Petrología y estimación de condiciones de presión y temperatura de metamorfismo de las anfibolitas de granate del complejo metamórfico Diego de Almagro, Región de Magallanes
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2017
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es
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Universidad Andrés Bello
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Licencia CC
Resumen
En el presente trabajo, se realizó un modelamiento termodinámico en el sistema MnNCKFMASHTO con software Perple_X, con la composición de una muestra de mesosoma de una migmatita anfíbolítica de la Unidad Lázaro del Complejo Metamórfico Diego de Almagro ubicado en la Patagonia chilena.
Se determinó el peak de metamorfismo para esta roca, indicado por la intersección de isópletas de Ca y Mg# en clinopiroxeno a los 640°C-12 Kbar, utilizando una composición de una inclusión de diópsido en granate y que se asume como relicto del peak metamórfico. Este resultado indica un origen migmatítico de la roca, con una cantidad de 12% vol. de fundido. Las principales fases predichas por el modelamiento termodinámico son granate y diópsido, ambos forman parte de un 70% vol. en la roca. Otras fases menores son biotita, epidota, anfíbola, magnetita, cuarzo y rutilo.
Análisis petrográficos indican evidencias de fusión parcial en estas rocas, donde se distinguen leucosomas trondhjemíticos con granate peritéctico. El melanosoma se dispone paralelo al leucosoma y corresponde a acumulaciones de anfíbola y en menor cantidad granate.
Se determina una trayectoria P-T-t antihoraria en base a observaciones petrográficas, termobarométricas y geocronológicas. El evento del peak metamórfico queda acotado a los 640°C-12 Kbar en transición facies Anfibolita-Granulita con edades entre los 165-160 Ma. Posteriormente ocurre una etapa temprana de enfriamiento cercanamente isobárico a los 150 Ma culminando a los 470°C-13 Kbar en facies Esquistos Azules a los 80 Ma.
Esta trayectoria antihoraria junto con datos presentados en este estudio, concuerdan con una fusión parcial de corteza oceánica caliente en las etapas iniciales de la subducción durante el Jurásico Medio en el margen oeste de Gondwana.
The present work is based on a thermodynamic modeling associated to a MnNCKFMASHTO chemical system which was accomplished by using Perple_X software. This thermodynamic modeling was obtained from the composition of the mesosome sample that belonged to a amphibolitic migmatite from Lázaro Unit in Diego de Almagro Metamorphic Complex located at the chilean Patagonia. The metamorphic peak for this rock is indicated by the intersection of Ca and Mg# isopleths on clinopyroxene at 640°C-12 Kbar conditions, using the composition of a diopside inclusion in garnet and it is assumed to be a relict of the metamorphic peak. These results indicates a migmatitic origin of this sample, with a calculated 12 vol% of melt. Main phases of the predicted mineral assemblages are garnet and diopside, both forming part of c. 70 vol% of the rock. Other minor phases are biotite, epidote, amphibole, magnetite, quartz and rutile. Petrographic analyzes indicate evidence of partial melting in these rocks, trondhjemitic leucosomes with peritectic garnet can be identified. Melanosomes are amphibole accumulation with small amounts of garnet and are parallel to the leucosomes. The P-T-t path is counterclockwise based on petrographic, thermobarometric and geochronological observations. The event of the metamorphic peak is delimited at 640ºC and 12 Kbar, on the amphibolite-granulite facies transition dating between 165-160 My. Later, an early nearly isobaric cooling stage occurs at 150 My, culminating at 470ºC-13 Kbar conditions, which is related to a blueschist facies at 80 My. This counterclockwise P-T-t path and the given data are consistent with partial melting of hot oceanic crust at initial subduction stages in the middle Jurassic at the west margin of Gondwana.
The present work is based on a thermodynamic modeling associated to a MnNCKFMASHTO chemical system which was accomplished by using Perple_X software. This thermodynamic modeling was obtained from the composition of the mesosome sample that belonged to a amphibolitic migmatite from Lázaro Unit in Diego de Almagro Metamorphic Complex located at the chilean Patagonia. The metamorphic peak for this rock is indicated by the intersection of Ca and Mg# isopleths on clinopyroxene at 640°C-12 Kbar conditions, using the composition of a diopside inclusion in garnet and it is assumed to be a relict of the metamorphic peak. These results indicates a migmatitic origin of this sample, with a calculated 12 vol% of melt. Main phases of the predicted mineral assemblages are garnet and diopside, both forming part of c. 70 vol% of the rock. Other minor phases are biotite, epidote, amphibole, magnetite, quartz and rutile. Petrographic analyzes indicate evidence of partial melting in these rocks, trondhjemitic leucosomes with peritectic garnet can be identified. Melanosomes are amphibole accumulation with small amounts of garnet and are parallel to the leucosomes. The P-T-t path is counterclockwise based on petrographic, thermobarometric and geochronological observations. The event of the metamorphic peak is delimited at 640ºC and 12 Kbar, on the amphibolite-granulite facies transition dating between 165-160 My. Later, an early nearly isobaric cooling stage occurs at 150 My, culminating at 470ºC-13 Kbar conditions, which is related to a blueschist facies at 80 My. This counterclockwise P-T-t path and the given data are consistent with partial melting of hot oceanic crust at initial subduction stages in the middle Jurassic at the west margin of Gondwana.
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Tesis (Geólogo)
Palabras clave
Petrogénesis, Chile, Magallanes, Rocas Metamórficas