The P-T-X(fluid) evolution of meta-anorthosites in the Eastern Granulites, Tanzania

摘要: Meta-anorthosite bodies are typical constituents of the Neoproterozoic Eastern Granulites in Tanzania. The mineral assemblage (and accessory components) is made up clinopyroxene, garnet, amphibole; scapolite, epidote, biotite, rutile, titanite, ilmenite and quartz. Within feldspar-rich matrix (70–90% plagioclase), mafic domains with metamorphic corona textures were used for P–T calculations. Central parts these constitute high-Al clinopyroxene – which a common magmatic anorthosites therefore assumed to be relict. rims have diopsidic composition surrounded by garnet corona. Locally pyroxene amphibole scapolite suggesting that mixed CO2–H2O fluid was present during their formation. Thermobarometric calculations give following conditions peak individual meta-anorthosite bodies: Mwega: 11–13 kbar, 850–900 °C; Pare Mountains: 12–14 kbar, Uluguru 850–900 °C. evolution modelled using pseudosections. amount <0.5 mol.% bulk sufficient produce fluid-saturated assemblages at 10 kbar 800 °C. Pseudosection analysis shows most likely formed under undersaturated or close boundary saturation. stabilities dependent on Mode isopleths minerals change geometry drastically between fluid-undersaturated assemblages. coronae developed isobaric cooling peak. segment followed decompression as indicated growth plagioclase. estimated ∼0.3–0.5. Although meta-anorthosites different formation ages (Archean Proterozoic) they experienced same Pan-African overprint retrograde path. Similar evolutionary paths known from hosting granulites. presented data best explained tectonic model hot fold nappes brought aged surrounding rocks together deep crust an history.