Geology, age and field relations of Hadean zircon-bearing supracrustal rocks from Quad Creek, eastern Beartooth Mountains (Montana and Wyoming, USA)

Abstract

Quad Creek Paleoarchean (≤3250Ma) quartzites in southern Montana host Hadean (pre-3850Ma) detrital zircons. Although an accessible resource for investigating early Earth processes distinct from other better known ancient zircon localities, the outcrop-scale geological and geochemical context of these rocks has not previously been well documented. New (1:250) mapping reveals a varied suite of isoclinally folded, sheared and variably deformed chromite-bearing banded and massive quartzites, garnetiferous siliceous (migmatitic) paragneisses, amphibolite, quartz–biotite schists and quartz+magnetite rocks (banded iron-formation; BIF). Conventional ion microprobe U–Pb zircon ages of populations from different quartzites and a paragneiss show outgrowth rim ages on older inherited detrital igneous zircon cores that match documented regional metamorphic events evidenced elsewhere in the Wyoming Craton. Weighted mean 207Pb/206Pb ages for the youngest concordant zircon cores of igneous derivation indicate the Quad Creek sediments were deposited by about 3250Ma. Coupled with a large zircon 207Pb/206Pb age survey (n=1274), and an extended U–Th–Pb depth-profile of the oldest grain in our sample set, these data support the notion that the oldest crust tapped by these sediments was comparable in age to the ca. 4000Ma Acasta Gneiss Complex. This similarity is suggestive of both of a linkage between the Wyoming Craton and the Western Slave Province, and the lingering influence of Hadean crust well into the Archean.