INFERENCES ON NORTHWESTERN U.S. MANTLE ARCHITECTURE FROM GEOCHRONOLOGIC AND ISOTOPIC STUDIES OF OWYHEE PLATEAU BASALTIC VOLCANISM

Time is a critical variable in understanding the evolution of long-lived continental intraplate basalt provinces. The 17 million year record of late Cenozoic basaltic volcanism in the Owyhee Plateau region, Oregon-Idaho-Nevada tristate, provides important insight into the nature of the lithospheric boundary between accreted and cratonic North America at the mantle level, as well as how the lithospheric and asthenospheric mantle were affected by subduction processes following terrane accretion. A variety of chemically distinct basaltic eruptive products are present in the Owyhee Plateau region, including: generally evolved basaltic lavas related to the Steens flood basalts, produced between 17-11 Ma; less differentiated olivine tholeiite compositions, produced between 11-0 Ma, including chemical types characteristic of both the Oregon Plateau and Snake River Plain, as well as compositions transitional between the two; and mildly alkaline basalts produced after 250 ka. Sr, Nd, and Pb isotopic characteristics of Owyhee Plateau basalts display systematic variations through time, yet are decoupled from bulk chemical characteristics, reflecting time-dependent variations in contributions from different lithospheric and sublithospheric mantle reservoirs. Three mantle source reservoirs are suggested, following R.W. Carlson (1984; Geochim. Cosmochim. Acta 48, 2357-2372): a depleted mantle source similar to the northern Pacific MORB source; a MORB-mantle source which experienced sediment-derived contamination as a consequence of Mesozoic-Cenozoic subduction; and an enriched mantle source in the subcontinental lithospheric mantle. While a mantle plume source cannot be conclusively excluded, it is not required to explain Owyhee Plateau basaltic magmatism. Rather, the pre-existing conditions resulting from terrane accretion and Sevier thrusting followed by Laramide low-angle subduction and subsequent slab removal provide the necessary chemical and isotopic inputs to yield the diversity of eruptive products and isotopic variability observed throughout 17 million years of Owyhee Plateau basaltic magmatism.