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 19 Ga or 50Ma with 19 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 50Ma. 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 19 Ga and were metamorphosed at 50Ma. All dated zircon is 19 Ga (except for 23^20 Ga inherited cores) and 19 Ga monazite makes up 490 % of the population in four samples. The remainder of the monazite is 50Ma and all monazite in one sample is 50Ma. Composition and zoning of 19 Ga zircon and monazite are uniform within samples, yet differ between samples, indicating growth from 19 Ga magmas that are unique to each sample. This relationship is unlikely if the grains are inherited because the host rocks are heterogeneous 23^21Ga gneisses. The 19 Ga zircon and monazite have zoning that is consistent with growth from magmas, whereas the 50Ma 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 19 Ga, and thus largely escaped Cordilleran tectonism