THE MOTLOUTSE COMPLEX AND THE ZIMBABWE CRATON LIMPOPO BELT TRANSITION IN BOTSWANA

The Archaean rocks of northeastern Botswana are divided into named lithostratigraphic complexes, each a different assemblage of stratiform lithologies, granitoids and gneisses, and each with differences in metamorphic grade, structural style, and metallic mineralisation. The Motloutse Complex lies between four other complexes. It includes granitoids and orthogneisses typical of the Zimbabwe Craton to the east, detached segments of the quartzite-carbonate-pelite association of the anomalous Matsitama schist belt to the west, and migmatites similar to those of the Limpopo Belt to the south, but is itself a distinctive lithological assemblage. It shows a higher grade of metamorphism than the adjacent cratonic terranes, but lacks the granulites of the Limpopo Belt. It is delineated by relatively abrupt changes in structural style and fabric orientation of which most, if not all, coincide with shear zones or faults. It is partly bounded against the Limpopo Belt by the Magogaphate Shear Zone, but also flanks the western boundary of the Limpopo Belt Central Zone, west of 27-degrees 35'E. The Motloutse Complex was formed in the Late Archaean between approximately 2.7 Ga and 2.5 Ga ago, in a zone of plutonism which separated a crustal fragment bearing the depositional basin of the Matsitama shelf-type sequences from the developing greenstone belts to the east. During regional deformation, elements of the Matsitama Belt and associated paragneisses were thrust from the southwest over the active magmatic zone; and were then infolded with, and underplated by, migmatites, orthogneisses and granitoids of the developing Motloutse Complex. The terranes which were thus accreted to the southwestern margin of the Zimbabwe Craton during this episode of deformation and crustal thickening (D1-D3) were then cut across by dextral movement on the Magogaphate Shear Zone during D4, so juxtaposing the Limpopo Belt with the Motloutse Complex. The area of the Motloutse Complex was previously regarded as a transitional boundary between the Craton and the Mobile Belt, but is here seen instead as an intracratonic accretionary assemblage on which the ENE-WSW "Limpopo" trend was partly superimposed during D4 shearing. Delineation of lithostratigraphic complexes elsewhere in the Zimbabwe Craton and the Limpopo Belt may be possible, and can be expected to clarify the pattern to which these provinces were assembled during the Archaean prior to D4 deformation.