Cathodoluminescence Study of Quartz Recrystallization in Contact-Metamorphosed Rocks of the Shimanto Supergroup, Kanto Mountains, Japan

Shimanto Supergroup sandstones and shales in the Kanto Mountains, Japan, have been contact metamorphosed to hornfels by intrusion of the Tokuwa Batholith. Garnet-biotite geothermometry shows that the metamorphic temperatures of samples collected at distances of ~1 m to 2600 m outward from the batholith range from about 700°C to 400°C. Cathodoluminescence (CL) imaging of quartz from these samples allows the CL characteristics of quartz to be evaluated as a function of known metamorphic temperatures. Contact metamorphism is shown to generate two basic kinds of CL features, homogeneous bright CL and mottled CL, which are characteristic respectively of metamorphism at high and low temperatures. Other CL features, such as dark and bright CL lines may be superimposed on mottled CL texture. CL imaging provide a basis for understanding recrystallization of quartz that involves changes in traceelement composition and crystal defect structures under the influence of contact metamorphism. Published studies indicate that most quartz in unmetamorphosed Shimanto Supergroup sandstones and shales was derived from volcanic and plutonic sources. Volcanic and plutonic quartz is characterized by distinctive CL textures such as fine-scale zoning. None of the quartz in metamorphosed Shimanto sediments that we investigated displays these CL textures, which indicates that original CL features were erased during an early dehydration stage of contact metamorphism. New quartz CL textures were subsequently acquired that reflect the conditions that prevailed during later stages of fluid infiltration at higher metamorphic temperatures.