VARIABLE AND HIGH 87SR/86SR DOMAINS WEST OF THE TERRANE-CRATONIC LITHOSPHERE TRANSITION AS DEDUCED FROM MID-MIOCENE SILICIC VOLCANISM IN EASTERN OREGON

Widespread mid-Miocene rhyolite volcanism of eastern Oregon mostly coeval with flood basalts of the Columbia River Basalt Province allows for mapping crustal domains using radiogenic isotopes. Rhyolites are thought to be derived in large part by partial melting of the crust and thus yield direct information on the composition of the crust. Silicic volcanism is expressed in the form of numerous domes and tuffs exposed over a wide area (~300 km in N-S dimension and ~100 km in E-W dimension) west of the craton boundary, which runs parallel but mostly east of the Oregon-Idaho state border as delineated by geophysical characteristics and isotopic transitions. Here, we mainly focus on initial ⁸⁷Sr/⁸⁶Sr ratios obtained from mid-Miocene silicic volcanic centers 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 Sr_i 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.