«By MOLATLHEGI LARTY LOSTMAN MOSEKI STUDENT NO. 208523856 Submitted in fulfillment of the academic requirements For the degree of Master of Science In ...»
1976) based on their age relative to each other and on regional deformation events (presence or absence of foliation) has been questioned (Bagai, 2002). Bagai (2002)’s work has shown that the G1-G5 system is not supported by U-Pb ages of granites. More recent studies in NE Botswana have been based on crystallization of granitoids rocks (magmatic zircons) and detrital zircons on supracrustal rocks (Table 1.7). These studies include McCourt and Armstrong, (1998), Kröner et al. (1999), Chavagnac et al. (2001) and Zeh et al. (2009) on the Phikwe and Mahalapye Complexes, Majaule and Davis (1998) on the Mosetse area, Bagai (2002, 2008) on the Francistown Arc Complex and unpublished data by B.K Paya (pers. com. 2010). The ages calculated and the respective authors’ interpretations are summarized in Table 1.7. In addition Holzer et al. (1999) published a series of PbSL ages on granitoid gneisses from the Phikwe and Motloutse complexes but the geological significance of these ages is unclear and they are not discussed further. More recently, McCourt et al. (2004) published detrital zircon ages obtained from quartzites from the Motloutse Complex. Additional unpublished U-Pb age data from granitoid rocks from the Phikwe and Motloutse areas were provided by 1B.K Paya (pers com. 2010).
1 Mr. Boikobo Paya, Permanent Secretary, Ministry of Minerals, Energy and Water Resources, Department of Geological Survey.
*NB. Lu-Hf data indicate presence of xenocryst zircons, evidence for crustal recycling (remelting of older crust) and depleted mantle source in Mahalapye and Phikwe Complexes, (Zeh et al., 2009).
2.1 INTRODUCTION The Archaean crust of the SFT area consists of two contrasting metamorphic sequences deformed together (see dissertation map in the dissertation, folded at the back of cover).
The first sequence comprises medium to high grade metasedimentary rocks (2metaquartzite-marble-amphibolite). The metasedimentary rocks have been subdivided into two main units comprising quartzite/quartz-mica schist association and a mixed metasedimentary association (Table 2.1). The rock units are metamorphosed and interfolded with each other such that their original relationship is difficult to establish.
Only resistant quartzite, marble and calc-silicates are well exposed. In many instances during this mapping, reliance was much more on float lithologies to establish the distribution of units.
Table 2.1: Subdivision of SFT Archaean crust into metasedimentary rocks and granitoid gneisses showing metamorphic facies and lithological units
The second constitutes the predominant rock assemblage and consists of granitoid gneiss.
The gneisses have been subjected to intensive metamorphism resulting in partial melting.
In support of the latter, Aldiss (1989) reports amphibolite facies metamorphism with 2 ‘Metaquartzite’ is a term commonly used by Geological Survey geologists in southern Africa but quartzite is a metamorphic rock after quartz arenite. Calling the metamorphic rock (recrystallized) rock ‘metaquartzite’ is misleading as it implies the protolith was quartzite. In view of this, it is preferred in the present study to identify the rock as “quartzite”.
44 anatexis (partial melting) in the gneiss terrane. The granitoid gneisses show evidence for partial melting and formation of migmatite in the banded tonalitic gneiss and the granitic gneiss. The broad distribution of the different rock types could be inferred from aerial photographic interpretation and ASTER imagery. Field relations between the metasedimentary belt and the granitoid gneisses are difficult to establish. The contacts are poorly exposed and characterized by flat lying outcrops. There is a lack of clear intrusive relations and no evidence for unconformable relationship between the metasedimentary rocks and the granitoid gneisses. The granitoids are nowhere seen to intrude the metasedimentary rocks and likewise no metasedimentary rocks were found as enclaves within the gneissic unit. However, both units are deformed and the deformation fabrics are compatible suggesting the current boundaries are tectonic. The geometry of the boundary is given by fabrics in the granitoid gneiss to the W and E of the metasedimentary belt. Structural interpretation of fold geometry and fabric relationships was hampered by the lack of good outcrop and in particular by the nature of most outcrops; typically flat rock pavements providing no opportunity to view structural elements in the vertical section.
2.2 METASEDIMENTARY ROCKS
2.2.1 QUARTZITE/QUARTZ-MICA SCHIST UNIT In outcrop the quartzite/quartz-mica schist unit comprises massive through foliated to finely laminated rocks. The foliation (S1) and lamination are defined by alignment of quartz and biotite grains in a predominantly NE-SW orientation and as a result the unit generally shows NE-SW trends with moderate to steep dips towards NW (Fig. 2.1). The laminated quartzite form thin layers that often grades into quartz-mica schist. Both lithologies are commonly iron-rich, though non-ferrugineous units also occur. Quartzmica schist forms micaceous partings within the quartzite
Texturally the quartzite ranges from very fine grained to medium grained. The massive quartzite units are typically milky white in colour although pale/dark grey and brown varieties are also found. The darker grey varieties contain concentrations of opaque minerals (Fig. 2.2).
Figure 2.2: Outcrop of massive pale grey quartzite from the quartzite/quartz-mica schist unit in the Gulushabe area.
46 2.2.2 MIXED METASEDIMENTARY UNIT An undifferentiated association of marbles and calc-silicate rocks interfolded with quartzite, amphibolites and granitoid gneiss were mapped in the Gulushabe area (dissertation map). The marbles are dull grey, brown, white-weathering, massive medium grained rocks (Fig. 2.3 A-C) and are associated with thin layers of calc-silicate rock. The calc-silicate protrudes on weathered surfaces usually parallel lamination. The marbles have internal laminations in thicker layers and alternations of marble and calc-slicate minerals both of which probably represent original bedding (Fig. 2.3 C). The marbles are composed of recrystallized carbonate minerals, most commonly calcite and dolomite (Tables 4.2 and 4.3).
A W of Gulushabe settlement B. N of Gulushabe settlement
C. ESE of Gulushabe settlement Figure 2.3 A-C: Typical marble outcrops from the Gulushabe area, A: massive dull grey marble unit from the western part of the Gulushabe structure.Strike and dip of foliation is B: Pale coloured marble unit north of Gulushabe area, near Shashe River. Bedding strikes;
310º/40° and C: Interdigitated marble and calc-silicate rock. bedding strikes; 256°/66º, eastern part of Gulushabe fold structure.
47 Contacts between the marbles and the amphibolites are sharp and concordant with the mineral layering. The stratigraphic relationship between the marbles, the quartzite and the amphibolite has not been established due to folding and thrusting deformation events that affected rocks in the area. Graphite schist occurs associated with the marble-calc-silicate rocks on top of a hill to the SE of Shashe Dam in Shashe Village. The graphite schist is very fine grained and weathers with a white tint (Fig. 2.4).
Figure 2.4: Graphite schist associated with marbles SE of Shashe Dam.
Strike and dip of foliation (S1) is 210°/55º.
The marbles vary from massive to layered types, the layering is the result of minor variations in composition of the marble. Nodules of marble occur in calc-silicate rocks.
The nodules are rimmed by thin layers of calc-silicate (Fig. 2.5). The nodules are elongated parallel to the foliation (S1) which is defined by alignment of quartz, muscovite, carbonate minerals and hornblende.
Marble and calc-slicate rocks form a low ridge E of the Shashe Village with amphibolite and graphite schist in the adjacent low lying areas. Prominent amphibolite bands are interrelated with marbles and calc-silicates in the vicinity of Gulushabe settlement. The outcrops are often covered by a thin layer of calcrete. In hand specimen, the mineralogy of these rocks comprises hornblende, plagioclase and quartz. The amphibolites are layered, fine to medium grained equi-granular rocks and are speckled white with plagioclase (Fig. 2.6). Contacts between the marbles and amphibolite have not been observed.
Biotite schist is of limited distribution and only occurs in the fold hinge and on the southeastern limb of the Gulushabe fold structure. The biotite schist is up to 2m thick and best exposed in a stream section adjacent to a marble horizon. On the southeastern limb, the biotite schist is associated with marbles, amphibolites, and quartzite. Foliation (S2) in schist in the hinge and on limb dips steeply WNW and carries deformed veins of quartz.
49 Elsewhere in the study area the metasedimentary rocks exposed include features of both the quartz-rich unit and the carbonate-bearing unit. For example close to the Shashe River sooutheast of Tonota amphibolite and marble occurs in association with tonalitic gneiss (Fig. 2.7). The contact between the rock units are sharp, concordant with the mineral layering and are emphasized by textural and colour dissimilarity.
Figure 2.6: Amphibolite unit displaying white spots of plagioclase feldspar, SE part of the Gulushabe fold structure.
Strike and dip of foliation (S1) is 220º/62°.
Figure 2.7: Interlayered amphibolite, marble and tonalitic gneiss, north of eastern closure of the Gulushabe fold structure, NE of Gulushabe settlement, strike and dip of foliation (S1) is 242º/82°.
Close to the Seswe cattlepost NW of Foley Village, pebble-bearing quartzite (Fig. 2.8 A) is intercalated with amphibolite (Fig. 2.8 B) and granitoid gneiss (Fig. 2.8 C and D). The pebble-bearing quartzite in particular carries a strong foliation (S1) and pebbles are flattened perpendicular to the foliation plane and elongated parallel to the foliation (Fig.
C D Figure 2.8 A-D: Examples interlayered rock units at Seswe area, NW of Foley Village A.
deformed pebble-bearing quartzite, 240/81°, B: amphibolite, strike/dip of foliation (S1);
242/80°, C: granite gneiss, strike/dip of foliation (S1); 243º/82° and D: deformed megacrystic granite gneiss, strike/dip of foliation (S1); 243º/79°, Compression perpendicular to foliation plane has resulted in elongation parallel to the foliation-an overall flattening (oblate strain).
Figure 2.9: Amphibolite interbedded with biotite-mica schist southern part of Foley structure.
Strike and dip of foliation (S1) is 220º/86º.
2.3 GRANITOID GNEISSES In the following account the general features of the granitoid gneisses are described and then the more significant lithologies are dealt with in more detail. Migmatitic gneisses are medium to coarse grained and textures are very variable. The migmatite is interlayered with banded gneiss and granitic gneisses and contacts are both gradational and sharp the latter due to shearing. Aldiss (1991) mentions occurrences of enclaves of calc-silicate gneiss, marble and quartzite in migmatite of the Shashe area but similar occurrences were not seen in the study area. Based on composition and texture the granitoid gneisses have been classified into 5 main types. The different variants of the granitoid are classified as Tonota biotite gneiss, megacrystic granite gneiss, megacrystic tonalite gneiss, banded tonalitic gneiss, pink gneissic granite and tonalite-trondhjemite gneiss. The Tonota biotite gneiss is considered to have been derived from a sedimentary protolith (Key 1976) but in 53 this work it is treated as an orthogneiss gneiss based the limited age range of the zircon grains and their physical character (section 4.3.4). Evidence for an intrusive relationship between the megacrystic granite gneiss, pink gneissic granite and tonalitic gneiss is given in sections (2.3. 2 and 2.3.5). The pink gneissic granite is not shown separately on the map because it occurs as non-mappable bands closely associated with the megacrystic granite gneiss. Although the foliation (S1) in the banded tonalitic gneiss, megacrystic gneiss and pink gneissic granite is typically concordant there are local cross-cutting relationships that probably represent original intrusive contacts.
2.3.1 TONOTA BIOTITE GNEISS The Tonota biotite gneiss is a sequence of biotite gneiss with small lenses and interlayers of amphibolite (Fig. 2.10 A and B) exposed in the vicinity of Tonota Village. The biotite gneiss is leucocratic, not banded, homogeneous, fine to medium grained, strongly foliated and includes grey to pink varieties. Compositionally the gneiss is a quartzo-felspathic rock containing a variable amount of biotite. Very sparse K feldspar megacrysts are present in the gneiss. The Tonota biotite gneiss contains veins of felsic material (comparable to white granitoid veins of Aldiss, 1989) in apparent concordance with the S1 foliation (Fig. 2.10 C). The rock is typically weathered.
C:strike/dip of foliation (S1); 330º/58° Figure 2.10 A-C : Outcrops of the Tonota biotite gneiss interlayered with thin layers of amphibolite south of Tonota (A and B). Note thin veins of quartz concordant to the foliation (S1) in the biotite gneiss in C.
The distribution of the gneissic and amphibolite components of the Tonota biotite gneiss is heterogeneous. Three areas of heterogeneity are recognised based on the proportion of amphibolite and biotite gneiss as well on textural basis. The northern zone is amphiboliterich and contains subordinate narrow interlayers of biotite gneiss. The amphibolite also encloses deformed fragments of biotite gneiss. The biotite gneiss is exposed in the form of koppies and boulders (Fig. 2.11).
55 A B Figure 2.11 A-B: Examples of outcrops of the Tonota biotite gneiss, nothern part of Tonota, E of Francistown main Road, strike/dip of foliation (S1): 278°/68º.