New evidence of Grenvillian crystalline basement in the Sierra de Pie de Palo, Western Sierras Pampeanas, Argentina: geotectonic implications

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

San Carlos de Bariloche

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

INGEIS - Universidad de Buenos Aires

Combined petrological, geochemical, isotopic and geochronological data shed light to understand the origin and evolution of a peraluminous garnet-bearing two-mica granitoid (El Tigre Granitoid: ETG) cropping out in south-western Sierra de Pie de Palo (31°31´30´´S - 68°15´12´´W), and to constrain the age of this intrusive phase. The ETG crops out as small size tabular felsic body emplaced in metasedimentary rocks of the Pie de Palo Complex. Its composition varies from granodiorite to tonalite and is moderately peraluminous (ASI 01.09-1.33; A/CNK >1.1; normative corundum; low CaO values between 1.72-2.41%). It falls in the granite-trondhjemite fields of the Ab-An-Or diagram. Trace elements contents show relatively low abundance of Rb, HFS elements such as Y, Nb, Ta, Ga and Zr, and high concentrations of Ba, Sr, and total LREE. Chondrite normalized REE pattern shows high slope with La/YbN = 9.48-55.32 and negative or absent europium anomaly. Relationships between trace elements suggest the classical setting of granitoids produced in convergent plate setting under a compressive tectonic regime. Obtained U-Pb SHRIMP crystallization age on zoned igneous zircon (1105.5  4.1 Ma), suggests that the ETG could be part of the magmatic complex forming the Grenvillian basement of the Western Sierras Pampeanas (WSP). The ETG isotopic signatures 87Sr/86Sr1105 ratios of 0.70543 and ENd of +4.2 indicate a rather immature source for its origin, with similiar initial isotopic ratios that those found in orthogneisses from elsewhere WSP. On the other hand, Sm-Nd model age(TDM) for the ETG is 1.20 Ga.