PETROLOGIC LINKAGES BETWEEN CONTEMPORANEOUS ASH-FLOW TUFFS AT NEWBERRY VOLCANO, CENTRAL OREGON

Ash-flow tuffs, representing the earliest caldera-forming eruption at Newberry Volcano, provide insights into petrogenesis of silicic magmas at a voluminous central volcano located ~60 km east of the Cascades in central Oregon. This study focuses on three tuffs exposed on the east flank of Newberry, including tuff of Tepee Draw (Qtp), ‘dacitic’ tuff (Qdt), and tuff of Orphan Butte (Qto). Most pumices from Qtp are rhyolite and homogeneous in hand sample (70.1-72.6 wt. % SiO₂), though some with conspicuous banding are less evolved (62.3-68.7 wt. %). Pumices from Qdt and Qto are dacite, with minimal SiO₂ variation (65.6-66.5 wt. %). On many variation diagrams, Qdt and Qto pumices plot as a group between the less evolved and more evolved Qtp samples. Electron microprobe analyses of plagioclase and pyroxene from select pumices define distinct populations, representing diverse magma compositions. Plagioclase phenocrysts from Qtp are subdivided into calcic (An_60-84) and sodic (An_20-44) groups. Banded pumices contain both calcic and sodic plagioclase, while sodic types only occur in the rhyolite pumices. Plagioclase phenocrysts from Qdt and Qto have a limited compositional range (An_36-55), plotting between the calcic and sodic groups in Qtp. For pyroxene, several varieties have been identified in Qtp pumices based on Mg content. Pyroxenes in the banded pumices are higher-Mg augite and enstatite, while those in the rhyolite pumices are lower Mg. In comparison, pyroxenes from Qdt and Qto display intermediate compositions between the higher and lower Mg groups. To provide age constraints, plagioclase mineral separates from Qtp and Qdt were dated using the ⁴⁰Ar/³⁹Ar technique. Analyses from plateau, single-crystal, and multi-crystal methods yield weighted mean ages of 0.250 ± 0.014 Ma for Qtp and 0.244 ± 0.024 Ma for Qdt.

The Newberry ash-flow tuffs record a dynamic scenario in which diverse magmas were erupted over relatively short time spans from the same evolving system. Qtp shows evidence that mafic magma was injected into a dominantly rhyolitic magma body and likely triggered the earliest caldera-forming eruption at Newberry. During eruption of Qtp, unerupted rhyolitic magma mixed with mafic magma to form dacitic magma, which in turn homogenized in the chamber, re-equilibrated, and subsequently erupted to form Qdt and Qto.