Regression Modelling of Metamorphic Reactions in Metapelites, Snow Peak, Northern Idaho

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Abstract
The second of two periods of regional metamorphism that affected pelitic rocks near Snow Peak caused complete re-equilibration of mineral assemblages and resulted in a consistent set of metamorphic isograds. Metamorphic chlorite and biotite occur in the lowest grade rocks. With increasing grade, garnet, staurolite, and kyanite join the assemblage, resulting in a transition zone containing all the above phases. At higher grade, chlorite, and finally staurolite disappear. Mass balance relations at isograds and among minerals of low-variance assemblages have been modelled by a non-linear least-squares regression technique. The progressive sequence can be described in terms of schematic T-XH2O relations among chlorite, biotite, garnet, staurolite, and kyanite at Ptotal above the KFMASH invariant point involving those phases. The first appearance of garnet was the result of an Fe-Mg-Mn continuous reaction. As temperature rose, the garnet zone assemblage encountered the staurolite isograd reaction, approximated by the model reaction: 3·0 chlorite + 1·5 garnet + 3·3 muscovite + 05 ilmenite = 1·0 staurolite + 3·1 biotite + 1·5 plagioclase + 3·3 quartz + 10·3 H2O. The staurolite zone corresponds to buffering along this reaction to the intersection where chlorite, biotite, garnet, staurolite, and kyanite coexist. The transition zone assemblage formed by reaction at this T–X H2O intersection which migrates toward more H2O-rich fluid composition with progressive reaction. The net reaction at the intersection is approximated by the transition zone reaction: 1·0 chlorite +1·1 muscovite + 0·2 ilmenite = 2·7 kyanite + 1·0 biotite + 0·4 albite + 4·2 H2O. Chlorite was commonly the first phase to have been exhausted and the remaining assemblage was buffered along a staurolite-out reaction, represented by the model reaction: 1·0 staurolite + 3·4 quartz + 0·4 anorthite + 1·4 garnet + 0·1 ilmenite + 7·9 kyanite + 2·0 H2O. Consumption of staurolite by this reaction resulted in the highest grade assemblage, which contains kyanite, garnet, biotite, muscovite, quartz, plagioclase, ilmenite, and graphite.

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