ORIENTATION, COMPOSITION, AND ENTRAPMENT CONDITIONS OF FLUID INCLUSIONS IN THE RAFT RIVER DETACHMENT SHEAR ZONE

Fluids are commonly invoked as a primary cause for weakening of the rheology of detachment shear zones. However, fluid related mechanisms (e.g. pressure-solution, reaction enhanced ductility, reaction softening, development of fabric, and the precipitation of phyllosilicates) are not well understood. Fluid-facilitated reaction and mass transport leads to rheological weakening and strain localization and the departure from experimental failure laws derived in the laboratory. This study focuses on the Miocene detachment shear zone associated with the Raft River metamorphic core complex. The detachment shear zone is localized in the Proterozoic Elba quartzite, which unconformably overlies an Archean basement complex, and consists of an alternating sequence of white quartzite (~90% quartz, ~10% muscovite) and muscovite-quartzite schist. Previous work demonstrates abundant evidence of chemical, physical, and isotopic water-rock interactions in the RRDSZ. Here, we investigate the relationship between fluid inclusions, microcracks, brittle fractures, and microstructures in the quartzite mylonite. Thin section analyses reveal the presence of micocracks, fluid inclusion planes, and isolated fluid inclusion clusters. Microcracks tend to be perpendicular to the mylonitc foliation. They are also filled with quartz, which has a distinct fabric, not as strong as the mylonitic fabric, suggesting that microcracks healed while the shear zone was still at conditions favorable for quartz crystal plasticity. Fluid inclusion planes are transgranular and occur in two conjugate orientations, at ~60º from the mylonitic foliation. Fluid inclusions are also present as intergranular clusters. Our results show ductile overprint of brittle microstructures, which suggest that, during exhumation, the detachment shear zone may have crossed brittle-ductile transition repeatedly, providing opportunities for fluid to permeate the detachment shear zone.