17:15 〜 18:45
[SMP22-P03] Monazite Th-U-Total Pb geochronology of the basement rocks in Malawi: Implications for Neoproterozoic thermal overprint on the Mesoproterozoic Irumide Belt
キーワード：Gondwana supercontinent、Ubendian Belt、Irumide Belt、Mozambique Belt、Monazite Th-U- Total Pb geochronology
The geology of southern Africa is composed dominantly of Archean cratons and Proterozoic orogenic belts formed by a series of continental collision events. These belts include the Ubendian (Paleoproterozoic), Irumide (Mesoproterozoic), and Mozambique (Neoproterozoic) Belts which occur along the boundaries of the cratons. Previous studies indicate that the three stages of the orogenic belts are distributed in north-central Malawi (Mozambique, Irumide, Ubendian Belts from south to north), but their boundaries are not clearly defined. In this study, we performed Th-U-total Pb dating of monazites in high-grade metamorphic rocks for the first time in this area for evaluating the relative distribution of the orogenic belts. From the southern part of the Irumide Belt adjacent to the Mozambique Belt, Mesoproterozoic ages of ca. 1100 -1050 Ma, which are consistent with the results of previous studies have been obtained as well as Neoproterozoic to Ordovician (ca. 610-480 Ma) thermal overprint age possibly related to the second metamorphism during the East African (ca. 650-620 Ma) and Kuunga (ca. 580-500 Ma) orogenies. Such prolonged metamorphic evolution could be due to the location of central Malawi around the junction between the two orogenic belts. The boundary between the Irumide and the Mozambique Belts is thus inferred as irregular probably because of heterogeneous distribution of the overprinted areas. Regarding the boundary between the Irumide Belt and the Ubendian Belt to the north, we obtained Paleoproterozoic ages of ca. 1990-1910 Ma, which is consistent with the timing of the Ubendian orogeny, from the northern part of the Irumide Belt. As the Paleoproterozoic rocks occur along a northeast-southwest zone, from Rumphi to Chilumba in northern Malawi we infer that the Ubendian rocks might have emplaced as a nappe onto the northern part of the Irumide Belt. As the Ubendian-age rocks also underwent thermal overprinting ca. 610 Ma this nappe structure might have formed during Neoproterozoic. Alternatively, it is also possible to discuss that the distribution of the Ubendian Belt might extend farther south toward the northern margin of Irumide Belt, suggesting that the ca. 600 Ma metamorphic ages obtained from the southern part of the Ubendian Belt to the northern part of the Irumide Belt could be attributed to thermal overprint of Neoproterozoic metamorphism, similar to what occurred in the southern part of the Irumide Belt. Such Paleoproterozoic, Mesoproterozoic, and Neoproterozoic metamorphic events recorded in the basement rocks in north-central Malawi is probably related to the amalgamation events of Columbia, Rodinia, and Gondwana supercontinents, respectively.