Sudbury impact turned the crust inside out

The 1800 Ma old Sudbury complex in eastern Canada is one of the largest repositories of nickel ores and contains commercial platinum deposits.  It has also been ascribed to a major impact that produced a crater over 200 km across.  The evidence is the common presence of shocked minerals and a sheet of very homogeneous, once molten rock, whose andesitic major-element composition suggests that it represents melting of the local upper crust.  However, the trace elements, including platinum group metals, have all the hallmarks of the lower crust (Mungall, J.E. et al. 2004.  Geochemical evidence from the Sudbury structure for crustal redistribution by large bolide impacts.  Nature, v. 429, p. 546-548.).  The melt sheet is mixed with upper crustal rocks, including sediments that formed in a shallow marine basin into which the meteorite plunged.  This suggests that impact not only affected the whole crust, but excavated it as well, so that a 30 km deep crater formed at the instant of collision.  The bulk of the homogenised crustal melt remained molten for long enough for complex fractional crystallisation to take place, thereby forming the classic layered Sudbury Igneous complex, in which the nickel ore bodies are located.  They may well represent relics of the impactor itself, that mixed with molten crust.

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