The orbicular granodiorite of Recoba Hill in the North Patagonian Batholith

A small body of orbicular granodiorite crops out on the Recoba Hill, immediately east of the town of Chaitén, in the mainland area called Chiloé continental, in southern Chile. The rock comprises cm-sized igneous cores with a single shell of fine-grained plagioclase-quartz-K-feldspar assemblages. It is hosted in a Miocene granodiorite, and it is crosscut by aplite and mafic dikes. No other mention of orbicular rocks has to date been reported for the >1,000 km long North Patagonian Batholith, suggesting that the conditions necessary for their formation were infrequent. Thermobarometric determinations indicate pressures lower than 2 kbar (less than 6 km depth) for its formation, a level much shallower than the estimated source depth of the older rocks of the batholith.pp.86-10

Visita técnica a sector estero La Sombra, comuna de Linares, región del Maule

Informe Técnico -- Unidad Ejecutora: Unidad de Asistencias Técnicas y Emergencias Geológicas, Santiago7 p

Visita Técnica por caída de rocas. Subida Alessandri, comuna de Viña del Mar, Región de Valparaíso

Informe Técnico -- Unidad Ejecutora: Unidad de Asistencias Técnicas y Emergencias Geológicas y Unidad de Peligros Geológicos y Ordenamiento Territorial12 p

Jean-Claude Vicente (1941-2022) Formador de varias generaciones de geólogos y gran explorador andino

pp.162-16

Lithofacies architecture within an intra-arc environment : A case study from the Permian-Triassic magmatic arc in the Chilean Frontal Cordillera (30-30.5° S)

In the Chilean Frontal Cordillera between 28.5 and 30.5° S, two volcano-sedimentary succesions of Late Guadalupian to Middle Triassic age croup out: the El Tapado (Late Guadalupian-Lopingian) and Guanaco Sonso (Late Lopingian-Middle Triassic) formations, representing the westernmost exposures of the Choiyoi Group and post-Choiyoi magmatism. Recently, these two units have been characterized in detail, shedding light into the tectonic and climatic context of the late stages of the Choiyoi magmatism, and, more generally, the geology of intra-arc successions in the southwestern Gondwana margin. In this work, we present a stratigraphy and lithofacies analysis of three stratigraphic sections studied in the La Laguna sector (30-30.5° S), comprising rocks of the El Tapado and Guanaco Sonso formations. The former is dominated by caldera-related rhyolitic to dacitic ignimbrites accumulated in depocenters associated with the extensional activity of major structures. Lacustrine and fluvial-alluvial deposits interbedded in this rock unit were mainly controlled by volcaniclastic input and subsidence dynamics, and record the transition from relatively humid conditions to semi-arid or even arid conditions during the Late Guadalupian-Lopingian, consistent with regional paleoclimatic observations. On the other hand, the Guanaco Sonso Formation presents mainly intermediate, proximal (near-vent) volcanic products that denote a varied volcanism in terms of composition and eruptive style, likely controlled by structures and tectonics. We conclude the southwestern Gondwana margin sustained high-explosivity, silicic volcanism (El Tapado) that transitioned into a more varied, mainly intermediate volcanism (Guanaco Sonso). This activity ultimately gave way to the basaltic-andesitic to bimodal products that characterized the final part of the pre-Andean stage during the Triassic in this region.pp.1-4

The Isomass Method : Verifying conserved elements in geochemically open geological processes

This contribution presents the Isomass method, aimed at verifying the conserved behaviour of elements in geochemically open systems even when the parent rock composition is lacking. The method estimates system size changes for a specific element that is assumed to be conserved by calculating the amount of material transfer for each of the other elements, thus verifying (or rejecting) the initial assumption. By analysing the calculated amount of material transfer (or daughter system size ratios), additional conserved elements (if any) can be identified. The Isomass method is used here to evaluate a set of numerically generated samples whose element concentrations derived after mass additions and losses are assigned to daughter rocks. In addition, it is also applied to three real datasets that examine soil formation, magmatic fractionation in a komatiitic lava, and hydrothermal metasomatism. The method is capable of: (1) determining which elements confirm their conserved behaviour for a variety of geological environments; (2) identifying which other elements are conserved, added or lost; (3) calculating the amount of material transfer; and (4) providing a measure of the extent of the conserved character of elements. The method illustrates that the whole-rock compositions of parent and daughter samples do not represent the actual material transfer that occurred during geochemically open geological processes, as conserved elements may appear enriched or depleted, and non-conserved elements may have concentrations that do not reflect the actual material transfers that took place. The Isomass method is therefore a proper and valuable tool for the verification of conserved elements and the investigation of material transfer in rocks.pp.117-13

Grutas del Palacio and other related Upper Cretaceous continental deposits (SW Uruguay) : Main sedimentary features and evidence for an old flooded forest

The Grutas del Palacio is the sector where the Asencio Formation (Upper Cretaceous), which crops out discontinuously in SW Uruguay, is best displayed. The Asencio Formation consists of red terrigenous sediments modified by pedogenesis-lateritic processes. It is constituted from bottom to top by the members Yapeyú (lacustrine) and Palacio (palustrine). The Palacio Member is characterized by the presence of numerous oval-like caves, <2 m high, as well as several column-like structures made up of ferruginized pisolithic aggregates. These structures are encased within sparsely laminated whitish sand-rich mudstone horizons and covered by iron-rich siliceous sandstones. Several interpretations have been proposed for the column-like structures. In this article we suggest that the Asencio Formation was accumulated primarily because of lacustrine processes, under relatively stable tectonic conditions and a warm, seasonally humid climate. The column-like structures would represent the casts of rotting tree trunks, which were later infilled by sand and iron-rich materials. The upper parts of these structures were probably affected by water erosion related to recurrent floods.pp.296-32

Evaluación de peligro de remoción en masa en la localidad de Yeco, ruta T-20, comuna de Mariquina, región de Los Ríos

Informe Técnico -- Unidad Ejecutora: Unidad de Geología Regional, Dirección Regional de Los Ríos, Valdivia16 p

The Andean Southern Volcanic Zone : A review on the legacy of the latest volcanic eruptions

The Andean Southern Volcanic Zone (SVZ) concentrates many of the most active volcanoes of the Andean continental arc, as well as the region’s most recent and impactful volcanic eruptions. In this contribution, we briefly revise the general characteristics of the SVZ volcanism and provide a synthesis of the scientific findings related to the latest volcanic eruptions (430 peer-reviewed publications with over 9,000 citations, with large-magnitude (VEI 4-5) eruptions being the most studied. Our study shows that SVZ research has been primarily focused on environmental and atmospheric impacts (29%), eruption descriptions and physical volcanology (20%), volcanic hazard and risk assessments (15%), and other investigations complementary to volcanology. Whereas the least silicic eruptions (e.g., Llaima 2008-2009 and Villarrica 2015) shed light on magma replenishment and degassing dynamics controlling eruption styles, intermediate eruptions (andesitic-dacitic) offered clues on either rapid or slow eruption initiation, with relevant findings on phreatic-to-magmatic style transitions and eruption triggering mechanisms. On the other hand, silicic (i.e., rhyolite-rhyodacite) eruptions provided unique observations on rapid magma ascent, high-rate magma extrusion, rheology, fragmentation processes, and style transitions. These recent eruptions have also inspired a new generation of tephrochronological, tephrostratigraphical, and physical volcanology studies, aimed at assessing the long-term (kyr-scale) evolution of the volcanic systems and their associated hazards. We debate how the knowledge gained from research and the long-term human coexistence with volcanoes are relevant to reducing volcanic risk in the SVZ. Finally, we discuss how challenges and opportunities emerging from other disciplines can complement our understanding of volcanism in this active region.pp.379-41

Tracing hotspot traces in the Andes

Two segments of subduction of the Nazca plate beneath the South American plate occur at low angles based on seismic hypocenter locations, approaching nearly horizontal below ~100 km in depth. In contrast with most of the rest of the subduction zone, the two segments, beneath central Chile, and central and northern Peru, lack active volcanoes along the crest of the Andes and have more subdued topography to the east of the Andean crest. Each low-angle subduction segment occurs to the east of the intersection of inferred mantle hotspot traces on the Nazca plate with the Peru-Chile Trench: the Nazca ridge (at the southern part of the Peruvian segment), and the Juan Fernández island-seamount chain (offshore the Chilean segment). A third inferred trace, the Galápagos-Carnegie ridge, may be correlated with a zone on incipient low-angle subduction beneath Colombia.The importance of such hotspot traces in contributing to low-angle subduction beneath the Andes is strengthened by updated South American-Nazca plate reconstructions, including three oceanic hotspot traces, in comparison with a new isotopic date compilation of igneous rocks from the mountain range. The Juan Fernández hotspot trace, reconstructed from Pacific-hotspot models to the Nazca-Farallon plate, encountered the subduction zone offshore southern Peru ~65 Ma, broadening arc volcanism to the east; the trace-trench intersection migrated gradually and then rapidly southward, widening the arc east to Bolivia and northern Argentina; it then stabilized about 13 Ma offshore central Chile, producing the contemporary low-angle Pampean segment. The Juan Fernández hotspot may also have been responsible for formation of the Manihiki Plateau on the Pacific plate much earlier, ~125 Ma. The Easter-Nazca hotspot trace intersected the subduction zone beneath Colombia before ~50 Ma and migrated southward beneath Ecuador beginning ~15 Ma, with progressive low-angle subduction implied by migrating volcanic cessation along the Andean crest to southern Peru. The Galápagos-Carnegie hotspot trace only recently encountered the subduction zone, apparently inducing a new low-angle segment and cessation of magmatism in Colombia. The reconstructions and magmatic history provided here strongly support a previously proposed genetic relationship of hotspot traces and low-angle subduction. Additionally, the reconstructions suggest remnants of older subducted traces in the asthenosphere may have sourced post-rift magmatism in eastern Brazil and Paraguay, which cannot be explained otherwise by simple hotspot mechanisms.pp.1-6