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Formation of chlor- and fluor-apatite in layered intrusions

Published online by Cambridge University Press:  05 July 2018

R. Grant Cawthorn
R. Grant Cawthorn*
Affiliation:
Department of Geology, University of the Witwatersrand, Private Bag 3, PO Wits 2050, Republic of South Africa
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Abstract

Apatite in most igneous intrusions has a high Cl/F ratio. However, chlor-apatite has been reported in the lower portions of the Bushveld and Stillwater Complexes. This has been used as evidence supporting the early separation of a Cl-rich discrete hydrous fluid in these intrusions. Mineralogical evidence is presented here to demonstrate that the Bushveld Complex, at least, formed from a nearly anhydrous magma, and did not release a hydrous fluid before apatite began to crystallize. It is suggested that apatite in the earliest cumulates equilibrated with trapped interstitial liquid, which converted it from the typical F-rich composition of cumulus apatite to a Cl-rich composition. This is an analogous process to that in which cumulus mafic minerals may become more Fe-rich on cooling and reaction with interstitial liquid.

Keywords

apatitelayered intrusionsCl/F ratioBushveld Complex
Type
Mineralogy
Information
Mineralogical Magazine , Volume 58 , Issue 391 , June 1994 , pp. 299 - 306
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1994

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