TEMPORAL EVOLUTION OF THE LAKE VICTORIA NEOARCHEAN GREENSTONE BELTS, TANZANIA

Neoarchean greenstone terranes record the construction and collapse of marginal continental and marine magmatic terranes and their amalgamation to an older Archean craton. In Tanzania, greenstone terranes fringing the Uganda craton were built through magmatic episodes between ~2860 and 2650 Ma, based upon U-Pb SHRIMP-RG ages of 62 rocks, along with 18 published U-Pb ages and 1 Sm-Nd isochron age. These packages are now structurally juxtaposed, and stitched by large, 2660-2640 Ma, low-Ca granites.

The oldest volcanic rocks are largely basaltic with lesser felsic rocks erupted between 2840-2820 Ma in the Musoma-Mara and Sukumaland belts. Slightly younger, ~2800-Ma rhyolitic volcanic rocks in the same belts form the next youngest event. Felsic volcanic rocks between 2780–2760 Ma are found in the Sukumaland and Nzega belts, and to a lesser extent in the Musoma-Mara belt. Between ~2725–2715 Ma, rhyolitic volcanism is widespread in the Sukumaland and Nzega belts. In the latter, felsic volcanism was contemporaneous with BIF sedimentation. At ~2710–2695 Ma, dacitic volcanism commenced; it overlapped BIF sedimentation in Sukumaland where trachyandesite also intruded the sedimentary rocks. Dacitic volcanism and BIF sedimentation continued to ~2660 Ma across the region. Post-magmatic erosion filled late basins with the Kavirondian Group.

Plutonic rocks also form discrete temporal clusters, which appear to overlap gaps in the volcanic record. The oldest plutons are ~2860–2810 Ma tonalite to granodiorite in the Musoma-Mara belt. A widespread, temporally distinct period of dioritic plutonism between ~2770–2730 Ma is present at least in the Sukumaland and Nzega belts, and likely in others. Within a ~15 Ma time frame around 2700 Ma, volumetrically small quartz-feldspar porphyry intrusions, emplaced across the region, are spatially associated with the structurally-controlled gold deposits. Syenite, based on field relations, appears to have intruded between 2670–2660 Ma; these rocks have yet to be successfully dated.

Collectively, the Tanzanian greenstone belts appear to have undergone a temporally evolution similar to that of other auriferous Neoarchean greenstone belts, the Abitibi and Yilgarn cratons.