«By MOLATLHEGI LARTY LOSTMAN MOSEKI STUDENT NO. 208523856 Submitted in fulfillment of the academic requirements For the degree of Master of Science In ...»
The minor shear zones cut the regional foliation (S1) at high angles and are often filled by syntectonic felsic veins. The minor shears zones show predominantly NNE-SSW trends (020°-040°) and have dextral displacement. These shear zones mostly lie on sub-horizontal smooth outcrop surfaces that make it difficult for changes in orientation to be measured, and are restricted to the granitoid gneisses i.e. have not been observed in the metasedimentary outcrops. They are orientated sub-parallel to the axial trace of the minor folds deforming the foliation (S1). Shear strain ( ) in the order of 3 to 4 was calculated for the shear zones in all domains using the formula = d/w where (w) represent width of the shear zone and (d) represent amount of displacement as illustrated in Figures 3.53 A, 3.55 B and 3.56 A. It should be noted however that the shear strain was calculated from values measured directly off the photograph.
3.4.1 DOMAIN 2 A Strike and dip of fold axial surface 180°/70°. B. Strike and dip of fold axial surface;
Figure 3.52: A-B: Plan views of drag folds (defined by felsic bands) developed in megacrystic granite gneiss (Domain 2) due to dextral shearing.
The small-scale shear zones occur along the limbs of the folds. N.B. the minor folds have steeply dipping to almost subvertical fold axial surfaces that lie parallel to the shear zones. Black arrows indicate sense of movement.
A.Trend of minor shear zone, 036°-216°. B. Strike and dip of minor shear zone, 225°/70°.
Megacrystic granite gneiss E of Gulushabe. Banded tonalitic gneiss inclusion in
Figure 3.53: Plan views of shear zones trending NE-SW deforming gneissic foliation (S1) and undeformed felsic material along the trace of the shear zone.
Calculated shear strain in A ~ 3. Note: the shear zone in B is restricted to the banded gneiss inclusion.
A.Trend of shear zone~065°. E of Gulushabe B. Trend of minor shear zone~060°. S of
Figure 3.54: A-B: Foliation (S1) and foliation parallel felsic bands are dextrally rotated into a NE-SW trending ductile shear zone developed in megacrystic granite gneiss.
Figure 3.55: A-B: NNE-SSW trending ductile shear zone dextrally deforming both the gneissic foliation (S1) and felsic bands in megacrystic tonalite gneiss.
Associated small-scale fold in B plunges 60º/040 sub-parallel to the trend of the shear zone.
Figure 3.56: Foliation (S1) and foliation parallel felsic bands deformed along A.
NNE-SSW trending ductile shear zone A in megacrystic granite gneiss. Rotation of veins into shear zone gives a clear dextral sense of movement.
3.5 DISCUSSION The foliation (S1) in the metasedimentary rocks of Domain 1 (the Foley synformal and the Gulushabe antiformal structures) is folded into NE to ENE trending folds with axial surfaces dipping to the NW (Table 3.1) and NNW (Fig 3.12 and 3.23). The development of these large scale folds is attributed to a NW or NNW orientated horizontal compression. There is a consistent geometry shown by these fold structures. The only difference between the fold structures is that, whereas the Foley structure has a steeply plunging NW (310º) to NNW (340º) elongation lineation defined by deformed quartz pebbles ((Figs. 3.3 to 3.6 and Fig 3.12), the prominent lineation associated with the Gulushabe structure is defined by deformed aggregates of quartz and feldspar (Figs. 3.16 to 3.19, 3.23), although locally an elongation lineation defined by the long axis of pebbles plunges NNW (Fig 3.15). Measured minor fold structures plunge NE to ENE in the Foley (Figs. 3.7 to 3.10) and ENE in the Gulushabe structure (Fig. 3.21 and 3.23). However, unlike these minor folds, fold axes given by the software for both the Foley and Gulushabe structures plunge NNE (Fig 3.12 and Fig 3.23). The NNE plunges of the fold axes are interpreted to reflect F3 deformation. F3 folds (e.g. Fig 3.11) and folds developed on the NW limb of the Gulushabe structure show a dominant NNE trend.
The foliation (S1) in Domain 3 dips NNW and trends ENE parallel to metasedimentary belt and it appears to be deformed around the NE end of the Gulushabe structure (Fig 3.43). The cause of this deformation is unknown but may reflect folding around the Gulushabe structure or deformation related to the Gulushabe shear zone. Figure 3.41 and
3.42 present evidence for a local sub-horizontal L tectonite in the vicinity of Foley settlement. Horizontal elongation lineation is normally related to strike-slip deformation but no kinematic indicators were found in Domain 3 to confirm the shear sense. This deformation 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 namely, the Magogaphate-Molabe-Lepokole shear system (Table 1.6, Paya, 1996). These SE dipping shear zones are thought to have been active at 118 the same time. The only difference is that whereas D4 structures in the Limpopo belt dip to the SE, the fabric in Domain 3 dips to the NNW.
The structural grain of Domain 4 is defined by foliation (S1) trending E-W to NNW and dipping N to NE. Two generation of folds thought to have formed by different tectonic events have been recognized in Domain 4. The first generation of folds has NW–striking axial planes dipping to the NE (Table 3.1). The folds verge to the SW indicating top to SW sense of movement. The folds are related to thrust sense shearing along the foliation (S1). The second generation folds (Set 2 of section 188.8.131.52) have axial planes striking NE to ENE and dipping steeply NW to WNW. Folds with a similar plunge direction have been identified in Domain 1 (50º/006º on Fig. 3.12) and Domain 2 (28º/017 ºon Figure 3.39). The axial planes of these folds are sub-parallel to foliation (S1) in Domain 1 and 2.
The map pattern and the stereographic plot in Figure 3.51A implies the Gulushabe shear zone and its N to NNE dipping foliation (S1) is folded by structures trending NE-SW and plunging to the NE. Structural evidence (this study) indicates the thrust sense shearing characterises the SW vergent Gulushabe shear zone which forms the boundary between the SFT area and the SE margin of the Tati greenstone belt. The shear zone transects the northeastern part of the SFT region and has affected rocks exposed along the Shashe river bed to the N of Gulushabe area. The SW rotation of folds in this zone indicates a SW directed ductile shearing event. The width of the shear zone is variable ranging from about 1 to 3 kilometres. The N to NE dip of the Gulushabe shear zone is in conflict with previous interpretations (Paya, 1996, Ranganai et al., 1999 and McCourt et al., 2004) which model this structure as an extension of the NE verging Shashe shear zone. The current interpretation is supported by the structural data collected along the boundary during the present study. Further work is required to resolve the regional kinematics. In the northern part of the SFT area, the foliation in Domain 2 is seen to truncate that of Domain 4, implying that the deformation in Domain 4 is older than that in Domain 2.
Based on the data collected during this study the geometry of the deformation features recognized in various structural domains can be compared. These features are summarized in Table 3.2. All rocks in each of the structural domains have been deformed. The metasedimentary rocks in Domain 1 have evidence at outcrop and map 119 scale for two phases of folding (F1, F2) with associated foliation. These folds plunge NE to ENE. An associated elongation lineation best defined by the long axis of pebbles in deformed pebbly quartzite plunges WNW and NNW. In addition to these co-axial events the bedding and foliation data plotted on Figures 3.12 and 3.23 define a fold axis plunging to the NNE. Folds related to this F3 event are recognized at map scale along the NW limb of the Gulushabe structure (Fig 3.14), at locality 12 on the dissertation map and Fig 3.11. The megacrystic granitoid rocks of Domain 2 and 3 are strongly foliated but minor folds of the foliation were only recognized in Domain 2. These folds plunge N to NNE and have axial surfaces that dip predominantly WNW. The granitoid gneisses of Domain 4 are characterized by a foliation that dips N and NE and has a well developed elongation lineation that plunges down dip. F2 folds deforming this foliation verge to the SW and are related to thrust sense displacement along the Gulushabe shear zone. A second set of folds plunge N-NNE parallel to the dip of the foliation and have axial surfaces dipping NW. Based on general orientation and plunge, these folds in Domain 4 may be correlated with the F2 folds of Domain 2 and the F3 folds.
4.1. INTRODUCTION U-Pb zircon geochronology was carried out on selected granitoid rocks from the study area in order to produce a temporal framework into which the magmatic events of the SFT region can be placed as well as determining deformation events present. This was done in an attempt to better understand the evolution of the granitoid rocks in the study area. The zircon grains separated from 5 samples of the main units recognised in the field were analysed by Dr. Richard Armstrong at the Research School of Earth Sciences (RSES) at The Australian National University using the SHRIMP method (Sensitive High Mass Ion Microprobe). The zircon descriptions that follow are based on his report to the Geological Survey of Botswana. Standard analytical techniques as outlined in numerous publications incorporating SHRIMP data were followed. An outline of how the SHRIMP method works and analytical procedure are given in Appendix 1. A global positioning system was used to record the coordinates of the sample locations.
It is important to mention that the mineral zircon contains high U and Th, and low Pb at the time an igneous rock forms. Its U-Pb signatures and internal structure remain undisturbed even at the high temperatures required for partial melting (e.g. Rubatto and Herman, 2003). The age of the zircon is therefore the age of the protolith to the host rock (zircon inheritance). Owing to its large size and low charge radiogenic Pb is expelled from zircon during recrystallization due to metamorphism, alteration and metamictisation (loss of radiogenic Pb due to U and Th decay). As a result, zircons yield discordant ages that plot off the Concordia curve as colinear data points. A regression line through these data points intercepts the Concordia curve. Plots of the Concordia curve are used as reference. The upper intercept gives the age of primary crystallization.
Cathodoluminescence (CL) imaging technique has been used to reveal the different internal zonation of zircons which is a requirement in distinguishing magmatically derived (igneous) zircons from non-magmatic (metamorphically grown) zircons. The widely accepted contention that magmatic zircons have Th/U ratios that are typically 124
0.2 whereas non-magmatic zircons have Th/U ratios 0.1 is followed in this study (e.g.
Bowring and Williams, 1999).
4.1 RATIONALE FOR STUDY The primary objective of the SHRIMP work was to establish the emplacement age of the protoliths to the tonalitic gneiss (MLM-SRP 3), megacrystic granite gneiss (MLM-SRP 1 and 5) and pink gneissic granite (MLM-SRP 2) thereby quantifying the relative age relationships recognised in the field. The tonalitic gneiss was dated to establish the crystallization age of the igneous protolith which following Bagai (2008) and Kampunzu et al. (2003) was produced during subduction along the southwest margin of the Zimbabwe craton. Zircon grains from the pink gneissic granite and the megacrystic granite gneiss were dated to constrain their emplacement ages. Mylonite from shear zones constrain the maximum age of deformation and displacement along these structures. In addition a sample of Tonota biotite gneiss (MLM-SRP 4) was processed to see if the zircon population provided any insight into whether the gneiss was derived from a sedimentary or magmatic protolith. The Tonota biotite gneiss is a fine grained leucogneiss with diffuse compositional layering and occurs associated with metasedimentary rocks around Tonota and Shashe Villages. The Tonota biotite gneiss is considered of sedimentary origin (e.g. Aldiss, 1991). The latter interpretation stems from its association with metasedimentary rocks and the local occurrence of sillimanite in the Tonota biotite gneisses. The age data obtained from these zircons were used to constrain the depositional or emplacement age of the protolith. The current chapter concludes by comparing U-Pb zircon ages from this study with published U-Pb zircon ages from the Tati greenstone belt, Matsitama belt and the Central Zone of the Limpopo belt. An evaluation of how age constraints on deformation features in the SFT area relates to those in the adjacent terranes is undertaken.
4.2 SAMPLE LOCALITIES The lithology and sample locations of rocks selected for U-Pb zircon age determinations are listed in Table 4.1 and plotted on the dissertation map. The results of U-Pb zircons isotope analysis are presented in Tables 4.2 to 4.6. Cathodoluminescence (CL) images of zircon populations and the Concordia diagrams are shown in Figures 4.1 to 4.10.