| Cooling and crystallization are two of the primary processes controlling the formation of igneous rocks. The relationship between these processes can be quantified by measuring textures from a magmatic intrusion with well-constrained thermal conditions. The Sudbury Igneous Complex (SIC) was chosen to examine this problem because it has a simple cooling history, and because drill cores provide detailed sample profiles through the complex.; The SIC is a 2.5 km thick igneous body formed during a large meteorite impact 1.85 billion years ago. It consists of two layers, granophyre and norite, established immediately after the impact event by gravitational separation and coalescence of a magmatic emulsion. Impact melting of heterogeneous continental crust produced a sheet in which discrete parcels of magma were mechanically interdispersed. The viscosity contrast between felsic and mafic liquids prevented mechanical homogenization and the density contrast between the melts caused gravitational separation. Over days to weeks, the emulsion separated into a felsic upper layer and a mafic lower layer.; Two-pyroxene thermometry suggests that the initial melt sheet temperature was as high as 1800°C. This corresponds to a Rayleigh number on the order of 1016 and indicates strong thermal convection. Convective cooling was rapid, reducing the melt sheet temperature to its liquidus in approximately 1,000 years. The post-convection thermal history of the SIC was investigated using four conductive cooling models: finite and infinite analytical solutions, a infinite Stefan solution, and a finite numerical solution. According to the numerical model, crystallization of the entire SIC occurred over 75,000 years.; Measured plagioclase crystal size distributions (CSDs) show regular variations through the SIC norite. The mean crystal length, calculated from the CSD, is directly proportional to crystallization times estimated using the numerical cooling model. Using the texture data and the conservation of mass condition, a scaling relationship was developed that relates the total number of crystals to the mean crystal length: NT = L¯ −3. This allows the CSD parameters to be directly related to spatial position using a cooling model and a crystal growth law. The results can be used to investigate crystallization kinetics or to predict textural variations in hypothetical intrusions. |
