FOUR MILLION YEARS FROM INTRUSION TO ERUPTION IN THE WHITE PINE STOCK AND EAST TRAVERSE MOUNTAINS PEBBLE DIKE SWARM, WASATCH MOUNTAINS, UTAH, U.S.A

The Little Cottonwood and White Pine stocks, part of the Eocene-Oligocene Wasatch intrusive belt, intruded the thickened crust of northern Utah following the Sevier orogeny. These plutons were subsequently bisected during Basin and Range extension. The stocks are now exposed as an east–west crustal section in the footwall of the Wasatch fault. Cogenetic volcanic products crop out in the east Traverse Mountains mega-landslide block, which contains a swarm of explosively emplaced pebble dikes with clasts of both Little Cottonwood and White Pine stocks in a fine-grained clastic matrix. These volcanic products, located in both the hanging and footwalls, 1) provide a piercing point to reconstruct the roof of the intrusions and 2) directly link the extrusive and intrusive parts of the system. Although the spatial connection has been established, the ages of the White Pine stock and pebble dikes are unknown. New LA-ICP-MS U-Pb zircon dates of seven samples provide a timeline of White Pine stock crystallization characterized by <1500 ppm U ~31–27 Ma zircons, pebble dike eruption characterized by >1500 ppm U ~27–26 Ma zircons, and post-eruptive crystallization of residual melt and/or hydrothermal alteration characterized by >>1500 ppm U ~27–24 Ma zircons). When placed in the context of the existing petrochronology from the Little Cottonwood stock (magma crystallization from ~33–28 Ma followed by ~26–25 Ma final melt crystallization/migration) and adjacent Alta stock (~35–33 Ma magma crystallization and ~33–24 Ma hydrothermal metamorphism), a coherent picture emerges of a long-lived, complex system with prolonged, large-scale mass and heat transfer from the upper crust to the surface. These new data demonstrate the several million-year timescales of pluton growth, magma residence, volcanic eruptions, and movement of hydrothermal fluids. Additionally, the new dates of the White Pine stock and Red Pine porphyry extend the range of magmatic activity to ~24 Ma, which strongly correlates with the timing of hydrothermal metamorphism in the adjacent Alta stock. Collectively these data show that mass, heat, and fluid transfer in the crust can occur over millions of years in a single plutonic-volcanic system. The dates also corroborate the pebble dike swarm as a piercing point for the east Traverse Mountains mega-landslide block.