17 Hazards and Resources in the Portland, Tualatin, and Willamette Basins of Oregon and Washington (Posters)

A PETROLOGIC OVERVIEW OF THE BORING VOLCANIC FIELD, CASCADE RANGE, USA

Paper number: 17-6
Program: THEME SESSIONS
Time: 8:00 AM – 6:00 PM

Authors

Abstract

Diverse mafic and intermediate lavas comprise the Pliocene-Quaternary Boring Volcanic Field in and around the Portland (OR) Basin. These lavas erupted in the forearc of the Cascades, but are similar to lavas found throughout a broad transect extending eastward at least 130 km. Some basalts display little or no subduction influence (low Ba/Nb and Th/Ta, plag+ol+sp-bearing mid-ocean ridge-like low-K tholeiite [LKT] + ocean island-like alkaline [OIB]), whereas others have typical subduction-related characteristics (high Ba/Nb, cpx+ol+sp±amph high-K calc-alkaline [HKCA] + calc-alkaline [CAB]). The sudden onset of predominantly LKT + OIB magmatism ca. 2.5 to 1.0 Ma was time transgressive from SE to NW across the Portland Basin following an ~10 Ma volcanic hiatus, and was succeeded by eruption of CAB, HKCA, and diverse basaltic andesite (BA) and andesite magmas. Low B/Nb in Boring lavas suggests minimal fluid addition to the mantle wedge in this part of the arc (Leeman et. al., 2004), and shallow mantle equilibration depths are inferred for Boring CAB + HKCA (Leeman et al., 2005). These characteristics, along with high Nb/Ta, Y/Yb, Ca/Al, and Zr/Ti ratios, support an origin for Boring CAB + HKCA via melting of fossil amphibole-bearing, subduction-metasomatized lithosphere. Melting was perhaps induced by advection of hotter deeper mantle that yielded LKT and OIB magmas beneath the northern OR Cascades during intra-arc rift propagation.

Compositional variations in evolved magmas appear to be generated by poorly constrained assimilation-fractionation-mixing processes in the lower crust. A lower crustal evolution is inferred from high Al2O3 and Sr concentrations, and high Sr/Y and Sr/Zr ratios in all evolved lavas. Basaltic andesite has a wide range in Ba/Nb, similar to that in LKT, CAB, and HKCA. All basalt types but OIB appear to have potential BA progeny but the genetic process is not simple fractionation. Andesite has Ba/Nb similar to CAB, but some exhibit extremely low Nb (~ 3 ppm). The latter andesite lacks an apparent differentiation path from any of the basalts or basaltic andesites, and is possibly a deep crustal melt.

Leeman et al., 2004, Chem. Geol., v.212, p.101; Leeman et al., 2005, JVGR, v.140, p.67.