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Siliceous dolomites in the Horsethief Creek aureole, S.E. British Columbia: fluid infiltration and mineral reactions

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Title: Siliceous dolomites in the Horsethief Creek aureole, S.E. British Columbia: fluid infiltration and mineral reactions
Author: Niermann, Maria C.
Degree Master of Science - MSc
Program Earth and Ocean Sciences
Copyright Date: 1998
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.
URI: http://hdl.handle.net/2429/9016
Series/Report no. UBC Retrospective Theses Digitization Project [http://www.library.ubc.ca/archives/retro_theses/]
Scholarly Level: Graduate

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