Monazite trace-element and isotopic signatures of (ultra)high-pressure metamorphism: Examples from 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 UHP monazites were fo...

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Bibliographic Details
Published in:Chemical geology 2015-08, Vol.409, p.99-111
Main Authors: Holder, Robert M., Hacker, Bradley R., Kylander-Clark, Andrew R.C., Cottle, John M.
Format: Article
Language:English
Subjects:
Concentration (composition)
Crystal structure
Feldspars
Gneiss
Lead (metal)
Melting
Monazite
Norway
Petrochronology
Strontium
Trace elements
Ultrahigh pressure
U–Pb
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Summary: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 7600ppm Sr, 110ppm non-radiogenic Pb, relatively weak negative Eu anomalies, and Y concentrations as low as 500ppm. Amphibolite-facies monazite that rims the UHP monazite in one sample contains Y concentrations up to 1.6wt.%, Sr as low as 13ppm, 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. •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.
ISSN:0009-2541
1872-6836
DOI:10.1016/j.chemgeo.2015.04.021