Plagioclase replacement textures in partially eclogitised gabbros from the Sanddal mafic-ultramafic complex, Greenland Caledonides

H. M Lang; J. A Gilotti

doi:10.1046/j.0263-4929.2001.00325.x

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Plagioclase replacement textures in partially eclogitised gabbros from the Sanddal mafic-ultramafic complex, Greenland Caledonides

Journal article

Peer reviewed

Plagioclase replacement textures in partially eclogitised gabbros from the Sanddal mafic-ultramafic complex, Greenland Caledonides

H. M Lang and J. A Gilotti

Journal of metamorphic geology, Vol.19(5), pp.495-515

09/2001

DOI: 10.1046/j.0263-4929.2001.00325.x

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Abstract

The Sanddal mafic-ultramafic complex (SMUK) is a cluster of variably eclogitized mafic and ultramafic bodies that comprise the westernmost known eclogite facies locality in the NE Greenland eclogite province (NEGEP). Although there are no true eclogites in the SMUK, three distinct textural types of plagioclase replacement are interpreted to record sequential stages in adjustment of SMUK olivine gabbronorites to eclogite-facies conditions. The earliest stage, in which plagioclase was replaced by omphacite/spinel symplectite before nucleation of garnet (Type 1A and 1B) has not previously been described. Documentation of this texture provides clear evidence that, at least in some cases, garnet nucleation is delayed relative to nucleation of omphacite and is a rate-limiting step for eclogitization. Type 1C domains were produced by scattered nucleation of garnet in the same sample, In Type 2 domains, plagioclase was replaced by a layered corona with an outer layer of garnet, an inner layer of omphacite and an interior of inclusions-rich plagioclase. In Type 3 domains, the omphacite layer was overgrown by the garnet rim, and omphacite is preserved only as inclusions in garnet. In more coarse-grained leucogabbros, recrystallization was more complete, plagioclase replacement textures were less localized and could not be divided into distinct stages. Plagioclase replacement in SMUK samples was not isochemical, and required diffusion of at least Mg and Fe from replacement of mafic phases in the surroundings. Strong compositional gradients in garnet reflect disequilibrium and were controlled by the different diffusion rates of Mg/Fe and Ca, different local chemical environments, and progress of the plagioclase breakdown reaction. The presence of small amounts of hydrous minerals (amphibole, phlogopite and clinozoisite) in local equilibrium in plagioclase domains of most SMUK samples indicates that a small amount of H (sub 2) O was present during high-P metamorphism.

Arctic region

Caledonides

chemical reactions

coronae

crystal zoning

eclogite facies

facies

feldspar group

framework silicates

gabbros

Greenland

Igneous and metamorphic petrology

igneous rocks

mafic composition

metagabbro

metaigneous rocks

metamorphic rocks

mineral assemblages

norite

northeastern Greenland

petrography

plagioclase

plutonic rocks

recrystallization

Sanddal ultramafic complex

silicates

textures

ultramafics

Details

Title: Subtitle: Plagioclase replacement textures in partially eclogitised gabbros from the Sanddal mafic-ultramafic complex, Greenland Caledonides
Creators: H. M Lang - West Virginia University
J. A Gilotti - West Virginia University
Resource Type: Journal article
Publication Details: Journal of metamorphic geology, Vol.19(5), pp.495-515
Publisher: Blackwell
DOI: 10.1046/j.0263-4929.2001.00325.x
ISSN: 0263-4929
eISSN: 1525-1314
Language: English
Date published: 09/2001
Academic Unit: Earth and Environmental Sciences
Record Identifier: 9984229300102771

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