PLEISTOCENE BASALT EVOLUTION IN THE NORTHERN BASIN & RANGE - HIGH LAVA PLAINS TRANSITION, SOUTHEASTERN OREGON

The chemical and physical processes taking place over the lifespan of a basaltic magmatic system are reflected in its eruptive products. This study focuses on small-volume basaltic volcanism in the Hawks Valley-Lone Mountain (HVLM) region of southeastern Oregon; one of many examples of abundant late Cenozoic northwestern United States basaltic volcanism that ranges from voluminous flood basalts to small-volume monogenetic systems. The HVLM is located at the transition from the northern Basin & Range to the High Lava Plains and preserves a history of episodic extension and magmatic system development since ~16.6 Ma. The youngest components of this history are a sequence of 0.7±0.2 Ma low-K, high-Al olivine tholeiite (HAOT) lava flows that erupted from a vent(s) along preexisting ~N-S trending zones of weakness, and the NNE and NW trending normal faults that displace these flows and vent(s). These simple and compound pahoehoe basalt flows cover an area of over 90 km² with 25 m of vertical exposure visible in the NNE trending fault bounding the western side of Hawks Valley. Approximately 40 samples from surface and vertical fault scarp sections have been examined. Detailed stratigraphically controlled sampling was performed at three scarp localities separated by distances of approximately 2.4 to 4.0 km with individual flow/lobe maximum thicknesses of 1 to 10 m and total exposure thicknesses of 9 to 25 m. Petrographically, the basalts are porphyritic to glomeroporphyritic with phenocrysts of olivine and plagioclase set in an intergranular to subophitic-ophitic, diktytaxitic groundmass composed of plagioclase, olivine, clinopyroxene and minor oxides. Whole rock major and trace element data show little variation although two groups are suggested based on SiO₂ and MgO contents. Only the low-Mg, high-Si group is represented in the stratigraphically controlled sample suite. Samples from the three scarp sections display relatively flat REE patterns (~10 times chondritic) and enrichments in Ba and Sr compared to NMORB. Minor up-section variations exist, notably increasing MgO and CaO with decreasing K₂O, but with no accompanying variation in incompatible trace element ratios. These characteristics suggest basalt evolution involving fractional crystallization and recharge by less evolved parental liquid.