The adakites of the Patagonian Cordillera: new geochemical and geochronological evidences

Abstract

The origin of many adakites ascribed to slab melting has been questioned with their formation being attributed to melting of thickened continental crust or forearc subduction erosion. Some of the best remaining candidates for a slab-melt origin are the Cerro Pampa adakites (47°55'SL) in Patagonia that erupted east of the inactive volcanic arc as the Chile Ridge collided with the Chile Trench at ca. 12 Ma. Support for a slab-melt origin for these hornblende-bearing dacites comes from high Sr concentrations (1330-2300 ppm) at 63-68% SiO₂, MORB-like ⁸⁷Sr/⁸⁶Sr (0.7028-0.7031), ¹⁴³Nd/¹⁴⁴Nd > 0.5129, and steep (heavy depleted) REE pattern (La/Yb = 30-37) in an area of thin crust. Two other adakite localities to the southwest that are also east of where the Chile ridge collided at ~ 12 Ma, have been recognized more recently (65-66% SiO₂, 1370-1440 ppm Sr, ⁸⁷Sr/⁸⁶Sr = 0.7032-7033, ¹⁴³Nd/¹⁴⁴Nd ~ 0.51289, La/Yb 28-30). These Puesto Nuevo (48°59'°S) and Chaltén adakites (~49°25'°S) adakites are to the east of the Pleistocene to Recent adakites in the Austral Volcanic Zone arc. New ⁴⁰Ar/³⁹Ar plateau ages obtained via laser incremental heating of hornblendes show that the Chaltén, Puesto Nuevo, and Cerro Pampa adakites decrease nortwards in age (14.50 ± 0.29, 13.12 ± 0.55, 11.39 ± 0.61 Ma), consistent with melting of the trailing edge of the Nazca plate as ridge collision progressed to the north. Their chemistry contrasts with non-adakitic Miocene retroarc andesites in the Cerro Moyano (~50°15'S) region, east of where the Chile ridge collided at ca. 14 Ma. A major difference in this southern region is the relative absence of large late Miocene plateau flows related to a slab window between the Nazca and Antarctic plates. This association suggests that Miocene slab melting in Patagonia required a thermal input from the asthenospheric mantle as well as a young, hot subducting plate. A similar case can be made between Austral Volcanic Zone adakites and extra-Andean Pliocene to Recent basalts.