GEOCHEMICAL CONNECTIONS BETWEEN THE IMNAHA AND GRANDE RONDE FORMATIONS OF THE COLUMBIA RIVER BASALT GROUP: THE DEVELOPMENT OF A LARGE MAGMA SYSTEM FOR A FLOOD BASALT PROVINCE

The Miocene Columbia River Basalt Group (CRBG) is an early manifestation of the Yellowstone-Snake River Plain hotspot. The Grande Ronde (GR) formation makes up most of the volume of the erupted CRBG lavas (~72%, 149,000 km³), but is basaltic andesite. The evolved composition requires either an unusual mantle source or crustal storage for its generation. However, its major chemical composition is quite restricted, and any crustal storage model must account for both the large volume and small compositional range. The underlying Imnaha Basalt is made up of two chemical types, American Bar and Rock Creek, the products of melting dominated by peridotite and pyroxenite mantle sources respectively. Individual Imnaha flows record distinct liquid lines of descent in Ni-Mg number trends in olivines from individual episodes of mantle melting and crustal transport [1]. The transition from the Imnaha to the GR thus represents a transition to an integrated and centralized crustal magma system, corresponding with a large increase in eruption rate and presumed magma flux. Based on radiogenic isotope and lithophile trace element ratios, the American Bar type represents the most likely candidate for a parental magma composition for the evolved and contaminated GR magmas.

We utilize the thermodynamic modelling software Magma Chamber Simulator [2], to reproduce the array of GR compositions and attempt to more rigorously understand the origin of the main phase of the CRBG. In the modelling, we consider major and trace-element compositions and radiogenic isotope ratios. Parameters include several Imnaha lava compositions as initial and recharge magma, variable magma water content, pressure, and wall rock compositions and temperatures. Wall rock lithologies include peraluminous and metaluminous granitoids and metasediments, which would have been available to Imnaha magmas. The simulations indicate that any individual Imnaha lava composition cannot reproduce the entire GR array. The GR parent magma is, then, either not represented among erupted Imnaha compositions, or else the GR has several ‘parents’. Alternatively, there may be an unerupted magma parental to both the Imnaha American Bar and the Grande Ronde lavas.

[1] Soderberg, Wolff (2023) Cont. Min. Pet. 178:11; [2] Bohrson et al (2014) J. Pet. 55 1685–1717