Formation of saline brines and salt in the lower crust by hydration reactions in partially retrogressed granulites from the Lofoten Islands, Norway

Anorthosites, gneisses, and mangeritic rocks from the Lofoten Islands in no rthern Norway contain Cl-rich amphibole (up to 3.5 wt percent Cl) and bioti te (up to 1.5 wt percent Cl) that developed mostly during regional metamorp hism at approximate to 600 degrees C and approximate to 9 kb by replacement of older anhydrous granulite facies mineral assemblages, (Na,K)CI halide s olid solutions are closely associated spatially with amphiboles that have t he highest measured Cl-contents, Rocks with Cl-rich amphiboles and biotites always have minerals that exhibit a wide range in OH/Cl; one sample from a shear zone contains virtually Cl-free am phibole within a few millimeters of Cl-rich amphibole coexisting with halide crystals. Textural evidence ind icates that amphibole and biotite evolved from samples; OH-rich, Cl-poor co mpositions are always much more abundant than Cl-rich ones, Cl-rich amphibo les and biotites occur exclusively in rocks that contain relict granulite f acies minerals (pyroxene and/or olivine) samples in which no granulite faci es minerals remain contain amphiboles and biotites without elevated Cl-cont ents, In one particular case, the restricted development of Cl-rich amphiboles an d biotites within a shear zone and the absence of these minerals from undef ormed lithologic equivalents outside the shear zone demonstrate that the am phibole and biotite developed by reaction between a preexisting granulite f acies mineral assemblage and an aqueous fluid rather than by fluid-absent m etamorphism or by magmatic processes, We infer from this example that Cl-ri ch amphibole and biotite with the same grain size and texture as in the she ar zone developed throughout the Lofoten Islands by fluid-rock reaction, Mi neral-fluid equilibria indicate that the aqueous fluid was variable in comp osition ranging from X-H2O (=[H2O/(H2O + Cl)] of 0.99 for equilibrium with the most Cl-poor amphiboles and biotites to 0.4 to 0.6 for equilibrium with the most Cl-rich minerals that coexist with salt, The most saline fluids c orrespond to NaCl equivalent concentrations of over 25 mol/l, The range in OH/Cl of amphiboles and biotite in individual samples and the restricted occurrence of el-rich minerals in rocks with relict pyroxene and /or olivine can be explained by a desiccation model in which small amounts of "normal" low-Cl crustal fluid infiltrate rocks with granulite facies min erals and is subsequently enriched in Cl by preferential consumption of H2O during the resulting hydration reaction, Saline fluids developed at the ex tremes of reaction progress and then produced the Cl-rich amphibole and bio tite compositions coexisting with halide crystals. The desiccation model fu rther explains variations in the whole-rock concentration of Cl in rocks fr om the Lofoten Islands, the absence of Cl-rich minerals from completely ret rogressed granulites that lack olivine or pyroxene, the measured frequency distribution of amphibole and biotite compositions in individual samples, a nd the calculated evolution of fluid composition with reaction progress. Ou r results have implications for interpreting measurements of elevated elect rical conductivity in the lower crust and for models of granulite genesis t hat appeal to fluids with low H2O activity.