MAGMATISMO GONDWANICO EN LA COMARCA NORDPATAGONICA, CORDILLERA NEUQUINA Y SUR DE CHILE |
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EL MAGMATISMO CARBONICO EN EL SUR DE CHILE |
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Tomado de Katja Deckart, Francisco Hervé, C. Mark Fannin3, Valeria Ramírez, Mauricio Calderón, Estanislao Godoy, 2014. U-Pb Geochronology and Hf-O Isotopes of zircons from the Pennsylvanian Coastal Batholith, South-Central Chile. Andean Geology 41 (1): 49-82. The Chilean Coastal Cordillera (33°-40°S) comprises a distinctive lithotectonic association representative of subduction zone processes. Critically important indicators of subduction are the late Paleozoic accretionary complex and the calc-alkaline Coastal Batholith. Although their origin within a subduction setting as exemplified for the Neogene Chilean active margin (e.g., Barazangi and Isacks, 1976; Hager and O’Connell, 1978; Jordan et al., 1983; Cahill and Isacks, 1992) seems straightforward (Hervé et al., 1988; Hervé et al., 2007a; Parada et al., 2007), the exact timing, nature and geodynamic significance of the Coastal Batholith in south-central Chile warrants further detailed investigations. The Coastal Batholith is a large and elongate composite intrusive body within slightly older metamorphic rocks of the accretionary prism (Hervé et al., 2013). The sedimentary protolith of these country rocks was deposited and then shortly after buried to a depth of about 8 to 10 km (at about 3 kbar; Willner et al., 2005) where it was intruded by quartzdioritic to granitic magmas. Trench-ward shift of arc magmatism from an easterly location apparently occurred as Mississippian (early Carboniferous) detrital zircons are present in the sedimentary rocks of the accretionary prism (Hervé et al., 2013). This shift may be explained by global tectonic plate reorganization, or tectonic processes such as the steepening of the subducted oceanic slab due to subduction of older and colder oceanic lithosphere. Other possible geodynamic scenarios (e.g., subduction rollback), however, are explored. In this study we report new SHRIMP U-Pb
zircon ages on distinct portions of the largely
north-south elongated Paleozoic Coastal Batholith
and the first Hf-O isotope data for this batholith.
2. Geologic Background
For the same dated samples 87Sr/86Sr whole rock initial ratios range between 0.70582 and 0.70605. Earlier, Cordani et al. (1976) presented a late Paleozoic age of 319±17 Ma (K-Ar recalculated) and a Juarssic age of 169±12 Ma (K-Ar recalculated) at Quintay. The Paleozoic sample yields an initial 87Sr/86Sr isotope ratio of 0.7132, the Jurassic sample is less radiogenic with an initial ratio of 0.7038. Two more Paleozoic K-Ar ages are presented in Hervé et al. (1988) for Algarrobo intrusives on biotite and hornblende yielding 287±7 Ma and 296±7 Ma, respectively. At Quintay and Reñaca, two further Jurassic ages on biotite are published (Hervé et al., 1988), at 159±4 Ma and 156±1 Ma, respectively. An additional whole rock Rb-Sr age was likewise obtained yielding 167±14 Ma with an initial 87Sr/86Sr whole rock isotope ratio of 0.70412. Furthermore, Gana et al. (1996) and Wall et al. (1996) published for the coastal area between Valparaíso and Cartagena several Middle to Late Jurassic K-Ar mineral ages on plutonic rock units. The Jurassic ages were attributed to thermal rejuvenation in rocks associated to the intrusion of Jurassic bodies. A U-Pb zircon crystallization age of 299±10 Ma for the Pomaire pluton further east (Gana and Tosdal, 1996), inland of Quintay, confirms the strong presence of Carboniferous (Pennsylvanian) rocks in this area (Cochoa Unit after Rivano et al. (1993). Earlier, Godoy and Loske (1988) published two U-Pb dates on zircon fractions in two distinct localities at Quintay; dates are 290 Ma (tonalite) and 309 Ma (gneissic granite). Additionally, Gana and Tosdal (1996) obtained a U-Pb zircon age of 214±1 Ma (Late Triassic) from the Cartagena gneissic diorite unit at Punta Suspiro, directly north of San Antonio harbor, about 25 km south of Algarrobo. Hervé et al. (1988) published a late Carboniferous, Pennsylvanian age for a coastal Santo Domingo granite sample, some 35 km south of Algarrobo. The Rb-Sr age is 308±15 Ma, with a whole rock initial 87Sr/86Sr isotope ratio of 0.70582. Further to the south, ca. 35 km northeast from the city of Concepción at the Dichato locality, a K-Ar biotite age of 306±6 Ma was published (Hervé et al., 1988). The same age (Rb-Sr isochron of 306±6 Ma) was obtained by Lucassen et al. (2004) in a diorite from the Cantera Giacomo, north of Concepción. Additionally at Concepción a biotite K-Ar age of 215±4 Ma (Triassic; Hervé et al., 1988) was indicated 85 km southwest of the city of Los Angeles and 70 km from the coastline at the Traiguén locality, a biotite K-Ar age of 298±4 Ma was published (Hervé et al., 1988). Furthermore, in the same area, the Nahuelbuta Mountains National Park, Hervé et al. (1988) obtained a biotite K-Ar age of 284±5 Ma. Glodny et al. (2008) present two-point Rb-Sr ages of 286.3±4.2 Ma (biotite-feldspar age) and 306.8±4.5 Ma (feldspar-muscovite age) on two intrusive igneous rocks of the same Nahuelbuta National Park region. Initial 87Sr/86Sr ratios (ranging between 0.7057 and 0.7098) and eNd values (between -2 and -4) were obtained for granitoids and enclaves in the Santo Domingo Complex (Parada et al., 1999). Similar enriched isotopic compositions were determined for granitoids of ca. 294 Ma (Hervé et al., 1988) from Nahuelbuta with initial 87Sr/86Sr ratios varying in the range of 0.705 to 0.715 and eNd values in the range of -2.5 and -7.5 (Lucassen et al., 2004). These results indicate a mixing of at least two isotopically different lithospheric sources, with crust similar in composition to the exposed metasedimentary host rocks (Lucassen et al., 2004). In the high Andes of Central Chile, Pineda and Calderón (2008) established through U-Pb zircon data a Carboniferous age of 328.1±2.8 Ma for a biotite-muscovite granite (Los Molles granite) and a Permian age of 294.2±2.3 Ma for a hornblendebiotite tonalite (El Pangue Tonalite), which belongs to the Elqui Limari Batholith (30-31ºS). Willner et al. (2008) presented U-Pb zircon ages of 326±12 Ma and 294±4 Ma, Carboniferous and early Permian times, for two leucogranite pebble ages from the scarcely developed coastal accretionary prism between 31°-32°S. These authors indicate that initial eHf signatures on the same zircons range between +3.6 to -3.1, respectively. The crustal residence times are of 1.3-0.97 Ga, Meso- to Neoproterozoic, with slightly older model ages for the somewhat younger leucogranite samples. Furthermore, they suggest that Carboniferous samples are either the result of mixing of juvenile, mantle-derived magma with older crust or of recycling of crustal material distinct from the older magmatic products of the coastal accretionary system. It is noteworthy that Martin et al. (1999b) dated late Paleozoic to Early Jurassic intrusive, volcanic and sedimentary rocks in the El Indio region close to the Argentinean border at 29-30°S. They interpreted their occurrence to be related to extensional processes that followed the cessation of Carboniferous to early Permian subduction along the western edge of Gondwana. |
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On the basis of our new data, the Pennsylvanian Coastal Batholith in south-central Chile is shown to have been emplaced in a very short period of time (20 Ma) and to be relatively homogeneous in crystallization age and both Hf and O isotopic compositions along 800 km length, regardless of rock types which vary from quartz diorite to granite. After a 150 Ma magmatic lull, Jurassic quartziferous gabbros of much more primitive isotopic composition were emplaced through the Paleozoic rocks of the Coastal Batholith in the northern study area. This evolution is very different to what is observed in the Mesozoic to Cenozoic batholiths in northern Chile (Dallmeyer et al., 1996) and in the South Patagonian batholith (Hervé et al., 2007b) which are typical Andean batholiths, the last with large age range -spanning from 155 to 5 Ma- and greater variation in Hf and O isotopic compositions (Fanning et al., 2010). This suggests a relative homogeneous source with a significant continental crust component for these magmas. The two stage Depleted Mantle model age calculations all yield Mesoproterozoic ages which indicate that the source(s) for the magmas from which the zircon crystallized have had a significant residence time in the crust. If this was the case, then the magmatic arc would have been formed on continental crust (d18O >5.3±0.6; Fig. 6), beyond the backstop of the accretionary complex to the west. This suggests that the Eastern Series, which the batholiths intrudes, was deposited over this continental crust or either has been tectonically transported from the west to its present position over the western edge of continental crust. Alternatively, part or all of the melting crustal material could correspond to sediments transported deep into mantle wedge by the subducted crust, as modeled by Gerya and Meilick (2011). The fast extinction of the arc, which appears to have lasted less than 20 Ma, was probably the consequence of changes in the subduction parameters, which displaced the melting conditions of the underlying crust from the site of the Coastal Batholith to the east. These changes could be related with the contraction of the Rheic Ocean that culminated with the collision of NW Gondwana and south-eastern Laurentia at ca. 320 Ma (McElhinny et al., 2003). It is interesting to consider that during the span in which the southcentral Coastal Batolith was emplaced Gondwana remained in a relative stationary position (Oviedo and Vilas, 1984) in contrast with fast movements and rotations before and after that time (Geuna et al., 2010). The Coastal Batholith was eroded to a similar level as the present one, before deposition of the Late Triassic (Norian) sedimentary rocks (Nielsen, 2005). Surprisingly, detrital zircon derived from the batholith are not found as constituents of the Permian accretionary prism which constitute the Western Series south of 38°S (Hervé et al., 2013), a probable indication that significant portions of the accretionary complex do not crop out. It is interesting to note that the evolution of the accretionary prism continued until the Triassic, when Late Triassic plutonic bodies beside some locally restricted post-subduction A-type granitoids intruded into the Western Series (Vásquez and Franz, 2008).
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EL MAGMATISMO CARBONICO DE LA CORDILLERA NEUQUINA Y DE LA COMARCA NORDPATAGONICA Tomado de: R.J. Pankhurst a, C.W. Rapela, C.M. Fanning, M. Márquez, 2006. Gondwanide continental collision and the origin of Patagonia. Earth-Science Reviews 76: 235–257. I-type granitoids of Carboniferous to Permian age form the core of the Coastal Cordillera of Chile from 33° to 38°S where they are seen in the Cordillera de Nahuelbuta (Fig. 1). Southeasterly extension of this belt and its metamorphosed envelope has been suggested, as far as the Piedra Santa complex near the northwestern boundary of the North Patagonian Massif (Franzese, 1995), but no supporting geochronology or geochemistry are available and until now it could not be traced any further. However, our results demonstrate the existence of a 120-km-long belt of Early Carboniferous I-type granodiorites in the eastern North Patagonian Massif, along the Cordón del Serrucho, between San Carlos de Bariloche and El Maitén (41–42°15′S) (Fig. 2). Two samples yield well defined Early Carboniferous (~Visean) crystallization ages of 323±3 and 330±4Ma; a sample of the El Platero tonalite in the Río Chico, about 50 km to the southeast, gave a comparable age of 329±4Ma (Fig. 6a). A two-point conventional 238U–206Pb zircon age of 321±2Ma has been independently reported for a sample from the same locality as MOS-043 (Varela et al., 2005). All three samples analysed, which are representative of continuous outcrop, are foliated metaluminous hornblende-biotite granitoids with low abundances of lithophile trace elements, rare earth element (REE) patterns typical of Andino-type calcalkaline arc rocks, positive εNdt values (+0.1 to +2.8), and low initial 87Sr/86Sr (0.7034–0.7046) indicating a long-term light-REE depleted source such as the upper mantle (Table 1). Other authors have previously suggested Carboniferous magmatism in the western areas of the North Patagonian Massif, but mostly on the basis of imprecise Rb–Sr whole-rock errorchrons or K–Ar geochronological data (see Varela et al., 2005 for a summary). Two previously undated granite bodies in the southwestern North Patagonian Massif have yielded Mid Carboniferous (Serpuhkovian/Bashkirian) crystallization ages: 314±2Ma from Paso del Sapo and 318± 2Ma from Sierra de Pichiñanes (Figs. 2 and 6b). These are both peraluminous S-type garnet-bearing leucogranites with high SiO2 contents, strongly depleted heavy- REE patterns, high K2O/Na2O ratios, unradiogenic εNdt values (−5.0 to −6.0 on four samples) and relatively high initial 87Sr/86Sr (0.7078–0.7098) indicative of generation by upper crustal melting (Table 2). They have multistage Sm–Nd model ages of 1500Ma that could represent the age of their deep crustal source region. These results show that in the southwestern part of the North Patagonian Massif a short-lived episode of subduction was followed by crustal anatexis during the Mid Carboniferous. The Cordón del Serrucho runs approximately N–S (Fig. 2), but the equivalence of the El Platero tonalite suggests that the arc was orientated more NW–SE, and the S-type granites occur somewhat to the northeast of this trend line. Although the wider distribution of I- and S-type granites in Patagonia is not clear, and the lack of outcrops may unfortunately prevent further elucidation, both are typically emplaced in the overriding plate above the subduction zone before and during plate collision, respectively, and their disposition thus strongly suggests subduction of an oceanic plate from the southwest. |
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MAGMATISMO PERMICO DE LA COMARCA NORDPATAGONICA |
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The North Patagonian Massif is characterized by large outcrops of Permian granitic rocks that was divided by Gregori et al. (2020) in three extensive magmatic belts. Their origin and evolution have been debated for a long time and there is still no consensus on these issues. The northeastern Gondwana magmatic belt (Fig. 1) strikes N300°-N340° for more than 200 km between the Atlantic coast and the Estación Falkner locality (Río Negro Province). The northwestern Gondwana magmatic belt (Fig. 1) strikes N70°-N93° for more than 300 km between Arroyo Pichi Leufu and the Planicie Baja area (Río Negro Province). The southwestern Gondwana magmatic belt (Fig. 1) of the North Patagonian Massif strikes N340°-N310° for more than 250 km from the head of the Arroyo Pichi Leufu to Sierra de los Pichiñanes (Chubut Province).
Geological map showing the configuration of the northern Patagonia and surrounding areas.
Several E–W and NW–SE-oriented structures at the latitude of the Río Colorado-Río Negro define the The supposed borders of the three Gondwana magmatic belts are indicated in yellow (Gregori et al. (2020)
The northeastern Gondwana magmatic belt The northeastern Gondwana magmatic belt strikes N300°-N340° for more than 200 km between the Atlantic coast and the Estación Falkner locality (Río Negro Province). Its NW part reaches 100 km at its widest point. One important part of the belt is covered by Jurassic rhyolitic ignimbrites and Tertiary basaltic rocks, so the real amount of granitic rocks is unknown. The outcrops are small and isolated, but several units were studied petrologically, geochemically and geochronologically. Undeformed and solid-state deformed bodies are observed. Las areas 1 a 7 de la figura de abajo se ubican en esta faja.
Los cuerpos intrusivos de las areas 1, 2, 3, 4 y 5 fueron agrupados bajo el nombre de Complejo Plutónico Pailemán INFORMACION GENERAL SOBRE EL COMPLEJO PLUTONICO PAILEMAN:
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1 Sierra Grande: Plutón Laguna Medina
La Granodiorita Laguna Medina se caracteriza por su homogeneidad litol ógica, lo que está en parte condicionado por la escasez y discontinuidad de afloramientos. Se trata de granodioritas con biotita, anfíbol y titanita, que exhiben xenolitos de esquistos cuarzo-micáceos, posiblemente pertenecientes a su roca de caja no expuesta. El muestreo de ésta unidad estuvo restringido a lo largo del tendido de un gasoducto al oeste de Laguna Medina y algunos morros aislados en los sedimentos de la laguna. En ambos cuerpos las texturas granosas magmáticas primarias están mejor preservadas en sectores alejados de las fajas de cizalla. En las zonas de mayor deformación el cuarzo presenta evidencias de deformación subsólida de alta temperatura, como microestructuras con patrón en tablero de ajedrez. Otras evidencias de deformación intracristalina son extinción ondulosa, láminas de deformación y bordes irregulares a lobulados a partir de procesos de migración en los bordes. La deformación intracristalina en feldespatos se evidencia en plagioclasas por maclas de albita con combamientos o escalonamientos y en microclino por maclas en enrejado distorsionadas. Además los feldespatos presentan reemplazo parcial por sericita-muscovita, carbonatos y prehnita. La biotita se exhibe combada, con extinción ondulosa y bordes desflecados, llegando en las fajas de cizalla a un completo reemplazo por clorita y carbonatos "tipo sandwich", agregados de epidoto pulverulento y quistes prehníticos, que indican metamorfismo en facies esquistos verdes. En cuanto al anfíbol varía entre fresco y casi totalmente reemplazado por una asociación de clorita + carbonatos + prehnita + albita (¿). Minerales accesorios comunes son titanita, apatita, circón, allanita y opacos. GEOLOGÍA Y GEOCRONOLOGÍA Rb-Sr DE GRANITOIDES DE SIERRA GRANDE, PROVINCIA DE RÍO NEGRO |
2 Los Berros-Peñas Blancas: Plutón La Verde, Plutón Peñas Blanca, Plutón La Laguna
El Plutón La Verde fue agrupado por Giacosa (1993) como integrante del Complejo Plutónico Pailemán del permo-triásico. Constituye un cuerpo ovalado cuyo eje se prolonga en dirección N-S. Presenta sus mayores afloramientos al este de Los Berros mientras que en su sector meridional, cerca de Arroyo Ventana, son más escasos. Hacia el Norte está cubierto por flujos ignimbríticos de la Formación Marifil y por basaltos de la Formación Somún Curá (Busteros et al . 1998). Posee numerosos tabiques y pendants de sus rocas de caja, como las milonitas porfiroclásticas graníticas de La Laguna y las metamorfitas del Complejo Mina Gonzalito (Giacosa1993). Cabe destacar la presencia de inclusiones de un granito porfírico no deformado, que posiblemente sea el protolito de las milonitas del plutón La Laguna. La facies más abundante del plutón La Verde es una granodiorita gris oscuro, biotítica-horblendífera a horblendífera-biotítica, de textura equigranular con variedades porfíricas en el borde de contacto con la caja. Este cuerpo carece en general de deformación pervasiva, por lo que ha sido agrupado dentro de los granitos no foliados del Complejo Plutónico Pailemán (Grecco y Grégori 2011), aunque en algunos sectores exhibe fajas de cizalla discretas tipo S-C (Busteros et al
1998). Además presenta numerosos enclaves microgranulares máficos
gabro-dioríticos. La granodiorita está intruida por una facies posterior microgranítica biotítica rosada. La granodiorita está compuesta por cuarzo, plagioclasa (andesina An30) y ortosa y microclino. Entre los minerales máficos se destacan biotita y hornblenda. Desde el punto de vista mineralógico las rocas se ubican dentro de los granitos calcoalcalinos de arco o ACG de Barbarin (1999), y granitos Tipo I asociados a subducción de Chappelly White (1974).
Las facies previamente mencionadas se hacen evidentes en el diagrama de clasificación normativo Ab-An-Or (O'Connor 1965, modificado por Barker 1979) donde a partir de datos geoquímicos de elementos mayoritarios las muestras definen estos dos grupos distintivos los granitos que poseen composiciones alcalino-cálcicas y las granodioritas El ASI (Aluminium Saturation Index) de Shand(1943) para las granodioritas varían entre 0,93 y 1,04 y para los granitos entre 1,00 y 1,18. Las granodioritas y granitos menos evolucionados son metaluminosos, mientras que los granitos más diferenciados son peraluminosos.
Las granodioritas poseen una relación (La/Yb) indicando mayor fraccionamiento en las rocas más evolucionadas.
En el diagrama de discriminación tectónica Y vs Nb (Pearce et al 1984) las muestras plotean dentro del campo de arco volcánico y granitos sin-colisionales. Se practicaron análisis U/Pb SHRIMP en circones pertenecientes a este plutón en los laboratorios del CPGeo, Instituto de Geociencias (Univ. de San Pablo, Brasil). Se obtuvo una edad concordia de 261,1 ± 2,0 Ma(1) con MSWD de 0,14 que se interpreta como la cristalización magmática del Plutón La Verde. El dato K-Ar en biotita de 253± 9 Madel mismo cuerpo (Busteros et al, 1998) es algo más joven y puede interpretarse como una edad de enfriamiento post-cristalización magmática. El dato de 261 Ma del plutón La Verde se encuentra dentro del rango de edades pérmicas de los otros cuerpos plutónicos del Complejo Pailemán en la región, como el Granito Arroyo Pailemán (Rb/Sr268±3Ma, Grecco et al 1994) y la Granodiorita Arroyo Tembrao 39Ar/40Ar,(Ar 266,1±1,5 Ma, Grecco y Grégori2011). Asimismo es más joven, aunque dentro del Pérmico, que el plutón Laguna Medina (U-Pb circón 291,2 ± 5,6 Ma, Varela et al 2008) de la región de Sierra Grande. Las edades modelo TDM Sm/Nd calculadas por el métodomultiestadio de DePaolo et al 1991) son meso-a paleoproterozoicas(1,2-1,8 Ga) para los granitos y paleoproterozoicas (1,6-1,8 Ga) para las granodioritas. Por el momento, y hasta no contar con más datos radimétricos, las edades del Complejo Pailemán indican unclímax de actividad magmática Gondwánica en el Pérmico, en sentido amplio. |
Grecco, L.E., Gregori, D. A., Rapela, C.W., Pankhurts, R. and Labudía, C.H. 1994. Peraluminous granites in the Northeastern Sector of the North Patagonian Massif". Symposium on Structural and Compositional Segmentation of the Andes. Concepción, Chile. VII Congreso Geol. Chileno, Actas II, IUGS Project 345. 1354-1359. Grecco, L. E. and Gregori, D. A., 2000. The Arroyo Pailemán Granite Stock: A peraluminous S-type intrusive, NPM, Argentina. Profil, Band 18. XVII Simposio sobre la Geología de Latinoamérica, Sttutgart, Alemania |
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4 Laguna Indio Muerto , Estación Munsters: Plutón La Malena, Plutón Mina San Martín
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5 Salinas del Gualicho:
COMPLEJO PLUTÓNICO PAILEMÁN Fue definida por Giacosa (1993) para reemplazar
al “Granito sierra Pailemán” de Stipanicic y
Methol (1972), debido a que “en primer termino sólo
algunos de los stocks son graníticos y en segundo
lugar la sierra de Pailemán esta constituida por rocas
volcánicas”.
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6 Ao. Nahuel Niyeu, Ao Treneta
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7 Ao Yaminué-Estación Falkner:
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FAJA MAGMATICA GONDWANICA NOROESTE La faja magmática Gondwaánica Noroeste tiene rumbos entre N70° y N93° y se extende por mas de 300 km entre el Arroyo Pichi Leufu y Planicie Baja area (Río Negro Province). En su parte oeste (Piedra del Aguila-Mencué- Arroyo Pichi Leufu), las rocas graníticas forman vastos y continuos afloramientos ocasionalmente cubiertos por volcanitas jurásicas. En este sector tiene un ancho de mas de 130 km. Fueron publidaos varios trabajo por Gregori y colaboradores donde se muestra la distribucion, litologia, quimica edad y origen de las rocas. En la parte central de la faja (La Esperanza) se realizaron varios trabajos de detalle que indican litologia quimica, edad y origen de las rocas. El extremo este de la faja tambien fue estudiada por Gregori y colaboradores. Otros sectores de la faja no fueron estudiados. Las areas 8 a 16 de la figura de abajo se ubican en esta faja.
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8 Cañadón Cullún Leufú, Sierras Blancas, Cañadón Mallín Chico, sector E Sierra de Queupu Niyeu Las rocas graniticas de este sector no fueron estudiadas en detalle pero posiblemente correspondan al Complejo Plutónico La Esperanza y se ubican en la carta Los Menucos
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9 Cañadón Soledad, Cañadón El Loro, Caita Có, Cerro Pangaré The Caita Có granite
Se ubica en la carta General Roca
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10 Zona La Esperanza, Loma Blanca, Cerro Pillahuinco Chico: Complejo Plutónico Volcanico Dos Lomas , Complejo Plutónico La Esperanza Complejo Plutónico La Esperanza y se ubican en la carta Los Menucos
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ZONAS 11, 12, 13, 14 Y 15 SE UBICAN EN LA CARTA PIEDRA DEL AGUILA
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IMAGEN SATELITAL DONDE SE UBICAN LAS ZONAS 11, 12, 13 Y 14
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11 Chasicó, Palenqueniyeu, Colán Conhué: Granito Chasico
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12 Zona Cañadón Michihuao, Cañadón Quili Malal, Lonco Vaca: GRANITO PALENQUENIYEU Núíicz y Cucchi (1985) propusieron el nombre
Granito Palenqucniyeu para los granitos biotítico-
hornbléndicos portadores de inclusiones
dioríticas, aflorantes al este de Mencué, en la zona de La Angostura, Palenqueniyeu, Carriyegua, Lonco Vaca y Planicie del Mirador.
Para correlaciones mas ajustadas ver el Batolito de Mencué (Gregori et al. 2020)
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13 Zona de Cañadón Quili Mahuida, Cañadón La Blancura, Cañadón Mencué, Cañadón Curru Mahuida: BATOLITO DE MENCUE In accordance with field recognition and microscopic descriptions, the
The Mencué Granodiorite Migmatites and mild foliated gray biotitic granodiorites outline small outcrops in the Cañadones but were never recognized in the flat area in-between, whereas the white to pink two-mica coarse-grained foliated granodiorites occupies nearly the whole surface of figures 2 and 3. The Mencué Granodiorite was intruded by the Cura Lauquén Granite and is therefore considered the first magmatic event. The migmatites crop out mainly in the northern part of the area, along the Cañadones La Blancura and Mencué, near the Jaramillo, R. Arias, and S. Arias houses (Figs. 2 and 3, 4a, and 4b).
The rocks exhibit alternating light and dark bands exposed in the walls of the canyons. In some sectors, the deformation in the migmatites is intense, with the folding and refolding of the bands (Fig. 4c).
Microscopically, the white bands are composed of orthoclase,
plagioclase, and quartz, while the dark ones mostly include oriented flakes of
biotite, muscovite and minor garnet. Accessory minerals are prismatic zircons,
garnet, allanite, rutile, and opaque minerals. The crystals of quartz show a granoblastic texture, forming "ribbon
quartz" structures where intracrystalline deformation, migration of grain boundaries and grain rotation was observed. Garnet occurs in euhedral crystals up to 2.5 mm thick, located in the dark bands.
La Blancura syenogranite. Granitic dikes Geochronological results Migmatite of the Mencué Granodiorite White to pink two-mica granodiorite of the Mencué Granodiorite
La Blancura Syenogranite
Geochemical characterization
The samples of Cura Lauquén Granite fall in the granite, granodiorite and even tonalite fields in most classification diagrams. In the case of La Blancura Syenogranite, the rocks plot as alkali feldspar granite, syenogranite, and monzogranite in the diagram from Streckeisen and Le Maitre (1979), (Fig.7a, e), whereas in the diagrams from Barker (1979) and Cox et al. (1979) samples fall in the granite field. In the alkali-SiO2 diagram from Irvine and Baragar (1971) all granitic rocks plot in the subalkaline field (Fig. 7f). Irvine and Baragar's (1971) AFM diagram (not shown) indicates a calc-alkaline trend for the granitic rocks. According to the Shand index (in Maniar and Piccoli, 1989), the intrusive rocks are peraluminous (Fig. 7g).
Increasing values of Nb*/NbN and Ta*/TaN are clearly shown in figures 9
a, b, and c. For the Cura Lauquén Granite they are Nb*/NbN: 1.9–4.1, Ta*/TaN:
3.3–5.8 (Fig. 9d). These values are in the range of grey granodiorite. Rare earth elements Protoliths and melting temperatures.
The northwestern Gondwana belt of the North Patagonian Massif
The second magmatic event includes ages of 269–275 Ma recorded in
the Paso Flores Tonalitic gneiss, the Mencué Granodiorite, the Palenqueniyeu
Granite and in the Prieto Granodiorite (Figs. 11a). Three samples of the Paso
Flores Tonalitic gneiss yielded an age of 273.1 ± 9.5 Ma (Varela et al., 2005).
North of the Pilahue locality (70 km NE of Varela et al., 2005 samples) Cucchi
et al. (1998) obtained a K-Ar age of 269 ± 10 Ma in the Palenqueniyeu Granite. The third magmatic event ranges between 268 and 259 Ma and
generated the Cura Lauquén Granite of the Mencué Batholith and the rhyolitic
domes and dikes of the Dos Lomas volcanic-intrusive complex (Figs 11a and
b). Although the outcrops of the Cura Lauquén Granite are relatively restricted
in the Mencué area, they cover an important surface of the Palenqueniyeu and
Colán Conhué area. The Dos Lomas volcanic-intrusive complex, located 100
km east of the Mencué Granites, was also studied by Llambías and Rapela,
(1984), Caminos, (1987); Caminos et al. (1988), Rapela and Llambías, (1999),
and Pankhurst et al. (1992, 2006). In the upper part of the plutonic-volcanic
section, Pankhurst et al. (2006) obtained an U-Pb age of 264 ± 2 Ma for the
rhyolitic domes and dikes. The fourth magmatic event (Figs. 11a) is represented by La Blancura Syenogranite of the Mencué Batholith, the Chasicó Granodiorite, the Calvo Granite and the ignimbritic rhyolites of the Dos Lomas volcanic-intrusive complex (250–239 Ma, Olenekian, Early Triassic). In the Chasicó area, located 70 km east of the studied area, (Figs. 11b) Dalla Salda et al., (1991) obtained a tentative Rb-Sr isochron of 239 ± 9 Ma for the Chasicó Granodiorite. The U-Pb ages in zircon for the Calvo Granite is 250± 2 Ma whereas for the ignimbritic rhyolites is 246 ± 2 Ma. (Pankhurst et al. 2006). These younger ages were not detected in the Mencué area.
Other magmatic belts of the North Patagonian Massif
Unlike the other Gondwana magmatic belts, this one exhibits only a few areas with ductile deformed granites. The northeastern Gondwana magmatic belt doesn´t show subductionrelated granites and their emplacement is related to an escape tectonic as indicated by Gregori at el (2008).
In the southern coast of Lago Ranco, the Futrono-Riñihue Batholith yields an U-Pb in zircon age of 304.9 ± 2.4 Ma (Deckart et al., 2012). Therefore, in southern Chile, a subduction system that includes an accretionary prism and arc-related granitic rocks was developed coevally with the emplacements of the Mencué and other Permian bodies in the western North Patagonian Massif (Fig. 11c)
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14 Zona Cañadón Pilahue, Cañadón Cura Lauquén, Cañadón Fita Ruin, Laguna Blanca: BATOLITO DE MENCUE |
Zona 15: Cañadón Chileno, Paso Limay: BATOLITO DE MENCUE
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16: Zona de Comallo-Neneo Ruca Se ubica en la carta Ingeniero Jacobacci
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FAJA MAGMATICA GONDWANICA SUDOESTE
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17 Zona Río Chico, Cañadón Chacay Huarruca, Sierra de Mamil Choique: Granitoides Mamil Choique Formación Mamil Choique (Volkheimer, 1965) equivalente a Formación El Platero (Volkheimer y Lage, 1981) y Granitoides Mamil Choique. Los Granitoides Mamil Choique consisten en tonalitas, migmatitas homogéneas, granodioritas y monzogranitos, de grano medio a grueso, biotíticos y biotilico-moscoviticos, con afloramientos de dimensiones batolíticas. Dalla Salda el al. (1994) segregaron de la Formación Mamil Choique en la acepción original de Ravazzolli y Sesana, a granitos de menor antigüedad relativa, que denominaron Granito Viuda de Gallo. El Granito Viuda de Gallo compone un stock que ofrece relaciones de intrusividad con el cuerpo mayor granodioritico. Son leucogranitos rosados, de grano medio, conteniendo biotita y moscovita. El contenido de máficos no excede el 7% en las muestras estudiadas; el feldespato potásico es microclino y representa un 22% a 45% de la roca; la composición de la plagioclasa presente es variable entre An3O% a An4O%. Las características geoquimicas ubican a estos granitos como los términos más diferenciados de una serie calcoalcalina; tienen un alto contenido de sílice (69% a 74%) y son rocas peraluminosas. Se considera que tiene la edad del Granito Mamil Choique.
ZONAS 17 Y 18 SE UBICAN EN LA CARTA INGENIERO JACOBBACCI
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18 Zona de Gastre, Sierra de Calcatapul, Sierra de Lipetrén
Granito Gastre, Granito Lipetrén. Granito Yancamil
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19 Zona Paso del Sapo, Sierra de Taquetrén Granito Sierra de Taquetrén |
20 Zona de Gan Gan, Coquelén, Colaleche |
21 Sierra Chata Diorita Mendez, Granito La Irene, Fm. Cerro del Ingeniero |
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BIBLIOGRAFIA SOBRE EL MAGMATISMO GONDWANICO
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V. A. y Cortés, J. M., 1984. Estructura e interpretación tectónica. IX Congr.
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C. W. and Caminos, R., 1987. Geochemical characteristics of the Upper Paleozoic
magmatism in the eastern sector of Northpatagonian massif. Revista
Brasileira de Geociencias, 17, (4): 535-543.
Rapela,
C. W. and Kay, S, M., 1988. Late Paleozoic to Recent magmatic evolution of
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Rapela, C.W., Pankhurst, R.J., and Harrison,
S.M., 1992, Triassic "Gondwana" granites of the Gastre district, North
Patagonian Massif: Trans. Roy. Soc. Edinburgh. 83: 291-304.
Rosenman, H. L., 1972. Geología de la región de Arroyo Los Berros (vertiente oriental de la Meseta de Somuncura), Provincia de Río Negro, República Argentina.
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C. M., 1999. Jurassic Granites in the northern portion of the Somoncura Massif,
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Congreso Geologico Argentino. Actas II: 175-177.
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DEFORMACION GONDWANICA EN EL NORTE DE LA PATAGONIA INCLUYENDO SECTORES ADYACENTES: SIERRA DE LA VENTANA, BLOQUE DE CHADI LEUVU, LAS MATRAS, COMARCA NORDPATAGONICA, SUDAFRICA, INCLUYENDO LA HIPOTESIS DE ALOCTONIA O AUTOCTONIA DE PATAGONIA
Interacción Terreno Panthalassan-Gondwana Hipótesis interacción Patagonia Gondwana Estructura transcontinental límite Patagonia Gondwana Arcos del N de Patagonia relacionados con la interacción Patagonia Gondwana
Hipótesis de Gregori-Kostadinoff
FIGURAS TOMADAS DE: Kostadinoff, J., Gregori, D. A. and Raniolo, A., 2005. Configuración geofísica-geológica del sector Norte de la provincia de Río Negro. Revista de la Asociación Geológica Argentina. LX: 368-376).
Results
Three areas with positive gravimetric anomalies, four with negative anomalies and several features characterized by steep gravity gradients (Figs 7A and 8A, B) were recognized during this study.
Positive anomalies
Chimpay–Gobernador Duval High
Residual anomalies within the range −27 to −14 mGal, a relatively positive anomaly occur from Chimpay to Gobernador Duval and Puelén (Fig. 7). Refraction shots by the YPF (Yapeyú to RN5, Fig. 6A) indicate a thickness of more than 1000 m for the Mesozoic Neuquén Basin near Gobernador Duval. In the Chimpay–La Japonesa area this basin reaches a maximum thickness of 300 m. In order to satisfy the observed anomalies a density contrast of +0.32 g/cm3 between the Mesozoic sedimentary rocks and those of the basement (Table 1) is required.
No outcrops of basement rocks with such high density weredetected in the Neuquén Basin. However, Grenvillian (~1.0 Ga) basement composed of tonalite and trondhjemite were studied by Sato et al. (2000) in the Las Matras area (Fig 1), which is located 130 km north of Gobernador Duval. We interpret the Chimpay–Gobernador Duval High to be composed of basicintermediate rocks (seismic velocity 5400–6600 m/s) covered by sedimentary rocks deposited in the Jurassic–Cretaceous Neuquén Basin. Such igneous rocks could also explain positive magnetic anomalies, which range between 250 and 175 nT (Fig. 9) in this area.
Río Colorado High
Several relatively positive residual gravity anomalies (6, −8 and −11mGal) appears between PichíMahuida, Cerro LosViejos and Río Colorado (Kostadinoff and Llambías (2002) (Fig. 7A). These anomalies are considered to reflect Pampean basement, which is composed of small outcrops of amphibolite, gneiss, micaceous schist, phyllite and metasandstone of the Las Piedras Metamorphic Complex and granite and granodiorite of the Pichí Mahuida Group (Linares et al., 1980; Tickyj et al., 2002). The anomalies are due to relative uplift of blocks similar to those that occur in the Sierras Pampeanas de San Luis y Córdoba. TheNW– SE margin of RíoColoradoHigh is coincidentwith a steep gravity gradient (near 1 mGal/km) between Cerro Los Viejos and 38° S (Figs 7A and 8A, B). Fig. 7B shows the measured and the modeled anomalies in the Río Colorado High considering a density of 2.65 g/cm3 for the basement rocks. According to the geometries of the anomalies and the outcrops, the Río Colorado High is believed to represent a block-faulted system.
Refraction shots by YPF (numbers 20 to 25, Fig.6B) show the existence of basement rocks with high seismic velocity (5000 to 6000 m/s), which are interpreted as the Las Piedras Complex. This basement deepens to the south, reaching 700 m below the surface near the Huincul Fault zone. Magnetic anomalies are relatively low in this area and suggest the absence of paramagnetic rocks in the basement (Fig. 9).
Choele Choel High
The Choele Choel High (Kostadinoff et al., 2005) is a positive residual gravity anomaly of +10 and +15 mGal (Fig. 7A and B) with steep (2 mGal/km) gradients (Fig. 8A, B). The northern boundary is defined by the Huincul Fault Zone and lineament v“A” (Kostadinoff et al. 2005). The southern boundary (Figs 7A, B, 8 and 10) is a complex, arcuate system that is formed by 3 lineaments, which are defined by steep (1–2 mGal/km) gravity gradients (B, C and E: Fig. 10). The model of Fig. 7B requires a slighter increase in the basement rocks density to explain the
measured anomaly and as showed in this figure the Choele Choel High represent an uplifted block.
Refraction shots by YPF in this area (Puesto Segatore to profile number 8, Fig. 6C) indicate the existence of rocks with seismic velocities of 4500 to 5320 m/s which are covered by Mesozoic–Cenozoic rocks. Such rocks exist at depths between 200 and 600 m in the eastern and southeastern parts of the Choele Choel High (Profiles 8 to RN19) and to a depth of 1000 m when approaching lineament “A”.
Candidates that fulfil such seismic velocity are the phyllite, muscovite-bearing schist, and metasandstone of the Las Piedras Complex that are exposed in the Río ColoradoHigh. The seismic velocities may also be explained by phyllite, metasandstone, marble and pelite of the Nahuel Niyeu and El Jaguelito formations (Fig. 3), which are temporally correlated with the Las Piedras Complex, and crop out in the Nahuel Niyeu–Valcheta area (Caminos and Llambías, 1984).
If ultramafic rocks were emplaced in a Pampean basement of this density (Table 1) an anomaly of more than 100 mGals could be expected. Most ultramafic belts in suture zones show values between +50 and +120 mGals (Gibb and Thomas, 1976, Kunar et al., 2004, Williams et al., 2006). However, this region does not display anomalies of this magnitude and the seismic profiles also do not suggest the existence of rocks with seismic velocities
sufficient to be sourced from ultramafic rocks.
The magnetic survey (Fig. 9) shows scarce magnetic anomalies, with maximum values of +75 nTand minimum values of−75 nT. These data also preclude the existence of mafic– ultramafic rocks in this area and are more consistent with a magnetic source such as the non-magnetic rocks such as Pampean low-grade metamorphic rocks.
Viedma High
Gravity anomalies from the Viedma High (Figs. 7A and 10) vary between 1 and 15 mGal, (Figs. 7A and 10), suggesting a density contrast caused by rocks with similar densities to that which caused the Choele Choel High. The contact between both of these gravimetric highs is defined by NW–SE and NE–SW lineaments that follow narrow belts with steep gravity gradients (Figs 7A, 8 and 10). Unfortunately, there are no seismic profiles
located in this area, but outcrops in the nearby Salinas del Gualicho area (Fig. 3) belong to the Nahuel Niyeu Formation (Figs. 3 and 10). Minor outcrops of the Sierra Grande Formation and Gondwana igneous rocks are also present. Drill holes at O'Connor and Elvira (Kaasschieter, 1965) intersected lowgrade metamorphic rocks to a depth between 400 and 475 m. (Fig. 7).
The positive gravity anomalies associated with the Viedma High extend northwards (Kostadinoff and Labudía, 1991 and Kostadinoff, 1992) and link with positive gravity anomalies located to the west of Sierras Australes (Figs. 2, 3, 7A and 10).
The Algarrobo drill hole, and refraction data near Bahía Blanca (Estancia El Recuerdo and Puerto Belgrano, Bonorino et al, 1987), indicates seismic velocities similar to those that are interpreted to be the source of the Choele Choel High (i.e. lowgrade metamorphic rocks of the Las Piedras Metamorphic Complex).
The distribution of positive anomalies in the Viedma High and in the Choele Choel High supports the interpretation that a belt of Pampean rocks extends from the southern part of the Buenos Aires province to northern Patagonia (Figs. 3 and 10).
Negative anomalies
Neuquén Basin
The Mesozoic Neuquén Basin (Figs 2, 7A and 10) is characterized by gravity lows between −54 mGal and −32 mGal. Seismic data (Fig. 6D) and drill hole information indicate that the sedimentary sequences reach an approximate thickness of 3000 m. Negative gravity anomalies (−41 mGal) near Chelforó represent small depocenters, whose thicknesses decrease to 750 m near Chimpay (Fig. 7A). Magnetic anomalies in this area
vary between −125 and +125 nT (Fig. 9) and are attributed to the presence of Fe-oxides in the Cretaceous Neuquén Group (400 m thick) that covers this portion of the basin.
Carapacha Basin
The scarce outcrops of the Permian continental Carapacha Basin and the volcanic rocks near Puelches in La Pampa province (Fig. 4) are coincident with negative gravity anomalies (−40 to −30 mGal: Fig. 7A). The density contrast between these rocks and those of the Pampean basement is relatively low (Table 1 and Fig. 7B), which led to Kostadinoff et al. (2001) to estimate a thickness of 2700 m for this basin.
This is substantially more than the 900 m thickness estimated by Melchor (1999). Based on these results, Kostadinoff and Llambías (2002) considered the subsurface extent of the basin to be greater than that suggested its surface extent.
Therefore, the gravimetric minimums of −41, −31 and−40 mGal located northeast of Gobernador Duval, as well as those located west of Cerro Los Viejos and Pichí Mahuida (−41 and −36 mGal), possibly represent an extension of the Carapacha Basin (Figs. 7A, B and 10). In the Carapacha Basin, the Gondwana volcanic rocks unconformably overlying the Paleozoic sedimentary sequences in the Carapacha Basin and are interpreted
to contribute to the negative anomalies. The magnetic anomalies (−275, −225 and −125 nT: Fig. 9) are due to the low susceptibility of the Paleozoic sedimentary rocks and the Gondwana acidic volcanic rocks.
Estancia El Caldén Basin
A relatively small negative residual gravity anomaly (−24 mGal: Fig. 7A) is located near the Estancia El Caldén
(Kostadinoff et al., 2005), north of the Huincul Fault Zone. A density contrast of −0.2 g/cm3 between the Pampean basement and the Mesozoic sedimentary units suggests a sedimentary thickness of 1700 m (Figs. 7A and 10). Refraction data in this area (Fig. 6E) indicate a greater than 1000 m thick sedimentary
pile. The seismic data suggest velocities consistent with Lower Cretaceous and Upper Tertiary sedimentary successions, which cover the Paleozoic rocks.
Ramos Mexías anomaly
The Ramos Mexías anomaly is part of a continental-scale anomaly (500 km in diameter), located in the south-western part of the North Patagonian Massif (Blitzkow et al., 1994). The Ramos Mexías anomaly corresponds to a large area of several negative gravity anomalies (−46 mGal to −35 mGal, Figs. 7A and 10) located south of the lineament “B) (Fig. 10). Outcrops include Pampean metamorphic rocks and Gondwana intrusive and volcanic rocks. The metamorphic rocks belong to the Nahuel Niyeu Formation and include micaceous and amphibolite schist, marble, and amphibolites. The intrusive rocks (minor tonalite, granodiorite and granite) belong to the Gondwana Navarrete Plutonic Complex. The volcanic rocks (acidic ignimbrite) are part of the Treneta Volcanic Complex (Caminos et al.,
2001) and the Triassic volcaniclastic Los Menucos Complex (Cucchi et al., 1999).
Lagunas Dulces anomaly
The Lagunas Dulces anomaly is a negative gravity anomaly (−20 mGal) that is located north-eastwards of San
Antonio Oeste (Figs. 7A and 10). This anomaly is interpreted to be caused by the Lagunas Dulces Granite (Kostadinoff, 1992). A drill hole at this locality intersected granitic rocks (249±10 Ma: Kaasschieter, 1965) at a depth of 370 m. Because the geometry and magnitude of this anomaly is similar to those coincident with outcrops of the Gondwana Navarrete Granodiorite, we interpret the Lagunas Dulces anomaly to be caused by intrusion of Gondwana granitic bodies in the Pampean basement.
Bajo de Valcheta anomaly
The Bajo de Valcheta anomaly is a negative anomaly of the order of −10 mGal, located between the Viedma High and the Ramos Mexías anomaly. The area is coincident with outcrops of the low-grade metamorphic rocks of the Nahuel Niyeu Formation (Figs. 7A,B and 10). According to the model in Fig. 7B a density of 2.65 g/cm3 is required to satisfy the observed anomalies. No significant magnetic anomalies (Fig. 9) can be recognized at this
location.
Laguna del Zorro anomaly
The Laguna del Zorro gravity anomaly (Figs. 7A and 10) is located to the north-west of the Viedma High and is bounded by lineament “C” (Fig. 10). The geometry and magnitude of the anomaly is similar to the Lagunas Dulces anomaly and is therefore interpreted to be sourced from the same or similar Gondwana intrusive body.
Belts of steep gravity gradients
In order to validate the continuity of the Huincul Fault zone and other structural lineaments proposed by Turner and Baldis (1978), we focused on areas with steep gravity gradients in the gravity anomaly map. Such steep gradients are usually interpreted as representing fault structures (Nettleton, 1976), boundaries of rocks with contrasting petrophysical characteristics or sutures (Gibb and Thomas, 1976, Wellman 1978, Coward and Fairhead
1980, Gibb et al. 1983). Differences in density or thickness between blocks are often responsible for steep gradients (Gibb and Thomas, 1976, Thomas, 1983, Kunar et al. 2004, Williams et al., 2006). Values between 0.5 and 2 mGal/km, were obtained using the gradient and second derivate operator. A 3D image generated using Surfer 8™ (Fig. 8B), show several lineaments defined by belts with steep gravity gradients.
Table 2 indicates the name, location, gradient, any surface evidence and possible age of each lineament, while their location is shown in Fig. 10.
Three main lineament orientations directions can be recognized: lineaments A and C are NE–SW trending; lineaments B, D and E are NW–SE trending; and lineaments F, G and the Huincul Fault zone are E–W trending.
The E–W lineaments exhibit more complex geometries than the other lineaments. Most lineaments form the boundary of the Choele Choel High (e.g. A, B, C, E and the Huincul Fault zone).
Huincul Fault zone
Because the Huincul Fault zone (Kostadinoff et al., 2005)
crops out in the Neuquén Basin (Windhausen, 1914 and Keidel, 1925), its configuration and kinematics are relatively well constrained. The Huincul Fault Zone displays dextral strike-slip movement (Orchuela and Ploszkiewicz, 1984) and a zig-zag along strike pattern (Figs. 2 and 10). This pattern suggests that both transtensional and transpressional conditions occurred during fault movement, resulting in the contemporaneous development of positive and negative flower structures. This fault system was active from Upper Paleozoic–Triassic times, as recorded by continental depocenters associated with the transtensional subsidence (Vergani et al. 1995).
Its eastwards continuity was suggested by Turner and Baldis (1978), and established by Orchuela and Ploszkiewicz (1984) based on interpretation of seismic reflection data. The high gravimetric gradients between Neuquén and Cubanea (Figs. 7, 8B and 10) support the continuity of the Huincul Fault Zone until 63° 40´W, with maximum gradients (2 m Gal/km) between the Neuquén Basin and 64° 45´W–39° 20´S.
The section between Neuquén and Chimpay shows a smooth releasing bend (Fig. 10), that changes to a restraining bend in the Chimpay 63° 40´W section. The gravity anomalies are discontinuous along the fault zone. In the area between Chimpay and Cubanea, the differences in magnitude of the anomaly between the Choele Choel High and the Carapacha Basin and the Río Colorado High mark the trace of this fault zone. In the section between Chimpay and Neuquén, such differences in the anomalies are less evident, but high gravimetric gradients (2 mGal/km) are interpreted to reflect the location of the fault zone.
Magnetic anomalies are diffusely coincident with the interpreted fault zone between 5 and 10 km north of Chichinales and Choele Choel. These anomalies are low-amplitude (175 to 75 nT, Fig. 9) and are interpreted to be related to the Albian– Coniacian red beds of the Neuquén Group, which contain abundant Fe-oxides (hematite). The sediments have a magnetic susceptibility of 0.006500 SI (Telford et al., 1990), which is
sufficient to explain the magnitude of the magnetic anomalies.
These low-magnitude anomalies do not compared magnetic anomalies typically observed in sutures zones because suture zones are generally composed by serpentinite zones associated with sheared peridotites, which have a magnetic response N500 nT (Ferraccioli et al., 2002).
The Huincul Fault zone is therefore unlikely to be a suture. In the Neuquén–Chichinales section (Fig. 10) the
Neuquén Group is 1000 m thick, but in the cliffs located south of the Rio Negro, only the upper section of this unit crop outs. In contrast, the area north of the fault zone, Upper Cretaceous–Lower Tertiary marine Malargue Group crop out, suggesting more than 500 m of vertical displacement across a fault zone.
Due to the outline of the Huincul Fault Zone, the section between Plaza Huincul and Picún Leufú (Fig. 10) and between Neuquén and Chimpay represent domains of dextral transpressive shear (Orchuela and Ploszkiewicz, 1984), where basement rocks are exposed in the fault zone. In the section between
Chimpay and 64° 15´W, deformation appears to be transtensional based on the existence of small depocenters associated to the fault zone.
Mylonitic belts and lineaments with steep gravimetric and high magnetic gradients in northern Patagonia, southern La Pampa and Buenos Aires provinces
Northern Patagonia is characterized by the presence of several mylonitic belts and regional structural lineaments, whose geometries, deformational mechanisms and age mostly remains unclear (Fig. 11). The better-known of these lineaments and mylonites include:
a) Pangaré and La Seña mylonites (Gregori et al., 2000), are formed by two belts, each 1 km wide, 15 km long, and comprising granitic augen-mylonites, protomylonites, mylonites and ultramylonites, (Figs 10, 11 and 12A) with strikes between 310° and 330°. They exhibit sinistral movement associated with an east–west compression
b) Peynecura mylonitic belt (Llambías et al., 2002) is more than 500mwide, 15 kmlong, and has aNNW–SSE strike (Fig. 11). The deformation is interpreted to have occurred during the Early Triassic because is related to foliated granites of such age. No information about strain orientation is available.
c) The El Jaguelito Fault zone is a 50 km long NW–SE dextral lineament that appears in the eastern area of the North Patagonian Massif (Fig. 11). According to Ramos and Cortés (1984), deformation occurred during Upper Paleozoic times.
d) Peñas Blancas and La Laguna mylonites are described by Giacosa (1996). They are 10 km long, and are temporal related to the foliated Permian-aged Peñas Blancas and La Laguna Granites. Four more NW–SE mylonitic belts were recognized during regional mapping in this area, but information on strain orientation is unavailable. The timing of deformation is interpreted to be Gondwanan because the foliated granites were intruded at the same time as mylonitization occur.
e) The Nahuel Niyeu lineament system is a major structure that is 25 km wide, and extends for 65 km long (Figs. 10 and 11) between the Salinas Trapalcó–Laguna Curicó Lineament (Figs. 1, 2 and 12A) and the Somoncura Plateau (Fig. 2). Nahuel Niyeu lineament includes the Arroyo Seco, Barda de Lucho, Arroyo Salado, Laguna Negra, Quiroga and Ramos lineaments, the Tardugno, Musters and Huanteleo faults and the Nahuel Niyeu, Railer and Rana thrust sheets (Fig. 10).
In the La Pampa province, a mylonitic belt was described in Cerro de los Viejos (Figs. 11 and 12A) by Tickyj et al. (1997). A further four belts of steep gravity gradients which were interpreted as major structures were described by Kostadinoff et al. (2001). These four belts include:
a) The Quehué lineaments is a 350 km long, 15 km wide. It is defined in the gravity data by a 2 mGal/km gradient which defines a structure interpreted to represent the boundaries of the Triassic Quehué basin.
b) The Valle Daza-Cuchillo Có lineament is a 310–320° trending belt that is 100 km long and characterised by a steep gradient in gravimetry data. This lineament is interpreted to represent the western boundary of the Rio Colorado High (Figs. 11 and 12A).
c) The Cerro los Viejos lineament extends in a NW-direction for more than 100 km from Cerro Los Viejos and is possibly related to the Cerro Los Viejos mylonite. According to Kostadinoff and Llambías (2002), the Cerro los Viejos lineament represents the contact between the Permian Carapacha Basin and the Río Colorado High. This lineament and its associated steep gravity gradient disappears in the central part of the La Pampa province, but is identified near Algarrobo del Aguila (Fig. 11) as a 50 km long, 15 km wide lineament with steep gravity gradient. This NW-trending structure is considered to be the boundary of a Triassic basin (Kostadinoff and Llambías, 2002).
d) The Macachín Rift includes Paleozoic and Mesozoic sedimentary sequences deposited in a NNW–SSE extensional basin (Figs. 11 and 12A). Outcrops of igneous and sedimentary Gondwanan rocks in the central part of the La Pampa province (Figs 2 and 4) follow WNWor NW trends, which suggest structural control.
In the Buenos Aires Province strike-slip faults located north and south have been used to explain the structure of the Sierras Australes. For example, Newton and Cingolani (1990) showed a series of WNW–ESE trending sinistral strike-slip faults near Pigue and Cerros Colorados (Figs. 11 and 12A), which are evident in gravity data (Alvarez, 2004).
In the Bahía Blanca area, Bonorino et al. (1987) showed the occurrence of several lineaments with steep gravity gradients south of Sierras Australes. Lineaments of importance include the Salitral de la Vidriera and Nueva Roma belts, which are strike-slip fault zones longer than 100 km in strike extent (Fig. 11).
Gondwana deformation in the Neuquén Basin, northern Patagonia and southern La Pampa and Buenos Aires provinces
Windhausen (1914), Keidel (1925) and Groeber (1929) described NE–SW structures transverse to the N–S Andean
Orogeny in the Neuquén Basin (Figs. 11 and 12A). According to these authors, as well as Herrero Ducloux (1946) and De Ferraris (1947) the stratigraphic nd structural evidences suggest folding during the Cenomanian or the Middle Jurassic.
They observed that the structures mimic the northwestern and northern border of the North Patagonian Massif and that they possibly formed during NW- and N-directed lateral translation of the massif. These structures were grouped in the “Dorsal de Huincul” (Huincul Fault zone) by De Ferraris (1947).
In the North Patagonian Massif, the Gondwana deformation is recorded by the Pangaré and La Seña mylonites. Clearly defined δ and σ type porphyroclasts of K-feldspar, and shaped lenses of Qz–K-feldspar, support sinistral movement. These belts and the associated foliated granite (Gregori et al., 2000) indicate maximum compression (σ1) in a WNW–ESE direction (Fig. 12A) during Middle to Upper Triassic (Saini-Eidukat
et al., in press).
In the El Jaguelito Fault zone, the fault pattern suggests a dextral transcurrence along N 310° (Fig. 12A) during Upper Paleozoic times (Ramos and Cortés, 1984). SE, S and SW vergence occurred along the Nahuel Niyeu lineament system (Fig. 12A), each of which was related to three phases of deformation, with pr inciple shortening directions in NE–SW, NW–SE and E–W directions during Permo-Triassic times (von Gosen,
2003).
In the Sierra Grande area, the Pampean El Jaguelito Formation contains evidence which supports a D1 event that was generated byNW–SE or E–W shortening (Giacosa and Paredes, 2001, von Gosen, 2002). In the same area, the Famatinian Sierra Grande Formation is folded around open synclines and anticlines that have N–S trending axial traces (Fig. 12A). The Permian Laguna Medina granite intrudes the Sierra Grande Formation and records
a ductile E–W shortening deformation event, which was coincident with deformation of the Sierra Grande Formation (Rossello et al., 1997, von Gosen, 2002).
Transport directions from NE to SW during the Gondwana deformation was accommodated along mylonites within the Peñas Blancas and La Laguna Granites (Giacosa, 2001, von
Gosen, 2002).
In the North Patagonian Massif, the Sierra Grande Formation, the Laguna Medina Granite and foliated granites and mylonites in the Estancias La Seña and Pangaré area all indicate shortening in aWNW–ESE or E–Wdirection (Fig. 12A). The mylonites in the Peñas Blancas and La Laguna Granites and thrust sheets and faults in the Nahuel Niyeu area are related to a N–S or SW–NE shortening (Fig. 12A) (Giacosa, 2001, von Gosen, 2002).
In the La Pampa Province, the Carapacha Basin (Figs. 4, 11 and 12A) exhibits a NW–SE elongation, suggesting basin development occurred during NE–SWdirection extension. Folding of the sedimentary successions and inversion in the Carapacha Basin (Melchor, 1999) and mylonitization of the Cerro de los Viejos granite (Tickyj et al., 1997) dated as Late Permian (254±2 Ma), suggests that both areas were affected by the same deformational phase (Fig. 12A) related to a NE–SW shortening.
In the Sierras Australes of Buenos Aires Province, the Pillahuincó Group (Carboniferous-Permian) was deformed (Fig. 12A) between the late Early Permian and the early Late Permian (Tomezzoli and Vilas, 1996). Deformation generated a sigmoidally-shaped mountain chain. Folding and overthrusting occurred during NE vergence (von Gosen et al., 1990). This deformation was related to sinistral transpressive deformation (Sellés Martinez, 1989) along theWNW–ESE Pigué transcurrent fault system (Alvarez, 2004) and the Salitral de la Vidriera and Nueva Roma transcurrent fault systems (Bonorino et al., 1987).
The deformation was interpreted to be related to transpressive movement of the “Patagonian block” relative to the South American craton during Permo-Triassic time (Martinez, 1980), although little supporting evidence was provided. Sellés Martínez (1989) considered the deformation in Sierras Australes to be due to a transpressive regime between the Patagonian block and the South American craton during the Late Permian oblique
subduction.
Detailed structural studies in the Sierras Australes define a sigmoidal belt, with fold trends rotating N60° clockwise, from WNW to NNW, a configuration attributable to dextral transpression (Cobbold et al., 1986, 1991, 1992). Inferred directions of principle shortening are oblique to the orogenic belt, and to the convergent southern margin of Gondwana and are interpreted to result from oblique (dextral) subduction beneath an Andean-type margin (Cobbold et al., 1992, Craddock et al., 1998).
Consequently, the sedimentary basins, the emplacement of the Gondwana intrusive and volcanic rocks and the regional structure north of the Colorado and Negro rivers are related to a first SW–NE extension (Fig. 4), which later switched to a shortening regime in a NE–SW or N–S direction (Fig. 12A).
In their structural analysis of northern Patagonia, Turner and Baldis (1978) identified several NW–SE lineaments in the area between Bajo del Gualicho and Golfo San Matías (Fig. 2). They suggested variable compressive stresses in the North Patagonian Massif including: west-shortening directed (Río Limay area); northeast-directed shortening (Salinas del Gualicho); eastdirected shortening (Sierra Grande); and southwest-directed shortening (El Cuy). The variation in stress directions is attributed to counterclockwise rotation of the North Patagonian Massif (Fig. 12A).
Deformation in the southern area of the Gondwana supercontinent
The first detailed reconstruction of the relative position of Africa and South America during the Upper Paleozoic was given by du Toit (1927). The continuity of an orogenic system was extended from South America to Australia, passing just south of the Permo-Triassic fold belts, introducing the name of Samfrau Orogenic Zone (du Toit, 1937). This belt is nearly equivalent to what we understand today as the Gondwanide Orogenic Belt (du Toit, 1937).
In the South African segment, the arrangement of the Gondwanide Orogenic Belt and the associated basins is strongly conditioned by the configuration of older basements blocks (Kaapvaal, ≥2.5 Ga, Namaqua-Natal, ≈1 Ga and Gariep, Saldania and Mozambique, 1–0.5 Ga). The Ordovician–Devonian Cape Basin follows a main W–E trend along the Saldania province with branches to the NWand NE. The Cape Fold Belt includes southern and western tectonic domains (Söhnge, 1983), separated by the Cape Syntaxis (Fig. 12b), across which the structural strike, and the intensity of deformation changes markedly (de Beer, 1995). The southern domain extends from the eastern margin of the Cape Syntaxis to Port Elizabeth and is characterized by east–west-trending, north-verging structures including bedding- parallel thrusts and north-verging folds (Fig. 12B).
No evidence for dextral shearing was found in this domain. In the western domain, strata is mildly deformed (de Beer, 1992). Folds and faults trend northwest to north (de Beer, 1990). North-trending slickensides developed on thrust planes (Ransome and de Wit, 1992) and stretching lineations (Cobbold et al., 1992) indicate a component of strike-parallel displacement.
The pattern of folds, faults and lineations are consistent with overall dextral strike-slip deformation (Cobbold et al., 1992). The Cape Syntaxis is a northeast-trending structural domain that separates the southern and western domains (de Beer, 1995). Based on a reconnaissance of the structure of the syntaxis and adjacent domains, Ransome and de Wit (1992) proposed a model in which the continental crust to the west of Cape Syntaxis was rotated clockwise during syntaxis formation, while crust immediately east is rotated counter-clockwise.
In the Malvinas (Falkland) Islands, Late Permian fold and thrust belt structures form the eastward continuation of the Cape Fold Belt when restored into a pre-Gondwana break-up configuration. Detailed structural studies by Curtis and Hyam (1998) and Hyam (1998) suggest that these structures developed in a complex strain system of combined NW–SE shortening and dextral shear.
In Antarctica, the Gondwanide Orogen extends through the Ellsworth–Whitmore Mountain block and the Pensacola Mountains (Curtis and Storey, 1996). During Gondwanide orogenesis the Ellsworth–Whitmore Mountain block is assumed to have occupied a site intermediate between the more easterly Falkland Islands and the more westerly Pensacola Mountains (Curtis and Storey, 1996). Structural analyses (fold orientation, shortening directions and strain partitioning) indicate contemporaneous development of gently plunging foreland-verging folds and
asymmetric, steeply plunging folds symptomatic of deformation within a highly oblique dextral transpressive belt (Curtis, 1997, 1998, Craddock et al., 1998).
Dextral transpressional structures are evident throughout much of the Gondwanide Belt, suggesting a component of dextral transpressional during the evolution of this orogenic belt. Johnston (2000) indicated that the east–west-trending domain of the Cape Fold Belt ends to the west and east against the northwest-trending South American and Antarctic portions of the Gondwanide Orogen respectively.
Tectonic model
The diverse shortening directions and lateral crustal translations during Gondwana time north and south of the interpreted boundary between northern Patagonia and Gondwana South America cannot be explained as the result of homogeneous stress regime (i.e. NE–SW compression due to the subduction and collision of Patagonia with the southern margin of Gondwana South America) as speculated by Ramos (1984), and von Gosen (2002, 2003).
All lineaments plotted in Fig. 11 (those both related and unrelated to mylonites, and the belts with steep gravity and magnetic gradients, presumably active during Gondwana times) suggest a lineament network that defines a block system (Fig. 12A).
This collage of continental crustal blocks and its movement was the result of a tectonic event that produced crustal fragmentation.
The geometry of individual blocks controlled the blockmovement and therefore the development of mylonite belts and other lineaments. In order to explain the variable stress regime in which zone of compressional and extensional tectonism were active at the same time over such a large area, we require movement of crustal
blocks along arcuate strike-slip faults (Fig. 12A).
This style of deformation is common during collisional orogeny, where frontal zones are deformed during indentor
tectonics, where the lateral terminations of the indentor involve extensive escape tectonics (Ratschbacher et al., 1991). The size and shape of the continental fragments involved, the orientation of the indentor, and the collision vector, influence the extent and geometry of indentation and later escape.
Modern examples of large-scale indentation-escape orogens are found in the Himalaya-Alpine belt. These models include the collision of the Arabian and the European plates (west-directed escape) and the east- and southeast-directed escape of southeast Asia following the collision of India and Asia (Tapponnier and Molnar, 1976). The transcurrent shear zones separating elongate crustal slices are the most important large-scale structural features
in these Himalaya–Alpine models.
Indentation-escape tectonics is widely observed and accepted in younger orogens, but has not been recognized to the same extent in older mountain belts, probably because the structures associated with such a tectonic regime are less obvious. In addition, subsequent orogenic eventsmay obscure the presence of an ancient indentation-escape regime (Ratschbacher et al., 1991).
Several examples of Precambrian indentation-escape tectonics have been described. In southern Africa, the proto-Kalahari craton acted as a southwest-directed indenter into juvenile Mesoproterozoic island-arc terranes at ca. 1100 Ma (Jacobs et al., 1993).
The associated escape tectonics regime in the orogen is shown by extensive craton-parallel ductile shear zones and the development of pull-apart basins in the preserved African foreland (Koras– Sinclair basins). Other example of indenter-escape tectonic has been reported in Australia, where the Yilgara craton indented
the Pilbara–Gawler craton during the late Paleoproterozoic Capricorn orogeny (Krapez, 1999). Another example is the East African–Antarctic orogen, which resulted from the collision of various parts of proto-East and West Gondwana during late Neoproterozoic–early Paleozoic time (between 650 and 500Ma).
The southern part of this Himalayan-type orogen can be interpreted in terms of a lateral-escape tectonic model. Small microplates (the Falkland, Ellsworth–Haag, and Filchner blocks) probably represent shear-zone-bounded blocks produced by tectonic translation during lateral escape, similar to those currently evolving in Southeast Asia.
Assuming a model of indentation-escape tectonics for northern Patagonia, the belts with steep gravity gradients described in this study represent transcurrent shear zones separating shear-zone bounded blocks (Figs. 11 and 12A). The block situated between lineaments C, D and E has possibly acted as a northwest-directed
indenter associated with west-directed movements of the area located south of the Huincul Fault zone. Several small blocks were displaced west and southwest, possibly the western sector of theChoeleChoel High and the areas located west of the lineAand southwest of line B, as recorded in the La Seña–Pangaré mylonites
and Nahuel Niyeu system (Fig. 12A).
The eastern sector of the Choele Choel High, between the Huincul Fault zone and lineament E was displaced to the east and southeast during lateral escape. In areas of transpression, such as along the Huincul Fault Zone between Estancia El Caldén and Chimpay (Fig. 10), rocks were faulted upwards to form positive
flower structures. Such a configuration can explain the uplift of the Choele Choel High against the Carapacha Basin and accounts for the change of detritus composition as detected in the Carapacha and Claromecó Basins (Rossello et al, 1997, Melchor, 1999).
It seems probable that some blocks in the North Patagonian Massif, located south of the Huincul Fault Zone, rotated counterclockwise during Permo-Triassic time (Fig. 12A). It is doubtful that the deformation can be explained in terms of the collision of two major crustal blocks, principally because there is no conclusive
evidence for such a collision.
The development of the Gondwanide deformation in northern Patagonia–Sierras Australes–La Pampa province in a location far from the active convergent margin of Gondwana is attributable to two factors: a) dextral margin-parallel translation of a crustal block or blocks (Fig. 12A and B) outboard of the Gondwanide
Orogen; and b) the oblique orientation of this area relative to the east–west trend (present coordinates) of the
orogen in the Cape Fold Belt, as shown in Fig. 12B.
Coincidences in timing of deformation and the orientation of principal stress directions, point to an evolutionary history that is similar to other portions of the Gondwanide Orogen (de Wit and Ransome, 1992; Cobbold et al., 1991, Vaughan and Pankhurst, 2008).
In the southern Canadian Rocky Mountain fold and thrust belt, Late Cretaceous and Tertiary deformation developed ~1500 km inboard of the active convergent margin (McMechan and Thompson, 1989). Oblique (dextral) subduction of oceanic plates beneath western North America resulted in the development of major dextral strike-slip faults within the overriding North American crust. The development of this fold and thrust belt 1500 km inboard of the cordilleran margin in response to oblique subduction-driven, margin-parallel dextral translation
was considered as an analogue for development of the Cape Fold Belt (Johnston, 2000). According to the evidence presented here, this model can also be applied to northern Patagonia. However, the relatively extensive width of this active margin is at present not fully understood.
Conclusions
North–South belts of Pampean and Famatinian rocks that cross the supposed boundary between the northern Patagonia and central Argentina regions support the presence of a common continental crust in both areas. No geological or geophysical evidences for oceanic crust formed by mafic– ultramafic rocks between both areas have been identified. The gravity and magnetic anomalies over the supposed boundary are incompatible with a suture composed of such rocks.
The continental crust in the studied area was sliced by displacements along the dextral Huincul Fault Zone and
several minor lineaments that represent strike-slip faults along the boundaries of several continental blocks. South of the Huincul Fault Zone, the indentation of these blocks, related to the translation and simultaneous counterclockwise rotation along a NW-direction produced escape of minor slices in W, SW, E, SE, and N directions. Such movements are recorded in Gondwana rocks and mylonitic belts around northern
Patagonia.
Transpression in the northern part of the massif possibly produced positive flower structures, e.g.: the Choele Choel High or those recognized in the Neuquén Basin.
The balance of evidence suggests that the Gondwanide Orogeny in the northern Patagonia area is due to the transtensive– transpressive deformation. The dextral strike-slip component detected in the northern Patagonia–Sierras Australes, Cape Fold Belt (Africa), and in the Ellsworth Whitmore Mountains block (Antarctica), may reflect margin-parallel strike-slip movements induced by oblique subduction (Cobbold et al., 1991, 1992) beneath the convergent southern margin of Gondwana.
Reinterpretacion de Ramos (2008) sobre aloctonía de Patagonia y configuración tectónica de Patagonia
Mas información en: Victor A. Ramos, 2008. Patagonia: A paleozoic continent adrift? Journal of South American Earth Sciences. 26: 235–251
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