Geochemical Comparison of the Saddle Mountain Basalts, Oregon High Lava Plains Basalts, Western Snake River Plains Basalts, and the Main Phase of the Columbia River Basalt Group.

Abstract:

The Columbia River Basalt Group (CRBG) forms part of the broader Yellowstone hotspot province, which also includes the bimodal volcanic regions of the Oregon High Lava Plains and the Snake River Plain in southern Idaho. Following the main phase of the CRBG (~17 – 16 Ma), basaltic activity split into 3 areas: the High Lava Plains, Snake River Plain and the Saddle Mountains Basalt (SMB), all beginning at approximately 15 ­– 14 Ma. The SMB was erupted onto the Columbia Plateau over a similar area to the main-phase lavas, but is isotopically distinctive, with a strong signature of ancient continental lithosphere. This isotopic signature is shared by the Snake River Plain basalts, whereas the high alumina olivine tholeiites of the High Lava Plains more closely resemble the main-phase CRBG. In detail, the High Lava Plains are most similar to a subset of main-phase CRBG with isotopic and trace-element signatures indicating derivation from depleted mantle. Other main-phase CRBG lavas carry an EM2 signature, thought to represent a mantle plume contribution, and most clearly seen among early CRBG (the Imnaha Basalt). Here, we review existing data and present new XRF and ICPMS data with the goal of better understanding the relationship of the post-15 Ma formations to each other and to the main-phase CRBG. Despite the isotopic imprint of continental lithosphere, the most primitive SMB have elemental signatures that overlap High Lava Plains and main-phase CRBG, indicating derivation from depleted peridotite mantle (for example, low MREE/HREE, Nb/Zr and Fe/Mn) or a mixture of depleted and EM2 mantle. The lithospheric signal among SMB and Snake River Plain lavas may be imparted by addition of very low degree partial melts from the subcontinental lithospheric mantle [1], and/or contamination by crustal storage [2]. In either case, the contaminating material exerts high leverage on isotope ratios due to generally low abundances of incompatible elements. Our data are consistent with a dominant common source of depleted to mildly enriched mantle for the Yellowstone province from inception at ~17 Ma through to ~6 Ma.

[1] Geology 36:51–54 (2008); [2] Geology 33:457–460 (2005)