Pocho volcanic rocks and the melting of depleted continental lithosphere above a shallowly dipping subduction zone in the central Andes

Abstract

The Late Miocene (7.9 to 4.5 Ma) Pocho volcanic field in Argentina occurs 700 km east of the Chile trench over the modern shallowly dipping Andean Wadati-Benioff zone near 32° S latitude in Argentina. The field is located in the Sierra de Cordoba which is the easternmost Laramide-style, block-faulted range in the Sierras Pampeanas (Pampean ranges). The arrival of the shallowly dipping slab initiated both volcanism and the uplift of the Sierra de Cordoba. Pocho rocks (52% to 68% SiO2; FeO*/MgO>2.2) comprise an older (7.5±0.5 Ma) high-K and a younger (5.3±0.7 Ma) shoshonitic series. Mineralogic data and fractionation models show that crystallization occurred under hydrous, oxidizing conditions, which were most extreme in the high-K series. An unusual pattern of successively lower REE at higher SiO2 concentrations can be modeled by sphene, apatite and amphibole removal. An arc-like trace element signature attributed to an arc component is strongest in the younger shoshonitic series. An important depleted lower crustal/mantle lithospheric source component in both series is indicated by non-radiogenic Sr and Pb isotopic ratios at ɛNd= 0 to + 2, low Rb/Sr ratios, and low U and Th concentrations. This depleted signature contrasts with the enriched one in potassic back-arc Central Volcanic Zone (CVZ) lavas over the steeper subduction zone to the north and is attributed to several processes in the shallow subduction zone. First, deep crustal (MASH) processes in the nearly normal thickness crust beneath Pocho incorporated depleted Proterozoic basement components, and not complexly mixed structurally thickened crustal components as in the CVZ. Second, the association of Pocho volcanism with the arrival of the slab allowed little time for modification of the mantle by subduction components. Third, Miocene shallowing of the subduction zone beneath the “flat-slab” required thinning of both the astenosphere and the subcontinental lithosphere. Thus, an important subcrustal component could be from blocks removed from the base of the lithosphere to the west and recycled into the asthenosphere. Similar magmatic sources would have existed during Laramide shallow subduction in western North America.