Extension and Explosivity during an Eccentric Era of the Early Oregon High Cascades

Abstract:

One of the most profound changes that the Cascade arc experienced since its inception ~40 Ma, was an eastward shift in volcanic activity starting at ~7.5 Ma, which initiated the modern High Cascades. The infant stages of this arc are exceptionally well preserved within the Deschutes Fm. (~7.4 - 4.0 Ma) of Central Oregon. In stark contrast to the effusive andesitic eruptions that dominated ancestral Cascade volcanism for the preceding 10 million years, the Deschutes Fm. contains over 120 (uncorrelated) tephra fall units and 130 ignimbrite units, indicating an unusually explosive period of volcanism. Conservative estimates of the cumulative volume for 14 regionally extensive ignimbrites is greater than 80 km³. Furthermore, ⁴⁰Ar-³⁹Ar dating of plagioclase from 7 ignimbrites indicate that this large volume was erupted in less than 1 million years (6.24 ±0.07 to 5.44 ±0.04 Ma).

Glass compositions of pumice (n=718) range from 54 to 76 wt. % SiO₂. Most ignimbrites contain multiple pumice populations, including banded pumice, which can span nearly 20 wt. % SiO₂, indicating involvement of multiple magma types. Two ignimbrites have a compositional gap between 62 and 68 wt. % SiO₂, possibly suggesting mingling of a mafic magma with a silicic one derived from partial crustal melting. Trace element (e.g. Nb, Ce, Th) compositions of rhyolitic pumice differ between northern- and southern-sourced ignimbrites, which may be indicative of disparate crustal sources of partial melts (i.e. Siletzia in the North). In addition, Deschutes Fm. rocks are enriched in FeO* and Zr/Sr compared to Quaternary Cascades, and are more similar to High Lava Plains. These trends and the absence of amphibole within the formation suggests hotter and drier magmatic conditions. We suggest that regional extension contributed to increased basaltic flux, leading to anatexis of previously un-melted crust beneath the new arc axis, thereby producing large volumes of silicic magma during this short explosive pulse.