CONSTRAINTS ON THE EMPLACEMENT AND PETROGENESIS OF SMALL VOLUME, INCOMPATIBLE ELEMENT ENRICHED LAVA FLOWS WITHIN THE WANAPUM BASALT, COLUMBIA RIVER BASALT GROUP DERIVED FROM THE INTEGRATON OF FIELDWORK WITH INTERNALLY CONSISTENT GEOCHEMISTRY

The Lookingglass and Shumaker Creek Members and the Basalt of Powatka are small (~20 to 35 km³) high-K₂O basaltic andesite flows that bracket the voluminous basalts of the Frenchman Springs Member, Wanapum Basalt Formation. This study draws upon ~35 years field work integrated with geochemical analyses produced in the UMass-Amherst geoanalytical lab. Relative to the voluminous main Wanapum lavas, incompatible elements are significantly enriched (by 1.3 to 2x) in these small volume flows while TiO₂ and Mg# are much lower. All Wanapum flows have similar incompatible element ratios (e.g., Zr/Nb, La/Y) and primitive mantle normalized trace element patterns with prominent Nb and Sr troughs. Rare earth element abundances in the high-K₂O basaltic andesites are elevated but similar to those of the main Wanapum flows.

Taken together, the trace and rare earth element abundances suggest that both the large volume main Wanapum and the small volume Lookingglass, Shumaker Creek, and Powatka flows were derived from similar primitive and ocean island basalt-like mantle sources and include contributions from both subducted material and assimilated lithosphere. The high-K₂O basaltic andesites probably represent lower degrees of partial melting of the mantle sources compared to the main Wanapum lavas. A subduction related signature is not surprising given the long history of subduction along the “Cascadia” subduction zone to the west. The nature of the assimilated lithosphere, however, is more enigmatic. Potential assimilants include cratonic lithosphere of the Idaho Batholith region and accreted Mesozoic oceanic lithosphere (e.g., Wallowa Terrane). The Shumaker Creek dike system lies within the Wallowa Terrane. Although dikes for the Lookingglass and Powatka flows have not been identified, flow distributions suggest that their vent systems are also within the Wallowa Terrane. All three vent systems are >100 km west of the North American craton edge as denoted by the near-vertical Western Idaho Shear Zone (and coincident ⁸⁷Sr/⁸⁶Sr = 0.706 line). Ce/Zr and Ce/Nb ratios are consistent with assimilation of either island arc lithosphere, cratonic lithosphere or the Idaho Batholith or all three. However, melts derived from the Idaho Batholith and craton would need to undergo significant W to NW flow in order to serve as assimilants.