EVIDENCE FOR SHALLOW DIFFERENTIATION AND MAGMA MIXING OF THE GRANDE RONDE FORMATION, COLUMBIA RIVER BASALT GROUP

The voluminous Grande Ronde Basalts (GR) of the Columbia River Basalt Group appear chemically evolved yet they contain <5% phenocrysts. Earlier studies proposed that the GR basalts are (1) near-primary melts generated by large-scale melting of eclogite and (2) that GR magmas were extremely hydrous (>4 wt. %) and rose rapidly from the mantle such that the dissolved water kept the magmas close to their liquidi. Our studies however reveal evidence of low-pressure fractionation. For example, (1) GR basalts form a tight cluster near the 1-atm multiply saturated curve on a normative pl-ol-cpx-q diagram; (2) plagioclase, augite, pigeonite, low-Al orthopyroxene (+relict olivine) occur as phenocrysts; (3) these phenocryst phases match the crystalline phases produced in the laboratory at 1 atm pressure (QFM buffer); (4) H2O content is < 0.4%. Furthermore, evidence of low pressure magma mixing can be found throughout the formation such as, pigeonite and orthopyroxene phenocrysts rimmed by augite and reversed and normally zoned cpx and plagioclase phenocrysts. Some of these phenocrysts appear to be resorbed and have inclusion rings. We therefore propose a petrologic model in which GR parent magmas underwent crystallization and mixing within the shallow crust, perhaps in a series of chambers or mush columns, where the magma fractionated in a sequence of steps as it move upward dropping and capturing phenocrysts along the way. The absence of phenocrysts can be explained by the efficient removal of phenocrysts, which can be attributed to a process known as Punctuated differentiation. These phenocrysts are formed at greater depths, but instead of forming a single thick cumulate pile of phenocrysts, the downward propagating upper solidification front captured many of them along the upper margin. The phenocryst-laden magma continued to rise via conduits, and was eventually emplaced in near-surface chambers. Once in these chambers, the density differences between the magma and the phenocrysts could be large enough so that some of the phenocrysts settled out leaving a fractionated magma with a lower viscosity. This lower viscosity magma is then more eruptible and has the fractionated appearance the GR lavas display. Subsidence generated as the magma chamber emptied produced the nearby Pasco Basin.