Protracted melting in the Archean crust: Evidence from the Northern Kaapvaal craton

• Ages of migmatites and granites in the Northern Kaapvaal Craton: 2930 to 2680 Ma. • Melts formed during 250 Myr, crust stayed molten for long. • Soft crust at the edge of a rigid craton. • Unique deformation style of Archean continental margins. On the northern edge of the Archaean Kaapvaal craton of South Africa, granulite-facies rocks are exposed. They contain abundant melt features in the form of leucosomes, in-source granites and small granite intrusions, as well as overprint not associated with clear petrological features and evidence of remelting of previously emplaced granites. At amphibolite facies conditions further South, large granite batholiths emplace during the same period. U-Pb zircon (and occasionally monazite) dating of 36 samples (and compilation of data on 65 previously published ones) allows to build a comprehensive database of the age of melting in the Kaapvaal crust. The database reveals continuous melting from ca. 2950 Ma to ca. 2650 Ma (culminating in the formation of spectacular anatectic metapelites at ca. 2710 Ma). This peculiar pattern casts light on the evolution of the Archaean continental crust. The long duration of the melting period rules out one single cycle of heating and cooling, but the lack of clear peaks that may be related to distinct tectonic phases contrasts with modern orogenic systems. The general image that emerges is that of an accretionary complex at the edge of a stable continent. Alternating phases of extension and compression allowed the formation of closure of sedimentary basins, and the burial and melting of all the lithologies in the region. In the generally hotter Archaean Earth, the system was both hotter, and cooled down more slowly, developing a long-lived “hot orogen” at the edge of the craton.