Eugenio Aragón

Temporal evolution and spatial variation of early tertiary volcanism in the Patagonian Andes (40°S–42°30′S)

Early to mid-Tertiary igneous activity in the Cordilleran Series (CS) of the Patagonian Andes between 40°S–42°30′S shows spatial variation and temporal trends that can be correlated with crustal thickness and slab depth. Volcanism in this region is concentrated in two sub-parallel arcs, the Pilcaniyeu Belt to the east and the El Maiten Belt to the west. Compilation of available K-Ar data and paleogeographic constructions suggests three major periods of volcanic activity. The first is a Paleocene-Eocene (60-42 Ma) event which developed primarily in the Pilcaniyeu belt; the second (Oligocene, 33-23 Ma) event is now exposed primarily along the El Maiten Belt. Miocene volcanics (16-11 Ma) have also been found in the northern sector of the El Maiten Belt. After an overall decline in the magnitude of volcanism, the third period of volcanic activity reached its maximum during the Pliocene-Pleistocene as large stratovolcanoes were built along the North Patagonian Cordillera. Temporal variations in the lower Tertiary CS, determined by superposition in selected cross-sections, indicate that the major episodes begin with silicic associations (ignimbritic, plinian, and obsidian rhyolitic facies) and end with intermediate and basic lava flows (stratovolcanoes and monogenetic cones). K2O and total alkalis decrease southward for a given silica content in the Pilcaniyeu and El Maiten Belts. The calc-alkaline andesites of the northern sector (39°30′S–41°30′S) are similar to many other Central Andes series, but towards the south (Cholila) there are also sequences with mild tholeiitic affinities. Spatial variations in major element composition seem to be related to the southward decrease in thickness of the underlying sialic crust. Variations in subduction geometry during the Cenozoic possible correlate with the compositional recurrence observed in the Cordilleran Series.

The Rio Chico Paleozoic crystalline complex and the evolution of Northern Patagonia

Abstract The Rio Chico Complex, which includes the Cushamen Metamorphics and the Mamil Choique Granitoids (Ordovician, 439±10 Ma), constitutes the crystalline basement of the Rio Chicoregion (southwestern North Patagonian massif). The metamorphics and migmatites resulted from a medium to high grade tectono-thermal event which caused local antexis of a protolith of graywackes, pelites and some quartz-rich sandstones. These rocks show a main deformational episode, trending mainly NNW, which controlled the post-Paleozoic structures. A Devonian age granite (387±17 Ma) intruded this complex. The Ordovician and Devonian granitoids, which are calc-alkaline, and per to metaluminous, may be related to anatexis during the Taconian and Acadian Laurentia-Gondwana collisional events. During the Permian (260±5 Ma) leucogranites, some bearing garnet, were intruded into the ancient orogenic belt following an important phase uplift. They are related to further melting of continental crust during extension in an intraplate environment, and are included in the Choiyoi acidic Province.

Thermal divide andesites–trachytes, petrologic evidence, and implications from Jurassic north Patagonian massif alkaline volcanism

Abstract The Andesitas Alvar Formation is part of the large Jurassic volcanic province of Patagonia (southern South America) that, in Jurassic time, participated in the disassembling of Gondwana in a tensional–transtensional regime with large-scale half-grabens. This large igneous province contains alkaline and calc-alkaline series of andesite–trachyte, trachyte–rhyolite, and andesite–dacite trends. Four-phase experimental diagrams of Qz–Or–Ab–An and Ab–An–Ol–Di correlate with the TAS diagram and explain the Andesitas Alvar Formations sodium-rich andesite–trachyte, which evolved through a thermal divide edge. This alkaline set initially included the paragenesis olivine–plagioclase–diopsidic augite, which was later replaced by hornblende–plagioclase–diopsidic augite. The andesite–trachyte trend is saturated in silica and rich in sodium and alumina. Diopsidic augite, plagioclase, and hornblende crystals are reverse zoned in magnesium, iron, calcium, and sodium, so the sodium-rich cores of plagioclase are not due to contamination, and the magmas mix several batches of partial melts of basic rock. Crystallization temperatures and pressures show that magma chamber emplacement and thermal reversal took place mainly at 4 kb. This is interpreted as the emplacement of collecting chambers at the brittle–ductile transition in the crust, where mixing and crystallization took place, followed by rapid extrusion in a transtensional tectonic environment.

Short-scale variability of the SCLM beneath the extra-Andean back-arc (Paso de Indios, Argentina): Evidence from spinel-facies mantle xenoliths

Abstract Cenozoic basalts carrying ultramafic mantle xenoliths occur in the Matilde, León and Chenque hills in the Paso de Indios region, Argentina. The mantle xenoliths from the Chenque and León hills mainly present porphyroclastic textures, whereas the Matilde hill xenoliths have coarse-grained to porphyroclastic textures. The equilibrium temperatures are in the range of 780 to 940ºC, indicating a provenance from shallow sectors of the lithospheric mantle column that were subjected to a relatively low heat ffiux at Cenozoic Era. According to the modal compositions of xenoliths, the mantle beneath Matilde and León hills was affected by greater than 22% partial melting, while less depleted peridotites occur in the Chenque suite (starting from 10% partial melting). Such an observation is confirmed by the partial melting estimates based on Cr#Sp, which vary from 8 to 14% for the selected Chenque samples and from 14 to 18% for the Matilde ones. The common melting trend is overlapped by small-scale cross cutting local trends that may have been generated by open-system processes, such as open-system partial melting and/or post partial-melting metasomatic migration of exotic Na-Cr-rich melts. The two main mineralogical reaction schemes are: i) the dissolution of pyroxenes and the segregation of new olivine in olivine-rich peridotites, and ii) the replacement of primary olivine by orthopyroxene±clinopyroxene in orthopyroxene-rich peridotites. These were produced by channelled and/or pervasive melt extraction/ migration. Enhanced pyroxene dissolution is attributed to channelling of silica- undersaturated melts, whereas the replacement of primary olivine by orthopyroxene±clinopyroxene points to reaction with silica-saturated melts. Late disequilibrium reactions identified in the xenoliths comprise: the breakdown of orthopyroxene in contact with the host basalt, and (rarely) reaction coronae on orthopyroxene, clinopyroxene and spinel linked to glassy veins. Such features are apparently related to the injection of melt, likely during entrainment into the host basalts and ascent to the surface.

The Exhumation of the Northern Patagonian Massif Gondwana Planation Surface Due to Uprising During the Oligocene

The altiplano (or high plain) of the Northern Patagonian Massif is a large, 100,000 km2 geomorphological unit that rose from sea level to at least 1,200 metres above sea level (m a.s.l.) in Early Oligocene times, as a consequence of epeirogenic uplift. This uniform tableland feature is essentially a Cretaceous planation surface carved on Paleozoic igneous and metamorphic rocks of the Northern Patagonian Massif. This planation surface had been preserved by a thin and scattered cover of Maastrichtian-Danian marine sediments and Late Oligocene-Early Miocene basaltic flows. Erosion since Middle Miocene times at this tableland has exposed much of the Gondwana planation surface and developed numerous basaltic plateaus by relief inversion.

Mohorovicic Discontinuity Depth Analysis Beneath North Patagonian Massif

The Mohorovicic discontinuity (Moho) is the surface that limits the Earth’s crust and mantle. It is of paramount importance in understanding and investigating the dynamics of the Earth’s interior. The GEMMA project (GOCE Exploitation for Moho Modeling and Applications), funded by the European Space Agency and Politecnico di Milano, has provided a high resolution map of the Moho surface (GEMMA Model), based on the inversion of homogeneous, well-distributed gravimetric data measured by the Steady-State Ocean Circulation Explorer (GOCE), which ensures a global coverage using gravity field. In the current paper, this Moho depth estimation (Riccardo Barzaghi, personal communication, April 20, 2012) is compared with other models based on both seismic and gravity observations, under the North Patagonian Massif (NPM). Said massif is an Argentinean plateau that stands out 500 to 700 m higher in altitude than the surrounding topography and was created by a sudden uplift without noticeable internal deformation (Aragon et al. (2011b) Upper mantle geodynamic constrains beneath the north patagonian massif, Argentina). The features described led us to analyze the crustal thickness in the area. The work describes different Moho models available in the area under study and their comparison with the GEMMA Model. The aim is to validate this well distributed, homogeneous data model in this area with sparse seismic data and check its usefulness to get more information about the Moho. According to comparisons with the different models, the crustal thickness in the study area varies between 36 and 46 km. The good agreement between the GEMMA Model and some of the other Moho models may account for the use of such model to study this little known area.

Paleo-landscapes of the Northern Patagonian Massif, Argentina

The dominant geomorphological unit of the Northern Patagonian Massif landscape is a regional planation surface, eroded across the crystalline basement (plutonic and metamorphic rocks), eruptive rocks of the Gondwana cycle (Early to Middle Carboniferous), and Jurassic volcanic rocks.