Assessing Inheritance of Zircon and Monazite in Granitic Rocks from the Monashee Complex, Canadian Cordillera
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| Authors: | Crowley, JL; Brown, RL; Gervais, F; Gibson, HD |
| Year: | 2008 |
| Journal: | J. Petrol. 49: 1915-1929 Article Link (DOI) |
| Title: | Assessing Inheritance of Zircon and Monazite in Granitic Rocks from the Monashee Complex, Canadian Cordillera |
| Abstract: | 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 50 Ma 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 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 19 Ga and were metamorphosed at 50 Ma. All dated zircon is 19 Ga (except for 2320 Ga inherited cores) and 19 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 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 2321 Ga gneisses. The 19 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 19 Ga, and thus largely escaped Cordilleran tectonism. |
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