Fuentes y evolución del magmatismo triásico del margen suroccidental de Gondwana: nuevos antecedentes petrogenéticos de la Alta Cordillera (28°S – 30°S), Chile.
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Date
2023
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Publisher
Universidad de Concepción
Abstract
Desde el surgimiento de la teoría de la tectónica de placas, Los Andes ha sido considerado como un ejemplo de un orógeno formado por subducción. El margen occidental de Sudamérica ha estado periódicamente activo desde el Paleozoico, y en base al registro geológico presente en el norte de Chile y Argentina, se han propuesto dos etapas orogénicas: Ciclo Gondwánico (Carbonífero Temprano – Pérmico Temprano) y Ciclo Andino (Jurásico temprano - Presente), que son marcadamente diferentes en sus estilos de deformación, metamorfismo y magmatismo. Entre ambas etapas, se desarrolló un periodo estacionario respecto al movimiento de las placas, llamado Ciclo Pre-Andino, propuesto como un periodo en el que cesa la subducción y prevalece una tectónica extensional que permite el desarrollo de un rift continental. Sin embargo, recientemente, nuevos antecedentes petrológicos y geoquímicos proveniente de las rocas pre-andinas de la Alta Cordillera Chilena muestran ser compatibles con una subducción activa, tales como extensos afloramientos de rocas ígneas con un amplio espectro composicional, calcoalcalinas y con un enriquecimiento en elementos de gran radio iónico en relación a los elementos de alto potencial iónico, además de anomalías negativas de Nb-Ta, generando algunas inconsistencias con el modelo actual. De este modo, el problema a abordar en esta investigación es determinar cómo cambiaron las condiciones tectónicas entre las dos etapas orogénicas que se han propuesto para la evolución del margen andino, a través del estudio del magmatismo pre-andino, expuesto en un área específica de la Cordillera Frontal Chilena (28°30’S – 30°30’S) donde extensos volúmenes de rocas volcánicas y plutónicas están excepcionalmente bien expuestas debido a la tectónica mesozoica y cenozoica y a las condiciones climáticas áridas. El magmatismo pre-andino fue caracterizado desde el punto petrográfico y geoquímico, tanto elemental como isotópico, evaluando de esta manera sus fuentes, y comparado con el magmatismo de los ciclos Gondwánico y Andino. Unidades representativas del magmatismo pre-andino fueron estudiadas en detalle, tales como los complejos plutónicos Chollay y Piuquenes, la Formación Pastos Blancos y los Sienogranitos Colorado, unidades que previamente habían sido catalogadas como parte del magmatismo del Choiyoi. El amplio espectro composicional en las unidades pre-andinas, junto a la presenta de texturas porfídicas, minerales máficos hidratados, una naturaleza calcoalcalina, enriquecimientos en LILE respecto a los HFSE, depresiones en Nb-Ta, Ti y P, además de enriquecimientos en Pb sugieren que el magmatismo pre-andino se generó en un contexto de subducción activa. La isotopía (Sr-Nd-Pb) arroja razones iniciales que son compatibles con una mezcla de dos fuentes, que corresponden a un manto deprimido y a la litósfera continental. Por otro lado, al comparar la geoquímica de los tres ciclos tectónicos es posible detectar que los cambios geoquímicos son graduales y que la señal de subducción se encuentra presente desde el Carbonífero al Jurásico (18°S - 40°S). Es posible identificar una disminución gradual de las razones vinculadas al espesor cortical (LaN/YbN y Sr/Y) a medida que las rocas ígneas son más jóvenes; además de una disminución del 87Sr/86Sri y un incremento del εNdi, lo que apunta a una disminución del componente litosférico/continental a la fuente de los magmas. Estos cambios, son explicados mediante modificaciones en la configuración de la zona de subducción, proponiendo que un escenario de roll-back del slab, sería lo más probable para el Ciclo Pre-Andino, que habría permitido el desarrollo de una tectónica extensional en la corteza y probablemente la pérdida litosférica por erosión termal o delaminación, disminuyendo esta fuente en la génesis del magmatismo pre-andino y aumentando la proporción de una fuente deprimida, de manera progresiva hasta el Jurásico.
Since the emergence of the plate tectonics theory, the Andes have been regarded as an example of a subduction orogen. The western margin of South America has been periodically active since the Paleozoic, and in northern Chile and Argentina, two orogenic stages have been proposed: Gondwanan Cycle (Early Carboniferous–Early Permian) and the Andean Cycle (Early Jurassic–Recent), which are markedly different in their styles of deformation, metamorphism, and magmatism type. Between both, a stationary period respect to the movement of the plates was developed, called Pre Andean Cycle, proposed as a period in which ceases subduction and extensional tectonics prevail, which allows the development of a continental rift. Nevertheless, recently, new petrological and geochemical background from the pre-andean igneous rocks of the Chilean High Andes, show compatibility with the development of this magmatism in a context of active subduction (extensive outcrops of igneous rocks of broad compositional range, calcalkaline nature and enrichment large-ion lithophile elements (LILE) in relation to high field strength elements (HFSE), in addition to negative anomalies of Nb-Ta, generating some inconsistencies with the current model. Thus, the problem to be addressed in this thesis is to determine how tectonic conditions changed in the transition between both orogenies that have been proposed for the evolution of the Andean margin and aims to be resolved through the study of part of the pre-andean magmatism, in a specific area in the Chilean High Andes (28°30’S and 30°30’S), where vast volumes of pre-andean volcanic and plutonic rocks are exceptionally well exposed due to the Mesozoic and Cenozoic tectonic events and arid climatic conditions. The pre-Andean magmatism was characterized from the petrographic and geochemical perspective, evaluating its sources, and compared with the Gondwanan and Andean magmatism. Representative units oh pre-andean magmatism were studied in detail, such as the Chollay and Piuquenes plutonic complexes, the Pastos Blancos Formation and Colorado Syenogranites, units that had previously been catalogued as part of Choiyoi Group. The wide compositional spectrum in the pre-andean units, together with the presence of porphyritic texture, hydrated mafic minerals, a calc-alkaline nature, LILE enrichment with respect to HFSE, Nb-Ta, Ti and P toughs, as well as Pb enrichments, suggest that pre-andean magmatism was generated in an active subduction context. The isotopy (Sr-Nd-Pb) yields initial ratios that are compatible with a mixture of two sources which correspond to a depleted mantle and the continental lithosphere. On the other hand, when comparing the geochemistry of the three tectonic cycles, it is possible to detect that the geochemical changes are gradual, and that the subduction signal is present from the Carboniferous to the Jurassic (18°S - 40°S). It is possible to identify a gradual decrease in the ratios related to crustal thickness (LaN/YbN and Sr/Y) as the igneous rocks are younger; in addition to a decrease in 87Sr/86Sri and an increase in εNdi, which point to a decrease in the lithospheric/cortical component at the source of magmas. These changes are explained by changes in the configuration of the subduction zone, proposing that a slab roll-back scenario would have allowed the development of extensional tectonics in the crust, and probably the lithospheric loss due to thermal erosion or delamination, decreasing this source in the genesis of pre-andean magmatism and increasing the proportion of a depleted source, progressively until the Jurassic.
Since the emergence of the plate tectonics theory, the Andes have been regarded as an example of a subduction orogen. The western margin of South America has been periodically active since the Paleozoic, and in northern Chile and Argentina, two orogenic stages have been proposed: Gondwanan Cycle (Early Carboniferous–Early Permian) and the Andean Cycle (Early Jurassic–Recent), which are markedly different in their styles of deformation, metamorphism, and magmatism type. Between both, a stationary period respect to the movement of the plates was developed, called Pre Andean Cycle, proposed as a period in which ceases subduction and extensional tectonics prevail, which allows the development of a continental rift. Nevertheless, recently, new petrological and geochemical background from the pre-andean igneous rocks of the Chilean High Andes, show compatibility with the development of this magmatism in a context of active subduction (extensive outcrops of igneous rocks of broad compositional range, calcalkaline nature and enrichment large-ion lithophile elements (LILE) in relation to high field strength elements (HFSE), in addition to negative anomalies of Nb-Ta, generating some inconsistencies with the current model. Thus, the problem to be addressed in this thesis is to determine how tectonic conditions changed in the transition between both orogenies that have been proposed for the evolution of the Andean margin and aims to be resolved through the study of part of the pre-andean magmatism, in a specific area in the Chilean High Andes (28°30’S and 30°30’S), where vast volumes of pre-andean volcanic and plutonic rocks are exceptionally well exposed due to the Mesozoic and Cenozoic tectonic events and arid climatic conditions. The pre-Andean magmatism was characterized from the petrographic and geochemical perspective, evaluating its sources, and compared with the Gondwanan and Andean magmatism. Representative units oh pre-andean magmatism were studied in detail, such as the Chollay and Piuquenes plutonic complexes, the Pastos Blancos Formation and Colorado Syenogranites, units that had previously been catalogued as part of Choiyoi Group. The wide compositional spectrum in the pre-andean units, together with the presence of porphyritic texture, hydrated mafic minerals, a calc-alkaline nature, LILE enrichment with respect to HFSE, Nb-Ta, Ti and P toughs, as well as Pb enrichments, suggest that pre-andean magmatism was generated in an active subduction context. The isotopy (Sr-Nd-Pb) yields initial ratios that are compatible with a mixture of two sources which correspond to a depleted mantle and the continental lithosphere. On the other hand, when comparing the geochemistry of the three tectonic cycles, it is possible to detect that the geochemical changes are gradual, and that the subduction signal is present from the Carboniferous to the Jurassic (18°S - 40°S). It is possible to identify a gradual decrease in the ratios related to crustal thickness (LaN/YbN and Sr/Y) as the igneous rocks are younger; in addition to a decrease in 87Sr/86Sri and an increase in εNdi, which point to a decrease in the lithospheric/cortical component at the source of magmas. These changes are explained by changes in the configuration of the subduction zone, proposing that a slab roll-back scenario would have allowed the development of extensional tectonics in the crust, and probably the lithospheric loss due to thermal erosion or delamination, decreasing this source in the genesis of pre-andean magmatism and increasing the proportion of a depleted source, progressively until the Jurassic.
Description
Tesis a optar al grado de Doctora en Ciencias Geológicas