Mineralogía y petrología del volcanismo permo-triásico del bloque de San Rafael en el área de Sierra Pintada, Provincia de Mendoza y su relación con las mineralizaciones de uranio.

Institución otorgante:

Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales

Fecha:

1999

Tipo de documento: 

info:eu-repo/semantics/doctoralThesis
 

Formato:

application/pdf

Idioma:

spa

Descripción:

The southern portion of South America formed part of the supercontinent of Gondwana during most of the Paleozoic, and a broad magmatic belt was active in thewest margin in the Carboniferous and Permo-Triassic. In the San Rafael Massif, Mendoza, Argentina a foreland basin developed behind the Carboniferous magmatic arc,which resulted in a thick accumulation of marine and continental sediments. Later, thesesediments (El Imperial Formation) were deformed during the San Rafael orogenic phasewhich was related to an eastward migration of the arc in the Early Permian. Volcanism is Permo-Triassic and postdates the deformation of the El Imperial deposits, but the earlyvolcanics and associated sediments (Cochicó Group) were synchronous with acompressive and/or transpressive stress regime, related to the San Rafael orogenic phase,indicating that subduction continued to the west. The volcanic sequences in the San Rafael Massif form part of the Choiyoi magmatic province. Field observations, inconjuction with mineralogy, petrology, major, trace and REE element data, indicate thattwo different suites can be distinguished within the Choiyoi, termed in here lower andupper Sections. The lower Section (Early Permian) comprises andesites, and dacitic tolow-silica rhyolitic ignimbrites with associated sediments (Cochicó Group) andrepresents the products of the subduction-related magmatic arc. The upper Section (Early Permian- Early Triassic) which is mainly composed of rhyolitic ignimbrites (Aguade los Burros Formation), andesitic dyke intrusions (Quebrada del Pimiento Formation),caldera-related high-silica rhyolitic ignimbrites and lava flows, dacitic to rhyoliticsubvolcanic bodies, some with garnet and topaz, and mildly alkalic basaltic andesites (Cerro Carrizalito Formation). This change in the character of volcanism is correlatedwith a shift from the compressive tectonic style to a progressive extensional regime, andis believed to represent the transition between Late Paleozoic subduction and Triassicrifting. A third suite of Middle Triassic age (Puesto Viejo Formation) could also bedistinguished. It is bimodal, comprises basaltic andesites, basalts and rhyolitic ignimbritesand would correspond to the peak of extension (rift ?) in the area. Dacitic to rhyolitic ignimbrites of the subduction-related Cochicó Groupcontain biotite as the main mafic mineral, and evolved from andesitic magmas by crystalfractionation of the observed mineralogy (Toba Vieja Gorda Member, Yacimiento Los Reyunos Formation). They are homogeneous sequences that could be considered as “monotonous intermediates” (Hildreth, 1981). Disequilibrium textures suggest thatmagma mixing and/or lower crust contamination were important processes. High-silica rhyolites of the upper Section are similar to bimodal aluminousrhyolites and to topaz rhyolites. They contain biotites, arnphiboles and pyroxenessuggesting lower pressures, higher temperatures and dryer conditions of crystallization. Rhyolites from different volcanic centers show distinctive geochemical patterns. Besides,some textures suggest magma mixing processes and zoned magma chambers. They showlow composition gradients suggesting crystal fractionation processes. The crust wasthinned, magma Chambers were emplaced at higher crustal levels and crustalcontamination and/ or melting seems to be a main process. They are transitional betweensubduction and continental intraplate conditions. Andesitic dykes emplaced in tensionalnormal faults (Quebrada del Pimiento Formation) can record the beginning of extensionin the Early-Late Permian. These andesitic dykes could probably be the product ofhibridization of a basaltic underplate and the lower crust. Extension could eventually berelated to changes in the subduction geometry due to diminishing rates of plate motions,which finally produced the end of plate convergence. Triassic high-silica rhyolites (Puesto Viejo Formation) are also similar tobimodal aluminous rhyolites, no mafic minerals were observed, and they seem to begenetically unrelated to spatially associated basalts and andesites. The rhyolites wouldrepresent crustal melts which were produced by heating due to mafic magmas emplacedat the base of the crust, whereas the basalts and andesites are the product of this basaltic “underplating” that fractionated and/or interacted with the crust. The relationship between silicic volcanic rocks (mainly volcaniclastic) anduranium mineralizations has been known for many years, because uranium concentratesin felsic igneous rocks, due to its highly incompatible character. These rocks were widelyrecognized as source rocks through devitrification processes of volcanic glass. In recentyears, however, their importance as potential hosts, mainly of those rocks related to acaldera cycle, was also evaluated: In order to be a potential host, post-magmaticprocesses, such as long-lived hydrothennal systems, are esential for uraniumremobilization and concentration within this environment. In the San Rafael Massif, the Choiyoi lower Section (Grupo Cochicó) hoststhe main uranium mineralization known, at present, in Argentina: the Dr. Baulíes-Los Reyunos deposit which is emplaced in sediments (Areniscas Atigradas Member)redeposited from dacitic and low-silica rhyolitic ignimbrites (Toba Vieja Gorda Member)with very low U background (up to 3 ppm). Field, mineralogical and geochemical data inthis case, points to the role of volcanism to be limited to that of source rock. Besides,this type of magma is not very suitable for yielding great amounts of U. In contrast, thecomposition of magmas, the tectonic setting and the presence of caldera-relatedvolcanics in the Choiyoi upper Section are more favourable conditions for uraniummineralizations to ocurr, according to worldwide examples, and seem to be a moreadequate exploration target. Mineralogical and chemical data from some known uraniummineralizations associated to these rocks (Rincón del Atuel, Pircas del Mesón) show thatsome hydrothermal activity was relevant in the remobilization and concentration of U. The paucity of sediments associated to the upper Section volcanics makes ignimbritesand other caldera-cycle rocks, a more favourable host.

Identificador:

https://hdl.handle.net/20.500.12110/tesis_n3148_Kleiman