西南日本内帯西部地域における中・古生代造構作用の研究

Study on th Late paleozoic - Early mesozoic Tectonic Developement of Western Half of the Inner Zone of Southwest Japan.

早坂 康隆

廣島大學地學研究報告

Geological report of the Hiroshima University

The Upper Paleozoic - Lower Mesozoic sequences widely develop in the Inner Zone of Southwest Japan just on the south of the Hida belt. Most of them are considered to be accretionary complexes formed in consuming plate-boundaries during Middle Permian to Late Jurassic age. In this paper the author has attempted to clarify evolutional proceses of those accretionary complexes.
The western half of the Inner Zone of Southwest Japan consists of the Chugoku - Maizuru belt and Tamba belt. The fundamental tectono-stratigraphic subunits of the Chuoku - Maizuru belt are classified as followes: 1) High-P/T type metamorphic rocks ("Sangun metamorphic rocks"). This subunit is further divided into several subgroups as described in later paragraph. 2) Non-metamorphic Paleozoic formations. This subunit is further divided into two subgroups following Hase(1964). (a) North zone Paleozoic formations as a Late Permian accretionary prism cosisting mainly of seamount - reaf-limestone complexes such as Akiyoshi, Taishaku and Atetsu chert -limestone - greenstone complexes, clastic rocks and acidic tuff. (b) Middle zone Paleozoic formations and maizuru group in the Maizuru belt as a Late Permian accretionary prism consisting mainly of Yakuno rocks and clastic rocks with subordinate acidic tuff and hemipelagic reddish-claystone. 3) Equivalents of theTamba group in the Tamba belt: Middle to Latest Jurassic accretionary coplexes consisting mainly of bedded chert, siliceous claystone, pebbly mudstone and greenstones with sub-ordinate limestone lenses. 4) Triassic and Jurassic formations of blackish to shallow water facies: subunit consisting mainly of relatively coarse-grained clastic rocks. They unconformably overlie the non-metamorphic Paleozoic formations. 5) Ultra Tamba zone (Ishiga, 1985; Caridroit et al., 1985) as a Triassic (probably) accretionary complex developed between the Maizuru belt and the Tamba belt.
The author has clarified that the Chugoku belt is divided into two units, NE unit and SW unit, by a narrow zone which cuts across the general distribution trend of the above-mentioned constituent subunits of the Chugoku blet from the Nichihara - Tsuwano district to the North of Hiroshima and consists of the equivalents of the Tamba group. The two units are different from each other mainly with respec to some important characteristics of the "Sangun metamorphic rocks": a) distribution trend of metamorphic rocks (Fig. 1), b) orientation trend of mineral lineations (Fig. 4), c) original rock assemblage (Fig. 5), d) radiometric age (Fig. 6), e) distribution frequency of lawso-nite-bearing schist (frequently found in the NE unit, but only one lacality in the SW unit), and f) volume of Yakuno rocks (abundant in the weekly metamorphosed Paleozic formations of the NE unit, but rare in those of the SW unit). Because Paleozoic formation of the Northern zone of the NE unit has essentially the same property as that of the SW unit, the author has pointed out that those two units appeared after the formation of the Paleozoic formation.
Recently, the radiometric age of "Sangun metamorphic rocks" has extensively been measured mainly by Shibata and Nishimura(1983, 1984, 1985), showing that the "Sangun metamorphic rocks" of the Northern district of the Chugoku belt are of 280 - 310 Ma, those of the Western district of 210 - 220 Ma and those of the Eastern district of 170 - 190 Ma (Fig. 6). The Northern district corresponds to the northern margin of the Chugoku belt (NE unit and SW unit), while the Western and the Eastern district correspond to the remaining part of the SW unit, and to that of the NE unit respectively. Metamorphic rocks of the Northern district are correlated with those of the Hida marginal belt. The author has found some fossiles such as Triassic Conodonts and Early Jurassic Radiolarians and high-P/T type minerals such as alkali and sodic-calcic amphiboles (Table 2) and lawsonites (Table 3), from the Hatto formation in the Mochigase district, Tottori Prefecture (Table 1), which has been considered to be a member of the "Sangun metamorphic rocks", showing that in the Chugoku zone there are Jurassic high-P/T type metamorphic sequences. Original rock assemblage of the Hatto formation consists mainly of bedded chert, greenstone, siliceous claystone and pebbly mudstone, being comparable with that of the Type II suite formations of the Tamba group which are of Jurassic age. The "Sangun metamorphic rocks" with such original rock assemblage are found only in the NE unit (Fig. 12), forming relatively low-grade metamorphic portion of them and commonly associating the equivalents of the Tamba group. Other "Sangun metamorphic rocks" in the NE unit, which consist mainly of elastic rocks and greenstones with subordinate limestones, are closely associated with the non-metamorphic Paleozoic formations. On the other hand the metamorphic rocks of the SW unit may form a unique tectono-stratigraphic subunit.
The author has studied the geology of the Maizuru zone of the Asako-cho district, Hyogo Prefecture, showing that the border zone between the Maizuru and the Tamba belt is structurally characterized by a pile nappe structure consisting of nappe of the Tamba groupe, nappe of the Ultra Tamba zone and three nappes of Yakuno rocks, and that the Yakuno rocks were produced by igneous activities of two stages. The first-stage Yakuno rocks form a pseudostratiform sequence, containning Pyroxenite member, Transitional zone, Lower Gabbro member, Upper Gabbro member and Dolerite member in ascending order, and correspond to the Yakuno ophiolite after Ishiwatari(1978). The second-stage Yakuno rocks consist mainly of tonalite and quartz-diorite, which discordantly intruded the first-stage Yakuno rocks. The constituent minerals such as clinopy-roxenes of the Lower Gabbro memger and amphiboles of the Upper Gabbro member of the first-stage Yakuno rocks have preferred lattice orientation, showing their metamorphic nature (Figs. 18, 19). The maximum Ti content of such metamorphic amphiboles increases towards the lowest horizon of the volcano-plutonic sequence, shwing increase of metamorphic grade from the greenshcist facies in the upper part of the Dolerite member to the hornblende-granulite facies of the lowest part of the Lower Gabbro member and the Pyroxenite member (Fig. 21). Such metamorphism may be ascribed to sea floor metamorphism accomplished before dismembering of the ophiolitic sequence of the first-stage Yakuno rocks. With referece to major element chemistry of whole rock and phenocryst clinopyroxene, it may be said tha the first-stage Yakuno rocks are comparable with either some E-type MORB or tholeiite of ocean plateau with relatively thick crust (Fig. 26). On the ohter hand, the second-stage Yakuno rocks appear to be regarded as arc-related igneous rocks (Fig. 24). It has been also pointed out that the greenstones of the Chugoku and Tamba belts associated with bedded cherts are commonly of seamount, ocean island and/or ocean plateau (Fig. 27). MnO/TiO 2 vs MnO/Al2O3 plots of the Triassic beded cherts of the Mino belt also sugest a possibility that they were produced in pelagic but relatively shallower environments such as ocean plateau, seamount or flank of ocean island (Fig. 30).
Thus the tectonic developement of the Chugoku, Maizuru and Tamba belts during Carboniferous to Jurassic age has been synthesized as below.
1) Early Carboniferous to Early Permian age
During Early Carboniferous age, on a sea floor (Akiyoshi ocean floor) appeared seamounts with reef-limestone complexes of Hiraodai, Akiyoshi, Taishaku, Atetsu and Omi. All rocks arround them deposited during this age was of pelagic type. During Late Carboniferous age, subduction of the Akiyoshi coean floor began at the front of the Hida - Hida marginal belt.
2) Middle to Late Permian age
Accretionary prisms of non-metamorphic Paleozoic formations related to the subduction of the Akiyoshi ocean floor had not began to form untill Middle Permian age, and their formation appears to have ceased during Latest Permian age. During this period, either T-T-R type or R-F-T type triple-junction arrived at the front of the Hida - Hida marginal belt, forming the first-stage Yakuno rocks (the Yakuno ophiolite) together with sedimentation of fine-grained terrigenous materials. This was followed by collision of young aseismic ridge. After this collision, the trench and vol-canic front were sifted oceanward. Hence, the collided first-stage Yakuno rocks were involved in arc-related igneous activities, forming the second-stage Yakuno rocks.
3) Triassic age
Because any volcanic activity has not been known throughout the Hida - Chugoku - Maizuru belt, it has been inferred that, after passing away of the tripple-junction, the subduction ceased during Early to Middle Triassic age. While acidic tuffs of small amount are found in the Upper Triassic formation, although their petrologic characteristics have not yet been clarified. Thus it seems probable that during Late Triassic age the plate boundary had turned into of highly oblique subduction, forming the accretion of rocks of the Ultra Tamba zone.
4) Jurassic age
During Early Jurassic age were produced such "Sangun metamorphic rocks" as the Hatto formation with high-P/T minerals, followed by a volumenous accretion of Type II suite formation of the Tamba belt during Middle Jurassic age. During the Latest Jrassic age, the Type I suite formation of the Tamba group had underthrusted bnenerth the Permian to Jurassic complexes of the Chugoku blet, forming a pile-nappe structure of the Chugoku and Tamba belts. The Early Cretaceous arc-volcanisms widely occured in these belts as a tectonic collage.
It has been clarified that, althogh the formation of the Chugoku and Tamba belts is ascribed to acrretion tectonics, any constituent material of normal type abyssal plane such as N-type MORB or typical pelagic sediment is not involved in the constituent rocks of those belts and they commonly contain oceanic materials derived from topographically prominent highs on oceanic plate such as seamount, ocean island and/or ocean plateau. This fact strongly suggests that only the collision of the seamount, ocean island and/or ocean plateau during Permian to Jurassic age was responsible for the formation of accretion complexes of the Chugoku and Tamba belts.

日本語

廣島大學理學部地學教室

[ISSN] 0073-2303

[NCID] AN00213676