Publication: Anfibolitas granatíferas de Sierra Albarrana (Córdoba). Termobarometría e implicaciones para el desarrollo del metamorfismo regional.
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Publication Date
1991
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González del Tánago y del Río, José
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Sociedad Geológica de España
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Abstract
Las anfibolitas granatíferas que afloran dentro de la formación de Micaesquistos de La Albariza - Bembezar
(Sierra Albarrana), en condiciones propias de la zona de la estaurolita (+ distena), contienen una paragénesis
principal (pico térmico) constituida por hornblenda + plagioclasa + almandino + cuarzo + ilmenita. En base a características texturales y a consideraciones quemográficas en el sistema Si02 - Ab03 - FeO - MgO - CaO - Na20 - H20, puede deducirse que la paragénesis anfibolítica alcanzó equilibrio textural y químico durante su crecimiento, así como que esta situación fue poco modificada por reequilibrios retrógrados. Las condiciones de cristalización de la paragénesis de las anfibolitas han sido estimadas en T = 537 ± 15°C y P = 4.3 ± 0.5 kb (termometría de intercambio Fe - Mg / Fe - Mn y equilibrios granate - rutilo - ilmenita - plagioclasa - cuarzo /GRIPS/ y granate - hornblenda - plagioclasa - cuarzo /GHPS/); no se observa una diferencia
significativa entre los valores obtenidos para centros y bordes de granates.
Las anfibolitas de Sierra Albarrana no muestran ninguna evidencia de una historia de mayor presión que la estimada para su paragénesis fundamental; la mineralogía conservada como inclusiones en porfidoblastos no difiere de la que aparece en la matriz, y la ausencia generalizada de rutilo permite estimar un límite bárico máximo para la trayectoria P - T de P < -6.5 kb (GRIPS). Por tanto, el máximo térmico debe ser esencialmente coincidente con el bárico, quedando ambos valores definidos por las condiciones P - T determinadas para la paragénesis anfibolítica; este hecho está a favor de un modelo de engrosamiento cortical homogéneo para el sector de cadena considerado. La evolución tectonometamórfica es similar a la que se registra en otros sectores del Macizo Ibérico en facies de las anfibolitas, aunque en la mayoría de los ejemplos comparables se alcanzaron tasas mayores de enterramiento. Teniendo en cuenta los valores P / T calculados, junto con el gradiente
moderadamente elevado ligado al pico térmico (35°C/km), el metamorfismo regional de Sierra Albarrana debe considerarse dentro del rango bárico menor del metamorfismo de presiones medias.
Garnet - bearing amphibolites outcroping within the La Albariza -Bembezar Mica Schist Formation (Sierra Albarrana), in the staurolite (+ kyanite) metapelite zone, contain a main assemblage (metamorphic peak conditions) constituted by hornblende + plagioclase + almandine + quartz + ilmenite. On the basis of textural characteristics and chemographic considerations in the Si02 - Ab03 - FeO - MgO - CaO - Na20 - H20 system, it can be deduced that textural and chemical equilibrium was attained during the crystallization of the amphibolitic assemblage, and similarly that this situation was only slightly modified by retrograde reactions. P - T equilibrium conditions of the peak assemblage have been estimated at T = 537 ± 15°C and P = 4.3 ± 0.5 kb (Fe - Mg / Fe - Mn exchange thermometry, a~d garnet - rutile - ilmenite - plagioclase - quartz /GRIPS/ and garnet - hornblende - plagioclase - quartz /GHPS/ equilibria). Significative differences between the obtained values for cores and rims of garnets were not observed. The Sierra Albarrana amphibolites show no evidence of any metamorphic stage higher in pressure than, that represented by the amphibolitic assemblage. The mineralogy included in porphyroblasts do es not differ from that which appear in the matrix, and the general absence of rutile allows an estimation of P < 6.5 kb (GRIPS) for the baric limit of the P - T path. Therefore, the thermal and baric peaks must essentially coincide, keeping the same P - T values than those defined for the amphibolitic assemblage; this fact point to a model of homogeneous thickening of the crust for the part of the Iberian Massif in which Sierra Albarrana is included. The tectonometamorphic evolution is similar to that seen in other amphibolite facies areas of the Iberian Massif, although the majority of comparable examples are characterized by higher pressure conditions. Taking into account the estimated P - T values, along with the moderately high gradient bound to the thermal peak (35°C/Km), the regional metamorphism of the Sierra Albarrana area must be considered within the lower baric range of the medium pressure metamorphism
Garnet - bearing amphibolites outcroping within the La Albariza -Bembezar Mica Schist Formation (Sierra Albarrana), in the staurolite (+ kyanite) metapelite zone, contain a main assemblage (metamorphic peak conditions) constituted by hornblende + plagioclase + almandine + quartz + ilmenite. On the basis of textural characteristics and chemographic considerations in the Si02 - Ab03 - FeO - MgO - CaO - Na20 - H20 system, it can be deduced that textural and chemical equilibrium was attained during the crystallization of the amphibolitic assemblage, and similarly that this situation was only slightly modified by retrograde reactions. P - T equilibrium conditions of the peak assemblage have been estimated at T = 537 ± 15°C and P = 4.3 ± 0.5 kb (Fe - Mg / Fe - Mn exchange thermometry, a~d garnet - rutile - ilmenite - plagioclase - quartz /GRIPS/ and garnet - hornblende - plagioclase - quartz /GHPS/ equilibria). Significative differences between the obtained values for cores and rims of garnets were not observed. The Sierra Albarrana amphibolites show no evidence of any metamorphic stage higher in pressure than, that represented by the amphibolitic assemblage. The mineralogy included in porphyroblasts do es not differ from that which appear in the matrix, and the general absence of rutile allows an estimation of P < 6.5 kb (GRIPS) for the baric limit of the P - T path. Therefore, the thermal and baric peaks must essentially coincide, keeping the same P - T values than those defined for the amphibolitic assemblage; this fact point to a model of homogeneous thickening of the crust for the part of the Iberian Massif in which Sierra Albarrana is included. The tectonometamorphic evolution is similar to that seen in other amphibolite facies areas of the Iberian Massif, although the majority of comparable examples are characterized by higher pressure conditions. Taking into account the estimated P - T values, along with the moderately high gradient bound to the thermal peak (35°C/Km), the regional metamorphism of the Sierra Albarrana area must be considered within the lower baric range of the medium pressure metamorphism