The morphologic parameters, pyroclastic deposits and evolution of maar-diatreme volcanoes are affected by the type of environment in which they are emplaced. End-member cases are a hard substrate ...(rocks) and a soft substrate (unconsolidated volcaniclastic or sedimentary deposits). In this paper, we present an example of a volcanic complex emplaced in a mixed hard-soft setting from the Pali Aike volcanic field (PAVF) near the Argentina-Chile border. The Plio-Pleistocene PAVF is an alkaline, mafic, back-arc monogenetic field which contains over 100 phreatomagmatic volcanoes. The studied volcanic complex contains two large coalescent maars overlain by scoria and spatter. The 1.4 x 1.3 km East Maar has better exposures than the shallower, 1.9 km-wide West Maar and seems to have been less modified by post-eruptive processes. The tephra rim of the East Maar was studied in detail and we infer it was produced mostly by base surges from phreatomagmatic eruption columns, with rare instances of intercalated scoria fall layers. Based on regional information, the general pre-maar stratigraphy is dominated by sedimentary and volcaniclastic rocks of the Magallanes Basin, including a thick poorly consolidated upper unit dating from the Miocene. These are overlain by Plio-Pleistocene fluvio-glacial deposits and PAVF lavas, some of which are exposed in the East Maar just below the phreatomagmatic deposits. All of these units are represented as lithic clasts in the tephra rim of the East Maar, the most abundant being the clasts from the earlier basaltic lavas and rock fragments derived from the glacial deposits. There is no specific evidence for a deep diatreme under the East Maar, and in this particular case, the mixed environment seems to have produced a maar-diatreme volcano typical of a soft substrate.
The Gulf of San Jorge (GSJ) is a semicircular basin, approximately 160 km long and 250 km wide, located in the central part of Patagonia between 45°S and 47°S, lacking any present-day major perennial ...tributaries. The grain size and bulk and clay mineralogical compositions as well as major and minor elements of 75 surface sediment samples from the GSJ and the adjacent continental shelf were investigated to define the spatial distribution, transport pathways, and potential sources of terrigenous material. To better constrain the origins of GSJ sediments, analyses were also performed on 14 terrestrial, riverine, and marine samples from potential source areas around the gulf and Patagonia. The mineral assemblage of surface sediments in the gulf, dominated by plagioclase, quartz, and clays, is a function of the primary continental volcanic geology of Patagonia. The significant concentration of volcaniclastic particles indicated by mineralogical signatures and scanning electron microscope images of sediments suggests a substantial contribution from rhyolitic volcanism to the modern sedimentation in the gulf. High amounts of smectite are carried into the GSJ by dust transport, whereas inputs of chlorite and illite seem to be associated with continental shelf current transport from southern Patagonia. Finally, our results suggest that 50% of the surface sediment in the GSJ is derived from external/oceanic inputs, 40% from inner gulf shores (i.e., erosion and runoff), and 10% from dust (i.e., aeolian transport).
The morphometric analysis of scoria cones allows studying the erosive processes and
evolution history of a volcanic region. Using TanDEM-X DEMs of 12 m resolution
we measured the morphological ...parameters of 157 the scoria cones of the Payenia
Volcanic Province, that together with absolute ages, let us calculate a rate of erosion
of 9.4°x Ma, this mean that the slope of the cones decreases 9.4° every Ma. This rate
is similar to others calculated in other volcanic fields around the world. With this rate,
the morphological ages of cones were calculated. Most of the ages are in the range
from 0.3 to 2.2 Ma.
El análisis morfométrico de los conos de escoria permite evaluar los procesos erosivos que los afectan e inferir la historia eruptiva de una región. Utilizando DEMs TanDEM-X de 12 m de resolución se midieron los parámetros morfológicos de 157 conos de escoria que integran diferentes campos volcánicos de la Provincia Volcánica de Payenia. Se calculó una tasa de erosión de 9,4° por Ma, es decir que la pendiente de los conos disminuye 9,4°cada millón de años. Esta tasa es similar a la obtenida en otros campos volcánicos monogenéticos con similares características climáticas. Con esta tasa se calcularon edades morfológicas para todos los conos estudiados, las que abarcan un rango entre 0,3 y 2,2 Ma.
The Pali Aike Volcanic Field (PAVF) represents the southernmost occurrence of the Cenozoic back-arc Patagonian Plateau Lavas. Its activity (Pliocene–Recent) started forming tabular lavas followed by ...the growth of about 470 essentially monogenetic volcanic centers (tuff-rings, maars, spatter and scoria cones). Azimuths of cone alignment, cone elongation and morphologic lineations show prevailing ENE–WSW and NW–SE trends. Erupted products consist mainly of alkaline basalt and basanite, with minor olivine basalt. PAVF rocks are quite primitive in composition (average Mg#=66, Ni=220
ppm and Cr=313
ppm) with relatively high TiO
2 (average 3.0
wt.%). Ultramafic garnet- and/or spinel-bearing xenoliths are found within PAVF volcanics. Chondrite-normalized REE patterns are significantly LREE-enriched and almost rectilinear (La/Yb)
N=10.9–21.0. Primordial mantle-normalized distributions of incompatible trace elements, as well as Sr and Nd isotope ratios (
87Sr/
86Sr=0.70317–0.70339,
143Nd/
144Nd=0.51290–0.51294), show values typical of intra-plate basalts, despite the fact that these rocks occur only 200
km east of the Andean Cordillera. Primary magmas were generated from a fertile garnet-bearing asthenospheric source at
P=1.9–2.9
GPa and
T=1420–1470°C.
The data suggest a geodynamic model that implies sub-slab asthenosphere flow through a slab window, which started opening below this sector of South America 14
m.y. ago as a consequence of the collision of the Chile Ridge with the Chile Trench. The trailing edge of the Nazca Plate crossed below the Pali Aike area at 9–10
Ma, that is 6–5
m.y. before the onset of the volcanic activity. We hypothesize that this time delay resulted from changes in the kinematics of the South America–Scotia transform plate boundary which only allowed the Pali Aike magmas to rise after about 4
m.y.
East of the Patagonian Andes, mafic volcanic rocks (mainly lava flows and scoriae) are exposed in the Sierra de San Bernardo fold belt and neighbouring areas (central Patagonia; 44.5–46° S, 69–71° ...W). They were erupted over a wide interval of time (late Eocene–Pleistocene; 14 new K–Ar ages), and show systematic chemical and Sr–Nd–Pb isotopic variations in time. The alkaline lavas (Mg number 57–66) erupted during the late Eocene and early Miocene, have an intraplate geochemical affinity, and have the highest 143Nd/144Nd and 206Pb/204Pb and the lowest 87Sr/86Sr ratios of the dataset. Their compositions indicate that their depth of equilibration in the mantle was greater than that of subsequent lavas. In contrast, the Plio-Pleistocene alkaline lavas (Mg number 58–71) are the most enriched in incompatible elements, still showing an intra-plate signature, and have the lowest 143Nd/144Nd and 206Pb/204Pb and the highest 87Sr/86Sr ratios. A distinctive group of early Miocene subalkaline lavas is characterized by slightly more evolved compositions (Mg number 56–59), coupled with very low incompatible element contents, flat LREE and fractionated HREE patterns (‘kinked’ pattern), and intermediate Sr–Nd–Pb isotope compositions. The Pleistocene basanites (Mg number 71–72) from the Cerro Ante monogenetic cone, on the easternmost slopes of the Patagonian Andes, have a marked orogenic geochemical signature and Sr–Nd–Pb isotope ratios that overlap with those of volcanic rocks from the adjacent active Andean arc. They originated in a mantle source extensively modified by the addition of materials from the subducting Pacific oceanic plates. We suggest that the wide chemical and isotopic variability of the Sierra de San Bernardo lavas reflects the upwelling of asthenospheric mantle beneath the study area, which induced lithospheric erosion and progressive involvement of enriched mantle domains in the genesis of magmas. In this context, late Eocene and early Miocene alkaline magmatism was dominantly sourced from the asthenospheric mantle, whereas Plio-Pleistocene alkaline magmas contain the largest proportion of an enriched lithospheric component. The peculiar compositional features of the early Miocene subalkaline lavas are interpreted in terms of high-degree mantle melting followed by melt–lithospheric mantle reaction processes. Based on current knowledge about the relative movement and decoupling between lithosphere and asthenosphere, we propose that the asthenosphere below the study area rose up to compensate for the westward drift of the mantle wedge coupled with the South American lithosphere.
During the Late Pleistocene-to-Holocene, the mafic Planchón volcano (35.2 °S, Southern Andes) experienced two important destructive events: a sector collapse to the west and a multiphase explosive ...eruption transforming the east summit area. We provide new field and laboratory evidence, including geochemical, geochronologic, and geological-morphological analysis, to reconstruct the evolution, triggering mechanisms, and physical parameters of these events.The lateral collapse (48~ka BP) was mainly predisposed by a tectonically westward-inclined substratum and rapid edifice growth rates (0.3–0.48 km3 ka-1). The resulting Planchón-Teno debris avalanche became valley-confined traveling at c. 260 km h-1 up to 95 km distance and forming an 8.6 ± 1.3 km3 deposit. The resulting 4.1 km wide amphitheater was later destroyed at c. 7 ka BP by the multiphase Valenzuela phreatomagmatic eruptions, forming a c. 2.5 km diameter caldera. The case of the Planchón volcano warns that rapidly growing mafic volcanoes imply a substantial catastrophic hazard increase for the surrounding areas.
This paper newly reports results of LA-ICPMS U-Pb dating for 282 zircon crystals separated from a Middle Miocene adakite in Cerro Pampa, southern Argentine Patagonia. With the exception of one spot ...age, 174 of the U-Pb concordia ages are markedly older (>94 Ma) than the cooling ages of the adakite magma (ca. 12 Ma). The presence of numerous exotic zircon crystals indicates that the adakitic magma carries up information related to the crustal components during its ascent through the entire crust underneath Cerro Pampa. The obtained concordia ages of exotic zircons, 94-1335 Ma, are divisible into five groups having distinctive peaks on a population diagram. The first (94-125 Ma) and second age groups (125-145 Ma) correspond to the age of plutonic activities that formed the main body of the South Patagonian Batholith. The third to fifth groups respectively correspond to activities of the El Qumado-Ibañez volcanic complex (145-170 Ma), plutonic rocks scarcely exposed in Central Patagonia (170-200 Ma), and the Eastern Andean metamorphic complex of Late Paleozoic to Early Mesozoic ages (200-380 Ma). Our data suggest that the crust underneath Cerro Pampa was formed mostly after 380 Ma, the majority forming during the Early Cretaceous to Middle Jurassic. The processes of crustal development ceased for ca. 80 m.y. until the activity of the Cerro Pampa adakite in ca. 12 Ma. In contrast to the existence of numerous Archaean-Palaeoproterozoic exotic zircons in Mesozoic plutonic rocks distributed in Andean Cordillera at around 46°S, no evidence was found for Archaean-Paleoproterozoic crust on the Cerro Pampa region at 48°S. This evidence suggests that two crusts must have aggregated along a boundary between 46°S and 48°S with the continental margin of Gondwana during Late Paleozoic times, as part of the amalgamation of Pangea.
The Crater Basalt Volcanic Field (CBVF) in northern Patagonia (∼42°S, Chubut province) is located 400 km eastward of the current Peru–Chile Trench and to the southwest of the Meseta de Somuncura, in ...a back-arc position in the extra-Andean Patagonia. The CBVF lava flows cover glaciation related terraces and present stream valleys. The occurrence of CBVF is related with the Gastre Fault System (GFS), a very significant tectonic feature about 40 km wide.
The CBVF volcanics cover a surface of ∼700 km
2, occurring mainly as lava flows and scoria cones. CBVF volcanics have relatively high contents of MgO (6–9 wt.%), Cr (136–289 ppm) and Ni (25–198 ppm), and classify as alkali basalts, basanites and trachybasalts. Geothermometry indicates a crystallization temperature of ∼1140 °C and a ƒO
2 of −
1.0 1 to 0.0 (log FMQ units).
The petrographic and geochemical characteristics of CBVF products suggest that the magma originated from a garnet-bearing lherzolite mantle source with asthenospheric characteristics, as a result of decompression, partial melting and ascent. These melts would have risen through a system of deep faults like the Gastre Fault System, following the last Quaternary glacial event.
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