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Outline • Economic Importance of Sudbury Complex • Geological History favoring both sides of debate. • Defense of large mantle-derived mafic intrusion. – Layered mafic intrusion. – Origion of unusual 𝑆𝑖𝑂2 content – Re/Os ratios. Economic Importance • Host to world’s largest known Ni-Cu sulfide ore. • Past production and reserves: 1.65𝑥109 tons. • Average ore composition: 1.2% Ni, 1.02% Cu. • Platinum Group Elements. Geological History Sudbury Igneous Complex *1.85 G.a. *1.8 G.a. Penkean Orogeny Canadian Shield Superior Province Southern Province Greenville Province Layered Mafic Intrusion • Eu anomalies Similiar to Bushveld, Stillwater, Skargaard complexs • Layers represent fractional crystallization. • Platinum group elements in lower layer. Figure from Kuo and Crocket: Showing layering similar to layered mafic intrusion. Origin of High 𝑆𝑖𝑂2 • Meteorite impact resulted in crustal melting. • Active Convergent Margin explains higher 𝑆𝑖𝑂2 content. • Composition of Proterozoic basalts from Superior Province in area indicate a silica rich mantle. • Xenoliths in sublayer indicate crustal-rich parental magma Re/Os Ratio • Re/Os can be sensitive tracer for meteoritic contamination. • High levels of terrestrial Os in Sudbury favor endogenic model. • Meteorite Defense argues instant vaporization. Popagai and Chicxulub impact sites. The Other Side: Differentiated Impact Melt Sheet Reasons Against Layered Mafic Intrusion • Strontium ratios for the various lithologies are not compatible with mantle-derived intrusions with a 1.85 Ga age – Ratios would need a mixture of up to 80% crustal material to achieve those levels • Neodymium ratios are typical for average upper continental crust at that time – These Nd ages correspond well with U – Pb zircon ages for local Archean rocks in the Sudbury area – Again incompatible with a significant mantle-derived component in the complex Reasons Against cont’d. • Layering suggests nothing because – compositional variations have been observed within smaller impact melt sheets (Morokweng) – average composition of the complex is similar to other known impact melt sheets Explanation on why layering isn’t commonly found • Not thick enough – Morokweng – 800m thick compared to Sudbury 2.5km thick • Probably too siliceous and/or viscous • Crystallized too quickly for differentiation to occur Reasoning against contamination melt • Crustal isotopic signature and the energy required (~1022J) for the amount of contamination to be unrealistic • Proper mixes of major elements from the target rocks have been found without contamination – One of these mixes is 93% average Archean Abitibi granite-greenstone terrain and 7% Huronian Lorrain arkose Reasoning for impact melt sheet • The basic stratigraphic relations at the Sudbury structure are nearly the same as those observed at other large complex impact structures Reasons for cont’d. • Primary Na-rich plagioclase are not common in layered igneous complexes • The coexistence of these unusual Na-rich plagioclase crystals with the Ca-rich ones cannot be achieved through normal processes of igneous crystallization - Plagioclases with reverse zoning and complex twinning have, however been described in the Manicouagan impact melt rocks Reasons for cont’d. • Shatter cones and other shock metamorphic features such as – planar deformation features (PDFs), or shock lamellae, in quartz within lithic clasts of the Onaping Formation – as well as the occurrence of high-pressure polymorphs, such as impact diamonds, in the Onaping Formation Comparisons to other large impact structures Conclusion • The debate on the origins of the Sudbury Igneous Complex still rages – Most recently, nearly all of the evidence has pointed to the impact melt sheet model – But still a few hold on to the notion of mantlederived layered mafic intrusion