Geochemical relationships in the Sudbury igneous complex; origin of the main mass and offset dikes

Petrological models relating the different rock types constituting the 1.85 Ga Sudbury Igneous Complex are constrained with extensive new geochemical data. We show that the main mass felsie noritc, transition zone quartz gabbro, and granophyre have similar atios of the highly incompatible trace elements (e.g., LaJ Sm = 4.5-7, LaJNb = 2.8-4.2, Tt•Zr = 0.04-0.05) and that these variations are consistent with the crystalliza-tion and differentiation f magma types largely (>80%) derived from the upper crust, with a smaller contribu-tion from a mantle soume. Although there is presently no conclusive proof that magma was generated in situ as a melt sheet produced by meteorite impact, we find no principal reason why this model should be rejected. However, we propose that a small contribution f mantle-derived pierifle magma is required to explain the abundant Ni, Cu, and platinum-group elements (PGE) in the Sudbury deposits, as well as the compositions of the ultramarie nclusions (MgO = 12-36 wt %; Foes_s7 olivines with 450-3,700 ppm Ni, and abundant chrome-rich spinel), and the magnesian composition f the marie noritc (8-14 wt % MgO) and the sublayer (6-12 wt % MgO). We believe that the main mass of the Sudbury Complex achieved its present composition through incorporatation of up to 20 percent mantle-derived picritic magma emplaced along crustal fractures produced by the impact event. These picritic magmas entered the melt sheet as a dense plume, vigorously