Title

Petrology of the Eocene Goble Volcanics, Southwest Washington: an Early Phase of the Cascade Arc

Faculty Advisor

Tepper, Jeffrey

Area of Study

Science and Mathematics

Publication Date

Summer 2019

Abstract

The Eocene Goble Volcanics (GV), comprised of >1000 km2 of subaerial lavas and tuffs, are one of the most voluminous igneous formations in SW WA. Whole rock K-Ar ages of 45 to 32 Ma (Beck and Burr, 1979) suggest these rocks are an early expression of the Cascade arc, but their location is anomalous, lying between oceanic Crescent Fm basalts of the Siltezia terrane to the west and younger arc rocks to the east. The goals of this research are to determine the chemical and Sr-Nd isotopic traits of the GV and better establish the tectonic setting in which these rocks formed.

Samples collected along a NW-SE transect across the GV fall into two groups. Group 1 consists of high-alumina tholeiitic basalts (48 wt.% SiO2, 17.6 - 18.2 wt.% Al2O3, Mg# 58 - 55) equivalent to the Kalama Basalt (Evarts, 1991). These lavas lack arc traits (Ba/Nb < 10) but are distinct from the Crescent basalts (e.g., lower TiO2, higher Al2O3) and similar to high alumina olivine tholeiites (HAOT) elsewhere in the Cascades (Leeman et al, 1991; Bailey and Conrey, 1992). Group 2 lavas, which appear to be younger, range from basaltic andesite to dacite (52 - 66 wt.% SiO2, Mg# 56 - 17), and have modest subduction signatures (Ba/Nb = 13 - 21) and greater LREE enrichment (La/Yb = 4.5 – 7.0 vs. 2.4 - 2.5).

The similarity of Group 1 lavas to HAOT suggests they formed in an extensional setting by melting of depleted mantle at shallow depth. The lack of arc traits in Group 1 suggests melting was driven by decompression rather than influx of a slab fluid, and extension may have been an upper plate response to re-establishment of subduction. Conversely, Group 2 magmas are products of melting of less depleted mantle that had been modified by slab-derived components. One GV sample with traits of both groups suggests a gradational transition between the two. Although more mapping and analysis are needed, available data indicate the GV spans >10 Ma and may record a transition in Cascade arc magmatism from an early phase driven mainly by decompression to a later phase influenced more by slab contributions.

Publisher

University of Puget Sound

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