Chapter 5 a Precise Zircon SHRIMP Age for the Post-Tectonic Entabeni Granite of the Limpopo Metamorphic Belt: Implications For
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The age of the Entabeni Granite Chapter 5 A Precise Zircon SHRIMP Age for the Post-Tectonic Entabeni Granite of the Limpopo Metamorphic Belt: Implications for the Age of the Soutpansberg Group. 5.1 Introduction The Entabeni Granite is an unfoliated granite that intrudes the central zone of the Limpopo Belt (Fig 5.1)(Barton et al., 1995). This granite is unconformably overlain by red beds and basaltic volcanic rocks of the Soutpansberg Group (Fig 5.1). The relative age of the Soutpansberg and Waterberg Groups has long been a matter of debate (Cheney et al., 1990; Bumby, 2000). During this study, two precise U-Pb zircon ages of ca. 2053Ma were obtained for quartz porphyry lava flows within the Lower Waterberg Group (chapter 4). Whole rock Rb-Sr ages of 1749±104Ma and 1769±34Ma for hydrothermally altered lava flows and sills from the Sibasa Formation (Barton, 1979) and a Pb-Pb whole-rock age that define an imprecise secondary isochron of 1809 +263/- 317Ma for the same samples (Cheney et al., 1990) are the only available geochronological data for the Soutpansberg Group. It would thus appear, as if the basal part of the Soutpansberg Group is younger than the lower part of the Waterberg Group. However, because of the poor quality of the geochronological data available for the Soutpansberg Group, it was decided to obtain a precise age on the Entabeni granite, to provide an accurate maximum age limit for the Soutpansberg Group that unconformably rests on the Entabeni Granite. Earlier work on the Entabeni granite yielded a discordant (35%) 207Pb/206Pb zircon age of 1957±3Ma (Barton et al., 1995). During this study it was attempted to obtain a precise zircon SHRIMP age for the Entabeni granite, enabling differentiation between the relative ages of the Soutpansberg and Waterberg Groups. 79 The age of the Entabeni Granite 80 The age of the Entabeni Granite 5.2 Geological and Geochronological Constraints The Limpopo mobile belt, with which the Entabeni granite is associated (Fig 5.1), is situated between the Archean Kaapvaal (De Wit et al., 1992) and Zimbabwe (Kusky, 1998) cratons. The belt is represented by an approximately 250 km wide ENE trending granulite facies terrane (Van Reenen et al., 1992) subdivided in a Northern Marginal Zone (NMZ), Central Zone (CZ) and Southern Marginal Zone (SMZ)(Fig 5.2). Peak granulite facies metamorphism reached 850°C at 9kbar in the SMZ of the Limpopo Belt (Van Reenen et al., 1987). The Hout River Shear zone forms the border between the SMZ and the Kaapvaal craton to the south. The Palala shear zone defines the boundary between the SMZ and the CZ to the north. Further to the north the Triangle shear zone forms the boundary between the CZ and NMZ (Fig 5.2). The Umlali shear zone is the border between the NMZ and Zimbabwe craton (McCourt and Vearncombe, 1992, Kamber et al., 1995). The structure and metamorphism of the Limpopo belt suggests that it may have formed in a similar tectonic setting to the modern day Himalaya Orogen (Treloar et al., 1992). Aeromagnetic maps indicate the presence of a large arcuate structure along the northern and western margins of the Zimbabwe craton (Fig 5.2) that represents the ca. 2.0Ga Magondi Belt (Hilliard, 1999). Apparently, this structure continues around the southern boundary of the Zimbabwe craton into the Limpopo Belt (Fig 5.2). However, there is marked difference in the magnetic signature along the southern boundary of the Zimbawe craton, in the main outcrop area of the Limpopo belt, when compared to the structure along the northern and western boundaries of the Zimbawe craton, which include the westernmost extent of the CZ of the Limpopo Belt (Fig 5.2). The Entabeni granite intrudes the CZ of the Limpopo metamorphic belt north of the Ntabalala shear zone, which in turn is thought to be the eastern continuation of the Palala shear zone (Fig 5.1). The Entabeni granite is a small, undeformed, peraluminous two mica, alkali granite. Previous studies have suggested an anorogenic origin for this granite (Du Toit, 1979; Barton et al., 1995). Previously, the Entabeni granite was reported to have a discordant (more than 35% discordant) zircon U-Pb age of 1957±3Ma (Barton et 81 The age of the Entabeni Granite 82 The age of the Entabeni Granite al., 1995). For this study, the Entabeni granite was sampled in a fresh road cut at 23°02’11.9”S; 30°13’02.4”E near the Timbadola saw mill (Fig 5.1). 5.3 Zircon geochronology The zircon grains of the Entabeni granite are mostly elongated, euhedral and display homogenous as well as oscillatory zoned internal structures (Fig 5.3). One of the grains was more than 500micron in length (Fig 5.3). Eighteen analyses were performed on 16 different grains (Table 5.1). Core and rim ages for the two grains that were analysed in two spots are within error of one another (Table 5.1). Most of the grains have an U content of 200-400ppm, unlike the grains that were analysed by Barton et al. (1995) that were reported to contain more than 800ppm of U. Of the 18 analyses, 16 yielded nearly concordant (within 6% discordancy) ages. In combination they give a precise 207Pb/206Pb age of 2021±5Ma (Table 5.1, Fig 5.4). This is regarded as the best estimate for the crystallization age of the Entabeni Granite. 5.4 Discussion A precise crystallization age of 2021±5Ma for the Entabeni granite provides a maximum age for the unconformably overlying Soutpansberg Group. This is an important finding because it illustrates that the basal Tshifhefhe Formation of the Soutpansberg Group (Fig 5.5)(SACS, 1980) is at least 35Ma younger than the lower part of the Waterberg Group (Fig 5.5). Quartz porphyritic lavas in the lower Swaershoek Formation of the Waterberg Group yielded ages of 2054±4Ma and 2051±8Ma (see Chapter 4, this thesis). Earlier whole rock Rb-Sr ages of 1749±104Ma and 1769±34Ma for hydrothermally altered lava flows and sills from the Sibasa Formation (Fig 5.5)(Barton, 1979) and Pb-Pb whole-rock data age 1809 +263/-317Ma for the same lava samples (Cheney et al., 1990) thus most probably correctly estimate post-depositional alteration of the rocks. Ages similar to those of the Sibasa Formation have been reported for dykes intruding the Southern Mariginal Zone of the Limpopo belt and NW trending dykes in the Barberton region. Barton (1979) reported Rb/Sr ages of 1899±107Ma and 1876±68Ma for the Sand River dykes that intrude the Southern Marginal Zones of the Limpopo belt. Layer et al. (1998) reported 40Ar/39Ar ages of 1902±6Ma and 1876±6Ma for NW trending dykes in the 83 The age of the Entabeni Granite 84 Table 5.1 Summary of SHRIMP U-Pb data for zircons from the Entabeni granite. (1) (1) (1) % Ppm ppm ppm 206Pb/238U 207Pb/206Pb % (1) (1) (1) 206 232 238 206 207 * 206 * 207 * 235 206 * 238 Spot Pbc U Th Th/ U Pb* Age Age Discordant Pb / Pb ±% Pb / U ±% Pb / U ±% errcorr 1.1 0.09 214 105 0.51 66.6 1,992 ±20 2,008.3 ± 8.2 1 0.12356 0.46 6.170 1.3 0.3621 1.2 .931 1.2 0.09 244 125 0.53 77.7 2,030 ±23 2,004.3 ± 9.0 -1 0.12328 0.51 6.290 1.4 0.3700 1.3 .935 2.1 0.06 237 113 0.49 75.3 2,030 ±23 2,024.6 ± 7.2 0 0.12470 0.41 6.363 1.4 0.3700 1.3 .956 3.1 10.01 3091 1029 0.34 271 565.7 ± 3.2 1,605 ±50 65 0.0990 2.7 1.252 2.7 0.09172 0.59 .215 4.1 0.05 226 125 0.57 70.8 2,001 ±13 2,020.0 ± 7.5 1 0.12438 0.42 6.241 0.87 0.3639 0.76 .873 5.1 8.38 809 580 0.74 241 1,776 ±11 2,070 ±86 14 0.1280 4.9 5.60 4.9 0.3171 0.74 .150 6.1 0.61 441 342 0.80 130 1,888 ±12 2,015.4 ± 9.3 6 0.12406 0.53 5.822 0.92 0.3404 0.75 .820 7.1 0.10 382 247 0.67 116 1,953 ±13 2,019.8 ± 7.0 3 0.12436 0.39 6.068 0.89 0.3539 0.79 .897 8.1 3.45 325 176 0.56 109 2,063 ±25 2,065 ±44 0 0.1276 2.5 6.63 2.9 0.3771 1.4 .492 9.1 0.01 335 205 0.63 104 1,987 ±12 2,031.8 ± 6.7 2 0.12521 0.38 6.234 0.78 0.3611 0.69 .874 10.1 0.03 256 170 0.69 79.6 1,991 ±12 2,029.2 ± 8.1 2 0.12503 0.46 6.239 0.83 0.3619 0.70 .835 11.1 0.71 341 300 0.91 101 1,902 ±11 2,015 ±11 6 0.12401 0.61 5.868 0.91 0.3432 0.68 .740 12.1 0.16 394 254 0.67 125 2,021 ±13 2,028.3 ± 7.9 0 0.12496 0.44 6.345 0.85 0.3682 0.72 .853 13.1 0.06 225 121 0.56 69.0 1,965 ±15 2,008.6 ± 9.0 2 0.12358 0.51 6.072 1.0 0.3564 0.90 .869 14.1 0.04 243 138 0.59 75.5 1,986 ±19 2,017.3 ± 8.4 2 0.12419 0.47 6.177 1.2 0.3607 1.1 .919 15.1 0.20 378 223 0.61 120 2,021 ±16 2,008.1 ± 9.5 -1 0.12355 0.54 6.273 1.1 0.3683 0.91 .861 15.2 0.17 290 163 0.58 92.0 2,021 ±21 2,005.5 ± 8.4 -1 0.12336 0.47 6.264 1.3 0.3682 1.2 .932 16.1 5.25 932 607 0.67 300 1,959 ±11 1,989 ±40 2 0.1222 2.2 5.98 2.3 0.3551 0.67 .290 * Errors are 1-sigma; Pbc and Pb indicate the common and radiogenic portions, respectively.