Thermal degradation and polymerization of carbonaceous materials in a metapelite–granitoid magma system in the Ryoke metamorphic belt, SW Japan

Some granitoid magmas in subduction zones are closely related to sedimentary rocks, due to partial melting or assimilation during generation and ascent. In the Ryoke high temperature (T)–low pressure (P) metamorphic belt of SW Japan, syngenetic carbonaceous material containing organic matter occurs in granodiorite which interacted with pelitic rocks under middle crust T and P conditions. Solid state carbonaceous materials in metapelites, injection gneiss, and granodiorite consist of graphite and organic matter. X-ray mapping by EPMA shows that granular carbonaceous material a few tens of microns in diameter consisting of C with or without N, Cl, and S occurs as inclusions in quartz, feldspar, biotite, and garnet. Micro laser Raman spectro-analysis shows that some carbonaceous inclusions in injection gneiss and metapelite are graphite. Peaks characteristic of phenyl-H, alkyl–CH3, –CH2–, CO, and –CC– stretching vibrations are present in FT-IR spectra of biotite flakes, indicating that hydrocarbon compounds are uniformly distributed in the crystal structure and/or along cleavages of that mineral. Particulate carbonaceous materials in minerals in the granodiorite contain long-aliphatic hydrocarbon bonds, oxygen-bearing organic compounds, and aromatic fragments. The distribution of these organic groups differs among feldspar, quartz, and biotite. Carbon isotopic ratios in the granodiorite are lighter than those of graphite and amorphous carbon in the metapelite, suggesting an origin from low molecular weight hydrocarbons released from the metapelite. Carbonaceous materials probably formed by hydrocarbon polymerization and solidification of gaseous hydrocarbons were trapped in the highly viscous granodiorite magma during crystallization and cooling, and under differing oxidation conditions. The presence of such organic matter in granitoids is important for the understanding of the behavior and circulation of volatile components in arc systems during subduction, accretion, and magmatism.