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 .vnGsn/Junlo ot mrcnErhSine 5(02 591–623 (2002) 15 Sciences Earth American South of Journal / Gosen von W.

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. 2 herein) and to the west of supported. Puerto Madryn (Chubut, Haller, 1976, 1978). The base of The clastic succession has been divided into two the unit is not exposed; however, Chernicoff and Caminos members, each containing an iron oolite horizon (Zanettini, (1996) assume an unconformable contact between the 1981). These horizons have their greatest thicknesses in the Nahuel Niyeu Formation and the higher metamorphic western part of the outcrop distribution and seem to thin out Yaminue´ Complex in the Valcheta-Nahuel Niyeu area. to the east. North of Puesto Monochio, only thin relicts of In the Sierra Grande area (Figs. 2 and 4), the El Jagu¨elito iron-rich sandstones and siltstones at the base of the Sierra Formation is a monotonous pile of clastic sediments that Grande Formation were found. consists of centimeters to several meters thick quartzites For the deposits of the Sierra Grande Formation, a alternating with layers of silty phyllites. It contains relicts of shallow marine environment has been interpreted by graded bedding, ripple marks, and load casts and is Huber-Gru¨nberg (1990) and Spalletti et al. 1991 and interpreted as a turbiditic succession. Mafic metamagmatics Spalletti, 1993. Their facies analyses and measurements of have been described by Huber-Gru¨nberg (1990). West of Mina Gonzalito, the El Jagu¨elito Formation is intruded by sedimentary transport directions suggest that the basin the Pen˜as Blancas granite (Ramos, 1975; Giacosa, 1997; deepened westward from an inferred shoreline in the east. Fig. 3 herein). The age of the phyllitic succession can On the basis of fossil findings in the western part, the age of broadly be assigned to the Late Proterozoic–Cambrian the Sierra Grande Formation has been assigned to the interval as indicated by a radiometric date from the Nahuel Silurian–Lower Devonian (Braitsch, 1965; Mu¨ller, 1965; Niyeu Formation (Linares et al., 1990) and the age pattern Castellaro, 1966; Cuerda and Baldis, 1971; Amos, 1972). At of inherited zircons (Pankhurst et al., 2001). several localities in the eastern and northern parts of the The Punta Sierra granitoids include granodiorites to succession, poorly preserved relicts of Lamellibranchiate granites from the Punta Sierra area, as well as granodiorites shell imprints were found during the present study. At two to tonalites, granites, and pegmatites/aplites in the surround- localities in the eastern area, scolithus pipes could be ing of Puesto Monochio (east of Sierra Grande; Fig. 4). detected, whereas Braitsch (1965) and Huber-Gru¨nberg Good exposures in the Arroyo Salado show that the (1990) have described them from one locality in the west. intrusive activity began with granodiorites to tonalites, South of Sierra Grande, in the area of the former mine followed by fine- to coarser-grained granites that partly (Mina Hiparsam, Yacimiento Sur), a reddish granite occur as dykes. The final stage of intrusive activity is intruded the quartzitic sandstones of the Sierra Grande indicated by the injection of pegmatite and aplite dykes. Formation (‘Laguna Medina granite’ or ‘Granito Hiparsa’ of From the Punta Sierra granitoids, isotopic data exist. A Varela et al. (1997)). The intrusion has been interpreted as Rb–Sr whole rock isochrone of the Punta Sierra granite at syntectonic (Rossello et al., 1997). It led to the formation of the Atlantic coast, with 483 ^ 22 Ma (Varela et al., 1997, a contact aureole in the Sierra Grande Formation, as 1998), indicates an Early Ordovician age of the intrusive described by Gelo´s (1977). To the southeast, north of activity. In the Arroyo Salado, Rb–Sr whole rock dating of Laguna Medina, the Laguna Medina granodiorite is the granodiorite and U–Pb dating of zircon fractions from exposed. Outcrops of the contact with the granite could a tonalite gave 467 ^ 16 and 476 ^ 4 Ma, respectively not be found. As the granite in the northeast, the 596 .vnGsn/Junlo ot mrcnErhSine 5(02 591–623 (2002) 15 Sciences Earth American South of Journal / Gosen von W.

Fig. 4. Geological map of the Sierra Grande area compiled and adapted after Zanettini (1981) and personal investigations. For location, see Fig. 2. W. von Gosen / Journal of South American Earth Sciences 15 (2002) 591–623 597 granodiorite intrudes the southernmost parts of the Sierra Sierra Grande area (Fig. 4). Cross-cutting relationships Grande Formation and led to contact metamorphism. between bedding (S0) and a penetrative S1 cleavage, as well Isotopic Rb–Sr whole rock dating of the Laguna Medina as the geometries of minor F1 folds of centimeters to meters granite yielded an unprecise date of 363 ^ 57 Ma (Varela size, show that the sediments were bent around F1-fold et al., 1997). A Rb–Sr whole rock date of the Laguna structures with amplitudes and wavelengths in the range of at Medina granodiorite (318 ^ 28 Ma; Varela et al., 1997), least several hundred meters. The B1 axes of the tight folds however, indicates cooling of the intrusion during (Late) gently plunge to ,Nor,S(Fig. 5), as is also indicated by Carboniferous times. Halpern et al. (1970) report a Rb/Sr the intersection lineation between the S0 and S1 planes (d1). date (? biotite) from a diorite of the Sierra Grande area of According to the steeply westward-dipping S0 planes on 252 ^ 5 Ma. In addition, the same date, according to Rb/Sr different fold limbs and the related S1 cross-cutting whole rock analysis of a granite from the same area, has relationships, F1 folds display a slight E-directed vergence, been found by Halpern, 1972 in Stipanicic and Linares, as is supported by the observations of Giacosa and Paredes 1975. Both older isotopic data point to the possibility that (2001).OnS1 planes, a clear L1 lineation, depicted by the Laguna Medina intrusives are younger and Permo- aligned sericite, could be detected only in the east, where it Triassic in age. Therefore, they could be equivalents of the steeply plunges toward ,WNW to NNW. Permian Pailema´n granites in the northwest (Busteros et al., On a micro-scale, the penetrative S1 foliation/cleavage is 1999). indicated by aligned sericite þ chlorite ^ microbiotite. It is To the west of Mina Gonzalito and west of the Jagu¨elito oriented parallel to or cuts across bedding (alternations of fault, the Pen˜as Blancas granite has been interpreted as part sericite-rich to quartz-rich layers) at small angles. Single of the Permian Pailema´n granites (Giacosa, 1997). For a quartz grains are affected by pressure solution and partly leucocratic part of the granite, however, a K–Ar biotite age display sericite beards in pressure shadows. In some parts, of 197 ^ 8 Ma (Linares in Busteros et al., 1999) suggests syn- to post-D1 recrystallization of quartz could be detected. that younger parts could exist. In contrast with the Pen˜as Clastic muscovite is aligned parallel to S1 and partly to Blancas granite, the La Laguna granite is porphyric with up entirely replaced by sericite. Single small biotite and to several centimeters long alkali feldspar phenocrysts in a muscovite þ sericite statically grew across S1 planes. biotite-rich matrix (compare Giacosa, 1997). The porphyric Finally, biotite is often converted into chlorite. In general, character continuously changes to finer grained types of the the rocks were affected by T conditions of the slight granite. Mylonitization of both granites has been discovered greenschist facies during D1 deformation. by Giacosa (1996), who recently described the Pen˜as In the phyllite succession of the El Jagu¨elito Formation, Blancas granite (Giacosa, 2001). The porphyric La Laguna the effects of the deformation in the Sierra Grande granite also occurs in the northwestern part of the strip of the Formation are indicated by single shear planes and fracture Pen˜as Blancas granite mylonite. Furthermore, it was found planes that cut across the S1 foliation at different angles. at the Ruta Provincial east of Arroyo de Los Berros. Thus, They partly led to crenulation of S1 planes or displacements the intrusion is not restricted to the present strip of granite of single beds. During shearing, S1 sericite was realigned, mylonites between Ea. La Laguna and Ea. San Roque. and some new sericite grew in C2 planes. In the pelitic parts, Despite the widely distributed mylonitization of the an S2 crenulation cleavage relates to microscale folds and intrusion, several fine-grained granitic dykes could be bending of bedding-parallel S1 planes. Pressure solution detected. They are up to several meters thick, record along the cleavage planes affects single quartz clasts in intrusive contacts, and are deformed together with the sericite-dominated layers. coarse-grained granite. West of the La Laguna granite mylonites, a medium- to 3.2. Punta Sierra granitoids and contact metamorphism fine-grained granite to granodiorite constitutes the La Verde pluton (Giacosa, 1997; Fig. 3 herein). It covers a wide area 3.2.1. Macrofabrics in the west to the Ea. La Verde in the north. A K–Ar biotite The different magmatics of the Punta Sierra granitoids age of the granodioritic part of the pluton has yielded intruded the simply deformed and metamorphosed clastics 253 ^ 9 Ma (Linares, in Busteros et al., 1999), which of the El Jagu¨elito Formation (Fig. 6) and led to a widely demonstrates that the intrusion belongs to the Permian distributed contact metamorphism. Porphyroblasts of anda- Pailema´n granites. lusite and (?) cordierite statically overgrew S1 cleavage planes. The exposed intrusive contact in the Arroyo Salado 3. Structure (southeast of Puesto Monochio) shows that, during intru- sion, the granodiorites to granites followed the steeply 3.1. El Jagu¨elito Formation inclined to subvertical bedding planes in the turbidite succession of the El Jagu¨elito Formation (Fig. 7a). They The clastic pile of sediments of the El Jagu¨elito Formation enclosed up to tens of meters long and deformed (D1) blocks has been affected by a compressive deformation (D1) in the and slices, which partly preserved their initial orientation. 598 .vnGsn/Junlo ot mrcnErhSine 5(02 591–623 (2002) 15 Sciences Earth American South of Journal / Gosen von W.

Fig. 5. Simplified geological sketch map of the Sierra Grande area with lower hemisphere and equal area stereoplots of structural elements in Sierra Grande and El Jagu¨elito Formations (white and grey plots, respectively). Map is based on Fig. 4a–h (encircled): locations of the stereoplots shown in Fig. 10. .vnGsn/Junlo ot mrcnErhSine 5(02 591–623 (2002) 15 Sciences Earth American South of Journal / Gosen von W.

Fig. 6. Geological profiles across the different units in the Sierra Grande area. For locations, see Fig. 4. 599 600 W. von Gosen / Journal of South American Earth Sciences 15 (2002) 591–623 W. von Gosen / Journal of South American Earth Sciences 15 (2002) 591–623 601

Toward the more central parts of the intrusion, enclosed related to the granitoid intrusions without emplacement- slices are successively rotated and decrease in size. related deformation. There was no indication that an Structures of intrusion-related deformation were only older internal fabric was rotated with respect to Se found in one outcrop next to the old Ruta 3 (Fig. 8a). ( ¼ S1). The porphyroblasts were converted into phyllo- There, local F2 folding within the phyllites is restricted to silicates prior to deformation within the Sierra Grande the contact and combined with single C2 shear planes. Formation. On the basis of the wide distribution of contact At the old Ruta 3, however, C2 shear planes, related to phenomena in the El Jagu¨elito Formation, it is reasonable open F2-fold structures (Fig. 8a), display biotite realigne- to assume the existence of one large pluton in the ment and recrystallization. Bent quartz layers record subsurface. The various kinds of igneous rocks exposed continued static grain growth with the formation of straight suggest that the pluton has a complex composition. This grain boundaries. Muscovite is partly recrystallized and interpretation contrasts with that for several different newly grown. This deformation and temperature overprint plutons in the area east of Sierra Grande (Busteros et al., at the contact with the Punta Sierra granodiorite cannot be 1999). related to those of the Sierra Grande Formation, as will be discussed subsequently. This suggests that local com- 3.2.2. Microfabrics of contact metamorphosed phyllites pression refers to the latest stage of pluton emplacement A few diffuse, lens-shaped sericite aggregates in the under decreasing temperatures, which explains why folding phyllites north of Puesto Monochio, near the intrusive is restricted to the contact area and dies out further west. As contact of the Punta Sierra granodiorite, represent converted shear planes comparable to those in the phyllites cut across porphyroblasts (? andalusites). Because the internal opaque the adjacent granodiorite, the latter must have been in a dust fabric is parallel to the external S1 foliation planes, solid state during compression. porphyroblast growth took place in static conditions after cessation of D1 deformation. In contact metamorphosed 3.3. Sierra Grande Formation phyllites at the old Ruta 3 and in the Arroyo Salado at the contact to the Punta Sierra granitoids, up to millimeters long With basal conglomerates and/or coarse-grained sand- relicts of porphyroblasts with diffuse boundaries toward the stones, the Sierra Grande Formation overlies the Punta matrix occur within the heated S1 foliation. They have oval Sierra granite and El Jagu¨elito Formation in the Sierra shapes and consist of an aggregate of sericite, muscovite, Grande area (Figs. 4, 6 and 7b; see also De Alba (1964) and quartz grains, and, partly, a biotite-rich core. Sericite and/or Huber-Gru¨nberg (1990)). The angular unconformity muscovite are not (or only diffusely) aligned parallel to the between the mostly subvertical bedding of the turbidite external S1. On a macro scale, undeformed porphyroblasts sequence and the basal clastics of the Sierra Grande display random orientations and are not aligned in S1.The Formation is exposed at several localities. existence of aligned quartz suggests that they were (?) andalusite poikiloblasts that grew statically, enclosed the 3.3.1. Field observations preexisting S1 foliation as internal planar fabric, and finally The D1 deformation of the Sierra Grande Formation is were replaced by (aligned) phyllosilicates. Outside the characterized by open F1 folding around NW–SE- to converted porphyroblasts, muscovite and biotite grew NNE–SSW-trending axes plunging north or southward statically at different angles across S1 planes. Quartz is (Figs. 4 and 5). The fold structures on a several hundred statically recrystallized with straight boundaries. meters to kilometers scale have shallowly dipping limbs All microfabrics support the field observations that the (up to 458). A steeper dip is mostly related to fault lines foliated phyllites have been affected by static heating cutting through the sequence (Fig. 6). The F1 synclines

Fig. 7. (a) View toward the west on granodiorite invading steeply inclined phyllites of the El Jagu¨elito Formation parallel to bedding and S1 cleavage (Arroyo Salado east of Puesto Monochio). (b) View toward the west on Punta Sierra granite (PSG) in the foreground overlain by subhorizontal sediments of the Sierra Grande Formation (SGF) in the background (Atlantic coast south of Punta Sierra). (c) Photomicrograph of fine-grained quartzitic sandstone of Sierra Grande Formation (north of Sierra Grande, ,3.5 km west of Ea El Porvenir; crossed polarizers). Quartz clast records a diagenetic to slight metamorphic overgrowth next to a sandstone clast, which is entirely impregnated by opaque dust (arrow). (d) Photomicrograph of S1 foliation in Sierra Grande Formation sandstone. Note quartz–sericite beards in pressure shadows at quartz clasts (crossed polarizers; basal parts of Sierra Grande Formation north of Sierra Grande, east of Ruta Nacional 3). (e) Photomicrograph of fabrics in Sierra Grande Formation sandstones in sinistral shear zone toward Punta Sierra granodiorite (,4 km NNW of Puesto Monochio). Clastic quartz grains are ductilely elongated in the subvertical bedding, which is cross-cut by younger brittle shear planes and a wider shear zone (arrow; crossed polarizers). (f) View toward the north on east-dipping quartzites of the contact metamorphosed Sierra Grande Formation above the contact with the Laguna Medina granite south of Sierra Grande. Bedding (S0) is cross-cut by steeply eastward-dipping S1 cleavage planes. (g) Photomicrograph of dextral shearing in Laguna Medina granite at the contact with the Sierra Grande Formation as indicated by S/C fabrics. Feldspar is entirely replaced by sericite (dextral shear zone of Fig. 12a, south of Sierra Grande; crossed polarizers). (h) Photomicrograph of undeformed andalusite porphyroblasts in contact metamorphosed siltstone of the Sierra Grande Formation east of Laguna Medina granite intrusion (south of Sierra Grande; plane polarized light). Scale bars in all photomicrographs are 1 mm long. 602 W. von Gosen / Journal of South American Earth Sciences 15 (2002) 591–623

Fig. 8. (a) Simplified, schematic, composite block sketch of intrusive contact of Punta Sierra granodiorite and granite in El Jagu¨elito Formation. Note that intrusives follow the steeply inclined bedding of the country rocks where emplacement-related deformation resulted in the formation of F2 folds and C2 shear planes (Arroyo below bridge of old Ruta 3, WNW of Puesto Monochio). (b) Simplified sketch of reverse fault planes in Sierra Grande Formation sandstones with sinistral components and combined with sinistral fault planes (fault line WSW of Arroyo Punta Sierra).

and anticlines are symmetric, and a slight E- or places, C1 shear planes could be related to the fold W-directed vergence is combined with and related to structures. In most cases, irregular fracture planes occur. high-angle reverse faults. Parallel to bedding planes, pressure solution affected In most parts of the Sierra Grande Formation, D1 contacts between quartz pebbles. At the contact between deformation took place under brittle conditions. At a few the Sierra Grande and El Jagu¨elito Formations north of W. von Gosen / Journal of South American Earth Sciences 15 (2002) 591–623 603

Sierra Grande (east of the Ruta Nacional 3), basal Grande Formation sediments are in sedimentary contact conglomerates and quartzitic sandstones are affected by a with the El Jagu¨elito Formation phyllites in the west, which penetrative S1 cleavage and partly record incipient ductile are intruded by the granodiorite further westward. A deformation of quartz. sinistral component can be inferred for the fault lines in The strike of F1-fold structures in the Sierra Grande the northwestern part of the Sierra Grande Formation near Formation follows that of the steeply inclined to subvertical the Ea. El Porvenir. In addition, one NNE–SSW-striking bedding and S1 cleavage planes of the El Jagu¨elito dextral fault to the northeast can be reconstructed. Formation (Figs. 4–6). A few outcrops show that reverse In a general view, measured fault planes and shear planes faulting acted along these planes in the underlying phyllites. mostly depict ,W- and E-directed reverse displacements, This suggests that the orientations of these structures in the sinistral displacements along N–S to NW–SE (partly W–E) subsurface controlled the geometries developing in the planes, and, to a minor extent, dextral planes (Fig. 10). In cover sediments. Therefore, it is assumed that discrete particular, the combination of W–E reverse displacements displacements along subvertical planes in the phyllites and sinistral displacements along N–S to NW–SE planes accommodated shortening during F1 folding in the cover (Fig. 9) suggests that the direction of compression was rocks. Continuous compression then led to high-angle oriented ,W–E to NW–SE. In support of this, folding was reverse faulting in which slices of the El Jagu¨elito combined with faulting, as can be seen in several outcrops Formation and cover rocks were displaced against Sierra and is indicated by the fold geometries (Fig. 6). Grande Formation clastics (Fig. 6). Gently inclined thrusts could only be found SE of Co. Rinco´n. They developed as E-directed imbricate faults from 3.3.2. Microfabrics a steeply W-dipping, high-angle reverse fault. From the The sandstone succession of the Sierra Grande Formation Sierra Grande Formation northeast of the Laguna Medina generally does not display a penetrative ductile foliation. granite, Ramos and Corte´s (1984) reconstructed a Pressure solution is widely distributed at quartz–quartz clast SE-directed thrusting in combination with folding. contacts and quartz–sericite boundaries. To a variable The few exposed faults display either a pure reverse extent, quartz is affected by undulation, bending, and partial displacement (west and southeast of Co. Rinco´n, Fig. 6)ora kinking. Sericite growth between quartz grains is reduced but combination of reverse movements with oblique sinistral to present. Some fine-grained sandstones record densely pure sinistral slip (WNW of Punta Sierra, Fig. 8b). North of packed quartz grains with straight to slightly curved grain Puesto Monochio, a vertical, N–S-striking slice of the boundaries. Quartz grain boundaries shift during compac- Sierra Grande Formation is sinistrally displaced against the tion, as demonstrated by quartz clasts with a clastic core, Punta Sierra granodiorite in the east (Fig. 9). The Sierra marked by a rim of opaque dust, and new quartz overgrowths

Fig. 9. Simplified schematic block profile across the Punta Sierra granodiorite, Sierra Grande Formation, and underlying El Jagu¨elito Formation north of Puesto Monochio. The granodiorite in the cast is sinistrally displaced against the Sierra Grande Formation sandstones. In the west, it intrudes the El Jagu¨elito Formation phyllites, which represent the sedimentary base of the Sierra Grande Formation. The phyllites are cross-cut by two oblique sinistral strike–slip faults and bent around an F2 fold structure. 604 W. von Gosen / Journal of South American Earth Sciences 15 (2002) 591–623

Fig. 10. (a)–(f) and (h) Lower hemisphere, equal area Hoeppener plots (Hoeppener, 1955) of shear planes and fault planes (small and big white dots, respectively) in Sierra Grande Formation with data from the El Jagu¨elito Formation and Punta Sierra granodiorite in (d). (g) Punta Sierra granodiorite and El Jagu¨elito Formation. Locations of the diagrams are shown in Fig. 5. In the Hoeppener plots, the lineation is projected on the pole of the related plane. Arrows indicate the relative sense of shear/displacement of the hanging wall unit.

(Fig. 7c). All fabrics can be related to burial, compaction and submicroscopical quartz. Sericite is aligned in the shear a temperature overprint in the anchizonal range. planes and micro shear zones, where it leads to pressure More intensely deformed sandstones display hetero- solution at quartz clast boundaries. geneous shearing in the matrix along micro shear zones. It is reasonable to suggest that sinistral shearing began Incipient shearing led to ductile quartz grain elongation, under T conditions of the anchizone/greenschist facies subgrain formation, and partly incipient dynamic recrys- transition and continued during decreasing temperature in tallization. It is indicated by sericite growth parallel to S1 the anchizone, which led to brittle deformation of quartz. planes and sericite beards at quartz clasts. More intense The oblique sinistral fault zone northeast of Puesto shearing and/or S1 cleavage planes are recorded by strictly Monochio (WNW of Punta Sierra), where deformed quartz aligned sericite parallel to S1 and pressure solution at clast grains do not record a flattening, provides such an example. boundaries. At those boundaries, quartz–sericite beards There, brittle shearing with brecciation of the quartzitic occur in pressure shadows at quartz clasts (Fig. 7d). In such sandstones is at an advanced stage. parts, quartz is slightly flattened, undulated, bent, and kinked but without clear recrystallization grains at the 3.4. Laguna Medina granitoids and contact aureole boundaries. The metamorphic overprint can be estimated to have reached the anchizone/greenschist facies boundary. 3.4.1. Field observations Single, younger, brittle micro shear zones cut across South of Sierra Grande, the Laguna Medina granite is bedding and several clasts. heterogeneously sheared at its eastern contact with the In the exposed sinistral shear zone against the Punta Sierra Grande Formation (Fig. 11). Ductile shear is Sierra granodiorite north of Puesto Monochio, quartz grains concentrated in a strip parallel to the contact with the Sierra in the Sierra Grande Formation sandstones are elongated, Grande Formation and dies out toward the west. In the bent, and kinked, and partly display subgrain formation with Sierra Grande Formation quartzites, bedding-parallel shear incipient dynamic recrystallization. Aligned sericite and also led to ductile deformation of quartz. In some parts flattened quartz grains depict a foliation that is subparallel to above the contact, the quartzites record an S1 cleavage that bedding and the sinistral strike–slip fault. At different cuts across bedding at great angles (Fig. 7f). Cleavage angles, the foliation is cross-cut by distinct shear zones formation is also combined with ductile deformation of (Fig. 7e). They contain angular pieces of ductilely deformed quartz. Deformation at the base of the Sierra Grande quartzite in a fine-grained cataclastic quartz matrix. In an Formation and top of the Laguna Medina granite may have advanced stage, shearing leads to brecciation. In the sinistral been related to the formation of the large, open F1 syncline shear zone, quartz clasts with various diameters are broken in the clastic succession to the east (Fig. 6, profile D–D0). and crushed. The matrix consists of a mixture of sericite and Shearing was an effect of fold-related flexural slip parallel to .vnGsn/Junlo ot mrcnErhSine 5(02 591–623 (2002) 15 Sciences Earth American South of Journal / Gosen von W.

Fig. 11. Simplified schematic block diagram of the eastern and southeastern contacts of the Laguna Medina granite with the contact metamorphosed Sierra Grande Formation south of Sierra Grande. Note that the southern occurrence of the Sierra Grande Formation is enclosed by the granite and granodiorite in the south (see text for further explanations). Lower hemisphere, equal area stereograms of structural elements are partly Hoeppener plots (Hoeppener, 1955), which depict the sense of shear along shear planes and mylonite foliation planes (arrows with small and big white dots, respectively). 605 606 W. von Gosen / Journal of South American Earth Sciences 15 (2002) 591–623 the bedding planes with the local formation of S1 cleavage probably were large magmatic biotite. It either follows the planes, as supported by a few shear bands on a microscale, S1 foliation or is randomly distributed. Large quartz grains which indicate a top-to-W sense of shear. are ductilely deformed (bending, kink bands, subgrains). The central and southern parts of the contact between both Smaller and sometimes elongated grains are partly to units are displaced by W–E- to WNW–ESE-striking and entirely replaced by undeformed recrystallization grains subvertical shear zones. They are interpreted as faults related that mostly have straight boundaries meeting at ,1208 to F1 folding in the east. One exposure of a shear zone triple junctions. Features of dynamic quartz recrystallization (Fig. 12a) shows ductile deformation of the granite with can be broadly related to the dextral shear zones cutting asymmetric feldspar s clasts and S/C fabrics in the central across the eastern contact. Feldspar is entirely replaced by part. Together with the displaced contact, they indicate a sericite and muscovite (Fig. 7g), which are aligned parallel dextral sense of shear under ductile conditions, which is to the S1 foliation or randomly distributed. A few larger supported by observations in the thin section (Fig. 7g). It muscovite plates in the foliation are bent and kinked. Within continued under brittle conditions with the formation of highly strained parts, however, recrystallization begins. In breccias. general, randomly oriented sericite and muscovite and static Equivalents of the shear zones were found in the granite recrystallization of quartz show that the temperature in the west. Their geometries, asymmetric feldspar s clasts, overprint outlasted shearing under static conditions. and S/C fabrics indicate a dextral sense of shear (Fig. 12b). To the west of the contact with the Sierra Grande The isolated ductile shear zones, however, continuously Formation, in a few parts of the granite, a developing S1 grade into magmatic fabrics. This suggests that shear zone foliation is marked by aligned magmatic biotite bent and deformation took place in the not entirely solidified granite. overgrown by new biotite of various sizes. Magmatic quartz In the southeastern and southern part of the granite, north records undulation, bending, kink bands, and small of Laguna Medina, contact metamorphosed slices of the recrystallization grains at the boundaries. The microfabrics Sierra Grande Formation quartzites are deformed together suggest that deformation was outlasted by biotite growth with the granite (Fig. 11). Their contacts with the granite and and recrystallization of quartz. Contrary to the eastern granodiorite are subvertical ductile shear zones that affect the granite margin, feldspar is not entirely replaced by sericite. magmatics as well as the quartzites. Strips of mylonites in the In the dextral shear zone at the eastern granite margin, shear zones are derived from both rock types. Kinematic the mylonite foliation is recorded by extremely elongated indicators (asymmetric feldspar clasts, S/C fabrics, shear magmatic quartz that is partly to entirely replaced by bands) demonstrate two sets of shear zones: (1) W–E- to subgrains and dynamically generated recrystallization WNW–ESE-trending shear zones record a dextral sense of grains. Sericite is aligned parallel to the foliation fabric or shear and cut across the southwesternmost occurrence of the randomly distributed. Because the foliation wraps around Sierra Grande Formation, which is enclosed in the granite to partly angular feldspar grains, their conversion into sericite granodiorite intrusion, and (2) NE–SW- to ENE–WSW- must have taken place mostly after foliation development. striking shear zones display a sinistral sense of shear and Therefore, feldspar was entirely crystallized prior to the mark both boundaries of the southernmost Sierra Grande onset of shearing, as is supported by the foliation in the Formation quartzites within the igneous rocks (Fig. 11). granite just beneath the contact with the Sierra Grande Shear zone deformation under ductile conditions affected Formation. Magmatic muscovite plates are bent, kinked, the Laguna Medina granite and granodiorite, as well as the and widely replaced by sericite and small muscovite. heated rocks of the Sierra Grande Formation. Furthermore, Coarse, elongated magmatic quartz, aligned muscovite, the Laguna Medina granodiorite to the south of the and biotite recrystallization grains depict the shear zone westernmost part of the Sierra Grande Formation records foliation in dextral shear zones of the Laguna Medina ductile deformation phenomena in distinct parts only. granite and granodiorite pluton. Both muscovite and biotite There, shear zones are combined with fold structures on a also grew across the foliation. Magmatic plagioclase and K- centimeter to decimeter scale. These structures continuously feldspar are partly aligned by their long axes in the foliation grade into magmatic fabrics. Therefore, D1 deformation plane. Deformed magmatic quartz records undulatory under ductile conditions in these rocks, in contrast with extinction, partial subgrains, kink bands, and strain-induced brittle deformation in the Sierra Grande Formation east of boundary migration. Quartz and magmatic feldspar are Sierra Grande, was accomplished by a heat transfer from the partly replaced by single, coarse recrystallization grains or not entirely cooled and solidified pluton. grain aggregates that depict granoblastic textures. Bound- aries between recrystallization grains are straight to slightly 3.4.2. Microfabrics curved. A variable and incomplete conversion of biotite recrystallization grains into chlorite is a secondary process. 3.4.2.1. Laguna Medina granite and granodiorite. At the The conversion of feldspar into sericite ^ muscovite is eastern margin of the Laguna Medina granite, the S1 widely reduced. foliation is depicted by aligned sericite, biotite, and All fabrics suggest that shear zone deformation affected ^muscovite. Biotite also occurs in aggregates, which the consolidated parts of the granite, as confirmed by the W. von Gosen / Journal of South American Earth Sciences 15 (2002) 591–623 607

Fig. 12. (a) Sketch map of the intrusive contact between the Laguna Medina granite and contact metamorphosed Sierra Grande Formation (eastern margin of the Laguna Medina granite south of Sierra Grande). Both units and the intrusive contact are dextrally displaced along a WNW–ESE-striking, subvertical shear zone that records ductile and final brittle deformation phenomena (see text for further explanations). (b) Simplified sketch of dextral, ductile shear zone in the northern part of the Laguna Medina granite (plan view). Shear zone deformation is condensed in a central, W–E-trending strip in which S/C fabrics and asymmetric feldspar clasts with s shapes occur. The shear zone dies out along-strike and, as diffuse S planes (dashed lines), grades into magmatic fabrics. deformation and recrystallization of quartz, feldspar, and 3.4.2.2. Sierra Grande Formation. East of the granite mica. In particular, the internal ductile strain of magmatic intrusion, the contact-metamorphosed quartzites in the quartz is an indicator of solid-state deformation. Compared outer parts of the contact aureole record quartz grain growth, with the dextral shear zone at the eastern margin of the which led to straightened grain boundaries. In a thin pelitic granite, however, shearing in the granite and granodiorite took layer, a few small sericite þ chlorite aggregates probably place at higher temperatures. (Compare Fig. 13.) represent converted andalusites. Further west, toward 608 W. von Gosen / Journal of South American Earth Sciences 15 (2002) 591–623 the intrusive contact, pelitic layers in metamorphosed dark aggregates (,0.25–0.5 mm) presumably represent con- siltstones contain andalusite idioblasts that randomly over- verted cordierites. In this rock, no fibrolitic sillimanite grew bedding planes (Fig. 7h). They are partly converted into could be detected. Relicts of porphyroblasts do not record chlorite þ sericite. Sericite ^ chlorite ^ quartz aggregates flattening or development of pressure shadows. Therefore, with diffuse boundaries in sandy layers may have been the rock has not been affected by a penetrative shearing, poikiloblastic andalusites. In the matrix of pelitic layers, and porphyroblasts grew across bedding. sericite, muscovite. and biotite are aligned parallel to Quartz-muscovite schists and foliated quartzites in this bedding but randomly grew across porphyroblast relicts. southern strip of the Sierra Grande Formation display Quartz is recrystallized with straight to slightly curved microfabrics of quartz and phyllosilicates, which indicates boundaries. The rocks may have been affected by bedding- heating during and after shearing. Small sericite aggregates parallel shear prior to the peak of contact metamorphism. (up to 1 mm long, ? former andalusite) are affected by S1 Porphyroblasts, however, do not record a syntectonic growth. planes. Fibrolitic sillimanite records the features previously In the basal parts of the Sierra Grande Formation described. Comparable microfabrics also occur in a dextral quartzites and silty schists, above the contact with the shear zone next to the Laguna Medina granite. Fibrolite is Laguna Medina granite, sericite and muscovite are aligned aligned in S1 planes and partly overgrown by quartz rims. in a bedding-parallel S1 foliation or cross-cutting S1 Gaps between the extended quartz clasts are filled with cleavage. Growth and recrystallization of phyllosilicates quartz grains that contain fibrolite needles with long axes took place during shearing and outlasted deformation under parallel to the S1 planes. Furthermore, fibrolite needles are static conditions. To a different extent, clastic quartz grains condensed within pressure shadows at quartz clasts, with with variable sizes are flattened in these planes. They can their long axes oriented parallel to the trace of the S1 display beard-like intergrowth of aligned, bedding-parallel foliation (Fig. 14b). In general, shearing took place during sericite and quartz within pressure shadows. Undeformed the high temperature overprint and was followed by the quartz recrystallization grains with straight to slightly static growth of large muscovite plates and quartz grains. curved boundaries are distributed at the rims of deformed Both minerals overgrew aligned fibrolite, which does not clasts and in the matrix, where they also occur in smaller record evidence of post-D1 growth. aggregates. Some record an internal strain and indicate Only in some areas, F2 crenulations or crenulation folds dynamic recrystallization, which suggests that shearing of affect the S1 foliation with all fabrics described. Polygonal the quartzites interfered with heating. fabrics in hinge zones suggest that muscovite recrystalliza- Single quartz–muscovite–sericite aggregates (1–2 mm tion outlasted crenulation folding. Poikiloblastic muscovite in diameter) represent former porphyroblasts (? andalu- plates statically grew across the crenulations. In hinge zones, sites). They are affected by S1 cleavage planes that wrap quartz is slightly deformed in a few parts, but in most cases, it around the porphyroblast relicts and by muscovite that grew is recrystallized syn- to post-F2 and records straight to slightly internally, mostly without preferred orientation. A ^planar curved grain boundaries between strain-free grains. internal fabric (opaque grains) in aggregates oriented at a Younger shear planes and shear zones only locally affect large angle with respect to external S1 foliation planes S1 planes in the Sierra Grande Formation at the eastern supports this theory. Therefore, the growth of andalusite margin of the Laguna Medina granite. On the one hand, porphyroblasts probably was followed by shearing and quartz is displaced by such micro shear zones (brittle replacement by muscovite ^ sericite. deformation); on the other hand, it records internal bending, In the southern part of the Sierra Grande Formation at the kinking, and subgrain formation. In the adjacent granite, eastern granite contact, fibrolitic sillimanite needles occur in undulation, bending, and kinking of bigger magmatic quartz marginal parts and the rims of quartz grains (Fig. 13 and 14a). partly also affect the recrystallization grains. These are the Quartz growth proceeded from the more central parts of the effects of comparable younger micro shear zones cutting clastic grains toward the margins and hence enclosed the across the heated structures of the main foliation at small fibrolite. Toward the quartz boundaries, long axes of fibrolite angles. In such shear zones, ductilely deformed quartz is are bent parallel with the external foliation. In the matrix, almost entirely replaced by subgrains and recrystallization fibrolite is aligned in S1 planes with sericite and ^biotite grains with submicroscopical sizes. Sericite and small, plates. Therefore, sillimanite growth took place prior to and internally deformed muscovite are aligned. Small-scale S/C during shearing. Some radial fibrolite aggregates between fabrics indicate a top-to-W sense of shear. These details leads quartz grains, however, suggest that distinct layers were not to the conclusion that shearing continued locally along affected by the final shearing, because sillimanite generally bedding planes of the Sierra Grande Formation. The shear did not overgrow the foliation fabric. zones probably represent the youngest structures and indicate In a contact-metamorphosed silty sandstone at the successive deformational stages at decreasing temperatures. southern tip of the Sierra Grande Formation enclosed in the Laguna Medina granitoids, widely distributed 3.4.2.3. Interpretation. These observations suggest that sericite aggregates (,0.5–2 mm) with relicts of idioblastic shearing in the Sierra Grande Formation took place at shapes probably were andalusites. Microbiotite þ chlorite elevated temperatures. Contact metamorphism began prior .vnGsn/Junlo ot mrcnErhSine 5(02 591–623 (2002) 15 Sciences Earth American South of Journal / Gosen von W.

Fig. 13. Simplified schematic block diagram of the eastern and southeastern contacts of the Laguna Medina granite with the contact metamorphosed Sierra Grande Formation south of Sierra Grande. Inset symbol diagrams depict the features of growth, recrystallization, and transformation of minerals at different locations (see text for further explanations). 609 610 W. von Gosen / Journal of South American Earth Sciences 15 (2002) 591–623

Fig. 14. Photomicrographs from thin sections except (d); scale bars are 1 mm long. (a) Foliated and contact metamorphosed quartzite of the Sierra Grande Formation (above southeastern contact with Laguna Medina granite, S of Sierra Grande, crossed polarizers). Thin layers between quartz clasts consist of sericite and fibrolitic sillimanite, which is also enclosed in marginal parts of quartz (arrow). (b) Contact metamorphosed quartzite of the Sierra Grande Formation from dextral shear zone against the southeastern part of Laguna Medina granite (south of Sierra Grande; crossed polarizers). Pressure shadow at quartz clasts is filled with quartz and fibrolitic sillimanite (arrow) aligned in the shear zone (S1) foliation. (c) Mylonite of the Pen˜as Blancas granite with s shapes of pressure shadows at feldspar clasts, indicating a top-to-SW sense of shear (SW of Ea Pen˜as Blancas; crossed polarizers). (d) Intrusive contact between La Verde granite (top) and La Laguna granite mylonite (bottom) northwest of Ea. La Laguna. (e) Mylonite of porphyric La Laguna granite south of Ea. La Laguna (crossed polarizers). Quartz ribbons are filled with quartz recrystallization grains and in contrast with thin layers of submicroscopical feldspar recrystallization grains. In the center of photograph, both represent parts of s shaped pressure shadows at feldspar porphyroclasts (bottom left and right), indicating a top-to-SW sense of shear. (f) Slight foliation in shear zone within La Verde granite west of Ea. La Laguna (plane polarized light). Note single isolated clasts of quartz and feldspar in the foliation fabric. to D1 with the formation of andalusite and ? ^ cordierite. growth in a shear zone can be explained by its position near During an increase in temperature, sillimanite (fibrolite) the center of the pluton. High-temperature deformation in grew, and shearing with foliation development began. High- shear zones of the granite and granodiorite with feldspar temperature conditions with sillimanite growth during recrystallization (Fig. 13), which outlasted shearing under deformation prevailed in the southern and southeastern static conditions, supports this interpretation. Field obser- parts of the contact area. There, sillimanite (fibrolite) vations have furthermore shown that gradual transitions W. von Gosen / Journal of South American Earth Sciences 15 (2002) 591–623 611 exist from the shear zones into magmatic fabrics, with some 3.5.2. Pen˜as Blancas granite poor alignment of feldspar in between. In comparison with To the west of the Jagu¨elito Fault (Fig. 3), the red Pen˜as the related dextral shear zone at the eastern margin of the Blancas granite is cross-cut by several brittle shear planes. granite, which records dynamic quartz recrystallization, To the west of Ea. Pen˜as Blancas, the granite is foliated in there must have existed a notable temperature gradient. distinct strips (Fig. 16). At short distances, the hetero- Thus, the irregular heat transfer from the cooling pluton genously foliated granite grades into granite mylonites led to variable temperature conditions during D1, as is also (Fig. 17a) that contain thin ultramylonite layers (Giacosa, indicated by the difference between the eastern contact area 2001). (no sillimanite detected) and the southeastern to southern In the NW–SE-striking mylonite foliation (Smy), asym- parts of the contact (clear sillmanite and ? cordierite; cf. metric elongated quartz grains and quartz-filled pressure Fig. 13). This hypothesis is supported by ductile defor- shadows at feldspar clasts display s geometries. As single mation with feldspar recrystallization in the Laguna Medina S/C fabrics and shear bands, they indicate a reverse, ,SW- granite and granodiorite and at the contact with the Sierra directed uplift of the northeastern hanging wall block Grande Formation in the south, which demonstrates that parallel to the pronounced mylonite lineation (Lmy), as heat transfer from the pluton was greater in the south, shown by observations in thin sections (Fig. 14c). This southeast, and central areas. At the eastern margin of the confirms the results of Giacosa (2001) who also found a granite, however, brittle deformation of feldspar, ductile NE-directed displacement in a strip of mylonites, located in deformation and recrystallization of quartz, and the growth the southeast. of single porphyroblasts (? andalusite) are the effects of The lineation is indicated by aligned sericite and reduced heat input. muscovite, elongated quartz, and pressure shadows at The final stage of dextral strike–slip shearing at the feldspar clasts. In the mylonites, the reverse sense of shear eastern margin of the granite is indicated by breccias, (Fig. 16) partly changes to an oblique dextral sense. In the though in the south, dextral shearing in ductile shear zones foliation, isolated relicts of isoclinal folds with axes parallel has been outlasted by static growth of quartz and feldspar to Lmy occur. Continuous t NE–SW compression led to a without evidence of brittle deformation. The ,300 8C local, ,SW-vergent folding of the mylonites (Fmy2) that isograde, necessary for dynamic quartz recrystallization, combined with the formation of ductile shear zones must have shifted during the final stage of dextral strike– (Fig. 18). The La Laguna granite also has been affected by slip faulting away from the eastern contact and toward the mylonitic shearing in this area. more central parts of the cooling pluton. In the south, a On a microscale, the foliation in the granite and granite successive drop in temperature must have taken place at the mylonite is marked by newly grown sericite, muscovite, and end of D1. Sillimanite did not overgrow either S1 or F2 biotite that flow around feldspar clasts and elongated structures and is followed by the growth of muscovite plates magmatic quartz. Aligned and deformed magmatic biotite and sericite, which do not record an internal strain. is partly replaced by recrystallization grains. Biotite growth outlasted deformation under static conditions. Strips of 3.5. Granitoids west of Mina Gonzalito elongated magmatic quartz are almost entirely replaced by dynamically generated recrystallization grains that con- 3.5.1. Granite at the Jagu¨ielito Fault tinued growth post-Smy only in some part. Shear therefore The fine- to medium-grained granite east of the Jagu¨elito was partitioned between layers of static growth of Fault (Fig. 3) is characterized by a diffuse and hetero- recrystallization grains and those in which dynamic geneous foliation that records continuous transitions into recrystallization prevailed. magmatic fabrics or clearly foliated parts. The granite K-feldspar porphyroclasts (f , 1 cm) or K-feldspar- mylonites described by Giacosa (1997) could not be found. plagioclase aggregates are bent, kinked, and broken. Joints Overall, quartz and feldspar recrystallization in foliated and cracks between displaced and extended fragments are parts suggests that foliation development in the granite took filled with quartz and/or sericite and ^epidote. To a minor place during a high-temperature overprint. Exposures at extent in some parts of the mylonites, intracrystalline the Ruta Provincial 61 show that the granite intruded the deformation zones are successively replaced by small eastern basement schists, both of which were foliated and feldspar recrystallization grains that diffusely fill transition subsequently folded around open fold structures (Fig. 15). zones between slightly rotated feldspar pieces and pressure The main foliation in the biotite muscovite schists is at least shadows. the second and can be compared with the heterogenous S1 Increasing shear led to a dominant and penetrative planar foliation in the granite. Brittle dextral faults, cutting across Smy foliation that obliterated all relicts of magmatic fabrics the succession, are related to the Jagu¨elito Fault (Fig. 3), (Fig. 14c). It contains isolated K-feldspar and plagioclase which dextrally juxtaposed the basement against the granitic porphyroclasts or feldspar aggregates. Ultramylonite strips area to the west (Ramos and Corte´s, 1984). At the fault line consist almost entirely of a penetratively and intensely and related faults to the southwest, no evidence for ductile foliated matrix with a few small relicts of feldspar and shear could be detected. quartz porphyroclasts. The foliation is recorded by aligned 612 .vnGsn/Junlo ot mrcnErhSine 5(02 591–623 (2002) 15 Sciences Earth American South of Journal / Gosen von W.

Fig. 15. Sketch map of the heterogenously foliated granite in contact with biotite muscovite schists east of the Jagu¨elito fault, west of Mina Gonzalito (small outcrops on the Ruta Provincial No. 61, see Fig. 3 for location). Outcrops are shown by darker grey shade; the further areal extent of the units is inferred. Note that both the granite and schists are bent around open syn- and antiform structures with subvertical axes. The younger fault lines are brittle structures. W. von Gosen / Journal of South American Earth Sciences 15 (2002) 591–623 613

Fig. 16. Sketch map of mylonite strips in Pen˜as Blancas granite and La Laguna granite west of Ea. Pen˜as Blancas based on field mapping (for location, see Fig. 3). Arrows in lower hemisphere, equal area Hoeppener plots (Hoeppener, 1955) from different localities depict the relative sense of shear of the hanging wall along foliation planes.

microbiotite and ^sericite, along with extremely elongated NW–SE-striking, steeply northeast-dipping Smy foliation. magmatic quartz replaced by dynamically generated The sense of shear partly changes to oblique dextral and recrystallization grains. Among thin layers, small epidote dextral strike–slip within small zones of one outcrop or in grains are accumulated and partly replace feldspar. different outcrops. East of the village of Arroyo de Los In certain areas, the muscovite–sericite Smy matrix was Berros, the La Laguna granite records a subvertical S1 affected by crenulations and a local crenulation cleavage foliation and a dextral to oblique dextral sense of shear related to Fmy2 folds. The growth of sericite, muscovite, and parallel to the L1 lineation (Fig. 19a). Surrounding Ea. La microbiotite accompanied and outlasted cleavage Laguna, the mylonites depict a top-to-SW sense of shear formation. Quartz grains are elongated in axial planes and with an oblique dextral component (Fig. 19b and c). recrystallize dynamically. Therefore, the La Laguna granite mylonite structures and their sense of shear are comparable to those of the Pen˜as 3.5.3. La Laguna granite Blancas granite mylonite (cf. stereoplots in Fig. 16). In a wide area southwest of Ea Pen˜as Blancas (around Ea Only a few granite dykes record another sense of shear. La Laguna, Fig. 3), the porphyric La Laguna granite and Their mylonite foliation can be traced into that of the fine-grained granite dykes are affected by heterogeneous surrounding porphyric granite. Asymmetric feldspar s mylonitization (Fig. 17b). Deformation led to ductile and clasts and widely distributed S/C fabrics indicate a sinistral brittle deformation of quartz and feldspar, respectively. In sense of shear (Fig. 17b), which can be explained by the its eastern part, the fine-grained, light La Verde granite initial orientation of the dykes at a distinct angle to the finite intruded the La Laguna granite mylonite (Giacosa, 1997; orientation of the mylonite foliation. During mylonitic present Fig. 14d),whichisaffectedbycontactmetamorphism. shearing, the dykes were rotated and, by sinistral shearing, Asymmetric pressure shadows at feldspar porphyroclasts forced into the finite orientation of the Smy foliation. and lens-shaped polycrystalline quartz aggregates in thin On a microscale, porphyroclasts of magmatic K-feldspar section display s geometries. Along with S/C fabrics and a and plagioclase are elongated in the Smy foliation. At the few shear bands, they indicate a reverse sense of uplift of the intrusive contact of the La Verde granite, K-feldspar is northeastern block that is parallel to the Lmy lineation on the almost entirely recrystallized and displays granoblastic 614 W. von Gosen / Journal of South American Earth Sciences 15 (2002) 591–623

Fig. 17. Simplified, schematic, composite block profiles. (a) Deformed Pen˜as Blancas granite west of Ea. Pen˜as Blancas. Note heterogeneous foliation development within the granite. The top-to-SW sense of shear is indicated by asymmetric feldspar clasts and shear bands in intensely foliated and mylonitized slices of the granite. (b) Deformed La Laguna granite south of Ea. La Laguna (for location, see Fig. 3). Note heterogeneous foliation development and oblique dextral sense of shear. Dykes of fine-grained granites are foliated, boudinaged, and subsequently folded together with the mylonitized granite host. One granite dyke with an orientation oblique to the mylonite foliation is sinistrally sheared (see text for further explanations). textures. Quartz- or biotite-rich layers also record widely brittle deformation of feldspar. Other former intrafeldspar distributed recrystallization grains. In some bigger K- deformation zones were filled with new, small feldspar grains. feldspar porphyroclasts, however, quartz-filled extension Away from the intrusive contact, K-feldspar and joints indicate that mylonite formation took place during plagioclase porphyroclasts are partly bent and broken; W. von Gosen / Journal of South American Earth Sciences 15 (2002) 591–623 615

did biotite growth. Undulation, bending, and grain boundary migration of quartz are related to younger shear zones.

3.5.4. Metamorphism of the La Laguna and Pen˜as blancas granite mylonites In the La Laguna granite mylonite, the contrast between fine-grained feldspar recrystallization and coarse quartz recrystallization grains in elongated ribbons suggests that the T overprint syn- to post-Smy was in the range of the middle greenschist facies, because feldspar recrystallization begins in highly strained parts. Other than in the contact zone with the La Verde granite, thick layers of granoblastic feldspar recrystallization grains do not occur in the mylonites. In an outcrop west of Ea La Laguna, comparable heated quartz fabrics occur, together with minor evidence of feldspar recrystallization. This suggests that mylonite formation generally operated in comparable temperature conditions. One interpretation of the regional situation could be that the heat transfer necessary for mylonite formation was related to an intrusion in the subsurface. After the cessation of mylonitic shearing, heating from the La Verde granite pluton led to the static growth of former feldspar recrystallization grains near and at the intrusive contact. This final T overprint, however, must have had a clear gradient in a short distance. Fig. 18. Simplified block diagram of structural details from mylonitized In contrast, all microfabrics in the Pen˜as Blancas granite Pen˜as Blancas granite west of Ea. Pen˜as Blancas. The top-to-SW sense of suggest a slight greenschist facies metamorphic overprint shear is indicated by asymmetric feldspar clasts and S/C fabrics. In the during mylonite formation (see also Giacosa, 2001). foliation, isolated relicts of isoclinal folds have axes (Bmy) parallel to the Dynamic quartz recrystallization is widespread. Incipient mylonite lineation (Lmy). Continuing compression led to the formation of feldspar recrystallization and static grain growth of quartz ,SW-vergent fold structures with axes (B ) at great angles to the L my2 my only occur in the northwestern part of the granite and lineation. Fmy2 folding was combined with the formation of local ductile shear zones. suggest a local rise in temperature and strain rate. Greenschist facies temperature conditions (T $ 300 8C) submicroscopical recrystallization grains appear in inra- continued through Fmy2 folding of the mylonite foliation in crystalline shear planes. Extension joints are filled with both granites, which is related to continuous shear and not a quartz. In the matrix, thin strips parallel to recrystallized separate event. Transformation of biotite into chlorite quartz ribbons consist of submicroscopic feldspar, quartz probably took place after the cessation of shearing. recrystallization grains, and sericite. Thus, feldspar was Mylonite formation in both granites is assumed to have deformed in the brittle–ductile transition without coarse the same timing because the La Laguna granite occurs in the recrystallization grains (Fig. 14e). Epidote and muscovite þ Pen˜as Blancas granite and both were sheared together sericite are conversion products of plagioclase and are (Fig. 16). There was an interference between mylonite concentrated in thin layers parallel to Smy. shearing and heat transfer. T conditions sufficient for quartz Elongated quartz often recrystallized either at the recrystallization retreated from the Pen˜as Blancas granite margins or completely (Fig. 14e). Recrystallization stati- during mylonite formation (only dynamic quartz recrystal- cally outlasted Smy (strain-free grains with straight grain lization) but covered the La Laguna granite during and after boundaries). Thick quartz ribbons consist of a granoblastic shearing (static growth of quartz recrystallization grains). In mosaic of recrystallization grains. Magmatic biotite is the latter, temperatures during deformation permitted aligned parallel to Smy, and its recrystallization outlasted incipient feldspar recrystallization. Near and at the contact deformation. Muscovite grows parallel to Smy. Old grains with the La Verde granite, elevated temperatures led to are partly to entirely recrystallized. static growth. All this implies an important but variable heat As in the Pen˜as Blancas granite mylonite, the foliation in transfer with both a vertical and a horizontal gradient. It is platy mylonites is bent around Fmy2 folds (Fig. 17b) with reasonable to assume that a subsurface intrusion and the La related Smy2 crenulation cleavage planes. The latter are Verde granite pluton were the sources of heating during marked by aligned chlorite as a conversion product of mylonite formation and final static grain growth, biotite. Quartz recrystallization statically outlasted Dmy2,as respectively. 616 W. von Gosen / Journal of South American Earth Sciences 15 (2002) 591–623

Fig. 19. Lower hemisphere, equal area Hoeppener plots (Hoeppener, 1955) depicting the relative sense of shear of the hanging wall. (a), (b), and (c) are from La Laguna granite, and (d) is from La Verde granite. For locations, see Fig. 3, except for (a), which is from outcrops at the Ruta Provincial east of Arroyo de Los

Berros. Data in (a) represent S1 foliation and L1 lineation, mylonite foliation and lineation are depicted in (b) and (c), and measurements in (d) are from shear planes in young shear zones.

3.5.5. La Verde granite aggregates that grew at the expense of biotite. In the matrix, West of the intrusive contact with the La Laguna granite, angular clasts of single quartz, polycrystalline clasts of the La Verde granite contains dm-sized angular fragments of feldspar and quartz, and single feldspar occur (Fig. 14f). this granite that do not record significant ductile deformation. Quartz-rich clasts were affected by bending, kinking, Therefore, mylonite formation prior to the intrusion of the La subgrain formation, strain-induced boundary migration, Verde granite affected the La Laguna granite pluton only in and incipient dynamic recrystallization. They also record distinct parts. The occurrence of the foliated La Laguna beards of sericite and microbiotite in the foliation. In granite far to the west, at the Ruta Provincial 61 east of feldspar clasts, single epidote grains occur. The micro- Arroyo de Los Berros, furthermore shows that the La Verde fabrics suggest that shear zone deformation took place in T granite represents the youngest intrusion after the cessation conditions of the slight greenschist facies or greenschist of shearing and mylonite formation in the Pen˜as Blancas and facies–anchizone transition. La Laguna granite plutons. The La Verde granite is not foliated. At single outcrops, it is cross-cut by millimeters to decimeter thick distinct 4. Timing of magmatism, deformations, and shear zones (see also Giacosa (1997) and Busteros et al. metamorphism (1999)). In single, subvertical, ,NNW–SSE-trending shear zones, asymmetric s tails at feldspar clasts and S/C fabrics Because the heterogenously foliated granite intrusion display a sinistral to oblique sinistral sense of shear east of the Jagu¨elito Fault is different than the Pen˜as Blancas (Fig. 19d). Some of these shear zones were also found at granite or its mylonites, it is assumed to be part of the the intrusive contact and in the La Laguna granite mylonite. basement complex west of Mina Gonzalito. Assuming a On a microscale, the matrix in one shear zone of the Late Paleozoic age, the dominant deformation in the granite consists of dynamically generated quartz recrystal- Gonzalito block has the same age, though this is not lization grains with submicroscopical sizes. A poorly supported by additional data. defined foliation is depicted by aligned sericite and Because the Early–Mid-Ordovician Punta Sierra grani- (micro) biotite. Chlorite occurs as single grains or grain toids in the Sierra Grande area intruded the simply W. von Gosen / Journal of South American Earth Sciences 15 (2002) 591–623 617 deformed, low-grade metasediments of the El Jagu¨elito Laguna and Pen˜as Blancas granites is older. Its local and Formation, it is assumed that the clastic sedimentary pile is distinct shear zones should represent the latest stage of Late Proterozoic–Cambrian in age. This is supported by an the magmatic deformational cycle with a Permian to Rb–Sr whole rock date from equivalent phyllites in the Permotriassic age. Valcheta area (Nahuel Niyeu Formation, 600 ^ 25 Ma; Linares et al., 1990) and the age pattern of inherited zircons of the El Jagu¨elito Formation (Pankhurst et al., 2001). 5. Regional implications In that the Punta Sierra granitoids do not record structures of a ductile deformation and brittle deformation The El Jagu¨elito Formation low-grade rocks can be can be related directly to the deformation of the Sierra lithologically compared to the Nahuel Niyeu Formation in Grande Formation, D1 deformation and metamorphism in the Valcheta area and Ectinitas El Jagu¨elito west of Mina the El Jagu¨elito Formation (and equivalents) must have Gonzalito. According to the Early to Mid-Ordovician age of taken place prior to the Ordovician. On the basis of these the intrusions of the Punta Sierra granitoids (Varela et al., data and the timing of the Punta Sierra intrusions, a 1997, 1998), which are not ductilely deformed, simple Cambrian age is probable. However, the basement west of deformation and metamorphism could have taken place Mina Gonzalito might be Proterozoic in age, with a during the latest stage of the Pan-African orogeny in the metamorphic overprint in the Early Ordovician (Pankhurst Cambrian. et al., 2001). The lack of a ductile deformation and regional On the basis of published isotopic data and age metamorphism in the Ordovician intrusions suggests that interpretations and because ductile deformation at the the Sierra Grande area has not been affected by the contacts between the Sierra Grande Formation and Laguna Famatinian tectonic and metamorphic overprint during Medina granite and granodiorite, as well as within the the Ordovician–Devonian time interval. In contrast, in the magmatics, occurred during cooling of the intrusions, a Late Sierras Pampeanas of western Argentina, Famatinian Paleozoic (probably Permian) age of the compressive deformation and metamorphism is widely distributed. In deformation is reasonable. The entire deformation in the the southeastern part of the La Pampa Province, north of Sierra Grande Formation must have the same age because Patagonia, the Chadileuvu´ block also records evidence of its structures at the western intrusive contact are directly Famatinian deformation, metamorphism, and plutonism related to folding and faulting in the east and because the (Llambı´as et al., 1996; Tickyj et al., 1999a; Sato et al., youngest parts of the Sierra Grande Formation succession 2000), whereas in the Las Matras area to the northwest (Fig. are not preserved. Rapalini’s (1996, 1998) paleomagnetic 1), an undeformed Grenvillian intrusion has been reported studies point to a late Early to Late Permian age of by Sato et al. (1998–2000). deformation of the Sierra Grande Formation. The minimum Therefore, there seems to be a continuation of the age of deformation is given by an undeformed mafic dyke Famatinian deformational belt from the Sierras Pampeanas that cuts across the ductile fabrics in the southern part of the southward into the La Pampa Province (Tickyj et al., 1999a, Sierra Grande Formation (south of Sierra Grande) and is b; Sato et al., 2000). According to the present evidence, related to the Jurassic Marifı´l Formation. however, this belt of deformation does not traverse the The pre-Jurassic age of the deformation of the Pen˜as boundary to the northeastern segment of the North Blancas granite is evidenced by intrusions of undeformed Patagonian Massif in the south. Thus Late Paleozoic dykes of the Jurassic Marifı´l Formation. Assuming a tectonic displacements of different blocks in the boundary Permian age of the granite, a Permian to Permotriassic area between Patagonia and Gondwana South America age for ductile shearing and mylonite formation is cannot be excluded. reasonable (see also Giacosa, 2001). The occurrence of These interpretations are supported by the shallow the porphyric La Laguna granite, which was affected by marine elastic sediments of the Sierra Grande Formation, mylonitic shearing during the same deformational event in which lie in sedimentary contact above the El Jagu¨elito the northwestern strip of the Pen˜as Blancas granite mylonite Formation and Punta Sierra granite. The scenario suggests suggests that its deformation took place at the same time. intense uplift, erosion, and peneplanation prior to the onset Hence and contrary to Busteros et al. (1999), mylonite of Silurian deposition of clastic sediments. The time span of formation in the La Laguna granite is not older than that of this stage is unknown because the onset of sedimentation in the Pen˜as Blancas granite. Because no exposed contacts the Sierra Grande Formation is not indicated by fossils. between the granites were found, however, the relative age Compared with the stratigraphic column of the Sierras relationships of both intrusions are unclear. Australes (Fig. 20a), the Sierra Grande Formation probably The intrusive contact with the La Laguna granite mylonite represents an equivalent of the Silurian–Lower Devonian suggests that the La Verde granite is the youngest pluton in parts of the Ventana Group. Therefore, either one wide the Pailema´n granitoids west of Mina Gonzalito. The K–Ar basin or at least two basins existed during the Silurian– biotite age of 253 ^ 9 Ma (Linares, 1994 in Busteros et al., Lower Devonian time interval. Differences are indicated by 1999) could indicate that mylonite formation in the La the situation in the Ordovician, characterized by granitoid 618 W. von Gosen / Journal of South American Earth Sciences 15 (2002) 591–623

Fig. 20. (a) Geological sketch map of the Sierras Australes (Buenos Aires Province). (b) Sketch map of the areal distribution of metamorphism in the Sierras Australes. Maps adapted after von Gosen et al. (1990, 1991); for locations, see Fig. 1. intrusions with subsequent uplift and erosion in northeastern South of Rı´o Colorado, two drill holes record Permian Patagonia and clastic sedimentation in the Sierras Australes granites that might be part of the northern margin of the (Buggisch, 1987; von Gosen and Buggisch, 1988). North Patagonian Massif. The study by Fryklund et al. (1996) shows that the North and northwest of Sierra Grande, the Sierra Grande Permian to partly Triassic parts of the Sierras Australes Formation occurs in the area west of Valcheta (Chernicoff continue along-strike toward the southeast in the subsurface and Caminos, 1996) and at Gran Bajo del Gualicho, WNW of into the offshore areas and exist in the Clarameco´ basin to San Antonio Oeste (Lizuain Fuentes and Sepulveda, 1978; the northeast (Fig. 1). Their drill data also show that the Fig. 1 herein). In addition to small occurrences south of granitic basement of the Sierras Australes’ clastic sediments Sierra Grande, equivalents of the Sierra Grande Formation exists to the west of the mountain chain in the subsurface. are reported from the base of the drill hole ‘Penı´nsula de W. von Gosen / Journal of South American Earth Sciences 15 (2002) 591–623 619

Valde´s es-1’ by Marinelli and Franzin (1996) whereas in the studies show a syntectonic Permian magnetization of the Rawson basin to the east, conglomerates and breccias in the Sierra Grande Formation, in accordance with the results basal parts of the ‘Tayra x-1’ well are assigned to the pre-rift from the Sierras Australes. According to this interpretation, Paleozoic. however, deformation in the Sierra Grande took place after Since the pioneer work of Keidel (1916), the clastic those in other areas previously described. succession in the Sierras Australes has been compared to Compressive deformation in the Sierras Australes was that of the Cape Fold belt and the Ellsworth Mountains (e.g. accompanied and outlasted by anchizonal metamorphism in Buggisch, 1987). Similarly, the Sierra Grande Formation the east and greenschist facies metamorphism with static can be compared to equivalent succession on the Falkland annealing of quartz in the west (von Gosen et al., 1990, (Malvinas) Islands (Nullo et al., 1996). From Cape 1991; Fig. 20b herein). The gradient in temperature Belgrano, Cingolani and Varela (1976) describe the overprint cannot be related to regional metamorphism unconformable contact between the clastics (Puerto Stevens only. Radiometric data from the Lo´pez Lecube granite Formation), which are thought to be Silurian or Devonian in west of the Sierras Australes proper (227 ^ 32 Ma, Rb–Sr, age and the underlying granite of the Proterozoic basement whole rock and 240 ^ 12 Ma, K–Ar, hornblende) are (Cape Meredith Complex). reported by Lo´pez Gamundı´ et al., (1995) are related to a The NW–SE-trending mylonite zones west of Ea. Pen˜as possible syntectonic intrusion. The situation can be Blancas (Pen˜as Blancas granite mylonite) and La Laguna compared to those south of Sierra Grande, where cooling (La Laguna granite mylonite) suggest the existence of of the Laguna Medina granitoids interfered with compres- important ductile shear horizons in the northeastern part of sive deformation, and the variable heat transfer inferred the North Patagonian Massif (von Gosen, 2002). Thrusting for the granite mylonites west of the Jagu¨elito Fault. It is led to ,SW- and NE-directed transports of the hanging wall reasonable to assume that heat transfer from a synkinematic blocks with a dextral component. Whether the dextral Permian–Triassic pluton in the subsurface of the Sierras Jagu¨elito Fault west of Mina Gonzalito was active during Australes enabled ductile deformation and subsequent static this stage of compression is unclear. heating of the western parts of the deformed sedimentary Regionally, this is comparable with the formation of a pile. In conjunction with the situation in the Valcheta area granite mylonite in the Cerro de los Viejos of the La Pampa NW of Sierra Grande (von Gosen, 2002), this shows that Province (north of the inferred boundary of extra-Andean compressive deformation across the northern boundary of Patagonia), which is described by Tickyj and Llambı´as Patagonia interfered with intrusive activity. (1994) and Tickyj et al. (1997, 1999a). Thrusting along the The magmatics in northern Patagonia can be related to NW–SE-trending mylonite zone led to NE-directed trans- either E-directed subduction along the Pacific margin in the ports. It has been assigned to the Carboniferous–Permian west (e.g. Davidson et al., 1987; Uliana and Biddle, 1987; boundary (Tickyj et al., 1999a) and interpreted as the result Herve´, 1988; Cingolani et al., 1991)orSW-directed of a NE-directed push (or collision) of Patagonia (Tickyj subduction beneath the northern Patagonia margin et al., 1997). The ,NE–SW compression also has been (Ramos, 1984, 1986). The first interpretation implies an estimated for the deformation in the Sierras Australes fold- autochthonous position of Patagonia. Arc plutonism far and-thrust bell of the Buenos Aires Province (Selle´s to the east of the trench would be related to flat slab Martı´nez, 1986; von Gosen et al., 1990). An estimated subduction. This model is supported by the results of ,NE–SW compression during the Late Paleozoic (prob- paleomagnetic studies of Rapalini (1998), which indicate ably Permian) acted over a wide area across the boundary that Patagonia did not undergo important latitudinal between Patagonia and Gondwana South America (Giacosa, displacements relative to South America since Devonian 2001). times. Gondwanide deformation then must be explained by In the Sierra Grande area, however, Late Paleozoic intraplate compression (Dalla Salda et al., 1990; Rossello deformation was the result of ,NW–SE to W–E et al., 1997), and the formation of a compressional back-arc compression. This part of the North Patagonian Massif basin, related to a wide, Andean-type active margin (Trouw probably was deformed during a second stage of com- and De Wit, 1999), would be a possibility. pression after that which occurred in the other areas. This In general, compression in this northeastern segment of interpretation is supported by several stages of compressive the North Patagonian Massif can be regarded as part of the deformation in the area west of Valcheta (von Gosen, 2002). Gondwanide deformation in the ‘Samfrau Orogenic Zone’ The results of paleomagnetic studies in the northeastern of Du Toit (1937), which is also recorded, for example, in and uppermost sedimentary unit (Tunas Formation) of the the Cape Fold Belt (So¨hnge and Ha¨lbich, 1983) and the Sierras Australes (Tomezzoli and Vilas, 1999; Tomezzoli, Falkland (Malvinas) Islands (Marshall, 1994; Curtis and 2001) show that magnetization was acquired before or early Hyam, 1998), and probably continues in the Ellsworth in the deformation and between the late Early Permian and Mountains (Dalziel et al., 1987; Curtis, 1998) and Pensacola the early Late Permian. There, deformation was diachron- Mountains (Ford, 1972) of Antarctica. The entire dimen- ous and propagated toward the foreland (Tomezzoli, 2001). sions of this Gondwanide fold Belt do not support its In the Sierra Grande area, Rapalini’s (1998) paleomagnetic interpretation as a local intraplate compression zone. Thus, 620 W. von Gosen / Journal of South American Earth Sciences 15 (2002) 591–623 extra-Andean Patagonia can be interpreted as part of a plate Table 1 that collided with Gondwana South America during the Late Simplified interpretative chart of events in the areas of Sierra Grande and Paleozoic (Permian). Within the Permian sediments of west of Mina Gonzalito (W of Jagu¨elito Fault) based on publications cited in the text and personal investigations the Sierras Australes, Cape Fold Belt, and Ellsworth Mountains, the initial cralon-derived clastic debris changed to arc/orogen-derived material, along with deposition of tuffs, thereby indicating the evolution of foreland basins (Collinson, 1991; Lo´pez Gamundı´ et al., 1995; Lo´pez Gamundı´ and Rossello, 1998). This suggests that the Sierras Australes and Cape Fold Belt were bordered to the south by an active plate margin (Lo´pez Gamundı´ and Rossello, 1998). Similar to the deformation in the Sierras Australes after SW-directed subduction beneath northern Patagonia (Ramos, 1984, 1986), the formation of the Cape Fold Belt may be the result of collision after a S- (De La Winter, 1984) or N- (Johnson, 1990) directed subduction. Due to widely distributed younger cover sediments at the northern margin of Patagonia, however, no ophiolites and/or ocean floor sediments in the inferred suture zone could have been detected. The relationships between the Late Paleozoic– Triassic intrusions in northeast Patagonia and a flat slab subduction along the western Patagonia margin are regarded as inconsistent with the distance of more than 1000 km to the trench and the calc-alkaline magmatism plus orogenic activity that is oblique to the Andean trend (Ramos, 1986). In the reconstruction and interpretation of Dalziel and Grunow (1992), the Gondwanide orogeny may have been caused by an arc–continent collision. They explain the collision of their Patagonia fore arc/arc terrane by the closure of a marginal basin mainly floored by stretched continental crust. Such an interpretation could imply that Patagonia detached from the SW Gondwana margin and subsequently collided during the Gondwanide orogeny. This might explain the deposition of the Sierra Grande Formation and its equivalents outside Patagonia, as well as the collisional are unconformably covered by Silurian–Lower Devo- features in northern Patagonian and across its northern nian clastics of the Sierra Grande Formation, which boundary. In such an interpretation, it remains unclear shows that the Early Paleozoic Famatinian deformation whether Patagonia was a far traveled terrane, as depicted by of western Argentina did not affect this sector of the Keppie and Ramos (1999), Figs. 7–9). North Patagonian Massif. Among others, the Sierra Grande Formation can be compared to equivalents in the Sierras Australes north of Patagonia, the Cape Fold 6. Conclusions Belt of South Africa, and the Falkland (Malvinas) Islands. The studied structures in the northeastern segment of the 3. The ,NW–SE to W–E compression of the Sierra North Patagonian Massif lead to the separation of several Grande Formation led to the formation of open fold stages of evolution as follows (schematically summarized in structures with high-angle reverse faults and sinistral Table 1): strike–slip displacements. Preexisting structures in the underlying El Jagu¨elito Formation phyllites controlled 1. The phyllitic succession of the El Jagu¨elito Formation is the developing geometries of structures in the cover estimated to be Late Precambrian–Cambrian. After sediments. simple deformation and metamorphism, it was intruded 4. Compressive deformation interfered with the cooling of by Ordovician granitoids in the Sierra Grande area that the Laguna Medina granitoids south of Sierra Grande are not ductilely deformed. and is assigned to the Late Paleozoic (probably 2. The Ordovician granitoids and El Jagu¨elito Formation Permian) interval. A comparable mechanism is assumed W. von Gosen / Journal of South American Earth Sciences 15 (2002) 591–623 621

for metamorphism in the Sierras Australes fold-and- Caminos, R., 1999. Hoja Geolo´gica Valcheta 4166-1 (1: 250000). Proyecto thrust belt north of Patagonia. Minero Rı´o Negro (Viedma). 5. The ,NE–SW compression in the area west of Mina Caminos, R., Llambı´as, E.J., 1984. El basamento cristalino. 9 Congreso Geolo´gico Argentino (San Carlos de Bariloche), Relatorio (Geologı´ay Gonzalito led to the formation of mylonites in the Pen˜m Recursos Naturales de la Provincia de Rı´o Negro) 1 (2), 37–63. Blancas and La Laguna granites. Ductile deformation is Caminos, R., Llambı´as, E.J., Rapela, C.W., Parica, C.A., 1988. Late suggested as Permian in age, followed by the intrusion Paleozoic–Early Triassic magmatic activity of Argentina and the of the La Verde granite. significance of new Rb–Sr ages from northern Patagonia. Journal of South American Earth Sciences 1 (2), 137–145. On a regional scale, ductile deformation of the granites is Caminos, R., Chernicoff, C., Varela, R., 1994. Evolucion tecto´nico- metamo´rfica y edad del Complejo Yaminue´, Basamento pre-andino comparable to that described for the granite of Cerro de Los norpatago´nico, Repu´blica Argentina. 7 Congreso Geolo´gico Chileno Viejos (La Pampa Province) and the Sierras Australes (Universidad de Concepcio´n), Actas II, 1301–1305. (Buenos Aires Province), both north of the boundary Castellaro, H.A., 1966. Guı´a Paleontolo´gica Argentina, Parte I: Paleozoico. between Patagonia and Gondwana South America. This Seccio´n III—Faunas Silu´ricas. Seccio´n IV—Faunas Devo´nicas. Pub- suggests that across the boundary, intense ,NE–SW licacio´n del Consejo Nacional de Investigaciones Cientı´ficas y compression took place during the same Gondwanide Tecnicas, 1–164. Chernicoff, C.J., Caminos, R., 1996. Estructura y relaciones estratigra´ficas period of time, which would indicate that separated extra- de la Fomacio´n Naheul Niyeu, Macizo Nordpatago´nico oriental, Andean Patagonia collided with Gondwana South America. Provincia de Rı´o Negro. Revista de la Asociacio´n Geolo´gica Argentina Deformation in the Sierra Grande area with ,NW–SE to 51 (3), 201–212. W–E compression is interpreted as a second-stage event Cingolani, C.A., Varela, R., 1976. Investigaciones geolo´gicas y geochro- during the Gondwanide deformational and magmatic nolo´gicas en el extremo sur de la Isla Gran Malvina, sector de Cabo Belgrano (Cabo Meredith), Islas Malvinas. VI. Congreso Geolo´gico history. Argentino, Actas I, 457–473. Cingolani, C., Dalla Salda, L., Herve´, F., Munizaga, F., Pankhurst, R.J., Parada, M.A., Rapela, C.W., 1991. The magmatic evolution of northern Acknowledgements Patagonia; new impressions of pre-Andean and Andean tectonics. Geological Society of America Special Paper 265, 29–44. I am grateful to E. Llambı´as, A. Sato. P. Gonza´lez (La Collinson, J.W., 1991. The palaeo-Pacific margin as seen from East Antarctica. In: Thomson, M.R.A., Crame, J.A., Thomson, J.W. (Eds.), Plata), and C. Prozzi (Bahı´a Blanca) for all the help, Geological Evolution of Antarctica, Cambridge University Press, support, and discussions during field work in Argentina. Cambridge, pp. 199–204. Many thanks to J. Ranalli (La Plata/Mendoza) for help and Corte´s, J.M., 1978. Primeros afloramientos de la Formacio´n Sierra Grande discussions during field work in Patagonia. Discussions with en la provincia del Chubut. 7 Congreso Geolo´gico Argentino, Actas 1, V. Ramos (Buenos Aires), C. Cingolani, L. Spalletti, and 481–487. Corte´s, J.M., 1981. El substrato precretacio del extremo noreste de la R. Varela (La Plata) are gratefully acknowledged. Thanks provincia de Chubut. Revista de la Asociacio´n Geolo´gica Argentina 36 also to E. Bouhier (Direccio´n de Minas, San Antonio (3), 217–235. Oeste/Rı´o Negro) for support during field work. I am Criado Roque, P., Iba´n˜ez, G., 1979. Provincia geolo´gica sanrafaelino- grateful to both reviewers, N. Froitzheim (Bonn) and pampeana. Segundo Sı´mposio de Geologı´a Regional Argentina V. Ramos (Buenos Aires), for constructive comments and (Academia Nacional de Ciencias, Co´rdoba) I, 837–869. suggestions that helped improve the manuscript. A car was Cuerda, A.J., Baldis, B.A., 1971. Silu´rico-Devo´nico de la Argentina. Ameghiniana 8 (2), 128–162. provided by the Centro de Investigaciones Geolo´gicas (CIG, Curtis, M.L., 1998. Development of kinematic partitioning within a pure- La Plata). Grants from the German Research Foundation shear dominated dextral transpression zone: the southern Ellsworth (DFG, Proj. Go 405/4-1 and 4-2) enabled this study to be Mountains, Antarctica. Geological Society of London Special Publi- carried out. The support of both institutions is gratefully cation 135, 289–306. acknowledged. 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