変成史と変成条件から見た領家変成作用の研究 : 南西三河高原を例として

In the southwestern part of Mikawa Plateau, central Japan, there widely develop the Ryoke metamorphic and granitic rocks. The former are composed of pelitic, psammitic and siliceous rocks, associated with lesser amounts of basic ones. Various Kinds of granitic rock, such as Shinshiro quartzdiorite, Mitsuhashi granodiorite and Busetsu granite, are distributed in the eastern and northern part of this area. Geological, petrographical and mineralogical characters of the metamorphic and plutonic rocks are investigated in detail. Metamorphic rocks in the surveyed area are litho-stratigraphically divided into the following four formations in the ascending order ; the Chigiri, the Hongusan, the Kashiyama and the Sakuraiji formations. The general trend of the foliation is ESE-WSW, dipping to the north. Total thickness of the formations are estimated to be at least several thousand meters. Macroscopically, the area is characterized by the prevalence of homoclinal structure. The area can essentially be inferred to be a regional metamorphic terrain, but the contact effects by the acting of granitic rocks can also be observed especially in the northern and eastern parts. On the basis of the mineralogical parageneses in the pelitic rocks, the area escaped from the contact effects can be subdivided into two zones of I and II. While the contact metamorphic area can be subdivided into two zones of Younger contact metamorphic Zone and Older one in connection with the time of granitic activity. Such a zonation of the regional metamorphic rocks is based upon the parageneses of metamorphic minerals recrystallized during the final phase of the progressive metamorphism which is contemporaneous with the deformation of the forth phase described below. Meanwhile, recognition of the deformation phase has been made based on the analysis of timing of appearance of biotite and muscovite in biotite schists (SEO & HARA, 1980). The history of deformation and metamorphism of the Zone I can be divided into following four phases :
The first phase; Deformation related to the formation of the schistosity of the first phase (= initial Si) which is recognized as Si in A-biotite flakes at present.
The second phase; Appearance of A-biotite frakes (= inclusion-rich cores) under static condition= mimetic recrystallization of A-biotite flakes on initial Si.
The third phase; Intracrystalline slip-deformation of A-biotite flakes under compression parallel to initial Si associating formation of small recrystallized biotite flakes (C-biotite flakes) in them.
The forth phase; Deformation related to formation of the schistosity of the second phase (= Se) = formation of elongated platelets of A-biotite dimensionally preferentially oriented parallel to Se which is related to its pressure solution under compression normal to Se but not to its intracrystalline slip deformation, and appearance of B-biotite flakes showing preferred lattice and dimensional orientation parallel to Se.
In this paper, the succession of metamorphic phases in the regional metamorphic zones (Zone I & Zone II) can be understood on the basis of analysis of microtextures of metamorphic minerals employing deformation-structures as time-markers in dating their appearance. The metamorphic history of this area in the Ryoke belt has been discussed by this technique. Common metamorphic minerals, such as plagioclase, K-feldspar, garnet, biotite, muscovite, cordierite, staurolite and tourmaline, have been analyzed by means of EPMA to clarify their chemical characteristics. Using the results, physical conditions of the Ryoke metamorphism are discussed from the viewpoints of mineralogical partition equilibria and experimentally determined metamorphic reaction curves. Based upon these petrological results coupled with the observations of textural characteristics, P-T path of Ryoke metamorphism in this area has been determined. Summarizing up the geological and petrological characters of constituent rocks, the geological history of metamorphism and plutonism of the Ryoke metamorphic terrain under consideration can be inferred as follows: The metamorphism of the first phase is characterized by the appearance of staurolite, garnet and biotite. These minerals are enclosed as inclusion minerals in the porphyroblasts (A-biotite, A-andalusite and A-cordierite etc..) formed during the metamorphism of the second phase. During the metamorphism of the second phase, which is characterized by static conditions of lower to moderate temperature, lower pressure and higher vapour pressure, such characteristic minerals as A-biotite, A-andalusite, A-cordierite, K-feldspar and garnet were formed. The metamorphism of the third phase, which occurred in relation to the deformation of the forth phase, is characterized by conditions of medium to higher temperature, lower pressure and lower vapour pressure. In Zone I of stratigraphically upper part, such minerals as B-biotite, B-andalusite, B-cordierite, and muscovite were produced. On the other hand, in zone II of stratigraphically lower part, sillimanite, cordierite, K-feldspar, garnet, biotite and muscovite were produced. Then basic (gabbroic) igneous plutonism occurred in the later stage of the metamorphism of the third phase. On the basis of the above results, it is shown that the abrupt depression of pressure occurs from the first to the second metamorphic phase, and the rise of temperature occurs between the second and the third metamorphic phase. Time and spatial relationship of the Ryoke metamorphism has been clarified in terms of polymetamorphism.