EVOLUTION OF JURASSIC MAGMATISM OF THE WESTERN CORDILLERA: THE RECORD FROM FALLOUT TUFFS AND BACK-ARC PLUTONS

Volcanic ash is widespread in Jurassic (~201-145 Ma) strata of the western interior of North America. Its age, distribution, and composition provide important constraints on the tectonic evolution of the Cordillera. Fallout tuffs range from about 184 Ma in the marine Gypsum Spring Formation of northern Utah and Wyoming to 150 Ma in the continental deposits of the Morrison Formation of Utah, Colorado, and Wyoming. Based on the number of tephra beds, volcanism appears to have peaked twice, once in Middle Jurassic ~170 Ma and again in Late Jurassic ~150 Ma, after which it fell off rapidly. Plutonism, particularly across central Nevada, appears to have followed a similar pattern. The compositions of the tuffs and their constituent minerals are consistent with eruptions from a continental-margin volcanic-arc related to subduction along the west coast. However, some ash beds in the Late Jurassic Morrison Formation have mineral compositions like rift-related A-type rhyolites. Pb-isotopic compositions of sanidines in these ash beds are most like those of southern California where granites and syenites with A-type affinities were intruded at the same time. The alkaline dikes of the Independence dike swarm and Osa Creek ring complex (eastern California) along with volcanic rocks in the Bisbee basin (southern Arizona) formed at this time in a transtensional plate margin. A-type granites were also intruded in the back arc region in what is now eastern Nevada and western Utah (e.g., 148-157 Ma Granite Peak, Dolly Varden, and Silver Zone Pass intrusions). This plutonism was preceded by more normal calc-alkaline plutons in the Jurassic arc in California and Nevada and in the retroarc region as far east as western Utah. To explain these observations, we suggest that a brief ignimbrite flareup from 157 to 150 Ma may have been driven by slab rollback and possible breakoff. This accompanied conversion of the plate boundary to a strike-slip margin after a preceding phase of folding and thrusting. The presence of ash beds and plutons with A-type characteristics mixed with those that have more typical subduction signatures confirms that the Late Jurassic was a geologic transition time in North America where subduction was changing to transtension along the western plate boundary.