Defining the contact zone between Archean-Proterozoic terranes with distinct geologic histories: The scenario from eastern Botswana

Abstract

Understanding when Archean and Proterozoic terranes of distinct geologic histories became proximal to each other is an important topic to investigate. This study aims to define the contact zone between the southern Motloutse Complex, an Archean medium-grade terrane with prominent Neoarchean-Mesoarchean rock units, and the northern Mahalapye Complex, a Paleoproterozoic high-grade granitoid-gneiss terrane, in eastern Botswana. The contact zone between the two terranes is obscured by cover rocks. Field reconnaissance aided with remote sensing and aeromagnetic imaging delineated the NW-SE trending Radisele-Mogome corridor beneath the cover rocks. Tonalite gneiss with amphibolite and layered ultramafic rock xenoliths dominate the NE part, and are common rock types in the southern Motloutse Complex. Granodiorite with diorite gneiss xenoliths dominate the SW part, and are extensions of the NW-SE elongate c.2.04 Ga Mokgware pluton in northern Mahalapye Complex. The c.2.02 Ga Mogome granite intrudes across the contact zone, and contains xenoliths of rocks from both complexes. Shear deformation and metasomatism variably overprint the rocks along the contact zone. U-Pb zircon and 40Ar/39Ar amphibole–biotite geochronology were carried out on representative rock types to understand the chronology of events along the contact zone. The 2056 ± 40 Ma (MSWD = 5.2; n = 56) diorite gneiss from the NE margin of the contact zone contains c.2.6 Ga and c.2.9 Ga xenocrystic zircons. No Archean zircons were found in the 2041 ± 88 Ma (MSWD = 8.4; n = 13) diorite gneiss from the SW margin of the contact zone. The 2036 ± 12 Ma (40Ar/39Ar amphibole) and 2039 ± 11 Ma (40Ar/39Ar biotite) ages, respectively from the NE and SW margins, argue for the c.2.04 Ga age to represent the first common event along the contact zone. Shear deformation and metasomatism are respectively dated at 1987 ± 7 Ma and 1951 ± 11 Ma (40Ar/39Ar biotite), and account for the Pb-loss in dated zircons. It is argued that the Archean Motloutse Complex and Paleoproterozoic Mahalapye Complex terranes came together at c.2.04 Ga, with the emplacement of syntectonic Mokgware granodiorite. The results highlight the factors that are important to define the contact zone between Archean-Proterozoic terranes with distinct geologic histories.