17:15 〜 19:15
[SMP26-P07] Neoproterozoic thermal overprinting on the Mesoproterozoic Irumide Belt in Malawi: New insight from monazite geochronology and geothermobarometry
キーワード：Irumide Belt、Mozambique Belt、Pseudosection、Amphibolite geothermobarometer、Monazite-xenotime geothermometer
The geology of southern Africa is composed dominantly of Archean cratons and Proterozoic orogenic belts formed by a series of continental collision events. Previous studies recognized three orogenic stages from north-central Malawi; Ubendian, Irumide, and Mozambique Belts from north to south. However, the boundaries of these orogenic units in northern Malawi are not clearly defined because most of the basement rocks in the region are still undifferentiated, and metamorphic P-T conditions are not available in regional scale.
In this study, we performed petrological and geochronological investigations on high-grade metamorphic rocks from Mzimba area in the central-northern Malawi, which corresponds to the southeastern part of the Irumide Belt (sensu stricto). The application of monazite Th-U-total Pb geochronology of the basement rocks from the region gave ages of ca. 1.0 to 1.1 Ga, which is consistent with available metamorphic ages from other regions of the Irumide Belt. We also obtained younger monazite ages of ca. 580 to 500 Ma from selected localities.
The application of the amphibole-plagioclase geothermometer and Ti-Al-in-amphibolite geothermobarometer to amphibolite and biotite-hornblende gneiss samples yielded temperatures of approximately 810 to 820℃ and pressures of 6 kbar. The application of the monazite-xenotime geothermometer to quartzite resulted in temperatures ranging from 750 to 770℃ and the metamorphic age at ca. 1.0 Ga. We also applied phase equilibria modeling on a pelitic gneiss (quartz + plagioclase + K-feldspar + garnet + biotite + sillimanite + magnetite + ilmenite) in NCKFMASHTO system using Perple_X software and obtained the peak condition of 660-750℃ and 6.5-7.6 kbar. These results suggest upper amphibolite- to granulite-facies metamorphic conditions throughout the Irumide Belt in Malawi. The conditions are higher than those from the Irumide Belt in Zambia (650℃ and <8 kbar; e.g., De Waele et al., 2006), but lower than the ultrahigh-temperature conditions (>900℃) for pelitic granulites from Jenda and Lundazi areas (Wakabayashi et al., 2024; Sakuwaha et al., 2024).
We also applied garnet-biotite and garnet-sillimanite-plagioclase-quartz geothermobarometers to garnet-bearing metamorphic rocks and found a decrease in P-T conditions from north to south, ranging from 6.9 kbar and 700℃ to 2.5 kbar and 590℃, indicating a wide variation in retrograde P-T conditions within the belt. It has to be noted that the garnet-biotite gneisses with lower P-T conditions from the southern part record monazite ages of 580–500 Ma as well as Mesoproterozoic ages, indicating the effect of regional Neoproterozoic thermal overprinting. The younger metamorphic event might be triggered by the activity of the Mozambique Belt related to the amalgamation of the Gondwana Supercontinent.
Reference
De Waele, B., Liégeois, J.-P., Nemchin, A.A., Tembo, F., 2006. Isotopic and geochemical evidence of Proterozoic episodic crustal reworking within the Irumide Belt of south-central Africa, the southern metacratonic boundary of an Archaean Bangweulu Craton. Precambrian Research 148, 225–256.
Sakuwaha, K.G., Tsunogae, T., 2024. Metamorphic P-T-t evolution of ultrahigh-temperature granulite facies metapelites from the Lundazi terrane, Irumide Belt, Eastern Zambia. Precambrian Research, 401, 107271.
Wakabayashi, S., Sakuwaha, K.G., Tsunogae, T., Santosh, M., Nyirongo, M. W., 2024. Sapphirine+ quartz assemblage from the Irumide Belt, northern Malawi: Implications for Mesoproterozoic ultrahigh-temperature metamorphism related to Rodinia assembly. Precambrian Research, 414, 107607.
In this study, we performed petrological and geochronological investigations on high-grade metamorphic rocks from Mzimba area in the central-northern Malawi, which corresponds to the southeastern part of the Irumide Belt (sensu stricto). The application of monazite Th-U-total Pb geochronology of the basement rocks from the region gave ages of ca. 1.0 to 1.1 Ga, which is consistent with available metamorphic ages from other regions of the Irumide Belt. We also obtained younger monazite ages of ca. 580 to 500 Ma from selected localities.
The application of the amphibole-plagioclase geothermometer and Ti-Al-in-amphibolite geothermobarometer to amphibolite and biotite-hornblende gneiss samples yielded temperatures of approximately 810 to 820℃ and pressures of 6 kbar. The application of the monazite-xenotime geothermometer to quartzite resulted in temperatures ranging from 750 to 770℃ and the metamorphic age at ca. 1.0 Ga. We also applied phase equilibria modeling on a pelitic gneiss (quartz + plagioclase + K-feldspar + garnet + biotite + sillimanite + magnetite + ilmenite) in NCKFMASHTO system using Perple_X software and obtained the peak condition of 660-750℃ and 6.5-7.6 kbar. These results suggest upper amphibolite- to granulite-facies metamorphic conditions throughout the Irumide Belt in Malawi. The conditions are higher than those from the Irumide Belt in Zambia (650℃ and <8 kbar; e.g., De Waele et al., 2006), but lower than the ultrahigh-temperature conditions (>900℃) for pelitic granulites from Jenda and Lundazi areas (Wakabayashi et al., 2024; Sakuwaha et al., 2024).
We also applied garnet-biotite and garnet-sillimanite-plagioclase-quartz geothermobarometers to garnet-bearing metamorphic rocks and found a decrease in P-T conditions from north to south, ranging from 6.9 kbar and 700℃ to 2.5 kbar and 590℃, indicating a wide variation in retrograde P-T conditions within the belt. It has to be noted that the garnet-biotite gneisses with lower P-T conditions from the southern part record monazite ages of 580–500 Ma as well as Mesoproterozoic ages, indicating the effect of regional Neoproterozoic thermal overprinting. The younger metamorphic event might be triggered by the activity of the Mozambique Belt related to the amalgamation of the Gondwana Supercontinent.
Reference
De Waele, B., Liégeois, J.-P., Nemchin, A.A., Tembo, F., 2006. Isotopic and geochemical evidence of Proterozoic episodic crustal reworking within the Irumide Belt of south-central Africa, the southern metacratonic boundary of an Archaean Bangweulu Craton. Precambrian Research 148, 225–256.
Sakuwaha, K.G., Tsunogae, T., 2024. Metamorphic P-T-t evolution of ultrahigh-temperature granulite facies metapelites from the Lundazi terrane, Irumide Belt, Eastern Zambia. Precambrian Research, 401, 107271.
Wakabayashi, S., Sakuwaha, K.G., Tsunogae, T., Santosh, M., Nyirongo, M. W., 2024. Sapphirine+ quartz assemblage from the Irumide Belt, northern Malawi: Implications for Mesoproterozoic ultrahigh-temperature metamorphism related to Rodinia assembly. Precambrian Research, 414, 107607.