systems within the greenstones have been extensively disturbed. However isotopic data from the surrounding granitic terrain do constrain the age of the intervening greenstone succession, which rests on gneissic "basement", exposed only in a small area, giving an age of 2865±135 Ma, but was intruded by tonalitic and injection gneisses yielding analytically indistinguishable ages of 2688±138 Ma and 2622±168 Ma, respectively. A felsite from the Bulawayo Belt gives 2615±28 Ma, which is approximately consistent with the constraints deduced for the Harare Belt. Sm:Nd model ages suggest that the two main felsic metavolcanic formations were extruded in the period around 2.74 to 2.62 Ga. Thus the felsic metavolcanics overlap in age with the tonalite-granodiorite plutonics; they are geochemically similar and may well be co-eval. Both the bimodal greenstone succession and these early tonalitic intrusives were strongly deformed by a major tectonic event at about 2.62 Ga. Emplacement of post-tectonic, unfoliated, potassic granite plutons or sheets, at about 2.60 Ga, marked the last major Archean thermal event in the region and the stabilisation of the Zimbabwe Craton.
- Publication:
-
Journal of African Earth Sciences
- Pub Date:
- 1988
- DOI:
- 10.1016/0899-5362(88)90022-X
- Bibcode:
- 1988JAfES...7..795B