Did Patagonia collide with Gondwana in the Late Paleozoic? Some insights from a multidisciplinary study of magmatic units of the North Patagonian Massif

The origin of Patagonia and its relations with the South American crustal blocks to the north have been a matter of debate for decades. We report results from a multidisciplinary study centered on Paleozoic granitoids exposed in the northeastern corner of the North Patagonian Massif. Microstructural and magnetofabric studies reveal two suites of granitoids. Late Carboniferous (?) granitoids (Yaminué Complex, Tardugno Granodiorite, Cabeza de Vaca leucogranite) were emplaced and subsequently deformed in a major NNE-SSW compressive stress regime that also provoked top-to-the-SW thrust deformation in shallow crustal levels. Gravity and geobarometric studies show that the same major deformation event has been recorded at different crustal levels. The age and type of deformation of this event recorded across the northern boundary of Patagonia strongly supports a Late Carboniferous – Early Permian frontal collision between Patagonia and Gondwana. This major deformation event ceased by 281 Ma when the Navarrete Plutonic Complex, which shows mainly magmatic fabrics, was emplaced under a far-field WNW-ESE stress regime. Crustal continuity between the North Patagonian Massif and the Pampia and Arequipa- Antofalla terranes is suggested by similar Late Paleoproterozoic crustal model ages, comparable detrital zircon ages in Early Paleozoic successions, the apparent continuity of an Early Ordovician continental magmatic arc and paleomagnetic data. Reconciliation of this evidence with the Late Paleozoic frontal collision is obtained in a tectonic model that suggests that the North Patagonian Massif is a parautochthonous crustal block

Counterclockwise tectonic rotation of the Del Salto Formation (Lower Permian), San Juan Province

A palaeomagnetic study was made on the Early Permian Del Salto Formation, exposed in the Western Precordillera (31.3°S, 69.2°W). Eleven sites were sampled for siltstone and fine-grained sandstone in the middle-upper part of the succession exposed in the creek at kilometre 114, San Juan. Standard demagnetization techniques permitted the identification of a characteristic remanence that showed good within-site consistency of direction at eight sites. Application of a tectonic correction showed that the characteristic remanence is pre-tectonic. A palaeomagnetic pole for the Del Salto Formation was computed, SC: 26.8°S, 357.0°E.δp = 4.4°, δm = 6.4°, N = 8. The position of SC indicates that the sampling locality underwent a counterclockwise rotation of about 40° around a vertical axis. The age of the rotation cannot be determined from the available data but it is most likely to have happened in either Permian or Late Cenozoic times.Fil:Rapalini, A.E. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Mena, M. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

Géodynamique andine : résumé étendus = Andean geodynamics : extended abstracts

Une étude paléomagnétique des formations Ponon Trehue (Ordovicien moyen) et El Imperial (Carbonifère supérieur), affleurant à travers le bloc de San Rafael, a mis en évidence une aimantation syntectonique acquise lors de l'évènement dit de San Rafael, d'âge Permien inférieur. La région affectée par cet épisode de réaimantation serait donc plus vaste que ne le laissait entendre des études précédentes. (Résumé d'auteur

The sanrafaelic remagnetization of precordillera in the permian: New evidence

A paleomagnetic study, carried out on 9 sites of the La Flecha Formation (facies Los Sapitos, Upper Cambrian, Marjuman-Steptoean) exposed along the la Angostura creek, Precordillera of La Rioja (29.5°S, 68.7°W), permitted to determine two magnetic components. One is postectonic and of low coercivities and unblocking temperatures (A, Dec: 7.5°, Inc: -40.3°, a95: 10.1°, N: 6), being interpreted as a recent remanence of probable viscous origin. A second, pretectonic, component was isolated in seven sites with opposite polarities and is likely carried by magnetite (B, Dec: 206.9°, Inc: 63.3°, a95: 12.8°, N: 7). The paleomagnetic pole position for this component (Lat: 63.8°S, Long: 244.6°E, dp: 15.9°, dm: 20.2°) suggests that the remanence was acquired by the end of the Permian and it is interpreted as produced by the regional remagnetization associated with the San Rafael orogenic phase found in other units of the Precordillera. An analysis of pole positions, type of magnetizations, polarities and geographic distribution of the remagnetized units suggests a pattern of time-space migration of the remagnetization from west to east, between the Early Permian and the Permian-Triassic. This pattern is consistent with a model of remagnetization by migration of fluids expelled from the orogenic area that reached rocks not involved directly in the deformation. The pole position obtained for La Flecha Formation also suggests the lack of major tectonic rotations in the study area since the Permian. © 2005 Asociación Geológica Argentina.Fil:Rapalini, A.E. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

Geochronologic evidence of a large magmatic province in northern Patagonia encompassing the Permian-Triassic boundary

The Los Menucos Complex (northern Patagonia) consists of ~6 km thick succession of acidic and intermediate volcanic and pyroclastic products, which has been traditionally assigned to the Middle/Late Triassic. New U/Pb (SHRIMP) zircon crystallization ages of 257 &177; 2 Ma at the base, 252 &177;2 Ma at an intermediate level and 248 &177; 2 Ma near the top of the sequence, indicate that this volcanic event took place in about 10 Ma around the Permian-Triassic boundary. This volcanism can now be considered as the effusive terms of the neighboring and coeval La Esperanza Plutono-Volcanic Complex. This indicates that the climax of activity of a large magmatic province in northern Patagonia was coetaneous with the end-Permian mass extinctions. Likely correlation of La Esperanza- Los Menucos magmatic province with similar volcanic and plutonic rocks across other areas of northern Patagonia suggest a much larger extension than previously envisaged for this event. Its age, large volume and explosive nature suggest that the previously ignored potential role that this volcanism might have played in climatic deterioration around the Permian-Triassic boundary should be investigated.Financial support for these investigations came from the Agencia Nacional de Investigaciones Científicas y Tecnicas (Argentina) through grants PICT2013-1162 to M.G.L.L. and PICT2015-206 to A.E.R. Tom as Luppo thanks CONICET for a 5-year Doctoral scholarship and C.I.M.D. would like to thank Fundaleu (Fundacion para la lucha contra la Leucemia)

Geochemistry and geochronology of the shallow-level La Esperanza magmatic system (Permian-Triassic), Northern Patagonia

The La Esperanza plutonic-volcanic complex is the largest Late Paleozoic-Early Triassic composite magmatic system of northern Patagonia. This paper reports new SHRIMP U-Pb zircon ages and K-Ar muscovite dating as well as whole-rock geochemical data for selected units. In addition, we present some new and reprocessed whole-rock Sr isotopic compositions. On the basis of the new and published data, three compositionally and isotopically distinct high-K magnesian calc-alkaline series were distinguished. Two of these are characterized by high Ba-Sr: (i) biotite and muscovite bearing rhyolites and granites (265 ± 2 Ma; 260 ± 2 Ma) and (ii) metaluminous amphibole-biotite bearing granodiorites (273 ± 2 Ma), monzogranites (255 ± 2 Ma), dacites (253 ± 2 Ma), and slightly peraluminous granites (dated herein as 251 ± 2 Ma). There is also a low Ba-Sr series of high-silica metaluminous rocks (granites and acid dike swarms; 250 ± 2 Ma and ≈244 ± 2 Ma). Geochemistry coupled with geochronology revealed a pulsatory multi-sourced open magmatic system with mafic magma replenishment and reactivation of crystal mushes that occurred before upward migration to upper crustal levels. Mafic magmas alternated with crust-derived magmas incrementally assembled in subvolcanic levels over 30 Ma. Zircon crystallization and mica cooling ages in the granite units allowed detection of two magmatic lulls, between 270 and 265 Ma and between 260 and 255 Ma. Both episodes coincide with a period of exhumation in upper crustal levels. The new temporal and geochemical constraints allow correlation of the La Esperanza plutonic-volcanic complex with the Los Menucos Group (258-248 Ma), encompassing a volume of magmatism comparable to a moderately sized large igneous silicic province. These mid-to-late Permian to Middle Triassic rocks record the transition between subduction-related magmatism (> 273 Ma) and post-orogenic extensional magmatism (< 250 Ma) in the Gondwana margin. Even though this magmatism would be coeval with the proposed collision of the Patagonia terrane, no expected syn-collisional magmatism or associated deformation were found in upper crustal levels. However, the different nature and melting conditions of the inferred sources of the magmas that crystallized before 270 Ma, between 265 and 260 Ma, and from 255 to 245 Ma, suggest that the La Esperanza plutonic-volcanic complex was assembled during a 30 Ma period of major plate reorganization.This work was supported by the Agencia Nacional de Promoción Científica y Tecnológica (Argentina) under grants PICT2013-1162 and PICT2016-2188, PICT2016-3148

The Gondwana connections of northern Patagonia

A multidisciplinary study (U-Pb sensitive high-resolution ion microprobe geochronology, Hf and O isotopes in zircon, Sr and Nd isotopes in whole-rocks, as well as major and trace element geochemistry) has been carried out on granitoid samples from the area west of Valcheta, North Patagonian Massif, Argentina. These confirm the Cambrian age of the Tardugno Granodiorite (528 ± 4 Ma) and the Late Permian age of granites in the central part of the Yaminué complex (250 Ma). Together with petrological and structural information for the area, we consider a previously suggested idea that the Cambrian and Ordovician granites of northeastern Patagonia represent continuation of the Pampean and Famatinian orogenic belts of the Sierras Pampeanas, respectively. Our interpretation does not support the hypothesis that Patagonia was accreted in Late Palaeozoic times as a far-travelled terrane, originating in the Central Transantarctic Mountains, and the arguments for and against this idea are reviewed. A parautochthonous origin is preferred with no major ocean closure between the North Patagonian Massif and the Sierra de la Ventana fold belt