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The coastal aquifer La Yarada has anthropogenic and geogenic contamination that adversely affect the quality of groundwater for population and agricultural use. In this scenario, multivariate statistical methods were applied in 20 physicochemical and isotopic parameters of 53 groundwater pumping wells in October 2020, with the aim of characterizing the hydrogeochemical processes that dominate the groundwater of the coastal aquifer and the factors that cause them to optimize the effective management of water resources, delimiting areas affected by more than one salinization process. The samples were grouped into three clusters (C1, C2, and C3) with cluster analysis, the spatial distribution of C2 and C3 (reclassified in stiff diagrams), evidenced hydrogeochemical facies associated with the flow and recharge directions governed by the structural lineaments (NE-SO), favoring some areas more than others, arising different facies and hydrogeochemical processes. Factor analysis was applied from three different approaches: (1) main elements, (2) trace elements, and (3) physicochemical and isotopic parameters; exposing 6 distinguishable hydrogeochemical processes in the aquifer and factors that cause them: (i) salinization—marine intrusion, (ii) fertilizer leaching and dissolution of (Ca2+, Mg2+), (iii) wastewater mixture (NO3−), (iv) reducing conditions (Fe, Mn, Al), (v) contributions of (B, Sr), (vi) conservative mixtures and dissolution (As, F). It was validated with water quality indices (WQI) according to the national limits, delimiting 67 km2 parallel to the coast with “bad” to “very bad” quality for human consumption and unsuitable for irrigation according to the Wilcox diagram thus pre-treatment in this area is indispensable.

The crisis of governance and governability in the use of groundwater in coastal aquifers in arid zones is an element that contributes to the depletion and deterioration of the groundwater quality, due to processes of marine intrusion. Under the current exploration conditions, the governance crisis is characterized in the overall consumption of groundwater by humans, their animals, and by their industrials. Interpreting current national and international regulations applicable to this type of system confirms this area of investigation. This problem goes beyond the laws in all its terms regarding the management regimes and limitations on the availability of groundwater resources. If immediate actions are not taken by the government, the crisis of the system could become irreversible, with the consequent economic damage that this involves.

Pino, E., Tacora, P., Steenken, A., Alfaro, L., Valle, A., Chávarri E., Ascencios D., & Mejía, J. (noviembre-diciembre, 2017). Efecto de las características ambientales y geológicas sobre la calidad del agua en la cuenca del río Caplina, Tacna, Perú. Tecnología y Ciencias del Agua, 8(6), 77-99, DOI: 10.24850/j-tyca-2017-06-06. Se busca determinar la calidad del agua en la cuenca Caplina tomando como componentes principales el análisis de la geomorfología, geología, medio ambiente e hidrología. Para ello, se tomaron los estudios realizados por el Proyecto Especial Tacna (PET), Autoridad Nacional del Agua (ANA), Autoridad Local del Agua (ALA), el Servicio Nacional de Meteorología e Hidrología (SENAMHI), entre otros. Se identificaron tres unidades geomorfológicas, tales como: la Cordillera Occidental, Puna y Flanco Disectado de los Andes con sus características propias; asimismo en base a la cartografía geológica se identificó las formaciones geológicas que inciden sobre la calidad del agua en la Cuenca Caplina. Se determinaron las fuentes con mayor influencia sobre la calidad del agua, siendo las geotermales Aruma y Paralocos que emanan aguas con alto contenido de Arsénico, Plomo y Sodio en concentraciones por encima de los valores del Estándar de Calidad Ambiental (ECA), incluso el trasvase de aguas de la fuente Barroso Chico hacia la Quebrada Ancoma con alto contenido de Hierro. Se identificó las interrelaciones entre la geología, el clima y la hidrología los cuales reflejan la variación de la concentración de los elementos en el agua en épocas de estiaje y épocas de avenida.

The Eastern Sierras Pampeanas were structured by three main events: the Ediacaran to early Cambrian (580–510 Ma) Pampean, the late Cambrian–Ordovician (500–440 Ma) Famatinian and the Devonian-Carboniferous (400–350 Ma) Achalian orogenies. Geochronological and Sm–Nd isotopic evidence combined with petrological and structural features allow to speculate for a major rift event (Ediacaran) dividing into two Mesoproterozoic major crustal blocks (source of the Grenvillian age peaks in the metaclastic rocks).This event would be coeval with the development of arc magmatism along the eastern margin of the eastern block. Closure of this eastern margin led to a Cambrian active margin (Sierra Norte arc) along the western margin of the eastern block in which magmatism reworked the same crustal block. Consumption of a ridge segment (input of OIB signature mafic magmas) which controlled granulite-facies metamorphism led to a final collision (Pampean orogeny) with the western Mesoprotrozoic block. Sm–Nd results for the metamorphic basement suggest that the TDM age interval of 1.8–1.7 Ga, which is associated with the less radiogenic values of εNd(540) (−6 to −8), can be considered as the mean average crustal composition for the Eastern Sierras Pampeanas. Increasing metamorphic grade in rocks with similar detrital sources and metamorphic ages like in the Sierras de Córdoba is associated with a younger TDM age and a more positive εNd(540) value. Pampean pre-540 Ma granitoids form two clusters, one with TDM ages between 2.0 and 1.75 Ga and another between 1.6 and 1.5 Ga. Pampean post-540 Ma granitoids exhibit more homogenous TDM ages ranging from 2.0 to 1.75 Ga. Ordovician re-activation of active margin along the western part of the block that collided in the Cambrian led to arc magmatism (Famatinian orogeny) and related ensialic back-arc basin in which high-grade metamorphism is related to mid-crustal felsic plutonism and mafic magmatism with significant contamination of continental crust. TDM values for the Ordovician Famatinian granitoids define a main interval of 1.8–1.6, except for the Ordovician TTG suites of the Sierras de Córdoba, which show younger TDM ages ranging from 1.3 to 1.0 Ga. In Devonian times (Achalian orogeny), a new subduction regime installed west of the Eastern Sierras Pampeanas. Devonian magmatism in the Sierras exhibit process of mixing/assimilation of depleted mantle signature melts and continental crust. Achalian magmatism exhibits more radiogenic εNd(540) values that range between 0.5 and −4 and TDM ages younger than 1.3 Ga. In pre-Devonian times, crustal reworking is dominant, whereas processes during Devonian times involved different geochemical and isotopic signatures that reflect a major input of juvenile magmatism. Fil: Steenken, André. Universität Greifswald; Alemania Fil: Lopez, Monica Graciela. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geocronología y Geología Isotopica. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geocronología y Geología Isotópica; Argentina Fil: Martínez Dopico, Carmen Irene. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geocronología y Geología Isotopica. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geocronología y Geología Isotópica; Argentina Fil: Drobe, Malte. Universität Göttingen; Alemania Fil: Wemmer, Klaus. Universität Göttingen; Alemania Fil: Siegesmund, Siegfried. Universität Göttingen; Alemania

Geochronological data pertaining to the structural evolution of the Sierra de San Luis provide important insights into the geodynamic history along the southwestern margin of Gondwana. In the Pringles metamorphic complex, metamorphic fabrics (S 1 ) were affected by two folding events (D 2 and D 3 ) related to the Ordovician approach of the Cuyania terrane. Contemporary formations of high-temperature shear zones record a prominent east-side-up displacement. Resumption of compression due to the Achalian collision with the Chilenia terrane was accommodated by the reactivation of the Famatinian high-temperature shear zones in greenschist facies conditions. K–Ar Ms ages point to their activity up to the Early Carboniferous. The post-Pampean structural evolution of the Pringles metamorphic complex is supported by new Sm–Nd and 207 Pb/ 206 Pb data, which agree with previously proposed early Cambrian sedimentation of the psammopelitic precursors. A Sm–Nd mineral isochron, together with published SHRIMP U–Pb zircon ages, bracket the emplacement of mafic and ultramafic intrusions that control the granulite facies metamorphism between 506 and 478 Ma. A late Cambrian emplacement of the crustal-derived Paso del Rey pluton is suggested by a 207 Pb/ 206 Pb zircon age at 491 ± 19 Ma. The syn-D 2 emplacement of these granitoids indicates that the D 1 to D 2 structural evolution predates the generally assumed Middle Ordovician accretion of the Precordillera/Cuyania terrane. K–Ar Hbl, Ms, and Bt ages and some Rb–Sr mica data record the cooling after the Famatinian metamorphic peak. Ms ages from large Ms booklets suggest a common cooling of the basement of the sierra at approximately 445 Ma, whereas normal-sized muscovites from gneissic rocks (K–Ar ages of ∼380 Ma) indicate slow cooling of the Pringles metamorphic complex with respect to the other basement domains. K–Ar Bt ages between 360 and 340 Ma capture cooling below approximately 300 °C of the basement. Differential cooling is interpreted to reflect the Achalian tectonic cycle as an event separate from the Famatinian process.

The application of the sensitive high-resolution ion microprobe (SHRIMP) U/Pb dating technique to zircon and monazites of different rock types of the Sierra de San Luis provides an important insight into the provenance and timing of deposition of the sedimentary precursors as well as the metamorphic and igneous history of the various basement domains. Additional constraints on the Famatinian metamorphic episode are provided by Pb/Pb stepwise leaching experiments on one staurolite and two garnet separates. The results indicate that the sedimentary precursors of the Conlara Metamorphic Complex have a maximum age of c . 590 Ma, whereas the Pringles Metamorphic Complex metasediments appear to be sourced from the Pampean orogen in the Early Cambrian. Folded xenoliths within the c . 496 Ma El Penon pluton suggest that the host Conlara Metamorphic Complex underwent a Pampean compression. From a 208 Pb/ 232 Th monazite age of 478 Ma for a migmatite from the Nogoli Metamorphic Complex, the structural evolution of this basement complex appears to be entirely post-Pampean. Onset of the Famatinian high-grade metamorphism, between c . 500 Ma and c . 450 Ma, follows a period of crustal extension on the western outboard of Gondwana and might not be related directly to a Mid-Ordovician accretion of the Cuyania Terrane.

The Sierra de San Luis constitutes the southernmost tip of the Eastern Sierras Pampeanas. Its Palaeozoic metamorphic basement units define a key location for the understanding of the accretional history along the proto-Andean margin of Gondwana. Although, it is largely accepted that the polyphase accretional history of the Sierras Pampeanas is preluded by the docking of the Pampean Terrane followed by the Famatinian Orogenic Cycle that involves subduction along the margin of Gondwana and the accretion of the Precordillera (Cuyania) Terrane and finally ceased with the collision of the Chilenia terrane, a vast amount of controversial information concerning the timing and mode of collisions as well as the origin of the different involved crustal fragments within the Eastern Sierras Pampeanas is published. In this paper, those different hypothesis are presented and evaluated under the light of new isotopic data of the Sierra de San Luis. Nd-systematics of the metasedimentary sequences of the Sierra de San Luis indicate that the studied sequences were developed on the Pampean Terrane. An Amazonian origin of the Pampean Terrane that was probably detached from the Arequipa Antofalla Craton is proposed. Furthermore, the correlation of two low-grade phyllitic belts (San Luis Formation) with the widespread Puncoviscana Formation is not supported by Sm-Nd data. It is suggested that the sedimentary precursors of the Pringles Metamorphic Complex and the topping phyllites were sourced on the Pampean Orogen and accommodated in a newly formed back arc basin during the early Famatinian. The cooling history of the basement complex is recorded by an extensive amount of K-Ar muscovite and biotite ages. A high variability in muscovite ages is only partly related to different intrusion times of two pegmatoid generations. Post Famatinian to Achalian crustal scale mylonite formation (-359 Ma) and a rotational exhumation of the central basement unit are causal for the observed K-Ar muscovite age pattern in the range from 395 Ma to 447 Ma. Therefore, the decrease in metamorphic degree from west to east is the result of the erosion level of a crustal profile from the mid lower crust to the upper crust. An even higher variability in K-Ar biotite cooling ages covering the range from 315 Ma to 418 Ma is related to the slow cooling after the Famatinian Orogenic Cycle or reheating during the Achalian Orogenic Cycle and consequent variable reset of the isotopic system. However, ages recorded by biotite booklets substantiate the hypothesis of a differential exhumation of the basement of the Sierra de San Luis.

The Defereggen–Antholz–Vals (DAV) Line marks the southern border of alpine greenschist facies metamorphism within the Austroalpine basement to the south of the western Tauern window. During the Oligocene sinistral-strike slip, coeval with vertical displacement, was accompanied by the syntectonic emplacement of the Rieserferner Pluton along the north side of the fault. To the north of the DAV Line Rb/Sr as well as K/Ar-biotite cooling ages continuously increase from 16 Ma at the Rensen Pluton in the west to 30 Ma at the eastern end of the Rieserferner Pluton. Similar trends are apparent from zircon and apatite fission-track data, which increase eastward from 19 to 29 Ma, and 13 to 16 Ma, respectively. Low cooling rates allow a direct transformation into exhumation rates assuming an average geothermal gradient of 28 °C km–1. The resulting exhumation rates of less than 1 mm year–1 do not affect the depth of isotherms in a geologically significant manner. Hence, variable cooling rates within the basement north of the DAV Line are a result of differential exhumation of individual blocks. High exhumation rates bracketed between the intrusion age of the Rieserferner Pluton at 31±3 Ma and the apatite partial annealing zone (PAZ) range between 0.4 and 0.6 mm year–1, whereas final exposure takes place at reduced exhumation rates of 0.2 mm year–1. East-side-down tilting of the block north of the DAV Line, as indicated by variations in exhumation rates, strongly affects the western part of the area studied between the Rensen and the Central Rieserferner Pluton whereas, further east, block rotation becomes less important. A total difference in exhumation of about 2.2 km corresponding to 5° tilting between the Western and Eastern Rieserferner, as indicated by K/Ar-biotite and zircon fission-track data, occurred before going through the PAZ for apatite fission-tracks. Furthermore, the significance of the cooling ages, especially for biotite, is tested by calculating a finite element approximation. Different numerical models indicate fast cooling of the intrusive melts prior to approaching the closure temperature of the K/Ar system in biotite.

The Sierra de San Luis forms the southern tip of the Eastern Sierras Pampeanas in central Argentina. Two narrow belts of low-grade phyllites and quartz arenites, i.e. the San Luis Formation, have accommodated part of the strain-related differential exhumation of the medium- to high-grade metamorphic domains that constitute to the basement complex of the sierra. Eleven phyllite samples were subjected to the K/Ar fine-fraction dating technique. Results are interpreted in relation to the Kübler index of the illites, which indicate epimetamorphic conditions for the majority of the samples. Obtained ages between 330 and 290 Ma cover a period of compressional tectonics in the late Mississippian (Visean/Serpukhovian boundary) followed by the subsidence during the formation of the Paganzo Basin in the provinces of La Rioja and San Luis. These tectonic movements are coincident with the Toco orogeny in northern Chile and southern Bolivia. This suggests that the older K/Ar ages document the compressional stage and that younger ages record the cooling of the basement during the subsequent extensional uplift of the basement. Fil: Wemmer, Klaus. Universität Göttingen; Alemania Fil: Steenken, André. Universität Greifswald; Alemania Fil: Müller, Stefan. Universität Göttingen; Alemania Fil: Lopez, Monica Graciela. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geocronología y Geología Isotopica. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geocronología y Geología Isotópica; Argentina Fil: Siegesmund, Siegfried. Universität Göttingen; Alemania

Whole-rock geochemical analyses using major and trace elements in combination with the Sm–Nd and Pb–Pb isotope systems, together with SHRIMP age dating on metasedimentary rocks from the Sierras de Chepes, the Sierras de Córdoba, the Sierra Norte and the San Luis Formation in the Sierra de San Luis, have been carried out to unravel the provenance and the geodynamic history of the Eastern Sierras Pampeanas, Central Argentina. The geochemical and the Sm–Nd data point to a slightly stronger mafic and less-fractionated material in the provenance area of the Sierras de Córdoba when compared to the other units. The TDM model ages from the Sierras de Chepes (~1.82 Ga) and the Sierra Norte (~1.79 Ga) are significantly older than the data from the Sierras de Córdoba (1.67 Ga). The Pb data are homogeneous for the different units. Only the 208Pb/204Pb ratios of some samples from the Sierras de Córdoba are higher. A late Pampean detrital zircon peak around 520 Ma from the Sierras de Chepes is in accordance with the new data from the San Luis Formation. This is similar to the literature data from the Famatina Belt located to the northwest of the Sierras de Chepes and also fits the detrital zircon peaks in the Mesón group. These maximum depositional ages were also reported from some locations in the Puncoviscana Formation but are absent in the Sierras de Córdoba. An improved model for the development of the Eastern Sierras Pampeanas in the area between the Sierras de Córdoba and the Puncoviscana Formation is provided. This gives new insights into the late Pampean development of the Sierra de San Luis and the complex development of the Eastern Sierras Pampeanas. This new model explains the younger detrital ages in the Puncoviscana Formation compared with the older ages of the Sierras de Córdoba. Another model of the Sierra de San Luis explains the younger depositional ages of the Pringles Metamorphic Complex and the San Luis Formation when compared to the Nogolí Metamorphic Complex and the Conlara Metamorphic Complex. Additionally, the rather fast change of the high-grade metamorphic conditions in the Pringles Metamorphic Complex and the low-grade metamorphic conditions in the San Luis Formation is explained by extension, the ascent of (ultra) mafic material and later folding and erosion. Fil: Drobe, Malte. Georg August Universität; Alemania Fil: Lopez, Monica Graciela. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geocronología y Geología Isotopica. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geocronología y Geología Isotópica; Argentina Fil: Steenken, André. Universität Greifswald; Alemania Fil: Wemmer, Klaus. Georg August Universität; Alemania Fil: Naumann, Rudolf. Deutsche GeoForschungsZentrum Potsdam; Alemania Fil: Frei, Robert. Universidad de Copenhagen; Dinamarca Fil: Siegesmund, Siegfried. Georg August Universität; Alemania

The application of the SHRIMP U/Pb dating technique to zircon and monazite of different rock types of the Sierras de Córdoba provides an important insight into the metamorphic history of the basement domains. Additional constraints on the Pampean metamorphic episode were gained by Pb/Pb stepwise leaching (PbSL) experiments on two titanite and garnet separates. Results indicate that the metamorphic history recorded by Crd-free gneisses (M2) started in the latest Neoproterozoic/earliest Cambrian (553 and 543 Ma) followed by the M4 metamorphism at ~530 Ma that is documented in the diatexites. Zircon ages of 492 Ma in the San Carlos Massif correlate partly with rather low Th/U ratios (

Provenance studies have been performed utilising major and trace elements, Nd systematics, whole rock Pb–Pb isotopes and zircon U/Pb SHRIMP data on metasedimentary rocks of the Sierra de San Luis (Nogolí Metamorphic Complex, Pringles Metamorphic Complex, Conlara Metamorphic Complex and San Luis Formation) and the Puncoviscana Formation of the Cordillera Oriental. The goal was the characterisation of the different domains in the study area and to give insights to the location of the source rocks. An active continental margin setting with typical composition of the upper continental crust is depicted for all the complexes using major and trace elements. The Pringles Metamorphic Complex shows indications for crustal recycling, pointing to a bimodal provenance. Major volcanic input has to be rejected due to Th/Sc, Y/Ni and Cr/V ratios for all units. The εNd(540 Ma) data is lower for the San Luis Formation and higher for the Conlara Metamorphic Complex, as compared to the other units, in which a good consistency is given. This is similar to the TDM ages, where the metapsammitic samples of the San Luis Formation are slightly older. The spread of data is largest for the Pringles Metamorphic Complex, again implying two different sources. The whole rock 207Pb/206Pb isotopic data lies in between the South American and African sources, excluding Laurentian provenances. The whole rock Pb–Pb data is almost indistinguishable in the different investigated domains. Only the PMC shows slightly elevated 208Pb/204Pb values. Possible source rocks for the different domains could be the Quebrada Choja in the Central Arequipa–Antofalla domain, the Southern domain of the Arequipa–Antofalla basement, the Brazilian shield or southern Africa. Zircon SHRIMP data point to a connection between the Puncoviscana Formation and the Conlara Metamorphic Complex. Two maxima around 600 Ma and around 1000 Ma have been determined. The Nogolí Metamorphic Complex and the Pringles Metamorphic Complex show one peak of detrital zircons around 550 Ma, and only a few grains are older than 700 Ma. The detrital zircon ages for the San Luis Formation show age ranges between 590 and 550 Ma. A common basin can be assumed for the Conlara Metamorphic Complex and the Puncoviscana Formation, but the available data support different sources for the rest of the Complexes of the Sierra de San Luis. These share the diminished importance or the lack of the Grenvillian detrital peak, a common feature for the late Cambrian–early Ordovician basins of the Eastern Sierras Pampeanas, in contrast to the Sierras de Córdoba, the PVF and the Conlara Metamorphic Complex. Fil: Drobe, Malte. Georg August Universität; Alemania Fil: Lopez, Monica Graciela. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geocronología y Geología Isotopica. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geocronología y Geología Isotópica; Argentina Fil: Steenken, André. Universität Greifswald; Alemania Fil: Frei, Robert. Universidad de Copenhagen; Dinamarca Fil: Naumann, Rudolf. Deutsches GeoForschungsZentrum Potsdam; Alemania Fil: Siegesmund, Siegfried. Georg August Universität; Alemania Fil: Wemmer, Klaus. Georg August Universität; Alemania

The Paleozoic granitoids of the Sierra de San Luis comprise the Ordovician tonalite suite (OTS; metaluminous to mildly peraluminous calcic tonalite–granodiorites) and granodiorite–granite suite (OGGS; peraluminous calcic to calc-alkaline granodiorite–monzogranites), as well as the Devonian granite suite (DGS; peraluminous alkali-calcic monzogranites) and monzonite–granite suite (DMGS; metaluminous alkali-calcic quartz monzonite–monzogranite ± granodiorite, mildly peraluminous alkalicalcic monzogranites). The OTS has relatively high K2O, CaO, and YbN and low Cr, Ni, Ba, Sr, Rb/Sr, Sr/Y, and (La/Yb)N, as well as negative Eu/Eu∗, high 87Sr/86Sr (0.70850–0.71114), and unradiogenic eNd(470Ma) (−5.3 to −6.0), which preclude an origin of variably fractionated mantle melts and favour a mafic lower crustal source. The OGGS consists of two granitoids: (1) high-temperature characterized by low Al2O3/TiO2, Rb/Sr, and (La/Yb)N, a smooth negative Eu/Eu∗, and relatively high CaO and (2) low-temperature with high Al2O3/TiO2 and Rb/Sr, low CaO, (La/Yb)N, and Sr/Y, and negative Eu/Eu∗. Melting of metagreywackes at pressures below 10 kbar with a variable supply of water could account for the chemistry of the high-T OGGS, whereas dehydration melting of biotite-bearing metasedimentary sources at low pressures is proposed for the low temperature OGGS. Melting of crustal sources relates to a contemporaneous mafic magmatism. Devonian magmatism is characterized by high Ba, Sr, K2O, Na2O, Sr/Y, and (La/Yb)N. Sources for the DGS include metasedimentary or metatonalitic protoliths. Biotite dehydration melting triggered by the addition of heat, supplied by mantle-derived magmas, is proposed. High Ba, Sr, LREE, MgO, Cr, Ni, Zr, and V of the monzonites suggest an enriched lithospheric mantle source. Low Yb and Y and high Sr and (La/Yb)N indicate a garnet-rich residual assemblage (P ⩾ 10 kbar). Melts for the peraluminous rocks may have derived from a metasedimentary or metaigneous source at lower pressures in a process dominated by biotite consumption and plagioclase in the residue. The Ordovician granitoids are synkinematic with compressive deformation related to the early stages of Famatinian convergence. The Devonian magmatism is synkinematic with a system of shear zones that were active during the Achalian cycle.

A major and trace element based characterization of the metasedimentary protoliths of three metamorphic units of Sierra de San Luis (Pringles Metamorphic Complex, San Luis Formation and Conlara Metamorphic Complex) is presented. Geochemistry indicates a dominance of shales in the protoliths of San Luis Formation, whereas greywackes and shales made up the Conlara Metamorphic complex and mainly greywackes, the Pringles Metamorphic Complex. Both major element data and trace element ratios (i.e. Th/Sc, Th/U,) indicate a source with an average upper crustal composition for the protoliths of the Pringles Metamorphic Complex, the San Luis Formation and the shales of the Conlara Metamorphic complex. A component with less evolved signature may be inferred for the metagreywackes of the Conlara metamorphic Complex. Mixed sourced detritus are indicated for the three units with clastic material resulting mainly from both andesitic and acidic/recycled detritus. The overall data consistently suggest a continental island arc and/or active margin setting as the more probable geodynamic scenario for the deposition of the sedimentary precursors of the studied units. In this context, a back-arc setting can account for the mixed nature of the inferred source areas with uplifted old basement and arc-related detritus as the end members of the mixtures. The inferred back-arc basin would have evolved through the Cambrian receiving the sediments derived from the Pampean Orogen to the east combined with probably some old crust exposures and to the west the source might have been controlled by the active continental margin. © 2003 Asociación Geológica Argentina. Fil:Cerredo, M.E. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.