Petrology of Whiteschists and Associated Rocks at Mautia Hill (Tanzania): Fluid Infiltration during High-Grade Metamorphism?

Abstract

Talc-kyanite schists (whiteschists), magnesiohornblende-kyanite-talc-quartz schists and enstatite-sapphirine-chlorite schists occur at Mautia Hill in the East African Orogen of Tanzania. They are associated with metapelites and garnet-clinopyroxene-quartz metabasites. Geobarometry (GASP/GADS equilibria) applied to the latter two rock types indicates a peak pressure of P = 10-11 kbar. These results are confirmed by the high fO(2) assemblage hollandite-kyanite-quartz and late-stage manganian andalusite that contains up to 19.5 mol. % Mn2SiO5. Maximum temperatures of T = 720degreesC are inferred from late-stage yoderite + quartz. A clockwise P-T evolution is constrained by prograde kyanite inclusions in metapelitic garnet and late-stage reaction rims of cordierite between green yoderite and talc that reflect conditions at least 3-4 kbar below the peak pressure. Oxidizing conditions are recorded throughout the metamorphic history of the whiteschists and chlorite schists, as indicated by the presence of haematite coexisting with pseudobrookite and/or rutile. Increasing water activity near peak pressures is thought to have led to the breakdown of the high-pressure assemblages (Tlc-Ky-Hem and Mg-Hbl-Ky-Hem) and the subsequent formation of certain uncommon minerals, e.g. yellow sapphirine, Mn-andalusite, green and purple yoderite, piemontite and boron-free kornerupine. The proposed increase in water activity is attributed to fluid infiltration resulting from the devolatilization of underlying sediments during metamorphism.