The Sudbury igneous complex; a differentiated impact melt sheet
The Sudbury igneous complex; a differentiated impact melt sheet (in A special issue devoted to the mineral deposits of the Sudbury Basin, C. M. Lesher (editor) and P. C. Thurston (editor))
Economic Geology and the Bulletin of the Society of Economic Geologists (November 2002) 97 (7): 1521-1540
An updated terminology is proposed for the Sudbury igneous complex (from top to bottom): upper unit, middle unit, lower unit and contact sublayer. The bulk composition of the Sudbury igneous complex, from N Range data, is granodioritic. Continuous and gradational mineralogical and geochemical variations between the lithological units are evidence that the complex behaved as a single melt system. All the Sudbury igneous complex lithologies have the same light to heavy REE ratio and an overall pattern of increased LREE light REE and depleted HREE. The occurrence of primary hydrous minerals (hornblende and biotite), deuteric alteration, and abundant micrographic and granophyric intergrowths demonstrate that the melt was rich in H (sub 2) O. The granophyric and other far-from-equilibrium textures are most likely due to rapid crystallization triggered by exsolution of a volatile phase. The Sudbury igneous complex differs from traditional layered mafic complexes in the following aspects: it has an overall intermediate composition, a hydrous nature, a crustal isotopic signature, normative corundum and an unusually large volume of granophyre. The Sudbury complex differs from known terrestrial impact melt sheets only by its great thickness and the presence of chemical, and therefore mineralogical layering. Reported here are the occurrences of plagioclase xenocrysts with complex twinning and zoning patterns and planar deformation features in quartz xenocrysts. The well-known ore deposits of the Sudbury region are directly related to the genesis of the Sudbury igneous complex. Some ores precipitated from the Sudbury melt, whereas others were concentrated by hydrothermal fluids that percolated through the crystallized complex. It is concluded that the Sudbury igneous complex is the best exposed and only well-documented terrestrial impact melt sheet to have differentiated.