Ocurrence and significance of the polymorphs of Al2SiO5 in Metamorphic rocks of the Santander Massif, Eastern Cordillera (Colombian Andes)

Abstract

The pelitic and semipelitic rocks of the Santander Massif metamorphic basement have been affected by a Barrovian type of metamorphism, which has occurred under high-temperature and medium-pressure conditions. The polymorphs of Al₂SiO₅, Kyanite, andalusite, and sillimanite, are each stable over a characteristic range of pressure and temperature and provide useful means of estimating conditions of metamorphism recorded by these rocks. In the staurolite-Kyanite zone, in some cases the polymorphs of Al₂SiO₅, Kyanite and andalusite, occur as porphyroblasts, the last one generally with a poikiloblastic character.Textural evidence indicates the formation of kyanite after and andalusite, And -> Ky. During the process of prograde metamorphism of the Santander Massif metamorphic rocks, the sillimanite first appear after kyanite or iin muscovite adjacent to garnet, replacing biotite or garnet, and occurs as aggregates of fribrolite, sometimes penetrating quartz grains. A reaction such as staurolite + muscovite + quartz could produce the first appearance of sillimanite, as well as the disappearance of the staurolite from the chemical system