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The use of phosphorus contents in yielding estimates of the proportion of trapped liquid in cumulates of the Upper Zone of the Bushveld Complex

Published online by Cambridge University Press:  05 July 2018

R. Grant Cawthorn  and
Kevin L. Walsh
R. Grant Cawthorn
Affiliation:
Department of Geology, University of the Witwatersrand, 1 Jan Smuts Avenue, Johannesburg, 2001, South Africa
Kevin L. Walsh
Affiliation:
Department of Geology, University of the Witwatersrand, 1 Jan Smuts Avenue, Johannesburg, 2001, South Africa
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Abstract

Phosphorus contents in cumulus rocks occurring close to the level of apatite appearance in the basic rocks of the Bushveld Complex, South Africa, provide a method of calculating the proportion of intercumulus component in these rocks. Previous experimental studies have accurately constrained the phosphorus content of magmas when apatite becomes stable. The ratio of the phosphorus content in the cumulates immediately below the appearance of apatite to this liquid composition defines the proportion of trapped liquid.

Application of this method to rocks from the uppermost mafic rocks of the Bushveld Complex leads to the conclusion that there is from 1 to 6 per cent intercumulus component. Many of these rocks are multiphase cumulates and in such rocks estimation of intercumulus component from textural criteria is difficult.

If crystals grow In situ on the floor of the magma chamber such small proportions of interstitial component can be produced without appealing to excessive diffusion and circulation of magma through an unconsolidated crystal pile. The geometry of the intrusion as well as its size might have a major influence on the proportion of the liquid ultimately solidifying within a cumulus rock.

Keywords

layered complexesintercumulusapatitebottom growth
Type
Geochemistry
Information
Mineralogical Magazine , Volume 52 , Issue 364 , March 1988 , pp. 81 - 89
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1988

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