I am working on a pelitic rock metamorphosed in amphibolite facies. It is mainly composed of garnet, staurolite, quartz and plagioclase.
The figure shows a grain of garnet which has an overgrowth on the porphyroblast (Fig1). There is a significant variation of Mn from core to
rim in the porphyroblast whereas the overgrowth composition is different (shown in the fig 1).
The garnet is swapped by schistosity defined by biotite. This indicate it's pre-tectonic in nature. Interestingly, the overgrowth forms as
fragments of garnet grain (Fig1.) rather than a continuous one. My question is can it be a result of rotation of the garnet during the
overgrowth formation? In such a case does the overgrowth texture represents syn-tectonic nature?
Also, staurolite is not swapped by biotite and forms optically continuous small grains. Therefore, they are post tectonic in nature. The
feature is not prominent in fig 1 but it is visible in fig 2(different slide). I interpret the mechanism of staurolite formation as dissolution-
reprecipitation. Can there be any other mechanism? In such case is it logical to use the overgrowth composition as reactant while modeling
staurolite forming reactions?
It will be very helpful if you can suggest some literatures related to such textures.