EL MOLINO PLUTON: A GRANITE WITH REGIONAL METAMORPHISM WITHIN EL JAGÜELITO FORMATION, NORTH PATAGONIAN MASSIF

GONZALEZ, PABLO; SATO, A.M.; VARELA, R.; LLAMBÍAS, E.J.; NAIPAUER, M.; BASEI, M.A.S.; CAMPOS, H.; GRECO, G.A.

Bariloche, Argentina.

Simposio; VI South American Symposium on Isotope Geology; 2008

ABSTRACTEl Jagüelito and Nahuel Niyeu Formations make up the low grade metamorphic basement of EasternNorth Patagonian Massif, Argentina. In addition to their better known psammo-pelitic and minor maficigneous protoliths, we are now describing a new felsic igneous protolith, a small granite body intruded in theEl Jagüelito Formation, before the regional metamorphism took place. The granite show a chilled marginagainst its country rock, and the hornfels in the contact aureole is characterized by muscovite and biotiteporphyroblasts that are subsequently affected by regional deformation. Granite body and country rock, thus,share the same S1 foliation with a dextral shear component. The deformation has transformed the granite intogneissic and mylonitic rocks under low grade thermal conditions. SHRIMP U-Pb dating of 14 pointsperformed in prismatic zircon crystals without metamorphic overgrowth yielded a magmatic crystallizationage of 472 ± 5 Ma. This age constraints the original deposition of El Jagüelito Formation to Cambrian up tothe earliest Ordovician times. However, the age of metamorphism, which should be later than 472 Ma, doesnot 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 geotectonicinterpretations.