Geology and petrogenesis of the Island Park caldera of rhyolite and basalt, eastern Idaho

The Island Park caldera, in the northeastern part of the Snake River Plain, is an elliptical collapse structure 18 by 23 miles in diameter that was dropped from the center of a shield volcano composed of rhyolite ash flows.The western semicircle of the caldera margin is a single scarp in the northwest and a composite scarp in the southwest.Rhyolite domes and lava flows were extruded along the western rim during and after the period of collapse.The eastern semicircle of the caldera scarp has been covered completely by rhyolite ash flows, domes, and lava flows that were extruded along it.The caldera is filled, in upward succession, by rhyolite ash flows, interbedded rhyolite ash flows and olivine-basalt lava flows, and flows of olivine basalt alone.Rhyolite domes protrude through the basalt.The exposed rocks are middle ( ? ) and late Pleistocene in age.Basalt of the Snake River Plain, which laps onto the caldera shield from the west, was erupted from vents near the Island Park caldera during late Pleistocene and Recent time.The eastern part of the caldera is overlain by rhyolite ash :flows and lava :flows of late Pleistocene age from the Yellowstone Plateau.The rocks of the caldera are bimodal, consisting of uniform olivine basalt on the one hand and uniform highly silicic rhyolite on the other.The basalt is holocrystalline and diabasic and consists of labradorite, subcalcic augite, and magnesian olivine.The rhyolite is largely vitric ; ash :flows are mostly welded ; and crystals of sanidine, high-quartz, and oligoclase are ubiquitous.A single :flow of latite was found on the caldera rim.Eleven specimens were analyzed for both major and minor elements.The Island Park caldera is part of the Snake River-Yellowstone province of intense Pliocene and Quaternary volcanism of olivine basalt and rhyolite.In this province, as in other bimodal volcanic provinces, rhyolite and basalt erupted from vents interspersed in both time and space, and simultaneous eruptions of both liquids from the same or nearby vents are known to have occurred.In the Island Park caldera, the eruptive sequence and geometry suggest that the large magma chamber contained liquid rhyolite overlying liquid olivine basalt.Several kinds of evidence indicate that the rhyolite of this and other bimodal volcanic provinces has formed by differentiation of basaltic magma.This differentiation cannot be explained in terms of fractional crystallization, but it can be explained in terms of an original tholeiitic basalt magma separating by liquid fractionation into rhyolite and olivine basaltic liquids in the proportion of about 1 to 5. Such fractionation may possibly occur in the uppermost mantle or lower crust ; there; rising tholeiite magma might split into immiscible p}lases, one rich in volatiles and fusibles (rhyolite) and the other rich in refractories (olivine basalt), owing to instability of the initial homogeneous liquid caused by pressure decrease during ascent in a region of abnormally high thermal gradient.