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Compositional variation in högbomites from north Connemara, Ireland

Published online by Cambridge University Press:  05 July 2018

N. S. Angus  and
R. Middleton
N. S. Angus
Affiliation:
Department of Geology, The University, Hull, HU6 7RX
R. Middleton
Affiliation:
Department of Geology, The University, Hull, HU6 7RX
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Abstract

Högbomite occurs in two contrasting mineral assemblages within the Currywongaun-Dough-ruagh intrusion of north Connemara: a cordierite-rich pelitic xenolith and an orthopyroxenite. In the latter, högbomite and green spinel form blebs within magnetite-ilmenite grains. The högbomite displays significant compositional variation from grain to grain: TiO2 (3.0–6.3%), FeO (21.6–21.3%), MgO (10.0–7.5%), ZnO (3.6–2.4%). This chemical heterogeneity appears to represent variable degrees of partial substitution of Mg and Zn by Ti, in the replacement of spinel by högbomite. By contrast, in the cordierite-hornfels, the högbomite compositions are more notably enriched in iron: TiO2 (4.7–7.0%), FeO (29.6–24.3%), MgO (4.2–6.2%), ZnO (2.7–2.1%). This iron-rich högbomite appears to have formed primarily by interaction between opaque ore and adjacent cordierite, rather than by replacement of spinel.

Two high-grade metamorphic episodes appear to be necessary for högbomite growth, one determining chemical composition and the other appropriate physical parameters. In the Connemara occurrences thermal metamorphism and partial melting, coupled with contamination of the surrounding magma, controlled the formation of mineral assemblages rich in Fe, Mg, Al, Ti, and Zn. Emplacement of the intrusion was accompanied by amphibolite facies regional metamorphism and it is to this metamorphic event that the growth of högbomite may be attributed.

Keywords

högbomiteorthopyroxenitecordieritehornfelsConnemaraIreland
Type
Research Article
Information
Mineralogical Magazine , Volume 49 , Issue 354 , December 1985 , pp. 649 - 654
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1985

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