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| Titel |
The Daskop Granophyre Dyke: Inhomogeneous clast distribution and chemistry |
| VerfasserIn |
Elizaveta Kovaleva, Matthew S. Huber, Andrew Somers, Stuart Bateman |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250140387
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| Publikation (Nr.) |
 EGU/EGU2017-3768.pdf |
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| Zusammenfassung |
| The Vredefort Granophyre is present in the central basement of the Vredefort impact structure as a set of dykes up to 9 km long and up to 65 m wide and is considered to be the remnant of the impact melt sheet (e.g. French et al. 1989; French and Nielsen 1990). The dykes intruded into the floor of the structure’s core during the crater modification and settling stages (e.g. Therriault et al. 1996). Granophyre is typically considered a well-homogenized and uniform melt (e.g., Nel 1927; Gibson and Reimold 2008). This study presents new insights into the chemical variety and inhomogeneous clast distribution of the Vredefort granophyre.
The Granophyre dyke on the farm Daskop is located in the core of the impact structure and hosted by granitic gneiss of the Archean basement. The clast distribution was mapped in the eastern half of the dyke. Additionally, non-destructive geochemical methods (handheld µXRF and LIBS systems) were used to obtain chemical analysis of the dyke along strike.
The map of clast distribution in the granophyre dyke reveals an inhomogeneous content of clasts, with a consistently higher concentration of clasts along the southern contact. This distribution suggests that either 1) the dyke orientation is non-vertical, allowing gravitational settling to affect the distribution of the clasts after the dyke intruded; or 2) that clasts were preferentially entrained along the southern margin of the dyke. Clast frequency also differs along strike. Many elongated clasts are oriented parallel to the dyke walls, indicating flow. We have also documented linear structures resembling flow channels. These structures are strictly parallel to the dyke walls and have a finer texture than the host granophyre. These may represent differentiation of the melt during crystallization.
Chemical inhomogeneity of granophyre dyke has also been documented along strike. Such chemical variation may reflect local differences in the relative amounts of target rocks incorporated into the melt (e.g. French and Nielsen 1990).
References
French B.M., Nielsen R.L. (1990) Vredefort bronzite granophyre: chemical evidence for origin as a meteorite impact melt. Tectonophysics 171:119–138.
French B.M., Orth C.J., Quintana L.R. (1989) Iridium in the Vredefort Bronzite Granophyre - Impact melting and limits on a possible extraterrestrial component. Proceedings, 19th Lunar and Planetary Science Conference. pp.733–744.
Gibson R.L., Reimold W.U. (2008) Geology of the Vredefort impact structure, a guide to sites of interest. Pretoria: Council for Geoscience. 181 p.
Nel L.T. (1927) The geology of the country around Vredefort – An explanation of the geological map. Pretoria: South Africa Geological Survey. 134 p.
Therriault A.M., Reimold W.U., Reid A.M. (1996) Field relations and petrography of the Vredefort Granophyre. South African Journal of Geology 99:1-21. |
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