The geology and tectonic setting of the Shashe-Foley-Tonota area (Central Motloutse Complex), NE Botswana.
Moseki, Molatlhegi Larty Lostman.
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The well-developed NE-SW structural grain of the Shashe, Foley and Tonota (SFT) region is used to separate it from the NW-SE trending structures that characterise the Matsitama belt and adjacent area previously described as the Shashe belt. The study area is divided into 4 domains showing different geometrical and geological characteristics. Domain 2 (Shashe Dam shear zone) and Domain 3 lie to the NW and SE of the metasedimentary belt (Domain1). Domain 4 (the Gulushabe shear zone) is to the NE of the metasedimentary rocks. The rocks are characterised by NNE to ENE striking foliation present in both the the metasedimentary belt (supracrustal rocks) and the granitoid rocks. The metasedimentary rocks are deformed into large map scale NE to ENE trending folds structures (the Foley synform and the Gulushabe antiform) that deform bedding (So) and foliation (S1). The deformation involved NW-SE or NNW-SSE horizontal compression and is explained by two main episodes of NE-ENE coaxial (F1/F2) folding followed by a younger phase (F3) that produced NNE trending folds. The effect of F3 folding is reflected in stereographic plots of poles to bedding and foliation in Domains 1and Domain 2 which indicate folds plunging to the NNE. The foliation and the shape of deformed pebbles in the pebbly-quartzite is a product of oblate strain (flattening) in response to NW or NNW horizontal compression. This pebble elongation is considered to reflect the end result of F1 and F2 folding produced by oblate strain. Likewise, the shape of the K-feldspar megacrysts in the megacrystic granite gneiss in Domain 2 can be attributed to flattening (pure shear) rather than simple shear. The kinematics of the deformation features recognized are not compatible with the accretion-linked models proposed by previous workers for the SW margin of the Zimbabwe craton. Field based intrusive relationship studies indicate the granitoid gneisses were derived from igneous protoliths. Neoarchaean U-Pb zircon ages obtained confirm the order of granitoid emplacement obtained from field based studies. The ages obtained are 2724±48 Ma (Tonota biotite gneiss), 2698.9±9.2Ma (tonalite gneiss), 2647±24 Ma (megacrystic granite gneiss) and 2631.5±4 Ma (pink granite). Granitoid magmatism occurred between about 2724 Ma (biotite gneiss) and 2631 Ma (pink granite), a duration of ~ 94 Ma. The foliation (S1) in the tonalitic gneiss (2699 Ma) and the megacrystic gneiss (2647 Ma) predate intrusion of the pink gneissic granite (2631 Ma). Since S1 in the granitoid rocks is equated with S2 in the metasedimentary sequence, deformation fabrics in both the metasedimentary sequence and granitoid rocks predate 2631 Ma. The U-Pb ages obtained in this study, together with previous U-Pb zircon ages for granitoids from adjacent parts of the Motloutse Complex, the Limpopo belt and the Mosetse Complex indicate a geotectonic link between the terranes during the interval 2.6-2.7 Ga Domain 3 fabric is parallel to large scale ENE-WSW trending ductile dextral strike-slip shear zones (Regional D4 structures) that define the northern boundary to the Central Zone of the Limpopo belt but no kinematic indicators were found in the domain to confirm the shear sense. Structural evidence (this study) indicates thrust sense shearing characterises the SW vergent Gulushabe shear zone (Domain 4) which forms the boundary between the SFT area and the SE margin of the Tati greenstone belt. The Gulubashe shear zone dips N to NE thus deviating from the regional scale WNW/SSE trending system of thrust sense ductile shear zones which dips SW. The Gulushabe shear zone has the geometry of a back-thrust in an overall NE vergent system. The last deformation event is marked by widespread development of NE trending minor shear zones. The minor shear zones were not found in the pink granite gneiss implying that they are older than 2631 Ma. The development of these minor ductile shear zone is constrained between 2647 Ma and 2630 Ma. The structural history of the rocks and the kinematics of the deformation features recognised are given in table 3.1. Stable isotope analysis has shown that carbonate rocks (dolomites and calc-silicates) from the SFT region have high positive ð13C values (4.8 to 14.2‰). Such elevated ð13C values suggest a Palaeoproterozoic (2.4-2.1) age. However in the SFT area the regional foliation (S2) present in the granitoid gneisses and metasedimentary rocks is older than 2631 Ma indicating that despite the high ð13C values, the metacarbonate rocks are Neoarchaean in age.
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