The influence of recent lead loss on the interpretation of disturbed UPb systems in zircons from metamorphic rocks in southwest Sweden

Abstract

Zircons from progressively metamorphosed tonalites and granodiorites from the approximately 1.65-Ga-old, calc-alkalic Åmål suite (SW Sweden) have been analyzed with the UPb method. The data are discordant when plotted on concordia diagrams. The regressed discordias give geologically meaningless intercepts with the concordia even though the linear arrangements of the points are good. Zircons separated from rocks from four different localities (20 km apart) yield identical upper intercepts (within limits of error). The isotopic system could have been opened at various times. Extensive magmatic activity occurred in the area about 1.56 and 1.25 Ga ago. Most rocks are polymetamorphic, and all of them are metamorphosed in the amphibolite facies. Conventional RbSr isotopic analyses on whole-rock samples as well as RbSr small-slab studies show that the isotopic system in the Åmål granitoids was strongly overprinted approximately 1.4 and 1.0 Ga ago. Analyses of zircons treated with stepwise leaching with phreatic water, HF and with air abrasion strongly suggest that the UPb system in the zircons was influenced by a recent lead loss caused by natural processes. A three-stage mathematical model for zircon evolution is developed. This model is consistent with the intrusion of the Åmål suite and crystallization of the zircons at 1.65 Ga ago followed by a thermal event involving new growth of zircons at 1.4 Ga and finally, a recent lead loss. The thermal event about 1.0 Ga ago did not appreciably influence the UPb isotope system in the zircons; however, sphene gives a nearly concordant age corresponding to this event. This model is also consistent with the data obtained by fission-track analyses on apatites and with scanning electron microscope investigations.