The Rodeo de la Bordalesa Tonalite Dykes as a Lower Devonian Magmatic Event: Geochemical and Isotopic Age Constraints

One of the ‘pre-Carboniferous units’ from the San Rafael Block is the sedimentary Río Seco de los Castaños Formation, which is distributed in isolated outcrops within the Block. At the Rodeo de la Bordalesa area two small intrusives in the mentioned unit were mapped, composed of tonalitic rocks, lamprophyre (‘spessartite-kersantite’) and aplite dykes.We present in this paper, geochemical and isotopic data from the gray tonalitic rocks with abundant mafic enclaves and late magmatic aplite veins. The country rocks are a folded sequence of feldspathic sandstones, wackes, and shales. The Rodeo de la Bordalesa tonalite dykes are characterized by high to medium potassium concentration, with metaluminous composition and I-type calc-alkaline signature. The 401 ± 4 Ma U–Pb zircon age corresponds to the emplacement time and it is confirmed by the K–Ar biotite age. The Rb–Sr whole rocks and biotite age of 374 ± 4 Ma could be related to deformation during the ‘Chanic’ tectonic phase. Nd model ages (TDM) show an interval between 1 and 1.6 Ga, indicating Mesoproterozoic age derivation, whereas the negative εNd is typical from crustal sources. The crystallization age for the Rodeo de la Bordalesa tonalite corresponds to a Lower Devonian time and suggests that part of the Late Famatinian magmatic event is present in the San Rafael Block. The dykes are contemporaneous with the large peraluminous batholith in Pampeanas Ranges, with the transpressional shear belts during ‘Achalian’ event and could be correlated with the Devonian magmatism present in the southern part of the Frontal Cordillera. The geochemical and geochronological data allow us to differentiate the Rodeo de la Bordalesa tonalite from the mafic rocks exposed at the El Nihuil area.Facultad de Ciencias Naturales y Muse

Silurian-Devonian Land-Sea Interaction within the San Rafael Block, Argentina: Provenance of the Río Seco de los Castaños Formation

The Río Seco de los Castaños Formation (RSC) is one of the ‘pre-Carboniferous units’ outcropping within the San Rafael Block assigned to Upper Silurian–Lower Devonian age. We review the provenance data obtained by petrography and geochemical-isotope analyses as well as the U–Pb detrital zircon ages. Comparison with La Horqueta Formation is also discussed. The main components of this marine fine-grained siliciclastic platform are sandstones and mudstones. The conglomerates are restricted to channel fill deposits developed mainly at the Lomitas Negras location. A low anchizone for the RSC was indicated by illite crystallinity index. From the geochemical proxies described above (Manassero et al. in Devonian Change: Case studies in Palaeogeography and Palaeoecology. Geological Society, 2009) a provenance from an unrecycled crust with an average composition similar to depleted compared with average Upper Continental Crust is suggested. TDM ages are within the range of the Mesoproterozoic basement and Palaeozoic supracrustal rocks of the Precordillera-Cuyania terrane. εNd values of the RSC are similar to those from sedimentary rocks from the Lower Palaeozoic carbonate-siliciclastic platform of the San Rafael Block. These data suggest an Early Carboniferous (Mississipian) low-metamorphic (anchizone) event for the unit. It is correlated with the ‘Chanic’ tectonic phase that affected the Precordillera-Cuyania terrane and also linked to the collision of the Chilenia terrane in the western pre-Andean Gondwana margin. As final remarks we can comment that the studied RSC samples show dominant source derivation from Famatinian (Late Cambrian-Devonian) and Pampean-Brasiliano (Neoproterozoic-Early Cambrian) cycles. Detritus derived from the Mesoproterozoic basement are scarce. U–Pb data constrain the maximum sedimentation age of the RSC to the Silurian–Early Devonian.Centro de Investigaciones Geológica

Roberto Caminos (1931-1997) un geólogo regional con base petrológica: desde macro- a microescala

Graduated at Buenos Aires University, Roberto Caminos (1931-1997) was a distinguished regional geologist. The mapping of igneous and metamorphic terrains was based on his solid knowledge of petrology. He surveyed several geologic sheets, a work starting on the field with the use of a hand lens, and ending in the laboratory with the use of a microscope. This combination of mega- to meso- and microscopic scales gave him flexibility when mapping igneous and metamorphic rocks. Field surveys used to last several months and a lot of data were then gathered. His main contributions dealt with Sierras Pampeanas, Precordillera in La Rioja Province, Frontal Cordillera, San Rafael Block, North Patagonian Massif and Fueguian Andes. He divided the northwestern Sierras Pampeanas into two belts: (1) a western belt characterized by relative abundance of metamorphic rocks with calcareous origin and mafic to ultramafic rocks, and (2) an eastern belt characterized by the presence of granitic batholiths and metamorphic rocks with siliciclastic origin. These differences would later be explained by the collision of the Cuyania terrane against Gondwana. In Frontal Cordillera, he chronostratigraphically reassigned the previous Lower Carboniferous units into the Late Carbonifeous - Permian interval, and recognized the importance of the San Rafael orogenic phase. Moreover, he described the Huarpic phase in detail, and with this the final structures of the Gondwanide cycle in Frontal Cordillera. In the North Patagonian Massif he grouped the Permian magmatic rocks into a plutonic complex unconformably followed by a plutono-volcanic complex. In the Isla de los Estados he described the metamorphism affecting the Lemaire Formation, with the first mention of stilpnomelane, an indicator of dynamic metamorphism equivalent to prehnite-pumpellite facies.Roberto Caminos (1931-1997), recibido en la Universidad de Buenos Aires, fue un notable geólogo regional. Su sólida base petrológica era apropiada para mapear terrenos ígneos y metamórficos. Relevó varias hojas geológicas, cuyo desarrollo comenzaban en el campo con lupa y finalizaban en el laboratorio con microscopio. Así combinaba escalas megascópicas con escalas meso- a microscópicas, método que le dio versatilidad en el mapeo de unidades ígneas y metamórficas. Los trabajos de campo duraban varios meses y se recogía mucha información. Sus contribuciones más importantes fueron en Sierras Pampeanas, Precordillera de La Rioja, Cordillera Frontal, Bloque de San Rafael, Macizo Norpatagónico y Cordillera Fueguina. Subdividió las Sierras Pampeanas Noroccidentales en dos fajas, occidental y oriental. La occidental la distinguió por la abundancia de metamorfitas de origen calcáreo y rocas máficas y ultramáficas. La oriental, en cambio, la identificó por la presencia de batolitos graníticos y metamorfitas con protolitos areno-arcillosos. Estas dos fajas servirían posteriormente para explicar la colisión de Cuyania con Gondwana. En Cordillera Frontal reasignó las unidades carboníferas tempranas al Carbonífero Tardío-Pérmico y reconoció la fase orogénica San Rafael. Además, describió en detalle la fase Huárpica, y al hacerlo caracterizó la finalización de la estructuración de la Cordillera Frontal. En el Macizo Norpatagónico subdividió las rocas magmáticas pérmicas en un complejo plutónico y en otro plutono-volcánico, separados por una discordancia. En isla de los Estados describió el metamorfismo de la Formación Lemaire, mencionando por primera vez estilpnomelano, indicador de metamorfismo dinámico y equivalente a la facies de prehnita-pumpellita

Ambientes y procesos generadores de las sedimentitas portadoras de hierro en la plataforma Silúrico-Eodevónica de la Patagonia, República Argentina

Marine strata of the Upper Silurian - Lower Devonian Sierra Grande Formation in northeastern Patagonia (Argentina) comprise silty to very coarse quartzites, siltstones, quartzglomerates and ferrilites.Based on sediment - body geometries, mechanical and biogenical structures and textures, eight sedimentary facies were recognized and grouped into five facies associations.The studied deposits were formed in a shallow open marine shelf dominated by wave and storm processes. Most (sandstone and conglomerate) facies were deposited between the beach and the upper - shore -face. The offshore deposits are represented by heterolithic facies formed under alternating fair weather and storm conditions. Paleocurrent measurements of directional structures (wave-ripples and cross beds) suggest that the local position of the shoreline was northeast-southwest. They also show that subtidal shad bars were mainly induced by longshore currents.Iron concentration was favoured by open and shallow marine environments, very slow sedimentation rate, warm paleoclimatic conditions in the source areas and a general sea level rise during the deposition of the Sierra Grande Formation.Las sedirnentitas marinas de la Formación Sierra Grande del Silúrico tardío-Devónico temprano del NE de la Patagonia, comprenden arenitas cuarzosas (ortocuarcitas) de grano mediano a grueso, limolitas, cuarzoglomerados y ferrilitas. Basado en la geometría de cuerpos sedimentarios, estructuras mecánicas y biogénicas y también texturas, fueron reconocidas ocho facies sedimentarias agrupadas en cinco asociaciones de facies.Los depósitos estudiados fueron formados por procesos de acumulación en ambientes marinos abiertos, con buen desarrollo de las zonas de plataforma poco profunda dominadas por procesos de olas y tormentas.La mayoría de las facies fue depositada entre la playa y el frente de costa superior. Los sedimentos de mar afuera están representados por facies heterolíticas formadas bajo condiciones alternantes de buen tiempo y tormentas. Las paleocorrientes (óndulas de olas y capas entrecruzadas) sugieren que el rumbo local de la línea de costa fue NE-SO. También muestran que las barras de submarea fueron principalmente inducidas por corrientes longitudinales.Las concentraciones ferríferas fueron favorecidas por ambientes marinos abiertos y de poca profundidad, lento índice de sedimentación,condiciones paleoclimáticas cálidas en las zonas fuentes y un levantamiento general del nivel del mar durante la depositación de la Formación Sierra Grande

Geochemical variations in Cenozoic back-arc basalts at the border of La Pampa and Mendoza provinces, Argentina

Back-arc volcanism was active in central-western Argentina (provinces of Mendoza and La Pampa) from the Miocene through historic times. The rocks of 39 monogenetic volcanoes located in this area were studied in order to define their geochemical characteristics. The dominant rock texture is porphyritic, with intergranular, pilotaxitic and hyalophitic groundmasses. The most frequent phenocrysts are olivine followed by olivine–plagioclase–clinopyroxene. Their SiO2 content varies between 42.3 and 51.2 wt.%, the most abundant rocks are trachybasalts, followed by basalts and basanites, all of them alkaline. The rocks display enrichment of incompatible elements that varies according to the geographic location and age. There is an increase in incompatible element concentrations from the southern and central to the northern zones. Also, in the northern part of the study area, the behavior of incompatible elements varies with time; the incompatible element ratios of the Plio–Pleistocene rocks show arc signature, while the rocks of the Miocene De la Laguna volcano show intraplate affinity. We conclude for this sector that the mantle source region was modified after the generation of Miocene magmas by subduction-related fluids. These fluids are related to a late Miocene episode of subhorizontal subduction, i.e., after the generation of the rocks of De la Laguna volcano.Centro de Investigaciones GeológicasConsejo Nacional de Investigaciones Científicas y TécnicasUniversidad Nacional del Su

The Mesoproterozoic Basement at the San Rafael Block, Mendoza Province (Argentina): Geochemical and Isotopic Age Constraints

This work provides new petro-geochemical and isotopic information to constrain the crustal evolution of the Precambrian Cerro La Ventana Formation. The Rb–Sr, Sm–Nd, Pb–Pb, and U–Pb isotopic data obtained as well as their petrological and geochemical features are reported. These data are useful to discuss relationships with equivalent Mesoproterozoic units located along the Cuyania terrane in the proto-Andean Gondwana margin. The type section of the basement rocks of the Cerro La Ventana Formation is located in the south-eastern part of the San Rafael Block, Mendoza Province known as Leones-Ponón Trehué-La Estrechura region. Equivalent crustal fragments are also included in this basement, such as ductile-deformed rocks of the El Nihuil Mafic Unit that are intruded by Ordovician undeformed dolerites. The basement exposed along the type section corresponds to a metamorphosed volcano-plutonic complex with hardly any sedimentary protolith. Main rocks are tonalites and foliated gabbros and quartz diorites that pass to amphibolites, and minor granodioritic–dioritic orthogneisses, with abundant angular microgranitoid enclaves now deformed and stretched intruded in mafic to felsic metavolcanics with porphyritic relic textures. The studied samples classified as tonalites and some close to the field of granodiorites following a calc-alkaline trend. Gabbroic samples from the El Nihuil mafic unit show a more tholeiitic signature. The bulk of samples from the Cerro La Ventana Formation plot within the field of metaluminous rocks; although a few are in the peraluminous field. Main groups of samples plot as low-Al TTD field; however, some of them show high Sr/Y ratios which are typical of high-Al TTD. The Mg#/K ratio is higher in the Cerro La Ventana Formation compared with Las Matras TTG series suggesting a minor differentiated grade for the first one. The chondrite-normalized REE diagrams for Leones samples have Eu anomalies rather positive and gabbros from El Nihuil region display patterns with positive Eu anomalies typical of plagioclase-rich igneous rocks. The Rb–Sr data defined an isochron with 1148 ± 83 Ma, initial 87Sr/86Sr = 0.70292 ± 0.00018. The low initial ratio is indicative of a slight evolved Mesoproterozoic source. An acceptable isochron was obtained using Sm–Nd methodology indicating an age of 1228 ± 63 Ma. The model ages (TDM) are in the range 1.23–1.64 Ga with εNd(1200) in between −0.94 and +4.7 recording a ‘depleted’ source, less evolved than CHUR for the time of crystallization. In a 207Pb/204Pb diagram the samples plot similarly to rocks from the basement of Cuyania Terrane (Pie de Palo Range and crustal xenoliths) showing a distinctive non-radiogenic signature. The tonalitic lithofacies located at the Leones River type section was chosen for zircon U–Pb TIMS dating and the obtained crystallization age was 1214.7 ± 6.5 Ma. The in situ U–Pb (LA-ICP-MS) zircon data done in two different laboratories on samples from El Nihuil mafic unit (tonalitic orthogneisses) plotted in a Concordia diagram, record an intercept at 1256 ± 10 Ma and in Tera-Wasserburg diagram an age of 1222 ± 6.9 Ma. With these isotopic data we confirm the Mesoproterozoic age for the basement of the San Rafael Block. The obtained ca. 1.2 Ga is quite similar to those belonging to the basement of other regions from the Cuyania allochthonous terrane.Centro de Investigaciones Geológica

U-Pb detrital zircon data from the paleozoic Sierra Grande Formation, North Patagonian Massif, Argentina

The Sierra Grande Formation is a siliciclastic sedimentary unit with oolithic iron levels of economical importance which crops out in the Argentinean North Patagonian Massif. It was deposited within an open marine environment, with littoral to sub-littoral conditions. A Silurian-Lower Devonian age is assigned based on fossil content. It is mainly composed of medium to fine quartz arenites, wackes, silts, shales and conglomerate beds. In order to establish the source rocks for the Silurian-Devonian basin and also to constrain the sedimentation age, different U-Pb geochronological studies have been carried out on detrital zircons. The zircons have been studied with comparative morphology analysis using SEM images to asses the different population types. For dating using U-Pb isotopes in zircon detrital grains, the high resolution SHRIMP and LA-ICP-MS methodologies were used. The predominant obtained age groups are: Neoproterozoic (547–991 Ma), Cambrian (497-546 Ma) and Ordovician (443-495 Ma). All the analyzed samples also comprise Mesoproterozoic ages of about 1009 to 1382 Ma and smaller proportion Paleoproterozic (1641 to 2248 Ma) and Neo-Archean (2649-2657 Ma) ages. It is important to note that Silurian ages (428 and 440 Ma) were obtained from a group of zircon crystals, constraining the sedimentation age of the Sierra Grande basin to be not older than Middle Silurian. With these isotopic data it is possible to discuss the provenance of the Silurian-Devonian siliciclastic sedimentary cover of the North Patagonian Massif.Centro de Investigaciones Geológica

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

Ordovician K-bcn Ionites have now been recorded from >20 localities in the vicinity of the Argentine Prccordillera. Most occur in I he 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 t. victorias iunatus grnplolite (Oe. evae conodont) Zone, and the youngest in the middle Llanvirn P. etegans (P. .tuecicas) 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, tilanilc and Fe-Ti oxides. The proportions of the mineral phases and variations iii their crystal chemistry are commonly unique to individual (or small groups of) K-benlonite beds. Glass melt inclusions preserved in quartz are rhyolitic in composition The sequence is unique in its abundance of K-benlonite beds, but a close association between the Precordillera and other Ordovician sedimentary basins cannot be established.Theash distribution is most consistent with palacogeographical 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 Famalinian orogeny. The Puna-Famatina terrane northeast of the Precordillera might have served as the source of the K-bcnlonite ashes, possibly in concert with active arc magmatism on the Gondwana plate itself.Centro de Investigaciones Geológica

Faja milonítica El Cortijo: puesta en valor de su patrimonio geominero y propuesta de un nuevo sitio de interés geológico en Tandil, Buenos Aires

El basamento del Sistema de Tandilia reúne a las rocas más antiguas de la Argentina, las cuales guardan una valiosa información de los procesos geológicos ocurridos hace aproximadamente 2.200 Ma. Entre estas rocas ígneo-metamórficas (Complejo Buenos Aires) se encuentran las que integran la Formación El Cortijo, que contrastan notoriamente con el resto por sus características mineralógicas, metamórficas y estructurales. Esta Formación está constituida por rocas típicas de una cuenca oceánica desarrollada durante el lapso Neoarqueano - Paleoproterozoico, fuertemente milonitizadas. Aflora en las cercanías de la ciudad de Tandil, a lo largo de una faja de 3 kilómetros con rumbo este-oeste (aquí denominada Faja milonítica El Cortijo), a la que se accede por la Ruta Nacional 226. Tandil es una ciudad reconocida por su origen minero, actualmente con una fuerte impronta turística y una infraestructura óptima para ofrecer diversos circuitos geomineros. Esta contribución tiene como objetivo presentar un nuevo sitio de interés geológico en Tandil, que junto con los otros circuitos geoturísticos urbanos, manifieste el valor geológico de la región. Esto conduciría a su protección y podría impulsar la creación de un futuro Parque Geológico, producto de la integración de todos los geositios conocidos además del aquí propuesto. El trabajo realizado incluyó un relevamiento del área mediante imágenes satelitales y actividades de campo que permitieron seleccionar un conjunto de canteras labradas en la faja milonítica que constituye el geositio propuesto. Este sector de la provincia de Buenos Aires reúne todos los atributos necesarios para establecer un sitio de interés geológico como el sugerido, considerando sus riquezas paisajísticas y geológicas relacionadas con la evolución geológica y tectónica del basamento de Tandilia