SHRIMP dating of titanite from metasyenites in the Central Zone of the Limpopo Belt, South Africa

Abstract

SHRIMP dating of titanite from metasyenites in the Central Zone of the Limpopo Belt yields a mean 207 Pb/ 206 Pb age of 2010.3 +/-4.5 Ma calculated from twenty-three analyses. This age, combined with petrographic and field observations, suggests the metamorphism in the syenites occurred during the regional Palaeoproterozoic event.Keywords: Limpopo Belt; SHRIMP; titanite; geochronology; Palaeoproterozoic. IntroductionA unique body of (meta)-syenite intrudes the Alldays Gneiss and outcrops in an elongate, 6.25 x 1.25 km body WNW of Alldays within the Central Zone of the Limpopo Belt, South Africa (Fig 1a + b). A recent study by Rigby et al. (2008a) reveals the metasyenites underwent a metamorphic evolution characterized by a maximum pressure of 7-8 kbar and ~770 o C. The subsequent retrograde path involved a simultaneous P-T decrease along a decompression-cooling path to 4 kbar and ~550 o C. These P-T estimates are 'intermediate' between the high-grade conditions reported by Zeh et al. (2004) and Rigby (2009) for metapelites near Messina and the amphibolite-facies conditions reported by Zeh et al. (2005a,b) and Chudy et al. (2008) for rocks from the Venetia area. Collectively, this data led Rigby et al. (2008a) to postulate the existence of a metamorphic field gradient. However, there are two main tectonometamorphic episodes known in the Central Zone of Limpopo Belt, one in the Neoarchean (e.g. McCourt & Armstrong, 1998; Kroner et al., 1998;Bumby et al., 2001;Bumby & van der Merwe, 2004; Zeh et al., 2007; Millonig et al., 2008; Perchuk et al., 2008; Van Reenen et al., 2008; Gerdes & Zeh, 2009; Zeh et al. 2009) and one in the Palaeoproterozoic (e.g. Jaeckel et al., 1997; Holzer et al., 1998; Kroner et al., 1999; Zeh et al., 2004; Rigby et al., 2008b;Chudy et al., 2008;Eriksson et al., 2009; Gerdes & Zeh, 2009; Rigby et al., 2010; Eriksson et al., 2010a and b; Millonig et al., 2010) and without robust geochronological constraints it is not clear in which event the metasyenites were metamorphosed. Additionally, due to this uncertainty, the existence of the proposed metamorphic field gradient by Rigby et al (2008a) is, at present, mere speculation. In this short communication we present new U-Pb SHRIMP data obtained from titanite in the metasyenites to delineate their age and support the metamorphic field gradient hypothesis.

The volcano-sedimentary succession of the Archean Sodium Group, Ventersdorp Supergroup, South Africa: Volcanology, sedimentology and geochemistry

“…Our overview of the Ventersdorp Supergroup and the comparison to the Sodium Group between Prieska and Britstown demonstrate significant similarities between these volcanosedimentary successions, as already proposed by SACS (1980), van der Westhuizen et al (19871989 and is poorly constrained (Kramer et al, 2006;Rigby et al, 2008;Rigby and Armstrong, 2010;Bumby et al, in press). On the other hand, the mantle plume model as discussed by Hatton (1995), or Eriksson et al (2002), does not explains the bimodal volcanism and the reappearance of komatiites at the top of the Ventersdorp succession.…”

Meso-Archaean and Palaeo-Proterozoic sedimentary sequence stratigraphy of the Kaapvaal Craton

“…Boshoff et al 2006;van Reenen et al, 2008;Millonig et al, 2008) and a final ~2.03 Ga metamorphic overprint (e.g. Boshoff et al, 2006;Zeh et al, 2007;Perchuk et al, 2008;Rigby et al, 2008b;van Reenen et al, 2008;Rigby, 2009;Rigby & Armstrong, 2010). Conversely, the Southern Marginal Zone appears to have an Archaean-only history, with no evidence of 2.0 Ga metamorphism (Eriksson et al, in press).…”

Petrology and geochronology of sapphirine‐bearing granulites from the Limpopo Complex in eastern Botswana: Implications for Palaeoproterozoic long‐lived high‐pressure/ultrahigh‐temperature metamorphism and rapid exhumation

“…Zeh et al (2007) proposed that the initial Hafnium model ages between 3.01 and 3.18 Ga reveal that the Zanzibar Gneiss formed from the remelting of substantial amounts of an older Archean crust. Similar Mesoarchean (e.g., Hofmann et al, 1998; Kröner et al, 1999; Mouri et al, 2009; Xie, Kröner, Brandl, & Wan, 2017; Zeh et al, 2010), Neoarchean (e.g., Holzer, Frei, Barton Jr., & Kramers, 1998; Millonig et al, 2008; van Reenen et al, 2008; Xie et al, 2017; Zeh et al, 2007), and Palaeoproterozoic (e.g., Boshoff et al, 2006; Buick, Williams, Gibson, Cartwright, & Miller, 2003; Rajesh, Safonov, Belyanin, & Kramers, 2014; Rigby & Armstrong, 2011; Rigby, Basson, Kramers, Grἅser, & Mavimbela, 2011) thermal events have been reported from the BBC in South Africa.…”