Assessing Inheritance of Zircon and Monazite in Granitic Rocks from the Monashee Complex, Canadian Cordillera

Zircon and monazite from granitic sheets and dikes in the Monashee complex, Canadian Cordillera, were investigated to determine whether igneous crystallization occurred at 1·9 Ga or 50 Ma with 1·9 Ga inherited zircon and monazite. Four of the five samples are weakly deformed to undeformed, despite occurring in a gneiss dome at the structurally deepest exposed level of the orogen that elsewhere was strongly deformed and partly melted at 50 Ma. Based on U–(Th)–Pb ages from zircon and monazite, field relationships, and mineral composition and zoning, we conclude that the granitic rocks crystallized at 1·9 Ga and were metamorphosed at 50 Ma. All dated zircon is 1·9 Ga (except for 2·3–2·0 Ga inherited cores) and 1·9 Ga monazite makes up >90% of the population in four samples. The remainder of the monazite is 50 Ma and all monazite in one sample is 50 Ma. Composition and zoning of 1·9 Ga zircon and monazite are uniform within samples, yet differ between samples, indicating growth from 1·9 Ga magmas that are unique to each sample. This relationship is unlikely if the grains are inherited because the host rocks are heterogeneous 2·3–2·1 Ga gneisses. The 1·9 Ga zircon and monazite have zoning that is consistent with growth from magmas, whereas the 50 Ma monazite has variable composition and zoning that suggest growth from diverse metamorphic fluids. The results demonstrate that part of the Monashee complex was last strongly deformed and partly melted at 1·9 Ga, and thus largely escaped Cordilleran tectonism.