PRIMITIVE HIGH-AL OLIVINE THOLEIITES OF THE PACIFIC NORTHWEST REVISITED – INSIGHTS FROM COMBINED ELEMENTAL AND ISOTOPIC DATA

Primitive high-Al olivine tholeiites (HAOT) constitute a small volume, but very wide-spread volcanic rock type in the Pacific Northwest extending from Cascadia, throughout the Oregon High Lava Plains, and continuing along the Snake River Plain to Yellowstone. Across this areal extent, primitive (MgO > 8%) HAOT show remarkably limited compositional variation. Those from Cascadia range to slightly higher Rb/Nb than those from the east suggestive of some subduction influence, yet Ba/La ratios are nearly constant over 14 degrees of longitude. HAOT to the east show slightly elevated Ce/Yb compared to those from the west. In spite of their compositional similarity, HAOT show a dramatic change and greatest diversity in Sr, Nd and Pb isotopic composition between -117 and -118 degrees longitude – the well-known “0.706 line” – in the Owyhee Plateau region near the Idaho-Oregon border. 87Sr/86Sr ranges from 0.7031 – 0.7045 to the west and 0.7055-0.7080 to the east of the line in rocks that have essentially the same concentrations of highly incompatible elements such as Ba and Rb. The 0.706 line has long been interpreted as the western boundary of Precambrian North America with the obvious implication that the more evolved radiogenic isotopic compositions to the east reflect input of older crustal or lithospheric mantle materials. Given the compositional similarity of primitive basalts across this boundary, however, the mechanism by which this isotopic signal of older continent is imparted on young basalts is not clear. Element ratios sensitive to addition of typical felsic crust show no correlation with isotopic composition, so direct crustal contamination is not an obvious explanation. The slightly more LREE enriched signature of eastern HAOT, suggestive of increasing depth of melting, is a gradual trend with longitude in contrast to the rapid change in isotopic composition at the 0.706 line. We suggest that the isotopic shift reflects the dynamic nature of basalt generation where rising melts continually interact and equilibrate with surrounding mantle rocks through most of their ascent. The primitive basalts with elevated 87Sr/86Sr gain this signature by interacting with cratonic mantle with these isotopic characteristics, but maintain essentially a constant composition driven by equilibration with surrounding peridotite.