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.