The Sierra de Los Llanos intrusion is a Lower Paleozoic pluton of meta-aluminous granites (with tholeiitic and calc-alkaline features) representative of batholiths of the Sierras Pampeanas, here associated to small bodies of cordieritic granite and metamorphic septa that upgrade from weak contact to strong grade of metamorphism, emplaced at low depth. The first data (spatial and temporal associations, etc.) suggest a genetic relation to a heterogeneous source, with contamination, by crustal materials and other processes, of mantelic arc magmas