DETRITAL ZIRCON AGES FROM SOUTHERN NORWAY - IMPLICATIONS FOR THE PROTEROZOIC EVOLUTION OF THE SOUTHWESTERN BALTIC SHIELD

An ion-microprobe (SIMS) U-Pb zircon dating study on four samples of P recambrian metasediments from the high-grade Bamble Sector, southern N orway, gives the first information on the timing of discrete crust-for ming events in the SW part of the Baltic Shield. Recent Nd and Pb stud ies have indicated that the sources of the elastic metasediments in th is area have crustal histories extending back to 1.7 to 2.1 Ga, althou gh there is no record of rocks older than 1.6 Ga in southern Norway. T he analysed metasediments are from a sequence of intercalated, centime tre to 10-metre wide units of quartzites, semi-metapelites, metapelite s and mafic granulites. The zircons can be grouped in two morphologica l populations: (1) long prismatic; (2) rounded, often flattened. The B SE images reveal that both populations consist of oscillatory zoned, r ounded and corroded cores (detrital grains of magmatic origin), surrou nded by homogeneous rims (metamorphic overgrowths). The detrital zirco ns have Pb-207/Pb-206 ages between 1367 and 1939 Ma, with frequency ma xima in the range 1.85 to 1.70 Ga and 1.60 to 1.50 Ga. There is no cor relation between crystal habit and age of the zircon. One resorbed, in ner zircon core in a detrital grain is strongly discordant and gives a composite inner core-magmatic outer core Pb-207/Pb-206, age Of 2383 M a. Two discrete, unzoned zircons have Pb-207/Pb-206 ages of 1122 and 1 133 Ma, representing zircon growth during the Sveconorwegian high-grad e metamorphism. Also the mu m wide overgrowths, embayments in the detr ital cores and apparent ''inner cores'' which represent secondary meta morphic zircon growth in deep embayments in detrital grains, are of Sv econorwegian age. The composite-detrital-metamorphic zircon analyses g ive generally discordant Pb-206/U-238 versus Pb-207/U-235 ratios and m aximum Pb-207/Pb-206 ages of 1438 Ma. These data demonstrate the exist ence of a protocrust of 1.7 to 2.0 Ga in the southwestern part of the Baltic Shield, implying a break in the overall westward younging trend of the Precambrian crust, inferred from the southeastern part of the Baltic Shield.