isograde de la sillimanite

OBS: Un isograde représente une ligne sur une carte résultant de l'intersection d'une surface isograde avec la topographie. Elle représenterait une réaction métamorphique et serait implicitement diachrone à cause du temps requis pour mobiliser la chaleur, les fluides et les réactions dans les roches.
OBS: isograde de la sillimanite : terme en usage à Ressources naturelles Canada - Secteur des sciences de la Terre.

CONT: The transition from kyanite to andalusite is known as the andalusite isograd, while the transition from andalusite to sillimanite is called the sillimanite isograd.
CONT: Note that if were making a geologic map, we could draw a line on the map that separates the pelitic rocks containing only Pyrophyllite from those containing Kyanite + Quartz. Such a line (a surface in 3 dimensions) is called an isograd (iso - same, grad - grade). In this case, since it represents the first appearance of Kyanite, we call it the Kyanite Isograd ... If we continue walking along the direction that T & P increased in these rocks during metamorphism, we would eventually come to another place where in the pelitic rocks the mineral assemblage changes. This time the change is from Kyanite + Quartz to Sillimanite + Quartz. Again, we can draw a line on the map that indicates this change in mineral assemblage, this time calling it the Sillimanite Isograd. Just like before, this represents the point on the phase diagram, above, where the geothermal gradient intersected the boundary for the reaction ... Because the sandstone and the limestone do not contain Kyanite, the isograd does not appear in these rocks, but we can still extrapolate its position across the map or outcrop.
OBS: isograd: A line or surface (either imaginary or in a diagram) joining points where the rock originated under the same conditions of pressure and temperature, as indicated by similarity of facies or metamorphic grade.