Pliocene intraplate-type volcanism in the Andean Foreland at 26°10'S, 64°40'W (NW Argentina); implications for magmatic and structural evolution of the Central Andes

The Antilla magmatic complex (26°10'S, 64°40'W, NW Argentina) attests to magma eruption at ca. 4.7 Ma in the Central Andes backarc region, 300 km E of the active arc. The Antilla lavas have an alkaline, predominantly mafic composition and record the most primitive isotopic ratios (87Sr/86Sr=0.704360 and 143Nd/144Nd=0.512764) of Central Andes Neogene backarc volcanism between 24°S and 27°S. They show trace-element patterns recalling backarc Pliocene-Quaternary intraplate mafic rocks, but they show lower silica and higher alkali contents, and are interpreted to derive from the depleted subcontinental mantle. A revision of the structural and volcanological characteristics of the Central Andes between 24°S and 27°S shows that this region was, during the Miocene-Pliocene, the site of lithospheric processes that account for partial melting in the mantle wedge, in the subcontinental mantle, and in the continental crust. The existing geophysical and petrological data agree with a model in which magma production was related to a process of lithospheric delamination. The Antilla rocks are the easternmost volcanic products with intraplate characteristics, located beside a large zone of partial melting of the continental crust, at the intersection of the NW-trending Archibarca and NE-trending Tucuman transversal lineaments. Their age of 4.7 Ma corresponds to the acme of mafic monogenetic and silicic ignimbrite volcanism in the backarc at the same latitude. This provides new constraints for the spatial and temporal reconstruction of deformation events in the crust and of the lithospheric delamination process and its bearing on magmatic activity.