Carmen I. Martínez Dopico, Mónica G. López de Luchi, Klaus Wemmer y Augusto E. Rapalini
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Rev. Asoc. Geol. Argent. vol.70 no.1 Buenos Aires ene./mar. 2013
El Complejo Plutónico La Esperanza está situado en el sector norte del Macizo Norpatagónico y es parte del cinturón magmático Paleozoico Superior del norte de la Patagonia. Dentro de este complejo se agrupan mayormente granodioritas y monzogranitos biotítico-anfibólicos (Granodiorita Prieto) o biotíticos (Granito Donosa), siendo 273 Ma la edad de cristalización de los primeros. En este trabajo se utilizan las composiciones minerales de biotitas y anfíboles para describir la naturaleza del magma y estimar la presión y temperatura del emplazamiento de estos cuerpos plutónicos. En base a la química de las biotitas, el Complejo Plutónico La Esperanza presenta una signatura magmática calcoalcalina con tendencia a la peraluminosidad, la cual puede ser asociada a ambientes transicionales orogénico- anorogénico. Las presiones de emplazamiento calculadas para la Granodiorita Prieto oscilan entre 0,6 y 2,0 kbar registrándose un aumento de la presión en la facies de granitoides inequigranulares aflorante al este de la localidad de La Esperanza. El termómetro hornblenda-plagioclasa indica que este cuerpo se emplazó a una temperatura de entre 630 y 790ºC en condiciones de alta fugacidad de oxígeno. Estos resultados, en combinación con edades de enfriamiento, indican un emplazamiento somero en un ambiente con un régimen termal elevado que habría permanecido en actividad desde el Pérmico medio al menos hasta el límite permotriásico.
Chemical composition of biotite and hornblende and cooling ages as indicators of the emplacement conditions of the La Esperanza Plutonic Complex (Upper Permian), North Patagonian Massif
La Esperanza Plutonic Complex crops out in the northern part of the North Patagonian Massif in the Upper Paleozoic magmatic belt of northern Patagonia. This magmatic complex is mainly composed of 273 Ma biotite and amphibole bearing (Prieto Granodiorite) or biotite bearing (Donosa Granite) granitoids. In this study, biotite and amphibole mineral compositions are used to describe the nature of the granitic magma and to estimate the pressure and temperature at which these plutons were emplaced. Based on biotite chemistry, La Esperanza Plutonic Complex represents a calc-alkaline magma with a trend of increasing peraluminosity, which could be related to a transitional orogenic to anorogenic setting. Emplacement pressures for Prieto Granodiorite vary between 0,6 and 2,0 kbar, with higher values in an inequigranular facies which crops out to the east of La Esperanza locality. Hornblende-Plagioclase thermometer yields emplacement temperatures ranging from 630 to 790ºC at high oxygen fugacity conditions. These results, combined with good qualitycooling ages, propose a shallow emplacement in an elevated thermal regime that might have lasted from Middle Permian to Permotriassic times.
La Esperanza Plutonic Complex crops out in the northern part of the North Patagonian Massif in the Upper Paleozoic magmatic belt of northern Patagonia. This magmatic complex is mainly composed of 273 Ma biotite and amphibole bearing (Prieto Granodiorite) or biotite bearing (Donosa Granite) granitoids. In this study, biotite and amphibole mineral compositions are used to describe the nature of the granitic magma and to estimate the pressure and temperature at which these plutons were emplaced. Based on biotite chemistry, La Esperanza Plutonic Complex represents a calc-alkaline magma with a trend of increasing peraluminosity, which could be related to a transitional orogenic to anorogenic setting. Emplacement pressures for Prieto Granodiorite vary between 0,6 and 2,0 kbar, with higher values in an inequigranular facies which crops out to the east of La Esperanza locality. Hornblende-Plagioclase thermometer yields emplacement temperatures ranging from 630 to 790ºC at high oxygen fugacity conditions. These results, combined with good qualitycooling ages, propose a shallow emplacement in an elevated thermal regime that might have lasted from Middle Permian to Permotriassic times.
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