Orogenic magmatism along the transition Precordillera-Sierras Pampeanas, between 29° and 31° S, Argentina

Abstract

New geochemical data and petrological analysis of Neogene volcanic units from the Pampean Flat Slab Segment (29°-31°SL) are presented in this contribution. Rocks were collected along a cross-section from the Western Sierras Pampeanas to the Eastern Precordillera. The outcrops delimit areas with a sub-parallel disposition to the orogenic front and main Andean regional structures. The study involves several volcanic rocks of the Formación Cerro Morado, sampled from the Huachi eruptive center and the Helvecia and Portezuelo de Los Burros mine districts, with thick lava flows and pyroclastic rocks. Lavas and breccias from several medium size outcrops near Huaco are also analyzed. To the NE, near the town of Guandacol, the Unidad Volcánica El Corral crops out. These rocks constitute the easternmost outcrops of the Precordillera in San Juan. We also studied rocks from the La Ciénaga volcano of the Sierra del Pocho Volcanic Field within the Sierras Pampeanas of Cordoba, which are the easternmost magmatic expression of the volcanic arc in this shallow subduction zone of the Central Andes. The new geochemical and isotopic data obtained from Formación Cerro Morado and lava flows belonging to the Unidad Volcánica El Corral, and its comparison with new geochemical data of the La Ciénaga volcano of the Sierra de Pocho show that these rocks result from the melting of two sources coming from two different types of crustal basements (Precordillera and Sierras Pampeanas). We found a magmatic association with similar geochemical features within the Formación Cerro Morado in Huachi region and Helvecia mine and Agua de Los Burros. It shows abundant hydrothermal fluids, which are often related to mineralization of economic interest. On the other hand, the rocks of the Formación Cerro Morado from Huaco, of the Unidad Volcánica El Corral and El Pocho Volcanic Field have a similar isotopic signature that allows us to infer the participation of the underlying Sierras Pampeanas Crust in their development. This feature leads us to interpret the continuity of the Sierras Pampeanas Basement Crust below the Western Precordillera.