PB-LOSS PATTERNS IN ZIRCONS FROM A HIGH-GRADE METAMORPHIC TERRAIN AS REVEALED BY DIFFERENT DATING METHODS - U-PB AND PB-PB AGES FOR IGNEOUSAND METAMORPHIC ZIRCONS FROM NORTHERN SRI-LANKA

We report U-Pb and Pb-207/Pb-206 ages for multigrain fractions, single grains, and grain domains in zircons from high-grade igneous and sedi mentary rocks of the Vavuniya Charnockite Province (VCP) in northern S ri Lanka, using the ion-microprobe SHRIMP, a new vapour digestion tech nique (VDT) and evaporation. Charnockitic, enderbitic and mangeritic g neisses are derived from 1-type tonalitic, syenitic, granodioritic and granitic precursors for which the U-Pb systematics of zircons as dete rmined by SHRIMP suggest crystallization ages between approximately 10 00 and approximately 1100 Ma. Many translucent, round and multifacette d zircons in these rocks are not of metamorphic but of igneous origin and acquired their shape through partial zircon dissolution and recrys tallization. The large majority of zircons exhibit severe and variable Pb-loss some 540-580 Ma ago which we ascribe to granulite-grade metam orphism, almost immediately followed by local retrogression. Most of t he zircons lost Pb from the entire grain with no domains remaining tha t reflect the original Pb isotopic composition. It was therefore impos sible to obtain meaningful Pb-207/Pb-206 evaporation ages for such gra ins, and this cautions against use of this method in zircons that expe rienced severe Pb-loss. However, zircons formed during high-grade meta morphism apparently retain their primary Pb isotopic composition and w ere dated successfully by the evaporation technique. yielding ages bet ween 553 +/- 26 and 560 +/- 30 Ma. Conventional multigrain analysis an d small fractions following VDT for two samples, although much more pr ecise than the SHRIMP data, yielded only strongly discordant data poin ts with patterns reflecting variable Pb-loss. These data alone would n ot have made it possible to obtain reliable primary zircon crystalliza tion ages. Pb-207/Pb-206 evaporation ages for detrital zircons from tw o garnetiferous gneiss samples of sedimentary origin in the centre of the VCP revealed a maximum age of 1329 +/- 18 Ma. whereas a metaquartz ite near the southern margin of the VCP contained zircons with Pb-207/ Pb-206 ages between 1857 +/- 14 and 2543 +/- 1 4 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 (Milise nda et al.. 1994) suggest the VCP to be part of the Wanni Complex, a d istinct crustal domain in northern Sri Lanka characterized by relative ly young ages in comparison with much older ages in the adjacent Highl and Complex.