MINERAL COMPOSITION AND PHENOCRYST ZONING IN PRIMITIVE ABSAROKITE AT PEPPER MOUNTAIN, BORING VOLCANIC FIELD (BVF), OREGON

One basalt end member of the BVF of the Portland Basin is high-K basalt or absarokite. One such absarokite crops out at Pepper Mountain on the eastern side of the BVF and has a primitive composition with a Mg# of 65, 167 ppm Ni, 280 ppm Cr, and olivine (Fo_87-88.5) just as expected for equilibrium olivine. It is distinguished from other Boring basalts by its high incompatible element composition (Ba =1307 ppm, Sr = 2993 ppm), high LILE/HFSE ratios, and high La/Yb of 41.

Although unexpected given the primitive nature and unzoned, equilibrium olivine, the absarokite reveals some intriguing mineralogical aspects. It contains ~10% phenocrysts of subequal proportions of olivine (with spinel inclusions) and clinopyroxene (cpx). All cpx crystals have a complex growth history and fall in two broad categories. One has coarsely sieved interiors overgrown by euhedral rims. Resorbed interiors and rims are compositionally alike (Mg# = 84-87; Cr₂O₃ = 0.2–1.0 wt%) except the very outermost rim that is more evolved consistent with crystallization during solidification. The other cpx type has similar high Mg# and Cr overgrowths. However, the overgrowth occurred on cpx that is petrographically and compositionally different. Such cpx cores have an unusual dark cloudy microscopic appearance and have much more evolved compositions (Mg# 60-71) combined with higher Al₂O₃ (4-6 wt.%) than high Mg# cpx (2-4 wt.% Al₂O₃). Incompatible trace element data also show distinct differences between cores and rims.

The compositional variation of spinel inclusions in olivine is also striking. All spinels have low Fe²⁺/Fe* of ~0.3 – the lowest among spinels from the BVF. However, other parameters vary strongly from the most primitive spinels with Mg# of ~55 and Cr# of ~65 to the most evolved with Mg# of ~27, Cr# of ~10 and high TiO₂ (~7 wt%). Evolved spinels are often located near fractures that can be extremely subtle while primitive spinels are not.

The above data suggest that Pepper Mtn. absarokite magma, originating from a quite refractory source, must have interacted with deeper crustal components during which more primitive cpx was resorbed and evolved cpx cores were inherited followed by an absarokite recharge that led to ascent during which euhedral rims grew. Copious reequilibration of spinel inclusions suggests this may have occurred deep in the crust as well.