DATING CONTINENTAL SUBDUCTION AND EXHUMATION WITH MONAZITE IN THE WESTERN GNEISS REGION, NORWAY

Monazite U–Pb and trace-element data were gathered from six high- to ultrahigh-pressure (UHP) samples from the Western Gneiss Region, Norway, using LASS (laser-ablation split-stream ICP-MS) to investigate variations in monazite composition during high-pressure metamorphism. The cores of monazite grains were found to contain higher Sr, higher non-radiogenic Pb, weaker negative Eu anomalies, and lower Y+HREE compared to the rims. The high-Sr–Pb–Eu and low-Y+HREE monazite cores are interpreted to record growth or recrystallization at UHP, whereas the rims are interpreted to record amphibolite-facies metamorphism during exhumation. This hypothesis is supported by independent geo- and thermochronological constraints on the metamorphic evolution of the terrane. The UHP monazite composition (high Sr–Pb–Eu, low Y+HREE) is interpreted to result from growth in the absence of feldspar but abundance of garnet. Due to complexities of Eu partitioning resulting from its multiple valence states, this multi-element monazite proxy for feldspar-absent, high-pressure metamorphism is considered more robust than the Eu anomaly alone.