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Zaffarana C.B.,University of Buenos Aires | Lopez de luchi M.G.,CONICET | Lopez de luchi M.G.,Institute Geocronologia | Somoza R.,University of Buenos Aires | And 6 more authors.
Journal of South American Earth Sciences | Year: 2010

The Central Patagonian Batholith is a suite of acid and meso-silicic rocks cropping out in central Patagonia. The emplacement of these rocks has been proposed to be related to the activity of a system of dextral transcurrent faults, the NW-SE Gastre Fault System. This fault system has been ascribed a transcontinental magnitude and a ∼500 km dextral displacement during Gondwana dismembering in Jurassic times. However, the timing, kinematics and amount of displacement of the Gastre Fault System are still controversial. In this work we have visited two localities which were subject of controversial observations, in order to perform petrographical, microstructural and anisotropy of the magnetic susceptibility studies to contribute to the ongoing discussion. The results mostly agree with the findings von Gosen and Loske (2004) in that rocks spatially and temporally associated to the Gastre Fault System do not show evidence supporting the existence of a major dextral fault system active during Jurassic times. © 2010 Elsevier Ltd.

Rovere E.I.,Servicio Geologico Minero Argentino SEGEMAR | Violante R.A.,Servicio de Hidrografia Naval | Rodriguez E.,Laboratorio Geologico LCV S.R.L. | Osella A.,University of Buenos Aires | De La Vega M.,University of Buenos Aires
Latin American Journal of Sedimentology and Basin Analysis | Year: 2012

Tephrology is a broad term that comprises all the aspects related to "tephra" studies (stratigraphy, chronology, petrology, sedimentology, chemistry, Froggat and Lowe, 1990; Lowe and Hunt, 2001) (Fig. 1). In Argentina, tephrological studies have significantly increased recently as a result of the increment in the Southern Andes volcanic activity affecting the country in the last two decades (E.g.: Corbella et al., 1991a,b; Stern, 1991; Mazzoni and Destéfano, 1992; Nillni et al., 1992; Gonzalez Ferrán, 1993; Naranjo et al., 1993; Scasso et al., 1994; Nillni and Bischene, 1995; Haberle and Lumley, 1998; Villarosa et al., 2002; Kilian et al., 2003; Naranjo and Stern, 2004; Orihashi et al., 2004; Stern, 2004; Scasso and Carey, 2005; Daga et al., 2008; Watt et al., 2009; Martin et al., 2009; Leonard et al., 2009; Rovere et al., 2009, 2011; Wilson et al., 2009, 2012). The eruption of Quizapú volcano (Volcanic Complex Azul-Descabezado Grande, Province of Talca, Chile, 36,67°S-70,77°W, maximum height of 3788 m a.s.l.), that occurred on April 10, 1932, represented one of the largest eruptions worldwide in the 20th Century. It affected extensive regions of Argentina as well as many coastal areas of the Southwestern Atlantic Ocean as a result of the prevailing westerly winds, and specifically impacted dramatically in regions located nearby the source volcano (Department of Malargüe, Province of Mendoza, west-central Argentina, Fig. 2). The wide spreading of the resulting tephras and its easy reconnaissance in the field provides a great opportunity for detailed studies about the eruption and its products. Results on the eruptive aspects and tephras dispersion and deposition from this eruption were published by some authors (Lunkenheimer, 1932; Kittl, 1933; Walker, 1981, Hildreth and Drake, 1992, González Ferrán, 1993; Ruprecht and Bachmann, 2010; Ruprecht et al., 2012). In this contribution the sedimentological, mineralogical and chemical characteristics of the tephra deposits occurring at the Llancanelo Lake and surroundings, located 140 km east (downwind) of the Quizapú volcano, are studied based on grain-size, petrographic and electron microscope analysis (SEM) as well as semiquantitative chemical determinations by Energy Dispersive Spectrometer (EDS). The obtained results, when compared with the results of analyses performed by other authors in tephras from the 1932 eruption of the Quizapú volcano, allow attributing the studied tephra layer to this eruption. On these bases, diverse aspects related to the depositional and post-depositional aspects of the tephras are herein discussed, as well as some environmental changes produced by the eruption. On the other hand, this paper contributes to a systematic and comparative classification of volcanic hazard in health and society that serves as base-studies for better understanding other more recent Southern Andes eruptive events that affected Argentina (Hudson, Copahue, Chaitén, Llaima, Peteroa and Puyehue-Cordón Caulle volcanoes). The eruption of Quizapú volcano in 1932 was one of the most important events among a long history of activity of this volcanic complex (Smithsonian Institution, 2012). It had a plinian character and threw into the atmosphere enormous amounts of tephras varying between 5 and 30 km3 according to different authors (Kittl, 1933; González Ferrán, 1993; Hildreth and Drake, 1992; Ruprecht and Bachmann, 2010), producing a dramatic impact in society, agriculture and local economies in the downwind neighboring affected regions (Abraham and Prieto, 1993; González Ferrán, 1993). The tephra deposits were very uniform in thickness with a notable decreasing grain-size tendency with distance from the source volcano, ranging from 6 cm in neighboring areas and reaching silt and clay sizes around 100 km east (Kittl, 1933; Hildreth and Drake, 1992). The horizon of tephras was recognized as a regional level in a number of natural outcrops pits and excavations, as well as in sediment cores recovered from short drillings (Fig. 3). The tephra level was affected by compaction and post-depositional transformations after 80 years of burying and exposure to weathering and pedogenetic processes, although most of the original characteristics are very well preserved. The sedimentary sequence in which the tephra level is included was recognized regionally by surface and subsurface surveys based on geoelectrical methods and short drillings (Violante et al., 2010; Osella et al., 2010, 2011; de la Vega et al., 2012). The sequence is composed of light brown sandy-silty sediments of lacustrine and eolian origin with high volcaniclastic content and interbedding of buried soils and evaporites (Rovere et al., 2010a,b; D'Ambrosio et al., 2011). In some profiles (P19 and P42, Fig. 3) located in marginal areas east of the lake, the tephra layer overlies lacustrine deposits and is in turn covered by eolian deposits; this indicates that the lake borders were filled with tephra during the eruption and definitively desiccated, and were later covered by eolian deposits probably as a result of the aridity of the climate that followed the eruption. On the other hand, in the lacustrine plain west of the lake the tephra layer was not found; a possible explanation for this is either post-depositional erosive processes or not deposition, as some places could have been, at the moment of the eruption, part of the lacustrine body with higher water energy, and therefore the ash was dispersed without leaving any recognizable deposit. Northwest of the lake, the tephra deposit was found overlying a buried soil containing burned vegetation remains (profile P45, Fig. 3), suggesting high temperatures of the ash fall with consequent burning of vegetation, as it was also documented in other regions of the world (Carson et al., 1990; Seymour et al., 1993). In the lacustrine coastal plain of the lake, tephra layers were found overlying eolian deposits (profiles P5, P21 and P26, Fig. 3). Tephra's grain-size indicates varied sizes between very fine and medium sand. Sediments are poorly sorted and statistical grain-size distributions (Table 1, Fig. 4) are bimodal with two well-marked populations separated at the size-range of 3-3,5 φ (88- 125 μm). Population 1 is coarser with mode between 1 and 2 φ (250 to 500 μm), whereas Population 2 is finer with mode between 4 and 7 φ (63 to 8 μm). This bimodal distribution is typical for distal tephras (Bonadonna and Houghton, 2005; Rose and Durant, 2009). The lower-sized population contains the "respirable particles" (PM10 <10 μm, Horwell et al., 2003, Horwell and Baxter, 2006). Optical microscopy allowed obtaining the bulk mineralogical composition and details of the ash shards. Bulk composition is: 59% volcanic glass, 40% crystals (in decreasing order: plagioclases, magnetite, hornblende, pyroxenes, quartz, olivine and ilmenite) and 1% lithoclasts (possibly andesitic volcanic pastes). Glass is mainly composed of fibrous, pumiceous shards with vesicular microcavities, most of them tubular and elongated with minor amount of cuspate, blocky and platy individuals (Figs. 5, 6 and 7). Besides, the minerals contain vesiculated glass adhered to the crystals. SEM analyzes were aimed at observing details of the particle's shapes and surface characteristics. They are all of varied shapes ranging from equidimensional, elongated (prismatic) and irregular, from rounded to angular with sharp edges, with striations and different degrees of vesicularity (Figs. 6 and 7). Glass shards show a major composition of light brown glass (possibly sideromelano) although dark glass is also present, and they show some coating. Its vitreous textures were defined following the clasification by Miwa et al. (2009), as massive with two types of surfaces, smooth-uniform (S-type) and not-smooth-irregular (NS-type) with alveoli and hollows (Fig. 7). The coating consists of highly cohesive small particles (<25 μm, and hence they correspond to the "respirable" sizes) which can be partially adhered by some melting process to the larger particles. EDS revealed predominance (in decreasing order) of SiO2 (up to ~70%), Al2O3 (up to ~15%), with lesser amounts of K, Na, Ca, Zn, Mg, Cu, Fe y Ti (Fig. 7, Table 2). The three last mentioned components are abundant as oxides included in the ash. K is an important component in accordance to the high K content of the Volcanic Complex Cerro Azul - Descabezado Grande - Quizapú (Backlund, 1923), which seems to have been proportionally increased in percentage by desilication of the tephra during transport (Aomine and Wada, 1962). On the other hand, high concentrations of Cu were found in some samples (Fig. 8, samples P5 III and P20 I in Table 2), what is preliminary associated to postdepositional alteration of tephras by weathering and transformation in alofana and halloysite with incorporation of high Cu content. © Asociación Argentina de Sedimentología.

Poma S.,University of Buenos Aires | Zappettini E.O.,Servicio Geologico Minero Argentino SEGEMAR | Quenardelle S.,University of Buenos Aires | Santos J.O.,University of Western Australia | And 3 more authors.
Andean Geology | Year: 2014

We have carried out zircon U-Pb SHRIMP dating and Hf isotope determinations as well as geochemical analyses on three plutonic units of Gondwanan magmatism that crop out in NW Argentina. Two episodes of different age and genesis have been identified. The older one includes gabbros and diorites (Río Grande Unit) of 267±3 Ma and granitoids (belonging to the Llullaillaco Unit) of 263±1 Ma (late Permian, Guadalupian); the parent magmas were generated in an intraplate environment and derived from an enriched mantle but were subsequently contaminated by crustal components. The younger rocks are granodiorites with arc signature (Chuculaqui Unit) and an age of 247±2 Ma (middle Triassic-Anisian). Hf isotope signature of the units indicates mantle sources as well as crustal components. Hf model ages obtained are consistent with the presence of crustal Mesoproterozoic (mainly Ectasian to Calymnian (TDM(c) =1.24 to 1.44 Ga-negative Hf(T)) and juvenile Cryogenian sources (TDM=0.65 to 0.79 Ga-positive εHf(T)), supporting the idea of a continuous, mostly Mesoproterozoic, basement under the Central Andes, as an extension of the Arequipa-Antofalla massif. The tectonic setting and age of the Gondwanan magmatism in NW Argentina allow to differentiate: a. Permian intra-plate magmatism developed under similar conditions to the upper section of the Choiyoi magmatism exposed in the Frontal Cordillera and San Rafael Block, Argentina; b. Triassic magmatism belonging to a poorly known subduction-related magmatic arc segment of mostly NS trend with evidence of porphyry type mineralization in Chile, allowing to extend this metallotect into Argentina.

Perez D.J.,University of Buenos Aires | D'Odorico Benites P.E.,University of Buenos Aires | D'Odorico Benites P.E.,ArPetrol Argentina S.A | Godeas M.C.,Servicio Geologico Minero Argentino SEGEMAR
Revista de la Asociacion Geologica Argentina | Year: 2010

The advanced spaceborne thermal emission and reflection radiometer (ASTER) was used to identify different deposits of hydrothermal alteration which indicates that several important lithological groups can be mapped in areas with good exposure by using spectral-matching techniques. Different methods are tested in order to identify and map zones with hydrothermal alteration minerals using the ASTER dataset. These areas are often referred to having large quantities of clay minerals which can be detected using multispectral imagery. Several authors have developed different procedures to map these hydrothermal minerals. Among the simplest ones, band combinations and band ratios have proven to be very useful tools for identifying targets. Lithology indexes point to reinforce the spectral response of this group of minerals working with band products and ratios. So far, these techniques do not need a full image correction. Other methods here employed require further processing of the ASTER scene, especially when spectral data are used. These techniques include spectral angle mapper (SAM) classification and minimum noise fraction (MNF) transforms to segregate noise and reduce computational requirements. Spectral data used in this paper were collected from field samples using SWIR (short wave infrared) reflectance spectroscopy and derived from the scene itself. These mapping methods have been tested in areas of known hydrothermal alteration occurrences, e.g. Los Pelambres, El Pachón and Altar, and in other sector of Santa Cruz region like Carnicerias and La Coipa; all of these at the south westernmost part of San Juan province. The result of this work is here presented as a series of images showing lithology indexes and an expected mineral assembly.

Zappettini E.O.,Servicio Geologico Minero Argentino SEGEMAR | Coira B.,National University of Jujuy | Santos J.O.S.,University of Western Australia | Cisterna C.E.,CONICET | Belousova E.,Macquarie University
Journal of South American Earth Sciences | Year: 2015

Las Lozas volcanic succession, cropping out in the southwestern Puna, Catamarca province, Argentina, consists of an intracontinental volcanic sequence of Carboniferous age. The lavic members are predominantly rhyolites, and subordinated andesites and basalts. The volcaniclastic layers consist of monomictic and polymictic breccias with structures denoting processes of lava fragmentation. They constitute a bimodal suite, ranging from basalts to high silica rhyolites. A new U-Pb SHRIMP age of 320±2 Ma for a rhyolite allows extending the Mississipian magmatism of the region to the lower Pennsylvanian. Hf data point to juvenile sources of Cryogenian age with no evidence of older reworked crustal contamination. © 2015 Elsevier Ltd.

Chernicoff C.J.,CONICET | Zappettini E.O.,Servicio Geologico Minero Argentino SEGEMAR | Santos J.O.S.,University of Western Australia | Santos J.O.S.,Brazilian National Council for Scientific and Technological Development | And 2 more authors.
International Geology Review | Year: 2016

We present U-Pb Sensitive High Mass Resolution Ion MicroProbe (SHRIMP) data of unexposed igneous-metamorphic basement rocks from two areas of the southeastern Río de la Plata craton (RPC; Buenos Aires city) located within the Buenos Aires-Piedra Alta (BAPA) terrane - and the Tapalqué area (Buenos Aires province) - located within the Tandilia terrane, and discuss the tectonic evolution of that portion of the craton based on both the new data and previous work. The newly obtained geochronological data of drill cores indicate that: (a) arc magmatism occurred at 2164-2186 Ma corresponding to early Trans-Amazon (early Rhyacian) arc magmatism; (b) the age of collision between the BAPA and Tandilia terranes is inferred to have commenced at ca. 2110 Ma; (c) peak metamorphism occurred at ca. 2069 Ma; and (d) the presence of rocks related to the RPC is confirmed under Cenozoic sediments in a large area between Martin Garcia Island and Tandil. We envisage an early Rhyacian divergent double subduction scheme between the BAPA and Tandilia terranes - i.e. N-dipping towards BAPA and S-dipping towards Tandilia - prior to the late Rhyacian collision between these two terranes. © 2015 Taylor & Francis.

Medina R.A.,University of Buenos Aires | Codignoto J.O.,Servicio Geologico Minero Argentino SEGEMAR | Codignoto J.O.,CONICET
Revista del Museo Argentino de Ciencias Naturales, Nueva Serie | Year: 2013

This paper discusses the recent geomorphological evolution of the Paraná River Delta on the basis of cadastral information, old topographic and nautical charts and satellite images. This evolution is linked to the consequences of global warming in the region of the Río de la Plata, particularly to the erosive processes. These processes are very active in areas close to the delta, especially in the Samborombón Bay. However, in the Paraná Delta not there was a decrease in their rhythm of progradation, on the contrary, it has remained constant or even has quickened. This is probably due to increased sedimentary load by the Paraná River in response to the increase of precipitation recorded since the 1970s in the region center and north of Argentina. As this is a direct consequence of global warming, or local climate variability, it may be said that the climate factor is also responsible for the aforementioned progradational dynamics.

Rubinstein N.,University of Buenos Aires | Gomez A.,University of Buenos Aires | Mallimacci H.,Servicio Geologico Minero Argentino SEGEMAR
Revista de la Asociacion Geologica Argentina | Year: 2012

The Arroyo La Chilca-Zanjón del Buitre alteration zone, San Rafael Massif, Mendoza. The Arroyo La Chilca-Zanjón del Buitre alteration zone is located in the San Rafael Massif (35°24'8" S; 68°19'25,8" W), Mendoza. Previous works defined preliminary this deposit as a porphyry copper system genetically linked to the gondwanan magmatism. This alteration zone is hosted by a sequence composed of ignimbritic breccias and ignimbrites of trachyandesitic composition with interbedded tuffaceous sandstones and andesitic lavas flows intruded by trachyandesitic-dacitic and andesitic hypabyssal bodies. The lithology and geochemistry of the volcanic sequence confirm the link with the Choiyoi Magmatic Cycle lower section (Lower Permian). The alteration zone consists of a potassic halo with a parageneses of K-feldspar-quartzmagnetite overprinted by phyllic alteration with an assemblage of quartz-muscovite-pyrite-(rutile) and quartz -pyrite veins. A late carbonatization process overprints both the potassic and phyllic alteration. Outside the alteration zone pyrite-chalcopyrite- galena-sphalerite veins with quartz gangue crop out. Metals geochemistry shows moderate Ag and Au anomalies and little Cu, Mo, Pb and Zn anomalies. Statistical analyses allow defining one Cu-Mo mineralizing stage and another Mo mineralizing stage only recognized in Zanjón del Buitre. Moreover, two Zn mineralizing stages, one of them probably genetically related to the Cu stage, were also recognized.

Zaffarana C.B.,University of Buenos Aires | Lagorio S.L.,Servicio Geologico Minero Argentino SEGEMAR | Somoza R.,University of Buenos Aires | Somoza R.,CONICET
Andean Geology | Year: 2012

A paleomagnetic study on nine samples from lavas from the Upper Cretaceous Tres Picos Prieto locality (43°50'S and 70°3'W), Patagonian Plateau Basaltic Province, revealed that a ca. 300 m thick pile of basalts accumulated during three discrete (i.e., temporally separated) volcanic episodes. The chemistry of these lavas shows characteristics compatible with both subduction and intraplate magmatism, with the former having a more important contribution in the younger lavas. Overall, we interpret these rocks to be transitional basalts generated in a supra-subduction environment. The Late Cretaceous to Cenozoic Patagonian Plateau Basaltic Province crops out from eastern Patagonia to the cordillera, and their genesis have been associated with different tectonic processes, even for spatially separated rocks of the same age. Considering that the development of the magmatic province is contemporary with the stage of westward drift of South America, we propose that magma generation and upwelling due to rapidly shearing asthenosphere inducing circulatory flow associated to asperities in the lithosphere-asthenosphere interface (shear-driven upwelling) can be evaluated as a further potential mechanism to account for many outcrops of these backarc to intraplate lavas.

Codignotto J.O.,Servicio Geologico Minero Argentino SEGEMAR | Codignotto J.O.,CONICET | Dragani W.C.,CONICET | Dragani W.C.,University of Buenos Aires | And 6 more authors.
Continental Shelf Research | Year: 2012

The coastal area of Samborombón Bay ends in a short cliff which, during the last decades, has been undergoing an increased retrogression of approximately 8.2m decade -1. The aim of this paper is to investigate whether this accelerated erosion can be related to an apparent wind-wave climate change, which has been recently reported for the Río de la Plata region. A numerical study with SWAN wave model for the period 1971-2005 drives to positive trends in the frequency of occurrence and heights of waves propagating from the E and ESE. Particularly, the number of cases of high waves from those directions displays a significant increment. In addition, previous papers have reported an increment of the frequency, height and duration of the storm surges in the Río de la Plata, so as a rise of the mean sea level in the region. It is concluded that the combination of those three factors acting together constitutes a powerful and effective mechanism which is likely responsible for the observed increasing erosion in Samborombón Bay. © 2012 Elsevier Ltd.

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