HIGH PRESSURE, AL RICH MEGACRYSTIC PYROXENES IN LAVAS OF NORTHEASTERN OR: EVIDENCE OF DEEP CRYSTAL MUSH?

The Magpie Basalts (MB) of northeastern OR are located in the Magpie Table 7.5-minute quadrangle and make up small volume lavas (<5 km³) which lay on top of andesite and rhyolite lavas of the Strawberry Volcanics (SV). The most proximal, dated SV andesite and rhyolite lavas are 15.6 and 16.2 Ma, respectively, providing a maximum age for the MB. The MB lavas are primitive, tholeiitic, mildly alkaline basalts with ~8.0 to 8.8 wt% MgO and ~47.0 to 48.5 wt% SiO₂. They can be distinguished from the SV by clear differences in incompatible trace elements; MB have elevated HFSE (Nb, Ta, Th, and U, 34.6, 2.13, 2.86, and 0.8 ppm respectively) and lower Pb (1.65 ppm) relative to the SV. Furthermore, MB have large (>4 cm) megacrysts of clinopyroxene (cpx) with spinel inclusions. The cores of the cpx are high-Al (~8 - 10 wt% Al₂O₃) with low TiO₂ (<1 wt%) with Mg# of 79-83. Rims of the cpx are chemically akin to the groundmass with moderate Al (~ 3-5 wt% Al₂O₃) with Mg# of 74. Cores and rims of the cpx are in equilibrium with the liquid (K_D(Fe-Mg)^cpx-liq = 0.27 ± 0.03) indicating that the differences is controlled by temperature and pressure rather than liquid composition. Thermobarometry of the cpx indicate a high-pressure origin. The cores were originally stored between 11-15 kbars (~35-45 km) at 1210-1238°C, then moved to a second storage between 3-6 kbars (11-20 km) at 1110-1137°C before erupting to the surface. Laser ablation trace and REE analysis of cpx indicate that at least two chemically distinct populations exist. One population is more enriched in trace elements and REEs (e.g. Cr: 60.1 to 6, La: 2.3 to 1.0 and Dy: 4.1 to 2.3 ppm) and match the rims of the second population, which have slightly more depleted cores. It is possible that the cpx are recording variable storage depths in multiple mush zones below the Moho between 35 to 45 km depths. The differences seen in the trace elements in the cpx may reflect the chemical heterogeneity of the magmatic plumbing system.