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Prahuaniyeu, located on the NW margin of the Somoncura Large Igneous Province in northern Patagonia, is one of two known localities in Argentina where mantle-derived garnet- and spinel-bearing peridotites occur associated with alkali basalts; the other locality is the Pali Aike volcanic field of southern Patagonia. Most of the Prahuaniyeu garnet-bearing peridotites are fertile in terms of their Al 2 O 3  and CaO contents, whereas the spinel-bearing peridotites cover a wide range from fertile to depleted compositions. Whole-rock light rare earth element (LREE) enrichment in the garnet-bearing peridotites and partly in the spinel-peridotites is consistent with intergranular percolation of the host basalt melt, as hydrous phases are not present and the clinopyroxenes and garnets are not enriched in LREE. Lack of slab-derived component(s) in the metasomatic products rules out the participation of a subducted slab in the generation of these basalts.  In situ  clinopyroxene analyses suggest that a group of spinel-peridotites experienced cryptic metasomatism by carbonatitic melts. Non-metasomatized garnet- and spinel-peridotites have experienced fractional melting ranging from 1 to 3% and from 5 to 12%, respectively. The Prahuaniyeu xenoliths lie on an elevated geotherm (high temperatures at low pressure) implying convective heat transport. The two most fertile samples, which indicate apparent internal ‘ages’ between c. 10 and 30 Ma for the sub-Prahuaniyeu lithospheric mantle, suggest resetting of the Sm–Nd isotopic system under a high-temperature regime and most probably reflect closure of the system following this ‘high- T  event’, which can be related to extensive magmatic activity within the Somoncura province, starting in the Eocene and finishing in the Miocene.

Heterogeneous Lithospheric Mantle beneath Northern Patagonia: Evidence from Prahuaniyeu Garnet- and Spinel-Peridotites