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

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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 > or =800 ppm at distances >2.5 km from the contact, to 1.5 km), these rocks have extremely uniform delta 15 N air near +3.7 per mil, whereas closer to the contact (< or =1 km) delta 15 N is shifted to higher values (up to +8.7 per mil). The coupled decreases in N content and increases in delta 15 N are compatible with the removal of N having low delta 15 N in fluids during continuous, prograde devolatilization reactions involving the break-down 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 SiO 2 /TiO 2 , SiO 2 /Al 2 O 3 , and the ratios of other major oxides to TiO 2 and Al 2 O 3 ) 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 K 2 O 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 15 N of +1.0 to +4.8 per mil. The N concentrations and delta 15 N 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 delta 15 N and delta 13 C 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.

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