The Franciscan subduction complex of California is considered a type example of a subduction-accretion system, yet the age of subduction initiation and relationship to the tectonic history of western North America remain controversial. Estimates for the timing of Franciscan subduction initiation are largely based either indirectly on regional tectonic arguments or from the ages of high-grade blocks within mélange. Many of the high-grade blocks record counterclockwise pressure-temperature paths with early amphibolite overprinted by later eclogite and blueschist; however, their origin and significance with respect to subduction initiation have been debated. In contrast, some high-grade blocks show evidence for clockwise pressure-temperature paths and an early eclogite assemblage overprinted by later amphibolite. Zircon U-Pb ages from inclusions in garnet and Lu-Hf estimates of initial garnet growth ages from these samples record early eclogite metamorphism at~176 Ma. Matrix zircon U-Pb ages and Lu-Hf estimates of final garnet growth ages record a barroisite-amphibolite assemblage overprint of eclogite at~160 Ma. Combined with petrologic data and existing geochronology, the data suggest that (1) Franciscan subduction was underway by no later than 180 Ma, (2) continuous subduction metamorphism occurred for at least 100 Ma, and (3) Franciscan subduction initiation predated the formation of the overlying Coast Range Ophiolite, supporting models that form the ophiolite above an east dipping Franciscan subduction zone.

The Mineral King pendant in the Sierra Nevada batholith (California, USA) contains at least four rhyolite units that record highsilica volcanism during magmatic lulls in the Sierran magmatic arc. U-Th-Pb, trace element (single crystal spot analyses via sensitive high-resolution ion microprobe-reverse geometry, SHRIMP-RG), and bulk oxygen isotope analyses of zircon from these units provide a record of the age and compositional properties of the magmas that is not available from whole-rock analysis because of intense hydrothermal alteration of the pendant. U-Pb spot ages reveal that the Mineral King rhyolites are from two periods, the Early Jurassic (197 Ma) and the Early Cretaceous (134-136 Ma). These two rhyolite packages have zircons with distinct compositional trends for trace elements and δ 18 O; the Early Jurassic rhyolite shows less evidence of crustal infl uences on the rhyolites and the Early Cretaceous rhyolite shows evidence of increasing crustal infl uences and crystal recycling. These rhyolites capture evidence of magmatism during two periods of low magmatic fl ux in the Sierran Arc; however, they still show that magmas were derived from interactions of maturing continental crust, increasing from the Early to Late Jurassic. This fi nding likely refl ects the transition of the North America margin from one of docking island arcs in the Early Jurassic to one of a more mature continental arc in the Early Cretaceous. This also shows the utility in examining zircon spot ages combined with trace element and bulk isotopic composition to unlock the petrogenetic history of altered volcanic rocks.