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Siliceous dolomites in the Horsethief Creek aureole, S.E. British Columbia: fluid infiltration and mineral reactions Niermann, Maria C.
Abstract
Potassic siliceous dolomites in the Horsethief Creek aureole, S.E. British Columbia, Canada record peak contact-metamorphic conditions of ≈2.2 kbars and 650 °C in the inner aureole. The protolithic dolostones have varying but small amounts of disseminated quartz (≤ 5 volume percent) and K-feldspar (≤15 volume percent) but have interbedded numerous chert nodules and layers. Subvertical joints and veins cut across layering. Six isograds can be identified: phlogopite; tremolite+K-feldspar; diopside; diopside+phlogopite; diopside+dolomite; and at highest grade forsterite. The prevalence of isobarically divariant mineral assemblages, paucity of high XCO2 mineral assemblages, widespread development of hydrous minerals often in high abundance (up to 65 volume percent) and a ≈7 per mill (0/00) depletion (from ≈23 0/00 to ≈16 0/00) in δ18O are all evidence for substantial syn-metamorphic fluid flow. The almost complete de-dolomization of the inner and middle aureole suggests that metamorphism was not isochemical and that silicametasomatism took place. Heterogeneity in the distribution of index minerals and the potassic phases reflect varying bulk composition as well as variable fluid infiltration. Infiltration-driven near-isothermal reactions are documented in the rocks in form of numerous samples showing a texture interpreted to record prograde reaction of diopside to tremolite. A petrographic break is observed at the tremolite+K-feldspar isograd. While rocks up-grade of this isograd are dominated by isobarically divariant assemblages, rocks down-grade are characterized by isobarically univariant assemblages. This break may record a sudden change in infiltration character. The proposed model of contact metamorphism in the impure siliceous dolomites of the Horsethief Creek aureole is: 1) Intrusion of the Horsethief pluton triggered metamorphic reactions in the siliceous dolomites and the underlying pelitic rocks. 2) Buoyant fluids evolving in the metapelites at temperatures >530 °C rose along subvertical joints in and across the different lithologies and infiltrated the dolostone. 3) Infiltration of silica-bearing aqueous fluids triggered pervasive reaction in the siliceous dolomites. Rocks in the outer aureole did not experience significant infiltration because temperatures were too low to trigger the dehydration reaction in the underlying metapelites. Plutonic fluids may only have played a minor role, and bedding parallel flow likely was also only minor.
Item Metadata
Title |
Siliceous dolomites in the Horsethief Creek aureole, S.E. British Columbia: fluid infiltration and mineral reactions
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Creator | |
Publisher |
University of British Columbia
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Date Issued |
1998
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Description |
Potassic siliceous dolomites in the Horsethief Creek aureole, S.E. British Columbia, Canada
record peak contact-metamorphic conditions of ≈2.2 kbars and 650 °C in the inner aureole. The
protolithic dolostones have varying but small amounts of disseminated quartz (≤ 5 volume
percent) and K-feldspar (≤15 volume percent) but have interbedded numerous chert nodules and
layers. Subvertical joints and veins cut across layering. Six isograds can be identified: phlogopite;
tremolite+K-feldspar; diopside; diopside+phlogopite; diopside+dolomite; and at highest grade
forsterite.
The prevalence of isobarically divariant mineral assemblages, paucity of high XCO2 mineral
assemblages, widespread development of hydrous minerals often in high abundance (up to 65
volume percent) and a ≈7 per mill (0/00) depletion (from ≈23 0/00 to ≈16 0/00) in δ18O are all evidence
for substantial syn-metamorphic fluid flow. The almost complete de-dolomization of the inner
and middle aureole suggests that metamorphism was not isochemical and that silicametasomatism
took place. Heterogeneity in the distribution of index minerals and the potassic
phases reflect varying bulk composition as well as variable fluid infiltration. Infiltration-driven
near-isothermal reactions are documented in the rocks in form of numerous samples showing a
texture interpreted to record prograde reaction of diopside to tremolite.
A petrographic break is observed at the tremolite+K-feldspar isograd. While rocks up-grade
of this isograd are dominated by isobarically divariant assemblages, rocks down-grade are
characterized by isobarically univariant assemblages. This break may record a sudden change in
infiltration character.
The proposed model of contact metamorphism in the impure siliceous dolomites of the
Horsethief Creek aureole is: 1) Intrusion of the Horsethief pluton triggered metamorphic reactions
in the siliceous dolomites and the underlying pelitic rocks. 2) Buoyant fluids evolving in the
metapelites at temperatures >530 °C rose along subvertical joints in and across the different
lithologies and infiltrated the dolostone. 3) Infiltration of silica-bearing aqueous fluids triggered
pervasive reaction in the siliceous dolomites. Rocks in the outer aureole did not experience
significant infiltration because temperatures were too low to trigger the dehydration reaction in the
underlying metapelites. Plutonic fluids may only have played a minor role, and bedding parallel
flow likely was also only minor.
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Extent |
13910831 bytes
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Genre | |
Type | |
File Format |
application/pdf
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Language |
eng
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Date Available |
2009-06-12
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Provider |
Vancouver : University of British Columbia Library
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Rights |
For non-commercial purposes only, such as research, private study and education. Additional conditions apply, see Terms of Use https://open.library.ubc.ca/terms_of_use.
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DOI |
10.14288/1.0052316
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URI | |
Degree | |
Program | |
Affiliation | |
Degree Grantor |
University of British Columbia
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Graduation Date |
1999-05
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Campus | |
Scholarly Level |
Graduate
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Aggregated Source Repository |
DSpace
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Rights
For non-commercial purposes only, such as research, private study and education. Additional conditions apply, see Terms of Use https://open.library.ubc.ca/terms_of_use.