CRETACEOUS AND EOCENE-OLIGOCENE U-PB ZIRCON MIGMATITE AGES FROM THE EAST HUNBOLDT RANGE MEAMORPHIC CORE COMPLEX, NEVADA

We report new U-Pb zircon ion probe ages from migmatites at Angel Lake in the East Humboldt Range metamorphic core complex. Migmatites are from metasedimentary rocks in the lower limb of the Winchell Lake nappe at the lowest structural level of the complex. Five zircon separates were dated; two contiguous leucosome-melanosome pairs and one micaceous quartzite paleosome. CL images of leucosome and melanosome zircon reveal bright (low U) anhedral cores surrounded by broad dark (high U) euhedral rims with oscillatory zoning. Many zircon rims exhibit inner and outer growth zones separated by dissolution boundaries that truncate the oscillatory zoning of the inner zone. Largely concordant U-Pb dates (n = 36) from leucosome-melanosome zoned rims indicate two periods of zircon growth. Inner growth zones yield Cretaceous ages with protracted growth from 109 to 81 Ma and abundant zircon growth at 70 ± 2 Ma. Outer growth zones yield Eocene-Oligocene ages, zircon growth at 45 ± 1.7 Ma followed by protracted growth from 42 to 32 Ma. Rims from each zircon subpopulation (leucosome and melanosome) record both Cretaceous and Eocene-Oligocene ages. The majority of zircon core analyses (including quartzite) are discordant but we have resolved Mesoproterozoic upper intercept ages of about 1.2 Ga and 1.4 Ga, consistent with the inferred Neoproterozoic to Cambrian protolith age. We interpret our Cretaceous and Eocene-Oligocene rim ages as recording protracted growth of anatectic zircon during the evolution of the metamorphic core complex. Previous studies have reported Cretaceous U-Pb ages for migmatite leucosomes from the upper limb of the Winchell Lake nappe; McGrew et al (2000) reported a U-Pb TIMS age of 84.8 ± 2.8 Ma and Premo et al (2008) report ion probe ages between 90 and 71 Ma. Following the interpretations of McGrew et al, we attribute the Cretaceous ages to zircon growth during high grade metamorphism (800°C, > 9 kbar) and anatexis related to nappe emplacement during the Sevier orogeny. Our Eocene-Oligocene ages suggest that anatectic melts persisted in the structurally deepest part of the complex during Cenozoic extension and exhumation of the complex (to 630°C, 5 kbars) perhaps facilitated by heat transfer related to 40 to 29 Ma emplacement (Wright and Snoke, 1990) of diorite to granite into the core complex.