El Molino Pluton: a granite with regional metamorphism within El Jagüelito Formation, North Patagonian Massif

GONZÁLEZ, P.D.; SATO, A.M.; VARELA, R.; LLAMBÍAS, E.J:; NAIPAUER, M.; BASEI, M.A.S.; CAMPOS, H.J.; GRECO, G.A.

San Carlos de Bariloche

Simposio; 6th South American Symposium on Isotope Geology; 2008

CIG - INGEIS

El Jagüelito and Nahuel Niyeu Formations make up the low grade metamorphic basement of Eastern North Patagonian Massif, Argentina. In addition to their better known psammo-pelitic and minor mafic igneous protoliths, we are now describing a new felsic igneous protolith, a small granite body intruded in the El Jagüelito Formation, before the regional metamorphism took place. The granite show a chilled margin against its country rock, and the hornfels in the contact aureole is characterized by muscovite and biotite porphyroblasts that are subsequently affected by regional deformation. Granite body and country rock, thus, share the same S foliation with a dextral shear component. The deformation has transformed the granite into gneissic and mylonitic rocks under low grade thermal conditions. SHRIMP U-Pb dating of 14 points performed in prismatic zircon crystals without metamorphic overgrowth yielded a magmatic crystallization age of 472 ± 5 Ma. This age constraints the original deposition of El Jagüelito Formation to Cambrian up to the earliest Ordovician times. However, the age of metamorphism, which should be later than 472 Ma, does not consistently agree with previous age controls given by undeformed, posorogenic granitoid datings. Therefore, we suggest that more geological knowledge is necessary in order to improve regional geotectonic interpretations.