Sierras Pampeanas Research Articles

Uplift analysis on a pericratonic region: An example in the Sierras de Córdoba (29°–34°S), Argentina

The Sierras de Córdoba (Argentina) is a mountain belt located in the most distal (pericratonic) part of the South-Central Andean foreland, in the easternmost end of the Sierras Pampeanas (Pampean ranges) broken foreland. Although this region is considered an excellent example for settings where basement thrusting generates relief and intermontane basin systems, hundreds of kilometers away from the Cordilleran front, the driving mechanisms and main controls on topography are still poorly understood. Given that the landscape, and particularly the rivers, react to changes in climate and tectonics on timescales between 103 and 106 yrs, we used DEM-based morphometry (elevation patterns, local relief, and analysis of 242 longitudinal stream profiles) to evaluate the main controls on modern topography. We focus on the morphologies of the main streams and their tributaries using metrics of the channel concavity and normalized channel steepness index (ksn). The distribution of local relief, concavity and ksn values was compared with the present-day topography, tectonic structures, rock-type and modern rainfall patterns. The stream longitudinal profiles show homogeneous concavities and ksn background values, with some local anomalies. Local structures (east and west vergent thrusts) in the Sierras de Córdoba seem to be the most fundamental controls on the formation of these anomalies, slope-break knickpoints as well as deep incision (high local relief). This suggests localized and accelerated rock uplift rates, which produce localized increases in the gradient along the longitudinal stream profile. It is important to notice that the highest ksn values were reported on the eastern range flanks, coincident with the highest values of mean annual rainfall. However, high ksn values appear to be maintained even with greater rainfall. Our study provides new insights on the Neogene Sierra de Córdoba uplift evolution, which will assist future sampling and modelling (cosmogenic nuclides analysis, thermochronology and GPS analysis).

U[sbnd]Pb and Sm[sbnd]Nd constraints on miocene units in the Ischigualasto-Villa Unión foreland basin, Sierras Pampeanas, Argentina: Sedimentary provenance, landscape evolution coupling flat-slab subduction

Foreland basin systems are intimately linked to thrust belts and activation and reactivation of structures in association with convergent plate dynamics, capable of controlling regional uplifts, sedimentation, as well as drainage patterns among other processes. The Ischigualasto-Villa Unión basin (IVB) is a Miocene broken foreland, occurring in-between the Famatina-Sañogasta, Valle Fértil and Maz Ranges in the Western Sierras Pampeanas, Argentina, Central Andes. We conducted U–Pb geochronology by SHRIMP in igneous zircons and LA-ICPMS in detrital zircons as well as whole rock Sm–Nd analyses in the Quebrada del Médano and Desencuentro Formations – the uppermost units of the IVB. The new and compiled data provide new insights on the influence of flat-slab on the uplift of the Sierras Pampeanas and Precordillera and consequent shift of the drainage patterns in the basin infill. The SHRIMP age for the youngest zircons from a volcanic tuff of the middle portion of the Desencuentro Formation yielded a mean 206Pb/238U age of 8.49 ± 0.33 Ma whereas the other grains gave concordant ages from 10 to 1433 Ma. The detrital zircon ages from both Formations indicated that different sourced materials contributed to the basin infill, through time and space. The major age modes in the detrital populations were: 1) 1250-950 Ma, (2) 850-600 Ma, (3) 540-500 Ma, (4) 490-460 Ma, (5) 350-320 Ma, (6) 300-250, (7) 40 Ma and (8) 16-8 Ma. From a statistical point of view, we defined two similar aged groups with distinct major contribution provenance: a first cluster with main Cambrian-Ordovician ages and a second one with main Permian and Mesoproterozoic ages. The bulk picture suggests a critical change in the IVB provenance history since the mid-late Miocene, roughly consistent with the Pampean flat-slab subduction model that controls uplifts of the Western and Eastern Sierras Pampeanas and associated regional drainage shifts in Central Andes, as like geodynamic processes in Western North America. We interpret the statistic change in the distribution of the IVB zircons as product of: (1) the presence of axial rivers flowing parallel to the basement arches in the more distal foreland; (2) a disconnection between the IVB and the Bermejo basin related to the uplift of the IVB western margin; (3) the uprising of the IVB eastern ranges. Our findings are also consistent with the acquired TDM ages (1.2–0.9 Ga) and the coupled εNd(t) constraints (−6 to −5) for samples collected along the stratigraphy of both IVB Formations that indicated recycled Grenville-aged sources were likely overload by sourced rocks with high lithologic erodibility.

Late Holocene environmental changes from fluvio-aeolian morpho-sedimentary sequences (Santa María Valley, northwest Argentina)

The Santa María valley is located in the northern sector of the Sierras Pampeanas in Northwest Argentina. In spite of its arid characteristics, it was a densely populated area during the Pre-hispanic period. The populations developed an agropastoral economy, taking advantage of the wetter epochs that characterized some periods. The study of fluvio-aeolian records of the El Paso and Cafayate areas yielded interesting data for regional paleoenvironmental reconstruction. Geomorphological maps, field surveys, and archaeological digs were complemented with the detailed stratigraphic record of several points, together with radiocarbon and OSL dating. Alternating wetter and drier periods were inferred for the last 2,800 years. The wetter phases generated enough stability to allow soil formation and a better plant cover, thus favouring human settlements. For the paleoenvironmental interpretation, well defined European chronozones were used because there are no South American denominations. Within the sequence, some wetter periods stand out, such as the one between 2,800-2,500 BP, the intermediate Roman Period (80-280 AD), and the LALIA (Late Antique Little Ice Age) (536-660 AD), while the drier phases comprise the Roman Epoch and the Medieval Climate Anomaly. Moreover, dune active phases during the last millennium were determined for 1,000, 1,350, 1,650, and 1,770 AD approximately, separated by wetter periods recorded by the presence of a paleosoil (15th century) and historical data (17th and 18th centuries).

Holocene paleoenvironments from the Tafí Valley (northwest Argentina) through morpho-sedimentary and geoarchaeological record analyses

The Tafí Valley is a small basin located in the northern sector of the Sierras Pampeanas in Northwest Argentina. The geomorphological studies made in the area showed the presence of very representative Holocene morpho-sedimentary records, which are useful for regional paleoenvironmental reconstruction. The study of the main slope accumulations, fluvial terraces, and alluvial fans provided several geoarchaeological records, paleontological remains, and volcanic ash layers as well as radiocarbon and thermoluminescence datings. We used all these data to construct a paleoenvironmental sequence. This sequence was also accompanied by some previously obtained data and detailed geomorphological cartography. Four morphosedimentary units were identified. The oldest unit (H1) comprises the Early and Middle Holocene (ca. 13000 - ca. 4200 BP). It could be divided into two sub-stages (H1a and H1b), with different environmental signs. They appear separated by a tephra layer (V0) dated ca. 10000 BP. The end of sub-stage H1b is limited by another ash layer (V1) dated ca. 4200 BP. By that time, an incision process had already started separating this substage from the next one. Stage H2 spans from ca. 4200 BP to ca. 630 BP, and extends vastly throughout the valley. It was possible to identify several features of human settlements (Tafí and Santa María Cultures) corresponding to that period. The characteristics of the sediments indicate a strong anthropogenic influence during its formation. Finally, two minor units (H3 and H4) cover the last centuries of the Holocene sequence, separated by incision phases. The Holocene set was related to the regional and global paleoclimatic evolution.

Crystallochemical indexes and geothermobarometric calculations as a multiproxy approach to P-T condition of the low-grade metamorphism: The case of the San Luis Formation, Eastern Sierras Pampeanas of Argentina

In this work, crystallochemical indexes and geothermobarometric calculations are used together to estimate the P-T condition in low-grade metamorphic units of the San Luis Formation from Sierra de San Luis (Eastern Sierras Pampeanas of Argentina). Phyllites and slates from this formation record at least two episodes of white mica (Wm) and chlorite (Chl) growth associated with main D1-S1 (Wm2-Chl1) and local D2-S2 (Wm3-Chl2) stages. Mean KICIS values of 0.30 (Western Belt) and 0.23 Δ°2θ (Eastern Belt) were determined for the <2 μm fraction. Moreover, since the average grain size of the Wm2 exceeds 10 μm, KICIS values were also measured for the 10–20 μm fraction ranging between 0.22 and 0.24 Δ°2θ for both belts. For those fractions, the KICIS values indicate epizone conditions, associated with temperatures higher than 300 °C. These values are comparable to temperatures obtained from chlorite thermometers, 316°–416 °C in Western Belt and 261°-403 °C in the Eastern Belt. Pressure values for D1 stage constrained from chlorite – white mica – quartz – H2O equilibrium are 2.5–5.1 kbar in the Western Belt, and at 3.3–4.7 kbar in Eastern Belt. The pressures are consistent with white mica b parameter values measured in samples from Western Belt (9.006 to 9.025 Å) and Eastern Belt (9.003 to 9.020 Å), linked to intermediate thermal gradients (25–35 °C/km).The identification of increasing P-T condition paths in low-grade metasedimentary successions is not easy in the absence of higher grade rocks; in these situations, a systematic sampling and analysis of dozens of samples must be carried out. The very good correlation between crystallochemical index values (Kübler index and white mica b parameter) and thermobarometric calculations (empirical and semi-empirical chlorite thermometers and thermobarometry for chlorite – white mica – quartz – H2O equilibrium) presented in this work argues in favor of the development of a multiproxy study strategy in this type of low-grade metasedimentary successions that involve: (1) a systematic sampling of metapelite levels for X-ray diffraction study (mineralogical and crystallochemical indexes with a rapid implementation allowing a dense systematic sampling); (2) the recognition of variations in the P-T conditions from KI and b parameter values and the consequent selection of metapelitic samples for analyzing mineral chemistry and thermobarometric calculations. In order to facilitate these calculations, a user-friendly spreadsheet for crystallochemical indexes and chlorite thermometry calculations is also provided.

Mesoproterozoic to Paleozoic U-Pb SHRIMP II ages of granitoid bodies in NW Argentina and their consequences for the interpretation of the geological evolution of the Central Andes

Petrographic and geochemical studies combined with U-Pb SHRIMP II analyses of zircons from three granitoid rocks in northwest Argentina reveal new data to enhance the understanding of the margin of western Gondwana.The I-type granite at Pomán (‘Granito Pomán’), with a typical continental arc geochemical signature, has been intruded into schists and gneisses of the regional basement during the Lower Cambrian (535 ± 5 Ma; MSDW = 0.024). This supports the existence of a volcanic arc of Lower Cambrian age evolving possibly contemporary to some successions of the Puncoviscana Complex. Two deformed and metamorphosed, granitoid bodies (Chacritas and San Ignacio-Los Pinos granitoids) derived from metamorphic (metasedimentary?) rocks, contain few euhedral Mesoproterozoic zircon crystals, which show no sign of reworking. Most of these euhedral zircons have rims of younger ages (Neoproterozoic to Permian) with Th/U ratios mostly lower than 0.001. Both, the S-type granite from Chacritas and the San Ignacio-Los Pinos granitoid from Balcozna and their protoliths, may represent the previously unidentified local source for the abundant late Mesoproterozoic detrital zircons reported in Neoproterozoic to Paleozoic sedimentary sequences of NW Argentina. This would imply Mesoproterozoic basement, alike the Arequipa Massif (in Peru and Bolivia) and the Western Sierras Pampeanas, in northwest Argentina.

The strongly peraluminous A-type granites of the Characato suite (Achala batholith), Sierras Pampeanas, Argentina: Evidence of Devonian-Carboniferous crustal reworking

The largest igneous body in the Sierras Pampeanas (the Achala batholith) is composed by several magmatic intrusions. How many plutons form this batholith and its intrusive history remains unclear. The emplacement of the batholith began at ca. 380 Ma; the Characato suite, in the northern extreme of the body, represents a relatively young intrusive phase in its construction (ca. 360 Ma). This suite, composed mainly of two-mica monzogranites, correspons to a strongly peraluminous A-type magma (A/CNK > 1.1) with fractionated compositions (high Rb/Sr and Rb/Ba). The parental magma corresponds to a high temperature melt generated by biotite dehydration melting, which was then differentiated to form the Characato monzogranites. These monzogranites have crustal Nd and Srisotope signatures (εNdt −6.0 to −6.1 and 87Sr/86Srt 0.7074 to 0.7131) and abundant inherited zircons (ca. 90%). The inherited ages are mainly Early Cambrian, and to a lesser extent Mesoproterozoic, Neoproterozoic and Ordovician, all they recognized in the basement of the Sierras de Córdoba. The Nd-isotope composition, added to the inherited zircon ages of the Characato suite, indicate that these magmas derived from partial melting of a heterogeneous crust composed by the S-type Pampean granites and migmatites, Famatinian granitoids, and probably the low-to medium-grade metamorphic rocks of the Puncoviscan Series. The high variability and positive εHft values of the inherited Cambrian zircons found in the Characato suite could have been transferred from the Neoproterozoic grains of the Puncaviscana Series to the Early Cambrian S-type granites and migmatites during the Pampean orogeny.

Late Holocene plant use in the Sierras Pampeanas of Argentina: Evidence from phytoliths and starch grains

Archaeobotanical studies carried out in the southern Sierras Pampeanas of Argentina (San Luis Province) are reported, which add new information about the presence of cultigens in the area during the late Holocene and the variety of wild species used in this period. The presence of starch grains of Zea mays (corn), Cucurbita sp. (squash), undifferentiated tubers, and Phaseolus sp. (beans), as well as phytoliths of Panicoideae, Chloridoideae, Arundinoideae, Bambuseae, Cyperaceae, Asteraceae, Arecaceae and woody dicotiledons are documented from analyses on knapped tools. The obtained data allow discussing the diversity of the resources utilized and the importance of cultigens in prehispanic times in a context that is currently considered the southern limit of prehispanic food production economies.

Recurrent intrusive episodes in the Paleozoic metasedimentary upper crust during the Early Carboniferous time: The Veladero granitoid stock and the peraluminous andesite

The Veladero hill is a well-exposed upper crustal block located between the basement of Sierra de Umango (Western Sierras Pampeanas), and the Devonian - Carboniferous sedimentary sequence of Precordillera, at western Argentina. The hill is formed by schists, metaluminous granitoids and a subalkaline peraluminous andesite studied in this work. The country rocks are schists with mineralogy typically developed during a thermal event (e.g., cordierite, andalusite, biotite). The magmatic assemblage of the peraluminous andesite consists of K-feldspar + plagioclase + quartz + biotite + cordierite, with zircon, monazite, and oxides as accessory minerals. One representative sample from this rock was analyzed and has contents of SiO2 = 61%, Fe2O3t + MgO = 9.03, Na2O + K2O = 6.6%, and aluminium saturation index = 1.5, consistent with the mineralogy. A U-Pb zircon Concordia age of 347 ± 4 Ma was acquired by LA-MC-ICP-MS from zircon hosted in the peraluminous andesite, which together with an age of 342 ± 2 Ma recently published for the metaluminous Veladero granitoid stock, indicate that these two ages overlap within error. Combined mineral assemblage and Al-in-hornblende geobarometric data from the Veladero granitoid stock reveal that the granitic magma passed through ∼10 km of continental crust before reaching its final emplacement level. Field evidence indicates that the Veladero granitoid stock was emplaced under dominantly brittle deformation mechanisms and that the peraluminous andesite magma was generated and subsequently intruded into a previously emplaced magma mush close to solidification, making space by the displacement of large blocks of the host granite. Documented mingling relationships between granite and peraluminous andesite also support emplacement while the granite remained partially crystallized so that the time interval between the two was relatively short, probably on the scale of hundreds of thousands of years. We propose that the peraluminous magma was generated during the ascent of the metaluminous Veladero granitoid stock magma and that the most of the zircon crystallized in high middle crust from the Veladero granitoid stock. These zircons were subsequently recycled and incorporated in the peraluminous andesite. Zircons in the Veladero granitoid stock and the peraluminous andesite have indistinguishable morphology and support this interpretation. In situ Hf isotope data were obtained from igneous zircons in the peraluminous andesite and reveal a dominantly continental source (average εHf,t = −8.0, excluding one just positive value). εHf,t and TDM values from zircon in the Veladero granitoid stock and hosted in the peraluminous andesite are indistinguishable, which is consistent with their original crystallization in the Veladero granitoid stock. TDM values mainly range from 1.5 to 1.9 Ga (average = 1.7 Ga) and reveal a continental Gondwana signature. Consistently, detrital zircon ages from schists in which the igneous rocks were emplaced indicate Ordovician depositional ages and original Gondwanan provenance. However, based on previous studies a Devonian depositional age cannot be ruled out.

Reconstructing the thermal and exhumation history of the Sierras Pampeanas through low-temperature thermochronology: A case study from the Sierra de Velasco

Reconstructing the thermal and exhumation history of the Sierras Pampeanas through low-temperature thermochronology: A case study from the Sierra de Velasco

New insights on regional tectonics and basement composition beneath the eastern Sierras Pampeanas (Argentine back-arc region) from seismological and gravity data

The eastern Sierras Pampeanas (ESP) are the easternmost expression of a series of foreland uplifts in the Argentine back arc region (~30–34°S) and show spatial and temporal connections with the subduction of the Juan Fernández Ridge (JFR) under the South American plate. In order to get new insights on the mechanisms that control crustal regional tectonics, we computed teleseismic receiver functions (RF) and jointly invert them with Rayleigh-wave phase velocity dispersion curves. RFs allow resolving crustal thickness and intra crustal velocity variations with a good vertical resolution whereas surface wave information helps to constrain absolute seismic wave velocities.Our seismic images have been combined with crustal density modeling in order to further investigate if the shear wave velocity structure obtained from the RF-SW joint inversion could explain the observed gravity variations. Our results show a crustal thickness varying from 35–40 km (east) to 45–50 km (west) with a Moho step at ~66°W. This step regionally presents a NW-SE orientation and is parallel to the trace at the surface of the Valle Fértil–La Huerta (VFLH) lineament (Cuyania-Pampia boundary). Our images also reveal the presence of a high wave velocity (high density) lower crust west of this Moho step, beneath the eastern Sierras Pampeanas (at 66–67°W). This observation suggests the east-dipping extension at depth of the VFLH structure and the underthrusting of the Cuyania lower crust under the Pampia terrane along this structure. Finally, we evidenced localized low velocity zones located at about 10 km beneath late Cenozoic volcanic fields (Pocho, Morro). We believe that these low velocity zones correspond to old magma chambers associated to the recent, slab flattening-related volcanism in the ESP.

A Cambrian mixed carbonate–siliciclastic platform in SW Gondwana: evidence from the Western Sierras Pampeanas (Argentina) and implications for the early Paleozoic paleogeography of the proto-Andean margin

The Western Sierras Pampeanas (WSP) of Argentina record a protracted geological history from the Mesoproterozoic assembly of the Rodinia supercontinent to the early Paleozoic tectonic evolution of SW Gondwana. Two well-known orogenies took place at the proto-Andean margin of Gondwana in the Cambrian and the Ordovician, i.e., the Pampean (545–520 Ma) and Famatinian (490–440 Ma) orogenies, respectively. Between them, an extensive continental platform was developed, where mixed carbonate–siliciclastic sedimentation occurred. This platform was later involved in the Famatinian orogeny when it underwent penetrative deformation and metamorphism. The platform apparently extended from Patagonia to northwestern Argentina and the Eastern Sierras Pampeanas, and has probable equivalents in SW Africa, Peru, and Bolivia. The WSP record the outer (deepest) part of the platform, where carbonates were deposited in addition to siliciclastic sediments. Detrital zircon U–Pb SHRIMP ages from clastic metasedimentary successions and Sr-isotope compositions of marbles from the WSP suggest depositional ages between ca. 525 and 490 Ma. The detrital zircon age patterns further suggest that clastic sedimentation took place in two stages. The first was sourced mainly from re-working of the underlying Neoproterozoic metasedimentary rocks and the uplifted core of the early Cambrian Pampean orogen, without input from the Paleoproterozoic Rio de la Plata craton. Sediments of the second stage resulted from the erosion of the still emerged Pampean belt and the Neoproterozoic Brasiliano orogen in the NE with some contribution from the Rio de la Plata craton. An important conclusion is that the WSP basement was already part of SW Gondwana in the early Cambrian, and not part of the exotic Precordillera/Cuyania terrane, as was previously thought.

Petrological, geochemical, isotopic, and geochronological constraints for the Late Devonian–Early Carboniferous magmatism in SW Gondwana (27–32°LS): an example of geodynamic switching

We report a study integrating 13 new U–Pb LA-MC-ICP-MS zircon ages and Hf-isotope data from dated magmatic zircons together with complete petrological and whole-rock geochemistry data for the dated granitic rocks. Sample selection was strongly based on knowledge reported in previous investigations. Latest Devonian–Early Carboniferous granite samples were collected along a transect of ~ 900 km, from the inner continental region (present-day Eastern Sierras Pampeanas) to the magmatic arc (now Western Sierras Pampeanas and Frontal Cordillera). Based on these data together with ca. 100 published whole-rock geochemical analyses we conclude that Late Devonian–Early Carboniferous magmatism at this latitude represents continuous activity (ranging from 322 to 379 Ma) on the pre-Andean margin of SW Gondwana, although important whole-rock and isotopic compositional variations occurred through time and space. Combined whole-rock chemistry and isotope data reveal that peraluminous A-type magmatism started in the intracontinental region during the Late Devonian, with subsequent development of synchronous Carboniferous peraluminous and metaluminous A-type magmatism in the retro-arc region and calc-alkaline magmatism in the western paleomargin. We envisage that magmatic evolution was mainly controlled by episodic fluctuations in the angle of subduction of the oceanic plate (between flat-slab and normal subduction), supporting a geodynamic switching model. Subduction fluctuations were relatively fast (ca. 7 Ma) during the Late Devonian and Early Carboniferous, and the complete magmatic switch-off and switch-on process lasted for ~ 57 Ma. Hf TDM values of zircon (igneous and inherited) from some Carboniferous peraluminous A-type granites in the retro-arc suggest that Gondwana continental lithosphere formed during previous orogenies was partly the source of the Devonian–Carboniferous granitic magmas, thus precluding the generation of the parental magmas from exotic terranes.

ANÁLISIS MORFOTECTÓNICO EN EL PIEDEMONTE SUROCCIDENTAL DEL ANTEPAIS ANDINO, SIERRAS PAMPEANAS OCCIDENTALES, SAN JUAN, ARGENTINA

El retroarco andino, entre los 27°S y 33°S, se caracteriza por ser una de las regiones con mayor sismicidad cortical en el mundo, siendo epicentro de importantes terremotos en el territorio argentino. Esta sismicidad produce una intensa deformación, provocando cambios en la morfología del terreno. En el sector occidental de Sierras Pampeanas Occidentales, en la sierra de la Huerta, se utilizó un SIG para diferenciar unidades geomorfológicas y determinar parámetros cualitativos y cuantitativos que permitieran caracterizar las geoformas identificadas y su relación con la actividad tectónica. La tectónica andina, que afecta la región desde los últimos 5 millones de años, es el principal factor que controla el paisaje de la región. Se diferenciaron tres unidades morfoestucturales y fallamiento con actividad tectónica cuaternaria, presentando abundantes indicadores morfotectónicos, como niveles aluviales dislocados, variación en el diseño y desvío de los ríos, aumento en su grado de incisión, surgentes de agua, abras de viento e inversión del patrón de drenaje, entre otros. El análisis de los indicadores morfotectónicos permitió identificar cómo la deformación disminuye hacia el sur y aumenta de este a oeste. Finalmente, es posible determinar que la deformación continúa activa, modificando el paisaje actual del piedemonte occidental de la sierra de la Huerta.

Hybridization between I-type and S-type granites in the Ordovician Famatinian magmatic arc, Tafí del Valle, Tucumán, NW Argentina.

In the Tafi del Valle region, in northwestern Argentina, several intrusive bodies of lower Paleozoic age were emplaced in the metasedimentary Puncoviscana Formation, belonging to the Sierras Pampeanas. Four intrusive bodies outcrop in the study area: La Ovejeria, El Infiernillo, Loma Pelada and Los Cuartos. La Ovejeria and El Infiernillo intrusive bodies represent the I-type magmatism according to their major element contents and show cotectic evolutions similar to those described by Famatinian I-type batholiths. These moderately peraluminous adakitic trondhjemites have characteristic high Sr/Y ratios and low Fe 2 O 3 +MgO+MnO+TiO 2 contents. They are related to high-pressure conditions at the source, where dehydration melting of basaltic rocks may be involved and garnet is retained in the residue leading to generation of HREE depleted melts. The Loma Pelada granites show characteristics of S-type magmatism (low CaO and MgO, and high SiO 2 and K 2 O contents) typical of granites segregated in the last stages of magmatic differentiation, or anatectic granites. They show an increasing peraluminosity due to garnet entrainment and they are related to the anatectic melts generated in the Puncoviscana Formation. Both the Loma Pelada and Los Cuartos granitoids include samples with intermediate geochemical characteristics that range between those of the El Infiernillo and La Ovejeria and the regional metasedimentary rocks. These characteristics could be explained by assimilation processes involving the I-type intrusive magmas and the metasedimentary host rocks or by hybridization processes between trondhjemitic I-type magmas as La Ovejeria and El Infiernillo and anatectic S-type melts.

Sedimentology of the mid-Carboniferous fill of the Olta paleovalley, eastern Paganzo Basin, Argentina: Implications for glaciation and controls on diachronous deglaciation in western Gondwana during the late Paleozoic Ice Age

Both global and regional climate drivers contributed to glaciation during the late Paleozoic Ice Age (LPIA). However, the transition from icehouse to greenhouse conditions was asynchronous across Gondwana suggesting that, in some cases, regional controls played a significant role in deglaciation. Of particular interest to understanding changing LPIA climatic conditions, is the eastern Paganzo Basin. This region was flanked by ice centers in the Precordilleran and Sierras Pampeanas regions of Argentina on the west, and major ice sheets in the Paraná, Chaco-Paraná, and Sauce Grande basins to the east, all of which resided between ∼40 and 65° S latitude. Hypotheses on the occurrence of ice in the eastern Paganzo Basin are based on interpretations of the narrow, steep-walled, Olta-Malanzán paleovalley as carved by an alpine glacier or by an outlet glacier draining an eastern ice sheet, and that glaciers deposited coarse clastics within the paleovalley. However, we found no evidence for glaciation. Rather, gravel from prograding alluvial fans/fan deltas and rock falls ponded drainage resulting in lacustrine activity in the eastern end of the valley. A transition from either subaerially or shallow subaqueously deposited sandstones to marine mudstones in the western end of the Olta paleovalley suggest a marine transgression, which, in turn, was overlain by deposits of prograding Gilbert-type deltas. Dropstones were from rock falls off valley walls and rafting by lake ice rather than from icebergs. Therefore, we conclude that the climate in western Argentina resulted from uplift induced glaciation in the Precordilleran region and along the western margin of the Paganzo Basin, and the occurrence of a precipitation shadow to the east. The disappearance of the western glaciers during the mid-Carboniferous, prior to deglaciation elsewhere at the same paleolatitude, resulted from a westward shift in the position of the active margin, collapse of the glaciated upland(s), and an expansion of the precipitation shadow across the whole of western Argentina.

Alluvial to lacustrine sedimentation in an endorheic basin during the Mio-Pliocene: The Toro Negro Formation, Central Andes of Argentina

A 2400 m-thick sedimentary column belonging to the Toro Negro Formation was recorded along the Quebrada del Yeso, Sierra de Los Colorados (Vinchina Basin), La Rioja province, NW Argentina. The Vinchina basin is a good example of a closed basin surrounded by the Precordillera fold and thrust belt to the west and basement-cored blocks to the north, south (Western Sierras Pampeanas) and east (Sierra de Famatina). Seven facies associations (FA) are described and interpreted to represent fluvial, lacustrine and alluvial environments developed in the southern part of the Vinchina basin from the Late Miocene until the earliest Pleistocene. The depositional evolution of the formation was divided in four phases. Phase I (∼7–6.6 Ma) represents sedimentation in medial (FA I) to distal (FA II) parts of a southward directed distributive fluvial system with a retrogradational pattern. During phase II (6.6–6.1Ma), the distributive fluvial system was replaced by a mixed clastic-evaporitic shallow lake (FA III) in a high aggradational basin. In phase III (∼6.1–5 Ma) the eastward progradation of a fluvial system (FA IV) was recorded as a distal clastic wedge. Finally, phase IV (∼5-2.4Ma) records two depositional cycles of proximal clastic wedge progradation of fluvial-dominated piedmonts (FAV, FAVII) from the southwest (Sierra de Umango) and/or the west (Precordillera) with an intervening playa lake (FA VI).Two new U-Pb ages obtained from zircons in volcanic ash layers confirm the Late Miocene age of the lower member of the Toro Negro Formation and permit a tight correlation with the central part of the basin (Quebrada de La Troya section). The sedimentation rate calculated for the dated lacustrine-fluvial interval is higher than the corresponding one in La Troya area suggesting a higher subsidence in the southern part of the basin.During the Late Miocene (∼7-6.6Ma) the ephemeral drainage was controlled by an arid to semiarid climate and initially dissipated mostly internally as terminal fan/distributive fluvial systems descending from the north. A thick lacustrine interval developed in the southern part of the basin between ∼6.6 and 6.1 Ma during a period of high subsidence and closed drainage. Besides, this interval coincides with increased aridity recorded in other basins in the Northwest of Argentina. By ∼6.1 Ma the area started to receive the first coarse-grained sediments heralding the progradation of a clastic wedge from the southwest-west (Sierra de Umango and Precordillera) which fully developed during the rest of the Pliocene to the earliest Pleistocene (∼5–2.4 Ma). The 6.1–2.4 Ma interval records ameliorating climate conditions.

Internal framework and geochemistry of the Carboniferous Huaco granite pluton, Sierra de Velasco, NW Argentina

The A-type Huaco granite pluton of the Velasco range (Sierras Pampeanas of northwest Argentina) is formed by three coeval granitic facies and contains subordinate coeval-to-late facies, as well as enclaves, dikes and stocks that show different temporal relations, textures and compositions. The dominant facies (Regional Porphyritic Granite; RPG) is a porphyritic two-mica monzo- to syenogranite, with abundant microcline megacrysts up to 12 cm in size. It was emplaced in a dominant extensional setting and has a mainly crustal source but with participation of a mantle-derived component. The RPG transitions towards two coeval and co-genetic granite facies, at its margins (Border Granite; BG) and around Be-pegmatites (Adjacent Porphyritic Granite; APG). These two facies have a finer-grained texture and smaller and less abundant megacrysts. They are also monzo- to syenogranites, but a slight decrease in the biotite/muscovite ratio is observed from the BG to the RPG to the APG. Trace element modeling suggests that the RPG, BG and APG differentiated from the same magma source by fractional crystallization. Temporally older mafic (ME) and felsic (FE) enclaves are common in the pluton. The ME can be considered partially assimilated remnants of a mafic component in the genesis of the RPG, whereas the FE seem to be remnants of premature aplites. Other subordinate rocks intrude the RPG and are, hence, temporally younger: felsic dikes (FD), dioritic dikes (DD) and equiganular granites (EqG) are clearly posterior, whereas coeval-to-late Be-pegmatites (BeP) and orbicular granites (OG) formed during the final stages of crystallization of the pluton. The BeP, OG and FD indicate the presence of abundant water and volatiles. The EqG form small stocks that intrude the RPG and were possibly originated from purely crustal sources. The DD probably correspond to a younger unrelated episode of mafic magmatism.

Weathering assessment in the Achala Batholith of the Sierra de Comechingones, Córdoba, Central Argentina. III: appraising chemical weathering

La Trucha is a small (~ 1.9 km2), mountainous (~ 1300 m a.s.l.) granitic drainage basin, demonstrative of hundreds of second-order basins in the Achala Batholith of the Sierras Pampeanas of Cordoba, Argentina. Dominated by physical weathering and a weathering-limited denudation regime, the coarse- and fine-grained regolith has shown scant geochemical differences with the country rock. Erosion and sediment transport mainly occurs during torrential rain events (austral summer), whereas surface waters classify as dilute (750 < TΣ+ < 375 µeq L−1) or very dilute (375 < TΣ+ < 185 µeq L−1) water types. The dynamics of chemical weathering at La Trucha was approached through the use of major dissolved chemical components, comparing the performance of different methodologies in its assessment: mineral stability diagrams (i.e., chemical equilibria), the RE chemical weathering index, ternary diagrams, and PHREEQC inverse modeling. The latter approach allowed to probe into subtleties of the geochemical processes indicating that, besides the morphological dependency on mineral hydrolysis/dissolution or precipitation, there is a marked seasonal control that promotes or limits the occurrence of geochemical reactions.

Mississippian lamprophyre dikes in western Sierras Pampeanas, Argentina: Evidence of transtensional tectonics along the SW margin of Gondwana

In the Famatina range, Sierras Pampeanas of Argentina (SW Gondwana), subvertical calc-alkaline lamprophyric dike swarms crop out through >300 km. The dikes cut Ordovician units with a prominent NW-SE trending and are covered by continental sedimentary successions of Pennsylvanian to Permian age. The dikes show a strong structural control associated with Riedel fault systems. Detailed field analysis suggested a ∼N-S opening direction oblique to the attitude of dike walls and a left-lateral transtensional tectonics during the emplacement. 40Ar/39Ar geochronology of a lamprophyric sample defined a crystallization age (plateau; whole rock) of 357.1 ± 7.1 Ma (MSWD = 2.3). Coetaneous ductile zones with dominant strike-slip motion, documented along western Argentina for >600 km, suggest a regional event in SW Gondwana during the Mississippian. We propose that this deformation was the result of the counterclockwise fast rotation of Gondwana between 365 and 345 Ma, when the Famatina range and western Argentina occupied a sub-polar position. A transform margin along SW Gondwana better explains our (and others) data rather than a subduction margin. This scenario is also consistent with the occurrence of A-type granites and normal-fault basins within the foreland as well as bimodal volcanics.