The Victory Pluton, a Devonian aged granite in northeastern Vermont, is thought to have intruded during movement on the Monroe Fault. Metamorphic rocks in the contact aureole provide clues about the pressure and temperature changes during intrusion of the magma. The goal of this research project is to observe the metamorphic reaction textures within one sample from the eastern (uplifted) side of the fault to examine the temperature and pressure changes that occurred during metamorphism.
Five sites throughout the thin section were analyzed using a petrographic microscope and scanning electron microscope. There are two dominant porphyroblasts within the sample, garnet and cordierite. Sites 1-4 possess garnet porphyroblasts with cordierite and ilmenite inclusions and biotite and fibrolite surrounding the crystal in the pressure shadow. Site 5 contains cordierite porphyroblasts with plagioclase and biotite inclusions. Biotite and sillimanite are also contained in the pressure shadow surrounding the cordierite.
While the porphyroblasts vary between garnet and cordierite from each site, the minerals textures suggest that garnet and cordierite were consumed while biotite and sillimanite grew. This reaction takes place at temperatures between 600 and 800 degrees Celsius and pressures between 2 and 5 kilobars. This reaction is driven by either decreasing temperatures or increasing pressures. These reactions are consistent with cooling after the pluton intruded, or burial of the rocks after pluton intrusion, or both.
