GEOCHEMICAL VARIATIONS IN THE BACK-ARC, SOUTH PATAGONIAN LITHOSPHERIC MANTLE BENEATH PATAGONIA (SOUTH AMERICA): TOWARDS A GEODYNAMIC MODEL

Abstract

Samples of the mantle wedge of convergent plate regions are rare and direct evidence of the interaction between slabderived components and wedge is therefore unusual. Abundant occurrences in Patagonia of mantle xenoliths in alkaline volcanic centres located in the backarc region of the Andean subduction zone (Stern et al., 1990; Laurora et al., 2001; Rivalenti et al., 2004a,b), 300 – 600 km east from the Chile trench provide the possibility to study these processes. Research on Patagonia arc and back-arc magmas provides evidence that the mantle wedge, fertilised by slab components, played an important role either as the source of the arc magmas or as a component of the plateau and postplateau back-arc basalts. These latter consist of older (12-5 Ma) tholeiitic sequences that formed voluminous mainplateau sequences and younger (7-2 Ma), less voluminous alkaline lavas with OIB-like characteristics erupted mostly from monogenetic volcanic centres. Stern et al. (1990) recognised an eastward decrease of slab-related geochemical features in back-arc basalts and proposed a supra-slab model for their origin. More recently, a sub-slab model has been envisaged with partial melting of sub-slab sources and migration of magmas to the surface through slab-windows (D’Orazio et al., 2001; Gorring and Kay, 2001). Abundant occurrences of mantle xenoliths in alkaline volcanic centres located in the back-arc South Patagonian provide a general overview of the lithospheric mantle composition and a reconstruction of its depletion and metasomatic enrichment processes. Combined petrological and texture- related geochemical investigations allowed us to address some important and still debated questions: • which is the role of the lithospheric mantle in the determination of the East-West geochemical variability of the lavas? • is the continental lithospheric mantle a potential reservoir for the Southern Patagonia magmas? • is the eastward fading of the slab signatures in the basalts recorded also in the lithospheric mantle? • which is the geodynamic mechanism determining the upwelling of the asthenosphere in the Patagonian back-arc area (extensional tectonic stress, mantle plume, slab window)? • how can the geochemical composition of the mantle ultramafics be used to infer the composition of migrating melts ? In particular, did the mantle wedge interact with carbonatitic or adakitic metasomatic agents as it has been inferred for Gobernador Gregores (Gorring and Kay, 2000) or some westernmost occurrences (Kilian and Stern, 2002), respectively? Xenoliths from ten localities from the Rio Negro to the Santa Cruz provinces were examined to address the important issues. Major results based on the trace element signatures of minerals and glasses determined in-situ by SIMS and LA ICP-MS techniques as well as isotopic signatures of mineral separates from Pali-Aike and Gobernador Gregores xenoliths can be summarised in the following: • the compositional regional variations observed in Patagonia volcanics (transitional to OIB-like basalts) are much weaker in basalt-borne xenoliths; nevertheless, an eastwards fading of possible slab-related geochemical signatures is observed; • only few spinel-bearing xenoliths document the presence of highly depleted lithospheric mantle domains related to old partial melting episodes; • the majority of sp-bearing xenoliths provide textural and chemical evidence of significant re-crystallisation due to interaction with melts; in westernmost and northernmost localities cpx compositions suggest either equilibrium or reaction with arc-derived magmas and transitional basalts; • in southernmost and easternmost occurrences lithospheric mantle has been extensively percolated via reactive porous flow by uprising OIB-like asthenospheric melts, which probably erased at various extent the signatures of previous metasomatic events; • the thermo-chemical erosion of the lithosphere is particularly important at Pali Aike and Praguaniyeu, the only two occurrences of garnet-bearing xenoliths, from northern and southernmost Patagonia, respectively; noticeably, at Pali Aike the basalts are poured out in a rift system located in a back-arc setting; • glass veins and pockets in xenoliths do not represent distinct metasomatic agents; they formed by melting or infiltration of the peridotite assemblage caused by upraising basaltic magmas to the surface and their reaction with ambient lithospheric mantle; carbonates are likely produced by degassing of magmatic fluids; • evidence for plume activity and carbonatitic and/or adakitic metasomatism is generally weak; • compositional variability of xenoliths results from both the primary vertical heterogeneity and the chemical overprint caused by reactive porous-flow reactions (under variable melt/rock volume ratios) operated by the migration of earlier transitional basalts, later on followed by OIB-like melts generated in deeper mantle sources. Proposed geodynamic interpretation includes supra-slab and sub-slab models. Nevertheless, the documented upwelling OIB-type asthenosphere cannot be simply ascribed to slab window opening, but it is supposed to be related to concurrent processes including corner-flow in the asthenospheric wedge, vertical slab tear along subducted fracture zones and local enhanced lithospheric extension.