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

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 1.6Ga and 1.0Ga) 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 2–3.5GPa. 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 1.0–0.9Ga high-grade metamorphism and was late Proterozoic to early Paleozoic.