«By MOLATLHEGI LARTY LOSTMAN MOSEKI STUDENT NO. 208523856 Submitted in fulfillment of the academic requirements For the degree of Master of Science In ...»
The Geology and Tectonic Setting of the Shashe-
Foley-Tonota area (Central Motloutse Complex),
MOLATLHEGI LARTY LOSTMAN MOSEKI
STUDENT NO. 208523856
Submitted in fulfillment of the academic requirements
For the degree of Master of Science
In the School of Agricultural, Earth and Environmental Sciences
University of KwaZulu-Natal, Durban
The research work described in this dissertation was carried out in the School of Agriculture, Earth and Environmental Sciences, University of KwaZulu-Natal, Durban, from March 2008 to December 2010, and was continued at Lobatse (Botswana) during 2011 to 2013 under the supervision of Professor Stephen McCourt.
This study represents original work by the author and has not otherwise been submitted in any form for any degree or diploma to any tertiary institution. Where use has been made of the work of others it is duly acknowledged in the text
This dissertation does not contain other persons, writing, unless specifically acknowledged as being sourced from other researchers. Where other written sources
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Their words have been re-written but the general information attributed to them has been referenced Where their exact words have been used, then their writing has been placed in italics and inside quotation marks, and referenced.
This dissertation does not contain text, graphs or tables copied and pasted from the internet, unless specifically acknowledged, and the source being detailed in the dissertation and in the Reference sections Signed……………………………………….
ABSTRACTThe 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 pebblyquartzite 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 accretionlinked 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 i 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..
ii Contents CHAPTER 1: INTRODUCTION _______________________________________ 1
1.1 LOCATION AND ACCESS _______________________________________ 1
1.2 GEOLOGICAL SETTING OF THE STUDY AREA ____________________ 4
1.3 RATIONALE FOR THE STUDY ___________________________________ 9
1.4 GOALS _______________________________________________________ 12
1.5 KEY QUESTIONS ______________________________________________ 12
1.6 METHODOLOGY ______________________________________________ 13
1.7 PREVIOUS WORK _____________________________________________ 13 CHAPTER 2. LITHOLOGICAL UNITS ________________________________ 44
2.1 INTRODUCTION ______________________________________________ 44
2.2 METASEDIMENTARY ROCKS __________________________________ 45
2.3 GRANITOID GNEISSES _________________________________________ 53 CHAPTER 3. STRUCTURE __________________________________________ 71
3.1 INTRODUCTION ______________________________________________ 71
3.2 DEFORMATION FABRICS IN THE METASEDIMENTARY ROCKS(DOMAIN 1) _____________________________________________________ 75
3.3 DEFORMATION OF THE GRANITOID ROCKS _____________________ 93
3.4 MINOR SHEAR ZONES ________________________________________ 113
3.5 DISCUSSION _________________________________________________ 116 CHAPTER 4: U-PB ZIRCON GEOCHRONOLOGY ____________________ 124
4.1 RATIONALE FOR STUDY______________________________________ 125
4.2 SAMPLE LOCALITIES _________________________________________ 125
4.3 RESULTS ____________________________________________________ 126
4.4 DISCUSSION _________________________________________________ 147 CHAPTER 5. STABLE ISOTOPE GEOCHEMISTRY ___________________ 151
5.1 RATIONALE FOR STUDY______________________________________ 151
5.2 SAMPLE LOCALITIES _________________________________________ 152
5.3 RESULTS ____________________________________________________ 154
5.4 DISCUSSION _________________________________________________ 155
5. DISCUSSION AND CONCLUSIONS ______________________________ 159
6.1 CORRELATION OF THE SFT AREA WITH ADJACENT TERRANES __ 159
6.2 REGIONAL ANALYSIS AND IMPLICATIONS ____________________ 164 APPENDIX 1: OUTLINE OF THE SHRIMP METHOD ____________________ 171
APPENDIX 2: CARBON AND OXYGEN STABLE ISOTOPE ANALYTICALPROCEDURE ______________________________________________________ 174 ACKNOWLEDGEMENTS __________________________________________ 175 REFERENCES ____________________________________________________ 177
1.1 LOCATION AND ACCESS The area of study lies in NE Botswana to the SW of Francistown (Fig. 1.1) and includes three main Villages; Shashe and Tonota in the N and Foley in the S and one settlement (Makomoto) midway between Foley and Tonota. The Makomoto settlement is not shown in the Foley QD sheet and should not be confused with the Makomoto cattlepost that lies NW of the Shashe Dam in the Shashe QD sheet. For convenience the study area will be referred to as SFT, the abbreviation derived from the names Shashe, Foley and Tonota Villages respectively. Geographically the study area is defined by longitudes 27 23’E and 27.63 ’E and latitudes 21.30°S and 21. 69 S and covers an area of about 702 km2, covering 26 km N-S and 26 km E-W.
The Shashe and Motloutse rivers control the drainage system of the study area with the Shashe River transecting the northern part of the area and the Motloutse River the southern part. Both rivers flow to the SE. The Gaborone-Francistown main road and railway line run NE/SW through the centre of the study area. Access into the area is gained via a number of unsurfaced or graveled roads that branch off from the main road to the surrounding cattle posts and farm land. Numerous such roads and tracks join the Villages, running both parallel and across the main Gaborone-Francistown tarred road. A four-wheel drive vehicle is required for some of these roads especially during rainy seasons and was additionally useful for traverses across sandy areas, at river crossings and along the Shashe River as the latter hosts a reasonable amount of rock exposure. In terms of Quarter Degree Sheets (QDS), the area lies in the central and southwestern parts of the region enclosed by Shashe, Foley, Francistown and Phikwe map sheets (Fig. 1.2).
The geology of NE Botswana (Fig. 1.3) comprises Archaean basement, overlain by Mesozoic (Karoo Supergroup) and Recent (Kalahari Group) cover rocks. The SFT area is underlain by rocks of the Archaean basement which regionally forms part of the Azanian Craton a fragment of Archaean continental crust comprising, from S to N, the Kaapvaal craton, the Limpopo belt and the Zimbabwe craton (McCourt et al. 2004). The geology of the study area can be linked to the Zimbabwe craton and the Central Zone of the Limpopo belt; the rocks of the Kaapvaal craton being obscured by younger cover.
McCourt et al. (2004) describe the salient features of the Archaean basement in NE Botswana as follows. The Zimbabwe craton is a composite granite greenstone terrane comprising 26 individual greenstone belts and associated granitoids (e.g. Blenkinsop et al., 1995; Jelsma and Dirks, 2002). The south-western part of the craton extends into Botswana where it has been mapped as the Tati-Vumba, Maitengwe and Matsitama granite greenstone terranes (Fig. 1.3). The full extent of the craton in Botswana is unknown, as the western boundary is obscured beneath Phanerozoic cover rocks and the southern boundary ill defined (see Bennett, 1970; Aldiss, 1991). The Limpopo belt (e.g.
van Reenen et al., 2011) is an ENE trending granulite gneiss terrane situated between the Kaapvaal and Zimbabwe cratons. It is described in terms of three contrasting crustal domains referred to as the Southern Marginal Zone, the Central Zone and the Northern Marginal Zone (Cox et al., 1965). Within Botswana the Northern Marginal Zone is referred to as the Semolale Complex and the Central Zone is subdivided into the Phikwe and Mahalapye complexes (Aldiss, 1991; Carney et al., 1994; Holzer et al., 1999). The northern boundary of the Limpopo belt against the Zimbabwe craton is a matter of debate. Based on an interpretation of new gravity data, Ranganai et al. (2002) have interpreted the NW trending Shashe Mobile Belt (Bennett, 1970) as an extension of the Limpopo belt and recognised a continuous arc-shaped orogen (the Limpopo-Shashe belt) along the S and SW margin of the Zimbabwe craton (Fig. 1.3 and 1.4).
Figure 1.5: Geological map of the Limpopo belt (modified from Kramers et al.
(2011), Dikalate shear zone from (Paya et al., 1997) and the Shashe shear zone from Aldiss (1991).
Figure 1.6: Subdivision of the Archaean Archaean basement rocks in NE Botswana (slightly modified after Aldiss (1991) and McCourt et al.
(2004). Red square is an outline of study area.