RAPID MUSCOVITE GROWTH DURING INFILTRATION OF METEORIC FLUIDS INTO QUARTZOFELDSPATHIC SHEAR ZONES: AN EXAMPLE FROM THE CORDILLERA BLANCA SHEAR ZONE, PERU

Fluid infiltration into ductile shear zones is an important reactant for strain softening via hydrolytic weakening and metamorphic reactions. A major mechanism for hydrous weakening of quartzofeldspathic mylonites is the growth of muscovite at the expense of feldspar. However, this process can be enigmatic in shear zones that often have complex and protracted deformation histories. The Cordillera Blanca shear zone in central Peru presents an opportunity to explore these processes in a well constrained system with no shear zone reactivation or ductile overprinting. The southernmost segment of the ca. 5.4 Ma – present normal-sense Cordillera Blanca detachment fault juxtaposes high-strain, muscovite-rich, end-Miocene to early Pliocene quartzofeldspathic mylonites against muscovite-free, undeformed, late Miocene leucogranodiorite. δ²H values of syntectonic muscovite and deformed biotite indicate that fabric development occurred in the presence of high-altitude, surface-derived, and deeply circulated water. Fault-controlled meteoric fluid flow contributed to significant reductions in shear strength via rapid muscovite growth and correlative feldspar grain area reduction during hydrous deformation. This led to formation of S-C-C’ fabrics derived from interconnected networks of muscovite and/or fine-grained polyphase aggregates. Mica and aggregate networks decouple rigid feldspar from the quartz rich matrix and occasionally reach proportions of near 50 % muscovite. Textural relationships within the fabric suggest reductions in shear strength due to progressive consumption and grain area reduction of rigid feldspar, strain partitioning into newly developed muscovite and aggregate shear bands, and decreased connectivity of matrix quartz. ⁴⁰Ar/³⁹Ar mica dates from the shear zone and footwall granite indicate that growth and cooling of muscovite to below estimated deformation temperatures within the shear zone occurred within 500 Ka of initiation of the Cordillera Blanca detachment fault. These observations indicate that fluid-mediated growth of muscovite is a volumetrically significant process that exerts substantial control on the rheology of ductile shear zones early in their activation history.