Low HFSE Rhyolites from Oregon: Smoking Gun for an Origin Through Partial Melting of the Crust

Rhyolites are a significant component of continental magmatism of Oregon for the last 40 million years. Rhyolites of all time slices are compositionally diverse ranging from low to high-silica rhyolites, from less fractionated rhyolites with high Ba concentrations (≥1300 ppm Ba) and weak Eu anomalies (>0.4 Eu/Eu*) to strongly fractionated rhyolites with very low Ba contents (≤100 ppm) but strong Eu anomalies (≤0.2 Eu/Eu*), and rhyolites range from Fe-rich, A-type rhyolites to calc-alkaline varieties. In this study, we focus on one compositional end member of rhyolites that is characterized by very low concentrations of high field strength elements (HFSE) designated here as low HFSE rhyolites. Such low HFSE rhyolites contain Nb and Ta contents of 8–13 ppm and 0.8–1.1 ppm, respectively, and Zr and Hf concentrations of 60–150 ppm and 2.5–4 ppm, respectively. The significance of these low HFSE concentrations is that they are consistent with partial melting of observed crustal rocks but not with fractionation of observed mafic magmas.

Compilation of chemical data of Cenozoic mafic magmas across Oregon indicate that the vast majority of basaltic rocks contain Nb and Zr contents in excess of 5 and 100 ppm, respectively. Only the most primitive high-Al olivine basalts have concentrations as low as 1 and 40 ppm. Basaltic magmas generate rhyolitic liquids after ~90-95% crystallization (e.g., as seen in lava lakes). After such high amounts of crystallization using basaltic magmas from Oregon, Nb and Zr contents would be far in excess of what is observed in these low HFSE rhyolites as both elements will behave incompatibly (strongly to moderately) during fractionation to rhyolites. Also, there are chemical and petrographic arguments that neither late titanite nor zircon fractionation in rhyolites caused low HFSE contents. Although lesser amounts of fractionation would be needed starting with an andesitic composition, HFSE contents of most andesites are already on levels as in low HFSE rhyolites. This indicates pure fractionation scenarios from mafic magmas to yield low HFSE rhyolites are implausible. On the other hand, there are intermediate to silicic crustal rocks with appropriately low HFSE contents that can yield rhyolite liquids during partial melting with or without restitic zircon.