REACTION LOCALIZATION LEADS TO STRAIN LOCALIZATION AND SOFTENING IN REACTIVATED MYLONITIC ROCKS

Muscovite-bearing mylonitic rocks locally overprint a 15 km thick fault zone in central Argentina. Much of this fault zone is composed of drier, muscovite-free ultramylonite (UM, D2), that contains K-feldspar, sillimanite, garnet, and biotite porphyroclasts inherited from a migmatitic gneiss (D1) protolith. Fine-grained biotite, quartz, and plagioclase grains in the UM matrix lack a preferred orientation and define a strong network of intergrown grains that show no textural weakening. Well-preserved sillimanite and garnet porphyroclasts suggest that the UM developed under dry, amphibolite facies conditions.

Locally overprinting the UM is a wetter, quartz-bearing mylonitic, D3 fabric. Here, fine-grained muscovite and biotite + chlorite folia truncate early biotite + plagioclase textures, and muscovite beards have partially replaced relict sillimanite grains. Anorthite content of matrix (D3) plagioclase is uniformly ~An30, distinct from D1 plagioclase (An40), which suggests that a pervasive grain boundary fluid facilitated partial replacement by muscovite. Biotite is partially replaced by chlorite, but more completely chloritic where both minerals help define D3 fabrics. Replacement reactions partially balanced with typical compositions are: 12 Bt + 68 H+=7Mu + 10Qz + 39H2O, 15Plg(An40) + 7K+ + 14 H+=7Mu + 9Qz, and 5Bt + 23H2O=7Chl + 6Qz + 9K+ + 5H+. All reactions require the introduction of H+, indicating an open fluid system. All produce volume loss, showing sensitivity to high local stresses. Liberated quartz precipitated in ribbons that help define the mylonitic fabric. Chlorite, biotite, and quartz in porphyroblast pressure shadows also mark the sinks for ions and H2O released in these replacement reactions. Thus biotite + plagioclase in random, mechanically strong D2 orientations are replaced by syntectonic D3 muscovite + quartz ± chlorite that grew in an orientation favorable for strain localization; their precipitation with preferred orientations and distributions created a strong textural softening. Strain localization occurred only where reactions progressed with the infiltration of H2O, on a scale of 100s of µm. Therefore, local reactivation of the fault zone was a consequence of the localization (and incompleteness) of replacement reactions in a partially open, moist system.