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

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.Doctoral thesis（Science）submitted to Hiroshima University in 1986

Nappe structure of the Asaji metamorphic rocks, with special reference to geological structure of the basement complexes in Kyushu

The Asaji metamorphic rocks in the Notsuharu area, Oita Prefecture are composed mainly of pelitic rock, psammitic rock, chert, basic rock and serpentinite. Thelast forms serpentinite melange zone, including many blocks of metamorphic rocks, and shows an overturned fold of SE vergence with the NE plunging fold axis and NNW dipping axial plane. The metamorphic rocks are divided into two groups, separated from each other by the melange zone. One occupies the horizon below the melange zone and the other above. Carbonaceous materials from 32 pelitic rock samples were examined by TAGIRI'S (1981) X-ray diffraction method in order to compare their metamorphic grade. The results show the distinct discontinuity of metamorphic grade between two groups of both sides of the melange zone. It is concluded that the sequence above the melange zone is a nappe (Hikata nappe), and the SE vergenced overturned fold has been formed during the formation of the nappe structure. The Asaji metamorphic rocks are considered to be long to the Ryoke metamorphic rocks. In Kyushu, apparent continuity of the Sambagawa belt terminate at the Saga-noseki Peninsula just to the east of the Notsuharu area. It appears from the above that the nappe of the Ryoke metamophic rocks such as the Hikata nappe overlies the Sambagawa belt in Kyushu. Incidentally, the Nagasaki metamophic rocks would blong to the Sambagawa metamorphic rocks

P-t Path of Sediment Subduction-Underplating-Exhumation Process Related to the Formation of the Sambagawa Schists

The Valanginian accretionary complex and the Barremian accretionary complex of the Chichibu megaunit I of the Southern Chichibu belt in east Shikoku, which consist of prehnite-pumpellyite facies rocks and overlie the Albian accretionary complex of the Chichibu megaunit I and the Cenomanian-Turonian accretionary complex of the Shimanto megaunit, have been clarified by Hara et al. (1992) to be of the same age with reference to the subduction beginning age (youngest fossil age) as the Saruta nappe (I+II) schists and the Fuyunose nappe schists of the Sambagawa megaunit as high P/T type metamorphic rocks respectively. K-Ar ages of muscovites from the former two accretionary complexes, which are considered to have been roughly comparable with the exhumation beginning age, were determined in this paper to be 114 ± 6Ma and 108 ± 5Ma respectively. The exhumation beginning age appears to have been different by ca. 20Ma between the Chichibu megaunit I of subcretion depth of a few kilobars (less than 4kb) and the Sambagawa megaunit of subcretion depth of ca. 10kb with the same subduction beginning age. It would said that this is a rough estimate of P-t path of sediment subduction-underplating-exhumation process related to the formation of the Sambagawa megaunit

Some problems on Palaeozoic-Mesozoic tectonics inSouthwest Japan: Tectonics of metamorphic belts of high-pressure type

Tectonics of the Sangun belt and Sambagawa belt in Southwest Japan, which belong to the metamorphic belt of high-pressure type, have been discussed in this paper. Regarding the Sangun belt, the tectonics of the phases when the original rocks of the Sangun belt were deposited and the Sangun metamorphic field appeared have been analysed. As for the Sambagawa belt, the tectonics of the phases when the Sambagawa metamorphic field was placed under the condition of the highest temperature and then its collapse began have been analysed

SHRIMP U-Pb ages of the Karasaki mylonites in western Shikoku, Japan

唐崎マイロナイトの優白質角閃岩マイロナイトについて, ジルコンのSHRIMP U-Pb年代測定を行った結果, 1試料からの8粒子は114.2±3.9～102.3±4.5Ma(±1σ)の年代値を示した.マイロナイト化した優白質角閃岩は, SiO_2含有量が低い(49wt.%)ことと, 主として角閃石, 曹長石, 緑泥石および緑簾石のモード比の違いによる数百μm～数mm幅の顕著な層状構造を有することから, 玄武岩質火山岩または火山砕屑岩に由来する砂岩を原岩としている可能性が高い.優白質角閃岩マイロナイト中の丸みを帯びたジルコンは, ある程度遠方より運搬される過程で円磨された可能性が高いこと, マグマから晶出したジルコンに特徴的なオシレトリー累帯構造が認められることから, 砕屑性起源のジルコンであると判断される.また, その年代値が約110Maの狭い範囲を示す事実は, ジルコンが"古領家帯"に帰属すると考えられる真穴帯の大島変成岩や肥後帯の約110Ma深成岩類のみに由来する可能性を示唆する.したがって, 本論において初めて"古領家帯"中にアルビアン以降の堆積岩源変成岩が存在していることが明らかとなった.Zircon U-Pb isotope ages have been analyzed by the Sensitive High Resolution Ion MicroProbe (SHRIMP) in an leucocratic amphibolite mylonite from the Karasaki mylonites, western Shikoku, Japan which has been considered to belong to the "Paleo-Ryoke" belt. Eight zircon grains from the sample yielded U-Pb ages of Albian (Early Cretaceous) ranging from 114.2 ± 3.9 Ma to 102.3 ±4.5 Ma (± 1σ errors). The protolith of the mylonitized leucocratic amphibolite has been inferred to be basaltic volcanic sandstones, because the SiO_2 content is low (49 wt.%), and the leucocratic amphibolite (consisting of albite and epidote rich, and chlorite and epidote rich layers alternates with amphibole rich and chlorite rich layers on the order of a few mm, suggestive of original sedimentary layering. All the zircon grains can be inferred to be detrital based on both the facts that they were abraded possibly due to sedimentation processes, and that they show oscillatory zoning indicating igneous (primary) texture. The small range of U-Pb ages varying ca. 110 Ma suggests that the zircon grains in the volcanic sandstones were supplied only from the Oshima metamorphic rocks of the Maana belt and the Higo plutonic rocks of the Higo belt whose U-Pb zircon ages were dated as ca. 110 Ma. The fact clearly indicates that the metamorphic rocks (Karasaki mylonites) originated from post-Albian sediments also constitute as a new member of the "Paleo-Ryoke" belt

Tectonic Evolution of the Sambagawa Schists and its Implications in Convergent Margin Processes

The Sambagawa schists as high P /T metamorphic rocks are a member of Mesozoic accretionary complexes developed in the southern front of the Kurosegawa-Koryoke continent of Southwest Japan. The Mesozoic accretionary complexes are divided into four megaunits developed as nappes, Chichibu megaunit II, Sambagawa megaunit, Chichibu megaunit I and Shimanto megaunit in descending order of structural level. The Chichibu megaunit II consists of three accretionary units developed as nappes, late early Jurassic unit, late middle Jurassic unit and latest Jurassic unit (Mikabu unit) in descending order of structural level. The Chichibu megaunit I consists of five accretionary units developed as nappes, late middle Jurassic unit (Niyodo unit), late Jurassic unit, Valanginian unit, Barremian unit and Albian unit in descending order of structural level. The Shimanto megaunit, which just underlies the Chichibu megaunit I, is Cenomanian-Turonian accretionary unit and Coniacian-Campanian accretionary unit. The schists, which underlie the Chichibu megaunit II, all have been so far called the Sambagawa schists. These are divided into six units, Saruta unit, Fuyunose unit, Sogauchi unit, Sakamoto unit, Oboke unit and Tatsuyama unit in descending order of structural level, which show different tectono-metamorphic history and different radiometric ages from each other. The Sakamoto unit, Oboke unit and Tatsuyama unit have been assumed with reference to their radiometric ages and structural relations to belong to the late middle Jurassic accretionary unit of the Chichibu megaunit I (high pressure equivalent of the Niyodo unit), the Cenomanian-Turonian accretionary unit of the Shimanto megaunit and the Coniacian-Campanian accretionary unit of the Shimanto megaunit respectively in this paper. The upper member of the Sambagawa schists, Saruta unit, Fuyunose unit and Sogauchi unit, is therefore called the Sambagawa megaunit in this paper. The northern half and the southern half of the Sambagawa megaunit are intercalated as nappes between the Chichibu megaunit II and the Oboke unit and between the Chichibu megaunit II and the Sakamoto unit respectively. The constituent units of the Chichibu megaunit II, Sambagawa megaunit and Shimanto megaunit clearly show a downward younging age polarity, as compared with each other with reference to the oldest one of radiometric ages ( = metamorphic ages) of each unit. The Chichibu megaunit II and the Chichibu megaunit I show the same radiometric ages as compared between them with the same fossil age. The Saruta unit, Fuyunose unit and Sogauchi unit have therefore been assumed to be high pressure equivalent of Valanginian unit, that of Barremian unit and that of Albian unit of the Chichibu megaunit I respectively. These high pressure units were exhumed, separating the Chichibu supermegaunit into the Chichibu megaunit II and the Chichibu megaunit I and thrusting up onto the Chichibu megaunit I. On the basis of the growth history of amphibole in hematite-bearing basic schists of the Sambagawa megaunit, it has been assumed that the highest temperature metamorphism of the Fuyunose unit occurred, when it had been coupled with the Saruta unit which was exhuming, and that of the Sogauchi unit did through its coupling with the Fuyunose and Saruta units which were exhuming. In the subduction zone which was responsible for the formation of the Sambagawa megaunit, namely, the peak metamorphism of a newly subducted unit appears to have occurred when it had been coupled with previously subcreted units which were exhuming. It has been also clarified that the subduction of a new unit occurred mixing the lower pressure part of the pre-existing subcretion unit as tectonic blocks. There is a distinct difference in the oldest one of radiometric ages between constituent units of the Sambagawa schists, showing a downward younging age polarity. The oldest one of radiometric ages of each unit appears to approximate to the age of the ending of peak metamorphism and to the age (Eh age) of the beginning of its exhumation. Such the tectonics of the Sambagawa megaunit would be explained in term of two-way street model. Because the age (Sub age) of the beginning of the subduction of each unit can be assumed from its fossil age, the average velocity of the subduction and that of the exhumation of the Sambagawa megaunit in Shikoku have roughly been estimated to be ca. 0.9 mm/year and ca. 2.0 mm/year respectively. Deformation of quartz, whose style depends strongly upon strain rate, resulted in type I crossed girdle without conentration in Y even in the depth part of more than 10kb of the subduction zone, which was placed under temperature condition of much higher than 500°C, unlike the cases of magma-arcs where quartz c-axis fabrics with maximum concentration in Y are found in gneisses produced under temperature condition of lower than 500°C. Quartz deformation in the depth part of 15-17kb of the subduction zone appears to have occurred as dominant prism slip. The hanging wall of the Kurosegawa-Koryoke continent, which was placed at the depth of ca. 15-17 kb, thrust onto the Saruta unit at the depth of ca. 10-11 kb, accompanying intermingling of constituent rocks of the former and the latter and also mixing of various depth parts of the latter. The highest temperature metamorphism of the Saruta unit, which appears to have occurred under metamorphic condition of lower P /T than under that related to the formation of the general type of high P /T type metamorphic rocks, is ascribed to a contact metamorphism related to the overthrusting of the Kurosegawa-Koryoke continent. The thrusting of the Kurosegawa-Koryoke continent is ascribed to its collision with the Hida continent. The coupling of the previously subcreted Saruta unit with the newly subcreted Fuyunose unit occurred accompanying nearly isobaric cooling of the former. The great exhumation of the Saruta nappe (I + II) and Fuyunose nappe schists with great volume began together with the subcretion of the Sogauchi unit. The beginning age of the exhumation of the Sambagawa schists with great volume appears to coincide with that of the subduction of the Kula-Pacific ridge in Kyushu-Shikoku, which has been assumed by Kiminami et al. (1990). Namely, their great exhumation occurred with the progress of the subduction of the Kula-Pacific ridge with an eastward younging age polarity. The exhumation units, which were developed after the Mikabu unit, clearly show an eastward younging age polarity. Namely, these comparable with the Saruta unit, Fuyunose unit and Sogauchi unit are not found in central Japan and the Kanto Mountains. Rock deformation in the deformation related to the exhumation of the Sambagawa schists and their underlying schists appears to have commonly been of flattened type in mean strain. During the Ozu phase when the Kula-Pacific ridge subducted to the greater depth, the collapse of the Kurosegawa-Koryoke continent took again place, accompanying that of the pile nappe structures of the Sambagawa megaunit, Chichibu megaunit I and Oboke unit, and the thermal gradient along the plate boundary greatly changed, giving rise to medium P/T type metamorphism in the subduction zone (formation of the Tatsuyama nappe schists). The geological structures of the Sambagawa megaunit consist thus of two types of pile nappe structures, pre-Ozu phase pile nappe structures and Ozu phase pile nappe structures. The former is structures related to the coupling of the exhuming units ( = previously subcreted units) with the newly subcreted unit. The latter is structures showing the collapse of the former. The Ozu phase pile nappe structures are further divided into the pile nappe structures formed during the earlier stage (Tsuji stage) of the Ozu phase and these formed during the later stage (Futami stage). The former is disharmonic with reference to movement picture with the latter: The deformation related to the formation of the former, accompanying exhumation of the Oboke nappes, appears to contain a component of northward displacement, while that for the latter does a component of southward displacement. After the Ozu phase deformation the Sambagawa megaunit suffered the Hijikawa-Oboke phase folding, forming a series of sinistral en echelon upright folds. The relationship between the above-mentioned tectonic events of the Sambagawa megaunit and its surroundings and their radiometric ages is summarized as follows: [Original table is skipped. For more details, please refer to the full text.

Missing western half of the Pacific Plate: Geochemical nature of the Izanagi-Pacific Ridge interaction with a stationary boundary between the Indian and Pacific mantles

The source mantle of the basaltic ocean crust on the western half of the Pacific Plate was examined using Pb–Nd–Hf isotopes. The results showed that the subducted Izanagi–Pacific Ridge (IPR) formed from both Pacific (180–∼80 Ma) and Indian (∼80–70 Ma) mantles. The western Pacific Plate becomes younger westward and is thought to have formed from the IPR. The ridge was subducted along the Kurile–Japan–Nankai–Ryukyu (KJNR) Trench at 60–55 Ma and leading edge of the Pacific Plate is currently stagnated in the mantle transition zone. Conversely, the entire eastern half of the Pacific Plate, formed from isotopically distinct Pacific mantle along the East Pacific Rise and the Juan de Fuca Ridge, largely remains on the seafloor. The subducted IPR is inaccessible; therefore, questions regarding which mantle might be responsible for the formation of the western half of the Pacific Plate remain controversial. Knowing the source of the IPR basalts provides insight into the Indian–Pacific mantle boundary before the Cenozoic. Isotopic compositions of the basalts from borehole cores (165–130 Ma) in the western Pacific show that the surface oceanic crust is of Pacific mantle origin. However, the accreted ocean floor basalts (∼80–70 Ma) in the accretionary prism along the KJNR Trench have Indian mantle signatures. This indicates the younger western Pacific Plate of IPR origin formed partly from Indian mantle and that the Indian–Pacific mantle boundary has been stationary in the western Pacific at least since the Cretaceous

舞鶴構造帯に沿う大規模横ずれテクトニクスの研究

研究期間:平成17-19年度 ; 研究種目:基盤研究C ; 課題番号: 17540436p.58までを登録、原著には他論文を含む

Genesis and evolutional processes of the Paleozoic oceanic island arc crust, Asago body of the Yakuno Ophiolite, Southwest Japan

兵庫県朝来地域には，古生代海洋内島弧の中～下部地殻に由来する夜久野オフィオライト朝来岩体が露出する．本研究では夜久野古島弧の基盤地殻を構成する苦鉄質岩類の起源について，そしてSuda（2004）が報告したミグマタイトの産状分類に基づいた島弧花崗岩類の形成過程について，地球化学的手法を用いたモデル計算を行い検討した．結果，以下のことが示された．朝来岩体では背弧盆地殻に由来した基盤地殻が部分融解し，珪長質メルトが形成された．これら珪長質メルトは集積，上昇する過程で結晶分化しながらマグマへと成長し，キュームレイトに相当するものが主にミグマタイト中のリューコソムとして下部地殻相当域に，一方，残液に相当するものが岩脈，岩床として主に中部地殻相当域に残された．朝来岩体には，島弧下部地殻の部分融解過程と，島弧花崗岩質マグマの形成と発達による地殻の安山岩質化過程を示すまさにその現行段階が残されている．A middle to lower crustal section of Paleozoic oceanic island arc is exposed in the Asago body of the Yakuno ophiolite. Based on field occurrence, petrography, and geochemical modeling, we investigated the evolution of the Asago body and relevant magmatic processes. The Asago body consists of two stages of rocks. The first-stage rocks consist of metagabbro and schistose amphibolite that represent the basement to the Permian Yakuno paleo-island arc. The second-stage rocks are mainly arc granitoids that intrude the first-stage rocks. Mafic migmatites occur in the lower crustal section of the Asago body. Field occurrences and petrographic data suggest that the migmatites formed by the anatexis of first-stage rocks, and that segregation and accumulation of the anatectic melt resulted in morphological changes in the migmatites toward the middle crustal section. Geochemical data indicate that the first-stage rocks were derived from a basaltic magma of back-arc affinity, suggesting in turn that the Yakuno paleo-island arc was developed within a back-arc basin. Moreover, a low-K series (hornblende gabbro, quartz diorite, and tonalite) and high-K series (quartz monzodiorite and granodiorite) within the second-stage rocks were generated by high and low degrees of partial melting of first-stage rocks, respectively. We conclude that the Asago body is an example of the transition from oceanic to continental crust, related to the anatexis of mafic lower crust in an oceanic island arc setting.本研究は，本論文の筆頭執筆者が広島大学大学院ならびに九州大学に在籍中に行った研究の一部を発展させたものである