AGE AND CONDITIONS OF METAMORPHISM OF GARNET HORNBLENDITE ON MINE RIDGE, NORTHEASTERN OREGON: HIGH P/T METAMORPHISM IN A LATE-TRIASSIC SUBDUCTION ZONE

Hornblendite is an ultramafic (i.e., plagioclase-absent) rock that has been variably interpreted as igneous (e.g., cumulates or residual after hydrous partial melting) or metamorphic. Understanding the petrogenesis of hornblendite can have implications for the tectonic environment in which it formed. Blocks of garnet hornblendite occur in serpentinite matrix mélange on Mine Ridge, in the southern part of the Blue Mountains Province of northeastern Oregon. The blocks are coarse-grained, foliated, and contain the assemblage amphibole (magnesiohornblende, edenite, tschermakite, pargasite) + epidote + titanite + rutile + ilmenite + fluorapatite ± garnet (almandine-rich). Primary amphiboles are commonly rimmed with winchite, and minor phengite was observed in some samples. Thin (£1 cm) veins of pure albite crosscut the amphibole foliation in all samples. The mineralogy and fabric observed in these hornblendite samples indicate they are metamorphic, rather than igneous.

Hornblendite samples are mafic, with SiO₂ contents ranging from 41.5 to 51.1 wt.%. TiO₂ is in the range 1.5-2.1 wt.% and K₂O contents are low (0.37-0.80 wt.%). Tectonic discrimination diagrams (e.g., TiO₂-(MnOx10)-(P₂O₅x10) indicate the protolith of these rocks was MORB or IAT, although the high TiO₂ is more consistent with MORB. Pseudosection models using Theriak-Domino suggest the peak metamorphic assemblage was stable at P=800-1150 MPa and T=375-500 °C. Titanite grains (n=48) from one sample yielded a ²⁰⁶Pb/²³⁸U metamorphic age of 227.82±3.66 Ma.

The presence of winchite and phengite suggests that a MORB protolith underwent high-P/low-T prograde metamorphism, approaching the blueschist facies, during Late Triassic time. These rocks subsequently experienced retrograde metamorphism through the greenschist facies (late albite veins and actinolite). We suggest that oceanic lithosphere of the Baker terrane was dismembered as it was rapidly subducted beneath the Olds Ferry island arc terrane, and the resultant fragments were rapidly exhumed through the subduction channel. Results of this study show that the Mine Ridge hornblendite is the result of high P/T metamorphism of a basaltic protolith during the initial stages of the development of the Blue Mountains Province.