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Post-magmatic alteration in eudialyte from the North Qôroq centre, South Greenland

Published online by Cambridge University Press:  05 July 2018

Ian M. Coulson
Ian M. Coulson*
Affiliation:
School of Earth Sciences, The University of Birmingham, Edgbaston, Birmingham B15 2TT, England
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Abstract

The North Qôroq centre comprises a series of nested nepheline syenite intrusions and forms part of the mid-late Proterozoic Gardar province of South Greenland. Within the centre fractionation has produced varied rock types ranging from augite-syenite to lujavrite, a eudialyte microsyenite. Samples of eudialyte from the lujavrites of unit SN1B of the centre show evidence for two-stage alteration. This alteration ranges from slight modification along crystal margins to complete breakdown and replacement by new pseudomorphing phases. Modification to crystal margins is accompanied by increasing Nb and Zr contents and is related to metasomatism produced by the intrusion of younger syenite units of the North Qôroq centre. More extensive alteration is as a result of metasomatism followed by lower-temperature supergene alteration. Simplified reactions for this breakdown include eudialyte + metasomatic fluid = allanite + nepheline; eudialyte + metasomatic fluid = titanite + aegirine + møsandrite + wöhlerite; eudialyte + fluid = zirfesite + fluid. Mass balance calculations for altered compared with unaltered samples of lujavrite show that alteration took place at approximately constant volume with an overall increase in Fe (+2.41 g/100g), Si and K (+0.65 and +0.61 g/100g), whilst Na (−2.67 g/100g) and all trace elements, particularly La, Y, Nb and Zr (−5.6 to −166 g/10000g) are lost from the system.

Keywords

eudialytezirfesitealterationmetasomatismlujavritelgalikoGardarGreenland
Type
Mineralogy
Information
Mineralogical Magazine , Volume 61 , Issue 404 , February 1997 , pp. 99 - 109
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1997

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