Mineral Parageneses: The Building Blocks of Metamorphic Rocks

Abstract

Rocks consist of a number of different minerals. In an igneous rock various minerals crystallized from a slowly cooling magmatic melt constitute an equilibrium assemblage. Such an assemblage is called a mineral paragenesis. In general, all the minerals that are detectable within a single thin section belong to the mineral paragenesis that characterizes a given igneous rock. This is so because a magmatic melt is a homogenous solution of the many components that constitute such a rock. However, in metamorphic rocks derived from sediments the composition may not be the same even over a small volume. Therefore, it is well possible that all minerals observed in a single thin section do not belong to a single metamorphic paragenesis. Rather, two or even more mineral parageneses, i.e., associations of minerals coexisting in equilibrium, may be present in the area of one thin section. This is a point of great significance. In earlier petrographic work it was believed that the determination of all the minerals of a given rock is sufficient. This is not so. It now must be ascertained which of the minerals in a thin section are in contact. Only minerals in contact may be regarded as an assemblage of coexisting minerals, i.e., a paragenesis.¹