Paulina Abre

A Silurian-Devonian marine platform-deltaic system in the San Rafael Block, Argentine Precordillera-Cuyania terrane: lithofacies and provenance

Abstract The San Rafael Block is included as a part of the pre-Andean region, in the southern sector of the Argentine Precordillera–Cuyania terrane, within the western Gondwana margin. The Río Seco de los Castaños Formation (Upper Silurian–Lower Devonian) is one of the major marine-siliciclastic pre-Carboniferous units, and is interpreted as a distal to proximal silty platform-deltaic system. The dominant sedimentary processes were wave and storm action and the source areas were located to the east, close to the study area. The rocks are mainly of immature arkosic sandstones showing both recycled orogen and continental block provenances. Sedimentological characteristics of conglomerate-filled channels and an organic-matter-rich bed are described. X-ray diffraction analyses of the clay minerals from the sequences show that very low-grade metamorphic conditions acted during the Early Carboniferous. Geochemical analyses indicate moderate to strong weathering, and potassium metasomatism. Zr/Sc ratios lower than 22, no important enrichments of Zr, Th/Sc ratios, high Sc and Cr concentration and the Eu-anomalies indicate a provenance from a less evolved upper continental crust. TDM ages and ϵNd are within the range of the Mesoproterozoic basement and Palaeozoic supracrustal rocks from the Precordillera–Cuyania terrane. Probable sources, tectonic setting and land–sea interactions are discussed.

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 a NNE–SSW orientation 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 by K–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; eNd(0) record positive values ranging from +3.85 and +7.84; eNd(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.

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 Ponon Trehue 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.

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

The Rio Seco de los Castanos 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. eNd 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.

The Pavón Formation as the Upper Ordovician Unit Developed in a Turbidite Sand-Rich Ramp. San Rafael Block, Mendoza, Argentina

The Pavon Formation crops out in the central-east region of the San Rafael block; it is composed of massive green-reddish-grey sandstones, wackes, quartz sandstones, siltstones and shales. The sand-dominated facies show tabular bedding with sharp contacts and scarce syndepositional deformational structures, and were deposited in a turbidite sand-rich ramp. The graptolite fauna, in particular the presence of Climacograptus bicornis Biozone indicates a Sandbian age (Upper Ordovician ). Sandstone petrography records a stable craton or a faulted continental basement as probable source areas. Illite crystallinity index suggest anchimetamorphic conditions underwent by the sequence. Relatively high CIA values and K/Cs ratios indicate intermediate to advanced weathering. Geochemical provenance proxies display detrital compositions derived from an average upper continental crust, but relatively high abundances of compatible elements (Cr, Ni, V, Ti and Sc) along with low Th/Sc ratios suggest mixing with a less fractionated component. The Zr/Sc ratios indicate that recycling was not important. An ophiolitic source can be neglected based on Y/Ni and Cr/V ratios. Chemical analyses of detrital chromian spinels indicate that they were formed within mid-ocean ridge basalts and continental flood basalts. Nd model ages of Pavon sandstones scatter around 1.4 Ga, e Nd(t) (t = 455 Ma) values range from −0.4 to −4.1 and ƒSm/Nd is of −0.40 ± 0.06 on average, indicating an affinity to Grenvillian-age crust. Detrital zircon grains dated confirmed a main Mesoproterozoic source (with peaks at 1.1 and 1.4 Ga), with subordinated inputs from Neoproterozoic and Paleoproterozoic crystalline rocks. The complete provenance dataset suggest the basement of the San Rafael block (Cerro La Ventana Formation) as the main source of detritus, but derivation from the Western Pampeanas Ranges was also probable. The siliciclastic sequence was deposited in a foreland basin at latitude of around 26°S, and linked to the accretion of the Cuyania terrane towards west of Gondwana; this accretion caused uplift by thrusting of the Mesoproterozoic crust to the east at ca. 460 Ma.

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 Rio Seco de los Castanos 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 e Nd 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.

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

La Horqueta Formation is developed from the Seco de las Penas 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 eNd 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 Rio de la Plata craton and the Famatinian belt. Despite geochemical similarities, Rio Seco de los Castanos Formation display different proportions of detrital zircon ages, when compared to La Horqueta Formation .