The Umatilla Member of the Columbia River Basalts: a chemically unusual flood basalt

Abstract:

The Columbia River Basalt Group (CRBG) is the youngest (~16.8-6 Ma) flood basalt province on Earth. It consists of at least two distinct pulses of activity; the main phase during which ~99% of the total 210,000 km³ volume erupted, and the late-phase Saddle Mountains Basalt (SMB), which erupted nearly 1 m.y. after main-phase activity ceased at about 16 Ma. The Umatilla Member is the basal unit of the SMB and makes up ~30% of that formation. Previous work has established that it has unusual chemical features compared to other CRBG lavas: (1) its composition varies widely(53–58 % SiO2, <1–3.3 % MgO); (2) it has weakly alkaline character (basaltic trachyandesite – trachyandesite), whereas the CRBG is overwhelmingly tholeiitic; (3) it is exceptionally enriched in Ba, with up to 0.5 wt. % BaO. Our recent XRF and ICP-MS data have brought to light these additional characteristics: (4) the Umatilla Member has positive Eu anomalies, despite lacking any textural evidence for feldspar accumulation; (5) it has some unusual high field strength element abundances, most notably a Zr/Hf of ~47. The combination of positive Eu anomalies, high Ba, and alkaline character suggests assimilation of alkali feldspar, perhaps in the form of a partly molten crystal mush [1], consistent with regional large-scale silicic volcanism which occurred during the hiatus between main-phase activity and eruption of the Umatilla Member [2]. The elevated Zr/Hf can then be explained by melting of zircons from the same felsic rock or mush. These results suggest links between the temporal patterns of mafic and silicic volcanism in the early history of the Yellowstone hotspot.

[1] EPSL 540:116251 (2020); [2] Geosphere 11/2:1–10 (2015)