MINERALOGICAL EVIDENCE FOR MAGMA SOURCES, MIXING AND ASCENT OF BASALTIC TO ANDESITIC LAVAS OF THE BORING VOLCANIC FIELD (BVF), PORTLAND BASIN, OREGON

The 2.6 Ma–60 ka BVF of the greater Portland area comprises mostly low-K tholeiite (LKT) and calc-alkaline basalt (CAB) but also lesser OIB-type and high-K basalt (absarokite), diverse basaltic andesite (BA) and occasional true andesite.

The basalts are usually phenocryst poor (<5%) and all contain olivine phenocrysts with spinel inclusions. Olivine ranges from Fo₈₉ to Fo₇₄ within primitive magmas (Mg# 56-70, Cr = 170-400, Ni = 100-320 ppm) and tends to be more forsteritic in CAB and absarokite (i.e., Group II lavas). Primitive spinel (TiO₂<2.5 wt%; Mg#>40) within LKT (Group I lavas) has the lowest Cr# (~30), highest Al₂O₃ (40 wt%) whereas spinel from absarokite has highest Cr# (~70) and lowest Al₂O₃ (~7%). Compositional variations of olivine and spinel within single samples indicate that mixing of basaltic magmas is rather common, yet differences in primitive spinel compositions also suggest intrinsic compositional differences between magma sources of Group I and II lavas. Phenocryst-sized plagioclase frequently occurs but at very low abundances (1-3 crystals/thin section) and most show signs of resorption suggesting a xenocrystic origin and thus some crustal interaction. Clinopyroxene phenocrysts are largely restricted to CAB or absarokite. Zoning and composition of cpx indicate growth from basaltic melts but some are inherited from more evolved melts or crustal rocks as indicated by crystal cores with Mg# as low as 60 and low Cr contents. Sizable phenocrysts (>300 µm) also grew rather rapidly – likely during ascent – as evidenced by olivines with skeletal outlines or characteristic melt inclusion shapes and sector zoning in cpx.

Andesites are of both calc-alkalic and adakitic affinity, with the latter distinguished by higher maximum plagioclase An content (~89 vs. ~64). Andesites vary widely from phenocryst rich (plag, pyroxene, olivine and texturally akin to those erupted at nearby composite volcanoes like Mt. Hood) to nearly aphyric (≤ 1% phenocrysts) andesite that also contains rare pseudomorphs after amphibole. Thus intricate plumbing below large volcanic edifices appears to favor generation of phenocryst-rich arc magmas via mixing, stagnation, and crystallization whereas resorption and suppression of phenocryst growth seem important in generating the aphyric lithologies.