Pb-loss patterns in zircons from a high-grade metamorphic terrain as revealed by different dating methods: UPb and PbPb ages for igneous and metamorphic zircons from northern Sri Lanka

Abstract

Abstract We report UPb and 207 Pb 206 Pb ages for multigrain fractions, single grains, and grain domains in zircons from highgrade igneous and sedimentary rocks of the Vavuniya Charnockite Province (VCP) in northern Sri Lanka, using the ionmicroprobe SHRIMP, a new vapour digestion technique (VDT) and evaporation. Charnockitic, enderbitic and mangeritic gneisses are derived from I-type tonalitic, syenitic, granodioritic and granitic precursors for which the UPb systematics of zircons as determined by SHRIMP suggest crystallization ages between ∼ 1000 and ∼ 1100 Ma. Many translucent, round and multifacetted zircons in these rocks are not of metamorphic but of igneous origin and acquired their shape through partial zircon dissolution and recrystallization. The large majority of zircons exhibit severe and variable Pb-loss some 540–580 Ma ago which we ascribe to granulite-grade metamorphism, almost immediately followed by local retrogression. Most of the zircons lost Pb from the entire grain with no domains remaining that reflect the original Pb isotopic composition. It was therefore impossible to obtain meaningful 207 Pb 206 Pb evaporation ages for such grains, and this cautions against use of this method in zircons that experienced severe Pb-loss. However, zircons formed during high-grade metamorphism apparently retain their primary Pb isotopic composition and were dated successfully by the evaporation technique, yielding ages between 553 ± 26 and 560 ± 30 Ma. Conventional multigrain analysis and small fractions following VDT for two samples, although much more precise than the SHRIMP data, yielded only strongly discordant data points with patterns reflecting variable Pb-loss. These data alone would not have made it possible to obtain reliable primary zircon crystallization ages. 207 Pb 206 Pb evaporation ages for detrital zircons from two garnetiferous gneiss samples of sedimentary origin in the centre of the VCP revealed a maximum age of 1329 ± 18 Ma, whereas a metaquartzite near the southern margin of the VCP contained zircons with 207 Pb 206 Pb ages between 1857 ± 14 and 2543 ± 14 Ma. We interpret the latter rock as belonging to the Highland Complex which occurs adjacent to the VCP. Our zircon data in combination with Nd model ages (Milisenda et al., 1994) suggest the VCP to be part of the Wanni Complex, a distinct crustal domain in northern Sri Lanka characterized by relatively young ages in comparison with much older ages in the adjacent Highland Complex.