SHRIMP U-PB ISOTOPIC SYSTEMATICS OF ZIRCON FROM THE ANGEL LAKE ORTHOGNEISS, EAST HUMBOLDT RANGES, NEVADA: IS THIS REALLY ARCHEAN CRUST?

Polyphase zircons from the Angel Lake orthogneiss, previously interpreted as a nappe of Archean crust, were analyzed using the SHRIMP to better define its protolith age. Earlier isotopic work had yielded a U-Pb zircon, upper-intercept age of 2520 ± 110 Ma from a quasi-linear array of highly discordant ID-TIMS analyses. SHRIMP analyses on zircon from the same sample confirm the presence of mostly dark, variably metamict, well-rounded, Archean zircons with ²⁰⁷Pb/²⁰⁶Pb ages between ~2430 and 2580 Ma, yielding a mean ²⁰⁷Pb/²⁰⁶Pb age of 2516 ± 22 Ma. However, the Archean component in many grains is a core, encased within a Late Cretaceous to Early Tertiary growth of clear, typically low Th/U (<0.015), oscillatory-zoned, and commonly euhedral zircon. The younger zircon zones yielded essentially concordant ²⁰⁶Pb/²³⁸U ages between 72 and 90 Ma, consistent with magmatic ages from Lamoille Canyon to the south. Several analyses from rounded to subrounded cores yielded discordant ²⁰⁷Pb/²⁰⁶Pb ages at ~1615, 1850, 1880, 1960, 1990, 2140, and 2170 Ma, and one at 1460 Ma.

The observation that the vast majority of Archean zircon grains and cores in this sample are rounded to subrounded strongly suggests that they are detrital in nature. With a definitive provenance age range from ~2430 to 2580 Ma, the age of the sediment is certainly Proterozoic or younger.

Latest Cretaceous ages obtained from oscillatory-zoned outer portions (likely indicating growth within a magma) of a significant number of analyzed euhedral zircon grains indicate that the protolith of this orthogneiss was more likely a Late Cretaceous igneous rock derived at least in part by partial melting of a post-Archean sediment composed predominantly of Archean detritus. The actual crystallization age of the igneous protolith is arguable. Whereas the actual age is uncertain, an age around 90 Ma is suggested. If that age is correct, then either Pb-loss or partial resetting of the U-Pb isotopic systematics occurred in portions of some grains, producing younger and apparently concordant ²⁰⁶Pb/²³⁸U ages. The youngest ²⁰⁶Pb/²³⁸U age of 72 Ma would be a maximum age estimate for the disturbance in these zircons.