A NEW LOOK AT THE GENESIS OF THE WANAPUM BASALT, COLUMBIA RIVER BASALT GROUP

The Columbia River Basalt Group (CRBG) is a continental flood basalt province contained within the greater Yellowstone Hotspot volcanic province. The group consists dominantly of tholeiitic basalts and basaltic andesites beginning with the eruption of the Picture Gorge Basalt ~17.2 Ma and ending with the Saddle Mountains Basalt, the last eruption of which is dated at ~6 Ma. The main phase, during which most of the Group erupted, consists of Picture Gorge, Steens, Imnaha, and Grande Ronde Basalt (~17.2 – 16.07 Ma) formations. The Wanapum Basalt directly succeeds the voluminous Grande Ronde Basalt but marks a decrease in intensity of the CRBG with lower eruption rates and volumes. This formation contributed ~12,175 km³ (5.8%) of the total 210,000 km³ CRBG output [1] between 16.07 Ma and 15.85 Ma with an effusion rate of ~0.055 km³/yr (compared to 0.261 km³/yr for the Grande Ronde Basalt). Wanapum Basalt emplacement began with the Eckler Mountain Member (335 km³ total), which has the most primitive magmas in the formation (≤8.8% MgO). The major Wanapum members are Frenchman Springs (7600 km³, MgO wt % ~4), Roza (1300 km³, MgO wt% ~4) and Priest Rapids (2800 km³, MgO wt% ~4-5) [1]. These are interbedded with smaller volume, more evolved units. Preliminary trace element and isotope data from the Wanapum Basalt show broad trends of increasing La/Nb and ⁸⁷Sr/⁸⁶Sr with decreasing MgO, consistent with derivation of the major members from magmas represented by the Eckler Mountain flows via assimilation-fractional crystallization of continental crust, similar to the relationship between the older, main phase Imnaha and Grande Ronde Basalts [2]. However, rare earth element variations suggest a greater role for residual garnet in the genesis of the major members compared to the more primitive Eckler Mountain lavas, raising the possibility of distinct mantle sources. Results using the Magma Chamber Simulator [3] will be used to model the petrogenetic history of the Wanapum Basalt.

[1] Reidel et al., GSA SP 497, 1–43 (2013); [2] Wolff et al., Nature Geosci. 1, 177–180 (2008); [3] Bohrson et al., J. Pet. 55, 1685–1717 (2014).