ISOTOPIC COMPOSITIONAL CHANGES ACROSS SPACE, TIME, AND BULK ROCK COMPOSITION IN THE HIGH LAVA PLAINS AND NORTHWESTERN BASIN AND RANGE, OREGON

At least three episodes of Oligocene and younger volcanism are represented in the high lava plateau of central and southeastern Oregon. The most recent pulse, 12 Ma to Recent, is strongly bimodal, characterized by high silica (SiO₂ > 74 wt. %) rhyolitic tuffs and domes that generally young to the west and widespread, non-time transgressive tholeiitic basalts. These rhyolites can be separated into two geographic and three geochemical classifications: those within the physiographic High Lava Plains (HLP) tend to be higher in SiO₂ and FeO and lower in Al₂O₃, similar in character to Icelandic rhyolites (tholeiitic) while those in the northwestern Basin and Range (NWBR) are lower in SiO₂ and FeO and higher in Al₂O₃, more similar in character to silicic rocks in the Cascade Arc (calc-alkaline). Mildly peralkaline rhyolites represent a third geochemical group and are confined to a narrow spatiotemporal region within the HLP.

The NWBR is characterized by large displacement (> 1000 m) extensional normal faulting whereas the Brothers Fault Zone (BFZ), located in the HLP, is a zone of transtension and small normal faults, with offsets limited to < 10 m. While the rhyolites of the two provinces are geochemically distinct, 35 new whole rock radiogenic isotopic (Sr, Nd, Pb) results indicate the rhyolites of both regions have similar petrogenetic histories, despite differences in crustal stresses. Previous workers have shown little whole rock isotopic variation between primitive basalts and evolved basalts and that there is an increase in ⁸⁷Sr/⁸⁶Sr and decrease in εNd to the east, as one approaches the craton. Rhyolites in the eastern part of the study area have similar isotopic ratios to these basalts. Other than these eastern samples, most of the rhyolites and basalts have a narrow range of isotopic ratios falling between 0.7033 to 0.7043 ⁸⁷Sr/⁸⁶Sr and 5.5 to 3.0 εNd indicating that the rhyolites are crystal differentiates of basalts or partial melts of slightly older basalts. An array of rhyolites plots to higher ⁸⁷Sr/⁸⁶Sr at constant ¹⁴³Nd/¹⁴⁴Nd indicating assimilation of young, silicic upper crust and includes samples from both the HLP and NWBR. The peralkaline rhyolites have extreme Rb/Sr ratios (>300) but plot near the range given above. Rhyolites from the earlier two episodes of volcanism have similar isotopic ratios to coeval basalts.