METAMORPHISM OF OCEANIC ULTRAMAFIC ROCKS FROM WESTERN IDAHO

The accreted terranes of central Oregon and western Idaho contain a series of ultramafic occurrences. Geologic mapping demonstrates that slivers of serpentinites, talc schists, and metaperidotites are located within several sedimentary and metasedimentary units throughout the region. Ultramafic occurrences are commonly found within marine sediments, which terminate at the boundary of western Laurentia, an area titled the western Idaho suture zone. Antigorite-serpentinites in Idaho have enrichments in fluid-mobile elements consistent with global values for subduction-channel type serpentinites. Near the western Idaho suture zone, the metamorphic grade increases to upper amphibolite facies in contrast with the low-grade rocks outboard. Ultramafic rocks there are influenced by the increase in grade, forming concentric metasomatic horizons. Metamorphic thermobarometry performed on rocks surrounding the ultramafic occurrences confirms ultramafic phase equilibria coincide with the temperatures and pressures from thermobarometry estimates. Olivine-bearing assemblages, which were previously considered to be igneous in origin, are a result of an antigorite breakdown reaction based on regional thermobarometry (637 ˚C and 6.6 kilobars), textural relations, and phase equilibria. Overprinting antigorite grains surround clinochlore core crystals in the high-grade assemblages and have a unique two-layer crystal structure. We propose this texture occurs in-part due to the stability of clinochlore as the aluminum phase beyond the antigorite breakdown temperature, and subsequent retrograde metamorphism. These observations indicate the following sequence of events: 1) serpentinization of ocean basin ultramafic rocks; 2) obduction of serpentinites into accretionary sediments; 3) prograde metamorphism of the serpentinites during arc-continent suturing; and 4) retrograde metamorphism of the metaperidotites during exhumation.