Mg-rich metabasalts from the Southern Musoma –Mara greenstone belt: Possible evidence for mantle plume activity in the Tanzania craton?

Abstract

Mg-rich metabasalts from the Simba Sirori to Majimoto segment of the southern Musoma-Mara greenstone belt are the most magnesian lavas reported in the Archaean Tanzania Craton. The lavas yielded a Sm-Nd isochron age of 2768 ± 38 Ma (MSWD = 1.2) and corresponding !Nd(2.7 Ga) values in the range of + 0.66 to + 2.81. The lavas exhibit high MgO contents of up to 16 wt %, Cr (140 – 1590 ppm), Ni (80 – 510 ppm), values which are higher than those of primitive NMORB, but are comparable with those of Archaean komatiitic basalts and modern oceanic plateau basalts. Their REE patterns range from the light REE – depleted (La/SmCN = 0.46) to nearly flat (La/SmCN = 0.71 – 1.08) and light REE enriched (La/SmCN = 2.89). They also show negative anomalies of Nb and Ti relative to adjacent elements in multi-element spidergrams. The samples exhibit higher Th/Nb ratios (0.10 – 1.50, mean = 0.39) than those of NMORB (Th/Nb = 0.05) and tend to crustal values (Th/Nb = 0.80)
The higher MgO contents of the samples than those of primitive NMORB  requires a mantle source that can generate more magnesian lavas than the depleted mantle and a mantle plume is considered to be a viable source. Thus, the geochemical features of the Mg-rich basalts of Simba Sirori to Majimoto area can be explained by the fact that these rocks were generated by the contamination of komatiitic magmas by the felsic crust. The southern MMGB Mg-rich metabasalts provide a clue to the presence of komatiites, lavas that have not been documented in the Archaean craton of Tanzania. Such magnesian lavas are common in other cratons of the world including Barberton of South Africa, Belingwe of Zimbabwe, and Superior Province of Canada.