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Inclusion patterns in zoned garnets from Magerøy, north Norway

Published online by Cambridge University Press:  05 July 2018

T. B. Andersen
T. B. Andersen*
Affiliation:
Geologisk Institutt Avd. A., 5014 University of Bergen, Norway
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Abstract

Garnet porphyroblasts in metasediments from Magerøy crystallized during static metamorphism. They display three optical zones, each having characteristic inclusions and chemistry. The compositional zoning is related to prograde metamorphism (an inner zone, 1, and a graphite-bearing zone) and retrogression (the outer zone, 2). Inclusions of two types are present in zone 1: type 1 are equidimensional remnants of the matrix, preferentially included along planes of rapid growth; type 2 are tubular and represent recrystallized quartz grains concentrated along defects in the garnet lattice. The defects are lineage boundaries between growth segments related to screw dislocations on crystal faces. Crystal growth developed at relatively high degrees of supersaturation, but below the supersaturation required for the development of dendrites. The inclusions suggest rapid growth of zone 1, caused by heat flow from an adjacent interkinematic mafic/ultramafic intrusive complex. The graphite-bearing zone crystallized at the metamorphic peak, while the inclusion-free idioblasfic rim probably developed during retrograde metamorphism.

Type
Research Article
Information
Mineralogical Magazine , Volume 48 , Issue 346 , March 1984 , pp. 21 - 26
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1984

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