COMPETING CHEMICAL AND MECHANICAL PROCESSES PRODUCING GNEISSIC LAYERING IN A DEEP CRUSTAL SHEAR ZONE: AN EXAMPLE FROM SW MONTANA

Gneissic layering in high metamorphic grade tectonites likely forms from a balance between chemical and mechanical processes during strain localization. But which processes are most important and which contribute to potential feedback mechanisms is not always clear. We investigate these issues in a deep crustal Archean to Paleoproterozoic meta-gabbro that hosts localized cm-scale shear zones in and around Gallatin Canyon in SW Montana. Metamorphic conditions were 1.1 GPa and 800 °C and the shear zones preserve continuous gradients from statically metamorphosed host rock to protomylonite, mylonite and ultramylonite.

The host rock contains mm-scale domains that preserve igneous plagioclase and pyroxene surrounded by localized corona of metamorphic Grt + Hbl + Ilm. The mylonite is characterized by elongate core-mantle structures of partially recrystallized plagioclase and clinopyroxene and an anastomosing foliation. In the ultramylonite, pyroxene appears to have been completely replaced by hornblende, and well-ordered alternating bands rich in combinations of hornblende, plagioclase, scapolite, garnet and quartz form a laminated tectonite with layering parallel to the shear plane. Within the bands, a hornblende shape-defined steady-state foliation developed at 20-35 degree oblique to the shear plane but without a well-developed crystallographically preferred orientation. In contrast to the coronitic garnet in the host rock, garnet appears to have grown synkinematically in the ultramylonite as highly elongate trains parallel to the shear plane. Portions of the garnet trains were subsequently rotated clockwise, consistent with an overall dextral shear sense. In summary, processes with an important chemical component included the dominant reaction of Cpx + Pl to Grt + Hbl, plagioclase replacement by scapolite, and dissolution precipitation creep in the hornblende. Dominantly mechanical processes include evidence for the potential nucleation of cm-scale shear zones along brittle fractures, grain size reduction of clinopyroxene, and partial breakup and rigid-body rotation of garnet. Some of these processes may be purely competitive, but others likely contributed to feedback mechanisms that enhanced the gneissic layering in ways that would have been unachievable by any one process alone.