Spinel-facies mantle xenoliths occur in a diatreme cutting through the Neogene Southern Patagonia Plateau at Gobernador Gregores (Santa Cruz Province, Argentina). This plateau is in a back-arc position with respect to the Chile trench. Xenoliths differ in their whole-rock composition from other South America occurrences, having higher CaO/Al₂O₃ ratios and, in some samples, TiO₂ enrichment, whereas the Na₂O/Al₂O₃ variation range is similar. Three assemblages can be distinguished. Assemblage 1, in anhydrous protogranular lherzolites and harzburgites, contains clinopyroxene with a depleted major and trace element composition, indicating pre-metasomatic depletion processes. This assemblage fully recrystallized to Assemblage 2 (amphibole ± phlogopite ± Cl-apatite-bearing) during a metasomatic episode. This causes clinopyroxene to acquire geochemical characteristics often attributed to carbonate-melt metasomatism. Noticeably, amphibole is markedly enriched in Nb (up to 298 ppm), especially when depleted in Ti. A further event, related to decompression during xenolith uplift to the surface, induces closed-system (perhaps with the exception of CO₂ addition) disequilibrium melting of Assemblage 2, dominantly of amphibole. It is found in pockets (where amphibole is a residual phase) consisting of Na–Si-rich glass and carbonate (Mg-rich calcite) drops, and in veins originating from the pockets (Assemblage 3). Euhedral olivine, clinopyroxene and spinel crystallize only in the silicate glass. So do new, euhedral apatite crystals when glass is in contact with previous Assemblage 2 apatite. Textural evidence and comparison with experimental work suggest that silicate glass and carbonates are the result of unmixing of a former homogeneous melt. Because of the different flow rates of carbonate and silicate melt, the xenoliths become enriched in carbonate, which is found in the veins during their migration. Thus, the high CaO/Al₂O₃ ratio of whole rocks provides inconclusive evidence of carbonatite metasomatism. This factor, and other minor deviations from the expected results of carbonatite metasomatism, lead us to hypothesize an aqueous, Cl-rich fluid, possibly slab derived, as an alternative agent. Amphibole, resulting from reactive porous flow of this agent in the mantle, could fully explain the observed geochemical features, as indicated by estimates of its partition coefficients.

Interstitial, subhedral, slightly pink zircons have been separated from a diorite of the External Gabbro unit of the Finero Mafic Complex, which crops out in the northernmost tip of the Ivrea–Verbano Zone (IVZ, Southern Alps). They are characterized by oscillatory zoning under cathodoluminescence (CL), a high Th/U ratio (0·9–0·4), and associated high U, Th, Pb, rare earth element (REE), Ta, Nb, Li and P contents, consistent with a magmatic origin. The mean of the single concordant ages is 232 ± 3 Ma, which is here proposed to constrain the timing of intrusion of the Finero Mafic Complex. Most of these magmatic zircons exhibit an outer rim, up to 50 μm thick, with a bright emission, with transgressive and/or sub-concordant contacts to the internal, oscillatory-zoned domains, in which the Th/U ratio is decreased, as are the U, Th, Pb, REE, Ta and Nb contents. These rims give younger, slightly discordant ²⁰⁶Pb/²³⁸U ages, between 219 ± 3 Ma and 205 ± 3 Ma, which are interpreted to highlight a fluid-assisted recrystallization event. The gabbros also have an interstitial zircon population consisting of rounded, small, colourless, bright to milky crystals with blurred zoning patterns in CL. These zircons have low U, Th, Pb, Nb and Ta contents, and low Th/U ratio (down to 0·08). The overall metamorphic character, together with field evidence of the presence of blocks of paragneiss belonging to the crystalline basement at the roof of the External Gabbro unit, indicates that these zircons are inherited from local crustal rocks. They have U and Pb isotopic compositions that are strongly altered and discordant, showing an upper limit of the timing of recrystallization consistent with the Middle Triassic emplacement age obtained from the magmatic zircons. It is concluded that the Finero Mafic Complex can no longer be considered as a part of the huge Permian Mafic Complex of the central IVZ. It follows that the intrusive record preserved in the IVZ allows us to look at the interplay between the continental crust and mantle melts through time (i.e. Permian to Triassic). In this scenario, the modest crustal assimilation shown by the Finero Mafic Complex can be interpreted in the first instance as the consequence of the limited volume of intruded melt (less than 2 km in thickness). However, the possible role played by the decreased fertility of the metamorphic country rocks owing to a inferred Permian magmatic event must be better constrained. The intrusion of the Finero Mafic Complex may represent the deep-crustal counterparts of the Ladinian volcanism that is widespread throughout the Southern Alps. The geodynamic setting of this magmatism is still strongly debated. The results presented in this study, along with several other structural, petrochemical and age heterogeneities documented in the literature, suggest the occurrence of a ‘Finero-type IVZ’ (i.e. northern IVZ) and a ‘Balmuccia-type IVZ’ (i.e. central IVZ), with different magmatic and tectonic evolutionary histories.