«By MOLATLHEGI LARTY LOSTMAN MOSEKI STUDENT NO. 208523856 Submitted in fulfillment of the academic requirements For the degree of Master of Science In ...»
Paya (1996) working in the Bobonong area (Figs 1.1 and 1.5) including the area to the N of Magogaphate Shear Zone (Motloutse Complex) recognised 5 deformational episodes, each of which was associated with a period of intrusion and related metamorphism. The established relationship between deformation, metamorphism and magmatism recognised by Paya (1996) is shown in Table 1.6.
Recent ideas on the Limpopo belt include Ranganai et al. (2002). Ranganai et al. (2002) adopt a different definition of the Limpopo belt. The latter include terranes that are not at granulite facies metamorphism and by so doing ignore the rationale of Aldiss (1991) for excluding the Shashe area from the Limpopo belt. Ranganai et al. (2002) based on an interpretation of new gravity data redefined the regional extent of the Limpopo belt and regarded the area between the Shashe shear zone in the N and the Lechana fault (Figs 1.6,
1.7 and 1.8) in the south as part of the Shashe belt. The Shashe belt was interpreted as an extension of the Limpopo belt and the same 3-fold division used for the Limpopo belt
Based on similarity in ages of detrital zircon ages from quartzites, and also on a similar structural pattern characterized by NE verging folds, thrusts and ductile shear zones in the Matsitama and Topisi area (part of Motloutse complex), the southern part of the Mosetse Complex, including the Matsitama belt was combined with the Motloutse Complex into a single terrane that lies adjacent to the Francistown Arc Complex, and named the Matsitama-Motloutse Complex (Figs 1.7 and 1.8, McCourt et al., 2004). In this interpretation, the northern boundary to the Matsitama –Motloutse complex was the Shashe shear zone a structure also recognised by Ranganai et al. (2002). However there is a difference in the two interpretations. Ranganai et al. (2002) interpret the MatsitamaMotloutse complex as the northern marginal zone to the Shashe section of the LimpopoShashe belt. McCourt et al. (2004) regard it as part of an accretionary complex distinct from the Limpopo belt. The only agreement being that the Shashe shear zone forms the northern boundary with the Zimbabwe craton. McCourt et al. (2004) based on geochronology and geochemistry of tonalitic rocks in Mosetse settlement (Majaule et al.,
1997) and Vumba belt (Bagai et al., 2002) interpret the northern part of the Mosetse Complex as a continuation of the Francistown Arc Complex (Fig. 1.8).
By use of U-Pb zircon data from Khumkago Island, gneisses exposed southwest of Matsitama Village (Figs (1.8) that record Neoarchaean-tectono thermal activity (Majaule and Davis., 1998), McCourt et al. (2004) extended the Archaean crust of NE Botswana to the west as far as Khumkago Island (NW part of Fig. 1.8). McCourt et al. (2004) demonstrated that the metasedimentary sequences within the Matsitama-Motloutse Complex were deposited between 2661 and 2647 Ma with the provenance being the Francistown Granite Greenstone Complex and the regions behind it. This conclusion is at variance with a previous suggestion (Key, 1976) that the Shashe gneiss is older than the Tati greenstone belt. McCourt et al. (2004) suggested a NE- directed subduction below
Figure 1.8: McCourt et al.
(2004)’s subdivision of Archaean crust in NE Botswana (slightly modified). Geology from: Key (1976), Aldiss (1991) and Paya (1996). N.B the northern part
Accretionary tectonics involving the Matsitama-Motloutse Complex moving towards the southwestern margin of the Zimbabwe craton occurred between 2661Ma and 2647Ma.
McCourt et al. (2004) further concluded that the Francistown, Phikwe and MatsitamaMotloutse Complexes were assembled and juxtaposed during the Neoarchaean in response to Andean-type convergent margin accretion tectonics along the southwestern margin of the Zimbabwe craton. The idea of the Matsitama-Motloutse Complex is at variance with Bagai (2008)’s view that the Francistown Granite Greenstone Complex is a large arc complex which also encompasses the Matsitama belt on the western side of the Vumba greenstone belt.
Published geotectonic interpretations of the Zimbabwe craton in Botswana focused on petrogenesis (geochemical), geochronological and tectonic setting with little emphasis on structure (e.g. Bagai et al., 2002; Kampunzu et al., 2003). Majaule et al. (1997) suggested that the Matsitama belt formed in a back-arc environment by extensional tectonics during rifting at the margin of the Zimbabwe craton. The evolution of the Zimbabwe craton is linked to an Andean-type convergent margin (continental margin arc) and related thrusts and sedimentary belts related to northward directed subduction which occurred within the Limpopo-Shashe area between 3.8 and 2.65 Ga (Kampunzu et al., 2003; Bagai et al., 2002). In support of this, McCourt et al. (2004) suggested NE directed subduction below the Zimbabwe proto-continent at 2.65 Ga, with the Matsitama-Motloutse Complex as part of the lower plate The latter scenareos are not mutually exclusive. The arc behind which Majaule et al. (1997) put the Matsitama basin could have been the Francistown Arc. The difference lies in position of the Matsitama basin relative to this arc i.e. behind the arc as proposed by Majaule et al. (1997) or outboard of the arc as proposed by McCourt et al.
Jelsma and Dirks (2002) argue for a magmatic arc along the NW margin of the protoZimbabwe craton with associated SE directed subduction. The down-going slab would have been located to the NW of the Zimbabwe craton.
33 Bagai (2008) proposed that the downgoing slab was located in the Limpopo-Shashe belt and the overriding plate was the Tokwe continental crust (in Zimbabwe) and related blocks. TTG and sanukitoid suites display continental subduction zone signatures. Hf and Nd-isotopes from the Francistown Granite Greenstone Complex suggests a magmatic arc setting (Bagai, 2008). In addition, the production and emplacement of voluminous TTGs to sanukitoids and K-granites at ~2.7-2.6 Ga in the southern margin of the Zimbabwe craton is taken to indicate a shift from a flat subduction (for the production of TTGs) to a steep subduction (for the production of younger sanukitoids) and subsequent break off and detachment of the slab (Kampunzu et al., 2003; Bagai, 2008 and Fig. 1.9). The production of TTG rocks is related to partial melting of mafic igneous rock underplated in the lower crust (lower angle subduction), the sanukitoids (diorites and gabbros) are linked to partial melting of a sub-arc mantle wedge enriched in silica during ascent of the TTG rocks and the young K-granites resulted from partial melting of the TTG material.
The detachment allowed hot mantle to rise up, melting earlier TTGs to produce the Kgranites. Bagai (2008) observed an increase in the intensity of deformation fabric from the Tati greenstone belt, towards the boundary with the the Limpopo-Shashe belt and suggested a geotectonic link between terranes. According to Bagai (2008), the TTG gneisses in the SW complex (Fig. 1.9) were partially melted during a younger metamorphic event at 2639 ± 7.9 Ma. This period is within error with the age 2630 ± 4.7 Ma for the migmatisation of the Shashe gneiss (Bagai, 2008). The high grade metamorphic event is related to orogenesis in the Limpopo-Shashe belt linked to the final collision between the Zimbabwe craton and the Kaapvaal craton (Bagai, 2008). The collision event led to emplacement of syn-tectonic granite (granite augen gneiss) widespread within the Limpopo-Shashe belt.
34 Figure 1.9: Simplified map outlining the three plutonic complexes of the Tati granitoidgreenstone terrane (Slightly modified from Kampunzu et al., 2003; Zhai et al., 2006 & Bagai. 2008). The northern part of the study area (SFT) is outlined red for reference (strike and dip measurements based on the current research). High aluminum tonalitetrondhjemite subsuite (HA-TTG) is equivalent to tonalite-trondhjemite gneiss of dissertation map and Fig. 1.1 1.7.1 BOUNDING SHEAR ZONES Crustal scale ductile shear zones in the Limpopo belt (e.g. McCourt and Vearncombe,
1992) define the boundaries between the various zones within the belt and between the high grade of the Limpopo belt (e.g. the northern and southern boundaries of the Central Zone) and the adjacent cratons (e.g. the North Limpopo Thrust Zone and the Hout River Shear Zone). Investigations on geometries and kinematics of the shear zones and associated structural features help in better explaining the evolution of the boundaries (their temporal and regional significance, collision tectonics, metamorphic processes 35 involved and implications). According to Schaller et al. (1999), the Palala shear zone (Fig 1.5) records dextral strike-slip movements which occurred during the Palaeoproterozoic (1.97 1.92 Ga) whereas McCourt and Vearncombe (1987) interpret it as a sinistral strike-slip shear zone. The Palala shear zone trends ENE, dips N and lies along strike from the Tshipise shear zone which dips south so it is likely that the Palala shear zone represents a reactivation of part of the Tshipise shear zone at about 2.0 Ga (Kramers et al., 2011). An important consideration is whether the Palala shear zone extends along strike to the WSW as the Zoetfontein Fault or swings to the north to become the Mahalapye shear zone (Figs 1.6 to 1.8). Alternatively the Mahalapye shear zone is a separate component of the system that links to the Palala shear zone. The Mahalapye straightening/shear zone defines the southern limit of the Mahalapye complex (Holzer et al., 1999).
The Central Zone of the Limpopo belt is structurally bounded by the Palala-Tshipise shear zone and Lethakane-Magogaphate-Triangle shear system against the Southern and Northern Marginal Zones respectively (Fig. 1.5). The south dipping Magogaphate shear zone is described as a dextral shear zone trending ENE to WSW (e.g. Aldiss, 1991) and separates the Phikwe Complex from the Motloutse Complex to the N (Aldiss, 1991; Key et al., 1994; McCourt et al., 2004). The development of the Magogaphate shear zone is attributed to continued ductile deformation and migmatisation (Aldiss, 1991) during collision between the Zimbabwe and the Kaapvaal cratons. Kamber et al. (1995) obtained a Proterozoic age of ~ 2.0 Ga for the youngest deformation along the Triangle shear zone.
According to Paya (1996) the northern boundary to the Central Zone comprise two dextral shear zones namely the Molabe and Lepokole shear zones, thought to have been active at the same time (Fig. 1.7). These shear zones dip SE. Paya (1996) refers to them as lateral ramps that accommodated emplacement of what he called the Magogaphate block and this model requires the Shashe shear zone to act as the frontal ramp. However the Molabe shear zone lies N of the Magogaphate shear zone and the Lepokole shear zone lies south. If the rocks deformed by the Molabe shear zone are part of the Motloutse Complex then the Molabe shear zone cannot define the northern boundary of the Central Zone. The Lepokole shear zone is a 5km wide ENE-WSW trending ductile strike-slip shear zone N of Bobonong Village. The Molabe shear zone is a strike-slip ductile shear 36 zone that possesses a predominant ENE-WSW general shear foliation trend with steep to moderate dips to the south (60°-80º). These are considered to be splays of the main Magogaphate-Triangle shear system. The steep dipping Palala-Zoetfontein and Lethakane-Magogaphate-Triangle shear zone systems were formed or reactivated under high grade metamorphic conditions at about 2.0 Ga (Holzer et al., 1999; Chavagnac et al., 2001).
The boundary between the Mahalapye and the Phikwe Complexes is the NW-SE trending Sunnyside Shear Zone (McCourt and Vearncombe, 1987). Paya (1996) has shown that the displacement on the Sunnyside Shear Zone was dip-slip rather than strike-slip thus it cannot be a western continuation of the Palala shear zone. Key et al. (1994) mapped a NS trending sub-vertical shear zone along the western side of the Phikwe Complex, initially referred to as the Western Limpopo Shear Zone and later as the Dikalate Shear Zone (Paya et al., 1997). The Dikalate Shear Zone, (Figs 1.5 to 1.8) is described as a dipslip ductile E to SE verging thrust zone in the western extremity of the exposed Limpopo belt (Paya et al., 1997). The thrust zone records N-S foliations with NW-SE trends occurring to the south of Topisi area. This shear zone is interpreted as an extension of the Lethakane-Magogaphate-Triangle shear system and taken to represent the western boundary to the Phikwe Complex. A change of structural trends from the E-W Magogaphate shear zone trend to a N-S trend along the Dikalate Shear Zone is interpreted as a transition from lateral to frontal ramps (McCourt and Vearncombe, 1987). McCourt and Vearncombe (1987) suggest westwards emplacement of the Central Zone based on kinematics of the shear zone along its margins i.e. dextral displacement on south dipping Triangle and Magogaphate shear zones and sinistral displacement on N dipping Palala shear zone (Fig. 1.5). In this interpretation, sense of displacement and dip direction (geometry/attitude) is compatible and define a single surface and a single structure. Contrary to this suggestion, Paya et al. (1997) observed that the geometry of asymmetrical porphyroclasts and S-C structures along the Dikalate Shear Zone indicate eastwards thrusting. Support for this interpretation comes from Holzer et al. (1999).
According to Holzer et al. (1999), the Dikalate Shear Zone dips westwards, with a steeply westwards plunging lineation but the Magogaphate shear zone dips south and so