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Journal of Petrology Advance Access originally published online on August 27, 2004
Journal of Petrology 2004 45(10):1959-1981; doi:10.1093/petrology/egh044
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Journal of Petrology 45(10) © Oxford University Press 2004; all rights reserved

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

NIELS JÖNS* and VOLKER SCHENK

INSTITUT FÜR GEOWISSENSCHAFTEN, UNIVERSITÄT KIEL, 24098 KIEL, GERMANY

* Corresponding author. Telephone +49 431 880 3489. Fax: +49 431 880 4457. E-mail: nj{at}min.uni-kiel.de

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 fO2 assemblage hollandite–kyanite–quartz and late-stage manganian andalusite that contains up to 19·5 mol. % Mn2SiO5. Maximum temperatures of T = 720°C 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.

KEY WORDS: fluid infiltration; high-pressure amphibolite facies; East African Orogen; Pan-African; whiteschist


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