Monazite trace-element and isotopic signatures of (ultra)high-pressure metamorphism: Examples from the Western Gneiss Region, Norway

- U-Pb dates and HREE indicate monazite recrystallization at UHP.
- UHP monazites have high Sr and non-radiogenic Pb.
- High Sr-Eu2 +-Pb may result from feldspar breakdown at high pressure.

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 UHP monazites were found to contain up to 7600 ppm Sr, 110 ppm non-radiogenic Pb, relatively weak negative Eu anomalies, and Y concentrations as low as 500 ppm. Amphibolite-facies monazite that rims the UHP monazite in one sample contains Y concentrations up to 1.6 wt.%, Sr as low as 13 ppm, and no detectable non-radiogenic Pb. The UHP monazite composition (high Sr-Pb) is interpreted to result from growth in the absence of feldspar, possibly aided by increased compatibility of Sr-Pb-Eu2 + in the monazite crystal structure at high pressure. Sr in monazite as a proxy for feldspar stability may be a useful tool not only in studying high-pressure metamorphism, but also in determining timescales of melting and crystallization, when the amount of feldspar changes over time.