U-TH/PB GEOCHRONOLOGY OF THE NORWEGIAN ULTRAHIGH-PRESSURE TERRANE

Monazites were dated to constrain the formation and exhumation of the world’s largest ultrahigh-pressure province in Norway. We used a 193 nm ArF laser coupled to the Isoprobe at the University of Arizona to ablate material from 8 µm wide pits in thin sections and in grain separates. Calibration was established by interleaving analyses of a 425-Ma standard. Two sources of measurement error, the fractionation correction (2.4% 2s) and counting-measurement errors (3.2%), dominate the internal error of the Th/Pb age. The common-Pb correction, uncertainty in the 232Th decay constant, and uncertainty in the age of the standard add an additional ~0.74% (2s) error to the absolute age. Thus, a single analysis typically yields a 232Th/208Pb age with a precision of ± 17.0 Ma (2s) and an accuracy of ± 17.3 Ma (2s). The Norwegian monazites reach several hundred microns in diameter, enabling detailed age mapping with hundreds of laser spots and detailed elemental mapping using an electron probe. Individual spot ages range from ~2085 to ~360 Ma, preserving a rich 1400 Ma history. Individual grains preserve 232Th/208Pb age gradients of >100 Ma, but the 207Pb/206Pb and 238U/206Pb ratios show that these are discordant ages resulting from Pb loss. These same grains are homogeneous in their elemental composition: SiO2 (0.2-0.3 wt%), CaO (1.0-1.4 wt%), FeO (<0.6 wt%), Y2O3 (<2.5 wt%), La2O3 (14-16 wt%), Ce2O3 (28-30 wt%, Pr2O3 (2.9-3.1 wt%), SmO (1.7-2.2 wt%), Nd2O3 (11.8-12.6 wt%), Gd2O3 (1.1-1.5 wt%), ThO2 (2-5 wt%), and UO2 (0.2-0.6 wt%). Monazites from the high-pressure part of the orogen show Pb loss at 400-395 Ma, identical to observations by Tucker et al. (1990); the systematics of monazites from the foreland nappe stack in the Trondelag area are similar. These ages are slightly younger than eclogite U/Pb zircon + Sm/Nd ages and slightly older than muscovite 40Ar/39Ar ages, implying that they date the supra-Barrovian amphibolite-facies overprint that followed the UHP metamorphism. In contrast, monazites from allochthons immediately overlying high-pressure rocks in the Solund area give ages of 441.4 ± 5.5 Ma and 502.1 ± 9.2 Ma, and monazites included within garnets in allochthons in the Hornelen area yield ages of 465.8 ± 6.5 Ma and 437.5 ± 9.9 Ma, indicating multiple metamorphic events prior to the UHP metamorphism.