INVESTIGATING MODELS OF METAMORPHIC SOLE PRESERVATION THROUGH THERMOBAROMETRY AND THERMODYNAMIC MODELLING OF THE EASTON METAMORPHIC SUITE, NORTHWEST CASCADES, WASHINGTON

Exhumed subduction complexes are critical for understanding the tectonic history of convergent margins and the thermal evolution of early subduction processes. Thin slivers of high-grade rocks, often called metamorphic soles, preserved in many subduction complexes are formed during subduction initiation when the down going slab is heated and accreted to the overlying mantle wedge. Two primary mechanisms have been proposed to preserve these high-grade assemblages during continued subduction: 1) subduction refrigeration which cools and preserves the high-grade rocks at depth and 2) rapid exhumation which brings the high-grade assemblages to cooler and shallower levels. The Easton metamorphic suite of the Northwest Cascades is a Jurassic-Cretaceous subduction complex with an inverted metamorphic gradient of older accreted high-grade rocks above younger regional blueschist assemblages. Conventional thermometry and phase equilibrium modeling, combined with published geochronology, show that the peak temperatures of the high-grade assemblages decrease with age structurally downward, but at approximately constant pressure for ~30 Ma. The oldest rocks consist of blocks of Grt±Cpx amphibolite that was metamorphosed at 700-760 C (0.9-1.2 GPa) circa 203 Ma. These blocks are contained within foliated amphibolite that records peak metamorphic conditions of 630-700 C (0.8-1.1 GPa) at 183 Ma. The amphibolite is underlain by a high temperature Grt-Ab-Gln blueschist that was metamorphosed at 500-550 C (0.9-1 GPa) prior to 165 Ma. Previous studies on the structurally lower regional Shuksan blueschist indicate the unit was metamorphosed at 350-400 C (0.7-0.8 GPa) by 140 Ma. The combined P-T-t history of units within the Easton metamorphic suite suggest the high-grade units experienced an early phase of subduction refrigeration from <183 Ma to ~165 Ma, followed by partial exhumation to ~0.7-0.8 GPa at ~160 Ma, and were underplated by the regional (Shuksan) blueschist at ≤350 C circa 140 Ma.