40Ar/39Ar geochronology of mafic volcanism in the back-arc region of the southern Puna plateau, Argentina

Late Cenozoic back-arc mafic volcanism in the southern Puna plateau of Argentina offers insights into the state of the mantle under the world's second largest continental plateau. Previous studies of the mafic magmas in this region proposed a scenario of mantle melting due to lithospheric delamination and/or steepening of the subducting slab. However, few of the centers have been precisely dated, which limits any geodynamic interpretation. We present results of laser incremental-heating 40Ar/39Ar dating of 22 back-arc centers in the southern Puna, with emphasis on the Salar de Antofalla region where volcanic activity was most intense. Three localities yielded ages between 7.3 and 7.0 Ma which, along with 2 previous 7 Ma ages, firmly establishes that back-arc activity began as early as late Miocene. Volcanism continued through the Pleistocene but the peak was in the early Pliocene. This result has important tectonic implications. If, as previously suggested, magma genesis is related to lithospheric delamination, this process was underway by the latest Miocene in the southern Puna. Furthermore, since the mafic back-arc volcanism is considered to mark a change in fault kinematics from compressional to transtensional, the new age constraints indicate that this change took place in the early Pliocene. The spatial and age distributions of the mafic centers indicate that magmatism began, and remained focussed in, a region between Salar de Antofalla and Cerro Galan. This concentration is probably structurally controlled, as it corresponds to the intersection of the NW-SE striking Archibarca lineament zone and the sets of NNE-SSW faults that run parallel to the Salar de Antofalla basin. (C) 2008 Elsevier Ltd. All rights reserved.