Evidence for retrogression of garnet peridotite in large ultramafic bodies, with late CO2-infiltration, and formation of heazlewoodite, orcelite, awaruite, andradite(?), lime, and pentlandite, and possible UHP metamorphism, Washington, USA

   Chlorite pseudomorphs in metaperidotites are not unusual in the Alps and are often in the vicinity of garnet peridotites, for example in the Ulten Zone of northern Italy (Pellegrino et al., 2021). If no garnet survives, then considering the important high-pressure and tectonic implications of garnet peridotite, it is important to demonstrate the former presence of garnet.
    In Washington state, two similar ultramafic bodies inside the ~91-95 Ma Wenatchee Ridge Orthogneiss, a highly deformed tonalite pluton, contain cm-scale chlorite pseudomorphs consisting of Cr (1-6.5 wt%) clinochlore. The host rock typically contains Ol-Srp-Tr±Chl±En±Tlc±Cum±Chr±Mag. One body is typically foliated and contains highly flattened chlorite pseudomorphs with undeformed tremolite and cummingtonite, whereas the other body contains spectacularly deformed, including isoclinally folded, enstatite.
    Chlorite occurs especially as relatively fine-grained randomly oriented flakes within the pseudomorphs. Rarely, chromite grains form s-shaped patterns inside. There is locally a slight core-to-margin variation in Cr content. These pseudomorphs are interpreted as a result of hydrous fluids accessing the rocks during cooling and decompression, resulting in chlorite replacing garnet.
    Minor minerals present include ilmenite (minor geikielite or pyrophanite components), barite, pentlandite grains rimmed by awaruite inside magnetite grains, rare Ni-As grains (probably orcelite), chromite, and heazlewoodite in pentlandite.
    A remarkable aspect of the chlorite pseudomorphs is the presence of late, thin (tens of microns), foliation-parallel calcite veins (no magnesite or dolomite). Normally confined to the pseudomorphs, they increase in thickness from margin to core, indicating a mechanical connection to the chlorite. Assuming countervailing volume expansion from decompression and volume decrease from cooling, a small volume loss, approximately consistent with the volume of the calcite veins, occurs for decreases of approximately 0.4 GPa and 400 °C. Lack of pre-existing carbonate indicates CO2 was introduced via fluid infiltration, whereas Ca may have been liberated from diopside or tremolite breakdown.
    The veins are complex; some are composed purely of calcite, whereas others display fibrous, dilational characteristics and multiple minerals. A Fe-Ca-Si-O mineral (andradite?) is present locally. Small lozenges of probable lime, a rare and unstable mineral, occur. Lime has been reported from limestone xenoliths and pyrometamorphic settings, and is thought to form above 900 °C (Khoury et al., 2016), and readily reacts to portlandite. The veins must be late, forming from local Ca but an external CO2-rich fluid.
     We tentatively propose a P-T path from the Grt-Ol-En field through the Di-Chl-En-Ol field, and into the Tr-Ol field, and finally into the Di-Atg fields of Lakey & Hermann (2022). This is consistent with the near absence of diopside but very late Di+Atg after tremolite, and indicates replacement of garnet by chlorite above about 2 GPa. This could indicate origin of these bodies at >2 GPa and ca. 800 °C, and a decompression and cooling path merging with that of the terrane at 600-650 °C and 1 GPa. Such pressures and the required tectonism would be a new twist on the Cordilleran Orogeny in the U.S. Pacific Northwest.