Carlos A. Cingolani

Early Paleozoic orogenic belt of the Andes in southwestern South America: Result of Laurentia-Gondwana collision?

The late Precambrian to early Paleozoic age rock units of the Pampean ranges, the Puna, and the North Patagonian massif of southwestern South America constitute the Famatinian orogenic belt. They are interpreted as an Ordovician collisional belt between the Occidentalia terrane and the Gondwana craton. They include mafic and ultramafic belts of Neoproterozoic to early Paleozoic age. An intense tectonothermal event resulted from the collision; syntectonic granitoids represent crustal melting. In that collision syntectonic to late-tectonic foreland basins developed. The recently proposed juxtaposition of Laurentia and East Antarctica-Australia in the Neoproterozoic raises the possibility that Laurentia and western South America were close together in the early Paleozoic, and therefore that the Famatinian belt resulted from Laurentia-Gondwana collision. Occidentalia, which is bordered by a Cambrian carbonate platform similar to that of eastern North America, may be a sliver detached from Laurentia during Late Ordovician time.

El basamento cristalino de los Andes norpatagónicos en Argentina: geocronología e interpretación tectónica

Se presentan los resultados obtenidos en el estudio isotopico de rocas del basamento igneo-metamorfico de los Andes expuesto en Argentina, aproximadamente a los 40°S, en las provincias de Neuquen y Rio Negro. Fueron utilizadas metodologias isotopicas, para obtener informacion sobre la edad de emplazamiento de cuerpos igneos deformados ductilmente (U-Pb en circones), tiempo de enfriamiento luego de los procesos igneos y metamorficos que modelaron a este basamento (K-Ar en micas; U-Pb en titanita) y la historia cortical involucrada (Sm-Nd en roca total). No se obtuvieron edades proterozoicas, como lo sugerian anteriores determinaciones Rb-Sr y K-Ar. Los datos definen en la region dos eventos igneo metamorficos. El mas antiguo, conservado en tonalitas y granitos deformados de los alrededores de San Martin de los Andes, tiene edades U-Pb en circones de 420 a 380 Ma (Devonico) y edades K-Ar en micas de 375 a 310 Ma. El mas joven esta bien representado en los afloramientos al sur del Rio Limay, desde Paso Flores a Rio Chico-Cushamen y tiene edades U-Pb en circones de rocas metaigneas en torno a 280 Ma (Permico inferior) y edades de enfriamiento en micas proximas a 260-250 Ma. Ambos conjuntos se generaron en ambiente de arco magmatico y denotan la accion de dos episodios principales separados por ca. 100 Ma, posibles de relacionar con los Movimientos Chanicos y Gondwanicos, respectivamente. Las edades modelo Sm-Nd TDM, con valores entre 1907 y 1165 Ma, eNd para el momento de cristalizacion de las rocas de preferencia entre -3,0 y -8,4 y e Nd(0) preferentemente entre -6,9 a -11,9 sugieren reciclado de corteza proterozoica con escasa adicion de corteza juvenil durante el Paleozoico.

Two Paleoproterozoic Orogenies in the Evolution of the Tandilia Belt, Buenos Aires, as Evidenced by Zircon U-Pb SHRIMP Geochronology

The superposition of two orogenies during the evolution of the Paleoproterozoic Tandilia Belt, Argentina, produced complex geological structures in the granitic-gneissic-migmatitic Buenos Aires Complex. Ion microprobe (SHRIMP II) dating of 61 spots in 56 zircon crystals from 10 rock samples shows tectonic activity related to the accretionary Encantadas orogeny of 2.25-2.12 Ga, although a major overprint of the collisional Camboriú orogeny of 2.10-2.08 Ga is also recognized. Only one zircon age is Neoarchean. Two age peaks for the Tandilia Belt are ~2.16 Ga and ~2.08 Ga, which occur along most of the Brazilian shield, both in granite-greenstone and in granulite-facies terrains. All zircon crystals show complex internal structures, but magmatic and metamorphic ages are restricted to the Trans-Amazonian Cycle between 2234 Ga and 2065 Ma. The Trans-Amazonian Cycle is a dominantly juvenile event followed shortly thereafter by crustal recycling. The Tandilia Belt exhibits a comparable petrotectonic evolution to the Piedra Alta Terrane of Uruguay and to the basement of the Dom Feliciano Belt in southern Brazil. These results require a re-evaluation and integration of the models proposed for the evolution of the southern Brazilian shield in relation to western Gondwana and other Precambrian supercontinents. Although only one Archean age is registered in the dated zircon crystals, Nd isotopic data on 13 samples show that the Tandilia Belt has Neoarchean crust formation ages, similar to other rocks from the southern Brazilian shield.

Ordovician K-bentonites in the Argentine Precordillera: relations to Gondwana margin evolution

Abstract Ordovician K-bentonites have now been recorded from >20 localities in the vicinity of the Argentine Precordillera. Most occur in the eastern thrust belts, in the San Juan Limestone and the overlying the Gualcamayo Formation, but a few ash beds are known also from the central thrust belts. The oldest occur in the middle Arenig I. victoriae lunatus graptolite (Oe. evae conodont) Zone, and the youngest in the middle Llanvirn P. elegans (P. suecicus) Zone. Mineralogical characteristics, typical of other Ordovician K-bentonites, include a matrix of illite/smectite mixed-layer clay and a typical felsic volcanic phenocryst assemblage: biotite, beta-form quartz, alkali and plagioclase feldspar, apatite, and zircon, with lesser amounts of hornblende, clinopyroxene, titanite and Fe-Ti oxides. The proportions of the mineral phases and variations in their crystal chemistry are commonly unique to individual (or small groups of) K-bentonite beds. Glass melt inclusions preserved in quartz are rhyolitic in composition. The sequence is unique in its abundance of K-bentonite beds, but a close association between the Precordillera and other Ordovician sedimentary basins cannot be established. The ash distribution is most consistent with palaeogeographical reconstructions in which early Ordovician drifting of the Precordillera occurred in proximity to one or more volcanic arcs, and with eventual collision along the Andean margin of Gondwana during the mid-Ordovician Ocloyic event of the Famatinian orogeny. The Puna-Famatina terrane northeast of the Precordillera might have served as the source of the K-bentonite ashes, possibly in concert with active arc magmatism on the Gondwana plate itself.

The Rio Chico Paleozoic crystalline complex and the evolution of Northern Patagonia

Abstract The Rio Chico Complex, which includes the Cushamen Metamorphics and the Mamil Choique Granitoids (Ordovician, 439±10 Ma), constitutes the crystalline basement of the Rio Chicoregion (southwestern North Patagonian massif). The metamorphics and migmatites resulted from a medium to high grade tectono-thermal event which caused local antexis of a protolith of graywackes, pelites and some quartz-rich sandstones. These rocks show a main deformational episode, trending mainly NNW, which controlled the post-Paleozoic structures. A Devonian age granite (387±17 Ma) intruded this complex. The Ordovician and Devonian granitoids, which are calc-alkaline, and per to metaluminous, may be related to anatexis during the Taconian and Acadian Laurentia-Gondwana collisional events. During the Permian (260±5 Ma) leucogranites, some bearing garnet, were intruded into the ancient orogenic belt following an important phase uplift. They are related to further melting of continental crust during extension in an intraplate environment, and are included in the Choiyoi acidic Province.

First Late Ordovician Paleomagnetic Pole for the Cuyania (Precordillera) Terrane of Western Argentina: a Microcontinent or a Laurentian Plateau

Abstract Time and tectonic processes involved in docking of the Argentine Precordillera (Cuyania terrane) against SW Gondwana has been a matter of much debate. A paleomagnetic study on the Early Caradoc Pavon Formation, exposed in the San Rafael block, province of Mendoza, Argentina, is presented. After detailed thermal and alternating field demagnetizations two geologically significant magnetic components were defined. A widespread post-tectonic component (A) is present in most sites of the Pavon Formation, with dual polarities, and is coincident with the characteristic remanence isolated from a Permo-Triassic rhyolitic dome intruding the sediments. Its pole position (83.7°S, 271.0°E, dp = 6.8°, dm = 9.0° N = 11 sites) falls on the Late Permian-Early Triassic South American reference poles suggesting that this component was acquired during the Choiyoi magmatic phase. A second component (B) also shows dual polarities and a positive fold test suggesting a primary origin. Unblocking temperatures and rock magnetic experiments indicate that B is carried either by hematite or magnetite at different sites. Anisotropy of magnetic susceptibility results suggest a depositional fabric and no remanence distortion due to deformation or compaction. A paleomagnetic pole computed from this remanence (PV) falls on 3.6°N, 346.4°E (dp = 2.9°, dm = 4.6° n = 22 samples). It indicates a paleolatitude around 26°S for deposition of Pavon sediments and constrains the paleogeographic evolution of Cuyania during the Ordovician, which was still at subtropical latitudes by the Early Caradoc. PV is consistent with the Laurentian Late Ordovician reference pole if Cuyania remains attached to SE Laurentia for the Early Caradoc, while it shows a significant cw rotation with no paleolatitude anomaly respect to the Gondwana reference pole when kept in its present position in SW South America. These comparisons are interpreted in three possible alternatives for the paleogeographic and tectonic setting of Cuyania in the Late Ordovician.

Land plants in the Devonian Villavicencio Formation, Mendoza Province, Argentina

Plant assemblages are described from two localities, San Isidro Creek and Vaqueria, in the marine basal Devonian Villavicencio Formation in Mendoza Province, the Argentine Precordillera. Abundant smooth and more rarely axes with enations occur with rare fertile specimens and isolated swollen structures interpreted as sporangia. Sterile ‘leafy’ axes are placed in a new morphotaxon, Bowerophylloides, and a new genus, Isidrophyton, has been erected to accommodate sterile axes with vertically elongate, fusiform surface features and terminal sessile sporangia borne in pairs. The plants are at similar grades of organisation to those in coeval assemblages elsewhere, but cannot be assigned unequivocally to existing taxa. The locations of these Lochkovian assemblages are plotted on basal Devonian palaeocontinental reconstructions and it is concluded that the composition and the isolated position of the Argentina assemblages on the western margin of Gondwana at mid latitudes in a possibly cool temperate climate hint at a distinct phytogeographic unit.

Middle Devonian lycopsids from high southern palaeolatitudes of Gondwana (Argentina)

Fossil plants are described from the upper part of the Devonian Lolen Formation, Sierra de la Ventana, Buenos Aires Province, Argentina, in the area of Estancia Las Acacias. The sequence is composed mainly of dark grey shales, and fossils were found in a single horizon where thin inter- layered beds of fine reddish-brown micaceous sandstones appear where the environment of marine deposition became more shallow. The age of the Lolen Formation is presently established on the basis of brachiopods, these being characteristic elements of the Malvinokaffric realm from the Gondwana Lower Devonian (Emsian). The fossil plants are remarkably preserved given that they are in rocks that have undergone intense deformation. The plants are identified as Haplostigma sp. and Haskinsia cf. H. colophylla, and suggest a Middle Devonian age (Givetian) for the fossil-bearing levels. Haskinsia, identified on the basis of leaf morphology, is the first well-delimited Middle Devonian lycopsid genus described from Argentina, and the record from the most southerly palaeolatitude. During the Middle Devonian, Haskinsia was distributed in tropical, warm temperate and high southern latitude, ?cool temperate zones.

Laurentia-Gondwana collision: the origin of the Famatinian-Appalachian Orogenic Belt (a review)

Abstract Evidence from Cambrian to Devonian palaeogeography, the Laurentian provenance of Precordillera, and comparison of Lower Palaeozoic tectonic, metamorphic, and magmatic events, suggests a common collisional origin for the Taconian Appalachians and the Famatinian (Puna, Sierras Pampeanas, North Patagonian Massif) orogenic belt. Detached from Eastern Laurentia following collision, the Occidentalia terrane (west of the Famatinian orogenic belt) is partly covered by the Precordillera carbonate bank, which contains syn-collisional 460 Ma K-bentonites, related to the Puna and Famatinian arcs. In both the Appalachian and the Famatinian belts, a mostly Cambrian pre-collisional subduction regime developed at the western margin of Gondwana, following the opening of Iapetus. Laurentia-Gondwana collision produced deep thrusting and crustal melting. As in the Blue Ridge of the Appalachian Mountains, oceanic and mantle rocks, and slices of Grenvillian basement were emplaced along the western side of the Famatinian belt. This hypothesis gives continuity to a paired Taconian-Famatinian tectonic setting: a western Lower Palaeozoic platform overlying Grenvillian basement, and an eastern complex of a Lower-Middle Palaeozoic igneous-metamorphic belt with Proterozoic inliers.

Plant assemblages from SW Gondwana: further evidence for high-latitude vegetation in the Devonian of Argentina

Abstract Plant assemblages are described from the Talacasto and Punta Negra Formations, San Juan Province, the Argentine Precordillera. A detailed facies study involving trace fossil assemblages indicates that the plants were buried in shallow low- to high-energy marine environments, but not in turbidites as previously postulated. Apart from a single specimen resembling Cooksonia from the Lochkovian Talacasto Formation, the abundant compression fossils consist of short lengths of smooth axes, occasional defoliated lycopod stems, and very rare isolated sporangia. It is thus impossible to assign any to existing taxa with confidence nor are there sufficient distinguishing characters to erect new ones. Although the possible age of the formations in the Precordillera remains conjectural, the fossils themselves, even preserved in such fragmentary states, indicate much lower grades of organization than seen in coeval assemblages elsewhere, particularly on the Laurussian and South China palaeocontinents. Palaeocontinental reconstructions of Gondwana indicate that the localities are within 30° of the palaeo South Pole. Thus a global latitudinal gradient in vegetation is postulated for the Early Devonian with decreasing disparity and diversity at higher latitudes. An explanation for the latter is explored in terms of light regime and temperature in highly stressed environments.