Fluid Assisted Mixed Phase Deformation Beneath a Barrovian Thrust Sheet

Abstract:

The Taconic orogeny in eastern New York resulted in what has been called a classic Barrovian sequence of metamorphosed rocks ranging from shales to metapelitic gneisses. Layers of quartzite and marble separate the meta-pelitic rocks of the Barrovian sequence proper from the underlying Precambrian gneisses. Field observations of the direction of maximum extension in the quartzite indicate west verging shear, consistent with the inferred emplacement of the Taconic Allochthon. EBSD and compositional mapping show that the quartzite is actually a meta-arkose with up to 40% K-feldspar, minor albite as well as trace phases. Preliminary investigation shows that overall the meta-arkose layer is highly deformed to grain sizes of 20 - 30 micron for both quartz and K-feldspar. The deformation grade as well as the proportions of quartz and K-feldspar are variable across the layer. SEM imaging further indicates the involvement of fluids during deformation. While quartz has a weak crystallographic preferred orientation consistent with inferred P, T conditions, K-feldspar likely deformed by brittle processes. Additionally, pressure solution in the presence of free fluids resulted in phase mixing, with precipitation of quartz and K-feldspar in pressure shadows of larger grains. Quartz fabrics will be examined as a function of phase proportion and involvement of fluid inferred from trace phases and grain boundary porosity. The meta-arkose layer also contains evidence of increasingly localized deformation during retrograde metamorphism.