Bulk compositional controls on the preservation of age domains within metamorphic monazite: A case study from quartzite and garnet–cordierite–gedrite gneiss of Thor-Odin dome, Monashee complex, Canadian Cordillera

Monazite growth and/or recrystallization can record the timing of metamorphism and anatexis and there is potential that distinct intracrystalline chemical domains may record the timing of different metamorphic events and/or reactions. To evaluate whether chemical domains in monazite correspond to the timing of peak metamorphism and subsequent anatexis, and whether bulk-rock composition plays a role in the preservation of age domains within monazite, grains from garnet–cordierite–gedrite rocks and interlayered quartzite from basement paragneiss of Thor-Odin dome in the Canadian Cordillera were dated by the U–Pb method using the Sensitive High Resolution Ion Microprobe (SHRIMP). Monazites were chemically mapped for Y, U, Ca and Th on the electron microprobe prior to analysis, to identify chemical domains that may represent age domains, and to select SHRIMP analytical sites. Most grains show a range of chemical patterns, including sector zoning, concentric zoning, discontinuous or irregular patches with embayments and cores varying in composition relative to the rims. Three samples yield a range of 206Pb/238U dates from ca. 62 to 50 Ma. The distinct chemical domains in the monazite grains do not generally correspond to discernable age domains; however, there are rare exceptions. In some cases, chemically homogeneous monazite grains contain different age domains. Monazite growth or recrystallization occurred over ca. 10 Myr. The range of ages preserved varies with bulk-rock composition of the host rocks: older ages and larger age ranges occur in the garnet-bearing cordierite–gedrite samples, whereas the quartzite contains the youngest age population. As all samples experienced the same metamorphic conditions, the range in ages is attributed to variable mineral assemblages, such as the presence of garnet, staurolite, and/or cordierite, that occur due to differences in bulk composition. Determining the timing of metamorphism in polymetamorphic terrains can be greatly complicated by the preservation of a range of monazite ages, which can be dependent on the composition of the host rock.