Neoarchean arc magmatism and Paleoproterozoic high-pressure granulite-facies metamorphism in the southern Motloutse Complex, eastern Botswana: Implications for the western extension of the Limpopo Complex

Abstract

We provide the first record of the occurrence of high-pressure garnet-bearing amphibolite and mafic granulite hosted by amphibolites from the Maope area of the southern Motloutse Complex in eastern Botswana. The mineral assemblages of the rocks are garnet + edenite + plagioclase + ilmenite + rutile + quartz (garnet-bearing amphibolite), magnesio-hornblende + orthopyroxene + clinopyroxene + plagioclase + quartz (mafic granulite), and magnesio-hornblende + plagioclase + quartz + ilmenite + epidote + titanite (host amphibolite). The poikiloblastic garnet in the garnet-bearing amphibolite contains inclusions of edenite, ilmenite, quartz, and plagioclase as prograde minerals, while it is mantled by ferro-edenite + plagioclase coronae formed during retrograde metamorphism. The geochemical signatures 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), indicate subduction-related volcanic arc affinity, whereas the garnet-bearing amphibolite indicates subduction-related volcanic arc or within-plate basalt affinity. The peak metamorphic condition of the garnet-bearing amphibolite was constrained as 850–910 °C and 10.0–10.5 kbar based on phase equilibrium modeling in the Na2O-CaO-FeO-MgO-Al2O3-SiO2-H2O-TiO2-Fe2O3 (NCFMASHTO) system and geothermobarometry. Slightly lower conditions were obtained from the mafic granulite (745–825 °C) and amphibolite (610–750 °C). Zircon grains from the garnet-bearing amphibolite with lower Th/U ratios of 0.02–0.23 yielded a weighted-mean 207Pb/206Pb age of 2020.2 ± 4.9 Ma as the timing of high-grade metamorphism. In contrast, oscillatory-zoned zircons from the mafic granulite with higher Th/U ratios of 0.44–0.86 recorded a magmatic age of 2648.8 ± 9.8 Ma. As the age is consistent with previously reported age of a tonalite gneiss (2645 Ma) from the same region, we infer bimodal arc magmatism during Neoarchean. The results of this study indicate that the southern Motloutse Complex underwent high-pressure granulite-facies metamorphism with post-peak decompression along a single clockwise P–T evolution during Paleoproterozoic Era (∼2.02 Ga). Our integrated petrological and geochronological studies suggest that the southern Motloutse Complex is nearly equivalent to the Beit Bridge Complex as a possible western extension of a collisional suture (the Central Zone of the Limpopo Complex) between the Archean Kaapvaal and Zimbabwe Cratons. The occurrence of ca. 2.0 Ga high-pressure granulite-facies rocks from the southern Motloutse Complex might suggest multiple collisions of discrete crustal units during Neoarchean to Paleoproterozoic.