Veronika Tenczer, Department of Earth Sciences (Austria)
Christoph Hauzenberger, Department of Earth Sciences (Austria)
Georg Hoinkes, Department of Earth Sciences (Austria)
Harald Fritz, Department of Earth Sciences (Austria)
Urs Klötzli, Center for Earth Sciences (Austria)
Sospeter Muhongo, ICSU Regional Committee for Africa (South Africa)
Anorthosites are important constituents of the lower crust. In Tanzania a chain of anorthositic rocks occurs in the Eastern Granulites - a deep crustal sequence within the Neoproterozoic Mozambique Belt. Along this N-S trending chain four major occurrences of anorthosites are found. From North to South these are: The Ikongwe anorthosite (Pare Mountains), the Mwega anorthosite around Mkata, the Uluguru anorthosite (Uluguru Mountains) and the Iromue anorthosite (Mahenge Mountains). Zircon studies have shown that the northern anorthosites (Ikongwe and Mwega) have Archean formation ages (2.64±16 Ga) whereas the two southern bodies (Uluguru and Mahenge) have Proterozoic formation ages (800-900 Ma).
All bodies show Neoproterozoic deformation and metamorphic overprint. The zircons from the southern anorthosites have broad metamorphic rims with concordant ages around 650±15 Ma. The northern bodies in contrast show small metamorphic rims with less-defined Neoproterozoic intercept ages of 600±50 Ma. The rocks are comprised of: 70-90% plagioclase and 10-30% mafics or accessory components (clinopyroxene, garnet, amphibole; scapolite, epidote, biotite, rutile, sphene, ilmenite, quartz, K-feldspar). The mafic components display metamorphic corona textures that are allow inferring the P-T evolution. Central parts of these textures constitute high-Al clinopyroxenes - which are magmatic minerals commonly found in anorthosites. At the rim the clinopyroxenes have a diopsidic composition and are surrounded by a garnet corona. Locally the pyroxenes are surrounded by coexisting amphiboles and scapolites indicating that a mixed CO2-H2O fluid was present during their formation. Thermobarometric calculations give the following P-T data for the metamorphic peak: Pare Mountains: P=12.50 kbar, T=847°C; Mkata: P=12.0 kbar, T=759°C; Uluguru Mountains: P=13.80 kbar, T=857°C. Based on these data a quantitative P-T evolution was modelled using pseudosections for a given bulk rock composition. Crucial parameters for these calculations are the amount and the composition of the metamorphic fluid. A small amount of fluid (e.g. <0.5 mol% in the bulk composition) is sufficient to produce fluid saturated assemblages at ca. 10 kbar and 800°C. The corona textures most likely have formed at fluid undersaturated conditions or close to the boundary of fluid saturation. Especially the stability of garnet and amphibole is dependent on the amount of fluid. Isopleths of these two minerals change its geometry drastically between a fluid-bearing or fluid-absent assemblage. Clinopyroxene is less dependent on this parameter.
The garnet coronae developed along isobaric cooling following the metamorphic peak with a T-decrease of ca. 70-100°C. The cooling segment is followed by a decompression segment as indicated by the growth of amphibole and plagioclase. The ratio between H2O and CO2 is estimated as 1:1 and the XCO2 of the metamorphic fluid is typically around 0.3-0.5. (Supported by FWF T247-N10)