The interpretation of complex zircon U–Pb systems in Archaean granitoids and gneisses from the Jack Hills, Narryer Gneiss Terrane, Western Australia

Abstract

SHRIMP U–Pb ages of zircons from Archaean TTG granitoids and gneisses from the Jack Hills region of Western Australia show a range of complexity which reflects the origin and geological history of the rocks and the stability of the zircon U–Pb systems under metamorphic and magmatic conditions. Strong light rare earth element (LREE) enrichment and similarities in the major and trace element chemistry suggest that TTG gneisses formed by fractional melting of a garnet-bearing amphibolitic source. Maximum ages, of 3.75 to 3.6 Ga, obtained from low U zircon cores are interpreted as the period of formation of the TTG parent rocks. However, most zircons in these populations are complex, consisting of zoned and unzoned parts, and have disturbed U–Pb systems which form a concordant to discordant array between 3.7 and 3.3 Ga on a concordia plot, suggesting these grains experienced one or more disturbance events within this time span. The range of complexity of U–Pb systems observed in a number of the zircon populations is also present within individual zircon crystals indicating that secondary movement and loss of Pb and possibly U has occurred within and from zircons, causing variable isotopic discordance and yielding a range of apparent ages within a single grain. Disturbance of the zircon isotopic systems appears to be due to a combination of early Archaean events together with recent Pb loss. The presence of 3.3 Ga zircon rims around older cores in samples of tonalite and porphyritic granodiorite, together with the exclusive presence of 3.3 Ga zircons in one sample of porphyritic granite, is interpreted as evidence of a major magmatic and possibly metamorphic event at this time. The presence of ca 3.5 Ga ages in a number of unzoned zircons in the tonalite and two samples of granodiorite suggests that this could also have been a time of magmatism and zircon crystallization. Evidence for 3.3 Ga magmatism appears to be restricted to zircons from TTG rocks south of the Jack Hills Metasedimentary Belt, raising the possibility that the present position of the belt defines a major suture within the Narryer Gneiss Terrain. The post-tectonic monzogranites have LREE enriched patterns with strong negative Eu anomalies, similar to the older gneisses, but in contrast to these rocks, zircons from the monzogranites have relatively simple U–Pb systems, which define an age of 2654±7 Ma, and only rarely do zircons have inherited cores with nearly concordant ages of ca 3.6 Ga. The very low content of inherited zircon raises doubts about the formation of the granites by reworking of older TTG gneisses and an alternative explanation that the monzogranites formed from new 2.65 Ga crust, with minor 3.6 Ga crustal contamination, is seen as more likely.