Magnetic fabric of the plutonic Navarrete Complex: further evidence for a Late Paleozoic collisional event in Northern Patagonia?

APALINI, A.E., M.G.LOPEZ DE LUCHI, R.TOMEZZOLI, F.L. KLINGER, M. GIMENEZ, P. MARTINEZ

Santiago de Chile

Simposio; GEOSUR; 2007

The Paleozoic tectonic evolution of Patagonia and its relations with the Gondwanan blocks located to the north have been a matter of much debate in the last two decades. No consensus has been reached on whether the North Patagonian Massif (NPM) was accreted in the Late Paleozoic by a frontal collisional event against the Gondwana margin (e.g. Ramos, 1984) or if it shared a similar paleotectonic evolution with other Gondwana blocks during the Paleozoic (e.g. Dalla Salda et al., 1991). As part of a muldisciplinary research project, a magnetic fabric study was carried out on the Permian Navarrete plutonic complex (NPC) exposed along the NE margin of the NPM. This study was accompanied by detailed structural observations in the field and under microscope, geochemical analysis and systematic gravity surveys. The main aim is to characterize the geometry and emplacement processes of the complex in order to analyse the relation between magmatism and regional deformation.             NPC, recently dated as 283 Ma (U/Pb SHRIMP in Zr, Pankhurst et al., 2006), intrudes the lower grade Cambrian metaclastics of the Nahuel Niyeu formation. Extensive cover by younger plutonic and volcanic units does not permit to define the actual shape of this batholitic complex. The largest outcrops are located to the south of the Nahuel Niyeu (eastern outcrops) and the Ramos Mexia (western outcrops) railway stations. On the western border of the western outcrops, NPC is intruding the Late Paleozoic (ca. 300 Ma, Basei et al., 2002) Yaminué Complex (YC), which has been described (e.g. Llambías et al., 2002) as a highly deformed and metamorphosed plutonic complex.             The magnetic fabric was characterized by the anisotropy of magnetic susceptibility (AMS). Five to nine oriented cores were drilled at each of the 82 sites measured in the studied area. Sampling mainly concerned the NPC outcrops although some sites were also located in the Early Jurassic (?) Flores granite and the YC.             Three main domains were depicted in the NPC by field observations, petrography and the AMS study. Domains 1 and 2 are paramagnetic, whereas domain 3 is ferromagnetic. Domain 1 corresponds to the northern third of the western outcrops and it is characterized by a well defined NW-SE planar magnetic fabric moderately dipping to the NE with moderate plunging NNE lineations. This corresponds to submagmatic deformation and it is strikingly parallel to the deformational fabric exhibited by the orthogneisses and deformed granodiorites of the YC. This strongly suggests that this domain was intruded during final stages of the main deformational event (D1) affecting the YC. Domain 2 occupies the central part of the outcrops and shows a steep NNW-NS planar fabric and moderate to steep N-S lineations compatible with a much weaker second deformational event recorded in the YC consistent with an E-W compression. Domain 3 comprises the largest outcrops of the NPC and its magnetic fabric is also apparently governed by the same stress field or in some areas mainly governed by magmatic chamber processes (like the outcrops at puesto Navarrete, from where the U-Pb age was obtained) with no evidence of far-field stress control.             NPC is, therefore, mostly syn-kinematic. Domain 1 would have shared with the YC a partially common deformational history, and its intrusion probably took place at the final stages of YC deformation. A transition from a Late Paleozoic compressive event towards a period of quiescence or perhaps extension is apparently recorded by the successive domains of NPC. The significant event of NNE-SSW compression that affected this region, and which has been interpreted by various authors as associated to final frontal collision of Patagonia with SW Gondwana is then bracketed between 300 and 283 Ma (i.e. Early Permian). References Basei, M. et al., 2002. 15° Cong. Geol. Arg., Actas, 3: 117-122 Dalla Salda, L. et al., 1990. Comunicaciones, una revista de Geología Andina, 41, 55-64. Llambías, E. et al., 2002. 15° Cong. Geol. Arg., Actas, 3: 123-128. Pankhurst, R.  et al., 2006. Earth Science Rev., 76: 235-257 Ramos, V. 1984. 9° Cong. Geol. Arg., Actas, 2: 311-325.