MONAZITE PETROCHRONOLOGY FROM THE UHP WESTERN GNEISS REGION, NORWAY

To understand the significance of U/Th-Pb chronology, dates from accessory minerals need to be tied to petrological conditions such as pressure, temperature, and mineral assemblage. This work examines monazite composition, petrologic texture, and U/Th-Pb dates acquired via LASS (laser ablation split stream) geochronology from pelites across the ultrahigh-pressure (UHP) Western Gneiss Region, Norway (WGR), to evaluate the range in monazite compositions preserved in UHP terranes and determine the petrologic reasons for the observed compositions.

Proterozoic monazites (1600 & 1100-900 Ma) occur in amphibolite- to granulite-facies rocks across the WGR and as garnet inclusions in eclogite- and amphibolite-facies rocks. The oldest Paleozoic monazites have dates of 460 Ma (1 sample), sector zoning, and show typical HREE profiles (Gd/Yb <1000). Four samples have dates of 430-420 Ma, and a decrease in HREE with time (up to Gd/Yb >10000), but no trend in Eu/Eu*, compatible with garnet + plagioclase stable conditions during prograde metamorphism. Three samples from the Sørøyane UHP domain have dates of 410-405 Ma, depleted HREE (Gd/Yb >1000), weaker Eu anomalies, 2-3% common Pb, and, in one sample, high Sr contents relative to younger monazites in the same rock (0.6 vs <0.1 wt%). High Gd/Yb ratios, weak Eu/Eu*, and relatively high Sr concentrations are compatible with garnet-stable, plagioclase absent conditions during (U)HP metamorphism. Except for the samples listed above, most monazites measured from the WGR have concordant dates from 400-390 Ma with increasing HREE concentrations over time, compatible with garnet resorbtion during the decompression that followed the (U)HP metamorphism. Two of the 430-420 Ma and most of the 400-390 Ma monazites have rims that vary laterally in width and composition, in the form of finger- to scallop-shaped embayments. These rims have large variation in U/Th, U/Pb and Th/Pb ratios, suggesting that alteration, by a fluid or other mechanism, can create spurious U/Th-Pb spot dates by up to 10 Myr.