Monazite response to ultrahigh-pressure subduction from U-Pb dating by laser ablation split stream

•U-Pb dates and trace-element concentrations were measured from Caledonian monazite-bearing ultrahigh-pressure rocks.•Nearly all monazite from metasedimentary rocks is Caledonian.•A few record recrystallization during ultrahigh-pressure subduction.•Some Precambrian granulite-facies basement samples were incompletely reset during subduction and exhumation.•Caledonian dates young westward, compatible with a westward progression of unroofing.

To assess the response of monazite during subduction of continental crust to mantle depths, U-Pb isotopic ratios and elemental abundances were measured simultaneously by laser-ablation split-stream inductively-coupled plasma mass spectrometry (LASS) in rocks from the ultrahigh-pressure Western Gneiss Region of the Scandinavian Caledonides. Nearly seventy different samples of quartzofeldspathic basement and overlying metasedimentary rocks were studied. Pre-subduction monazite (chiefly 1.6 Ga and 1.0 Ga) is preserved locally in the structurally lowest, basement rocks because earlier, Precambrian tectonism produced coarse-grained, high-grade rocks that were resistant to further recrystallization in spite of syn-subduction temperatures and pressures of 650-800 °C and 2-3.5 GPa. A few of the monazite in the metasedimentary rocks atop the basement preserve syn-subduction U-Pb dates, but the majority continued to recrystallize during post-subduction exhumation and record a general westward decrease in age related to westward-progressing exhumation. The absence of Precambrian monazite in the metasedimentary rock atop the basement suggests that sedimentation postdated the 1.0-0.9 Ga high-grade metamorphism and was late Proterozoic to early Paleozoic.