Eclogite, high-grade blueschist, and amphibolite blocks occur within the Mesozoic Otter Point Formation of southwestern Oregon and are inferred to have been tectonically emplaced by eastward-directed overthrusting involving Colebrooke Schist and serpentinite.
Eclogite from southwestern Oregon is very similar in bulk chemistry and mineralogy to the well-studied eclogite of California.
Calculations of phase equilibria at load pressures of 7 and 10 kb and T = 400°C to 550°C suggest that many of the hydrates found in eclogite could have been stable at very low H2O fugacities. The lack of lawsonite and the presence of almandine-grossular garnet set a maximum limit on H2O fugacity for a given Ps-T.
Chemically, Group C eclogite from Oregon and California characteristically is nepheline normative and is enriched in normative diopside relative to basaltic compositions. The present chemistry of this eclogite may be the result of metasomatism in an ultramafic environment with low a SiO2 and high a Ca, but outside the stability field of serpentine.
The generally high jadeite content of clinopyroxene from Group C eclogite compared with Group A and Group B eclogite is largely a function of bulk rock chemistry. Crystallization under low a SiO2 conditions stabilizes jadeite in clinopyroxene at lower Ps for a given T.
