15:30 〜 17:00
[SMP25-P04] Neoproterozoic crustal evolution of southern Malawi in southern Africa: new insights based on petrology, geochemistry, and monazite geochronology of orthogneisses
キーワード：Key words: Pan African, petrology, P-T condition, monazite ages, geochemistry
The southern part of Africa comprises a collage of Archean to Paleoproterozoic cratons which are separated by Paleo to Neoproterozoic orogenic belts. These rocks are believed to have preserved long records of crustal evolution spanning from the Archean to Neoproterozoic. Malawi is located at the southern end of East African Rift Valley. This study was carried out on basement orthogneisses from the southern part the country. Southern Malawi is considered as part of the Mozambique Belt which exhibit mostly the Pan-African ages 950-450 Ma (e.g., Tsunogae et al., 2022), and this has been recognized in adjoining parts of Gondwana including areas of Brazil, Arabia, and Madagascar. Few previous studies have been conducted in the area. This study will principally focus on petrology, geochemistry and geochronology of the basement rocks to elucidate its tectonic evolution. Particularly, Malawi geology is important for being located near the junction between the East-African (ca. 600–550 Ma) and the Kuunga (ca. 560–530 Ma) orogenies, suggesting complex magmatic or metamorphic episode will be recorded in the rocks.
Based on field survey and petrographic observations, the dominant rock types of the basement rocks from Mulanje, Blantyre, Thyolo, Bvumbwe, and Mwanza areas, which is a focus of this study, are meta-sedimentary rocks, metabasites, and felsic orthogneisses that have undergone amphibolite- to granulite-facies metamorphism. TAS plot showed that whole-rock geochemical data of all orthogneiss samples (mafic granulite, granitoids, and biotite gneiss) plot on calc-alkaline field suggesting volcanic arcs affinity, whereas some mafic rocks plotted on tholeiitic field. This is consistent with trace elements spider diagram of the LILEs which is characterized by enrichment in some of the LILEs and depletion in HFSEs, also suggesting magmatic-arc affinity. The monazite U-Th-Pbtotal age dating of the orthogneisses indicate two age clusters as 650-610 Ma and 590-560 Ma, possibly suggesting two metamorphic events. These two age ranges might suggest two discrete metamorphisms, or a single prolong thermal event.
Based on the conventional garnet-clinopyroxene-plagioclase-quartz for the orthogneisses indicated high pressure and high temperature conditions of 10.8-11.1 kbars and 765oC-810 oC respectively, possibly related to the Pan-African collision related tectonic activities.
Reference
Tsunogae, T., Uthup, S., Nyirongo, M.W., Takahashi, K., Rahman, Md.S., Liu, Q., Takamura, Y., Tsutsumi, T., 2021. Neoproterozoic crustal growth in Southern Malawi: New insights from petrology, geochemistry, and U–Pb zircon geochronology, and implications for the Kalahari Craton–Congo Craton amalgamation. Precambrian Research 352, Article 106007, 1-20.
Key words: Pan African, petrology, P-T condition, monazite ages, geochemistry
Based on field survey and petrographic observations, the dominant rock types of the basement rocks from Mulanje, Blantyre, Thyolo, Bvumbwe, and Mwanza areas, which is a focus of this study, are meta-sedimentary rocks, metabasites, and felsic orthogneisses that have undergone amphibolite- to granulite-facies metamorphism. TAS plot showed that whole-rock geochemical data of all orthogneiss samples (mafic granulite, granitoids, and biotite gneiss) plot on calc-alkaline field suggesting volcanic arcs affinity, whereas some mafic rocks plotted on tholeiitic field. This is consistent with trace elements spider diagram of the LILEs which is characterized by enrichment in some of the LILEs and depletion in HFSEs, also suggesting magmatic-arc affinity. The monazite U-Th-Pbtotal age dating of the orthogneisses indicate two age clusters as 650-610 Ma and 590-560 Ma, possibly suggesting two metamorphic events. These two age ranges might suggest two discrete metamorphisms, or a single prolong thermal event.
Based on the conventional garnet-clinopyroxene-plagioclase-quartz for the orthogneisses indicated high pressure and high temperature conditions of 10.8-11.1 kbars and 765oC-810 oC respectively, possibly related to the Pan-African collision related tectonic activities.
Reference
Tsunogae, T., Uthup, S., Nyirongo, M.W., Takahashi, K., Rahman, Md.S., Liu, Q., Takamura, Y., Tsutsumi, T., 2021. Neoproterozoic crustal growth in Southern Malawi: New insights from petrology, geochemistry, and U–Pb zircon geochronology, and implications for the Kalahari Craton–Congo Craton amalgamation. Precambrian Research 352, Article 106007, 1-20.
Key words: Pan African, petrology, P-T condition, monazite ages, geochemistry