Internal zoning and U–Th–Pb chemistry of Jack Hills detrital zircons: a mineral record of early Archean to Mesoproterozoic (4348–1576 Ma) magmatism

Magmatic processes were important on the nascent Earth during the first 500 million years (Ma) after accretion, yet the causes and timing of this early magmatism are largely unconstrained, as no rocks from this period have been discovered. Rare >4000 Ma detrital zircons from Western Australia preserve the only direct geologic evidence of this early magmatism. To understand the genesis and history of these zircons, we present the results of a combined ion and electron microprobe, and SEM study of the age, Th–U chemistry, cathodoluminescence (CL) zoning patterns, and inclusions for a population of detrital zircons from Jack Hills, Western Australia, with 207Pb/206Pb ages ranging from 4348 to 1576 Ma. The majority of the zircons preserve primary growth features discernable by CL imaging, such as oscillatory and sector zoning, have Th/U ratios from 0.1 to 1.0, and several contain granitic mineral inclusions. Thus, aside from age they are largely indistinguishable from zircons produced in common felsic magmas. The Jack Hills zircons are therefore remnants of igneous rock-forming events that pre-date the rock record by up to 400 Ma. The 207Pb/206Pb age distribution pattern for zircons older than 3800 Ma from Western Australia suggests that early Archean magmatism was punctuated, both in terms of high frequency events and conspicuous gaps. The variable age distributions within different rock units in the Jack Hills demonstrate that Early Archean zircons were derived from multiple source rocks; samples from Eranondoo Hill contain up to 12% >4000 Ma zircons, suggesting either that the source rocks were nearby or represent a large terrane. Furthermore, younger 3700–3400 Ma rims on 4300–4000 Ma zircons are evidence that >4000 Ma crust survived long enough to participate in younger Archean tectonic events in the Yilgarn Craton of Western Australia. Mesoproterozoic igneous zircons in a quartzite 50 m from Eranondoo Hill are attributed to either sedimentation or tectonic interleaving of younger sediments no earlier than 1576 Ma. This previously unrecognized Proterozoic (or younger) geologic history calls into question previous estimates of the age of the Jack Hills sediments and demonstrates the heterogeneous distribution of >4000 Ma grains within the belt.