ESTABLISHING THE MAGMATIC CHEMICAL IDENTITIES OF GRANDE RONDE BASALT LAVAS, COLUMBIA RIVER FLOOD BASALT PROVINCE: RESOLVING COMPOSITIONAL VARIATIONS FROM MAGMATIC AND ALTERATION PROCESSES

Defining the magmatic compositions of Grande Ronde Basalt (GRB) lavas is essential to determining their chemostratigraphy and understanding their petrogenesis. Reidel and Tolan (2013, GSA SP 497, p. 117-153) have recognized 25 GRB members each of which represent multiple eruptions. Members have been characterized chemically mainly by ranges in TiO₂ and MgO abundances. Compositional differences between lavas of some sequentially erupted members can be small, particularly among the low-Mg members, and both immobile (e.g., Ti) and mobile element (e.g., Mg) abundances in a normalized analysis are susceptible to modification by alteration. In addition, Reidel and Tolan (2013) provide only average analyses for entire members, or occasionally for some subunits within members, thereby precluding comparisons with new data and independent evaluations of the chemostratigraphy. As shown by Sawlan (Geosphere, 2018, v. 14, p. 286-303), Al-Ti variations for the Sentinel Bluffs Member (SB) are useful in distinguishing negatively-correlated magmatic Al-Ti variations from alteration-generated, positively correlated Al-Ti variations to supra-magmatic values. Sawlan (2018) attributed magmatic Al-Ti variations, as well as variations of other major elements, to clinopyroxene and plagioclase fractionation. Here I examine >1,500 analyses of R₂ and N₂ GRB lavas by applying the mass analysis methodology introduced by Sawlan (2018). Magmatic negative Al-Ti variations are also observed among samples from low-Mg members, but such trends are offset to higher Ti and are not precisely colinear. In addition, the Al-Ti magmatic trends for some members span a similar TiO₂ range but are offset by differences in Al₂O₃ abundances, making it difficult to associate some altered low-Mg samples to a given member. This problem is addressed by expanding mass analysis to mobile major elements, which decrease linearly with decreasing sample mass, thereby enabling calculation of parent mobile element abundances using two or more magmatic Al-Ti baselines. The estimated parent compositions are then compared to those of unaltered samples defining the magmatic baselines. The objective is to develop reliable criteria for identifying and better characterizing the compositions of GRB members.