«By MOLATLHEGI LARTY LOSTMAN MOSEKI STUDENT NO. 208523856 Submitted in fulfillment of the academic requirements For the degree of Master of Science In ...»
1.3 RATIONALE FOR THE STUDY NW-SE trending structures characterize the granite greenstone belts of the southwestern Zimbabwe craton (Aldiss, 1991, Bagai 2008) and adjacent areas previously described as the Shashe belt (Bennett, 1970, Section 1.7). These structural trends also characterize the Matsitama belt. The study area (SFT) is part of the Motloutse Complex and it is an anomalously oriented domain that trends NE-SW. The primary rationale of the current project is to provide an explanation for (or explain) these regionally anomalous structural trends; i.e. to place the kinematics of the SFT area (Motloutse Complex) into the regional framework. McCourt et al. (2004) linked the Motloutse Complex to the Matsitama belt (and therefore introduced the term “Matsitama-Motloutse Complex”) based on similar rock types particularly the metasedimentary sequence but not on structural grain. It was assumed there was a kinematic link between NE vergent structures in the Matsitama belt and NE trending fabrics in the Motloutse Complex, but this was not explained.
Generally, the subdivision of Archaean crust in NE Botswana is unclear, largely due to complex structural history and debated field geological relationships. The origin, spatial and temporal relationships of this Archaean crust is not well understood. The Shashe area is underlain by Archaean granitoid gneisses and ‘complexly’ folded metasedimentary sequences (Carney et al., 1994) which form part of the Shashe belt, which have been linked to the Limpopo belt (Ranganai et al., 2002). The ‘complexity’ of the folding in the metasedimentary rocks of the Shashe belt has never been investigated. Therefore knowledge about fold geometries is limited. The structural pattern of fold structures in the metasedimentary belt and granitoid gneisses need to be investigated. The depositional age of the protoliths to the metasedimentary rocks and the emplacement age of the protolith to the granitoid gneisses are poorly understood. Earlier work (e.g. Bennett, 1970; Mason, 1973 b; Key et al., 1976; Smith and Phofuitsile, 1985; Aldiss (1989, 1991);
Paya, 1996; McCourt et al., 2004; Bagai, 2008) in certain selected sectors include limited data on the structural and tectonic evolution of the SFT region. Little is known about the structure and chronology of events in this region, or of the tectonic setting. Structural data are scant and precise geochronology is inadequate. Published age data in the area are very scarce and the results indicate a complex geological history underlining the need for a 9 systematic well planned geochronological study. Knowledge of the timing of granitoid gneisses and deformation processes is fundamental for understanding the evolution of the SFT region. It is important to recognise and constrain periods of magmatism as well as the geometry and timing of shear zones in the SFT area. A lack of precise age data from this area, particularly U-Pb is a major impediment in constraining the evolution of the area and its regional correlation. The metasedimentary sequences and the gneisses in the area have been correlated on the basis of lithological and structural similarities (Aldiss,
1991) and the few available age data (McCourt et al., 2004). The granitoid gneisses have a complex history and the only way to resolve their initial crystallization age and to decipher their subsequent evolution is through high precision U-Pb geochronology and Hf-Lu isotope geochemistry. Field relations between the granitoid gneisses are conflicting since contradictory relationships are reported (Aldiss, 1991). A field based intrusive-relation study is necessary as a follow up on these.
Defining the position of and understanding the nature and geometry of the Botswana section of the Zimbabwe craton-Limpopo belt boundary has long been a question for debate (e.g. Bennett, 1970; Mason, 1973 a; Key et al., 1976; Robertson and du Toit, 1981; Watkeys, 1983; Aldiss, 1991; Paya, 1996; Ranganai et al., 2002, McCourt et al., 2004). In addressing this problem, theories and ideas ranged from a transitional boundary (Bennett, 1970; Mason, 1973 a; Key et al., 1976; Robertson and du Toit, 1981 and Aldiss, 1991) to a shear zone boundary (e.g. Bennett, 1970; Key et al., 1976; Robertson and du Toit, 1981; Watkeys, 1983; Aldiss, 1991; Paya, 1996; Ranganai et al., 2002, Mc Court et al., 2004). The Shashe shear zone is described as a NW-SE trending NE vergent dip-slip ductile shear zone that possesses a predominant E-W shear foliation with moderate to steep dips to the S. Rotated porphyroclasts and s-c fabrics along the shear zone indicate S over N movement. The Shashe shear zone considered the tectonic boundary between the Zimbabwe craton and the adjacent Limpopo belt (Paya, 1996;
Ranganai et al., 2002) or between the Matsitama-Motloutse Complex and the Zimbabwe craton (McCourt et al., 2004) shows northward thrusting of the Shashe section of the Limpopo-Shashe belt (Paya, 1996) or Matsitama-Motloutse Complex (McCourt et al.,
2004) and has been interpreted as a westward extension of the North Limpopo Thrust 10 Zone in Zimbabwe (Paya, 1996; Holzer et al., 1999; Ranganai et al., 2002). The northward thrusting is taken to imply a northward transportation of the northeastern Botswana area (Paya, 1996) typical of the North Limpopo Thrust Zone. However, the documented direction of movement on these structures does not easily allow such a link.
Displacement on the Shashe shear zone is to the NE whereas displacement in the North Limpopo Thrust Zone is towards NW. Further work is required to check if indeed the Shashe shear zone extends into the study area and establish the geometry and the movement direction defining the SFT section of this boundary. With the present state of knowledge, it is not clear whether the NW-SE trending Shashe shear zone is folded to produce NE-SW striking structural fabric in the SFT region or the NE shears cut across the NW-SE fabric or vice-versa. There is need to map out the field relationship between the NE-SW deformation fabrics (shear zone) and the Shashe shear zone and the timing and shear sense of the structures.
Magmatism and deformation associated with the SW margin of the Tati greenstone belt (Fig. 1.1) is linked to convergent tectonics along the margin of the Zimbabwe craton i.e.
an Andean-type scenario (Kampunzu et al., 2003; McCourt et al., 2004; Bagai, 2008).
Based on geochemistry of the granitoids and related volcanics, the Tati greenstone belt was interpreted to have formed in a marginal arc due to subduction below the crust of the proto Zimbabwe craton. This arc was built on older crust (i.e. overriding plate) and the rocks of the Tati belt are not part of that older crust. Establishing whether SFT region is in the hanging wall of a NE vergent thrust and thus have been thrust over the margin of the Zimbabwe craton or in the footwall of the boundary with the Zimbabwe craton is of great importance in understanding the evolution of the area.
NE-SW directed compression related to southwesterly movements of the Zimbabwe craton (Table 1.4, Aldiss, 1991) would produce NW-SE striking planar fabrics but foliations and fold axes in the SFT region are trend NE-ENE. Aldiss (1991) proposed that during thrusting of the Motloutse Complex against the Zimbabwe craton, the crust forming the Motloutse Complex was highly ductile. This allowed rotation of structural elements e.g. folds into parallelism with the northeastwards movement direction during
1.4 GOALS In an attempt to improve the understanding of the basement geology of NE Botswana, the
following goals were set:
1. To study the structural geology of the rocks in the SFT region and based on observations draw conclusions about the structural history of the rocks and the kinematics of the deformation features recognised. Geochronology on zircon grains from selected rock types would be used to establish the minimum and maximum age of the deformation events recognised
2. To decide whether the structural and kinematic history of the rocks in the SFT region (Motloutse Complex) correlates best with that of the Central Zone of the Limpopo belt (Phikwe area) to the SE, the Tati granitoid-greenstone terrane (Francistown area) to the N or is different from both.
1.5 KEY QUESTIONS The present study focused on establishing and clarifying field relationships between the main rock units, using these relationships to identify granitoids and granitoid gneiss that can serve as time markers and obtaining new precise U-Pb age data from these units to constrain the deformation history of the SFT region. To this end the following key questions were addressed and answers to these questions will contribute to the understanding of the crustal evolution of NE Botswana during the Neoarchaean.
To establish the deformation history of the metasedimentary rocks of the study area To study the geometry, age and kinematics of the NE-SW trending zone of deformation fabrics that characterize the granitoid gneisses of the study area
To identify suitable time markers in the study area and use U-Pb geochronology on zircon to establish the minimum and maximum ages of the deformation events
1.6 METHODOLOGY This study was a field-based project involving lithological and structural mapping supported by laboratory work, specifically U-Pb zircon geochronology (Chapter 4) on granitoids and stable isotope geochemistry on carbonate rocks (Chapter 5). Thus a combination of geochronological data with structural and petrological information is used to unravel the evolution of the SFT region. The reliability of existing data was checked and augmented with newly acquired field and geochronological data. A combination of data from aerial photographs and 1: 50 000 ASTER imagery (ERSDAC, 2010) were used to delineate fold structures and photo-lineaments. Structural investigations entail observation and analysis of the major structural elements of the study area, delineation of shear zones, their morphology, evolution and significance. The geometry and the movement direction on shear zones were investigated. Planes have been measured as strike and dip (with strike being the maximum dip-90º) and written as XXXº/XXº.
Lineations have been measured as plunge and plunge direction and written as XXº/XXXº.
The orientation measurements recorded were taken using a Silva Compus and the Right Hand Rule was applied.
1.7 PREVIOUS WORK In the sections that follow, relevant published literature and/or internal reports of the Botswana Geological Survey related to the basement geology of NE Botswana are summarized. Gerrard (1963) subdivided metasedimentary rocks in the Foley area (Fig.
1.1) into a Metaquartzite Formation and a Mixed Amphibolite and Metasedimentary Formation. The Mixed Amphibolite and Metasedimentary Formation is described as an association of amphibolite, quartzite, pelite and marble that are individually too thin to be mapped separately. Gerrard (1963) observed that the two formations are metamorphosed and interfolded. This hindered interpretations regarding their stratigraphic relationship.
The Metaquartzite Formation was considered to be older than the Mixed Amphibolite and 13 Metasedimentary Formation but no substantial evidence in support of this interpretation is given. Gerrard (1963) noted that flaggy metaquartzite occurs towards the base of the sequence and that the proportion of quartz-mica schist increases towards the boundary with the Mixed Amphibolite and Metasedimentary Formation. The difference between the types of granitoid gneisses; grey biotite gneiss, porphyroblastic and augen gneiss, and pink granite gneiss was interpreted to be a result of “syn-kinematic feldspathisation and replacement”. Gerrard (1963) reported that lithologically similar gneiss units lie stratigraphically above and below the metasedimentary formations.
Cox et al. (1965) recognised the widely accepted zoned nature of the Limpopo belt and divided it into a Central Zone characterized by N-S structural trends, flanked by Northern and Southern Marginal Zones (Figs 1.3, 1.5) characterized by ENE regional trends. This zoned subdivision of the Limpopo belt was extended into Botswana as far as the Foley area. Crockett (1968) mapped the Shashe area but only identified lithological units without undertaking detailed structural investigations. Crockett (1968) and Bennett (1970) identified a zone of high grade metamorphism encircling the southwestern part of Zimbabwe craton. The authors suggested a separate Shashe Mobile Belt which wraps around the SW margin of the Zimbabwe craton (Fig. 1.4), lying between the Matsitama and Vumba belts.
There are divergent views on the extent of the Limpopo belt in Botswana due largely to the interpretation of the Shashe belt. The Shashe belt was regarded as a continuation or offshoot of the Limpopo belt consisting of reworked basement (cratonic material) with infolded remnants of metasedimentary cover rocks (Crockett, 1968; Bennett, 1970).
Mason (1973) suggested it was the retrogressed equivalent of the Northern Marginal Zone of the Limpopo belt in Zimbabwe. Key et al. (1976) rejected the existence of the Shashe belt and interpreted the terrane as a continuation of the Zimbabwe craton that was deformed and metamorphosed at high grade. In more recent literature (Ranganai et al., 2002), the Shashe belt is considered to be part of a continuous orogenic belt, the Limpopo-Shashe belt or part of a separate terrane termed the Matsitama-Motloutse Complex (McCourt et al., 2004).
14 Mason (1973) describes the Limpopo belt as an ENE trending granulite gneiss terrane characterized by repeated shear deformation, igneous intrusion and extrusion and viewed it as a zone of crustal weakness throughout geological time. The author observed that the Central Zone of the Limpopo belt is separated from the marginal zones by major shear zones. Mason (1973) divided rocks of the Motloutse area (Fig. 1.6) into “series” namely;
arenaceous series, calcareous series, volcanic series, banded gneiss series and granitic series. The division was based on lithological differences with no implication for stratigraphical age. Determination of stratigraphical succession in the area was considered impracticable because of the high grade of metamorphism and polyphase deformation. Mason (1973) suggests that a series of impure arenaceous and calcareous metasedimentary rocks and volcanic rocks of basaltic compositions, were regionally metamorphosed to produce paragneisses, quartzite, marbles, calc-silicate rocks and amphibolite.