«By MOLATLHEGI LARTY LOSTMAN MOSEKI STUDENT NO. 208523856 Submitted in fulfillment of the academic requirements For the degree of Master of Science In ...»
2.28) has an age of 2631.5 ± 4.4 Ma. The emplacement age of the pink granite is within error of the megacrystic granite gneiss suggesting they could relate to the same magmatic event. However, the pink gneissic granite occurs as dykes discordant to the foliation (S1) in the megacrystic granite gneiss indicating that the original porphyritic granite was metamorphosed and deformed (to produce the gneiss) prior to the emplacement of the pink granite. The crystallization ages of the granitoid gneisses ranges between 2724 ± 48 Ma and 2632 ± 4 Ma, spanning a period of about 93 million years. U-Pb zircon age of 2625 ± 16 Ma was obtained from mylonitic megacrystic granite gneiss sampled south of Tonota Village. This age (the age of the rock deformed in the shear zone) is interpreted to give the maximum age of shearing.
It is suggested that the metasedimentary belt in the SFT and Topisi area to the south, form part of the same supracrustal assemblage. This is based on their spatial distribution as well as their lithological similarity. Detrital zircon grains from a quartzite sampled in the Topisi area indicate the maximum age of deposition of these rocks was 2661± 8 Ma 148 (McCourt et al., 2004) and this age also helps constrain the timing of supracrustal deposition in the SFT area. The metasedimentary sequences within the MatsitamaMotloutse Complex were deposited between 2661 and 2647 Ma with the provenance being the Francistown Arc Complex (also Francistown Granite Greenstone Complex) and the regions behind it (McCourt et al., 2004). The metasedimentary rocks and the granitoid gneiss have the same deformation fabrics. These rocks are dominated by NESW orientated foliation (S1) that dips steeply to the NW. The foliation (S1) is cut by the pink gneissic granite (Fig. 2.27 and 2.28) thus deformation of the metasedimentary rocks and the granitoid gneiss occurred before intrusion of the protolith to the pink gneissic granite at 2631.5 ± 4.4 Ma. This age combined with the detrital zircon data from the Topisi area indicates that the deformation event responsible for the regional foliation (S1) in the SFT area occurred between 2661 Ma and 2632 Ma, a duration of about 30 million years.
Bagai et al. (2002) constrained the age of TTG magmatism and volcanism in the Vumba granite-greenstone terrane to between 2696 ± 4 Ma and 2647 ± 4 Ma. This age range is similar to that of Majaule and Davis (1998) of 2710 ± 10 Ma to 2646 ± 3 on tonalitic gneisses from the Mosetse area. The granitoid magmatism associated with the Neoarchaean evolution of the SFT area is thus constrained between 2724 ± 48 Ma and 2632 ± 4 Ma i.e. the same age range ( Fig. 4.11) with those obtained by previous work in the Mosetse (Majaule and Davis, 1998) and Phikwe (B.K. Paya, pers.com. 2010, unpublished research data) areas. Figure 4.11 shows that the age of the pink gneissic granite (2631.5 ± 4.4 Ma) is within error of the age of the megacrystic granite gneiss whereas the age of the tonalitic gneiss (2699 ± 9 Ma) is within error of the Tonota biotite gneiss (2724 ± 48 Ma) although the latter is poorly constrained. Coeval or slightly older ages in the range 2710 ± 19 to 2639.7 ± 7.9 Ma were obtained from igneous rocks from the Tati and Vumba greenstone belts (Bagai, 2002, Kampunzu et al., 2003, Bagai, 2008) of the Francistown Complex suggesting they could relate to the same magmatic event.
Bagai (2008) obtained a U-Pb zircon age of 2630 ± 4.7 Ma for both the leucosome component of the Shashe migmatitic gneiss and the protolith to the Shashe augen gneiss collected within the Shashe River bed i.e. along the tectonic boundary between the SFT 149 and the Tati granitoid greenstone belt. This places a maximum age on the deformation responsible for the augen gneiss and constrains collision/accretion along this boundary to around 2630 Ma (Bagai, 2008). The tectonic fabric in the megacrystic granite gneiss is cut by the pink gneissic granite which was emplaced at 2630 ± 5 Ma. Deformation and metamorphism in the SFT area is indistinguishable in age to the rocks dated by Bagai (2008).
Figure 4.11: A comparison of age data obtained from terranes in NE Botswana.
Note, only minimum and maximum emplacement ages from the different authors were selected.
Holzer et al. (1999) data included. The oldest zircon grain from the Tonota biotite gneiss at 2978 ± 7Ma has not been plotted as it is interpreted as an inherited grain.
150 CHAPTER 5. STABLE ISOTOPE GEOCHEMISTRY
5.1 RATIONALE FOR STUDY In this section new carbon and oxygen isotope data is presented for metacarbonate rocks from the SFT, Topisi, Monatshane and the Matsitama areas. At the end of this section the stable isotope signature of the samples from the SFT area are compared with those from Topisi, Monatshane and Matsitama areas in NE Botswana as well as those published for the Gumbu Group carbonates in the Musina area, eastern part of the Central Zone of the 13 Limpopo belt in South Africa. The Gumbu Group carbonates have high C values taken to suggest that the carbonates are younger than previously believed (i.e.
Palaeoproterozoic and not Archaean). Buick et al. (2003) obtained U-Pb dates on detrital zircon that were interpreted to indicate that the Gumbu Group is Palaeoproterozoic in age. These authors also demonstrated that the Gumbu Group metacarbonates have a 13 similar C signature to that recorded in the Duitschland Formation carbonates of the 13 Transvaal Supergroup and considered the high C to be diagnostic of Palaeoproterozoic and therefore may be used as a proxy for age. According to Master et al. (2010) the high 13 C values record a sea water signature and therefore reflect the carbonate composition of open ocean at 2.2-2.1 Ga. The composition of carbonate sea water changed at 2.2-2.1 Ga, prior to plate reorganization at 2.1-2.0 Ga (Master et al., 2010). The reorganization affected ocean circulation and redox thus availing conditions suitable for burial of organic matter. Previous geochronological studies and work presented thus far in this dissertation show that the metasedimentary rocks of the SFT area are late Archaean in age. Therefore stable isotope analysis of metacarbonates and calc-silicates rocks in the SFT area and equivalent units in adjacent areas was undertaken to establish whether a 13 high C is present in these rocks. If so, it would argue against the suggestion that such values are confined to the Palaeoproterozoic.
1515.2 SAMPLE LOCALITIES
A total of 27 samples of metacarbonate rocks collected from 22 sites in the SFT, Monatshane, Topisi and Matsitama areas were chosen for carbon and oxygen isotope analyses (Tables 5.1, 5.2 and 5.3). Their locations are shown in Table 5.1 and Figure 5.1.
Those in the SFT area are also shown on the dissertation map. At these localities the most common rock type is marble and both calcite and dolomite varieties are present (Tables
5.2 and 5.3). Samples were cut to remove weathered surfaces if necessary. They were sent to the Stable Isotope for Innovative Research (SIFIR) laboratory at the University of Manitoba, where they were analysed by Dr. Andrey Bekker. The descriptions in Appendix 2 are from his report.
Table 5.1: Sample number, location and GPS coordinates for samples analysed for carbon and oxygen isotope values.
5.4 DISCUSSION The metacarbonates yielded δ13C (Cc) and δ18O (Cc) values in the range 0.9‰ to +14‰ (relative to PDB) and +12.6 to +24.8‰ (relative to SMOW), respectively. The dolomite minerals have some δ13C (Cc) values slightly higher and some in the same range (overlapping) with those obtained from ferroan and non-ferroan calcite. The data for sample MLM 255 is anomalous (-0.7‰0) and it is not discussed further. from the Gulushabe areaThe δ13C values of carbonates from the SFT area (Gulushabe structure), show no significant difference compared to those from Matsitama (MLM 286), Monatshane (MLM 443) and Topisi (MLM 261) areas suggesting that they may be part of the same package.
There is a previous contention that the sedimentary protoliths to supracrustal rocks in the Central Zone were deposited at 3.3 Ga (e.g. Barton 1983, Kröner et al., 1999) but SHRIMP U-Pb detrital ages obtained on detrital zircon grains from metapelite and metapsammite samples from the Gumbu Group are no older than 2.68 Ga and possibly as young as 2.2 Ga (Buick et al., 2003). Buick et al. (2003) also report high δ13C and δ18O stable isotope data from the Gumbu Group carbonates. A plot of δ13C vs δ18O of carbonates from the present study yielded a pattern comparable to that obtained for carbonate data from the Gumbu Group (Fig. 5.2 A and B). In Figure 5.2 A, 12 samples 13 have C values less than or equal to 6 and are thus directly comparable with the samples from the Gumbu Group shown on Fig. 5.2 B. Eight of the 12 samples in Fig. 5.2
Figure 5.2 A: Plot of stable isotope data from SFT, Topisi, Matsitama and Monatshane areas showing variation between δ13C and δ18O.
NB: the unnamed data points are from the SFT area.
What is apparent however is that the majority of the samples analysed in the present 13 18 O values (4.8‰ to +14‰ and +12.6 to +24.8‰) than study have higher C and those documented by Buick et al. (2003) from the Gumbu Group (+2.3‰ to +7‰ and +10 to +19.2‰) but the significance of this requires further research. The sampled localities in the Gumbu Group are underlain by metasedimentary rocks dominated by quartzite, marble and calc-silicate with minor metapelites and amphibolite suggesting the protoliths of these rocks are the same as those in the SFT, Matsitama and Monatshane areas.
The maximum age of deposition of 2661 ± 8 Ma reported by McCourt et al. (2004) for quartzite from Topisi area (Matsitama-Motloutse Complex) is within error of the maximum age of 2680 Ma reported by Buick et al (2003) for the deposition of metapsammite from the Gumbu Group. This suggests that the maximum age of 157 deposition of the metasedimentary rocks in the current study is coeval with the maximum age of deposition of the protoliths to the Gumbu Group.
The similarity between stable isotope and detrital zircon ages obtained from the Gumbu Group rocks and those from the SFT, Matsitama and Monatshane areas suggest that the metasedimentary belts may represent parts of the same sequence possibly related to the same geodynamic cycle. Buick et al. (2003) concluded that the supracrustal rocks in the Central Zone of the Limpopo belt are not exclusively Archaean in age but that a younger Palaeoproterozoic sequence is also present.
In the SFT area:
1. Late Archaean U-Pb zircon ages were obtained from granitoid gneisses from the SFT area.
2. The U-Pb zircon age determinations show that deformation and metamorphism of the SFT metasedimentary rocks and the granitoids occurred in the Late Archaean. Deformation in the metasedimentary sequence and the granitoids rocks predates 2631 Ma (pink gneissic granite) since S1 foliation in the pink gneissic granite can be traced into the metasedimentary rocks.
3. Whereas the Gumbu Group metacarbonates may be Palaeoproterozoic in age, (Buick et al. 2003) those of the SFT area are Late Archaean in age.
6.1 CORRELATION OF THE SFT AREA WITH ADJACENT TERRANESAvailable geochronological data show that the Limpopo belt is a product of Neoarchaean orogenic event/orogeny and the Central Zone (CZ) was overprinted by a Palaeoproterozoic tectono-metamorphic event (McCourt and Armstrong 1998; Kramers et al., 2011)). There is no evidence for a ~2.0 Ga event in the Vumba granite-greenstone terrane (Bagai et al. 2002), the Francistown Complex (Bagai, 2008) or the Mosete Complex (Majaule and Davis, 1998). However, a zircon grain (analysis # 1 in Table 4.6) from sample MLM-SRP5 yielded an age of 2058 ± 11Ma, suggesting presence of a Palaeproterozoic component in the SFT area. Support for this is comes from Zeh et al.
(2009), who found Palaeoproterozoic zircon overgrowth in a sample from Motloutse Complex (sample SP1; Table 1.7).
Lithologically, there is no significant difference between supracustal rocks from the Mosetse Complex (Matsitama belt), the Motloutse Complex and the Phikwe Complex.
They are characterised by a carbonate-metapelite-quartzite association interlayered with granitoid gneisses. There is a general contention that the protoliths to the sedimentary rocks in the Central Zone of the Limpopo belt were deposited at ~3.3 Ga (e.g. Barton and Sergeev, 1997; Kroner et al., 1999). Supracrustal rocks in the Motloutse Complex (SFT) are younger (2661 Ga) than those in the Central Zone of the Limpopo belt and the Francistown Arc Complex (McCourt et al., 2004). McCourt et al. (2004) suggested that the source area for the metasedimentary rocks in the Matsitama and Motloutse areas was the Francistown Arc Complex.
The Zimbabwe craton is different to the other terranes in that the supracrustal rocks are dominated by greenstone belts lithologies, metavolcanics and minor metasedimentary rocks. Whereas rocks in the Zimbabwe craton are at greenschsist to amphibolite facies metamorphism, the Motloutse Complex was metamorphosed at upper amphibolite facies (Tables 1.3 and 6.1). Peak metamorphism in the Phikwe Complex occurred under granulite facies conditions but there has been widespread retrogression to upper
Table 6.1: Compilation of data for type of supracrustal assemblages, age of clastic sedimentation, age range of magmatism and grade of metamorphism in the SFT area relative to the Francistown, Phikwe and Mosetse Complexes.
Previous accounts of the structural history of the NE Botswana document 4 deformation events: D1 is linked to the pre-granitoid folding that caused inversion of the Tati and Matsitama belts (Litherland, 1975); D2 is the event responsible for the regional foliation;
and D3 and D4 are events that deform the regional foliation and associated structures with D4 being the ENE striking shear zones along the boundary between the Motloutse Complex and the Phikwe Complex i.e. the Magagophate, Molabe and Lepokole shear zones (Paya, 1996).