COMPOSITION AND PETROGENESIS OF THE BASALTS OF THE ISLAND PARK AREA, ID

The Island Park area is located in the southwestern portion of the nested Yellowstone I, Henry’s Fork, and Yellowstone II calderas in eastern Idaho. The area centers on the 1.2 Ma partially-filled Henry’s Fork caldera and is largely surfaced by mafic rocks. Like the Yellowstone II caldera, the Island park area records the transition from rhyolite- to basalt-dominated magmatism that characterizes the development of volcanic fields in the Yellowstone Snake River Plain volcanic province (YSRP). This study reports the compositions of the mafic rocks of the Island Park area and infers their petrogenesis by comparing their compositions to those of YSRP rocks for which petrogenetic models have already been developed. Like other mafic rocks of the YSRP those in the Island Park area are dominantly ferroan; calcic to calc-alkalic; low- to medium-K₂O; and Nb-rich. Three basic petrogenetic magma series have been identified for the YSRP: 1) A distinctly bimodal basalt-rhyolite (normal YSRP) series. This series is volumetrically dominant and consists of upwelling-mantle-derived basalts and associated anatectic rhyolites. Fractional crystallization and minor assimilation variably contribute to compositions of this series. 2) A magma mixing series. This series is much less common and is characterized by compositions derived from mixing of the basalt and rhyolite magmas of the bimodal series. 3) An extensive fractional crystallization series. This series is uncommon and consists of the fractionation products—including rhyolite—of the normal YSRP basalt of the bimodal series. The mafic rocks of the Island Park area dominantly belong to the normal YSRP series. Like the Yellowstone system, the Henry’s Fork system includes above-average abundance of the extensive fractional crystallization series. Unlike the Yellowstone system, the transition to mafic magmatism in the Henry’s Fork system occurred with very little magma mixing—both in volume and in the extent of mixing. These observations suggest that magmas belonging to the magma mixing and fractional crystallization series are more common during the transition from rhyolite- to basalt-dominated volcanism in the YSRP, and that the abundance and extent of mixing during the transition is determined by the size and persistence of the granitic magma body of the system.