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Stability of naujakasite in hyperagpaitic melts, and the petrology of naujakasite lujavrite in the llímaussaq alkaline complex, South Greenland

Published online by Cambridge University Press:  05 July 2018

T. Andersen  and
H. Sørensen
T. Andersen*
Affiliation:
Department of Geosciences, University of Oslo, P.O. Box 1047 Blindern, N-0654 Oslo, Norway
H. Sørensen
Affiliation:
Geological Institute, Copenhagen University, Øster Voldgade 10, DK-1350 København K, Denmark
*
*E-mail: [email protected]
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Abstract

Naujakasite (Na6(Fe,Mn)Al4Si8O26) is a characteristic, rock-forming mineral of hyperagpaitic naujakasite lujavrite (arfvedsonite + naujakasite + steenstrupine + albite ± relict nepheline ± microcline ± ussingite) from the Ilímaussaq complex, South Greenland, and is only known from this complex. The qualitative stability relationships of naujakasite-bearing, low-variance liquidus mineral assemblages at near-solidus conditions in lujavritic magma can be modelled within the six-component system Si-Al-Fe-Na-O-H. The modelling suggests that an uncommon combination of elevated water activity and low oxygen fugacity is needed to stabilize arfvedsonite + naujakasite mineral assemblages in highly sodic liquids. These conditions are incompatible with the stable coexistence of aegirine and arfvedsonite. They are met in the arfvedsonite-bearing lujavrites of Ilímaussaq, but apparently not in rocks of similar composition in the Khibina and Lovozero complexes, Kola Peninsula, where aegirine- or aegirine-arfvedsonite-bearing mineral assemblages are characteristic. Liquid lines of descent leading to crystallization of naujakasite have to pass through the albite+nepheline+arfvedsonite liquidus field in the Si-Al-Fe-Na-O-H system, reaching this field at the high-Na side of the albite-nepheline-arfvedsonite plane.

Keywords

nepheline syeniteagpaitic rocksnaujakasitewater activityoxygen fugacityIlímaussaq complexGreenland
Type
Research Article
Information
Mineralogical Magazine , Volume 69 , Issue 2 , April 2005 , pp. 125 - 136
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2005

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Footnotes

Contribution to the Mineralogy of llímaussaq no. 125

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