Cambrian magmatism and Cambro-Ordovician tectono metamorphic event in Nahuel Niyeu Formation, Northpatagonian Massif, Argentina

GRECO, G.A.; GONZÁLEZ, S.N.; GONZÁLEZ, P.D.; SATO, A.M.; LLAMBÍAS, E.J.; TASSINARI, C.C.G.

San Pablo

Simposio; 9th South American Symposium on Isotope Geology; 2014

Organizing Commettee

Metasediments of the Nahuel Niyeu Formation between Valcheta and Aguada Cecilio towns, Rio Negro Province, Argentina, has metaigneous sills interbedded. This igneous rocks has clear cut and concordant contacts with the relict sedimentary bedding (S0). Its composition is mainly microgabric and granodioritic in less proportion. A main tectono metamorphic event (D1-D2-M1) on greenschist facies affects both metasediments protolits as interbedded igneous rocks. Ordovician moscovite leucogranites , equivalents to the Valcheta Pluton (40Ar-39Ar 470 Ma, Gozalvez 2009) intrude with post orogenic character to the sills and metasediments. Zircons from a metagranodioritic sill were analyzed by U-Pb SHRIMP method at the Centro de Pesquisas Geocronologicas (CPGeo), Geosciences Institute, University of São Paulo, São Paulo, Brazil. Cathodoluminescence (CL), Secondary Electron (SE), and binocular lens images were used for choosing the sites for analysis based on internal structure and grade of fracturing of the grains. The minerals are prismatics, ambar color, colorless and transparent, present low luminiscence and show oscilatory and sectorial zoning which is typical of magmatic zircon. The U-Pb SHRIMP analysis yielded Cambrian Concordia Age of 513.6 ± 3.3 Ma. (MSWD = 0.84). This result was interpreted as the age of magmatic crystallization of the sill. From the above, Cambrian magmatism (513.6 ± 3.3 Ma), mainly basic composition was confirmed. This was coeval with the sedimentation of the protolits of the Nahuel Niyeu Formation whose maximum sedimentation age ranges from 515-507 Ma (Pankhurst et al. 2006 y Rapalini et al. 2013). Furthermore, the first and main tectono metamorphic event (D1-D2-M1) was bounded between 513.6 to 470 Ma. from sills and leucogranites crystallization ages.