Petrology of Whiteschists and Associated Rocks at Mautia Hill (Tanzania): Fluid Infiltration during High-Grade Metamorphism? |
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| Authors: | JONS NIELS; SCHENK VOLKER |
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| Institution: | INSTITUT FÜR GEOWISSENSCHAFTEN, UNIVERSITÄT KIEL, 24098 KIEL, GERMANY |
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| Abstract: | Talc–kyanite schists (whiteschists), magnesiohornblende–kyanite–talc–quartzschists and enstatite–sapphirine–chlorite schistsoccur at Mautia Hill in the East African Orogen of Tanzania.They are associated with metapelites and garnet–clinopyroxene–quartzmetabasites. Geobarometry (GASP/GADS equilibria) applied tothe latter two rock types indicates a peak pressure of P = 10–11kbar. These results are confirmed by the high fO2 assemblagehollandite–kyanite–quartz and late-stage manganianandalusite that contains up to 19·5 mol. % Mn2SiO5. Maximumtemperatures of T = 720°C are inferred from late-stage yoderite+ quartz. A clockwise P–T evolution is constrained byprograde kyanite inclusions in metapelitic garnet and late-stagereaction rims of cordierite between green yoderite and talcthat reflect conditions at least 3–4 kbar below the peakpressure. Oxidizing conditions are recorded throughout the metamorphichistory of the whiteschists and chlorite schists, as indicatedby the presence of haematite coexisting with pseudobrookiteand/or rutile. Increasing water activity near peak pressuresis thought to have led to the breakdown of the high-pressureassemblages (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 increasein water activity is attributed to fluid infiltration resultingfrom 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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| Keywords: | : fluid infiltration high-pressure amphibolite facies East African Orogen Pan-African whiteschist |
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