[S-MP25] 変形岩・変成岩とテクトニクス

[SMP25-P08] Neoarchean magmatism and Paleoproterozoic high-pressure granulite-facies metamorphism from the southern Motloutse Complex, eastern Botswana: Implications for the western extension of the Central Zone of the Limpopo Complex

*Thembiso Otladisang Basupi^1,2、Toshiaki Tsunogae^1,3、Kazuki Takahashi¹、Yukiyasu Tsutsumi⁴ (1.Graduate of Life and Environmental Sciences, University of Tsukuba、2.Department of Earth and Environmental Sciences, Botswana International University of Science and Technology、3.Department of Geology, University of Johannesburg、4.Department of Geology and Paleontology, National Museum of Nature and Science)

キーワード：Neoarchean, Paleoproterozoic, Motloutse Complex, High-pressure granulite, Garnet-bearing amphibolite, Mafic granulite

We provide the first record of the occurrence of high-pressure garnet-bearing amphibolite and mafic granulite hosted by the amphibolite of amphibolite facies – lower granulite facies from the Maope area of southern Motloutse Complex in eastern Botswana. Our results also provide for the first time Neoarchean magmatic age preserved in the mafic granulite. The mineral assemblages of the rocks are garnet + hornblende + plagioclase + ilmenite + rutile + quartz (garnet-bearing amphibolite), hornblende + orthopyroxene + clinopyroxene + plagioclase + quartz (mafic granulite), and hornblende + plagioclase + quartz + ilmenite + epidote + titanite (host amphibolite). The poikiloblastic garnet from the garnet-bearing amphibolite contains inclusions of hornblende, ilmenite, quartz, and plagioclase as prograde mineral and is mantled by hornblende + plagioclase coronae, which formed during retrograde metamorphism. In the mafic granulite, orthopyroxene is coarse-grained, and the hornblende occurs as a thin retrograde corona around the plagioclase and clinopyroxene. The geochemical signature of the mafic granulite and amphibolite, combined with the enrichment of large-ion lithophile elements and depletion of high-field strength elements (Nb and Ta), indicates subduction-related volcanic arc affinity, and the garnet-bearing amphibolite indicate subduction-related volcanic arc and within-plate basalt affinity. The peak metamorphic condition of the garnet-bearing amphibolite was obtained at 850°C–910°C at 10–10.5 kbar using phase equilibrium modeling in the Na₂O-CaO-FeO-MgO-Al₂O₃-SiO₂-H₂O-TiO₂-Fe₂O₃ (NCFMASHTO) system and was consistent with results from geothermobarometry. The conventional geothermobarometry results estimated from the mafic granulite and amphibolite are 745°C–826°C and 488°C–736°C at 7 kbar, respectively. The pressure-temperature (P-T) condition results suggest high-pressure garnet-bearing amphibolite, granulite-facies for the mafic granulite, and amphibolite-lower granulite facies for amphibolite. Zircon grains from the garnet-bearing amphibolite, with a discordance of <5% and Th/U of <2.5, recorded a metamorphic age of 2020.2 ± 4.9 Ma whereas the zircon from mafic granulite with a discordance of <5% and Th/U of 0.44 to 0.86 recorded a magmatic age of 2648.8 ± 9.8 Ma. The results of the P-T condition and the geochronology from the garnet-bearing amphibolite suggest that the southern Motloutse Complex is a high-pressure granulite terrane that underwent a single clockwise P-T evolution in the Paleoproterozoic Era (~2.02 Ga). Our results also suggested for a 2.6 Ga magmatic emplacement of mafic rocks recorded from the mafic granulite. The results further, suggest that the southern Motloutse Complex is nearly equivalent to the Beit Bridge Complex in the Central Zone of Limpopo Complex.