Thermal Metamorphism

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Geochronological evidence indicates that an early crustal component of the granitoids of the core of the Vredefort Dome was formed around 3.4 Ga ago (our group, unpublished SHRIMP U-Pb zircon data; Lana 2004). Major granitoid formation and high-grade (amphibolite or - in the innermost zone of the core - granulite facies) metamorphism occurred then between 3.2 and 3.07 Ga (Hart et al. 1999; Moser et al. 2001; Gibson and Reimold 2001b; Lana et al. 2003a-d). The next metamorphic stage in the evolution of this terrane was a thermal metamorphic overprint that has been described by a number of workers (e.g., Phillips and Law 2000, and references therein) from the Witwatersrand conglomerates. It is characterised by a mineral assemblage characteristic of lower greenschist facies metamorphism, with chloritoid as indicator mineral, corresponding to maximum thermal conditions of about 350 oC. Some researchers have speculated whether this event could be related to Ventersdorp magmatism at 2.7 Ga (Barnicoat et al. 1997), Transvaal basin development at 2.15-2.6 Ga (e.g., Robb et al. 1997), or to the Bushveld magmatic event at 2.06 Ga, prior to the impact event (Gibson and Wallmach 1995; Frimmel 1997a,b; Robb et al. 1997). Gibson and Wallmach (1995) and Stevens et al. (1997) discussed whether the pre-impact peak metamorphic grade was reached due to regional overprint related to the emplacement of the massive Bushveld magmas at 2.06 Ga ago, some 50 Ma prior to the impact event. That lower greenschist metamorphism regionally overprinted the impact-related pseudotachylitic breccias has been established through studies of such breccias from the outer collar of the Vredefort Dome and from the gold fields in the outer parts of the Potchefstroom Synclinorium (Foya 2002; Reimold et al. 1999b; see also section 4.3.).

The impact event at 2.02 Ga ago, whereby the rocks now exposed in the central core experienced local shock melting and widespread high to moderate degrees of shock metamorphism (Gibson and Reimold 1999, 2000, 2001b; Gibson 2001; Gibson et al. 2002), represents the next metamorphic phase for this region. Gibson (1996) and Gibson et al. (1997, 1998) established that the pseudotachylitic breccias in the Dome experienced post-impact, high-grade metamorphic overprint that marks a roughly radial temperature decrease from > 1000 oC in the center of the dome to 300 oC at its margin. In the wider region of the Witwatersrand Basin, two thermal metamorphic events can be recognized, being separated by the formation of pseudotachylitic breccia (e.g., Reimold et al. 1999b). However, instead of being represented by amphibolite facies grade, these two phases of metamorphism attained only lower greenschist facies grade. The pre-impact stage is characterised by chloritoid as the peak metamorphic (ca. 350 oC) indicator mineral, whereas the post-impact metamorphic assemblage is characterised by chlorite-dominated parageneses indicative of slightly lower metamorphic temperatures of about 300-330oC (Frimmel 1997a,b; Frimmel and Gartz 1997; Frimmel and Minter 2002; Gibson and Reimold 1999; Foya et al. 2000; Foya 2002). The post-impact metamorphism decreases in intensity outwards from the Vredefort Dome, from lower amphibolite (500-525 oC/0.3 GPa) grade in the inner parts of the collar to lower greenschist grade (300 oC/for an assumed pressure of 0.25 GPa - based on lithostratigraphic thicknesses) in the area of the gold fields.

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