Trace and rare-earth element composition of 2480 Ma detrital zircons in Proterozoic metapsammites from northwestern Arizona

Detrital zircon grains in the ~1740-1750 Ma Vishnu Schist and similar rock units in northwestern Arizona consist of up to 30% grains dated by U-Pb isotopic analysis at 2470-2490 Ma. These zircon grains are distributed over ~40,000 km2 and define an age peak at 2480.0 ± 27.3 Ma (2SE). These grains have yielded unusually consistent 207Pb/206Pb dates, with generally smaller analytical uncertainty and greater concordance to ideal U-Pb evolution than grains of other ages. A weighted mean age of 2480 ± 0.9 Ma (2SE) for this zircon population reflects consistent analytical results and high analytical precision but not the accuracy of the age. The source of these zircons has not been identified. To better characterize the unidentified source, we analyzed 45 of these grains for trace and rare-earth elements by laser-ablation mass spectrometry and scanned 16 grains with an electron microprobe to identify mineral inclusions. Mass spectrometer determinations of Sc/Yb and Nb/Sc support derivation from an oceanic-island igneous source. Electron microprobe scans revealed quartz in 5 of 16 grains, indicating a felsic source. The low variability in 207Pb/206Pb dates and a generally linear relationship between U and Th support zircon derivation from a single igneous unit or closely related set of units without xenocrystic zircons. A literature search for other zircon populations with similar age and U/Th ratios identified ~2480 Ma zircons in a Mesoproterozoic(?) metapsammite and conglomerate in southwestern Montana. This sandstone was deposited near the margin of the Wyoming craton and contains almost entirely 2400-3600 Ma zircons, unlike zircon grains in Vishnu Schist which include a large population of 1730-1900 Ma zircons. From this relationship, we infer that the 2480 Ma zircons in both areas were derived from a source in the Wyoming craton. We conclude that the 2480 Ma Vishnu zircons were derived from a felsic batholith that formed above and from hotspot magma related to the ~2450-2480 Ma Matachewan Large Igneous Province, that this batholith formed by mixing between a mantle-derived hotspot magma and assimilated Archean continental crust, and that the source rock was emplaced during initial rifting between the Wyoming craton and the Superior province.

Methods
Two samples of Paleoproterozoic psammite were collected for previous studies and the zircons in them were analyzed for U-Pb geochronology.  The grain mounts that were used for U-Pb analysis of zircons from Cottonwood Cliffs and Jerome Canyon (Doe, 2014, Colorado School of Mines, Ph.D. dissertation; Spencer et al., 2016, Geosphere) were re-used to determine the trace-element and rare-earth-element (TREE) content of 45 zircon grains that had been dated at ~2480 Ma. Analyses were conducted in 2022 at the Arizona Laserchron Center at the University of Arizona with a Photon Machines Analyte G2 laser (193 nm) connected to an Element2 multi-collector LA-ICP-MS (laser-ablation – inductively coupled plasma – mass spectrometer) equipped with a Jet pump and interface (Pullen et al., 2018; Gehrels, 2020). Primary calibration standards SL and FC were analyzed intermittently during sample analyses (set-up with sequential analyses of standards FC-SL-FC-SL-FC-SL, then 5 unknowns in groups separated by FC-SL, then ending with FC-SL-FC-SL). Secondary standards R33 and NIST612 were also analyzed occasionally during sample analyses. The precision of measurements of element concentrations is <5% for a typical zircon grain (Gehrels, 2020). The expected stoichiometric concentration of Si in zircon (153,230 ppm) is shown for all zircons because this value was assumed and then used to calibrate signal intensities for all other analyzed elements (Gehrels, 2020).