METAMORPHISM, PROVENANCE AGES, AND SOURCE CHARACTERISTICS OF PRECAMBRIAN CLASTIC SEDIMENTS IN THE BAMBLE SECTOR, SOUTHERN NORWAY - AR, SR, ND, AND PB ISOTOPE STUDY

An integrated study of the K-Ar, Rb-Sr, Sm-Nd, and U-Th-Pb isotopic sy stems was performed on metasediments from two subareas of the Bamble S ector of southern Norway. (1) Hisoy: metapelitic gneisses and quartzit es with interlayered amphibolites, metamorphosed in the granulite faci es. A basic igneous rock as a protolith of the Hisoy amphibolites is m ost probable. The metasediments and intercalated amphibolites from His oy are not of LILE-depleted character. (2) The Froland area: quartzite s and metapelitic gneisses, metamorphosed in the amphibolite facies. B iotite K-Ar ages of the Froland area range from 930 to 998 Ma, while b iotite K-Ar ages from Hisoy range from 1074 to 1169 Ma. The higher His oy ages are best explained by the presence of excess Ar. Whole-rock Rb -Sr results of eight metasediment samples define a poorly fitted corre lation line of 1214 +/- 20 Ma, indicating incomplete homogenization of the Rb-Sr system. No correlation line can be calculated from whole-ro ck Sm-Nd data, indicating even less homogenization in the Sm-Nd system . The majority of metasediment samples yield Sm-Nd model ages within t he range 1.7 - 2.0 Ga, with no difference between the Hisoy and Frolan d suites and with a frequency maximum between 1.8 and 1.9 Ga. Uranogen ic Pb defines a poorly constrained correlation line with an age of 126 2 +/- 110 Ma and a single stage model mu-value of 8.18. The combined P b and Nd data suggest that the sediments were derived from a LILE-enri ched ''upper crustal'' source (U-238/Pb-204 = 18 - 30), which was deri ved from a LILE-depleted mantle (U-238/Pb-204 = 7.9 - 8.0) some time b etween 1.7 and 1.9 Ga, and which must have had a lifetime of at least 300 Ma before deposition of the sediments. Sveconorwegian U-Pb differe ntiation was poorly coupled to Pb isotopic homogenization but leads to U-238/Pb-204 ratios in the range 12 - 65 and is the reason for the hi ghly radiogenic Pb compositions seen in some samples and for the prese nt-day correlation of the Pb isotopes.