Metamorphic devolatilization, mineral reaction, and fluid alteration in the Mica Creek area (British Columbia, Canada) inferred from whole-rock and mineral nitrogen isotope signatures

Abstract

The Neoproterozoic Horsethief Creek Group metasedimentary rocks in the Mica Creek area (British Columbia, Canada) are an important supracrustal sequence in the Canadian Cordillera. They contain a continuous range of metamorphic grades from slate, progressively to schists in the biotite, garnet, kyanite zones, and gneisses in the sillimanite – K-feldspar zone. This metamorphic sequence provides an excellent opportunity to examine the behavior of nitrogen (N) in a collisional metamorphic environment, which has not been well studied to date. Our results show that, despite a large variation in N/K ratios, slates (N/K = 20.9 – 43.2, δ15N = 4.1 – 6.0‰) and chlorite-zone phyllites (N/K = 19.1 – 64.7, δ15N = 4.2 – 5.8‰) have relatively constant δ15N values, which largely resemble the signature of the protolith. The biotite-zone schists show decreased N contents associated with increasing δ15N values (N/K = 1.7 – 29.1, δ15N = 9.9 – 14.8‰). This is consistent with metamorphic devolatilization of 15N-depleted NH3 at T < 500 ºC, resulting in 15N enrichment in the biotite-zone schist. In contrast, the garnet-zone (N/K = 3.8 – 37.4, δ15N = 8.1 – 9.6‰), kyanite-zone (N/K = 11 – 23, δ15N = 7.2 – 9.2‰) and sillimanite – K-feldspar zone (N/K = 4.0 – 6.0, δ15N = 7.1 – 8.1‰) samples show a trend of decreasing δ15N values as well as N content relative to the biotite zone. This trend cannot be explained by the metamorphic devolatilization effect but is rather an isotopic alteration by a 15N-depleted granitic melt and its associated fluid. A detailed look at the N isotope signatures of coexisting minerals in the metamorphic rocks shows that muscovite, biotite and plagioclase in some samples more or less deviate from N isotope equilibrium fractionations, with δ15N values of both plagioclase and muscovite being negatively correlated to those of biotite. This can be best explained by a kinetic isotopic effect associated with metamorphic reactions that use muscovite and plagioclase to produce biotite, suggesting that plagioclase has likely been involved in the metamorphic reactions.