FRACTURING, FLUID FLOW, AND DEEP CRUSTAL SHEAR ZONE NUCLEATION IN PALEOPROTEROZOIC METAGABBRO, SW MONTANA

Fluids likely play important roles during deep crustal strain localization, yet the specific processes active during the complex convergence of metasomatism, metamorphism, and deformation remain elusive. Precambrian metagabbro dikes in SW Montana containing abundant Grt ± Hbl-lined fractures and discrete cm-scale shear zones, offer ideal settings to study these processes. The shear zones preserve well-developed strain gradients from isotropic host to ultramylonite, across which the metasomatic impact rises with modal increases in synkinematic scapolite and hornblende. In the host, relict igneous Opx_ig, Pgt_ig, and high-Ca Pl_ig are surrounded by an early dry metamorphic (M₁) assemblage of Cpx₁, coronitic garnet, quartz and moderate-Ca Pl₁. Static fractures cut these early assemblages and are filled with Grt and locally Hbl. A second assemblage (M₂), post-dating the fractures, is preserved as Hbl haloes around pyroxene and a third generation of low-Ca Pl₂. In the mylonite, early Cpx₁ and Opx_Ig act as sigma and delta clasts, surrounded by dynamically recrystallized Cpx₂ + Hbl₂ mantles and elongate tails. Fine-grained Pl₂ and scapolite replace Pl_ig and Pl₁ in extended plagioclase lenses. Early pyroxene is completely absent in the ultramylonite, and well-ordered alternating bands rich in Hbl₂, Cpx₂, Pl₂, Scp₂, Grt₂ and/or Qz form a laminated tectonite. Phase assemblage modeling in the NCFMASH system yield P-T conditions of ~0.63-0.77 GPa and 550-660 °C during M₁ and 0.86-1.0 GPa and 660-740°C during M₂. In summary, the transformation from isotropic metagabbro to layered tectonite is facilitated by (1) polyphase metamorphism, (2) brittle fracturing, (3) fluid infiltration and elevated fluxing along fractures, (4) fractures locally acting as nucleation surfaces for deformation, which reduced grain size and “mixed” the relict M₁ and igneous clasts into the effective bulk composition, and (5) by fluid-induced chemical alteration that shifted the composition by increasing H₂O, Na, K, and CO₂ and decreasing Ti. Increasing pressure during M₂ may represent contraction during the ~1.8-1.7 Ga Big Sky orogeny, or it may alternatively provide clues about the nature of a less well understood period of tectonic activity in this region near the Archean-Proterozoic boundary at 2.55 Ga.