Mantle Processes and Sources of Neogene Slab Window Magmas from Southern Patagonia, Argentina

• Published in: Journal of Petrology
• Main Authors: GORRING, MATTHEW L., KAY, SUZANNE M.
• Format: Text
• Language: English
• Published: Oxford University Press 2001
• Subjects: ARTICLES; Antarctic; Antarctic Peninsula; Argentina; Austral; Baja; Chile Trench; Patagonia; Antarc*; Ocean Island
• Online Access: http://petrology.oxfordjournals.org/cgi/content/short/42/6/1067; https://doi.org/10.1093/petrology/42.6.1067

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; 87 Sr/ 86 Sr = 0·7035–0·7046; 143 Nd/ 144 Nd = 0·51290–0·51261; 206 Pb/ 204 Pb = 18·3–18·8; 207 Pb/ 204 Pb = 15·57–15·65; 208 Pb/ 204 Pb = 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, tholeiitic main-plateau sequence (48–55% SiO 2 4–5% Na 2 O + K 2 O) and a less voluminous ∼7–2 Ma alkaline post-plateau sequence (43–49% SiO 2 5–8% Na 2 O + K 2 O). Moderately high FeO T (9–11%), and low heavy rare earth element (HREE), Y, 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 from 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 ...