FALLOUT ZIRCONS AS GEOLOGIC FINGERPRINTS: UNLOCKING THE HISTORY OF YELLOWSTONE-SNAKE RIVER PLAIN HOTSPOT MAGMATISM USING U-PB AND HF ISOTOPES IN ZIRCONS

Hafnium isotopic compositions in zircons from volcaniclastic and ash interbeds in the Kimama and Kimberly drill cores on the Snake River Plain (SRP), and from the Beaverdam Formation to the south of the SRP, document the assimilation of varying lithospheric sources into rhyolite fallout tuffs during the transit of the North American plate over the Yellowstone Hotspot. These sources include, from west to east, the Grouse Creek, Farmington, and Wyoming provinces. Juvenile volcanic zircons were recovered in the Kimama core from two volcaniclastic interbeds near the bottom of the core, and from a surface outcrop of the Beaverdam Formation along Trapper Creek. In the Kimberly core, volcanic and inherited zircons were recovered near the top of the core from three volcaniclastic interbeds and one fallout ash.

U-Pb ages and euhedral morphology of late Miocene volcanic zircons from tuffs and tuffaceous sands in both drill holes are congruent with absolute ages of rhyolite and basalt flows sampled in the cores, indicating minimum transport and rapid sediment and ash deposition. The Bruneau-Jarbidge (12.7-10.5 Ma), Picabo (10.4-6.6 Ma), and Heise (6.6-4.4 Ma) volcanic fields are represented by populations of fallout zircons with ages of 12.4 to 12.3 Ma, 8.0 to 6.3 Ma, and 7.3 to 5.84 Ma, identified in the Beaverdam fm and the Kimberly and Kimama cores, respectively. Volcanic zircons were matched to their likely crustal source area using ɛ_Hf values. ɛ_Hf ranges of -11 to -5 document assimilation of relatively homogenous Archean Grouse Creek block and Wyoming craton, while ɛ_Hf ranging from -37 to -2 signal assimilation of the heterogeneous Paleoproterozoic Farmington zone. The U-Pb and ɛ_Hf isotope record in volcanic and inherited zircons erupted from the Bruneau-Jarbidge, Picabo, and Heise volcanic fields provides a fingerprint of crustal terranes and crustal assimilation during YSRP volcanism and constrains the boundaries of crustal terranes underlying the YSRP hotspot track. Our results indicate that the assimilation of crustal material plays a greater role in the origin and eruptive characteristics of YSRP magmas than previously thought.