PRECAMBRIAN MULTISTAGE CRUSTAL EVOLUTION IN THE BAMBLE SECTOR OF SOUTH NORWAY - PB ISOTOPIC EVIDENCE FROM A SVECONORWEGIAN DEEP-SEATED GRANITIC INTRUSION

The Precambrian Ubergsmoen augen gnesis in the Bamble sector of south Norway is a dry, syntectonic charnockitic intrusion, which was emplaced at amphibolite- to granulite-facies conditions in the Sveconorwegian orogeny (approximately 1120 Ma). The present-day Pb-206/Pb-204 and Pb-207/Pb-204 ratios show restricted variation (Pb-206/Pb-204 = 17.566-17.938, Pb-207/Pb-204 = 15.459-15.590 in whole rocks; and Pb-206/Pb-204 = 17.159-17.350, Pb-207/Pb-204 = 15.451-15.487 in feldspar), whereas the Pb-208/Pb-204 ratio has a wider range of variation in whole rocks (36.571-42.200), but not in feldspars (36.487-37.054). These values reflect a U-238/Pb-204 ratio (mu) of 2-6 since the Sveconorwegian, and a variable Th/U ratio (2-20). At the time of final U-Pb differentiation, homogenization of the Pb isotopes was imperfect. The data therefore do not allow Pb-Pb isochron dating, but are well suited for model calculations. The isotopic evolution of Pb can be reproduced by a three-stage model, involving: (1) a mantle stage ending at 1.9-1.6 Ga; (2) extraction of a crustal precursor at 1.9-1.6 Ga, and differentiation of a LILE-enriched component (mu = 14-22); and (3) anatexis, igneous differention, emplacement and subsequent metamorphism at approximately 1.12 Ga. The granitic intrusion acquired its low U/Pb ratio by magmatic differentiation processes, rather than by fluid-induced U loss at high-grade metamorphic conditions.