High temperature forearc metamorphism and consequences for sulfide stability in the Pacific Rim Terrane, British Columbia
Date
2021-06-25
Authors
Geen, Alexander C.
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Abstract
The Pacific Rim Terrane in British Columbia is a group of fault-bound forearc metasedimentary and metaigneous rocks subcreted to Wrangellia, comprising three lithological units: the Leech River Complex (LRC), the Pandora Peak Unit (PPU), and the Pacific Rim Complex. Of these three, the LRC and PPU were subject to an elevated thermal metamorphic event which locally overprinted typical low temperature, medium pressure forearc assemblages with low greenschist through amphibolite facies assemblages. The field study shows that biotite, garnet and staurolite isograds occur concentrically in the LRC, centered on the Leech River fault, which separates the Pacific Rim Terrane from the underlying Metchosin Igneous Complex of the Crescent terrane. Local thermal overprint in the PPU is sub-biotitic and is characterized by local replacement of prehnite-pumpellyite and lawsonite-bearing assemblages with muscovite ± chlorite. Multi-method geothermobarometry shows peak metamorphic temperatures from ~230 °C in the northern PPU to ~600 °C near the Leech River fault at ~4 kbar, and isotherms are continuous across the LRC-PPU boundary. The interfoliated Tripp Creek metabasites and Eocene Walker Creek intrusions do not control the distribution of isotherms, and syn-metamorphic felsic sills rarely have contact aureoles. Intercalated metabasites show two distinct rare earth element (REE) patterns, including NMORB-like light REE depletion among most Tripp Creek metabasites, and light REE enrichment in PPU metabasites. The lack of thermal aureoles associated with metabasites, and interlayered garnetite bands with negative Ce-anomalies attributed to seafloor hydrothermal processes, suggest the Tripp Creek metabasites are not syn-metamorphic sills and formed prior to accretion. The subcretion of then recently formed oceanic crust belonging to the Crescent terrane is identified as the probable cause of anomalously high temperature forearc conditions, as well as possible proximity to an Eocene mid ocean ridge. The high temperature metamorphic rocks in the Pacific Rim Terrane document the conversion of inherited primary pyrite to pyrrhotite in carbonaceous metasediments. S-inclusive pseudosections for LRC protoliths predict a low temperature (<420 °C) narrow pyrite desulfidation window that produces pyrrhotite and releases negligible S to the fluid phase. Conversely, sulfide petrography in the LRC shows pyrite can persist up to ~550 °C as inclusions in andalusite and staurolite porphyroblasts, as well as possibly in the rock matrix. S contents in carbonaceous pelites show a marked reduction at medium grade, associated with a dearth of visible sulfide in LRC phyllites. Sluggish pyrite desulfidation, pyrrhotite desulfidation, and terrane-scale S mobility are interpreted as the driver for mobility of intra-terrane sourced Au, leading to the formation of a hypozonal orogenic Au deposit in the central LRC.
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Keywords
metamorphism, Pacific Rim Terrane, sulfide, forearc, Raman spectroscopy, geothermometry, geobarometry, pyrite, pyrrhotite, thermal metamorphism, Leech River Complex, metasediment, PerpleX, thermodynamic model, geochemistry