Origin and stabilization of off-craton continental lithosphere recorded by cratonic mantle relics in Pali-Aike (southern Patagonia)

• Pali-Aike refractory peridotites formed in an Archean melting-temperature environment. • High melting degrees originally produced a buoyant off-craton continental lithosphere. • Metasomatism subsequently eroded the chemical buoyancy of the lithospheric mantle. • Long-term survival of lithospheric plates requires refractory mantle roots beneath them. • Off-craton lithosphere likely consists of reworked relics of cratonic mantle. The off-craton subcontinental lithospheric mantle (SCLM) is generally more fertile and younger than cratonic mantle. Melting conditions ascribed to its formation hardly produce iron-depleted, and hence low-density residual peridotites capable of stabilizing the lithospheric plate. Here, we document how major/trace element co-variations in off-craton mantle xenoliths from Pali-Aike (southern Patagonia) define different melt/rock reaction trends that can be back-tracked towards the composition of a high-Mg# (91.7), refractory (Al 2 O 3 = 1.0 wt.%, CaO = 0.46 wt.%) spinel-harzburgite, which represents the original protolith of the off-craton SCLM. Thermobarometric calculations support that this refractory protolith formed by ∼30% ( F ) melting at 3.0 GPa and 1555 ± 70 °C, conditions not achievable in the present-day ambient mantle but that extrapolate a paleo-potential temperature ( T P ) of 1540–1570 °C. This T P agrees with models predicting a moderately warm Archean-age mantle at the time of formation of Pali-Aike SCLM (2.5–2.7 Ga). Thermodynamic modeling with Perple-X of asthenospheric mantle (KLB-1) upwelling through the inferred paleo-adiabat yields an isentropic melting path that reproduces the P-T - F estimates of the SCLM protolith. Peritectic garnet removal by extensive melting at moderate pressures promoted the formation of a dehydrated and low-density lithospheric mantle root, which could survive conductive cooling and stabilize the overlying crust. Subsequent metasomatic refertilization counteracted the chemical buoyancy of continental peridotites, promoting the transformation of cratonic edges into thinner and fertile off-craton lithospheric mantle. Since the stabilization of buoyant lithospheric roots requires high-melting degrees typical of cratonic environment, we propose that most SCLM originally formed in the Paleoproterozoic-Archean, before being refertilized and thinned at off-craton regions.