RELATIONSHIPS BETWEEN THE WANAPUM AND GRANDE RONDE LAVAS OF THE COLUMBIA RIVER BASALT GROUP

The Grande Ronde Basalt (GRB, 149,000 km³, mostly basaltic andesite) is the largest formation of the Columbia River Basalt Group, accounting for 72% of the total [1]. Its evolved composition implies the existence of a similar volume of associated cumulates in the crust [2]. The overlying Wanapum Basalt (WB) has a much smaller volume (~12,000 km³) [1], effusion rate, and distinct major-element chemistry. It consists of basal, relatively small (10s to 100s km³) and primitive flows of the Eckler Mountain Member, overlain by the main Frenchman Springs, Roza and Priest Rapids Members (10³ – 10⁴ km³), interbedded with small, relatively evolved lavas (Lookingglass, Powatka, and Schumaker Creek Members). Chemical differences between WB and GRB have been attributed to different sources, aided by the perception that a significant (~300,000 yr) time gap separated the two formations [3,4]. Recent geochronological studies have eliminated the hiatus in eruptive activity [5], but major element chemistry still indicates differences in petrogenesis. For example, WB chemistry shows higher TiO₂, FeO, and P₂O₅ relative to the GRB (e.g. TiO₂ > 2.7 wt%, < 2.3 wt%, respectively) despite lower SiO₂ (WB mostly <53 wt%, GRB mostly >53 wt%) [6]. However, new data reveal strong chemical similarities between GRB and WB; the main WB units resemble the GRB in both incompatible trace elements and radiogenic isotopic ratios. The small-volume evolved units are enriched in incompatible elements and can be modeled as fractionates of GRB compositions. Trace element abundances and isotope ratios in the main units can be modeled as mixtures of GRB cumulates, evolved melts represented by the minor members, and new, more primitive magma. Major-element differences between the formations are resolved by considering the role of Fe-Ti oxides and apatite in the recycled cumulates. WB chemistry is consistent with an origin via remobilization, of mushy GRB-related cumulate with residual evolved melt, by new, more primitive magma represented by the Eckler Mountain flows.

[1] Riedel et al. (2013) GSA Spec. Paper 497, 1–43; [2] Morriss et al. (2020) Geosphere 16, 1793–1817; [3] Hooper & Hawkesworth (1993) J. Petrol. 34, 1203–1246; [4] Tolan et al. (1989) GSA Spec. Paper 239, 1-20; [5] Kasbohm et al. (2023) EPSL 617:118269; [6] Hooper (2000) G-cubed 1, 2000GC000040