Priscoan (4.00-4.03Ga) orthogneisses from northwestern Canada

Ancient crustal rocks provide the only direct evidence for the processes and products of early Earth differentiation. SHRIMP zircon U-Th-Pb dating has identified, amongst the Acasta gneisses of the western Slave Province, Canada, two metatonalites and a metagranodiorite that have igneous ages of 4002+/-4, 4012+/-6 and 4031+/-3 Ga respectively. These are the first identified Priscoan terrestrial rocks. A record of metamorphic events at 3.75, 3.6 and 1.7Ga also is preserved. These discoveries approximately double, to 40km², the area over which 4.0Ga gneisses are known to occur. A single older zircon core in one sample suggests that rocks as old as 4.06Ga might yet be found in the region. As early as 4.03Ga, terrestrial differentiation was already producing tonalitic magmas, probably by partial melting of pre-existing, less differentiated crust.