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Metamorphism and deformation on the northeast margin of the Shuswap metamorphic complex, Azure Lake, British Columbia Pigage, Lee Case

Abstract

Detailed structural and petrologic mapping near Azure Lake, British Columbia provides an overview of geologic relations along the northeast margin of the Shuswap Metamorphic Complex. Four phases of deformation have been recognized in the Shuswap Complex and the adjacent lower grade metasediments of the cover sequence. The first deformation consists of west-verging isoclinal folds plunging north and northwest. The second phase resulted in large upright folds with a shallow northwest or southeast plunge. The third and fourth phases are only locally developed as faults, fractures, and brittle folds trending north and northeast, respectively. Mineral assemblages range from garnet-biotite through first sillimanite zones of the Barrovian facies series. Metamorphic grade increases toward the southwest. Regional metamorphism is associated with the first phase of deformation. The Complex is separated from the adjacent cover sequence by a first phase tectonic slide. Structural and metamorphic discontinuities across this slide probably resulted from reactivation of the slide surface during the second phase of deformation. Microprobe analyses have been combined with linear regression techniques to outline probable sillimanite-forming reactions in pelites of the Complex. The regressions show that reaction textures are partly preserved because of the exhaustion of rutile as a reactant phase. Metamorphic conditions in the Complex are estimated from the mutual intersection of experimentally studied mineral equilibria. These conditions are: P=7600 ± 400 bars, T=705 ± 40°C, aH₂O =0.5 ⁺⁰•⁵­₀․₂ . Carbonate mineral assemblages initially buffered fluid phase compositions to high X CO₂ values near 0.75 during metamorphism. Therefore the fluid phase was not homogeneous in composition throughout all rock types during metamorphism. Whole rock Rb-Sr dates of 138 ± 12 Ma (all five samples) and 163 ± 7 Ma were obtained for granodiorite stocks in the Azure Lake area. Two biotite-whole rock ± hornblende dates of 119 ± 11 Ma and 77 ± 20 Ma indicate isotopic resetting. Initial Sr⁸⁷ -Sr⁸⁶ ratios vary from 0.7061 ± 0.0001 to 0.7103 ± 0.0002 for rock and mineral dates. These dated stocks cross-cut structural trends for the first two deformations and impose a hornfelsic contact aureole on regional metamorphic assemblages. Therefore regional metamorphism and deformation were completed by Late Jurassic time.

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