15:30 〜 17:00
[SMP25-P03] Mesoproterozoic ultrahigh-temperature metamorphism in northern Malawi: New insights from petrology, geochemistry, and geochronology
キーワード：サフィリン、温度圧力経路、イルマイデ造山帯
The Irumide Belt situated in southern Africa is a Mesoproterozoic orogenic belt that probably formed during the amalgamation of the supercontinent Rodinia. On the other hand, the Southern Irumide Belt is regarded as an extensively reworked region of the Irumide Belt due to the effects of the Neoproterozoic Pan-African orogeny. Therefore, the Irumide Belt might record multiple thermal events related to convergent tectonics during Proterozoic eon. However, limited data related to the Irumide Belt are currently available. Thus, further petrological and geochronological investigations of high-grade metamorphic rocks in the belt are necessary to unravel the tectono-thermal evolution of the region. In this study, we report for the first time the occurrence of sapphirine + quartz association in pelitic gneiss (sapphirine + quartz + plagioclase + K-feldspar + ilmenite + magnetite + sillimanite) from the Irumide Belt as a robust evidence of peak ultrahigh-temperature (UHT) metamorphism. The mineral equilibrium modeling technique constrained the peak conditions of pelitic gneisses as >890°C/6 kbar along a clockwise P–T path. U–Th–total Pb ages of monazite grains in the sapphirine-bearing sample are classified into several age clusters from ca. 2000 Ma to and 1100 Ma, and the youngest clusters indicate the timing of high-grade metamorphism. The ca. 1100 Ma event might correspond to the main collisional event of the Bangweulu Block and an unknown craton during Rodinia assembly.