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The Highwood Mountains potassic igneous province, Montana: mineral fractionation trends and magmatic processes revisited

Published online by Cambridge University Press:  05 July 2018

C. M. B. Henderson ,
F. R. Richardson  and
J. M. Charnock
C. M. B. Henderson*
Affiliation:
School of Earth, Atmospheric and Environmental Sciences (SEAES), Williamson Building, University of Manchester M13 9PL, UK Accelerator Science and Technology Centre (ASTeC), Daresbury Laboratory, Science and Technology Facilities Council, Warrington WA4 4AD, UK
F. R. Richardson
Affiliation:
School of Earth, Atmospheric and Environmental Sciences (SEAES), Williamson Building, University of Manchester M13 9PL, UK
J. M. Charnock
Affiliation:
School of Earth, Atmospheric and Environmental Sciences (SEAES), Williamson Building, University of Manchester M13 9PL, UK
*
*E-mail: [email protected]
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Abstract

Potassium-rich mafic dykes and lavas from the Highwood Mountains Igneous Province, USA were studied by electron-microprobe and bulk-rock analysis. For the mafic phonolites, compositional trends for olivine and augite phenocrysts and groundmass biotite, alkali feldspar and titanomagnetites are presented and substitution mechanisms discussed. Phenocrysts of biotite and augite in the minettes are also characterized, together with groundmass alkali feldspar and titanomagnetite. The alkali feldspars and biotites are commonly enriched in Ba. Olivine, clinopyroxene and biotite phenocrysts are generally quite magnesium-rich, which is consistent with the primitive natures of the least evolved rocks.

Bulk-rock major-element compositions are combined with modal and microprobe data for the principal phenocrysts to calculate model residual liquid compositions for mafic phonolites, minettes and a syenitic rock. On the basis of phase-equilibria, it is suggested that the main controls of differentiation are polybaric involving crystallization during transport of primary magmas from the mantle for the minettes, and low-pressure differentiation for the mafic phonolites. Whereas magma mixing might have contributed to petrogenesis, many of the disequilibrium features exhibited by clinopyroxene and biotite phenocrysts can also be attributed to pre-existing phenocrysts undergoing decompression melting during magma uprise from its mantle source, followed by rapid crystal growth and episodic volatile loss in sub-volcanic magma chambers.

Keywords

petrogenesis of K-rich basaltic rocksprimary biotite fractionationdecompression meltingmagma mixingphase equilibria
Type
Research Article
Information
Mineralogical Magazine , Volume 76 , Issue 4 , August 2012 , pp. 1005 - 1051
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2012

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