Detrital zircon from the Jack Hills and Mount Narryer, Western Australia:: Evidence for diverse >4.0 Ga source rocks

Geochronological, geochemical, and morphological analysis of detrital zircon in the Jack Hills and Mount Narryer metasedimentary belts, Western Australia, indicates the grains were derived from diverse rocks, including >4000 Ma sources that predate the oldest known terrestrial rock. In three metaconglomerate layers in the western part of the Jack Hills, 4200-3800 Ma zircon makes up 14% of the population, 3800-3600 Ma grains form only 2%, and 3550-3250 Ma zircon is dominant with a significant peak at 3380 Ma (U-Pb ages and trace element concentrations were obtained by laser-ablation microprobe inductively coupled plasma mass spectrometry). These grains are interpreted as being derived from similar rock types because they are indistinguishable in U concentration (50-200 ppm), internal zoning (both oscillatory and sector zoning within the same grain), and morphology (subequant fragments of grains). We conclude that a previously proposed evolved granitic source is unlikely because the zircon differs significantly in U concentration, internal zoning, and morphology from zircon in typical Archean granitic rocks, such as the 3730-3300 Ma granitic gneisses that surround the Jack Hills belt. More likely sources were intermediate composition plutonic rocks that were distal or perhaps destroyed or removed from the region during Neoarchean tectonism. In contrast, detrital zircon in quartzites and metaconglomerates at Mount Narryer appears to have been derived from granite based on elongate prismatic morphology, fine oscillatory zoning, relatively high U concentration (100-600 ppm), and xenotime and monazite inclusions. Ages are also different: 4200-3800 Ma zircon makes up only 3% of the Mount Narryer population (most grains are 4200-4100 Ma), 3800-3600 Ma zircon forms 31%, and peaks are at 3650, 3600, and 3500 Ma. Trace element concentrations are broadly similar, except Mount Narryer zircon generally has higher U, smaller Ce and Eu anomalies, and lower Nb/Ta. Mount Narryer zircon is interpreted as having local granitic sources because the <3800 Ma grains closely match the age and nature of zircon in the surrounding granitic gneisses, which may include a minor, currently undiscovered 4200-4100 Ma component. The diversity of ancient zircon suggests that Earth's crust was heterogeneous by 4200 Ma, having already differentiated into granitic and intermediate components, as is the case in modern continents.