Paleogene and Neogene magmatism in the Valle del Cura region: New perspective on the evolution of the Pampean flat slab, San Juan province, Argentina

Abstract

The Valle del Cura region is characterized by a thick volcanic and volcaniclastic sequence that records the Tertiary arc and backarc magmatic evolution of the Argentine Main Cordillera over the modern Pampean flatslab at 29.5–30°S. During the Eocene, a retroarc basin developed, represented by the Valle del Cura Formation synorogenic volcanosedimentary sequence, which includes rhyolites and dacitic tuffs. These silicic volcanic rocks have weak arc chemical signatures and high lithophile element concentrations and are isotopically enriched relative to the late Oligocene–early Miocene volcanic rocks that followed them. Their chemical characteristics fit with eruption through a thin crust. The Valle de Cura Formation was followed by the Oligocene–early Miocene Doña Ana Group volcanic sequence, which erupted at and near the arc front west of the border with Chile. The Doña Ana Group volcanic rocks have calc-alkaline chemical characteristics consistent with parental magmas forming in a mantle wedge and erupting through a normal thickness crust (35 km). Subsequent shallowing of the downgoing Nazca plate caused the volcanic front to migrate eastward. The volcanic sequences of the middle Miocene Cerro de las Tórtolas Formation erupted at this new arc front, essentially at the Argentine border. Two stages are recognized: an older one (16–14 Ma) in which magmas appear to have erupted through a normal thickness crust (30–35 km) and a younger one (13–10 Ma) in which the steeper REE pattern suggests the magmas last equilibrated with higher pressure residual mineral assemblages in a thicker crust. Isotopic ratios in the younger group are consistent with an increase in original crustal components and crust introduced into the mantle source by forearc subduction erosion. A peak in forearc subduction erosion near 12–10 Ma is consistent with when the main part of the Juan Fernandez Ridge began to subduct beneath the region. In addition to late Miocene Tambo Formation dacitic ignimbrites, the younger Cerro de las Tórtolas Formation volcanic rocks erupted at the height of contractional deformation in the Valle del Cura and to the east. The last important volcanic sequence to erupt in the Valle del Cura is the late Miocene Vacas Heladas Ignimbrite, the most isotopically enriched Tertiary magmas in the Valle del Cura that contain the highest proportion of crustal components. Subsequently volcanism ceased in the region in response to shallowing of the subduction zone.