Mantle processes and sources of neogene slab window magmas from Southern Patagonia, Argentina

Neogene plateau lavas in Patagonia, southern Argentina, east of the volcanic gap between the Southern and Austral Volcanic Zones at 46·5° and 49·5°S are linked with asthenospheric slab window processes associated with the collision of a Chile Ridge segment with the Chile Trench at 12 Ma. The strong ocean-island basalt (OIB)-like geochemical signatures (La/Ta <20;; Ba/La <20; ⁸⁷Sr/⁸⁶Sr = 0·7035-0·7046; ¹⁴³Nd/¹⁴⁴Nd = 0·51290-0·51261; ²⁰⁶Pb/²⁰⁴Pb = 18·3-18·8; ²⁰⁷Pb²⁰⁴Pb = 15·57-15·65; ²⁰⁸Pb/²⁰⁴Pb = 38·4-38·7) of these Patagonian slab window lavas contrast with the mid-ocean ridge basalt (MORB)-like, depleted mantle signatures of slab window lavas elsewhere in the cordillera, (e.g. Antarctic Peninsula; Baja California). The Patagonian lavas can be divided into a voluminous ∼ 12-5 Ma, tholeeitic main-plateau sequence (48-55% SiO₂; 4-5% Na₂O 〈 K₂O) and a less voluminous ∼ 7-2 Ma alkaline post-plateau sequence (43-49% SiO₂; 5-8% Na₂O 〈 K₂O). Moderately high FeO_T(9-11%), and low heavy rare earth element (HREE), γ, and Sc concentrations in all lavas are consistent with melt generation just below the garnet-spinel transition at a depth of ∼ 70 km. The main-plateau lavas from the western back-arc can be modeled by ∼ 10-15% partial melting of an OIB-like asthenospheric mantle source with additions form slab fluid-melt components coupled with crustal contamination (AFC). A three-stage petrogenetic model is envisaged: (1) decompression melting and source region contamination of an OIB-like subslab asthenospheric source by slab melts of the trailing edge of the subducted Nazca Plate; (2) minor contamination of slab window melts with arc components 'stored' in the supraslab mantle wedge and/or basal continental lithosphere; (3) further modification by addition of crustal components during magma ascent. The main-plateau lavas from the eastern back-arc can be modeled by ∼ 7% partial melting of the same asthenospheric source as the influence of arc components diminishes and the intensity of mantle upwelling into the slab window decreases. All post-plateau lavas can be modeled as 1-4% partial melts of the pristine OIB-like asthenospheric source in the widening slab window.