SILICIC VOLCANISM OF THE CRBG PROVINCE: EVIDENCE FOR VARIABLE AND HIGH SR ISOTOPE DOMAINS WEST OF THE ACCRETED TERRANE-CRATONIC LITHOSPHERE TRANSITION

Radiogenic isotope signatures of widespread, mid-Miocene rhyolites of eastern Oregon that mostly erupted during Columbia River flood basalt volcanism can be used to map crustal domains. 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 west of the craton boundary, which runs parallel but mostly east of the Oregon-Idaho border as delineated by geophysical characteristics and isotopic transitions. Armstrong et al. (1977) were the first to propose that the cratonic margin is preserved as a sharp isotopic boundary as defined as the ⁸⁷Sr/⁸⁶Sr_i=0.7060 line. Here, we focus on initial ⁸⁷Sr/⁸⁶Sr ratios of 23 mid-Miocene silicic volcanic centers in eastern Oregon.

Our data, in combination with literature data, indicate variable ⁸⁷Sr/⁸⁶Sr mostly along longitudinal sections, yet more similar ratios in latitudinal directions. Except for rare examples on the westward 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 ⁸⁷Sr/⁸⁶Sr_i ratios range widely from 0.7041 to 0.7057. In addition, silicic centers 100 km west of the proposed craton boundary are recording Sr isotope signatures greater than 0.7060. Highway 20, which runs predominately in an E-W direction in the study region, is the approximate boundary separating higher ⁸⁷Sr/⁸⁶Sr_i units to the south from lower ⁸⁷Sr/⁸⁶Sr_i units to the north. Further east of 118°W, this transition between lower and higher ⁸⁷Sr/⁸⁶Sr is trending more NE-SW and incorporates the higher ⁸⁷Sr/⁸⁶Sr_i units of Littlefield Rhyolite and rhyolite of Cottonwood Mountain.

In general, our more radiogenic Sr data are inconsistent with the previously determined ⁸⁷Sr/⁸⁶Sr_i=0.7060 and 0.7040 boundaries. The documented range in ⁸⁷Sr/⁸⁶Sr_i reflects isotopic heterogeneities in crustal source regions. Our preferred model is that more radiogenic crustal domains exist west of the cratonic boundary and correlate with more radiogenic crust of selected accreted crustal domains. Alternatively, the accreted crust/cratonic crust transition may be not as sharp as proposed.

Armstrong, R.L., et al., 1977, GSA Bull., 88: 397–411.