Intrusion of the Chahghand gabbrodiorite in regionally
metamorphosed rocks of the Sanandaj-Sirjan zone in NE of Neyriz has
caused contact metamorphism. A variety of pelitic hornfelses are formed as
a result of thermal metamorphism. Spatial distribution of minerals in the
Chahghand aureole defines three mineralogical zones namely cordierite,
andalusite and sillimanite zones. Study of metamorphic reactions and phase
relations indicate a pressure of 2.5 to 3.5 kbar and temperature of 500-600 oC
and ~700 oC for the andalusite and the sillimanite zones respectively. Apart
from crystallization of prismatic sillimanite in the sillimanite zone, fibrolite
is formed in the other zones. It seems that cation leaching and deformation
were the major processes in the formation of fibrolite in the Chahghand
aureole. Regarding appearance of fibrolite in altered rocks far from the
igneous contact, fibrolite is not crystallized within the sillimanite stability
field, but it may has crystallized at lower temperatures. Therefore
considering fibrolite in the rocks equivalent to sillimanite existence and
estimated pressure and temperature on this basis, at least the altered rocks
within the thermal aureoles is fraught with some problems.