TEXTURAL VARIATION IN ULTRAMAFIC MINERAL ASSEMBLAGES FROM WESTERN IDAHO

The Western Idaho Suture Zone is a tectonic lineament which juxtaposes Mesozoic Island-arc volcanic terranes against western Laurentia. Along this boundary are a series of metaultramafic slivers, which vary from serpentinites, to high grade metaperidotites. The metaultramafic rocks record a sequence of overprinting hydration events that is reflected by their mineral paragenesis. Early low temperature serpentinization consists of lizardite and nanoscale crystals of antigorite, which is overprinted by antigorite coarsening and progressive structural annealing. High temperature metaperidotites have recrystallized olivine and anthophyllite along with retrograde biopyribole, clinochlore, and antigorite. Retrograde antigorite from the high temperature metaperidotites are aluminous, and mantle clinochlore as epitaxial overgrowths. Structurally, the aluminous antigorite has a prevalence of two-layer stacking, which consistent with reports of this structure spanning since the late 1970s. Combined high-resolution transmission electron microscopy (HRTEM) and single-crystal X-ray diffraction (SC-XRD) evidence suggests that the aluminous antigorite structure contains lizardite-2H modules along with reversals on 6 and 8-membered tetrahedral rings.