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Abstract:

In the North Range of the ca. 1.85 Ga Sudbury impact structure, massive bodies of Sudbury breccia cut across the Archean Levack gneiss complex and Early Proterozoic Matachewan diabase dykes. The complex underlies the Sudbury Igneous Complex (SIC), which represents an impact melt sheet that lined the floor of the impact crater. The breccia occurs as 1–100 m wide irregular bodies that contain rounded clasts of diabase and tonalitic and dioritic Levack gneiss within a fine-grained to aphanitic, black to dark grey matrix. Mineral chemical analyses and Rietveld analyses based on X-ray diffraction patterns show that the breccia matrix contains microclasts of magnesiohornblende, oligoclase, and quartz, surrounded by metamorphic andesine laths, actinolite, Kfeldspar, albite, and chlorite, which crystallized during cooling of the SIC. On binary oxide-SiO2 diagrams, breccia values plot on a compositional mixing line between those of the more felsic tonalitic gneiss and the more mafic dioritic gneiss and diabase. The breccia has concentrations in rare earth elements (REEs) intermediate between those of the dioritic and tonalitic gneisses and a REE pattern with a negative slope that is less pronounced than those of the gneisses. This is due to the addition of a minor diabase component that has a slightly negative REE slope, suggesting that the breccia is a mix of comminuted tonalitic gneiss, dioritic gneiss, and minor diabase. Comminution and cataclasis of these source rocks occurred along pre-existing anisotropies and fractures that formed and were reactivated during the growth and collapse of the transient crater. The comminuted materials mixed together as large bodies of breccia that were injected in dilational sites that opened during the upward expansion and collapse of the crater.