Triassic post-orogenic dyke swarm in Northpatagonian Massif, Argentina.

GONZÁLEZ, SANTIAGO; GRECO GERSON; SATO ANA MARÍA; GONZÁLEZ PABLO DIEGO; LLAMBÍAS EDUARDO; VARELA RICARDO; BASEI MIGUEL

San Pablo

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

CPGeo-IG-USP, Brasil

Mesosilicic dykes were described in the eastern region of the North Patagonian Massif by many authors. They constitute a regional dyke swarm of NW-SE trend of about 200 Km length and 50 Km width. They cut the already deformed Early Paleozoic basement rocks, Permian Pailemán Plutonic Complex, and in turn they are cut by sub-volcanic bodies of the Jurassic Marifil Complex. Dykes are composed of porphyritic andesites to trachyandesites with seriated groundmass. They contains frequents enclaves of country rocks, such as Cambro-Ordovician high grade metamorphic rocks of Mina Gonzalito Complex and Permian plutonic rocks. Published radiometric data from the dykes are K-Ar ages between 207 and 221 Ma. Zircon crystals from a dyke near Mina Gonzalito Mine were analysed in the Centro de Pesquisas Geocronologicas (USP) by LA-ICPMS U-Pb method. Three age groups are recognised: (1) Neoproterozoic to Cambrian ages from zircons with axial ratio 2 to 3 and complex oscillatory zoning. (2) Triassic ages from crystals with axial ratio 1 to 1.5, and wide oscillatory or sector zoning.  (3) Jurassic ages from the borders of zircons with oscillatory zoning and axial ratio close to 1. On the basis of textural evidence, we consider the Concordia age of 241.5 ± 1.8 Ma (MSWD = 0.01) in the second group as the crystallization age of the dyke in the swarm.  The older ages are interpreted as inheritance from the Mina Gonzalito Complex which is the country rock of this dyke, and the younger age is assigned to an extensive Jurassic hydrothermal processes. The NW orientation of the swarm is parallel to the previous Gondwanic orogenic axis proposed for this region. The Triassic dykes are linked to the post-orogenic magmatism associated with the extensional collapse of the Late Paleozoic orogeny, which precedes the generalized, Jurassic continental extension in Patagonia.