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

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 crusta l fluids. In the aureole, Skiddaw Group metasedimentary rocks with relative ly uniform lithology and major-element compositions show a dramatic decreas e in N content toward the granite contact (from greater than or equal to 80 0 ppm at distances >2.5 km from the contact, to <410 ppm less than or equal to 0.55 km from the contact). Far from the intrusive body (>1.5 km), these rocks have extremely uniform delta(15)N(air) near +3.7 parts per thousand, whereas closer to the contact (less than or equal to 1 km) delta(15)N is s hifted to higher values (up to +8.7 parts per thousand). The coupled decrea ses in N content and increases in delta(15)N, compatible with the removal o f N having low delta(15)N in fluids during continuous, prograde devolatiliz ation 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 concentr ations (including SiO2/TiO2, SiO2/Al2O3, and the ratios of other major oxid es to TiO2 and Al2O3) with distance from the granitic contact is consistent with minimal change in major element composition during the contact metamo rphism. Ratios of whole-rock N, B, Rb, and Ba concentrations to whole-rock K2O content are believed to reflect the differing fluid-mica partitioning l and involving varying relative proportions of white mica and biotite) of th ese trace elements during devolatilization reactions. Greisenized Skiddaw Granite from a borehole is enriched in N (range of 17-2 25 ppm for whole rocks and white mica separates) relative to the unaltered granite (whole-rock <30 ppm), and has delta(15)N of +1.0 to +4.8 parts per thousand. The N concentrations and delta(15)N Of the wall-rocks and greisen ized granites, combined with C isotopic data (carbonate and carbonaceous ma tter) for the same rocks, are consistent with the mobilization of fluids ha ving low delta(15)N and delta(13)C(PDB) values from the devolatilized aureo le into the cooling intrusive body. Such transport is consistent with the p redictions of recent theoretical models of late-stage hydrothermal evolutio n in cooling intrusive systems.