Thermal Effect of Granite Pluton on Graphitization of Carbonaceous Material in the Kuga Group, Southwest Japan

Abstract

The Kuga Group of Triassic-Jurassic age is mainly composed of black pelitic rocks and chert which were thermally metamorphosed by intrusion of Cretaceous granite. Graphitization of carbonaceous material in pelitic rocks of the Kuga Group was investigated using X-ray diffraction method. Samples were collected from six boreholes, of which three boreholes reached to the granite. Crystallite thickness Lc(002) value increases toward the granite pluton and is 300A at a vertical distance of 200-300m from the contact. Near the contact Lc(002) is 400A or more. Lattice strain ec decreases with increasing Lc(002). Vertical and lateral changes in Lc(002) of carbonaceous material in the Kuga Group are illustrated in the geological section showing the isopleth of Lc(002). The isopleth of Lc(002) is generally parallel to the contact. These features suggest that heat transfer through the Kuga Group depends chiefly on thermal conduction. Graphitization of carbonaceous material in sedimentary rocks is applicable to the prediction of the subterranean pluton.