The rock name Ophiolite originated in classic Greek, and introduced into Latin as Ophites, it means snake-stone from appearance of this rock. In the 16th century, AGRICOLA named this rock Serpentaria, and the name succeeded to Serpentine and Serpentinite of the present days. Ophiolite is proposed by French geologist BRONGNIART (1813) as term for serpentine rock proper. In 1905, German geologist STEINMANN proposed a concept of Ophiolithischen Massengesteine (and at later as Ophilolith) for a group of mafic and ultramafic rocks in association with deep sea sediments found in mountain chains. This concept was neglected by many geologists till to 1960's, and then revalued by BAILEY and Ar. HOLMES follow development of theories on ocean floor spreading.In the present, Ophiolite sequence is idealized and applied to many rock groups of the world as evidences of ancient oceanic crust. Historical development of ophiolite concept is reviwed. Problems concerning on Ophiolite sequence from view point of stratigraphic geology are discussed.

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The Kamuikotan Belt, a tectonic belt of the centralaxial zone of Hokkaido, is characterized by the presence of numerous serpentinite masses. A serpentinite mass in the southern part of the belt is called the Saru-gawa Ultrabasic Massif. Based on the geologic structure, the massif is divided into the Western Unit and the Eastern Unit. The mutual relation of these two units indicates that the block including the Eastern Unit was thrusted to the block including the Western Unit. Petrography of the ultrabasic rocks in the two units is asfollows; original rock of the ultrabasic rock in the Western Unit is represented by the layerd-textured Iwanai-dake peridotite surrounded by serpentinite. The Iwanai-dake peridotite body consists of dunite, harzburgite, and olivine orthopyroxenite, all of which carry a small amount of clinopyroxenes. The original rock of serpentinite in the Eastern Unit is inferred to be composed mostly of dunite and harzburgite, which contain more clinoproxene than the Western Unit. In addition, lherzolite and olivine websterite are also noticeable in the Eastern Unit. Dike-formed microdiorite, quartz albitite, and clinopyroxenite are observed in the ultrabasic rocks of the both units, although these dike rocks, except microdiorite, are predominant in the Eastern Unit. Other massifs composed mainly of serpentinite are known in the northern part of the Kamuikotan Belt (e.g. the Pinneshiri massif, the Horokanai massif). It may be interpreted that these serpentinite massifs represent complexes composed of several different ultrabasic rock units judging from the mode of their distribution.

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Rb and Sr contents and Sr isotopic ratios were determined for the ophiolitic rocks in the Kamuikotan Zone, Hokkaido. The initial ratios of these rocks are asfollows; Sorachi Group basic rocks (0.7019-0.7045), ultrabasic rocks (dunite, harzburgite and serpentinite) (0.7069-0.7113), olivine clinopyroxenites (0.7037-0.7103), microdiorites (0.7039-0.7054), rodingite (0.7038), quartz albitite (0.7048), amphibolitic rocks (0.7032-0.7053), and trondhjemites (0.7028-0.7052). The genetical relationship between the Sorachi Group basic rocks and the associated ultrabasic rocks can be explained by considering them as the complementary products formed by partial melting of certain lherzolitic mantle material, which is composed of various minerals with different Sr contents and Sr isotopic ratios. It seems plausible to consider that the dike rocks such as olivine clinopyroxenite, microdiorite and gabbroic pegmatite were formed from the liquids trapped within the hot residual mantle diapir.

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In the Shizunai-Mitsuishi district, two zones in which geosynclinal basic rocks develop are well known, i.e., the Kamuikotan and Hidaka Western Marginal Zones. No fossils are found in this district, though a few fossils indicating Triassic to Early Cretaceous ages are reported in other districts of the Kamuikotan and Hidaka Western Marginal Zones. In the both zones, original structures such as pillow lava, massive lava, pillow breccia, sheet, dyke, hyaloclastic rocks and reworked volcanic sediments are recognized. However, the discrimination of original rocks in Hidaka Western Marginal Zone is difficult due to effects of shearing. Scarce vesicles are found in the lava in the Hidaka Western Marginal zone. The following volcanic sequence from lower to upper is found at some localities in the Kamuikotan Zone : volcanic breccia (autobrecciated lava ?) → irregular alternation of massive lava and pillow lava → autobrecciated lava or pillow breccia → hyaloclastic rocks and reworked volcanic sediments associated with cherts and shales. The thickness of one cycle mentioned above is several meters in minimum scale. In the Kamuikotan Zone, basic rocks along the Mitsuishi river and Pirashuke river have suffered glaucophane schist facies metamorphism and those along the Futamata river and Menashibetsu river prehnite-pumpellyite metagraywacke facies metamorplism. Therefore, it is estimated that the latter two masses represent the upper horizon, judging from their .lower pressure metamorphism.

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The southern subzone of the Maizuru zone is occupied by an ophiolite suite composed of such members as harzburgite, dunite, pyroxenite, gabbro, and basalt in ascending order. Pillow structure is not developed in the basalts, which is frequently interbedded with mudstone. Distinct sheeted dike complex is absent, and several quartz diorite bodies are present in the lower part of the basalt member. The gabbro member exhibits strong metamorphic characteristics, and mineral parageneses in the ultramafic members are also metamorphic. The upper three members are distributed symmetrically along the axis of a possible isoclinal syncline, but the lower two members occur only in its southern flank. Maximum thickness of the exposed sequence is estimated to be about 6 km. Layering features are pervasive through lower three members, and cryptic layering units have been recognized in the dunite member. Systematic change in the composition of olivine in these members provides another evidence to conclude that these members were originally formed by the igneous fractionation. On the other hand, the composition and its variation trend of the basic rocks indicate ocean floor affinity of the magma from which they were formed. Ferric-poor spinels found in quenched lava flows in the basalt member not only confirm this affinity, but also suggest comagmatic relationship between the mafic and ultramafic members. The ophiolite formed by such igneous processes subsequently suffered metamorphism. Preliminary analysis of mineral parageneses show that basic rocks in the upper, middle and lower parts of the basalt member belong to the prehnite-pumpellyite, greenschist, and epidote amphibolite facies respectively, and that the underlying gabbro member belongs to the amphibolite facies. Furthermore, medium pressure granulite facies condition was attained below the gabbro member. Such a metamorphism may have taken place under a high thermal gradient near the site of the igneous formation of the ophiolite. Thus, it is concluded that the Yakuno ophiolite may be an on-land remnant of an oceanic crustmantle constructed at sediment-available sea floor. This conclusion necessarily requires a new dynamic model involving the formation and deformation of this oceanic crust-mantle to explain the development of the Permo-Triassic orogeny of inner southwest Japan.

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The ophiolite-like complex in the Green-Tuff geosyncline consists of basic effusive and intrusive rocks, and fine grained sedimentary rocks such as siliceous and black mudstones, all of which have been formed along the frontal margin of the initial basin in the geosyncline at the early stage. They may be regarded to correspond to the ophiolite, though the ultrabasic rock is not observed. A scheme on the development of the geosyncline in the Green Tuff region is presented in Fig. 1. Similar scheme can be applied to the Mesozoic and Paleozoic geosynclines in the Japanese islands.

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A short review on the tectonic elements around arc-trench system investigated by many authors is given. A generalized traverse section of arc-trench system of intermediate evolutional stage with Benioff-Wadachi plane inclined at angle of 45° is presented in Fig. 1. Wedge mantle is divided into four zones based on experimental data in the system MgO-SiO2-H2O (GREENWOOD, 1963; YODER, 1967) and the thermal structure of suture zone calculated by OXBURGH and TURCOTTE (1970), assuming that H2O is a perfectly mobile and excess component. Hydrated wedge mantle, caused by dehydration of subducted hydrous plate, penetrates upwards along the thrust and is uplifted associated with some fragments of oceanic crust metamorphosed under the condition of glaucophane schist facies. Among the many thrusts in the arctrench gap two thrusts are noted : one is connected with the trench-slope break and the other extends to the basement high. As the result tectonic blocks of glaucophane schists and enclosing serpentinite appear on the ground surface, forming the dismembered ophiolite belt, which runs parallel to the axis of the trench.

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The green rocks in the Shimanto belt are distributed at many places in the Cretaceous and Paleogene formations in Kyushu and Shikoku. They show a succession of massive basaltic lava, pillow lava, pillow breccia, hyaloclastite, red chert with or without radiolaria, red tuff, and greenish tuff. The following common occurrence is recognized: the green rocks are always conformably surrounded by "pebbly shales", which are intensively sheared and contain pebbles of dismembered sandstones and cherts. Fragments of sandstones in "pebbly shales" are contrasted with sandstones in neighbouring turbiditic layers of fine alternation of sandstones and shales, which are always adjacent to "pebbly shales" with a tectonic contact. Sandstone fragments in "pebbly shales" are lithic arenite type and those in turbiditic layer are "greywacke" type. Therefore, it is conceivable that "pebbly shales" were originally pelagic and deformed to "tectonic melange" after deposition within deep sea realm. Ultramafics are found in the green rocks of the Sakihama body in the Muroto Formation. The green rocks of the Sakihama body are more than 1,000 m in thickness and composed mainly of pillow lava at the upper and middle parts, whereas they consist of dolerite, microgabbro, picritic rocks and serpentinized ultramafic rocks at the lower part. These green rocks are considered to show a kind of "ophiolitic" igneous sequence. Many dykes of dolerites, which were probably feeder channels of surrounding pillow lavas, are found along the Aki River in the Terauchi area, Aki City. They intruded into the "pebbly shales" parallel or oblique to their foliations. Therefore, it is concluded that the green rocks and "pebbly shales" in the Shimanto belt were formed in the same deep sea realm, whereas turbiditic layers of alternated sandstones and shales, which are in tectonic contact with the green rocks and "pebbly shales", were deposited in shallow sea environment of the continental margin. This conclusion will throw light on the geodynamics and developmental mechanism of the "Shimanto geosyncline".

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Geochemical characteristic of Japanese Paleozoic to Mesozoic geosynclinal volcanics, except Hokkaido, are re-examined by using new data of their transition metal contents and other chemical data reported previously. It is revealed that the Japanese geosynclinal volcanics, except those in the Southern Kitakami belt, do not resemble the island-arc tholeiite, and that basaltic volcanics in the Mikabu and the Motai belts seem to be similar to the oceanic tholeiite. The majority of the Southern Kitakami volcanics are comparable with calcalkaline volcanics occurring in island arcs and continental margins of the present-day earth. The geochernical data on the volcanics in the Mikabu and the Motai belts suggest that two belts were located in the axial parts of rift zones like the Red Sea Trough or marginal sea in the Western Pacific Ocean.

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In the Mt.Ufu-san and Mt. Tonmaku-yama area, the Mikabu green rocks and several ultrabasic masses are widely exposed. The Mikabu green rocks (Basic meta-volcanics) form a formation of volcanogenic sediments and volcanic rocks and they are named the Okuyama Formation. They have suffered the glaucophanitic type of metamorphism. Ultrabasic masses consist of five large wehrlitic masses (Yamayoshida, Tsugeno, Kareshuku, Tonmaku-yama and Ufu-san), which are collectively named the Ufu-san Ultrabasic Complexes, and many small serpentinite masses. The wehrlitic masses, with an exception, show the layered structure due to the rock-type arrangement, that is, wehrlite in the lower part, frequent alternation of gabbro and wehrlite in the middle part and hornblendite in the upper part. Some masses show dome-like structure with concentric arrangement of the rock-types. Before the emplacement of the complexes, the submarine basaltic volcanism of the Okuyama Formation had taken place. The formation is divided in terms of the rock association from the lower to ther upper horizon into (a) the Lava-gabbro-rich zone, (b) the Lava-breccia-tuff-rich zone and (c) the Tuff-rich zone. Since gabbroic intrusives and dyke swarms are dominant in the lava-gabbro-rich zone, it is considered that this zone represents the central area of volcanism. Its activity was violent and successive, as non-volcanic sediments are not found in the Okuyama Formation. After the sedimentation of original rocks of the pelitic and/or basic schists (the Shibukawa Formation) overling conformably the Okuyama Formation, the Permian formation of limestone, chert, and shale (the Iinoya Formation) might have been deposited successively. The metamorphic events occurred at or after the emplacement of the ultrabasic complexes, whereas the tectonic movement forming main faults took place after the metamorphic events.

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Concerning to the green rock complex of the Paleozoic geosynclinal domain in the eastern part of Shikoku, the sequence of the igneous events is inferred. The events are divided into 4 stages in the area. The hornblende-clinopyroxene gabbro which occur within the olistostrome as numerous blocks, cobbles and pebbles are considered to be the oldest igneous body in the area. The gabbroic rocks belong to the olistolith and might be fragments of the basement rocks, a part of the oceanic crust. Secondly, the picritic basalts which form a lower hyaloclastite formation were erupted as the product of the initial magmatism on this basement. After the short interruption, the new basaltic lavas were extruded through the lower hyaloclastite formation, producing the upper hyaloclastite formation. These lavas are tholeiitic in chemistry and were differentiated in some extent. Small lenticular masses of ultrabasic rocks, pyroxenite, hornblendite and gabbros which are found within thelower hyaloclastite formation represent the cumulate rocks produced in the small magmapools of this basaltic magma. In addition, the sheeted dyke complexes associated with a lot of lava dykes (lava channels) of this stage are found within the lower hyaloclastite formation. The volcanic sediments are predominant in the Chichibu Zone on the southern side of the Mikabu Zone. These sediments were derivedfrom the Mikabu Green Rock Complex, and were deposited as conglomerate-sand facies grading into shale facies. After the consolidation of these sediments, the alkaline gabbroic rocks of the fourth stage intruded into the sediments.

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