La Caldera de colapso del Cerro Aguas Calientes, Salta, Argentina: evolución y esquema estructural

A new emission center of extensive pyroclastic flows has been recognized and interpreted, near San Antonio de los Cobres. The eruptive parameters and the volcanic history of the volcanic complex are pointed out. A caldera collapse controlled by left-lateral strike-slip faulting is proposed. The age of this caldera is 10-10.5 Ma. The least vesiculated magma volume is 200-250 km3. The identified pyroclastic flows are Verde Ignimbrite, Chorrillos Ignimbrite, Tajamar Ignimbrite and Abra del Gallo Ignimbrite. The volcanic history started with explosive events and opening of vents. The initial collapse of the caldera was asymmetric and only the eastern side developed. The eruption of the Verde Ignimbrite took place in this stage in a central vent. The continuous collapse and the opening of rim vents are related with the Chorrillos, Tajamar and Abra del Gallo ignimbrites. The Ignimbrite Tajamar is the principal collapse with intracaldera characteristics. The extracaldera facies is represented by the Ignimbrite Abra del Gallo. The collapse is completed in a short period of time, followed by a generalized resurgence in the caldera floor. Regional detumes-cence and probably the intrusion of a dome produced this resurgence. The postcaldera activity is represented by hydrotermal - geothermal activity. All of this process took place in a short period of time, ca 10-10.5 Ma. The field and petrographic data are coherent with the direction determinations of the Magnetic Anisotropy Susceptibility, and with the calculated temperatures of emplacement. The magmatic chamber geometry and the collapse of a caldera were controlled by the regional structural scheme. A WNW-ESE trend strike-slip fault system with left lateral movement called Calama-Olacapato-El Toro Lineament caused the local distension. These strike-slip faults continued and deformed the north side of the caldera. The magmatic system could have migrated to the west originating the Quevar volcanic complex (8Ma)

Descripción geoquímica y geocronológica de secuencias volcánicas neógenas de Trasarco, en el extremo oriental de la Cadena Volcánica Transversal del Quevar (Noroeste de Argentina)

New geochronological data (34) and 39 new geochemical analyses have been made in the Neogene backarc volcanic sequences in the “El Quevar Transversal Volcanic Chain” defined by Viramonte et al (1984a). This chain starts at the present N-S arc with WNW-ESE trend. New volcanic centres have been recognised and analysed at the eastern end of this chain. The different pulses from each volcanic centre (Aguas Calientes, Acay, El Morro - Organullo and Tocomar) are petrographically and geochemically homogeneous. This suggests that the magma chambers related in each volcano, were geochemically homogeneous and did not have crystal enriched sectors. The Acay eruptive centre is an exception of this assumption; a compositionally differentiated magmatic chamber could be inferred. The isotopic composition of the Aguas Calientes emission centre has a crust signature in its origin, related with melts generated by crustal fussion. The geochronological determinations show volcanic pulses at 17-19 Ma, 13-12 Ma, 10 Ma, 7-6 Ma, 1-0.5 Ma in this region of the Central Andes

Introducción a la geología de los Andes Centrales Meridionales: El noroeste argentino

Bajo el título genérico de"Geología de los Andes Centrales Meridionales: El Noroeste Argentino" se recopilan un conjunto de trabajos realizados en los últimos años en sectores del Noroeste A rgentino. Las investigaciones realizadas se localizan en la región andina de la provincia de Salta y territorios adyacentes. Este volumen monográfico reúne trabajos que constituyen el esfuerzo conjunto de veinticinco autores de diferentes países, europeos y suramericanos, trabajando en proyectos de colaboración internacional

Petrology of the Luingo caldera (SE margin of the Puna plateau): A middle Miocene window of the arc-back arc configuration

We describe the petrographic characteristics, whole-rock geochemistry and mineral chemistry of rocks from the Pucarilla-Cerro Tipillas Volcanic Complex with emphasis on the rocks belonging to the middle Miocene Luingo caldera, located in the south-eastern portion of the Central Volcanic Zone (CVZ) of the Andes. We modelled the petrogenesis of the Luingo caldera rocks as a mixture of ca. 20% crustal magmas and 80% of mantle magmas by AFC with recharge processes. A comparison of Luingo geochemical data with the composition of Miocene-Pliocene volcanic rocks from the broad area, points to major thickening events during the middle Miocene for the western portion and during the upper Miocene for the eastern portion of the Southern CVZ. In the eastern sector (~. 66\ub0W) the mantle source appears to change from a spinel-lherzolite type for the middle Miocene to a garnet-lherzolite type for the upper Miocene-Pliocene magmas. The areal distribution of the volcanic products led to the recognition of approximately equivalent areas covered by volcanic rocks both in the eastern and in the western Puna borders. This indicates a broad arc, which was structurally controlled at the proto-Puna/Puna margins, whose geochemical differences are related with variations in crustal thicknesses and heterogeneous mantle sources from west to east

Provenance and tectonic implications of Orán Group foreland basin sediments, Río Iruya canyon, NW Argentina (23° S)

Foreland basins are important recorders of tectonic and climatic processes in evolving mountain ranges. The Río Iruya canyon of NW Argentina (23° S) exposes ca. 7500 m of Orán Group foreland basin sediments, spanning over 8 Myr of near continuous deposition in the Central Andes. This study presents a record of sedimentary provenance for the Iruya Section in the context of a revised stratigraphic chronology. We use U-Pb zircon ages from six interbedded ash layers and new magnetostratigraphy to constrain depositional ages in the section between 1.94 and 6.49 Ma, giving an average sedimentation rate of 0.93 ± 0.02 (2σ) km Myr−1. We then pair U-Pb detrital zircon dating with quartz trace-element analysis to track changes in sedimentary provenance from ca. 7.6 to 1.8 Ma. Results suggest that from ca. 7.6 to ca. 6.3 Ma, the Iruya watershed did not tap the Salta Group or Neogene volcanics that are currently exposed in the eastern Cordillera and Puna margin. One explanation is that a long-lived topographic barrier separated the eastern Puna from the foreland for much of the mid-late Miocene, and that the arrival of Jurassic-Neogene zircons records regional tectonic reactivation at ca. 6.3 Ma. A second major provenance shift at ca. 4 Ma is marked by changes in the zircon and quartz populations, which appear to be derived from a restricted source region in Proterozoic-Ordovician meta-sediments. Considered in conjunction with the onset of coarse conglomerate deposition, we attribute this shift to accelerated uplift of the Santa Victoria range, which currently defines the catchment's western limit. A third shift at ca. 2.3 Ma records an apparent disconnection of the Iruya with the eastern Puna, perhaps due to defeat of the proto Rio-Iruya by the rising Santa Victoria range. This study is one of the first applications of quartz trace-element provenance analysis, which we show to be an effective complement to U-Pb detrital zircon dating when appropriate statistical methods are applied.Fil: Amidon, William H.. Middlebury College; Estados UnidosFil: Luna, Elisa Edith. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de San Juan. Facultad de Arquitectura, Urbanismo y Diseño. Instituto Regional de Planeamiento y Hábitat; Argentina. Middlebury College; Estados UnidosFil: Fisher, G. Burch. University Of California, Santa Barbara;Fil: Burbank, Douglas W.. University of California; Estados UnidosFil: Kylander Clark, Andrew R. C.. University of California; Estados UnidosFil: Alonso, Ricardo Narciso. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Salta; Argentina. Universidad Nacional de Salta; Argentin