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

• Author / Creator

  Li, Yingzhou

• 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.

• Subjects / Keywords

  • Nitrogen isotope
  • Metamorphic rocks
  • Geochemistry
  • Metamorphic devolatilization
  • Geology

• Graduation date

  Spring 2019

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/r3-rz9j-zd59

• License

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