Miguel Angelo Stipp Basei

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.

Ediacaran 2,500-km-long synchronous deep continental subduction in the West Gondwana Orogen

The deeply eroded West Gondwana Orogen is a major continental collision zone that exposes numerous occurrences of deeply subducted rocks, such as eclogites. The position of these eclogites marks the suture zone between colliding cratons, and the age of metamorphism constrains the transition from subduction-dominated tectonics to continental collision and mountain building. Here we investigate the metamorphic conditions and age of high-pressure and ultrahigh-pressure eclogites from Mali, Togo and NE-Brazil and demonstrate that continental subduction occurred within 20 million years over at least a 2,500-km-long section of the orogen during the Ediacaran. We consider this to be the earliest evidence of large-scale deep-continental subduction and consequent appearance of Himalayan-scale mountains in the geological record. The rise and subsequent erosion of such mountains in the Late Ediacaran is perfectly timed to deliver sediments and nutrients that are thought to have been necessary for the subsequent evolution of sustainable life on Earth.

WAS THERE AN EDIACARAN CLYMENE OCEAN IN CENTRAL SOUTH AMERICA

Previous studies have proposed that a major suture resulted from the collision between the Amazonian and São Francisco-Congo cratons during the Cambrian, following the closure of a supposed Clymene Ocean. The proposal tentatively located this ocean along the Araguaia and Paraguay belts at the eastern margin of the Amazonian Craton, and its southern extension reached the Pampean belt in Argentina. In the present study we will argue that the existence of Ediacaran-Cambrian oceanic lithosphere in central South America is highly unlikely. West Gondwana was assembled during the convergence between the Amazonian, West African, São Francisco-Congo and Rio de La Plata cratons as well as the Saharan Metacraton, leading to the closure of the Goiás-Pharusian Ocean during the Neoproterozoic. Final closure and continental collision resulted in the development of the Transbrasiliano-Kandi mega-shear zone that cuts through several mobile belts, but leaves the cratonic areas totally untouched. Consistent results of radiometric dating along the Transbrasiliano (TB) mega-shear in South America and of metamorphic rocks of the Brasília Belt have indicated that the Neoproterozoic collision finished at ca. 620 Ma. After isostatic uplift, cooling, and denudation, between 590 and 500 Ma, emplacement of undeformed K-rich postorogenic granites represented the main tectonic event. At this time or afterwards, a series of small extensional sedimentary basins formed in graben troughs, most of which are within the TB tectonic corridor. They all were of extensional character, contrasting clearly with the convergent tectonics occurring within the coeval Pampean Orogen in Argentina. The main arguments showing that an Ediacaran to Cambrian oceanic closure in central Brazil is untenable include: (i) the assembly of West Gondwana was completed by ca. 600 Ma, when the convergence between the Amazonian, São Francisco and Rio de La Plata cratons had already ended. After this, there is no geological evidence of an oceanic lithosphere (for example, ophiolites, magmatic arcs, et cetera), ruling out the possible existence of an Ediacaran or Cambrian Clymene Ocean in Central Brazil; (ii) the Gurupi and Araguaia belts in Brazil, as well as the Bassaride and Rokelide belts in West Africa, are regarded as aulacogenic-type systems formed within an intraplate tectonic setting. Their tectonic history precedes the collision between the Amazonian and São Francisco-Congo cratons, as demonstrated by the linear structures of the Transbrasiliano megashear which truncate the N-S structural trends of the Araguaia Belt; (iii) there is a close correlation between the Corumbá Group of the Paraguay Belt in Brazil and the Arroyo del Soldado Group in Uruguay. These sedimentary sequences belonged to the same Ediacaran continental shelf and this is a powerful indicator for an Ediacaran connection between the Amazonian and Rio de La Plata cratons, which precludes the existence of a wide ocean (for example, the Clymene) between them. On the other hand, the tentative correlation between the Sierras de Cordoba and the Paraguay Belt cannot be accepted, because these are far apart and there is no similarity in lithology, metamorphism, or structural trends; (iv) the Puga paleopole is the most important evidence for the hypothesis of the Cambrian Clymene Ocean, however the age of about 600 Ma for this paleopole, taken on the basis of Sr and C isotopes, is loosely constrained. In addition this is located at low latitude, not far from the present pole, and therefore could be related to a younger remagnetization; (v) the Pampean Orogen is made up of medium- to high-grade metamorphic rocks constrained between 560 Ma and 520 Ma and therefore was tectonically active during most of the Cambrian. However, at this time, an oceanic lithosphere is not evident in the vicinity of the Paraguay belt, and in central Brazil extensional rather than convergent tectonic processes have been observed.

Migração de orógenos e superposição de orogêneses: um esboço da colagem brasiliana no Sul do Cráton do São Francisco, SE - Brasil

The Meridional Tocantins Orogen represents a collision nappe pile involving three different tectonic environments. A magmatic arc domain developed in the Paranapanema continental active margin (Socorro-Guaxupe Nappe); a subducted continental domain (Andrelândia Terrane); and a passive margin domain and/or related to the Sanfranciscana plate (Carrancas and Lima Duarte Nappe systems). The magmatic activity related to the subduction started at 670 Ma and its metamorphism took place around 625Ma (active margin geothermal pattern modified in the subducted domain). The magmatic arc was active during flysch-type sedimentation in the Andrelândia terrane. Deformation and metamorphism related to the collisional process migrated from E to NE between 620 and 580 Ma. In the western internal nappe the tectonic process was very short-lived, less than 7 Ma, with some 612 Ma post-tectonic plutonism. On the other hand, in the eastern nappe the same process lasted ca. 20 Ma. Exhumation in the subducted terrane was very fast (ca. 2,2 mm/year). Superposed magmatic activities controlled by extensional regimes (Itu Granite Belt) followed orogen exhumation and preceded installation of continental-marine successor basins at 570 Ma. The Aracuai Orogen produced by convergence between the eastern passive margin of the Sanfranciscana plate and the Juiz de Fora Terrane (a microplate in an active margin environment) exhibits a metamorphic peak in the internal domain at around 560 Ma. The migration of the Aracuai nappes towards the cratonic domain (thermal metamorphic event ca. 530 Ma) reflects the superposition of the Mantiqueira Orogenic System. In the Cambrian the basement of the cratonic border was regenerated and included in the Aracuai orogen which involved the foreland in a thin-skinned thrust system well documented in the Quadrilatero Ferrifero. Oblique collision between the Mantiqueira Orogenic system and the proto-continent formed during the Meridional Tocantins orogeny was responsible for greenschist metamorphism at about 550 Ma in the successor basins. After this, the collisional process migrated northwards until 500 Ma. In the southern part of the Sao Francisco Craton the Brasiliano orogenic collage represents the interaction of several short orogenic steps in a long, convergent tectonic process involving plate consumption with eastward polarity and westward orogenic migration that lasted until the Cambro-Ordovician limit. The latest documented pre-collage metamorphism observed in the cratonic basement occurred between 2.03 and 2.06 Ga, probably under extensional conditions.

Las Matras Block, Central Argentina (37°S-67°W): the Southernmost Cuyania Terrane and its Relationship with the Famatinian Orogeny

Abstract The Las Matras Block in Central Argentina constitutes the southernmost part of the Cuyania terrane, which was accreted to the southwestern margin of Gondwana during the Early to Mid Ordovician Famatinian orogeny. The Grenville-aged rocks of the Las Matras Block are represented by the tonalitic to trondhjemitic Las Matras pluton. A new U-Pb conventional zircon age of 1244±42 Ma confirms previous Sm-Nd and Rb-Sr isochron ages of this pluton. Mineral composition data are consistent with the tonalitic-trondhjemitic character of the pluton, and constrain its emplacement level to 1.9 to 2.6 kb. This shallow level of emplacement and the undeformed character of the pluton are distinctive features of this southernmost basement. A regional comparison indicates that the igneous-metamorphic evolution of the Grenville-aged basement rocks of the Cuyania terrane occurred over a period of more than 200 million years, with ages older than 1200 Ma up to those close to 1000 Ma. The shallowest crustal level is found in Las Matras, suggesting a southward shallowing of the exposed level of basement. The deformation and metamorphism associated with the collisional Famatinian orogeny affect both the Cuyania terrane and the adjacent western margin of Gondwana, and the Gondwana margin was also the locus of the related arc magmatism, but the compressive effects of the collision decrease in intensity toward the south. The Famatinian metamorphism and magmatism continue even further south into the Patagonia region, but the southern continuity of the Cuyania terrane into this region remains uncertain.

Litoquímica, geocronologia U-Pb e Geologia isotópica (Sr-Nd-Pb) das rochas graníticas dos batólitos Cunhaporanga e Três Córregos na porção sul do Cinturão Ribeira, Estado do Paraná

Durante a evolucao da porcao sul do Cinturao Ribeira no Estado do Parana, o Ciclo Brasiliano-Pan Africano foi responsavel pela geracao de um grande volume de rochas graniticas com destaque para os batolitos Cunhaporanga (BCP) e Tres Corregos (BTC). Estudos litoquimicos e isotopicos revelaram a existencia de tipos graniticos distintos entre dois batolitos e internamente aos mesmos, definidos por unidades graniticas. A principal diferenca entre os dois batolitos esta no comportamento dos alcalis com o BCP, a NW mais potassico e o BTC, a SE mais sodico. Os dados isotopicos mostram claramente a participacao de fontes infracrustais distintas de longa residencia crustal, evidenciadas pelos isotopos de Pb em K-feldspato, pelos valores negativos de µNd (-11 a -13 para o BCP e -17 a -19 para o BTC) e pelas idades Tdm (1,8 a 2,0 Ga para o BCP e 2,2 a 2,4 Ga para o BTC). Estas fontes estariam situadas em uma crosta continental paleoproterozoica com a participacao de fontes mantelicas e supracrustais na genese das rochas graniticas do BCP e BTC. A Unidade Ribeirao Butia do BCP e as Unidades Arrieiros e Paina do BTC estao representadas predominantemente por monzogranitos e granodioritos e raros tonalitos, metaluminosos, calcio-alcalinos de alto a medio K, formados entre 630 a 620 Ma em ambiente de margem continental ativa. As Unidades Pirai do Sul e Santa Rita do BCP sao constituidas por monzogranitos fracamente peraluminosos, de alto K e as Unidades Sao Sebastiao e Conceicao do BTC sao constituidas respectivamente por quartzo-monzonitos e monzo a sienogranitos fracamente metaluminosos, de alto K. Estas unidades foram formadas em ambiente sin- a tardi-colisional entre 620 e 590 Ma.

Proterozoico medio y Paleozoico inferior de la sierra de Umango, antepaís andino (29°S), Argentina: edades U-Pb y caracterizaciones isotópicas

En el basamento cristalino de la sierra de Umango se han distinguido varias unidades igneo-metamorficas, formadas principalmente entre el Proterozoico medio y el Paleozoico inferior. Incluye ortogneises graniticos, una sucesion de esquistos y marmoles, anfibolitas, y tres plutones graniticos de reducido tamano. Se han realizado analisis quimicos de granitoides y anfibolitas, y determinaciones de edad por los metodos U-Pb y K-Ar. La edad U-Pb en circon del Ortogneis Juchi (1108 ± 13 Ma) ratifica la asignacion al Proterozoico medio por anterior datacion Rb-Sr (1030& ± 30 Ma). La edad U-Pb en circon del Granito El Penon (473 ± 17 Ma) es ligeramente mayor que la obtenida por el metodo Rb-Sr (469 ± 9 Ma), aunque concordante si se tiene en cuenta el error de las determinaciones. Las edades K-Ar sobre muscovitas del Granito El Penon (409 ± 14 y 381 ± 11 Ma) son relacionadas a deformacion y metamorfismo ocurridos en el Devonico. Los isotopos de Nd y Sr permitieron establecer diferencias entre un conjunto de unidades relacionadas con el Ciclo Grenvilliano y otro con los Ciclos Pampeano-Famatiniano. Los datos obtenidos en la sierra de Umango indican que en las Sierras Pampeanas Occidentales hay nucleos de un basamento del Proterozoico medio (Ciclo Grenvilliano), mas una sucesion de esquistos y marmoles con anfibolitas, y plutones graniticos de reducido tamano, formados en el lapso Proterozoico superior-Devonico (Ciclos Pampeano-Famatiniano). La estructura interna de mayor relevancia, formada en el Ordovicico-Devonico muestra clara vergencia tectonica occidental y estaria relacionada con laminas de corrimiento (nappes) de una tectonica colisional. Las mismas se proyectarian desde el interior orogenico (Sierras Pampeanas Orientales) hacia el contiguo antepais de una de las placas interactuantes. Las sedimentitas clasticas del Ordovicico superior-Devonico en Precordillera (cuenca de antepais) completan el escenario geodinamico.

Timing of deformation in the Sarandí del Yí Shear Zone, Uruguay: implications for the amalgamation of Western Gondwana during the Neoproterozoic Brasiliano–Pan‐African Orogeny

U-Pb and Hf zircon (sensitive high-resolution ion microprobe -SHRIMP- and laser ablation-inductively coupled plasma-mass spectrometry -LA-ICP-MS-), Ar/Ar hornblende and muscovite, and Rb-Sr whole rock-muscovite isochron data from the mylonites of the Sarandi del Yi Shear Zone, Uruguay, were obtained in order to assess the tectonothermal evolution of this crustal-scale structure. Integration of these results with available kinematic, structural, and microstructural data of the shear zone as well as with geochronological data from the adjacent blocks allowed to constrain the onset of deformation along the shear zone at 630–625 Ma during the collision of the Nico Perez Terrane and the Rio de la Plata Craton. The shear zone underwent dextral shearing up to 596 Ma under upper to middle amphibolite facies conditions, which was succeeded by sinistral shearing under lower amphibolite to upper greenschist facies conditions until at least 584 Ma. After emplacement of the Cerro Caperuza granite at 570 Ma, the shear zone underwent only cataclastic deformation between the late Ediacaran and the Cambrian. The Sarandi del Yi Shear Zone is thus related to the syncollisional to postcollisional evolution of the amalgamation of the Rio de la Plata Craton and the Nico Perez Terrane. Furthermore, the obtained data reveal that strain partitioning and localization with time, magmatism emplacement, and fluid circulation are key processes affecting the isotopic systems in mylonitic belts, revealing the complexity in assessing the age of deformation of long-lived shear zones.

The Acampamento Velho Formation, a Lower Cambrian Bimodal Volcanic Package: Geochemical and Stratigraphic Studies from the Cerro Do Bugio, Perau and Serra De Santa Bárbara (Caçapava Do Sul, Rio Grande Do Sul, RS - Brazil)

Abstract During the late stages of the Brasiliano orogenic cycle (Lower Cambrian), the Camaqua Basin was gradually filled by the alkaline-trending, bimodal volcanic rocks of the Acampamento Velho Alloformation. This volcanic package consists of two facies associations: the lower one composed of andesites and basaltic andesites, and the upper one of rhyolitic rocks. The rhyolitic association comprises alternating pyroclastic rocks (lapilli, tuffs and welded tuffs) in the middle section and flows at the top. Geochemical evidence, especially trace elements and REE, confirmed the stratigraphic succession proposed herein for the volcanic rocks, as well as their co-genetic relationships and the fractional crystallization of the felsic sequence. The Acampamento Velho Formation seems to have been generated in an extensional regime preceding the collision of the Rio de la Plata and Kalahari continental plates. This extensional regime probably occurred during subduction of the Adamastor oceanic plate beneath the Rio de la Plata plate in a retroarc setting.

U-Pb evidence for late Neoarchean crustal reworking in the Southern São Francisco Craton (Minas Gerais, Brazil)

The Passa Tempo Metamorphic Complex is one of several metamorphic complexes that form the Archean sialic crust of the southern Sao Francisco Craton. It encompasses hypersthene-bearing gneissic rocks, with subordinate NW- or EW-trending mafic-ultramafic bodies and granodioritic to alkali-granitic, weakly foliated, and light-colored granitoids. These granitoids are the product of generalized migmatization that followed granulite-facies metamorphism. To determine the ages of the granulite-facies metamorphism and granitoid genesis, we obtained U-Pb ages on zircon extracted from the mesosome and leucosome of the migmatitic gneisses. For the mesosome, a discordia that intercepts Concordia at 2622 ± 18 Ma is interpreted as a minimum age for granulite-facies metamorphism. For the leucosome, the upper intercept of discordia at 2599 ± 45 Ma corresponds to migmatization and granitoid genesis. Contemporaneous metamorphism and magmatism have been documented elsewhere in the Sao Francisco Craton, especially in the southern portion, demonstrating vast and vigorous reworking of sialic crust by the end of the Neoarchean.