Growth history of garnet in pelitic schists of the Sanbagawa metamorphic terrain in central Shikoku

Abstract

During the prograde metamorphism of the Sanbagawa metamorphic terrain in central Shikoku, garnet has grown in the pelitic schists whilst maintaining surface equilibrium with coexisting minerals such as chlorite and epidote, etc. In the quartz-albite layer, it has grown euhedrally. In the micaceous layer, its crystal morphology varies according to the different minerals in contact and their different orientations. Garnet enclosed in or sandwiched between micas is always ellipsoidal, extended parallel to the basal planes of the micas. However, when the edge of a garnet has intruded at an acute angle into the micas, euhedral garnet has grown. It is concluded that the growth of garnet at the interface with micas ceased when the {110} surface of garnet came into contact with the basal plane of the micas in the tectonic c-axis direction. Other models, such as chloritization-resorption, pressure-solution, and mechanical-erosion or grinding, do not adequately explain the origin of the flattened garnet. During retrograde metamorphism, chloritization and resorption of garnets took place in the matrix, resulting in the rough and irregular surface of garnet. Garnets included in albite porphyroblasts have been largely unaffected by the retrograde metamorphism.