Muscovite-rich pseudomorphs after staurolite as a record of fluid infiltration during prograde metamorphism

Jennifer Whittington; Barbara L Dutrow; C. Tom Foster

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Abstract

Muscovite-rich pseudomorphs after staurolite as a record of fluid infiltration during prograde metamorphism

Jennifer Whittington, Barbara L Dutrow and C. Tom Foster

Abstracts with programs - Geological Society of America, Vol.37(7), p.227

Geological Society of America, 2005 annual meeting

10/2005

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Abstract

During metamorphism, evidence of the prograde path is often obliterated by peak metamorphic conditions. However, pseudomorphs that form during prograde metamorphism may provide insights into heat and mass (fluid) transport along this portion of the P-T-X-t path. Fifteen sillimanite zone samples each containing 2-5 muscovite-rich pseudomorphs after staurolite were investigated from pelitic schists of the Farmington area, west-central Maine, USA. Pseudomorphs are easily recognized by an increase in average grain size ( approximately cm) and a change to randomly-oriented muscovite-rich ( approximately 30-75%) modal mineralogy. SEM cathodoluminescence images, X-ray maps, and electron microprobe analyses characterize the mineral distribution, modes, and compositions within the pseudomorphs and the surrounding matrix. These data provide the basis for textural modeling studies and for testing open versus closed system behavior of pseudomorph formation. Three compositional types of muscovite-rich pseudomorphs are observed: quartz-rich (15-30% qz with <1% pl, sil); plagioclase-rich (20-30% pl with < 1% qz, sil); and plagioclase-sillimanite-rich ( approximately 30% pl, 15-20% sil with <1% qz); in addition to those that are ms-rich ( approximately 75% ms). A biotite-rich ms-poor mantle surrounds most pseudomorphs. Irreversible thermodynamic modeling of pseudomorph development using analyzed mineral compositions and previously determined diffusion coefficients for metapelitic rocks suggests modes of approximately 75% ms, approximately 10% bio, approximately 10% pl, and approximately 5% qtz replace the staurolite in a system closed to fluids. These modes are substantially different than those observed. If the system is allowed to be open to H (sub 2) O, K, Na, and Al, modal variations in biotite, sillimanite, plagioclase and quartz comprising the pseudomorphs can be reproduced. Additionally, changes in cathodoluminescence from core to rim in pseudomorph quartz grains suggest two growth stages characterized by differing trace element distribution. These data suggest that metapelitic pseudomorphs retain evidence for an infiltrating fluid phase and its composition during prograde metamorphism.

Trace Elements

United States

cathodoluminescence

diffusion

electron probe data

Farmington Maine

Franklin County Maine

Igneous and metamorphic petrology

infiltration

Maine

metamorphism

mica group

mineral composition

models

muscovite

nesosilicates

orthosilicates

P-T-t paths

prograde metamorphism

pseudomorphism

SEM data

sheet silicates

silicates

staurolite

thermodynamic properties

west-central Maine

X-ray data

Details

Title: Subtitle: Muscovite-rich pseudomorphs after staurolite as a record of fluid infiltration during prograde metamorphism
Creators: Jennifer Whittington - Louisiana State University
Barbara L Dutrow
C. Tom Foster
Resource Type: Abstract
Publication Details: Abstracts with programs - Geological Society of America, Vol.37(7), p.227
Conference: Geological Society of America, 2005 annual meeting
Publisher: Geological Society of America (GSA)
ISSN: 0016-7592
Alternative title: Geological Society of America, 2005 annual meeting
Language: English
Date published: 10/2005
Academic Unit: Earth and Environmental Sciences
Record Identifier: 9984240787402771

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