Delineating the dismembered remnants of Archean layered complexes in medium-high-grade terranes: Scenario from the Motloutse Complex, eastern Botswana

Abstract

Ultramafic-mafic rocks in medium-to high-grade metamorphic terranes commonly occur as fragmented units of once continuous layered intrusions. A number of factors can complicate the delineation of the dismembered remnants and reconstruction of the original lithostratigraphy. This study focuses on remnant ultramafic rocks of the Mesoarchean Lechana layered complex in the southern Motloutse Complex, eastern Botswana. In view of the limited exposures, including the prominent cover rocks, geologic characterization was supplemented with geophysical techniques. The NE-SW traverse covered in the study includes the Tamasane and Sapolamorori hills, on either side of the cover rocks. Both hills are comprised of layered ultramafic rocks, with tonalite gneiss and amphibolite forming the surrounding rocks. The inferred layered lithostratigraphy includes serpentinized dunite, peridotite, peridotite-pyroxenite, pyroxenite-peridotite, and pyroxenite. Magnetitite seams constitute the main mineralization, and are associated with the serpentine-rich rocks. Aeromagnetic data provide insights into the bulk magnetic susceptibility of the ultramafic lithologies along the transect. The highest magnetic susceptibilities occur as semi-circular closed patches. The two anomalies at either end of the transect correlate with the layered ultramafic bodies at the Tamasane and Sapolamorori hills. The rest approximately correlates with high Ni ± Cr ± Cu soil anomalies reported from the region. This redefines the known extent of the Lechana layered complex beneath the cover rocks. The study highlights the potential of integrating geological and geophysical data to delineate the dismembered remnants of Archean layered complexes.