Publication Type:Journal Article
Source:Lithos (Oslo), Elsevier, Amsterdam, International, Volume 51, Number 4, p.271-292 (2000)
Keywords:Canada, chemical composition, country rocks, dikes, eastern canada, emplacement, enrichment, intrusions, isotope ratios, Isotopes, metal ores, metals, mineral composition, mineralization, Nd-144/Nd-143, neodymium, nickel ores, Ontario, Paleoproterozoic, Petrology, phenocrysts, Precambrian, proterozoic, radioactive isotopes, rare earths, Samarium, Sm-147/Nd-144, Stable isotopes, Sudbury District Ontario, Sudbury Igneous Complex, upper Precambrian, Whitewater Group
The mineralized sublayer at the base of the Sudbury Igneous Complex (SIC) consists of two variants, the noritic contact sublayer and radial and concentric quartz dioritic offset dykes. Both are characterized by the presence of significant quantities of Ni-Cu-PGE sulphides and by a prominent population of recrystallized diabasic-textured and melanocratic to ultramafic fragments. The two variants of the sublayer contain compositionally distinct inclusion populations and inclusion-bearing matrices. Contact sublayer and offset dykes hosted by north range granitoid footwall can be distinguished from those hosted by south range basaltic and metasedimentary footwall environments. The compositional variation in SIC rocks can be described in terms of contributions from exposed crustal rocks and differentiation of the resultant melt(s). The basaltic inclusion population is characterized by hornfels recrystallization of the plagioclase, and is geochemically and isotopically identified with Huronian basalts which comprise the south range footwall, with (Ce/Yb) (sub N) ratios of around 2.5 and epsilon (sub Nd) (super 1850) between -2 to -5. The melanocratic inclusions in the sublayer are typically coarse-grained and undeformed, with incompatible element contents and radiogenic isotopic compositions intermediate between those of the basaltic inclusions and those of the melt sheet, which has (Ce/Yb) (sub N) ratios of around 10 and epsilon (sub Nd) (super 1850) around -9. Calculated crystallization models are consistent with derivation of the ultramafic inclusions by crystallization from a magma produced by mixing of molten basaltic footwall with basal melt sheet. It is proposed that the sublayer appeared as the marginal facies of a meteorite impact melt sheet as a result of footwall melting following the impact. This basal layer was progressively enriched in sulphides and mafic cumulates from above through differentiation during cooling. Offset dykes were emplaced and the magmas effectively removed from the system. Subsequently, continued evolution of the marginal facies produced the more mafic inclusions in the contact sublayer. No extracrustal (e.g., mantle) component is envisioned in this model to explain the silicate compositional distributions in the SIC, and mafic crustal rocks in the target zone are implicated as the metal source for the SIC deposits.
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