Permalink : http://hdl.handle.net/2115/36739

Newly found dumortierite occurring as a minor constituent mineral in argillaceous gneisses of the Abukuma axial metamorphic belt, northeast Japan, is described. At present, dumortierite has been known from only three localities. The mineral in question is found in feldspar porphyroblastic gneiss of the Takanuki metamorphics and argillaceous diaphthoritic gneiss. All of the known dumortierites occur in leucocratic seams within the host gneisses. Dumortierite is found in interstices of the major constituent minerals of the leucocratic seams, and replaces neighboring plagioclase or cordierite, indicating that the dumortierite formed at the latest stage of formation of the leucocratic seams. As a result of the petrogenetic studies on these host gneisses, it may be concluded that the dumonierite-bearing leucocratic seams are the products of metamorphic differentiation which proceeded through porphyroblastesis and produced the leucocratic and melanocratic layers. It is likely that boron was initially contained in the source rocks of these gneisses and concentrated into the leucocralic seams. The concentration of boron may have reached high enough to form dumortierite at the latest stage of the formation of leucocralic seam.