Crystallization from Stratified Magmas in the Honningsvåg Intrusive Suite, Northern Norway: a Reappraisal

Christian Tegner, Brian Robins and Henning S. Sørensen
Danish Lithosphere Centre, Øster Voldgade 10, 1350 København K, Denmark.
Department of Geology, University of Bergen, 5007 Bergen, Norway
Department of Earth Sciences, University of Aarhus, 8000 Aarhus C, Denmark

Abstract: Wedge-shaped layers of ultramafic and mafic cumulates in Intrusion II of the Caledonian Honningsvåg Intrusive Suite suggest crystallization on an inclined magma chamber floor from a compositionally-zoned and density-stratified magma.

Cyclic unit 8 (140–100 m thick) consists of a distally-thinning olivine gabbro (denoted paoC) macrolayer overlain by a distally-thickening gabbronorite, pahC. New mineral data in four traverses across cyclic unit 8 show systematic compositional changes; the Mg# of the mafic phases decreases upwards through the unit and distally, both along the base and along the paoC/pahC interface.

A crystallization model based on an effectively continuously-zoned magma chamber with numerous, relatively thin, double-diffusive magma layers is proposed. Differential migration of horizontal isopleths (e.g. Mg# and aSiO2) in response to fractional crystallization and assimilation of country rock can explain the variations in the Mg# of the cumulates.

Keywords: magma chamber • cumulates • double-diffusive convection • cyclic unit • Honningsvåg • Norway

Mineralogical Magazine; February 1996 v. 60; no. 398; p. 41-51; DOI: 10.1180/minmag.1996.060.398.04
© 1996, The Mineralogical Society
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