Thermal Structure and Melting Conditions of the Mantle Beneath Salta Rift, NW Argentina: A Combined Empirical and Forward Petrological Approach

Abstract Alkali basalts and basanites, which carry xenoliths from the mantle and lower crust, are associated with late Cretaceous extensional structures in the Salta continental rift. The suite of mantle xenoliths is primarily composed of spinel lherzolites and harzburgites, with subordinate carbonatized peridotites and pyroxenites. These rocks provide an exceptional snapshot of the in situ thermal state of the subcontinental lithospheric mantle and underlying asthenosphere. The rapid ascent of these basic magmas was necessary to keep the dense mantle nodules suspended, thereby limiting magmatic differentiation and chemical interaction between xenoliths and magmas. While the mineralogy of the rift basalts reflects crystallization conditions of 1200 °C/1.5 GPa prior to eruption, the whole-rock chemistry reveals information about the original formation conditions, as the basalts preserve a near-primary composition without interaction with the continental crust, as indicated by Th–Nb ratios. This offers a unique opportunity to connect information from natural rock observations with melt activity models developed for ultrabasic compositions. The trace element chemistry of the volcanic rocks is homogeneous, indicating an enriched peridotitic source (high Nb/Ti gradients), low percentages of partial melting (high La/Yb ratios) and residual garnet (high Ti/Yb–Nb/Yb ratios), suggesting a deep origin. Thermodynamic models of mantle xenoliths further constrain the P–T melting conditions for generating the basic melts, indicating a potential mantle temperature of 1450–1460 °C and initial pressures of ~3 GPa. Despite the relatively high mantle potential temperature—about 100–110 °C above the ambient mantle temperature—there is no evidence of thermally driven active upwelling. Consequently, the accumulated melt volume remained very low <10 vol %. The lithosphere-asthenosphere boundary had to remain relatively stable, between 100 and 80 km depth, to produce these very low melt fractions from a lherzolite peridotite source. This is consistent with the local evolution in northwestern Argentina, where rifting was interrupted at an early stage, and the observed volume of basic magmatism is small. The composition of the mantle in this region is likely representative of the mantle beneath a wider region of the continent between ~6 and 26°S, where similar xenoliths and lavas have been reported. This upper mantle composition can be used to test regional rifting mechanisms and associated magmatism. The origin of rift basalts and their xenoliths in northwestern Argentina suggest a hot and therefore weak lithosphere, which could have significantly influenced the style of later Andean deformation.