[S-MP26] Supercontinents and Crustal Evolution

[SMP26-05] Petrogenesis of multilayer corona textures around olivine in metagabbros from the Mozambique Belt in central Malawi

*Mirei Kume¹, Toshiaki Tsunogae¹, Mzee Nkhwachi Nyirongo¹ (1.University of Tsukuba )

Corona textures in metamorphic and magmatic rocks refer to reaction textures characterized by the concentric growth of new mineral/minerals around a relic mineral, resulting from rapid recrystallization due to sudden changes in temperature and/or pressure. The formation mechanisms of corona textures are crucial for understanding metamorphic and/or cooling processes of the host rock. Olivine in gabbroic rocks often exhibits corona textures along its grain boundaries with plagioclase, and both garnet-bearing and garnet-free coronae are reported. Garnet-bearing corona such as orthopyroxene + garnet + hornblende has been reported from metagabbros in high-grade metamorphic terranes (e.g., Grenville metamorphic belt, Canada) with peak pressure-temperature (P-T) conditions of 8-10 kbar and 700-750 °C (Grant, 1988). In contrast, garnet-free corona (e.g., spinel + hornblende) are found in magmatic terranes such as Black Hill region, Australia (Turner and Stüwe, 1992) and Vestre Dale, Norway (de Haas et al., 2002). These garnet-free coronae require low-pressure (<1 kbar) and high-temperature (~980 °C) conditions. Therefore, detailed evaluations of corona textures in gabbros/metagabbros are useful for unraveling tectono-thermal evolution of metamorphic/magmatic terranes.
In this study, we investigated the petrogenesis of multilayer corona textures in metagabbros from the Neoproterozoic Mozambique Belt in central Malawi. Their geochemical features suggest that the parental magma was derived from a volcanic arc setting. Olivine adjacent to plagioclase in the metagabbros is surrounded by orthopyroxene corona, and it is further mantled by outer corona composed of garnet and pargasite, suggesting the progress of the following reaction:

olivine + plagioclase + H₂O → orthopyroxene + garnet + pargasite.

Using Perple_X software, mineral equilibrium modeling was conducted for a metagabbro sample in the Na₂O-CaO-FeO-MgO-Al₂O₃-SiO₂-H₂O-TiO₂ (NCFMASHT) system to construct P-T pseudosections. The results indicate that the protolith gabbro with the primary magmatic assemblage of olivine + plagioclase + ilmenite crystallized at temperature exceeding 1100 °C. The gabbro subsequently underwent amphibolite-facies metamorphism at approximately 700 °C and 5.6 kbar, forming orthopyroxene + garnet + pargasite multilayer coronae. The metamorphic condition is consistent with the results of hornblende-plagioclase geothermometry (670-710 °C) applied to a biotite-hornblende gneisse from adjacent locality, suggesting rapid cooling for more than 400 °C. The results of this study indicate that the gabbro intruded into the middle-crustal level, and the intrusion probably coincides with the timing of peak metamorphism of the Mozambique Belt in central Malawi (571.3 ± 2.0 Ma based on Th-U total Pb dating of uraninite). The peak pressure condition of 5.6 kbar is consistent with the results of previous studies that the garnet-bearing corona textures indicate syn-metamorphic gabbro intrusion into middle- to lower-crustal level and subsequent near-isobaric rapid cooling due to thermal relaxiation.

References
de Haas, G.J.L., Nijland, T.G., Valbracht, P.J., Maijer, C., Verschure, R., Andersen, T., 2002. Magmatic versus metamorphic origin of olivine-plagioclase coronas. Contrib. Mineral. Petrol. 143, 537-550.
Grant, S.M., 1988. Diffusion models for corona formation in metagabbros from the Western Grenville Province, Canada. Contrib. Mineral. Petrol. 98, 49-63.
Turner, S.P., Stüwe, K., 1992. Low-pressure corona textures between olivine and plagioclase in unmetamorphosed gabbros from Black Hill, South Australia. Mineral. Mag. 56, 503-509.