Study of retrograde metamorphism in the calc-silicate rocks from the Alvand aureole

The Alvand batholith is intruded into the Jurassic argillaceous and carbonate rocks and has caused contact metamorphism. The calc-silicate hornfelses and varieties of pelitic and mafic hornfelses are produced in this regard. Based on the modal percentage, the calc-silicate rocks of the area can be divided into different groups. The present study shows that the calc-silicate hornfelses of the Hamadan area are metamorphosed during two metamorphic phases, the prograde and the retrograde events. The prograde metamorphism formed anhydrous minerals such as pyroxene, garnet and vesuvianite. Following this phase, by reduction of temperature the retrograde metamorphism occurred and tremolite-actinolite, zoisite, epidote and to some extent hematite were formed. The rocks resulted from this metamorphism are epidote calc-silicate, epidote-tremolite-actinolite calcsilicate, zoisite calc-silicate and . . . .
Retrograde metamorphism caused alteration along the grain boundaries and along the cleavage plane of the minerals (especially garnet). Furthermore, some textures, such as symplectite texture, were developed in the rocks, which is in accordance with decreasing temperature.