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Petrogenesis of low-δ18O quartz porphyry dykes, Koegel Fontein complex, South Africa

  • Chris Harris
  • Kwenidyn Mulder
  • Saheli Sarkar
  • Benjamin Whitehead
  • Sherissa Roopnarain
Original Paper

Abstract

This paper investigates the origin of low-δ18O quartz porphyry dykes associated with the 144–133 Ma Koegel Fontein Igneous Complex, which was intruded during the initial phase of breakup of Africa and South America. The 25-km diameter Rietpoort Granite is the largest and youngest phase of activity, and is roofed by a 10-km diameter pendant of gneiss. Quartz porphyry (QP) dykes, up to 15 m in width, strike NW–SE across the complex. The QP dykes that intruded outside the granite have similar quartz phenocryst δ18O values (average 8.0‰, ± 0.7, n = 33) to the granite (average 8.3 ± 1.0, n = 7). The QP dykes that intruded the roof pendant have quartz phenocrysts with more variable δ18O values (average 1.6‰, ± 2.1, n = 55). In some cases quartz phenocrysts have δ18O values as low as − 2.5‰. The variation in δ18O value within the quartz crystal population of individual dykes is small relative to the overall range, and core and rim material from individual quartz phenocrysts in three samples are identical within error. There is no evidence that quartz phenocryst δ18O values have been affected by fluid–rock interaction. Based on a ∆quartz−magma value of 0.6‰, magma δ18O values must have been as low as − 3.1‰. Samples collected along the length of the two main QP dykes that traverse the roof pendant have quartz phenocryst δ18O values that range from + 1.1 to + 4.6‰, and − 2.3 to + 5.6‰, respectively. These δ18O values correlate negatively (r = − 0.96) with initial 87Sr/86Sr, which can be explained by the event that lowered δ18O values of the source being older than the dykes. We suggest that the QP dykes were fed by magma produced by partial melting of gneiss, which had been variably altered at high temperature by 18O-depleted meteoric water during global glaciation at ~ 550 Ma. The early melts had variable δ18O value but as melt pockets interconnected during melting, the δ18O values approached that of average gneiss. Variable quartz phenocryst δ18O values in the same dyke can be explained by vertical emplacement, at variable rates of ascent along the dyke. The lateral variation in quartz, and hence magma δ18O value at a particular point along a single dyke would depend on the rate of ascent of magma at that point along the dyke, and the ‘age’ of the particular magma batch.

Keywords

Koegel Fontein Low-δ18O magma Quartz porphyry dyke O-isotope heterogeneity 

Notes

Acknowledgements

We thank the National Research Foundation for funding this project through Grants to CH. We also thank Fayrooza Rawoot, Petrus le Roux, Kerryn Gray, Phil Janney, Jonathan van Rooyen and Christel Tinguely for their assistance in preparing and analysing samples. We are very grateful to Coenie de Beer for sharing his knowledge of Koegel Fontein, provision of maps and samples, and for introducing CH to the area. Albie and Adèle Pool are thanked for their hospitality, and we are grateful to them and to Braam van Niekerk for giving us access to their land. George Smith and Camille Olianti assisted with fieldwork, and Juliana Troch made many helpful comments on a preliminary version of the manuscript. Reviews by Jade Star Lackey, Ben Ellis, Ilya Bindeman and David Zakharov helped to sharpen our interpretations and presentation.

Supplementary material

410_2018_1457_MOESM1_ESM.pdf (588 kb)
Supplementary material 1 (PDF 588 KB)
410_2018_1457_MOESM2_ESM.xls (81 kb)
Supplementary material 2 (XLS 81 KB)

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Authors and Affiliations

  1. 1.Department of Geological SciencesUniversity of Cape TownRondeboschSouth Africa

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