Volume loss, fluid flow and state of strain in extensional mylonites from the central Mojave Desert, California

Compositional changes in mylonitic rocks from the central Mojave metamorphic core complex, California, indicate that large volume loss (20-70%) attended formation of mylonite along the Waterman Hills detachment fault. Strong silica depletion and significant mobility of normally immobile elements imply large fluid/rock ratios during mylonitization, possibly as a result of multiple-pass hydrothermal convection connecting near-surface and mid-crustal alteration regimes. Dissolution of quartz during mylonitization indicates that the fluid was undersaturated with respect to silica, and therefore was probably not magmatic or connate. Large volume loss in a shear zone should result in apparent flattening strains, yet mylonites throughout the complex, including the mylonites that record large volume loss, exhibit constrictional strains. This inconsistency raises questions about the applicability of the shear-zone model to Cordilleran metamorphic core complexes.