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