Ordovician mylonites from Mesoproterozoic granitoid, Sierra de Pie de Palo, Western Sierras Pampeanas, San Juan Province

CASTRO DE MACHUCA, BRÍGIDA; ARANCIBIA, GLORIA; PONTORIERO, SANDRA; PREVILEY, LORENA; MORATA, DIEGO

San Carlos de Bariloche

Simposio; VI South American Symposium on Isotope Geology (VI SSAGI); 2008

INGEIS - Universidad de Buenos Aires

A crustal peraluminous granitoid of Mid-Proterozoic age and mylonites-ultramylonites derived from it, were recognized at Quebrada del Tigre (31º31'30''S-68º15'12''W) on the southwestern end of the Sierra de Pie de Palo. The El Tigre Granitoid (ETG) is represented by narrow lenticular vein-like bodies intruding concordantly with the foliation of the host rocks of the Pie de Palo Complex. Textural and mineralogical features indicate that the ETG experienced retrograde amphibolite to greenschist facies metamorphism after igneous crystallization, followed by later deformation along ductile shear zones and partial dynamic recrystallization under lower-T conditions. Local ETG mylonite zones are a few centimeters- to a few decimeters wide and have southeast-dipping foliation oriented N25º-90°E. Notorious mineral stretching indicates a main strike-slip component and kinematic indicators evidence dextral sense of movement with dominant west-southwest transport direction. ETG shear zones form part of the extensive network of structures related to the regional Las Pirquitas overthrust. In order to constrain the age of shearing event to the mylonites, a K/Ar age was obtained on very fine-grained recrystallized micaceous matrix from the mylonites. Considering that mylonitic microstructures suggest shearing under greenschist facies conditions, the K/Ar age of 473±10 Ma (Lower-Middle Ordovician) is interpreted as close to the mylonitic event. This event was probably related with ductile thrusting associated with uplift during initial terrane collision and prograde Famatinian metamorphism in Sierra de Pie de Palo.