Orbicular rocks in Hida metamorphic belt, Japan

Abstract

Various types of orbicular rocks are known from more than ten places in granites and metamorphic rocks of the Hida metamorphic belt. The most fundamental structure of an orbicule is concentric structure. The orbicules, especially the concentric structure, would have grown in fluid state in the almost latest stage of the Hida metamorphism accompanied by granite activity. The reason is as follows. a) The orbicules are always embedded in the host rocks which are supposed to have been derived from fluid, such as magma or partial melt of the metamorphic rocks. The presence of agmatitic inclusions, fragmental inclusions of broken orbicules or fluidal structure support fluidal origin of the host rocks. b) The contrasting nature of each concentric shell, compositional variation of the constitutive minerals, or partial defect in a shell layer probably caused by mechanical break down or assimilation by the matrix, may be referred to fluctuation of chemical conditions during the growth of the orbicule. c) Deformed orbicules are often found in many types of orbicular rocks. Some have concave surface partly. These deformed orbicules might have grown up jostling to each other in a narrow space and afterwards, removed and separated by the movement of the fluidal matrix. d) The orbicules have no substantial relation to their nuclei. Although the nature and chemical composition of the nuclei are divergent, the orbicules are relatively similar in mineral and chemical compositions. The orbicules would have grown up neither by interaction between the nuclei and the matrix, nor by metasomatic process in a solid state but by cyrstallization from the fluid itself. e) Pegmatitic veinlets often accompany the orbicules intimately. They cut or wind around the orbicules. The nature of the orbicule-forming fluid is not yet elucidated. Any restricted chemical composition is not required to form various types of orbicules, judging from the diversity of the host rocks as well as the orbicules, from gabbro to adamellite. The writer is of the opinion that there should be a single genetic explanation satisfactory for all the orbicules in the Hida metamorphic belt. Because the orbicules are considered to have been formed almost coevally, probably about 180 million years ago, and their geographic distribution is confined in a narrow area, so small as 40×100km2. Briefly speaking, within a narrow area in the Hida metamorphic belt, the orbicular rocks might have grown up abundantly and also coevally. Mechanism of the orbicule growth will be discussed in detail in a forth coming paper.