87SR/86SR OF MID-MIOCENE SILICIC VOLCANISM IN EASTERN OREGON: EVIDENCE FOR VARIABLE AND HIGH SR DOMAINS WEST OF THE TERRANE-CRATONIC LITHOSPHERE TRANSITION

Widespread rhyolite volcanism mostly associated with voluminous mid-Miocene flood basalts of the Columbia River Basalt Province allows for mapping crustal domains using radiogenic isotopes. Here we use Sr isotopes but will supplement with other isotopic systems to study the source regions of rhyolites. Rhyolites of Oregon are thought to be mainly derived by partial melting of the crust and thus yield direct information of the make-up of the crust from which they are derived. Mid-Miocene rhyolite volcanism is expressed in the form of numerous silicic domes and tuffs that crop out over a wide area (~300 km in N-S dimension and ~100 km in E-W dimension) mostly west of the cratonic crust boundary located near the Oregon-Idaho state border. This boundary is delineated by geophysical methods and isotopic transitions. Our ongoing study is currently focused on determining the ⁸⁷Sr/⁸⁶Sr ratios of bulk rocks and mineral separates obtained from samples of mid-Miocene silicic volcanic centers in eastern Oregon.

Preliminary data indicate variable ⁸⁷Sr/⁸⁶Sr mostly along longitudinal sections, yet more similar values in latitudinal directions. In general, however, data indicate disagreement of isotopic ratios in relation to the long known 0.706 and 0.704 (⁸⁷Sr/⁸⁶Sr) boundaries. This calls into question how sharp the crustal transition really is or to what extent more radiogenic crustal domains exist west of the cratonic boundary that may well correlate with more radiogenic crust of select accreted terrane domains.