BASALTIC VOLCANISM OF THE SNAKE RIVER PLAIN AND ITS RELATION TO THE YELLOWSTONE HOTSPOT: AN OVERVIEW

The Snake River Plain (SRP) volcanic province of southern Idaho comprises two distinct terranes. The ESRP, which marks the path of the Yellowstone hotspot, is a gentle structural depression that rises to the NE until it merges with the Yellowstone Plateau. Gravity and magnetic anomalies in the ESRP define a texture normal to the trend of the plain, but parallel to volcanic rift zones and adjacent Basin and Range structures. Basalts of the ESRP form an axial volcanic high that post-dates rhyolite calderas of the Yellowstone hotspot and is cut by volcanic rift zones as young as 2 ka. In contrast, the WSRP is a true graben with >3 km of structural relief, and gravity and magnetic anomalies that trend parallel to its axis. Basalts of the WSRP formed in two phases: after eruption of syn-rift rhyolites (7-9 Ma) and again at <2.2 Ma; between phases the western rift was the site of a long-lived lake system.

Basalts of the ESRP are tholeiites with MgO similar to MORB (5-10%) but with higher Fe, Ti, P, and Al. The range in incompatible element ratios at constant MgO implies a range of parent magmas derived from a similar source by variable degrees of melting. Fe8 values (?13) imply deeper melting or a source higher in FeO than MORB asthenosphere, while Na8 values (2.4-3.2) imply moderate but variable partial melting. Partial melting models indicate 5-10% melting of a source similar to that of E-MORB. Isotopic compositions fall on a mixing line between OIB and continental lithosphere (EM2). Basaltic lavas of the WSRP are similar to those of the ESRP, but range to higher Fe-Ti and lower Na-K; the oldest lavas (7-9 Ma) are especially low in alkalis. Around 0.7-0.8 Ma an abrupt change in lava composition occurs, with tholeiites replaced by slightly alkaline, K-rich basalts that are much lower in Fe, Ti and REE. This change represents melting of a new mantle source region similar to that of ocean island basalt, as shown by their isotopic compositions.

ESRP basalts represent plume-derived melts that interacted extensively with old continental lithosphere and with consanguineous mid-crustal mafic sills. WSRP tholeiites derived from modified plume head residue that filled a sublithospheric conduit beneath the graben. This modified lithosphere was rejuvenated during Basin and Range extension, followed by penetration of asthenosphere-derived melts after 0.7 Ma.