THE ORIGIN OF THE GRANDE RONDE BASALT OF THE COLUMBIA RIVER BASALT GROUP: LARGE MAGMA SYSTEMS, EVOLVING CRUSTAL STORAGE REGIMES, AND THERMODYNAMIC SIMULATIONS

At ~149,000 km³, the Grande Ronde Basalt (GR) accounts for ~69% of the volume of the Columbia River Basalt Group. Despite this, GR lavas are both somewhat evolved (mostly basaltic andesite) and quite restricted in chemical composition. An origin for the GR by crustal contamination of more primitive basaltic magma, represented by the underlying Imnaha Basalt, has been proposed [1]. The Imnaha Basalt is composed of two chemical types, American Bar and Rock Creek, which arise from differences in mantle source lithology. Olivine compositions indicate that each Imnaha flow package represents a distinct melting event, hence these magmas were isolated in time and space [2]. The transition from the Imnaha to the GR represents the establishment of an integrated and centralized crustal magma system, corresponding to a large increase in eruption rate and presumed magma flux [3]. Major, trace element, and isotope chemistry suggest that the American Bar type was the parental magma for the evolved and contaminated GR magmas.

Any model for the origin of the GR must account for its large volume and small compositional range, and be physically plausible. We use the Magma Chamber Simulator thermodynamic modelling software [4] to reproduce the array of GR compositions and attempt to more rigorously test the contamination hypothesis for the petrogeneisis of the GR. In the modeling, we consider major and trace-element compositions and radiogenic isotope ratios. We use 3 different Imnaha lava and 1 primitive GR lava as magma compositions in different simulations. Pressures of ≤ 2 kbar and starting magma water content of < 1 wt. % produce the closest results to GR major-element trends, regardless of starting magma. Trace-element trends, particularly enrichment of Ba, require wholesale assimilation of country rock (stoping) rather than, or in addition to, incorporation of wallrock partial melts. Any one Imnaha lava composition cannot reproduce the entire GR array. The GR ‘parent’ magma is, then, either not represented among erupted Imnaha compositions or, more likely, was a fluctuating mixture of several parental Imnaha-like liquids.

[1] Wolff et al. (2008) Nat. Geosci. 1, 177-180; [2] Soderberg, Wolff (2023) Cont. Min. Pet. 178:11; [3] Wolff and Ramos (2013) GSA Spec. Pap. 497, 273-291; [4] Bohrson et al (2014) J. Pet. 55, 1685–1717