LAUSR

Abstract The lithospheric mantle beneath Archean cratons is conspicuously refractory and thick compared to younger continental lithosphere ( Jordan, 1988 , Boyd, 1989 ; Lee and Chin, 2014), but how such thick lithospheres formed is unclear. Using a large global geochemical database of Archean igneous crustal rocks overlying these thick cratonic roots, we show from Gd/Yb– and MnO/FeOT–SiO2 trends that crustal differentiation required continuous garnet fractionation. Today, these signatures are only found where crust is anomalously thick (60–70 km), as in the Northern and Central Andes and Southern Tibet. The widespread garnet signature in Archean igneous suites suggests that thickening occurred not only in the lithospheric mantle but also in the crust during continent formation in the late Archean. Building thick crust requires tectonic thickening or magmatic inflation rates that can compete against gravitational collapse through lower crustal flow, which would have been enhanced in the Archean when geotherms were hotter and crustal rocks weaker. We propose that Archean crust and mantle lithosphere formed by thickening over mantle downwelling sites with minimum strain rates on the order of 10−13–10−12 s−1, requiring mantle flow rates associated with late Archean crust formation to be 10–100 times faster than today.