Other
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27..300t&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 300
Other
Scientific paper
The formation of the Vredefort dome, a structure more than 100 km diameter in the center of the Witwatersrand basin, is still the subject of lively controversy. It is widely accepted that it was formed in a single sudden event, accompanied by the release of gigantic quantities of energy. It is, however, debated if this central event was a cryptoexplosion triggered by internal processes, or an impact of an extraterrestrial body. Ages obtained on rocks from the Vredefort structure cluster at 2.0 Ga (cf. Walraven et al., 1990). Granophyre, a unusual melt rock forming dykes in the Vredefort dome and thought to be related to the Vredefort event, has a Pb-Pb zircon age of 2.002 +- 0.052 Ga (Compston and Nicolaysen, 1990). Pseudotachylite, a melt breccia first discovered and extremely abundant in the Vredefort structure, like granophyre is widely regarded as a result of the dome forming process. We dated eight pseudotachylite samples from a bedding-parallel fault zone across the north-central Witwatersrand basin with the ^40Ar-^39Ar stepheating technique. Six samples have well-defined age plateau at 2.00 Ga (EL-19, EL-19B, VCR/PT and 10D300: 1.99 Ga; EL-27: 2.00 Ga; EL-28B: 2.02 Ga, errors +- 0.01 Ga). The initial apparent ages of another sample, 10D05, exceed 2.0 Ga and are accompanied by low K/Ca ratios. This feature is followed by more normal K/Ca ratios and a plateau age of 2.03 Ga. Sample EL- 30 is extremely K-poor (20 ppm) and thus largely contaminated by excess Ar (K-Ar age: 4.7 Ga). Because of the well developed plateau of a rather large number of Witwatersrand samples which yield identical ages, we do not believe that the 2.0 Ga ages are reset ages caused by thermal overprinting. The ages are consistent with the ^40Ar-^39Ar age of a Vredefort pseudotachylite (USA29, 2.00 Ga (Reimold et al., 1990a)). Therefore the Vredefort event and the Witwatersrand pseudotachylite are related--at least in time. Taking all our results into account, Witwatersrand pseudotachylite formation occurred 1.997 +- 0.005 Ga ago (0.019 Ga if the hornblende standard uncertainty is included). This is a constraint for the Vredefort event, if it caused pseudotachylite formation. While these results are consistent with a single-stage formation of the dome, previous ^40Ar-^39Ar studies of pseudotachylites from other localities in and around the dome yielded (for four out of six specimens) ages significantly below 2.0 Ga (Reimold et al., 1990a). Such recent formation ages would even question the single stage formation of the Vredefort dome and the associated pseudotachylites (Reimold et al., 1990a). ^40Ar-^39Ar ages lower than 2.0 Ga are not restricted to pseudotachylite either: pseudotachylite host rocks (Reimold et al, 1990b) and rocks from the Lindequesdrift intrusion (Hargraves, 1987) show post-2.0 Ga events, mineral separates of rocks from the Rietfontein complex suggest disturbances 1.17 Ga ago (Allsopp et al., 1991). To ascribe to thermal overprinting the widely different ages appears to be problematic because the local distribution of these rocks requires widely different temperature histories at different, but close localities within the dome. To avoid this dilemma Reimold et al. (1990a) proposed several pseudotachylite forming events at Vredefort. In this case it should be clear why there are no pseudotachylites older than 2 Ga. Future investigations are still needed to finally clarify the nature and the duration of post 2.0 Ga processes within the Vredefort structure. Acknowledgement: We gratefully acknowledge the generous support from Anglo American Prospecting Services that permitted the analysis of Witwatersrand pseudotachylites, and the permission to report the results. References: Allsopp, H.L., Fitch, F.J., Miller, J.A., and Reimold, W.U. (1991) Suid afrikaanse Tydskrif vir wetenskap 87. 431-442. Compston, W. and Nicolaysen, L.O., in Walraven et al. (1990), unpubl. Hargraves, R.B. (1987) S. Afr. J. Geol. 90. 305- 313. Reimold, W.U., Jessberger, E.K., and Stephan, T. (1990a) Tectonophysics 171. 139-152. Reimold, W.U., Stephan, T., and Jessberger, E.K. (1990b) VII. Int. Conf. on Geochronol. Cosmochronol. and Isot. Geol., Canberra, Geol. Soc. Austr. Abstr. No. 27, p. 82. Walraven, F., Armstrong, R.A., and Kruger, F.J. (1990) Tectonophysics 171.23-48.
Boer Rudolf H.
Jackson Megan C.
Jessberger Elmar K.
Kunz Jacob
Reimold Wolf Uwe
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