COMPOSITIONAL TRENDS IN HIGH LAVA PLAINS AND NORTHWEST BASIN AND RANGE RHYOLITES, COMPARISON TO SNAKE RIVER PLAIN, CASCADES AND ICELAND: PARTIAL MELT, FRACTIONATION, OR BOTH?

The High Lava Plains (HLP) and Northwest Basin and Range (NWBR) are a single, bimodal, volcanic province with time-transgressive rhyolitic eruptions that track form southeastern to central Oregon over the past 12 Ma. Basalts from the area are not time-transgressive. The NWBR contains large displacement (> 1000m) crustal-scale normal faults whereas the HLP is a zone of transtension with fault offsets < 50m across the Brothers Fault Zone. Compared to coeval rhyolites of the NWBR, the HLP rhyolites are more diverse, including high-Fe and peralkaline varieties. HLP rhyolites are generally higher in SiO₂, FeO*, TiO₂, REE and HFSE although there is significant overlap at higher silica values. Fe enrichment in HLP rhyolites is similar to the lower range in values from the Snake River Plain and Iceland. In contrast, the compositional range of NWBR rhyolites is more similar to that of the Cascades and parts of the BR.

We evaluate 4 models to produce “parental” low silica rhyolites from which high silica rhyolites are evolved mainly by fractional crystallization (FC). The models are 1) partial melts of amphibolite (+/- biotite) accreted terranes, 2) partial melts of recently intraplated basalts (gabbros), 3) extensive FC of mafic parents, and 4) assimilation-fractional crystallization (AFC) processes.

Parental low silica NWBR rhyolites are likely a product of AFC processes as there are more intermediate compositions represented and some elements (Na, Sr) are highest in these samples. Results indicate that assimilation is subordinate, with FC likely playing the dominant role in the formation of NWBR rhyolites. Most parental HLP rhyolites are a product of partial melts, and for the compositionally diverse eastern areas, we favor partial melts of amphibolitic protolith over that of intraplated basalts, mainly based on K₂O values. Protracted volcanism, either in the western region or at longer-lived polygenetic centers in the HLP indicates that over time, more partial melt is produced. This heat flux creates a feedback in the crust, resulting in both a more mafic crust and more partial melt, and is the likely cause of much of the volcanism in the HLP, including the voluminous ignimbrites. Many intermediate rocks in the HLP are clearly mixtures of end member compositions while a few rhyolites might be formed by extensive FC.