Pb- and Nd-isotope systematics of stromatolitic limestones from the 2.7 Ga Ngezi group of the Belingwe Greenstone Belt: Constraints on timing of deposition and provenance

Pb-Pb isochrons have been obtained for stromatolitic limestones from the late Archaean Belingwe Greenstone Belt of Zimbabwe, providing direct age constraints on the deposition of these shallow water marine sediments. Samples from the Manjeri Formation and stratigraphically higher Cheshire Formation yield age estimates of 2706 ± 49 Ma (MSWD = 11.5) and 2601 ± 49 Ma (MSWD = 0.93), respectively. These data are in agreement with published U-Pb zircon and Pb-Pb whole rock ages of associated volcanics, and we, therefore, interpret our Pb-Pb ages as representing the timing of early diagenesis, thus providing a minimum age for carbonate precipitation. A 2543 ± 70 Ma age (MSWD = 5.1) for one sample from the Cheshire Formation is considered to reflect either late-stage diagenesis, a craton wide thermal/chemical disturbance or tectonic activity along the crustal-scale Mtshingwe fault. Calculated model μ 1-values for Manjeri and Cheshire limestones are 8.40 ± 0.02 and 9.02 ± 0.01, similar to values for approximately 3.5 and 2.9 Ga Archaean basement units adjacent to the Belingwe Belt. Negative ε_Nd (T_deposition) and f_Sm/Nd suggest derivation of the REE from old, LREE enriched continental crust. Two-stage Nd model ages for the carbonates indicate that precursor rocks were extracted from the mantle at around 3.5 Ga (Cheshire Formation) and 3.3 Ga (Manjeri Formation), in good agreement with mantle-extraction ages for local basement units (model T_DM: 3.5-3.3 Ga).