Faculty Advisor

Tepper, Jeffery

Area of Study

Science and Mathematics

Publication Date

Summer 2019

Abstract

Modern Cascade arc magmatism began ~45 Ma, shortly after accretion of the Siletzia terrane culminated at ~50 Ma. The earliest expressions of this magmatism are several petrologically-diverse volcanic units in SW Washington including: (1) the Goble Volcanics (GV) / Hatchet Mountain Formation (HM), (2) the Pe Ell Formation (PE), and (3) scattered exposures of unnamed basalts (UB). These rocks, all dominantly subaerial lavas, occur west of the modern arc where they are interbedded with marine and deltaic sedimentary units, suggesting eruption in a forearc or volcanic front setting. Goals of this study are: (1) to characterize the elemental and Sr-Nd isotopic compositions of these early Cascades lavas, (2) to determine whether there are chemical differences between the HM and GV, and (3) to investigate the long-term chemical evolution of the Cascade arc by comparing the compositions of these early lavas with those of younger Cascade arc rocks.

Although mapped separately, the HM and GV are chemically indistinguishable. GV ranges from basalt to dacite (48.1%- 66.7%) encompassing the range of silica for HM (50.4%- 60.1%). Both units range from tholeiitic to calc-alkaline (up to 1.75 wt. % K2O) and have HFSE depletions and LILE enrichments characteristic of an arc setting. PE basalts are alkaline (shoshonite series) with OIB-like spidergram trends and La/Yb > 25. The UB are Mg rich tholeiites (45.6-46.3 wt.% SiO2, 7.6-10.9 wt.% MgO) with moderate REE fractionation (La/Yb = 5.5-18.4) and OIB affinities (Ba/Nb = 4 – 7). These primitive basalts are chemically similar to the Basalt of Wolf Point (Evarts, 2001) and the Quaternary low K tholeiites of Leeman et al (2005).

The compositional diversity of these lavas implies significantly different mantle sources / melting conditions. The PE and UB both require an OIB-like source that contained garnet. The higher La/Yb ratio for PE suggests a source with more garnet, perhaps the result of a lower degree of melting. Application of the Lee et al (2010) thermobarometer to UB magmas yields T = 1572-1590°C and P = 3.5-4.2 Gpa (105-126 km depth). These conditions suggest an asthenospheric source, possibly associated with a slab tear. The arc traits of the MF and GV imply a mantle wedge source that had been modified by slab-derived components.

Mellam

Publisher

University of Puget Sound

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