ORIGIN OF SNAKE RIVER PLAIN - YELLOWSTONE (SRPY) BASALTIC MAGMAS: LITHOSPHERIC VS. ASTHENOSPHERIC CONTRIBUTIONS

SRPY hot-spot magmatism comprises (1) early large-volume, age-progressive rhyolitic activity, followed by (2) basaltic volcanism following local rhyolite cycles. Formation of the rhyolites by crustal melting requires significant heat - presumably from concurrent and even more voluminous mafic intrusions. Volume, time, and compositional constraints on origin of SRP basalts bear on the contribution of a sub-lithospheric mantle plume as the driving mechanism.

Compositions. Primitive albeit Fe-rich SRPY olivine basalts (ca. 10% MgO, 200 ppm Ni, 400 ppm Cr; Mg# ca. 60) have ‘evolved' Sr-Nd-Pb isotopic compositions even though their elemental compositions are similar to those in oceanic basalts (though notably distinct from subduction-related basalts). They have REE patterns consistent with melting of shallow (spinel lherzolite) mantle. These features cannot generally be explained by crustal contamination, and are more consistent with melting of aged (ca. 2.5 Ga) lithospheric mantle. Moreover, sharp isotopic and tectonic discontinuities coincide along the northern and western boundaries of the SRP, impling that isotopic characteristics are inherited from distinct lithospheric source domains. Only high 3He/4He values (Ra>11), characteristic of oceanic hot-spot basalts, clearly implicate sub-lithospheric contributions. Although heat and volatiles may originate in part from sublithospheric depths, melting dominantly occurred within a compositionally distinct lithospheric mantle keel.

Melt production. Anomalously high SRPY melt production compared to adjacent regions to the N and S, requires either that (1) the underlying mantle is unusually warm (e.g., elevated potential temperature perhaps related to ascending plume-like asthenosphere), or (2) the source region is relatively fertile (e.g., higher melt productivity at normal temperatures). Basin and Range style extension (cf. Harry & Leeman, 1995, JGR) can induce significant and concurrent melting of lower lithospheric mantle, but only if this domain contains easily fusible (i.e., eclogitic or pyroxenitic) veins or ‘impurities' or is hydrated by earlier subduction processes. Direct melting of upwelling asthenosphere may be repressed by presence of an initially thick (> 100 km) lithospheric lid associated with cratonic North America.