Insights into the Paleozoic tectonic evolution of the North Patagonian Massif. Start up hypothesis on a sequence of collisional events

LÓPEZ DE LUCHI, M.G.; RAPALINI, A.E.; MARTÍNEZ DOPICO, C.I.; CERREDO, M.E.; WEMMER, K.

Neuquén

Congreso; XVIII Congreso Geológico Argentino; 2011

Asociación Geológica Argentina

The igneous and metamorphic evolution of the northern border of the North Patagonian Massif (NPM) during the Cambrian-Permian interval involves deformational events during Lower Paleozoic times, a Silurian extensional regime and compressive conditions in the Late Paleozoic (Ramos 2008, Rapalini et al., 2010). Gondwanide orogeny is represented in this area (Pankhurst et al., 2006). An extension of the Famatinian and Pampean events towards the NPM was proposed by many authors (see Rapalini et al., 2010 and references therein). This constitutes a remarkable difference with the westernmost sector of the NPM where pre-Silurian magmatism has not been reported. Widespread Permian and Triassic magmatism is present in both sectors with variable controls by the regional deformational events (Von Gosen 2003, 2008). The geodynamic evolution and linking geological processes between the orogenic belts in the N and W NPM together with the Sierras Pampeanas, Río de la Plata craton and other blocks are still a matter of debate (Ramos 2008). In order to constrain the previous tectonic models for the origin of northern Patagonia (Pankhurst et al., 2006; Gregori et al., 2008, Rapalini et al., 2010, among others) and its relations with the South American crustal blocks, geology, geochemistry, isotopic dating and tectonic characterization of the NPM lead us to point out that: (1) The northern belt of the NPM might have experienced at least two different metamorphic and deformational events. The first, prior to Lower Ordovician, is represented in the low to medium metasedimentary sequences of the Nahuel Niyeu and El Jagüelito formations (Pankhurst et al., 2006). These rocks are intruded by the 470-450Ma underformed strongly peraluminous Valcheta pluton (Tovher et al., 2008, among others) which is thought to be formed from the melting of Muscovite- rich pelites and by the undeformed highly hybridized 475Ma metaluminous granodiorites-tonalites of Arroyo Salado pluton in the coastal area. The proper distinction between Nahuel Niyeu and El Jagüelito formations needs to be investigated since at least its maximum depositional ages differs in 25 Ma and its inheritance detrital zircon patterns (Pankhurst et al., 2006) show similar age peaks than the “post-Pampean” basins of the Pampean Ranges. The second deformational event, which is recorded in the orthogneisses of the Yaminué Complex, can be bracketed between ~300Ma (the U-Pb age of a facies assigned to the Yaminué Complex, Basei et al., 2002) and the age of the undeformed Navarrete and La Esperanza plutonic complexes (281Ma - 273Ma , Pankhurst et al., 2006, López de Luchi et al., 2010a). These two magmatic and deformational events are clearly separated by a period of sedimentation recorded in the Silurian-Devonian iron-bearing Sierra Grande sandstones. (2) In the western to southwestern area, the first stratigraphic record is represented by an ill-defined Late Silurian (420-400 Ma) magmatic event (Pankhurst et al., 2006; Varela et al., 2005) recorded in hybrid tonalites and granodiorites nearby San Martín de los Andes and a Late Devonian (385-360 Ma) medium- to high grade metamorphic event that affected the metasedimentary rocks known as Cushamen and Colohuincul Formations. Ostera et al. (2001) recognized these events through Rb-Sr isochrons. More recently López de Luchi et al. (2010b) confirmed these ages for foliated leucogranites interlayered within the Cushamen metasedimentary sequence in the Cañadón la Angostura-Colonia Cushamen area. This Devonian metamorphism can also be recognized in the major peak on the detrital zircon inheritance pattern obtained by Hervé et al. (2005) for the Cushamen Fm metasediments in the Colonia Cushamen area as well as in the data provided by Lucassen et al. (2004) further north. Carboniferous magmatism and metamorphism in Cushamen Fm. would correspond to the convergence and subsequent collision of the Deseado Massif according to Pankhurst et al. (2006). Widespread 295-260 Ma Permian magmatism (e.g. Mamil Choique granitoids) seems to have involved an early extended water fluxed partial melting of lower crustal mafic and felsic sources associated with a compressive event which turns into dehydration melting of shallower felsic sources in the late stages. This evidence can be reconciled with a two-side sequence of collisional events that might have affected the NPM during Paleozoic times. Even though Early Paleozoic metamorphism and magmatism is recorded exclusively in its NE sector, the meaning of these events is not clear. There is strong evidence in favor of the authochtonous origin of the NPM respect to Gondwana, based on Famatinian ages for the magmatism and high grade metamorphism at the NE sector of the NPM (Pankhurst et al., 2006), comparable detrital zircon inheritance spectra for contemporaneous metaclastic units (Martínez Dopico et al., 2010) and fauna content (Naipauer et al., 2010). The broadly discussed evidence of a Late Paleozoic frontal collision with the southwestern margin of Gondwana (Ramos, 1984) can be reconciled with the authochtonous origin assuming a para-autochthonous evolution of the NPM in the Paleozoic that includes a rifting event during Ordovician-Silurian from a neighbouring position to its post-collision location. The possibility of a Mid-Paleozoic small oceanic basin between NPM and Gondwana is permissible by the available paleomagnetic data (Rapalini, 1998) as well as a subsequent development of a southward subduction zone prior to the collision with the Río de la Plata craton and Pampia terrane during Mid-Late Carboniferous times. In this interpretation, geochemical fingerprint on the Yaminué Complex lead us to conclude that it might represent remnants of a short-lived magmatic arc and syn collisional granitoids, whereas the younger Navarrete/La Esperanza Plutonic complexes represent post collisional granitoids possibly the products of slab break-off processes (Pankhurst et al., 2006). Devonian ages on the western belt are coincident with those proposed for the collision of the Chilenia terrane with Gondwana (Willner et al., 2008). Following this, our working hypothesis considers that the Cushamen/Colohuincul formations may represent the southern continuation of the accretionary prism whereas Devonian San Martín de los Andes tonalites and granodiorites may be the magmatic arc developed due to this convergence. Widespread Permian granitoids (Mamil Choique Fm and equivalents) and its N-NW trending deformation, could be related to the closure of the Sierra Grande basin along the NE margin of the NPM. Geodynamic scenario for the western belt during the 380/360Ma – 300Ma interval is loosely constrained. Circa. 330Ma ages (Pankhurst et al., 2006) could record some kind of resetting of the isotopic system and/or the P-T decompression paths related to an extensional regime postdating the Late Devonian peak metamorphic conditions (López de Luchi and Cerredo, 2008). In our view, Paleozoic deformational events can be associated with at least two collisional events: Devonian and Late Carboniferous developed at opposite margins of the North Patagonian Massif.