VARIABLE AND HIGH 87SR/86SR DOMAINS WEST OF THE TERRANE-CRATONIC LITHOSPHERE TRANSITION AS DEDUCED FROM MID-MIOCENE SILICIC VOLCANISM IN EASTERN OREGON
Our data, in combination with data from the literature, indicate variable 87Sr/86Sr mostly along longitudinal sections, yet more similar ratios in latitudinal directions. Except for rare examples on the west side, dispersion of 87Sr/86Sr ratios among both silicic and basaltic rocks occurs eastward of 118.6°W. For example, rhyolites in the Owyhee region between 117.10°W and 117.25°W retain 87Sr/86Sr ratios ranging from 0.70413 to 0.70566. The most radiogenic Sri ratio of 0.70787 in our study is obtained on a plagioclase separate from Buchanan Dome complex located near the western boundary of our study area. Feldspar separates and fresh groundmass of samples from adjacent centers yield similar 87Sr/86Sr ratios.
In general, our more radiogenic Sr data are inconsistent with the long known 0.706 and 0.704 (87Sr/86Sr) boundaries. Our preferred model is that more radiogenic crustal domains exist west of the cratonic boundary and correlate with more radiogenic crust of select accreted terrane domains. Alternatively, the crustal transition may be not as sharp as proposed.