Nitrogen-isotope record of fluid-rock interactions in the Skiddaw Aureole and granite, English Lake District

Abstract

The Skiddaw Granite and its contact metamorphic aureole in the English Lake District provide an excellent opportunity to test the capability of the N-isotope system to trace devolatilization and large- scale transfer of crustal fluids. In the aureole, Skiddaw Group metasedimentary rocks with relatively uniform lithology and major-element compositions show a dramatic decrease in N content toward the granite contact (from ≥ 800 ppm at distances >2.5 km from the contact, to <410 ppm ≤0.55 km from the contact). Far from the intrusive body (>1.5 km), these rocks have extremely uniform δ¹⁵N_air near +3.7‰, whereas closer to the contact (≤1 km) δ¹⁵N is shifted to higher values (up to +8.7‰). The coupled decreases in N content and increases in δ¹⁵N are compatible with the removal of N having low δ¹⁵N in fluids during continuous, prograde devolatilization reactions involving the break­down of white mica and the stabilization of biotite-, cordierite-, and andalusite-bearing assemblages. In the same metasedimentary rocks, the lack of obvious trends in major-element concentrations (in­cluding SiO₂/TiO₂, SiO₂/Al₂O₃, and the ratios of other major oxides to TiO₂ and Al₂O₃) with distance from the granitic contact is consistent with minimal change in major element composition during the contact metamorphism. Ratios of whole-rock N, B, Rb, and Ba concentrations to whole-rock K₂O content are believed to reflect the differing fluid-mica partitioning (and involving varying relative proportions of white mica and biotite) of these trace elements during devolatilization reactions. Greisenized Skiddaw Granite from a borehole is enriched in N (range of 17-225 ppm for whole rocks and white mica separates) relative to the unaltered granite (whole-rock <30 ppm), and has δ¹⁵N of+1.0 to +4.8‰. The N concentrations and δ¹⁵N of the wall-rocks and greisenized granites, combined with C isotopic data (carbonate and carbonaceous matter) for the same rocks, are consistent with the mobilization of fluids having low δ¹⁵N and δ¹³C_PDB values from the devolatilized aureole into the cooling intrusive body. Such transport is consistent with the predictions of recent theoretical models of late-stage hydrothermal evolution in cooling intrusive systems.