«By MOLATLHEGI LARTY LOSTMAN MOSEKI STUDENT NO. 208523856 Submitted in fulfillment of the academic requirements For the degree of Master of Science In ...»
Coward et al. (1976) report that the Southern Marginal Zone of the Limpopo belt consists of high grade, intensely deformed and metamorphosed TTGs with minor greenstone sequences whereas the Northern Marginal Zone is dominated by enderbite and charnoenderbitic orthogneisses with minor greenstone sequences and granitoid rocks.
The Southern Marginal Zone was interpreted to be a reworked counterpart of the northern Kaapvaal craton. Coward et al. (1976) interpreted the Northern Marginal Zone as a wide ductile shear zone with about 200 km dextral displacement. The ductile shear zone was extended westwards into the Foley area and therefore the geology of the Foley area was placed in the northern part of the Limpopo belt. However, Key and Hutton (1976) place the Foley area outside the Limpopo belt in the sense that regional scale shearing has died out west of 27° 30E. Coward et al. (1976) observed that the metasedimentary rocks within the Limpopo belt have been through the same deformation sequence as those on the craton and appear to be of similar age but different sedimentary facies. According to Coward et al. (1976) the tectonic phases affecting the Limpopo belt can be traced into the Zimbabwe craton and the margins of the Limpopo belt are difficult to define.
15 Key et al. (1976) described the Shashe gneiss as a unit composed of granitic augen gneiss and migmatitic gneiss. The migmatite zone was considered to mark the transition zone between the Zimbabwe craton and the Limpopo belt. Key (1976) suggested that the Shashe gneiss is older than the Tati greenstone belt but provide no evidence in support of this interpretation. Barton and Key (1981) and Barton (1983) suggest that the earliest geological events in the Foley area have probably been completely overprinted by the subsequent development of the Limpopo belt, which they linked to plate tectonics processes. Barton and Key (1981) based on gravity data, suggested the presence of a uniform, thick, low density crust in the Foley area and the possibility that that the supracrustal rocks in the Foley area unconformably overlies older basement (Rb-Sr ages of 3600 and 2900 Ma, Hickman, 1978) at depth.
Smith and Phofuitsile (1985) define the geological position of the Foley area as lying in an intermediate position between the Zimbabwe craton and the Limpopo belt. The authors report that in the Foley area, there is no distinction between the Central Zone and the Northern Marginal Zone as defined by Mason (1973) and Key (1976) further east.
The Foley area comprises thick meta-arkoses and meta-psammites overlain by a shallow water, continental shelf type, sequence of quartzite and meta-pelites. According to Smith and Phofuitsile (1985), the tectono-metamorphic sequence affecting the supracrustal rocks in the Foley area is in many ways intermediate between the higher grade parts of the Northern Marginal and Central Zones of the Limpopo belt (as described by Key,
1976) and the Zimbabwe craton (Lintern, 1982). McCourt and Vearncombe (1987) contend that the Northern Marginal Zone and the Southern Marginal Zones of the Limpopo belt do not extend into Botswana such that in Botswana, the Limpopo belt is confined to the area between the Magogaphate and the Mahalapye Shear Zones (McCourt et al., 2004, Fig. 1.6).
Published research on the Tati and Matsitama greenstone belts of the Zimbabwe craton and adjacent areas in NE Botswana include papers by Coward and James (1974); Coward et al. (1976); Key et al. (1976); Smith and Phofuitsile (1985) and Paya (1996). The papers document up to 5 deformational phases that include thrusting, isoclinal folding, 16 open folding and strike-slip shearing. The main findings are summarised in Tables 1.1, 1.2, 1.4, 1.5 and 1.6. Coward and James (1974) recognised an early NE vergent fold and thrust event (D1) which accounts for the stratigraphic inversion in the Tati and Matsitama greenstone supracrustal belts. The authors report that this event predated intrusion of foliated granitoids in the Tati greenstone belt and development of the regional pervasive foliation fabrics in both supracrustals and granitoid lithologies (D2). The second deformation (D2) structures and fabrics were re-orientated and locally overprinted during D3 and D4. Coward et al. (1976) linked the crustal shortening observed in the southwestern margin of Zimbabwe craton to collision, closing of basins, and thrusting of nappes over a region of high heat flow. Key (1976) recognised five (5) phases of deformation in the Tati greenstone belt in the Francistown area, and concluded that although the structural style differs, the same deformational phases also appear in the terrane to the S of the Tati greenstone in the Phikwe area. In support for this interpretation, Coward et al. (1976), Key (1976), Key et al., 1976 and Smith and Phofuitsile (1985) suggest that the phases of deformation recognised in the Shashe area generally correspond to those described in adjacent areas, which are themselves basically similar. A comparison of the tectonic histories of the Zimbabwe craton, Limpopo belt and the zone between these terranes (Key, 1976) is given in Table 1.1. The information shown in Tables 1.1 to 1.6 (1.4 excluded) is copied directly from the literature cited.
Smith and Phofuitsile (1985) correlated deformation structures and metamorphism in the Foley area as shown in Table 1.2.
D4 Local, close and open folds, Local kinks in micas, Growth of chlorite, epidote, actinolite, quartz N-S trend, plunge S. often if fractures and veins.
Later Cataclasites, Faulting Epidote, quartz, chlorite in fractures 19 In addition to the general references sited above, there are a number of publications that focus on the tectonic framework of NE Botswana. According to Aldiss (1989) the Shashe area is located about 50 km NW of the Limpopo belt and lies in the most southwesterly exposed portion of the Archaean Zimbabwe craton. Within Botswana Aldiss (1989) restricted the Limpopo belt to the region of granulite facies metamorphism around Phikwe area (Fig. 1.6). No granulites were identified in the Shashe area. The terrane underlying the Shashe area was therefore not regarded as part of the Limpopo belt but was considered a zone of reworked Zimbabwe craton. Following McCourt and Vearncombe (1987)’s observation that subdividing complex orogenic belts into domains of unique character is an important approach in their description and interpretation, Aldiss (1991) divided the Archaean crust of NE Botswana into a number of lithostratigraphic complexes, each of which exhibit distinct characteristic features with respect to rock assemblages, relationships between metasedimentary belts and the granitoid gneisses that surround them as well as the type and nature of mineralization, metamorphism and structural style. The complexes defined by Aldiss (1991) are; the Francistown Granite Greenstone Complex, the Mosetse Complex, the Motloutse Complex, the Phikwe Complex and the Mahalapye Complex (Figs 1.6 &1.7). The Francistown Granite Greenstone Complex represents the southwesternmost exposed portion of the Zimbabwe craton in Botswana (Aldiss, 1991; Francistown Arc Complex of McCourt et al., 2004; Bagai 2008). The Phikwe and the Mahalapye Complexes form part of the Central Zone of the Limpopo belt in Botswana (Aldiss, 1991). The Mahalapye Complex defines the southwestern edge of the Central Zone in Botswana. There is general consensus that it formed by reworking of the granulites of the Central Zone at ~
2.0 Ga (e.g. McCourt et al., 2004). The recognition of these complexes suggests a component of crustal accretion in the growth of the Zimbabwe craton and Limpopo belt during the Archaean. The main features of these complexes are summarised in Table 1.3.
Aldiss (1991) working in the Shashe area reports that Matsitama-type metasedimentary rocks (i.e. similar to those in Matsitama belt (Fig. 1.1) and granitoid gneisses are interlayered along southwest dipping thrust sense shear zones. The shear zones are characterized by foliated augen gneisses bearing an upright mineral elongation lineation.
In keeping with this observation, Key et al. (1994) recognised early NE vergent folds and thrusts refolded along NE trending axial surfaces and associated with a southwest plunging mineral elongation lineation in the Topisi area (Fig. 1.6). Aldiss (1989) reports that in the Shashe and Tonota areas the metasedimentary rocks and adjacent granitic gneisses are both highly foliated, with mylonitised fabrics developed along and adjacent to the contacts.
24 The boundary with the Francistown Granite Greenstone Complex is shown by the change in regional strike of planar fabrics from NW-SE in the Francistown Granite Greenstone Complex to N to NNE in the Motloutse Complex (Aldiss, 1991, Table 1.3). According to Coward et al. (1976) and Key et al. (1976) the metasedimentary rocks in the southwestern part of the Zimbabwe craton were subjected to the same deformation as the enclosing gneisses. In support of this Aldiss (1991) proposed that the metasedimentary rocks from Matsitama belt were incorporated into migmatites of the Motloutse Complex during northeasterly thrusting. The detached portions of the supracrustals are nowhere seen to be thrust over the migmatites in the Motloutse Complex. Aldiss argues the detached portions of the supracrustal rocks are in thrust contact with the granitoid gneisses. Aldiss (1991) suggests that the granites were emplaced syn-tectonically, as intrusive sheets along thrusts contacts separating the metasedimentary rocks and the granitoid gneisses. Aldiss (1991) concluded that the Motloutse Complex formed due to convergence between the Zimbabwe craton and a structural segment to the southwest.
The metasedimentary rocks in the postulated terrane to the southwest were thrust northeastwards over the margin of the Zimbabwe craton. This resulted in crustal thickening and high heat flow that promoted migmatisation, anatexis and ductile deformation. Sheets of anatectic granite were intruded syn-kinematically along thrust zones. The overthrust metasedimentary rocks were infolded with the underlying migmatites and gneiss. The protolith to the gneisses that surround metasedimentary belts in the Matsitama and Motloutse Complexes is unknown. The gneisses are a product of ductile deformation under high grade metamorphic conditions. Nevertheless, many of the gneisses have been interpreted as metamorphosed supracrustal lithologies dominated by felsic metavolcanics and metasedimentary rocks. Thus they have been considered to be of both sedimentary (Key et al., 1976) and volcanic (Majaule and Davis, 1998; Aldiss,
1989) origin. Aldiss (1991) deformational episodes in the Motloutse Complex are summarized in Table 1.4.
Key et al. (1994) working in the Topisi area established that the Central Zone of the Limpopo belt has a western limit which coincides with the concave east arcuate trace of the extended Magogaphate shear zone. The area to the W and NW of the Magogaphate shear zone was named the Western Zone of the Limpopo belt, an area that coincides with 25 Aldiss (1991)’s Motloutse Complex. According to Key et al. (1994) the Western Zone of the Limpopo belt was characterized by repeated oblique and head-on collision between the Central Zone and the Zimbabwe craton and was interpreted as a “tectonically modified imbricate system”. Key et al. (1994) recognised a sequence of events based on superposition of different generations of planar and linear structures. Large scale folds were interpreted as products of early ductile thrusting. The area is characterized by N-S trending folds some of which refolded to define interference patterns shown by different metasedimentary rocks e.g. at Sesweu hill. The metasedimentary rocks define N-S trending folds interpreted to be fold interference patterns due to F3 refolding of F2 structures.Key et al. (1994) note that the metasedimentary rocks are dominated by quartz and quartz-mica schist with subordinate amphibolite, marble and calc-slicates. These rocks are spatially associated with grey gneiss regarded as paragneiss, augen gneiss, ultramafic rocks and anorthosites. The relationship between the metasedimentary rocks and the granitoid gneisses is not clear but evidence suggests that they are sheared.
According to Key et al. (1994) the first deformation recognised in the Topisi area was a gneissification process defined by mineral layering in granitoid rocks. The formation of the gneissic fabric preceded deposition of the metasedimentary rocks based on the observation that the metasedimentary rocks do not have that gneissic fabric. Key et al.
(1994) suggested the presence of an older basement to the metasedimentary rocks. The second deformation (D2) is represented by development of early NW-SE trending regional folds (implying north-eastward direction of thrust). These D2 folds are characterized by a strong down-dip lineation in quartzite that plunges to the southwest and SW-NE plunging lineation in the granitoid gneisses. This is succeeded by a D3 phase that produced N-S to NE-SW trending folds that plunges towards NE. The D3 phase generated interference folds e.g. at Sesweu Hill.
26 Table 1.4: Summary of regional deformation in the Motloutse Complex. Tabulated from Aldiss (1991).
Deformational Phase D1 Inversion of Tati belt and parts of the Matsitama belt metasedimentary rocks by NE verging recumbent folding and thrusting (Litherland, 1975; Coward and James, 1974; Key et al., 1976;
Aldiss, 1991) Thrusting and nappe formation D2 Considered the main fabric producing event (Litherland, 1975; Coward and James, 1974; Key et al., 1976 and Aldiss, 1991; Paya 1996). Maximum compressive stress sub-horizontal and trending NE-SW (Key et al., 1976). NE-SW compression in response to SW movement of the Zimbabwe craton relative to the Limpopo belt after thrust emplacement of the Matsitama-type metasedimentary rocks in the Motloutse Complex, formation of NE folds with NE-SW trending axial surface possibly with large-scale movement-parallel folds developed from northeasterly verging isoclines and syn-tectonic granites intruded along thrust zones (Aldiss, 1991). Tati greenstone belt shortened by 60% (Coward et al., 1976).
The fourth deformational phase (D4) is characterized by ductile shearing while the fifth (D5) is associated with development of NW-SE open folds and crenulation structures (Key et al., 1994). A summary of the deformational history of rocks in the Topisi area is given in Table 1.5.