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A stable isotopic analysis of syn-tectonic fluid regimes in the dogtooth, western, and main ranges of southeastern British Columbia Knoop, Stuart R.

Abstract

Stable isotope data from syn-kinematic veins, cataclasites, and wall rocks in the Dogtooth, Western, and Main Ranges of southeastern British Columbia document fluid migration across local and large-scale (100's of meters to 10's of kilometers) distances during Mesozoic contraction. Sampling was conducted across a ~30 km long western transect, which includes siliciclastic-dominated assemblages of the Dogtooth Range and carbonate units of the Western Ranges, and a ~5 km long eastern transect in carbonates and meta-pelites of the western Main Ranges. Syn-tectonic veins throughout the study area are comprised primarily of quartz and calcite and conform to three morphologies: i) laminated, bedding-parallel veins and syntaxial tension gashes, ii) boudinaged, cleavage-parallel veins, and iii) cataclasites and fault-parallel veins. In the Dogtooth Range, the oxygen isotopic composition of whole rock carbonates is ~5 % lower than accepted sedimentary protolith values. Vein calcite is generally depleted in ¹⁸O, whereas vein quartz is equally enriched and depleted compared with host rocks. Fluid-assisted isotopic exchange between siliciclastic and less-abundant carbonate assemblages across outcrops has lowered carbonate signatures and yielded quartz vein values that are more homogeneous than those of wall rocks. Hydrogen isotopes suggest that the fluids involved were metamorphic in origin with a possible meteoric component. In comparison, the Western Ranges exhibit an abrupt, mean δ¹⁸O increase of ~ 4.5 % and a trend of gradually heavier values with distance up-section and eastward from the Dogtooth Range. Neither variation in age nor mineral abundance can fully explain this phenomenon. Moreover, isotope and solvus thermometry indicate that veins were in thermal equilibrium with their host rocks (~300°C - 380°C). Hinterland-driven infiltration of low-61 80, low Xco2 fluid into the Western Ranges best explains this trend, and analytical modeling places time-integrated fluid fluxes at 104 - 105 mol H20/cm2 In the Main Ranges, oxygen values are more heterogeneous and reflect multiple fluid sources. Vein signatures as low as 6 %o indicate minor focussing of meteoric fluids downward into some fault zones. Variable vein quartz and wall rock carbonate values in other outcrops, in conjunction with a metamorphic hydrogen signature from vein chlorite, suggest that lower-81 80, metamorphic fluids were channeled upward under conditions of heterogeneous time-integrated fluid flux.

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