T65 Multidisciplinary Studies of Cascade Volcanism and its Tectonic Setting II

Sunday, 18 October 2009
1:30 PM - 5:30 PM
E143/144

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

Paper number: T65-4
Program: Technical Sessions
Time: 2:15 PM – 2:30 PM

Authors

Abstract

Diverse mafic and intermediate lavas form 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 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 and was succeeded by eruption of CAB, HKCA, and diverse basaltic andesite (BA) and andesite magmas. The chronology suggests the heat source for the younger CAB + HKCA + BA magmatism was the earlier dry magmatism. 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 or uppermost mantle. 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, OIB, CAB, and HKCA. All basalt types 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 mafic lavas, and likely incorporates a large component of crustal melt.

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