«By MOLATLHEGI LARTY LOSTMAN MOSEKI STUDENT NO. 208523856 Submitted in fulfillment of the academic requirements For the degree of Master of Science In ...»
Biotite gneiss dominates the outcrop pattern in the area to the immediate south of Tonota, with amphibolite occurring as thin coherent layers while further south along the Tholotsane River, there are occurrences of weathered medium to coarse grained biotite gneiss interlayered with minor amphibolite. In all the outcrops visited the amphibolite and the biotite gneiss form concordant layers of contrasting composition and contacts between them are sharp, emphasized by colour dissimilarity. Fabrics present in the amphibolite and the biotite gneiss are the same. Although the Tonota biotite gneiss is typically characterised by thin layers of felsic material, there are also minor exposures of homogeneous foliated granitoid gneiss without the felsic layers (Fig. 2.12).
56 Figure 2.12: Homogeneous foliated Tonota biotite gneiss devoid of thin felsic layers.
Photograph from southeastern part of Tonota Village near Shashe River. Strike/dip of foliatio340/86°.
The amphibolite and theTonota biotite gneiss lie in concordance with the Shashe metasedimentary rocks and the stratigraphic relationship between the two is unclear as contacts relationships have not been observed. The contact between the Tonota biotite gneiss and the megacrystic granite gneiss NNE of Thombane Hill (locality 44 on dissertation map) appears to be tectonic, given by kinematic indicators in the megacrystic granite gneiss which suggest top to ESE sense of thrusting. The area underlain by Tonota biotite gneiss is poorly exposed and largely inaccessible since it falls within the centre of the Tonota Village, with numerous homes.
Three main types of amphibolites associated with the Tonota biotite gneiss are identified.
These are: 1. dark massive amphibolites that include thin veins of leucogranitic material (Fig. 2.13). Such veins are mostly concordant, but discordant veins also occur 2. Foliated speckled plagioclase amphibolite which is grey in colour (2.14 A). Some of the amphibolite is coarse grained in texture and could represent metamorphosed gabbroic or coarse grained dolerite sheets emplaced in the gneiss sequence prior to deformation (Fig.
57 Figure 2.13: Dark massive amphibolite (type 1) with concordant thin white veins of leucocratic material, SE of Tonota, strike and dip, 270º/70°.
A: Type 2: strike/dip of foliation (S1), 210º/56º B: Type 2, NNE of Tonota Village, 270/84º N of Tonota Village to the East of main tarred road.
W of main tarred road.
Figure 2.14 A-B: Speckled (spotted white) plagioclase amphibolite (Type 2) of the Tonota biotite gneiss unit exposed SE of Shashe Village.
Figure 2.15: Amphibolite with pillow-like structures (type 3) suggesting a volcanic protolith, NNE of Tonota to the E of Francistown main road, 260º/42°.
2.3.2 MEGACRYSTIC GRANITE GNEISS This rock unit crop out extensively throughout the map area. It forms low lying whalebacks roughly concordant with the regional foliation (S1). The megacrystic granite gneiss flanks the metasedimentary belt to the NW and SE but the contacts are not exposed thus making it difficult to define the field/age relationship between the two rock units. The megacrystic granite gneiss is interdigitated with banded tonalitic gneiss in the northwestern part of the metasedimentary belt. The megacrystic granite gneiss displays variable strain such that its appearance varies between gneissic granite, with very little evidence of strain through to granitic gneiss, in which deformed K-feldspar megacrysts define an obvious and often well developed augen texture (Fig. 2.16 A) to mylonitic granite gneiss that is locally ultramylonitic with less than 20% megacrysts and an intense 59 foliation (S1). In zones of high strain, the foliation defined by biotite and K-feldspar megacrysts is intense (2.16 B). The unit is a medium to coarse grained rock with varying content of K-feldspar megacrysts. The gneiss shows a high degree of compositional segregation with light-coloured feldspar megacrysts alternating with biotite-rich zones.
Foliation (S1) planes defined by an alignment of dark mica grains are asymmetrically distributed around the megacrysts. The megacrysts are interpreted to be porphyroclasts (i.e. where the K-feldspar megacrysts have become oval or augen shape) after original phenocrysts.
A: Augen gneiss near Shashe Dam, B: Intensely foliated megacrystic granite gneiss, N of strike and dip of foliation, 200º/68º Tonota Village, strike and dip of foliation, 198º/88º.
Figure 2.16 A and B: A: Plan view of megacrystic granite displaying augen texture.
B: an intense foliation (S1) in megacrystic granite gneiss, note reduction in grain size in zones of higher strain.
The megacrystic granite gneiss intrudes the banded tonalitic gneiss and both units are intruded by coarse unfoliated felsic melt. Enclaves of tonalitic gneiss occur within megacrystic granite gneiss (Fig. 2.17). The observation indicates that the tonalitic gneiss is older than megacrystic granite gneiss. The megacrystic granite gneiss and the pink gneissic granite share similar foliation (S1) trends implying that the pink granite either intruded syn-tectonically or the foliation forming event continued after emplacement of
Fig. 2.17: Enclave of tonalitic gneiss in megacrystic granite gneiss S of Shashe Dam. Note the foliation (S1) in the megacrystic granite gneiss on (top left of photograph) is at a high angle to that in the enclave and the two rock types are separated by coarse unfoliated felsic material, NW of Tonota to the south of Shashe dam, strike and dip of foliation in megacrystic granite to the bottom right of photograpgh, 210 º/54º.
Fig. 2.18: Dyke of pink gneissic granite in the megacrystic granite gneiss concordant to foliation (S1) in megacrystic granite gneiss, south of Shashe Dam, strike and dip of foliation,
2.3.3 MEGACRYSTIC TONALITE GNEISS The unit is distinguished from megacrystic granite on the basis of grain composition and texture, in differing trends of structural fabric and spatial distribution in the SFT area.
The megacrystic tonalite gneiss texturally resembles megacrystic granite gneiss and comprises K-feldspar megacrysts set in a medium to coarse grained dark grey matrix of quartz, plagioclase feldspar, biotite and minor k-feldspar (Fig. 2.19). It is a homogenous, coarse grained foliated rock with about 5-30% by volume of K-feldspar megacrysts. The unit has more plagioclase feldspar than K-feldspar in the matrix as compared to megacrystic granite gneiss. However, horizons of megacrystic granite gneiss occur intercalated with megacrystic tonalite gneiss and likewise, sheets of megacrystic tonalite gneiss occur within main outcrops of megacrystic granite gneiss. Whereas megacrystic granite gneiss is widely distributed across the study area and dominated by NE-SW trending foliation (S1) fabric, megacrystic tonalite gneiss is restricted to a narrow zone in the north-eastern part of the study area named the Gulushabe shear zone and is dominated by E-W to WNW trending structural fabric. As in the megacrystic granite gneiss, in areas of higher strain the megacrysts are flattened perpendicular to foliation.
2.3.4: BANDED TONALITIC GNEISS The banded tonalitic gneiss occurs throughout the area but particularly in the NW part where it occurs intercalated with megacrystic granite gneiss along a zone of high strain (shear zone). It is best exposed at Thombane Hill, west of Makomoto settlement. The unit generally appears medium to dark grey in colour, medium to fine grained in texture, banded (Fig. 2.20) and felsic granitoid veins are boundinaged (Fig. 2.21). Mineralogy is dominated by biotite, plagioclase and quartz. The tonalitic gneiss is intruded by the megacrystic granite gneiss (as previously mentioned in section 2.3.2) and a felsic granitoid that is typically concordant with the foliation (S1).
Amphibolite bands are found enclosed by the tonalitic gneiss often occurring as boudinaged layers. The boudin neck is cut by a younger felsic dyke (Fig. 2.22A). The amphibolite bands are gneissic with the foliation (S1) in the amphibolite parallel to that in the tonalitic gneiss. Although both units are gneissic and highly strained, there is local evidence to suggest the tonalitic gneiss is intrusive into the amphibolite. This evidence
A B Figure 2.22 A: Boudinaged amphibolite with cross-cutting coarse felsic unit SW of Shashe Village, predominant foliation (S1) at this locality is orientated NNE-SSW; strike and dip, 212º/58°. B: Tonalitic gneiss discordant to foliation in amphibolite unit. Note the obvious cross-cutting relationship between the felsic material in the amphibolite enclave to the left of the hammer in B indicating intrusive relationship (amphibolite is older).
The banded tonalitic gneiss shows sheared contacts with the amphibolite and display pinch and swell structures (Fig. 2.24).
Figure 2.24: Sheared contact between banded tonalitic gneiss and amphibolite, indicated by a red dotted line.
Note thinning of the amphibolite horizon and pinch and swell structure to the left of hammer, 232º/60°.
Figure 2.25: Felsic bands lying both concordant and discordant to the foliation (S1) in tonalitic gneiss.
NB: Enclave of tonalitic gneiss on the right of photograph is enclosed by a white felsic material (product of partial melting). Tonalitic gneiss is older, southern part of Gulushabe structure. Strike and dip of foliation is (S1), 230º/40°.
2.3.5 PINK GNEISSIC GRANITE The pink gneissic granite is medium to coarse grained and homogeneous. It is exposed as dykes crosscutting the steeply dipping foliation (S1) in both the megacrystic granite gneiss unit (Fig. 2.26) and tonalitic gneiss (Fig. 2.27). This field relationship demonstrates that the pink gneissic granite is younger than both the megacrystic granite gneiss and the tonalitic gneiss. In addition based on evidence in Fig. 2.27, the tonalitic gneiss was migmatised and deformed prior to intrusion of the pink granite
Figure 2.26: A.
dyke of pink gneissic granite unit cutting across megacryrstic granite gneiss unit, E of Gulushabe structure. Strike and dip of foliation (S1) is 220º/66°. B: Steeply dipping dyke of pink gneissic granite in megacrystic granite gneiss. Photograph B was captured at locality B in Fig. 2.27 A.
Figure 2.27: Pink gneissic granite cutting deformation fabrics in banded tonalitic gneiss exposed SW of Tonota.
Strike and dip of foliation (S1) is 232º/60°.
68 2.3.6 TONALITE-TRONDHJEMITE GNEISS The tonalite-trondhjemite gneiss exposed in the NE part of the SFT area is an extension of the Tati Pluton (or SW Complex, Fig. 1.9, Bagai 2008). The Tati pluton comprises foliated to banded gneisse that flank the Tati greenstone belt. Only a small part exposed to the N of the Gulushabe fold structure was mapped during this study. The tonalitetrondhjemite gneiss displays a penetrative deformational fabric that has been linked to convergent tectonics along the SW margin of the Francistown Arc Complex (e.g.
McCourt et al., 2004; Kampunzu et al., 2003). The deformation fabric in the tonalitetrondhjemite gneiss trends NW-SE, parallel to the trend of the Tati greenstone belt. The intensity of the tectonic fabric increases towards the boundary with the Gulushabe shear zone. The common mineral assemblage of the tonalite-trondhjemite gneiss is biotite, plagioclase, alkali feldspar and quartz. The proportions of alkali feldspar to plagioclase are variable resulting in granitic to tonalitic compositions. This gneiss has not been investigated in detail during the current study. The reader is referred to Bagai (2008) and references therein for detailed geochemical, petrological and geochronological descriptions of the tonalite-trodhjemite gneiss.
2.4 DISCUSSION The metasedimentary rocks in the SFT area comprise quartzite/quartz- mica schist with amphibolite, marble, calc-silicate rocks and and minor graphite schist. In some places, the metasedimentary rocks are intercalated with the granitoid gneisses (Figs 2.7 and 2.8).
Foliation (S1) generally strikes NE to ENE and dip predominantly NW and WNW. The metasedimentary rocks in the adjacent Phikwe Complex (Limpopo Central Zone) to the east are characterized by abundant paragneisses and amphibolites with minor carbonates (Key et al., 1994; Aldiss, 1983. They are generally orientated ENE with varied dip directions (Carney et al., 1994; Aldiss,1991).The metasedimentary rocks occur interlayered with the paragneisses and the contacts are deformed (Carney eta., 1994;
Aldiss, 1991). Those in the Tati greenstone belt are enclosed by intrusive plutons and the contacts are tectonised. They are dominated by mafic and ultramafic metavolcanic rocks overlain by intermediate to felsic rocks (Bagai et al., 2002; Key et al., 1994). They general show NW to NNW strikes with steep dips (Aldiss, 1991). Based on lithology the metasedimentary rocks in the SFT may be correlated with those of the Phikwe Complex.
69 Within the study area, the megacrystic granite gneiss is nowhere seen to cut across the foliation (S1) in the banded tonalitic gneiss thus the relative age of the deformation fabrics in these two units is unknown. The lack of this evidence could suggest that the protolith to the megacrystic granite gneiss (presumably porphyritic granite) intruded into tonalitic gneiss and then was deformed implying two tectonometamorphic events separated in time by intrusion of the protolith to the megacrystic granite gneiss. These could also imply that porphyritic granite intruded into tonalitic (the protolith to the tonalitic gneiss) and then they were deformed and metamorphosed together but the age relationship was preserved. This would indicate one tectonometamorphic event with the age of the “porphyritic granite” being the maximum age of that event.