Petrología y geoquímica del complejo Tortuga en el seno Año Nuevo, isla Hoste e (55ºS – 55º 20’ S)
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2019
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es
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Universidad Andrés Bello
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En Patagonia se encuentran tres complejos ofiolíticos los cuales han sido interpretados como los vestigios del piso oceánico máfico de una cuenca marina de trasarco llamada cuenca de Rocas Verdes. De norte a sur, estos complejos son: el Complejo Sarmiento (50ºS – 52ºS), el Complejo Capitán Aracena (53ºS – 55ºS) y el Complejo Tortuga (55ºS – 56ºS). Dentro del Complejo Tortuga, específicamente en las islas Gordon y Hoste, existen afloramientos aislados, de gabros, basaltos y diques doleríticos los cuales queda la incertidumbre si estos corresponden a la parte superior de un complejo ofiolítico o a una secuencia volcánica del tipo arco de islas. En el presente aporte se muestra que en términos de elementos mayores todas las rocas ígneas de los complejos ofiolíticos y las formaciones Yaghán y Hardy muestran una tendencia al campo toleítico, no obstante, en relación con los elementos traza, las rocas del presente estudio presentan patrones entre signaturas MORB e IAT. Las variaciones en los patrones de LREE encontrados indican que: Las rocas ígneas de los complejos Sarmiento y Capitán Aracena presentan valores de (La/Sm)n de 1,6 y 1,4 en promedio, mientras que las rocas ígneas asociadas a las formaciones Yaghán y Hardy presentan valores de (La/Sm)n de 1,4 en promedio, todas similares a un IAT y, por otro lado, las rocas ígneas asociadas al Complejo Tortuga presenta valores de (La/Sm)n de 0,5, cercano a un N-MORB. Las rocas volcánicas intercaladas de las formaciones Yaghán y Hardy, ubicadas en las cercanías del Complejo Tortuga presenta valores de Ba/Nb>70, más altos que las ofiolitas, las cuales presentan valores de Ba/Nb entre 34 a 70 en promedio, los cuales incluso son más altos que los de un N-MORB (2,7), lo cual indica variaciones en la anomalía de HFSE. A partir de esto, se deduce que estas unidades presentan diferentes fuentes mantélicas, las cuales se habrían formado cercanas entre sí. Las fuentes que habrían formado complejos Sarmiento y Capitán Aracena corresponden a una cuña mantelica metasomatizada y por otra parte el Complejo Tortuga habría sido formado en una fuente en transición entre una cuña mantelica metasomatizada y un manto tipo MORB deprimido, lo cual sería indicativo de un menor efecto de los componentes de subducción. Además, los resultados geoquímicos del presente aporte indicarían que las formaciones Yaghán y Hardy, tendrían un origen cercano al Complejo Tortuga. En la actualidad han sido reconocidos basaltos asignados al Complejo Tortuga cubriendo rocas asignadas a la Formación Hardy, por lo cual, si el Complejo Tortuga y las formaciones Yaghán y Hardy están en contacto, no solo responderían a aspectos estructurales. Para conciliar estos datos, se sugiere que la fuente de las ofiolitas habría ido cambiando durante la apertura de la cuenca desde una cuña mantelica metasomatizada hasta un manto tipo MORB deprimido. Finalmente, los datos presentados apoyan la hipótesis de que la apertura de la cuenca de Rocas Verdes fue mayor al sur respecto al norte.
In Patagonia there are three ophiolitic complexes which have been interpreted as the vestiges of the mafic ocean floor of a marine backarc basin called the Rocas Verdes basin. From north to south, these complexes are: the Sarmiento Complex (50ºS - 52ºS), the Captain Aracena Complex (53ºS - 55ºS) and the Tortuga Complex (55ºS - 56ºS). Within the Tortuga Complex, specifically on the Gordon and Hoste Islands, there are isolated outcrops of gabbros, basalts and doleritic dykes which remain uncertain if they correspond to the upper part of an ophthalmic complex or to a volcanic sequence of the arch type of islands. This contribution shows that in terms of larger elements all igneous rocks of the ophiolitic complexes and the Yaghan and Hardy formations show a tendency to the tholeiitic trend, however, in relation to the trace elements, the rocks of the present study have patterns between MORB and IAT signatures. The variations in the LREE patterns found indicate that: The igneous rocks of the Sarmiento and Capitan Aracena complexes have values of (La/Sm)n of 1.6 and 1.4 on average, while the igneous rocks associated with the formations Yaghán and Hardy have values of (La/Sm)n of 1.4 on average, all similar to an IAT and, on the other hand, the igneous rocks associated with the Tortuga Complex have values of (La/Sm)n of 0.5, close to an N-MORB. The volcanic rocks interspersed with the Yaghan and Hardy formations, located near the Tortuga Complex, have values of Ba/Nb>70, higher than the ophiolites, which have Ba/Nb values between 34 to 70 on average, which they are even higher than those of an N-MORB (2.7), which indicates variations in the HFSE anomaly. From this, it follows that these units have different mantellic sources, which would have formed close to each other. The sources that would have formed Sarmiento and Capitan Aracena complexes correspond to a metasomatized mantle wedge and on the other hand the Tortuga Complex would have been formed in a transitional source between a metasomatized mantle wedge and a depleted MORB mantle, which would be indicative of a minor effect of subduction components. In addition, the geochemical results of this contribution would indicate that the Yaghan and Hardy formations would have an origin close to the Tortuga Complex. At present, basalts assigned to the Tortuga Complex have been recognized covering rocks assigned to the Hardy Formation, so if the Tortuga Complex and the Yaghan and Hardy formations are in contact, they would not only respond to structural aspects. To reconcile these data, it is suggested that the source of the ophiolitic would have been changing during the opening of the basin from a metasomatized mantle wedge to a depleted MORB mantle. Finally, the data presented support the hypothesis that the opening of the Rocas Verdes basin was greater to the south compared to the north.
In Patagonia there are three ophiolitic complexes which have been interpreted as the vestiges of the mafic ocean floor of a marine backarc basin called the Rocas Verdes basin. From north to south, these complexes are: the Sarmiento Complex (50ºS - 52ºS), the Captain Aracena Complex (53ºS - 55ºS) and the Tortuga Complex (55ºS - 56ºS). Within the Tortuga Complex, specifically on the Gordon and Hoste Islands, there are isolated outcrops of gabbros, basalts and doleritic dykes which remain uncertain if they correspond to the upper part of an ophthalmic complex or to a volcanic sequence of the arch type of islands. This contribution shows that in terms of larger elements all igneous rocks of the ophiolitic complexes and the Yaghan and Hardy formations show a tendency to the tholeiitic trend, however, in relation to the trace elements, the rocks of the present study have patterns between MORB and IAT signatures. The variations in the LREE patterns found indicate that: The igneous rocks of the Sarmiento and Capitan Aracena complexes have values of (La/Sm)n of 1.6 and 1.4 on average, while the igneous rocks associated with the formations Yaghán and Hardy have values of (La/Sm)n of 1.4 on average, all similar to an IAT and, on the other hand, the igneous rocks associated with the Tortuga Complex have values of (La/Sm)n of 0.5, close to an N-MORB. The volcanic rocks interspersed with the Yaghan and Hardy formations, located near the Tortuga Complex, have values of Ba/Nb>70, higher than the ophiolites, which have Ba/Nb values between 34 to 70 on average, which they are even higher than those of an N-MORB (2.7), which indicates variations in the HFSE anomaly. From this, it follows that these units have different mantellic sources, which would have formed close to each other. The sources that would have formed Sarmiento and Capitan Aracena complexes correspond to a metasomatized mantle wedge and on the other hand the Tortuga Complex would have been formed in a transitional source between a metasomatized mantle wedge and a depleted MORB mantle, which would be indicative of a minor effect of subduction components. In addition, the geochemical results of this contribution would indicate that the Yaghan and Hardy formations would have an origin close to the Tortuga Complex. At present, basalts assigned to the Tortuga Complex have been recognized covering rocks assigned to the Hardy Formation, so if the Tortuga Complex and the Yaghan and Hardy formations are in contact, they would not only respond to structural aspects. To reconcile these data, it is suggested that the source of the ophiolitic would have been changing during the opening of the basin from a metasomatized mantle wedge to a depleted MORB mantle. Finally, the data presented support the hypothesis that the opening of the Rocas Verdes basin was greater to the south compared to the north.
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Tesis (Geólogo)
FINANCIADO POR PROYECTO FONDECYT N.º 1161818
FINANCIADO POR PROYECTO FONDECYT N.º 1161818
Palabras clave
Petrología, Chile, Magallanes, Geoquímica