EVOLUTION OF THE SUDBURY IGNEOUS COMPLEX: COLD IN THE SOUTH, HOT IN THE NORTH

The Sudbury Igneous Complex is a large granodioritic hypabyssal pluton produced by melting of the crust due to bolide impact 1849 Ma ago. The North Range and South Range of the SIC show different crystallization sequences and compositional trends. The evolution of the SIC can be modeled satisfactorily as an open system process involving equilibrium crystallization, fractional crystallization, sulfide liquid immiscibility, and assimilation of the glassy roof rocks of the Onaping Formation. Operation of these processes on a single quartz monzodiorite offset dike magma can produce the entire observed spectrum of rock modes and compositions in both the North and South Ranges of the SIC. The differences in the evolution of the North and South Ranges can be accounted for by proposing that immediately after impact, the South Range reached internal thermal equilibrium approximately 100 C below its liquidus temperature, containing about 40 wt% crystals and about 0.2% immiscible sulfide liquid. The resulting magma differentiated by a transient process of sulfide and crystal settling to produce hundreds of meters of norite cumulates with identical crystal compositions, and bulk compositions varying only due to a progressive upward decrease in the proportion of trapped liquid ('South Range norite'). The remaining South Range magma then evolved by closed system fractional crystallization to form a cryptically layered diorite cumulate ('felsic norite'). In contrast, the North Range magma was originally undercooled by < 30 C, containing < 5 wt% crystals of orthopyroxene and <0.2% immiscible sulfide. The early-formed pyroxene crystals and sulfides were swept into depressions to form the mineralized 'mafic norite'. The North Range magma then evolved by fractional crystallization over the next 100 degrees of cooling accompanied by a continuous process of assimilation of the Onaping Formation, to produce a cryptically-layered norite-diorite cumulate ('felsic norite'). In the latest stages, after assimilation stopped, the felsic residue crystallized from the top down in both the North and South Ranges ('granophyre'). These scenarios can all be modeled quantitatively using the MELTS model, respecting mass balance constraints and honoring the entire available major and trace element lithogeochemical data set.