Cretaceous and Eocene U-Pb zircon migmatite ages from the East Humboldt Range metamorphic core complex, Nevada

Zircon separates from migmatite samples from the East Humboldt Range (EHR), northern Nevada were U-Pb dated using secondary ionization mass spectrometry. Cathodoluminescence images of zircons show bright anhedral cores with abundant euhedral overgrowths surrounding them.;The overgrowths give U-Pb dates indicating two periods of zircon growth. Inner rims have a Cretaceous age population at 70.6 ± 2.6 Ma. Outer rims give Eocene ages that either fall in to a single-age population at 45 ± 1.3 Ma or are part of a range from 41-31 Ma. Discordant zircon cores have upper intercepts between 1.8 -1.0 Ga.;Cretaceous and Eocene ages are interpreted as recording protracted growth of anatectic zircon during Mesozoic thrust burial and Cenozoic regional extension. McGrew et al. (2000) reported a U-Pb TIMS age of 84.8 ± 2.8 Ma on zircons from the EHR, while Premo et al. (2008) reported ion probe ages of 90-71 Ma from zircons in the same area.