Evolution of the sublayer of the Sudbury Igneous Complex: geochemical, Sm-Nd isotopic and petrologic evidence

The mineralized sublayer at the base of the Sudbury Igneous Complex (SIC) c onsists of two variants, the noritic contact sublayer and radial and concen tric quartz dioritic offset dykes. Both are characterized by the presence o f significant quantities of Ni-Cu-PGE sulphides and by a prominent populati on of recrystallized diabasic-textured and melanocratic to ultramafic fragm ents. The two variants of the sublayer contain compositionally distinct inc lusion populations and inclusion-bearing matrices. Contact sublayer and off set dykes hosted by north range granitoid footwall can be distinguished fro m those hosted by south range basaltic and metasedimentary footwall environ ments. The compositional variation in SIC rocks can be described in terms o f contributions from exposed crustal rocks and differentiation of the resul tant melt(s). The basaltic inclusion population is characterized by hornfels recrystalliz ation of the plagioclase, and is geochemically and isotopically identified with Huronian basalts which comprise the south range footwall, with (Ce/Yb) (N) ratios of around 2.5 and epsilon(Nd)(1850) between -2 to -5. The melano cratic inclusions in the sublayer are typically coarse-grained and undeform ed, with incompatible element contents and radiogenic isotopic compositions intermediate between those of the basaltic inclusions and those of the mel t sheet, which has (Ce/Yb), ratios of around 10 and epsilon(Nd)(1850) aroun d -9, Calculated crystallization models are consistent with derivation of t he 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 e ffectively removed from the system. Subsequently, continued evolution of th e marginal facies produced the more mafic inclusions in the contact sublaye r. No extracrustal (e.g., mantle) component is envisioned in this model to explain the silicate compositional distributions in the SIG, and mafic crus tal rocks in the target zone are implicated as the metal source for the SIC deposits. (C) 2000 Published by Elsevier Science B.V. All rights reserved.