«By MOLATLHEGI LARTY LOSTMAN MOSEKI STUDENT NO. 208523856 Submitted in fulfillment of the academic requirements For the degree of Master of Science In ...»
The deformation history described in the present account is similar to that described from Topisi, Phikwe and Francistown areas. However, no evidence was found in the SFT area for top to NE directed thrusting (first reported in NE Botswana by Litherland (1975) and also reported in the Topisi, Phikwe and Francistown areas (Key 1976; Key et al., 1994).
The compression which produced NNE-ENE trending folds (F1/F2/F3) in the SFT area is comparable with deformations reported in neighbouring areas e.g. D2/D3 in Foley area (Table 1.2), D3 in the Motloutse Complex (Table 1.4), D3 in the Topisi area (Table 1.5) and D2 in the Bobonong area (Table 1.6).
The SFT and Topisi areas are characterised by ENE trending map scale folds defining Type 3 interference structures (this study; Key et al., 1994) e.g. the Gulushabe structure in the SFT area and the Sesweu hill structure in Topisi area. The two areas have the same supracrustal assemblage dominated by quartzite and quartz-mica schist and minor amphibolite, marble and calc-silicates. The granitoids gneisses are very similar. The grey gneiss regarded as paragneiss by Key et al. (1994) may be an equivalent of the Tonota biotite gneiss in the SFT area while the unit described as porphyritic granite (locally augen gneiss) is represented by the wide spread megacrystic granite gneiss. However, ultramafic rocks and anorthosites reported in the Topisi area are absent in the SFT area.
Compilation of data for type of supracrustal assemblages, age of clastic sedimentation 161 and age range of magmatism and grade of metamorphism in the SFT area relative to the Francistown, Phikwe and Mosetse Complexes is given in Table 6.1. A comparison of new age data for granitoid gneisses in the SFT region with publishe ages for granitoids and supracrustals rocks from the Topisi (McCourt et al., 2004) the Matsitama areaMosetse area (Majaule and Davis, 1998) and the Francistown Complex (Bagai, 2008) suggest that the deposition of supracrustal rocks in these terranes is coeval or slightly younger than spatially associated granitic intrusions. Magmatism occurred between about
2.6 and 2.7 Ga in these terranes. The similarity of U-Pb zircon ages for Neoarchaean granitoid gneisses in the SFT area (Motloutse Complex), Zimbabwe craton and Limpopo belt suggest a close link in their evolution between 2.6 and 2.7 Ga.
Stereographic plots of poles to foliation and lineation data from the northern part of the Topisi Sheet (Figures 6.1A and B, Key et al., 1994) are analogous to those from the SFT area. Poles to the S1 and S2 foliation from the north eastern part of the Topisi sheet (Fig
6.1A) are very similar to those of S1and S2 from the SFT area (Figures 3.12A and 3.23A) and the pole to the best fit great circle for these data defines a fold axis plunging NNE as in the study area. The plot of the intersection lineation data from the Topisi area (Fig.
6.1B) and lineation data from the Gulushabe structure (Fig. 3.23A) is also to the NNE and thus coincident with the fold axis. This is strong evidence that the geometry of the youngest fold structures in the SFT and Topisi areas is very similar. It is obvious from the projections that fold axes plunge NNE (Fig 6.1A). The above mentioned similarities suggest similar geodynamic setting for the SFT and Topisi areas both of which fall within the Motloutse Complex.
The age data for the rocks of the SFT area, those adjacent parts of the Motloutse Complex as defined by Aldiss (1991) and in the SW part of the Zimbabwe craton (Bagai,
2008) suggest a close link in the evolution of these areas between 2.7 and 2.6 Ga. It is therefore probable that conclusions reached by Kampunzu et al. (2003) and Bagai (2008) for the granitoid rocks associated with the Tati and Vumba greenstone belts on the SW part of the Zimbabwe craton can be applied to the granitoid rocks of the SFT area. The tonalitic gneisses of the SFT area are therefore can interpreted as equivalent to the tonalitic component in the TTG gneiss of the Tati granitoid greenstone terrane thought to have been produced by partial melting in a flat subduction zone and the megacrystic granite gneiss may be part of the young K-rich granites produced by partial melting of the TTG material as reported by Kampunzu et al. (2003) and Bagai (2008).
6.2 REGIONAL ANALYSIS AND IMPLICATIONSAn analysis of a mosaic of published geological maps allows the recognition of a regional scale WNW/SSE trending system of thrust sense ductile shear zones that is traceable from the southeastern corner of the Mosetse sheet, onto the northeastern part of the Matsitama sheet across the Shashe sheet (and thus through the study area), into the SW corner of the Francistown Sheet and eventually onto the Magogaphate/ Bobonong Sheet (Fig. 6.2). Within this system the regional dip of the thrust sense shear zones (Mosupe shear zone, Mooke shear zone and Shashe shear zone) is to the SW (Table 3.1 and Fig.
6.2) but in the study area the Gulubashe shear zone deviates from this orientation and dips N to NE suggesting it has a geometry of a back-thrust in an overall NE verging system.
Aldiss (1989) reports that NE trending foliation in the Shashe area extends into the Tati Siding Granite exposed N of the SFT area, in the Francistown area (Fig. 6.2). The age of 2734 ± 39 Ma (Zeh et al., 2009) obtained from the Tati Siding granite (referred to as Tati TTG in Table 1.7) provides a maximum age for the deformation along the Shashe Dam shear zone. To the west of the SFT area, biotite gneiss associated with metasedimentary rocks of the Matsitama belt is separated from the banded tonalitic gneiss of the Francistown Granite Greenstone Complex by a zone of deformation characterized by horizontal or gently plunging lineation (L-S tectonite fabrics; Aldiss, 1989). The lineation plunges SW and is inclined at angles of 45 degrees and often less than 10 degrees on the foliation. The same lineation also characterizes the contact between the southwestern outcrops of the Jankie gneisses (Fig. 6.2) with the Motloutse Complex migmatites. The lineation developed during regional deformation which imparted an LS-fabric on both units. The contrast in tectonic style suggests that the zone characterized by LS-tectonite may be a major tectonic break, along which a subhorizontal motion is indicted by the attitude of the lineations. This zone is herein named the Jamataka shear zone (after Jamataka settlement, Fig. 6.2), and corresponds to the boundary of the Mosetse and Motloutse Complexes as defined by Aldiss (1991). See Table 6.1 for details. The orientation of the Jamakata shear zone is similar to the Shashe Dam shear zone but in contrast to the Jamakata shear zone there is no clear evidence for simple shear displacement along the Shashe Dam shear zone. This requires further investigation.
Figure 6.2: Sketch map showing the mosaic of Quarter Degree Geological Sheets covering the basement of NE Botswana and the position of the regional NW-SE trending thrust sense shear zone system across these sheets; the various sectors of the thrust system are identified by name.
Barbs on the thrust sense shear zones indicate the hanging wall block in each case. Motloutse and Phikwe Blocks after Paya (1996). Mosupe shear zone after Majaule (1999), Mooke shear zone after Aldiss (1989), Shashe shear zone after Paya (1996).
167 The Motloutse Complex which includes the SFT area is interpreted to be a crustal block now attached to the SW margin of the Zimbabwe craton by accretion tectonics (Aldiss 1991; McCourt et al., 2004). The complex formed in an area of folding, granitoid intrusion, ductile thrusting and strike-slip deformation located between the Matsitama belt to the NW, the Tati greenstone belt to the immediate N, the Phikwe Complex to the E and an unknown terrane to the SW, now hidden beneath the Karoo cover. The metasedimentary rocks from the unknown terrane were thrust northeastwards over the magmatic arcs developed at the margin of the Zimbabwe craton, resulting in crustal thickening and high heat flow that prompted migmatization, anatexis and ductile deformation (Aldiss (1991). The overthrust sedimentary rocks were infolded with the granitoid gneisses. Key et al. (1994) report NE directed thrusting (indicated by SW plunging lineation in quartzite) in quartzite from the Topisi area. The current study however found no evidence for NE thrusting in the SFT area. The SW dipping thrust sense shear zones reported by Aldiss (1991) from the Shashe area were not found. The geological evolution of the SFT area is thus difficult to accommodate in the accretionlinked models for the SW margin of the Zimbabwe craton as suggested by Aldiss (1991), Kampunzu et al. (2003), McCourt et al. (2004 and Bagai (2008). Based on the foliation, lineation and fold data collected during mapping, the age data on zircon grains from selected granitoids and the stable isotope data from the metacarbonate rocks, the
conclusions from the present study are as follows:
The study area is characterised by NNE to ENE striking foliation present in both the supracrustal rocks (the metasedimentary belt) and the granitoids.
The metasedimentary rocks of Domain 1 are deformed into large map scale NE to ENE trending folds structures deforming bedding (So) and foliation (S1).
The foliation in the metasedimentary rocks is parallel to bedding and there has been flattening normal to the foliation plane and elongation within the foliation. 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
Deformation of the metasedimentary belt involved NW-SE or NNW-SSE compression producing S1. This was succeeded by a coaxial deformation (F2) possibly involving top to ESE thrusting.
Poles to bedding (So) and foliation (S1), local minor folds and map scale folds associated with the Gulushabe define NNE plunging F3 folds.
Foliation in the megacrystic granite gneiss is defined by preferred orientation of Kfeldspar megacrysts. The shapes of the megacrysts are related to flattening.
The foliation in Domain 4 is at a high angle to the foliation in Domains 1, 2 and 3, trending E-W to WNW in Domain 4 and NNE to ENE in Domains 1, 2 and 3.
The granitoid rocks SW of the Shashe Dam are intensely foliated suggesting a ductile ductile shear zone although there is no strongly developed lineation or shear sense indicators.
U-Pb zircon dating of the granitoid rocks indicate granitoid magmatism occurred during the late Archaean. The crystallization ages of the proliths to the granitoid rocks range between 2724 ± 48 Ma and 2631 ± 4. Ma, spanning a period of about 93 Ma.
The protolith to the Tonota biotite gneiss was emplaced first (2724 ± 48 Ma), followed by emplacement of the protolith to the tonalitic gneiss (2698.9 ± 9.2 Ma), megacrystic granite gneiss (2647 ± 24 Ma) and pink gneissic granite (2631 ± 4.4 Ma).
The latter occurs that as dykes cutting the foliation (S1) in the megacrystic granite gneiss (Figs 2.26) and the banded tonalitic gneiss (Fig. 2.27).
U-Pb zircon age of 2625 ± 16 Ma was obtained from mylonitic megacrystic granite gneiss in Domain 2, S of Tonota Village. This U-Pb zircon age is interpreted to give the maximum age of local ductile shearing S of Tonota.
The last deformation event is marked by widespread development of minor shear zones in the granitoid gneiss (section 5.3.4). The NNE to NE trending shear zones (020°-040°) have been interpreted as a distal expression of the event responsible for the Magogaphate shear zone (Key 1976, Aldiss 1991, Paya, 1996).
The minor shear zones were not found in the pink granite gneiss implying that they developed prior to emplacement of the original pink granite i.e. they are older than 2631 Ma. The development of these minor ductile shear zone is constrained between 2647 Ma and 2630 Ma.
The carbonate rocks (dolomites, marble 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 (3A. Bekker, pers. com, 2011) for the carbonate rocks but this is incompatible with field data that indicates the structures that deform the metasedimentary rocks are older than 2631 Ma and thus the carbonate rocks are late Archaean in age.
3 Andrey Bekker, Dept. of Geological Sciences, University of Manitoba, Winnipeg, Canada.
The Sensitive High Resolution Ion Microprobe (SHRIMP) is a large-diameter, doublefocusing secondary ion mass spectrometer (SIMS) sector instrument produced by Australian Scientific Instruments in Canberra, Australia. The SHRIMP microprobe focuses a primary beam of ions on a sample sputtering secondary ions which are focused, filtered and measured according to their energy and mass. The SHRIMP is primarily used for geological and geochemical applications. It can rapidly measure the isotopic and elemental abundances in minerals at a micrometre-scale and is therefore well-suited for the analysis of complex minerals, as often found in metamorphic terrains, some igneous rocks, and for relatively rapid analysis of statistical valid sets of detrital minerals from sedimentary rocks. The most common application of the instrument is in uraniumthorium-lead geochronology, although the SHRIMP can be used to measure other isotopic and elemental abundances. The SHRIMP determines the ages of crystals by measuring their lead and uranium contents. SHRIMP works by firing a beam of oxygen ions (electrically charged oxygen atoms) at just one spot on the crystal. They hit the crystal and knock off atoms of all the elements in the crystal, including atoms of uranium and lead. These atoms are sucked away by electrical forces and then separated into their different types by magnetic forces (a process called mass spectrometry). The atoms of lead and uranium are counted and the age of the zircon at the target spot is calculated.