Figura 5 - uploaded by Brígida Castro de Machuca
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a) Diagrama de clasificación de rocas SiO 2 vs. Na 2 O + K 2 O (Le Maitre et al. 1989) normalizado en base anhidra. La línea de trazos color gris corresponde a la curva de Irvine y Baragar (1971), que separa los campos alcalino y subalcalino; b) Diagrama de clasificación Nb/Y vs. Zr/Ti según Pearce (1996, modificado de Winchester y Floyd 1977); c) Diagrama de discriminación de ambiente Nb/Y vs. Ti/Y (Pearce 1982); d) Diagrama triangular La/10-Y/15-Nb/8 para discriminación de ambiente (Cabanis y Lecolle 1986).
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La Basanita El Retamo, localizada en el sector norte de la sierra de Valle Fértil, corresponde a una nueva evidencia del vulcanismo alcalino de edad triásica para esta región. Es interpretada como el remanente de una colada lávica que suprayace al basamento cristalino del Complejo Valle Fértil, topográficamente elevada por la tectónica Andina. Tien...
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Context 1
... normalizado en base anhidra, donde las muestras estudiadas se proyectan en el campo de las tefritas y basanitas, coincidiendo con rocas de naturaleza alcalina según Irvine y Baragar (1971). En el diagrama de elementos inmóviles Nb/Y vs. Zr/ Ti (Pearce 1996, modificado de Winchester y Floyd 1977) las muestras se agrupan como basaltos alcalinos (Fig. ...
Citations
... El sustrato sobre el que se apoya la Ignimbrita Vilama se observa en forma adecuada en el sector oriental (Fig. 1). Las principales unidades que lo forman son la Formación Acoite (Turner 1964), integrada por sedimentitas marinas ordovícicas, que afloran en Mina Pabellón y en el bloque de Pirquitas; el Grupo Salta (Turner 1959), integrado por sedimentitas continentales cuya edad general es cretácica inferior-eocena, aflorantes en la zona de Mina Bonanza ); y varias unidades de rocas volcánicas miocenas, siendo las principales: el Complejo Volcánico Pairique ); las Ignimbritas Granada (Aquater 1979, Coira et al. 1996 y Lagunillas (Soler 2005, Caffe et al. 2008); y algunas lavas y pi-roclastitas de los cerros Caucani, Colorado y Solterío (Ramírez et al. 2008). Por encima de la Ignimbrita Vilama se emplazan varios centros volcánicos andesíticos y dacíticos (Fig. 1). ...
La Ignimbrita Pululus -situada en la Puna norte de Argentina, cerca de la frontera con Bolivia- fue previamente definida como un escudo ignimbrítico relativamente joven (Plioceno-Pleistoceno) cuya erupción había sucedido por varios millones de años al colapso de la caldera Vilama. Estas inferencias estaban basadas principalmente en los rasgos morfológicos del cerro Pululus, localidad en donde aflora la ignimbrita homónima. En esta contribución se presentan argumentos estratigráficos, petrográficos y geoquímicos que invalidan la existencia de dicho centro eruptivo, y que permiten concluir que la Ignimbrita Pululus es en realidad la Ignimbrita Vilama, principal unidad eruptada durante el colapso de la caldera Vilama. A partir de ello resulta necesario rectificar el esquema estratigráfico de la región, por lo que se propone el abandono del término "Ignimbrita Pululus" y que sus afloramientos sean considerados como parte la Ignimbrita Vilama. Se identificaron además nuevas unidades estratigráficas en el cerro Pululus, de las cuales se describen su posición estratigráfica y características litológicas y petrográficas. Entre ellas se destaca un intrusivo subvolcánico cuyo emplazamiento produjo una notable estructura dómica en la zona apical del cerro Pululus, al cual proponemos denominar Dacita Pululus. A partir del cambio del esquema estratigráfico propuesto surgen importantes consecuencias que modifican el conocimiento geológico de la región. Ellas pueden resumirse en: a) la forma de "escudo" del cerro Pululus sería un carácter sobreimpuesto por deformación (por intrusión) debajo de la Ignimbrita Vilama, por lo tanto se deja de lado la hipótesis de erupción en escudo; b) se define un nuevo borde topográfico de la caldera Vilama; y c) la mineralización de estaño del cerro Pululus estaría asociada a la intrusión de la Dacita Pululus, hecho que permitiría descartar el modelo metalogenético previo vinculado a la desgasificación de la ignimbrita.
... El sustrato sobre el que se apoya la Ignimbrita Vilama se observa en forma adecuada en el sector oriental (Fig. 1). Las principales unidades que lo forman son la Formación Acoite (Turner 1964), integrada por sedimentitas marinas ordovícicas, que afloran en Mina Pabellón y en el bloque de Pirquitas; el Grupo Salta (Turner 1959), integrado por sedimentitas continentales cuya edad general es cretácica inferior-eocena, aflorantes en la zona de Mina Bonanza ); y varias unidades de rocas volcánicas miocenas, siendo las principales: el Complejo Volcánico Pairique ); las Ignimbritas Granada (Aquater 1979, Coira et al. 1996 y Lagunillas (Soler 2005, Caffe et al. 2008); y algunas lavas y pi-roclastitas de los cerros Caucani, Colorado y Solterío (Ramírez et al. 2008). Por encima de la Ignimbrita Vilama se emplazan varios centros volcánicos andesíticos y dacíticos (Fig. 1). ...
The Pululus Ignimbrite -located in the north Puna of Argentina, near the international border with Bolivia- was previously defined as a relatively young (Pliocene-Pleistocene) ignimbritic shield which erupted several millions of years after the collapse of the Vilama caldera. This conclusion was mainly based on the morphology of the cerro Pululus, the location where the ignimbrite crops out. In this contribution we present stratigraphic, petrographic and geochemical arguments which refute the existence of such an eruptive centre and so give support to the conclusion that the Pululus Ignimbrite is actually the Vilama Ignimbrite, i.e. the principal stratigraphic unit erupted during the collapse of the Vilama caldera. In order to rectify the stratigraphic framework of the region, we propose to abandon the term "Pululus Ignimbrite" and to consider their outcrops as being part of the Vilama Ignimbrite. Some new stratigraphic units in Cerro Pululus were also identified; their stratigraphic position is here described, as well as lithologic and petrographic characteristics. The most important of these units is a subvolcanic intrusive -to which we propose to name Pululus Dacite- whose emplacement deformed the Vilama Ignimbrite in the shape of a dome in the highest zone of cerro Pululus. The proposed change in the stratigraphic framework also bears important consequences for volcanological and metallogenic issues in the region. They are summarised in: a) the "shield" shape of Cerro Pululus is an superimposed character due to intrusion under the Vilama Ignimbrite, then the hypothesis of a central eruption is no longer valid; b) a new topographic border of the Vilama caldera is defined; and c) the tin mineralization at cerro Pululus is probably associated to the intrusion of the Pululus Dacite, so the previous metallogenic model which linked the tin to the degassing of the ignimbrite could be discarded.
Reconstructing thermal histories in thrust belts is commonly used to infer the age and rates of thrusting and hence the driving mechanisms of orogenesis. In areas where ancient basins have been incorporated into the orogenic wedge, a quantitative reconstruction of the thermal history helps distinguish among potential mechanisms responsible for heating events. We present such a reconstruction for the Ischigualasto‐Villa Unión basin in the western Pampean Ranges of Argentina, where Triassic rifting and late Cretaceous‐Cenozoic retroarc foreland basin development, including Miocene flat‐slab subduction have been widely documented. We report results of organic and inorganic thermal indicators acquired along three stratigraphic sections, including vitrinite reflectance and X‐ray diffractometry in claystones and new thermochronological [(apatite fission‐track and apatite and zircon (U‐Th)/He)] analyses. Despite an up to 5 km‐thick Cenozoic overburden and unlike previously thought, the thermal peak in the basin is not due to Cenozoic burial but occurred in the Triassic, associated with a high heat flow of up to 90 mWm‐2 and less than 2 km of burial, which heated the base of the Triassic strata to ~160°C. Following exhumation, attested by the development of an unconformity between the Triassic and Late‐Cretaceous‐Cenozoic sequences, Cenozoic re‐burial increased the temperature to ~110°C at the base of the Triassic section and only ~50°C 7 km upsection, suggesting a dramatic decrease in the thermal gradient. The onset of Cenozoic cooling occurred at ~10‐8 Ma, concomitant with sediment accumulation and thus preceding the latest Miocene onset of thrusting that has been independently documented by stratigraphic‐cross‐cutting relationships. We argue that the onset of cooling is associated with lithospheric refrigeration following establishment of flat‐slab subduction, leading to the eastward displacement of the asthenospheric wedge beneath the South American plate. Our study places time and temperature constraints on flat‐slab cooling calls for a careful interpretation of exhumation signals in thrustbelts inferred from thermochronology only.
We present new data about a major eruption-spreading approx. 110 km3 ashes over 440.000 km2-long thought to have occurred around 4200 years ago in the Cerro Blanco Volcanic Complex (CBVC) in the Central Andes of NW Argentina (Southern Puna, 26 • 45' S, 67 • 45' W). This eruption may be the biggest during the past five millennia in the Central Volcanic Zone of the Andes, and possibly one of the largest Holocene eruptions in the world. Discrimination and correlation of pyroclastic deposits of this eruption of Cerro Blanco was conducted comparing samples of proximal (domes, pyroclastic flow and fall deposits) with distal ash fall deposits (up to 400 km from de vent). They have been characterized using optical and electron microscopy (SEM), X-ray diffraction, particle-size distribution by laser diffraction and electron microprobe and HR-ICP-MS with laser ablation for major and trace element composition of glass, feldspars and biotite. New and published 14C ages were calibrated using Bayesian statistics. An one-at-a-time inversion method was used to reconstruct the eruption conditions using the Tephra2 code (Bonadonna et al. 2010, https://vhub.org/resources/tephra2). This method allowed setting the main features of the eruption that explains the field observations in terms of thickness and grain size distributions of the ash fall deposit. The main arguments that justify the correlation are four: 1) Compositional coincidence for glass, feldspars, and biotite in proximal and distal materials; 2) Stratigraphic and geomorphological relationships, including structure and thickness variation of the distal deposits; 3) Geochronological consistency, matching proximal and distal ages; and 4) Geographical distribution of correlated outcrops in relation to the eruption centre at the coordinates of Cerro Blanco. With a magnitude of 7.0 and a volcanic explosivity index or VEI 7, this eruption of ∼4200 BP at Cerro Blanco is the largest in the last five millennia known in the Central Volcanic Zone of the Andes. The implications of these results go far beyond having an excellent chronostratigraphic marker to reconstruct the Holocene geologic history of a large area of South America. Besides the effects directly associated with eruptive process, a deposit of tephra is very ephemeral and rapidly is reworked and redeposited. The interaction of the huge amount of ashes of this eruption with the wind and water in the large watersheds of the region must mobilize enormous amounts of both particulate and chemical elements to the large Chacopampean Plain. How impacted this eruption on the environmental, pollen, faunal and archaeological mid-Holocene records are features currently under study. On the other hand, the occurrence of Holocene volcanism in the southern Puna leads to consider new scenarios of volcanic hazard over large and densely populated areas in South America.
