Journal of South American Earth Sciences 15 (2002) 591–623 www.elsevier.com/locate/jsames Polyphase structural evolution in the northeastern segment of the North Patagonian Massif (southern Argentina) W. von Gosen* Geological Institute, University of Erlangen-Nu¨rnberg, Schlossgarten 5, D-91054 Erlangen, Germany Received 1 March 2001; accepted 1 July 2002 Abstract Structural analyses in the northeastern segment of the North Patagonian Massif (southern Argentina) show that the simply deformed and metamorphosed phyllitic succession of the Late Precambrian–Cambrian El Jagu¨elito Formation has been intruded by Ordovician granitoids that are not ductilely deformed. The unconformable cover of the Silurian–Lower Devonian Sierra Grande Formation suggests that the Early Paleozoic Famatinian deformation of western Argentina did not affect this sector of the North Patagonian Massif. The ,NW–SE compression of this succession led to the formation of open fold structures combined with high-angle reverse and sinistral strike–slip faults. Deformation interfered with the cooling of the Laguna Medina granitoids and is assigned here to the Late Paleozoic interval (probably Permian). A comparable mechanism is assumed for the metamorphism in the Sierras Australes fold-and-thrust belt north of Patagonia. The ,NE–SW compression in the area west of Mina Gonzalito led to the formation of mylonites in the Pen˜as Blancas and La Laguna granites. It is suggested that ductile deformation is Permian in age and took place along important shear horizons. On a regional scale, it is comparable to that of the Cerro de Los Viejos granite (La Pampa Province) and the Sierras Australes fold-and-thrust belt (Buenos Aires Province) north of the inferred suture between Patagonia and Gondwana South America. This suggests that, on both sides of the boundary, intense compression took place during the same Gondwanide period and that extra-Andean Patagonia collided with Gondwana South America. The deformation in the Sierra Grande area is interpreted as a second-stage event during the Gondwanide deformational and magmatic history. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Deformation; Metamorphism; Paleozoic; Patagonia 1. Introduction these are Valcheta-Yaminue´, Pailema´n-Mina Gonzalito, and Sierra Grande (Figs. 1 and 2). In southern Argentina, the North Patagonian Massif The main problem regarding Patagonia is related to its covers an area south of the Colorado and Neuque´n basins geotectonic interpretation. Ramos (1984) proposes a model between the Atlantic coast on the east and the foothills of in which Patagonia is interpreted as a terrane, accreted to the southern margin of Gondwana South America during the Patagonian Cordillera on the west (Fig. 1). The Carboniferous–Early Triassic times (see also Ramos southern margin of the massif is indicated by the Tecka- (1986) and (1988)). His interpretation is also based on the Tepuel basin (Rı´o Chubut). A W–E to NW–SE-striking intense Gondwanide deformation in the Sierras Australes fault system has been assumed as the northern margin fold-and-thrust belt, north of the covered boundary of against the Colorado basin (Turner and Baldis, 1978). This Patagonia, combined with Permo-Triassic intrusive activity extra-Andean area of Patagonia is widely covered by in northern Patagonia and intense volcanism (Choiyoi Triassic, Jurassic, and Tertiary volcanics (Stipanicic and Group) in the La Pampa Province. The strip of latter Methol, 1980), whereas Proterozic to Early Paleozoic rocks volcanics can be traced from the Cordillera southeastward to and Late Paleozoic–Mesozoic intrusions are only exposed the La Pampa Province and has equivalents in the North in a few areas. In the northeastern segment of the massif, Patagonian Massif (e.g. Linares et al., 1980; Kay et al., 1989; Lo´pez Gamundı´ et al., 1995; Llambı´as, 1999). The * Tel.: þ49-9131-852-2699; fax: þ49-9131-852-9295. belt of Late Paleozoic–Triassic intrusions in northeast E-mail address: [email protected] (W. von Gosen). Patagonia has been interpreted as a magmatic arc related to 0895-9811/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S0895-9811(02)00111-6 592 W. von Gosen / Journal of South American Earth Sciences 15 (2002) 591–623 Fig. 1. Simplified map of the main geological and tectonic units in the area between the North Patagonian Massif, Sierras Australes, and Las Matras block (compiled and adapted after Criado Roque and Iba´n˜ez (1979), Rossi and Zanettini (1986), Tickyj and Llambı´as (1994), Fryklund et al. (1996), Tickyj et al. (1997), Caminos (1999) and Sato et al. (1999, 2000)). Frames depict the locations of maps in Figs. 2 and 20. Inset map also shows location of the North Patagonian Massif (NPM). W. von Gosen / Journal of South American Earth Sciences 15 (2002) 591–623 593 Fig. 2. Geological sketch map of the northeastern segment of the North Patagonian Massif, compiled and adapted after Rossi and Zanettini (1986) and Giacosa (1997). For location, see Fig. 1. Frames depict the locations of maps shown in Figs. 3 and 4. SW-directed subduction beneath Patagonia (Ramos, 1984, South America since the Early Paleozoic. Rossello et al. 1986). Selle´s Martı´nez (1989) proposes a sinistral trans- (1997) also argue against an allochthony of Patagonia and pressive collision between the Patagonia terrane and postulate a dextral transpression under N–S contraction that Gondwana South America. affected northeast Patagonia, the Sierras Australes, and This contrasts with the opinion of Dalla Salda et al. Tandilia during the Late Paleozoic–Early Mesozoic. (1992a,b, 1993, 1994), who assume that the belt of Early Additional arguments for an autochthonous position of Paleozoic (Famatinian) deformation, metamorphism, and Patagonia are given by, for example Cingolani et al. (1991) plutonism in the Sierras Pampeanas continues southward into (see also discussions in Rapela and Kay (1988)). They relate northern Patagonia and thereby traverses the inferred terrane the Early to Late Paleozoic intrusive activity in the North boundary depicted by Ramos (1986, 1988). Their arguments Patagonian Massif, as well as acid volcanics, to an ‘inner are based on comparisons of basement rocks, Early Paleozoic cordilleran arc’ formed during E-directed subduction intrusive history, and interpreted isotopic data. According to beneath the western margin of Gondwana during the this model, Patagonia would have been part of Gondwana Devonian–Permian (compare also Davidson et al., 1987; 594 W. von Gosen / Journal of South American Earth Sciences 15 (2002) 591–623 Fig. 3. Geological sketch map of the area between Arroyo Salado, Mina Gonzalito, and north of Sierra Pailema´n, compiled and adapted after Giacosa (1997) and personal investigations. For location, see Fig. 2. The locations of Figs. 15–17b, and 19a–c are shown. Herve´, 1988). This is indirectly supported by Caminos et al. 2. Lithological units (1988), who propose a post-Middle Carboniferous uplift and cratonization of the massif. The oldest unit in this northeastern segment of the North To support one of these models, structural analyses focused Patagonian Massif is represented by the metamorphic on two parts of the northeastern sector of the North Patagonian complex west and northwest of Mina Gonzalito (Figs. 2 Massif: Sierra Grande and west of Mina Gonzalito (Fig. 2). and 3). It consists of micaschists with amphibolites, For both, the sequential order of deformational events is marbles, and several metagranitoids and has been assigned described subsequently. On the basis of the results, some to the Precambrian (e.g. Rosenman, 1972; Ramos, 1975; preliminary regional implications are provided. Caminos and Llambı´as, 1984; Linares et al., 1990; Caminos W. von Gosen / Journal of South American Earth Sciences 15 (2002) 591–623 595 et al., 1994; Varela et al., 1998). To the west, it is juxtaposed (Varela et al., 1997, 1998), which suggests an Early to Mid- against the Pen˜as Blancas granite along the dextral Jagu¨elito Ordovician age (according to the time scale of Gradstein Fault (Ramos and Corte´s, 1984). Just east of the fault, a and Ogg (1996)). NW–SE-trending granite is exposed parallel to the fault In the northeastern part of the North Patagonian Massif, line. It has been interpreted as part of the Pailema´n granites the Sierra Grande Formation is exposed at Sierra Grande in the north and west (Giacosa, 1997), which are Permian in and west of Valcheta. Small occurrences of equivalents of age (e.g. ‘Granito Arroyo Pailema´n’, 268 ^ 3 Ma, Rb–Sr, the formation are reported in the areas WNW of San whole rock, isochrone, Grecco et al., 1994; granite ‘Sa. Antonio Oeste (Gran Bajo de Gualicho; Lizuain Fuentes Pailema´n’, 270 ^ 10 Ma, Nun˜ez et al., 1975: in Valles and Sepulveda, 1978) and south of Sierra Grande (Ea. El (1978)). Refugio; Corte´s, 1978, 1981; Ea. Giordano; Zanettini, 1981, In the northeastern part of the North Patagonian Massif, Fig. 2 herein). the El Jagu¨elito Formation generally consists of phyllites, The pile of elastic sediments mostly consists of grey to quartzites, metagreywackes, and some greenstones. This white sandstones. Ripple marks, cross-beds, and conglom- succession has been named ‘Ectinitas El Jagu¨elito’ in the eratic layers are widely distributed. Although the clastic area west of Mina Gonzalito (Ramos, 1975). The El succession in the eastern part of the outcrops mostly consists Jagu¨elito Formation in the Sierra Grande area has been of grey to white quartzitic sandstones, the western equiva- compared to the Nahuel Niyeu Formation in the Valcheta lents are grey sandstones with an increasing amount of area (Caminos and Llambı´as, 1984; Giacosa, 1987, 1994, grey siltstones. The conglomerates contain angular to 1999; Chernicoff and Caminos, 1996). Small occurrences subangular clasts of quartzite, polycrystalline quartz, and have been described from the Ea El Refugio (south of Sierra phyllites (El Jagu¨elito Formation) and are mostly matrix Grande, Corte´s, 1981; Fig.