Facies sedimentarias de la formación río seco de los castaños (silúrico-devónico) del bloque de San Rafael, Mendoza

Fil: Manassero, Marcelo Jorge. CIG- Centro de Investigaciones Geológicas, Facultad de Ciencias Naturales y Museo, Universidad Nacional de La Plata, 1-644, 1900 La Plata, ArgentinaFil: Cingolani, Carlos Alberto. División Científica de Geología-Facultad Ciencias Naturales y Museo, Universidad Nacional de La Plata, Paseo del Bosque s/n, 1900 La Plata, ArgentinaFil: Abre, Paulina. CIG- Centro de Investigaciones Geológicas, Facultad de Ciencias Naturales y Museo, Universidad Nacional de La Plata, 1-644, 1900 La Plata, ArgentinaFil: Uriz, Norberto J.. División Científica de Geología-Facultad Ciencias Naturales y Museo, Universidad Nacional de La Plata, Paseo del Bosque s/n, 1900 La Plata, Argentin

Facies sedimentarias de la Formación Río Seco de los Castaños (silúrico-devónico) del Bloque de San Rafael, Mendoza

Se presentan los resultados de recientes estudios sobre aspectos sedimentológicoestratigráficos de la Formación Río Seco de los Castaños (González Díaz, 1964, 1972 y 1981), asignada originalmente como ‘pre-Carbónico’ del Bloque de San Rafael. Su sección tipo se encuentra en el denominado Cañón del Atuel, entre Valle Grande y Dique Nihuil. Asomos importantes se presentan también en otros sectores como Agua del Blanco, Lomitas Negras, Ruta 144 y Rodeo de la Bordalesa.Centro de Investigaciones Geológica

Sedimentary Provenance Analysis of the Ordovician Ponón Trehué Formation, San Rafael Block, Mendoza-Argentina

The present chapter deals with provenance analysis of a carbonate-siliciclastic Ordovician sedimentary unit of the San Rafael block, named the Ponón Trehué Formation (Darriwilian to Sandbian). This is the only sequence which exhibits a direct contact with the Mesoproterozoic basement through an unconformity, not only within the San Rafael block, but rather for the entire Cuyania terrane. When combining different provenance proxies, such as petrography, whole-rock geochemistry, Sm–Nd data, Pb–Pb analyses, and detrital zircon dating, it can be deduced that the source rocks are characterized by: (i) an upper continental crust composition, (ii) a subordinated influence of a more depleted composition, (iii) a dominantly Mesoproterozoic age, (iv) sedimentary recycling did not conspicuously affected the detrital source, and (v) weathering was relatively strong. All these characteristics point to the Mesoproterozoic Cerro La Ventana Formation basement as a main source of detritus to a restricted basin infilled during the Ordovician.Centro de Investigaciones Geológica

Provenance analysis of Devonian peripheral foreland basins in SW Gondwana, case of the Gualilán Group, Precordillera Argentina

We carried out a sedimentary provenance analysis of the Devonian Gualilan Group, Central Precordillera of western Argentina, focusing on the siliciclastic record of the Talacasto (Lower Devonian) and the Punta Negra (Lower-Middle Devonian) formations. Provenance is determined based on petrography, heavy minerals, whole-rock geochemistry, Sm–Nd isotopes, and U–Pb detrital zircon dating. Sandstones are composed of quartz, feldspar, and metamorphic and plutonic lithoclasts. The Gualilan Group underwent moderate weathering indicated by CIA values and Th/U ratios below 76 and 4.9, respectively. Th/Sc, Zr/Sc, Cr/V and La/Th ratios, negative Eu-anomalies and REE patterns point to felsic source compositions. The heavy minerals and zircon morphologies indicate that high-grade metamorphic and igneous rocks provided the bulk of detritus. TDM ages between 1.40 and 1.46 Ga and eNd(t) values ranging from − 9.21 to − 11.13 constrain the signature of the sources. U–Pb detrital zircon ages for the Talacasto Formation are equally distributed between the Famatinian (Late Cambrian–Late Devonian), Pampean–Brazilian (Neoproterozoic–Early Cambrian) and Grenvillian-Sunsas (Mesoproterozoic) orogenic cycles. Detrital zircon ages of the Punta Negra Formation were dominantly supplied from Mesoproterozoic rocks, linked to the Grenvillian-Sunsas orogeny. The data indicate a provenance from rocks located eastwards of the basin represented by the basement of the Pampean Ranges. The peripheral foreland basin developed during the post-collisional regime that followed the accretion of Cuyania against Gondwana. Whereas eastern and western Pampean Ranges provided detritus to the Talacasto Formation, the Punta Negra Formation was sourced by the western Pampean Ranges, implying exhumation and erosion of such basement during the Devonian.Facultad de Ciencias Naturales y Muse

La Horqueta Formation: Geochemistry, Isotopic Data, and Provenance Analysis

La Horqueta Formation is developed from the Seco de las Peñas River to Agua de la Piedra creek within the San Rafael block and was deposited in a marine environment. It comprises dominantly metasandstones, although metasiltstones, metapelites, and rare metaconglomerates are also present. The base of the succession is not exposed and it is superposed through unconformity by Upper Carboniferous units. La Horqueta Formation is folded and shows cleavage. Provenance analyses based on whole-rock geochemistry and isotope data is the main focus of the work. Whole-rock geochemical data point to a derivation from unrecycled upper continental crust, based mainly on Th/Sc, Zr/Sc, La/Th, and Th/U ratios and rare earth element (REE) patterns (including Eu anomalies). Sc, Cr, and V concentrations and low Th/Sc ratios are indicative of a source slightly less evolved than the average upper continental crust. The εNd values are within the range of variation of data from the Mesoproterozoic Cerro La Ventana Formation, which is part of the basement of the Cuyania terrane outcropping within the San Rafael block. The Rb-Sr whole-rock data indicate that the low-grade metamorphism and folding events are Devonian in age. U-Pb detrital zircon ages suggest main derivation from the Mesoproterozoic (“Grenvillian-age”) basement of the San Rafael block and the Pampean–Brasiliano cycle, as well as a detrital input from the Río de la Plata craton and the Famatinian belt. Despite geochemical similarities, Río Seco de los Castaños Formation display different proportions of detrital zircon ages, when compared to La Horqueta Formation.Centro de Investigaciones Geológica

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

Lower Paleozoic ‘El Nihuil Dolerites’: Geochemical and Isotopic Constraints of Mafic Magmatism in an Extensional Setting of the San Rafael Block, Mendoza, Argentina

The ‘El Nihuil Mafic Unit’ is exposed at the Loma Alta region northwards of the El Nihuil dam. This igneous body consists mainly of mafic rocks assigned to the Precambrian and Lower Paleozoic according to different authors. The mafic unit shows an elongated shape with aNNE–SSWorientation on the western side of the San Rafael Block (SRB), developed for a length of 17.5 km and with a maximum width of 4.2 km and is composed of deformed gabbros, amphibolites, and tonalites that represent the Mesoproterozoic continental crust, and dykes and sills of undeformed Lower Paleozoic porphyritic dolerites. We present the petrology, geochemistry, isotope data, and determinations of emplacement conditions of the dolerites that could represent a sliver of Cuyania-Chilenia terranes suture. The dolerites show classical porphyritic texture, with elongated subhedral plagioclase (andesine) and clinopyroxene phenocrysts. Geochemical analyses of El Nihuil Dolerite samples indicate that the rocks are MORB-type basalts. In the P2O5 versus Zr diagram, the dolerites plot in the tholeiitic field similarly to western Cuyania basalts, and in the Th–Hf/3–Ta tectonic discrimination diagram the dolerite dykes plot mainly as E-MORB. Dolerite samples were dated byK–Ar (whole rock) systematic and the ages are 448.5 ± 10 and 434.2 ± 10 Ma (Upper Ordovician and close to the Lower Silurian boundary). The dolerites represent the unique Lower Paleozoic mafic rock outcrops within the SRB. Nd(TDM) ages are in between 0.51 and 0.80 Ga; εNd(0) record positive values ranging from +3.85 and +7.84; εNd(t) record +4.27 to +12.42. 87Sr/86Sr ratios are in between 0.7032 and 0.7050 in agreement with values for ocean ridge tholeiites. These mafic rocks are interpreted as a part of a dismembered ‘Famatinian ophiolite belt’ emplaced during the Lower Paleozoic extensional environment within a thinned Mesoproterozoic continental crust on western Cuyania terrane.Centro de Investigaciones Geológica

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

Provenance of ordovician to silurian clastic rocks of the Argentinean precordillera and its geotectonic implications

D. Phil.A Mesoproterozoic basement and a Cambrian-Ordovician carbonate platform characterize the Precordillera terrane. These characteristics and its distinct geologic history mark a difference between this suspected exotic-to-Gondwana terrane and the Gondwanan autochthonous, leading to speculation that the Precordillera was derived from Laurentia. The surprising similarities of the carbonate sequences between the Precordillera and certain parts of southeast Laurentia suggest a common geological history. However, other models interpret the origin of the Precordillera terrane as being para-autochthonous with respect to Gondwana. All these models are still controversial. A combination of several methodologies including petrography and heavy minerals analysis, geochemistry, Sm-Nd and Pb-Pb isotopes and zircon dating were applied to several Ordovician and Ordovician to Silurian units of the Precordillera terrane. Geochemistry and petrography indicates that all the Formations studied have similar characteristics, with at least two sources providing detritus to the basin. The dominant source has an unrecycled upper continental crust composition whereas the other component is more depleted. The study of detrital chromian spinels suggests that mid-ocean ridge basalts, continental intraplate flood basalts and ocean island basaltrelated rocks were among the sources for the detrital record of the Precordillera terrane. Nevertheless, the mafic sources and their ages remain unknown. Nd isotopes account for negative εNd values and TDM ages in a range of variation found elsewhere within Gondwana and basement rocks of the Precordillera. The Sm/Nd ratios of certain samples indicate fractionation of LREE. Pb isotopes indicate that a source with high 207Pb/204Pb was important, and point to Gondwanan sources. Detrital zircon dating constrain the sources as being dominantly of Mesoproterozoic age (but with a main peak in the range 1.0 to 1.3 Ga), with less abundant populations of Neoproterozoic (with a main peak in the range 0.9 to 1.0 Ga), Palaeoproterozoic, Cambrian and Ordovician ages in order of abundance. i The uniformity shown by the provenance proxies indicate that there were no important changes in the provenance from the Lower Ordovician until the Early Silurian. Several areas are evaluated as sources for the Precordillera terrane. The rocks that fit best all the provenance constraints are found within the basement of the Precordillera terrane and the Western Pampeanas Ranges. Basement rocks from the Arequipa-Antofalla area (Central Andes) also match the isotopic characteristics, but a northern source is less probable, except for the Western tectofacies. On the other hand, areas such as Antarctica, Falklands/Malvinas Microplate, the Natal-Namaqua Metamorphic belt and the Grenville Province of Laurentia can be neglected as sources. The proposal of these areas as sources is in agreement with palaeocurrents and facies analyses and suggests proximity between them and the Precordillera since at least the Late Arenig to Early Llanvirn. This has important implications for the proposed models regarding the geotectonic evolution of the Precordillera terrane. The models would need to be adjusted to the here proposed youngest timing of collision