PETROLOGY AND GEOCHRONOLOGY OF THE EOCENE BASALT OF FROST MOUNTAIN: THOLEIITIC VOLCANISM IN THE CENTRAL CASCADES, WASHINGTON

The Eocene was a time of great geologic change across the Pacific Northwest, with compositionally diverse magmatism extending as far inland as Montana. In central Washington volcanic rocks of this age include the Naches Formation (NF), the Teanaway Formation (TF), the Taneum Formation (TNM) and the Basalt of Frost Mountain (BFM). This study focuses on characterizing the petrology, eruptive style and geochronology of the BFM and the tectonic setting in which they erupted.

The 30 km² BFM is a NW trending unit, ~25 km long, consisting dominantly of subaerial flows and diabase with lesser hyaloclastite and tuff. The age is poorly constrained by K-Ar dates ranging from 53 - 42 Ma (Tabor et al., 1984). BFM samples are bimodal with a gap between mafic-intermediate rocks (48.5 – 65.1 wt. % SiO₂ ; Mg# = 0.24 – 0.58) and tuffs (78.7 wt. % SiO₂ ; Mg# = 0.16). The rocks are tholeiitic, which sets the BFM and nearby TF apart from all other Eocene units in the vicinity. Mafic BFM samples (SiO2 < 53 wt. %) generally lack Ta-Nb depletions characteristic of arc magmas, whereas those with higher silica (SiO2 > 53 wt. %) have arc signatures. All samples have relatively unfractionated REE (La/Yb = 1.1 – 5.4) and negligible Eu anomalies (except the felsic tuff), and most plot as MORB on tectonic discrimination diagrams.

The nearby TF is also tholeiitic and bimodal, which has led to the suggestion that it may be correlative with the BFM (Tabor et al., 1984). However, the two units differ significantly in major and trace element composition, BFM having lower SiO₂ (avg = 52.9 vs. 57.0) and higher Al₂O₃ (avg = 16.0 vs. 12.9). In addition, samples with MORB-like spidergrams are not present among the TF. These differences indicate that the BFM and TF are separate magmatic systems, although they likely formed by similar processes.

The presence of lavas with both arc and MORB-like spidergram signatures and the bimodal character of BFM are consistent with a model in which hot mantle ascending through a rupture in the slab promoted melting of both aesthenospheric mantle and mantle wedge. Ascent of mantle derived basalts may have triggered subsequent crustal melting that produced felsic melts. Ongoing Sr-Nd isotopic analyses and U-Pb dating will shed further light on the origin of the BFM.