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f O- and H-isotope Study of the Cretaceous Koegel Fontein Complex
- Publisher: European Association of Geoscientists & Engineers
- Source: Conference Proceedings, 11th SAGA Biennial Technical Meeting and Exhibition, Sep 2009, cp-241-00116
Abstract
Koegel Fontein is a 30 km diameter anorogenic igneous complex that intruded Namaqualand gneisses during the initial phase of break-up of Africa and South America. It is notable for the presence of quartz porphyry dykes that formed from low-δ18O magmas. Many of the igneous rocks show petrographic evidence for alteration and have had their δ18O values lowered by fluid-rock interaction at high temperatures. Partial melting and/or assimilation of this material produced the low- δ18O magmas. Whole-rock δD and δ18O values indicate equilibrium with meteoritc water with a δ18O value around -10 ‰, which is abnormally negative given the relatively low latitude at the time of intrusion. A combination of an elevated volcanic edifice and the ‘continental effect’ is the most likely explanation.