Melting and metasomatism/refertilisation processes in the Patagonian sub-continental lithospheric mantle: A review

Abstract

The large amount of geochemical variability in peridotitic and pyroxenitic xenoliths exhumed by Eocene to present-day magmas in Patagonia provides a unique opportunity to investigate the depletion and enrichment processes affecting the lithosphere in proximity of subduction settings and their relationships with magmatism. In this study, a review of the geochemical features of the Patagonian ultramafic xenoliths coupled with new SrNd isotopic data on clinopyroxene separates from representative suites enabled us to delineate the main melt extraction and enrichment (metasomatism vs. refertilisation) processes acting in these sub-continental lithospheric mantle (SCLM) portions. Our findings show that clinopyroxenes and orthopyroxenes in Patagonian harzburgites/lherzolites lie on two discrete Al2O3/Mg# trends and record variable partial melting degrees (1–30%), progressively decreasing southward, where the mantle domains experienced melt extraction in the past. The occurrence of both negative and positive correlations between (La/Yb)N, SrN, and Al2O3 in clinopyroxenes suggests that large part of the SCLM beneath Patagonia was affected by both metasomatic (low melt-fluid/rock ratio) and refertilisation (high melt-fluid/rock ratio or long-lasting low melt-fluid/rock interaction) processes, while a few portions - particularly in Southern Patagonia - were devoid of significant enrichment processes. A comprehensive study of the major/trace element composition and rare earth element patterns of clinopyroxene from both cumulitic and mantle xenoliths led us to hypothesize that tholeiitic enrichment was followed by infiltration of transitional/alkaline agents in the Northern and Central Patagonian SCLM. The percolation of melts with transitional affinity can be identified in some Central Patagonian localities. In Southern Patagonia clinopyroxenes record a slight interaction with alkaline and proto-adakitic agents; the latter occurred at Cerro Fraile, the locality nearest the trench. The SrNd xenolith isotopic signature led to speculation that enriched and depleted mantle components interacted variably in the SCLM domain beneath Patagonia. This work suggests that a detailed and comprehensive study of ultramafic xenoliths can help identify the evidence produced by magmatic events occurring at the surface, as both a serial affinity and a temporal sequence.