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Petrogenesis of the Eshowe melilitites

By Elizabeth Anne Colgan

Abstract

Includes bibliography.The Eshowe melilitites intruded marginal cratonic crust at approximately 80my. Their intrusion followed after a long period of extensive rift tectonism associated with the breakup of Gondwanaland. The intrusives represent the final phase of alkaline and basaltic magmatism that commenced at about 200my. This magmatism was probably related to mantle processes responsible for the continental fragmentation and was controlled by a fluctuating mantle thermal regime. The Eshowe melilitites intrude an area of attenuated crust in an essentially rift valley setting. Petrological, and chemical evidence suggest that the Emtilombo melilitite does not represent crystallisation of a primary magma. The magma was generated in the asthenosphere but reacted with incompatible element and probably CO₂ and H₂O enriched lithosphere and perhaps crustal sources on its way to the surface. The Emtilombo melilitite contains microxenoliths of mantle derived spinel peridotite and of a suite of dunitic rocks that are believed to be high pressure cumulates of earlier alkaline magmas. These latter trapped melts would have introduced metasomatising agents into the lithosphere. The dunitic melts are believed to represent earlier intrusions related to the Eshowe alkaline volcanism. The chemistry of the olivine phenocrysts and microphenocrysts and complex zonation patterns on olivine xenocrysts (macrocryst and some complex phenocrysts) suggest the Emtilombo magma formed by mixing of several batches of melt. The 'parental magmas' to the Eshowe occurrences are therefore considered to consist of a mixture of asthenospheric and lithospheric components and a variety of different melts. The 'parental magma' to the Emtilombo dyke was an incompatible element enriched ultramafic melt that contained microxenoliths of spinel lherzolite and the dunitic suite of rocks. Changing oxygen fugacities are believed to be controlled by a loss of volatiles at relatively shallow depths in the mantle or lower crust. These changes are reflected by the spinel chemical trends and perovskite crystallisation. The change occurred after complex zonation patterns had developed on the olivines. The microphenocryst olivines are believed to be the only population of grains that crystallised from the Emtilombo 'parental magma'. Mode of emplacement of the melilitites is probably influenced by the nature and volume of magma in the intrusion, its volatile content, and to some extent the nature of the country rock. The Eshowe melilitites show a wide variety of intrusive modes and also demonstrate how late stage processes, possibly during or post consolidation, influence the geochemistry of the rock type

Topics: Geochemistry
Publisher: Department of Geological Sciences
Year: 1992
OAI identifier: oai:open.uct.ac.za:11427/17331

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