Subsurface Structure of Miocene Large-Scale Caldera Cluster: Illustrated Descriptions of Geology of the Okueyama Volcano-Plutonic Complex, Southwest Japan

日本大学文理学部自然科学研究所研究紀要 No.49 （ 2014） pp.197 － 230

Subsurface Structure of Miocene Large-scale Caldera Cluster: Illustrated Descriptions of Geology of the Okueyama Volcano-plutonic Complex, Southwest Japan

Masaki TAKAHASHI

（Received November 16, 2013）

The geology of the Okueyama volcano-plutonic complex is described by showing a lot of photographs taken in the field. The subsurface structure of the cauldron cluster is well exposed in the steep mountainous land with deep valleys. Good exposures of lavas and welded tuffs filled the cauldron make it possible to observe the internal structure of the cauldrons. The intrusions, such as small stocks of granitic rocks and dikes of tuffisite, felsites and granite porphyry, feeding the eruptive products, show the close relationships between the volcanic rocks and plutonic rocks. The roof and wall contacts of the large-scale batholithic granite is well exposed and the three dimensional shape of a granitic batholith is well preserved. The internal structure of the solidified fossil large-scale magma chamber can be observed along the section about 1,000m from the roof.

Keywords: volcano-plutonic complex, cauldron, welded tuff, composite volcano, lava, ring-dike, batholithic granite

yama volcano-plutonic complex will be described by 1. Introduction showing field evidences, namely abundant photographs The Okueyama volcano-plutonic complex is a member taken in the field. of the igneous complexes of the middle Miocene Setou- chi-Outer Zone magmatic belt (Fig.1), which was active 2. Geology of the Okueyama volcano-plutonic during a short time-span around 14Ma (Shibata,1978). complex The Okueyama volcano-plutonic complex is located in the The Okueyama volcano-plutonic complex comprises six northeastern part of the central Kyushu, Southwest Ja- main stages: (1) the older cauldrons (the Sobosan and pan, straddling the border between the Miyazaki and Oita Katamukiyama cauldrons) with voluminous densely weld- prefectures, which exhibits the subsurface structure of a ed dacitic to rhyolitic pyroclastic flow deposits and lavas, middle Miocene large-scale caldera cluster (Okumura et (2) older granitoids I forerunning the intrusion of batho- al., 1998; Murao and Matsumoto,1992; Takahashi, 1986; lithc granite (3) the caldera-filling flat-shaped composite Ono et al.,1977; Matsumoto and Miyahisa, 1973). The volcanoes with andesitic to dacitic lavas, pyroclastic and Okueyama volcano-plutonic complex is situated in the dis- volcaniclastic deposits, filling the caldera depressions, (4) sected mountainous land called the Sobosan-Okueyama older granitoids II probably related to the caldera-filling mountain range with an area of about 30×40km and alti- composite volcanoes, and (5) the younger cauldron (the tudes of about 1,300 to 1,700m; excellent exposures along Okueyama cauldron) with voluminous densely welded the deep valleys and steep slopes and cliffs of the moun- rhyolitic pyroclastic flow deposit and a ring dike of granite tains display good views of the subsurface structure of porphyry accompanying felsite and tuffisite, and (6) final the caldera cluster. In this paper, the geology of the Okue- intrusion of granite with a batholithic dimension.

Department of Geosystem Sciences, College of Humanities and Sciences, Nihon University: 3-25-40 Sakurajosui, Setagayaku, Tokyo 156-8550, Japan

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Fig. 1 Map showing the location of Okueyama volcano-plutonic complex. The Okueyama volcano-plutonic complex belongs to the middle Miocene Setouchi-Outer Zone magmatic belt. Numerals denote K-Ar ages (Ma).

2-1. Older cauldrons and related volcanic rocks of densely welded pyroclastic flow deposits with total (1) Basement rocks thickness of exceeding 570m, intercalating three thick The basement rocks of Okueyama volcano-plutonic dacitic lava flows with thickness of about 30 to 85m. The complex are the sedimentary complex of Cretaceous Shi- bottom of SDT inside of Sobosan cauldron is directly in- manto super-group and Mesozoic Sanbosan and Chichibu truded by the batholithic granite. The estimated total super-groups, which were the members of Cretaceous to eruptive volume of Sobosan dacitic pyroclastic flow depos- Jurassic accretionary sedimentary complex. The Miocene its and related lavas probably exceeds about 300km3 DRE Mitate formation consisting of sandstone and conglomer- (Takahashi, 1986). ate overlie them with an angular unconformity, indicating that the sedimentary basin was present just before the (3) Katamukiyama cauldron and related volcanic rocks. volcanic activity of Okueyama volcano-plutonic complex, After the formation of Sobosan cauldron, a thick lava but it was uplifted rapidly to form terrestrial land before flow of aphyric rhyolite with maximum thickness of about the onset of volcanic activity. 300m effused and spread over the wide area of eastern portion outside of Sobosan cauldron (Photos. 8 to11), (2) Sobosan cauldron and related volcanic rocks however, which is not observed in the boring core drilled The magmatic activity of Okueyama volcano-plutonic inside of the Sobosan cauldron. The aphyric rhyolitic lava complex began with the eruption of Sobosan daicitic pyro- (the Katamukiyama aphyric rhyolite: KRL) spread only clastic flow deposit (SDT). The Sobosan cauldron (20× outside of the Sobosan cauldron, but it must have distrib- 13km) was collapsed by the eruption of voluminous dacit- uted inside of the Katamukiyama cauldron because the ic magma. The out-flow sheet of SDT is composed of a KRL is cut by the ring fault encircled the Katamukiyama simple cooling unit and a single flow unit with maximum cauldron. The estimated eruptive volume of KRL exceeds thickness of about 150m (Photos. 1 to 7). The drilling per- about 60km3 (Takahashi,1986). formed in the Sobosan cauldron reveals that the SDT in- The lava plateau of KRL is overlain by the Katamukiya- side of Sobosan cauldron consists of more than six units ma dacitic pyroclastic flow deposits (KDT) consisting of

（ 154 ） ─ 198 ─ Subsurface structure of Miocene large-scale caldera cluster: Illustrated descriptions of geology of the Okueyama volcano-plutonic complex, Southwest Japan densely welded tuff without remarkable erosional interval lowest part of SDT. The bottom boundary is inclined, (Photos. 12 to14). The KDT comprises at least three ma- showing that the pyroclastic flow deposit is valley filling. jor units. The lower unit consists of five flow units of Photo.4 Columnar cooling joints of the densely welded densely welded tuff with abundant lapilli of lithic frag- Sobosan dacitic pyroclastic flow deposit near the base- ments, the thickness of which is 10 to 50m. The middle ment (the Takahatadani valley in Mitate district, at 800m member is composed of three flow units of densely weld- in altitude). ed tuff characterized by abundant crystal fragments with Photo.5 Columnar cooling joints of the densely welded thickness of 15 to 75m. The upper member comprises two Sobosan dacitic pyroclastic flow deposit near the base- flow units of densely welded tuff with abundant lapilli of ment (the Takahatadani valley in Mitate district, at 800m lithic fragments, whose thickness is about 100m. in altitude). The KRL and KDT are absent in the boring core drilled Photo.6 Extremely flattened essential lenses or fiannme inside of the Katamukiyama cauldron; the Sobosan andes- in the densely welded Sobosan dacitic pyroclastic flow de- itic to dacitic lavas and pyroclastic to volcaniclastic rocks posit (the Nakantani valley in Mitate district, at 1,060m in are directly intruded by batholithic granite. The estimated altitude). total volume of KDT exceeds about 360km3 DRE (Taka- Photo.7 Essential lenses or fiannme in the densely hashi,1986). The Katamukiyama cauldron (12×6km) welded Sobosan dacitic pyroclastic flow deposit (the Ku- which is presently filled with andesitic to dacitic lavas and wazurudani valley in Mitate district, at 700m in altitude). pyroclastic to volcaniclastic rocks was probably formed by The densely welded lava-like welded tuff is thermally the eruption of voluminous KDT. metamorphosed by the intrusion of batholithic granite to hornfels and shows purple-colored appearance. 2-2. Explanations for photographs of the older cauldrons and related rocks (Photos. 1 to 14) (2) Katamukiyama aphyric rhyolitic lava and Katamuki- (1) Sobosan dacitic pyroclastic flow deposit yama dacitic pyroclastic flow deposit Photo.1 View of the volcanic edifices erupted from the Photo.8 Folded flow banding in the Katamukiyama older cauldrons, constructing Mt.Katamukiyama (1,605m) aphyric rhyolitic lavas (lithoidite) (the Kuwazurudani val- (the Kuwazurudani valley in Mitate district, at 700m in al- ley in Mitate district, at 750m in altitude). titude). The foreground cliff comprises a thick pile of Photo.9 Folded flow banding in the Katamukiyama densely welded Sobosan dacitic pyroclastic flow deposit aphyric rhyolitic lavas (lithoidite) (the Kuwazurudani val- (SDT). The SDT is overlain by the Katamukiyama aphyric ley in Mitate district, at 750m in altitude). rhyolitic lava (KRL). The KRL is further overlain by the Photo.10 Flow banding in the Katamukiyama ahyric Katamukiyama dacitic pyroclastic flow deposits (KDT), rhyolitic lava (lithoidite) (the Kuwazurudani valley in Mi- constituting the summit of Mt. Katamukiyama. tate district, at 750m in altitude). Photo.2 Bottom of the densely welded Sobosan dacitic Photo.11 Brecciated fragments of the Sobosan dacitic pyroclastic flow deposit (SDT), which is settled on the densely welded pyroclastic flow deposit included in the bedded sandstone of Mitate formation (M) (the Kuwazur- basal part of the Katamukiyama aphyric rhyolite lava (the udani valley in Mitate district, at 670m in altitude). The Kuwazurudani valley in Mitate district, at 730m in altitude). SDT and sandstone are thermally metamorphosed by the Photo.12 Densely welded Katamukiyama dacitic pyro- intrusion of batholithic granite. Aplite veins derived from clastic flow deposit including abundant angular fragments the undersurface batholithic granite intrude both the of the Katamukiyama aphyric rhyolite and densely welded sandstone and SDT. Sobosan dacitic pyroclastic flow deposit (near the summit Photo.3 Bottom of the densely welded Sobosan dacitic of Mt. Katamukiyama, at 1,450m in altitude). pyroclastic flow deposit, settled on the sandstone of Mi- Photo.13 Densely welded Katamukiyama dacitic pyro- tate formation (M) (the upper reaches of Hinokagegawa clastic flow deposit including abundant angular fragments river, at 650m in altitude). Platy joints parallel to the bot- of the Katamukiyama aphyric rhyolite and densely welded tom surface (indicated by an arrow) are developed in the Sobosan dacitic pyroclastic flow deposit (near the

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Mt.Katamukiyama, at 1,400m in altitude). The matrix is itic to dacitic composite volcanoes (Takahashi et al.,2014 black and glassy. in press). The older granitoids are intruded by the felsite Photo.14 Small essential lenses are scattered in the ma- dike and batholithic granite. trix of densely welded Katamukiyama dacitic pyroclastic flow deposit with abundant lithic fragments (the Shirozu- 2-4. Explanations for photographs of the caldera- dani valley in Shirozu district, at 700m in altitude). filling andesitic composite volcanoes and related intrusions (Photos. 15 to 34) 2-3. Caldera-filling andesitic composite volcanoes and (1) Sobosan andesitic composite volcanoes related intrusions. Photo.15 Katamukiyama cauldron filled with andesitic Andesitic to dacitic magmas erupted to form flat shaped lavas and pyroclastic to volcaniclastic rocks (viewed from composite volcanoes consisting of thick lavas and pyro- the Uwahata district). The Mt. Katamukiyama outside of clastic to volcaniclastic rocks, which probably filled the Katamukiyama cauldron comprises the densely welded topographic depression inside of the Sobosan and Katamukiyama dacitic pyroclastic flow deposit, Katamuki- Katamukiyama cauldrons. (Photos. 15 to 32). The distri- yama aphyric rhyolitic lava and densely welded Sobosan bution of andesitic lavas and pyroclastic to volcaniclastic dacitic pyroclastic flow deposit downwards from the top. rocks is mostly restricted to inside of the cauldrons. At Photo.16 Thick lavas and pyroclastic rocks comprising first, porphyritic andesitic lavas and pyroclastic rocks the Sobosan andedsitic composite volcano erupted in the erupted and filled the western half of Sobosan cauldron Sobosan cauldron (the upper reaches of Torokugawa riv- (the Sobosan andesitic composite volcano 1 (SACV1)). er, at 1,000m in altitude). The thickness of upper lava of Next, aphyric andesitic lavas and pyroclastic rocks ef- aphyric andesite is about 50m. AA: aphyric andesitic lava; fused and filled the eastern half of Sobosan cauldron and PA: porphyritic andesitic lava the western half of Katamukiyama cauldron (the Sobosan Photo.17 Thick lavas and pyroclastic to volcaniclastic andesitic composite volcano 2 (SACV2)). Then, porphyrit- rocks comprising the Sobosan andedsitic to dacitic com- ic andesitic lavas and pyroclastic rocks erupted in the posite volcano erupted in the Sobosan cauldron (the up- eastern half of Katamukiyama cauldron (the Sobosan an- per reaches of Torokugawa river, at 1,000m in altitude). desitic composite volcano 3 (SACV3)), overlaying the AA: aphyric andesitic lava; PA: porphyritic andesitic lava; aphyric andesitic volcanic rocks of the SACV2. At last, PC: pyroclastic and volcaniclastic rocks dacitic lavas and pyroclastic rocks emplaced on both the Photo.18 Columner joints of an andesitic lava flow (the SACV1 and SACV2. upper reaches of Uenogawa river, at 1,150m in altitude). The pyroclastic to volcaniclastic rocks are composed of Photo.19 Lava of aphyric andesite (the Ogochidani val- pyroclastic flow deposits, hyaloclastites or autobrecciated ley in Uwahata district, at 800m in altitude). lavas, and secondary volcaniclastic lake deposits. The oc- Photo.20 Dacitic block and ash flow deposit, the matrix currences of hyaloclastites to autobrecciated lavas and of which is densely welded (the southeastern flank of volcaniclastic lake deposits show the presence of small Mt.Shojidake, at 1,100m in altitude). lakes in the caldera depression at the eruption. Photo.21 Lapilli tuff probably produced by the pyroclas- The estimated total volume of Sobosan andesitic to tic flow. Clasts of aphyric andesite and fragments of volca- dacitic composite volcanoes exceeds about 210km3 DRE nic rocks related to the older cauldrons are included in (Takahashi,1986). the matrix (the upper reaches of Uenogwa river, at 850m The older granitoids II comprises small stocks of in altitude). quartz monzodiorite and granophyre. The OBG2-1 is a Photo.22 Fragment of the Katamukiyama aphyric rhyo- compositionally zoned stock, in which the marginal lite in the lapilli tuff, which includes a fragment of densely quartz mozodiorite is intruded by the central granophyre. welded tuff of the SDT (the upper reaches of Torokugawa The quartz monzodiorite is similar to andesite and grano- river, at 1,000m in altitude). phyres to dacite in chemical compositions, suggesting Photo.23 Autobrecciated lava of andesite, which was that the older granitoids are the feeder of Sobosan andes- probably subaqueous (the upper reaches of Kawachigawa

（ 156 ） ─ 200 ─ Subsurface structure of Miocene large-scale caldera cluster: Illustrated descriptions of geology of the Okueyama volcano-plutonic complex, Southwest Japan OKG3 BTL FKG Katamukiyama cauldron OMG OKG1-2 Okueyama cauldron HNG2 OBG1 HNG1 UHG 10km Sobosan cauldron 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Fig. 2 Geologic map of the Okueyama volocano-plutonic complex. 1: batholithic granite; 2: granite porphyry; 3: felsite; 4: tuffisite; 5: Kunimidake rhyolitic pyroclastic flow deposit; 6: older granitoids I and II; 7: Sobosan dacitic lava; 8: Sobosan aphyric andesitic lava; 9: Sobosan porphyritic andesitic lava; 10: Sobosan andesitic to dacitic pyroclastic and volcaniclastic rocks; 11: Katamukiyama dacitic pyroclastic flow deposit (abundant in crystal fragments); 12: Katamukiyama dacitic pyroclastic flow deposit (abundant in lithic fragments); 13: Katamukiyama aphyric rhyolitic lava; 14: Sobosan dacitic pyroclastic flow deposit; 15: faults; OBG1: Obira granite 1; OMG: Okumura granite; HNG1:Hinokage granite 1; HNG2: Hinokage granite 2; FKG: Fujikochi granite; OKG1-2: Okueyama granite 1-2; OKG3: older granitoids II (hyperthene biotite granodiorite); BTL: Butsuzo tectonic line

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river, at 650m in altitude). tact. Photo. 24 Stratified volcaniclastic lake deposit, which Photo.33 Polarizing photomicrograph of quartz monzo- was thermally metamorphosed to hornfels (the upper diorite (crossed polar). The width of photograph is about reaches of Torokugawa river, at 950m in altitude). 0.1cm. Photo. 25 Closed view of the lake deposit consisting of Photo.34 Polarizing photomicrograph of granophyre an alternation of coarse and fine grained volcaniclastic (crossed polar). The width of photograph is about 0.1cm. tuffaceous rocks (the upper reaches of Torokugawa river, at 950m in altitude). 2-5. Older granitoids I Photo. 26 Volcaniclastic bed is overlain by andesitic py- The older granitoids I (OKG3) comprising hyperthene roclastic flow deposit. The upper surface of volcaniclastic biotite granodiorite emplaced at the upper reaches of bed is irregular, showing it was soft when the pyroclastic Horigawa river near the Horigawa dam, which is intruded flow deposit was settled. It is suggested that the pyroclas- and thermally metamorphosed by the batholithic granite tic flow deposit was subaqueous and a member of the lake (Photos. 57 to 61). The mineral assemblage and chemical sediments (the northwestern flank of Mt.Yurugiyama, at composition of older granitoids I are similar to the S-type 600m in altitude). mafic granites, and it is suggested that the magmatic ac- Photo.27 Volcaniclastic lake deposit comprising volca- tivity of older granitoids I is closely related to the dacitic nic sandstone and lapilli tuff (the Ogochidani valley in pyrclastic flow deposits erupted from the older cauldrons. Uwahata district, at 500m in altitude). The stage of emplacement of older granitoids I is ambigu- Photo.28 Section of the Sobosan andesitic to dacitic ous, but the chemical characteristics of older granitoids I composite volcano viewed from the Obira. The peak at suggest that its activity is closely related to the formation righthand is the summit of Mt.Sobosan (1,758m). The vol- of older cauldrons (Takahashi et al.,2014 in press). canic and basement rocks are intruded by the stocks of quartz monzodiorite (QMD) and granophyre (OBG2), 2-6. Explanations for photographs of the older which is probably the feeder of andesitic to dacitic com- granitoids I (Photo.57 to 61) posite volcano, felsite dikes and batholithic granites Photo.57 Older granitoids I (OKG3) occurs near the (OBG1). wall contact of batholithic granite (near the Horigawa Photo.29 Peak of the Mt.Sobosan consisting of the So- dam). Gr: batholithic granite; OG: older granitoids I bosan andesitic to dacitic composite volcano (from the (OKG3) Mt.Shojidake, at 1,700m in altitude). Photo.58 Xenoliths of pelitic to psamitic sedimentary Photo.30 The Sobosan mountains comprising the Sobo- rocks of the Shimanto super-group in the older granitoids san andesitic to dacitic composite volcano (along the I (OKG3) consisiting of hypersthene biotite granodiorite Okutakegawa river). Both the volcanic and basement (the Horigawa, at 350m in altitude). rocks are intruded by the batholithic granites (Gr) and Photo.59 Xenoliths of pelitic to psamitic sedimentary other intrusions. rocks of the Shimanto super-group in the older granitoids I (OKG3) (the Horigawa, at 350m in altitude) (2) Older granitoid II Photo.60 Polarizing photomicrograph of hypersthene Photo.31 Volcanic rocks of the Sobosan andesitic to (OPX) biotite (Bi) granodiorite (OKG3) (plane polar). dacitic composite volcano are intruded by the older gran- The width of photograph is about 0.1cm. itois II (quartz monzodiorite and granophyre) (OG) (the Photo.61 Polarizing photomicrograph of hypersthene upper reaches of Okutakegawa river, at 600m in altitude). biotite granodiorite (OKG3) (crossed polar). The width of Photo.32 Contact boundary between the marginal quatz photograph is about 0.1cm. monzodiorite (dark colored) (QMD) and central granophyre (light colored) (GRP) (the upper reaches of Okutakegawa river, at 700m in altitude). The quartz monzodiorite is intruded by the granophyre with sharp con-

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2-7. Younger cauldron and related volcanic rocks and tic flow deposit, which means the Kunimidake rhyolitic intrusions. pyroclastic flow deposit comprises several different flow (1) Okueyama cauldron and related volcanic rocks and cooling units (the upper reaches of Kawachigawa river, The younger cauldron (32×23km), named the Okueya- at 650m in altitude). ma cauldron, was oval shaped and constructed by the eruption of voluminous Kunimidake rhyolitic pyroclastic (2) Ring dike of granite porphyry and related dikes flow deposit (KRT). The KRT are densely welded and Photo.39 Felsite dike (FS) intruding the Katamukiyama now distributed in the western part of Sobosan cauldron, aphyric rhyolitic lava (KRL) (near the Kuwazurudani val- overlying the western foot of Sobosan andesitic to dacitic ley in Mitate district). composite volcanoes (Photos. 35 to 38). The Sobosan and Photo.40 Boundary between the felsite dike and sand- Katamukiyama cauldrons were subsided again when the stone of the Shimanto super-group (the Moritanidani val- Okueyama cauldron was depressed. The estimated total ley in Tobiishi district). volume of KRT exceeds about 370km3 DRE (Takahashi, Photo.41 Boundary between the felsite dike (FS) and 1986). batholithic granite (Gr) at the roof contact (the upper reaches of Utouchikotani valley in Kamishishikawa district, (2) Ring dike of granite porphyry and related dikes at 1,300m in altitude). Linear flow bandings are devel- A ring dike and associated dikes of granite porphyry oped in the felsite. were intruded along the margin and fractures inside Photo.42 Composite dike of felsite and tuffisite (the Okueyama cauldron (Fig.2) (Photos. 39 to 56). The intru- Moritanidani valley in Tobiishi district). A thick arrow in- sion of felsite (ahyric rhyolite) and tuffisite was followed dicates the contact boundary between the composite dike by the granite porphyry forming a ring dike and associat- and host sandstone of Shimanto super-group. The felsite ed dikes; the intrusion of felsite preceded that of tuffisite. is intruded by the grey tuffisite (T1) and the grey tuffisite The tuffisite is probably the feeder of KRT. The tuffisite is intruded by the black tuffisite (T2). intruded at least twice, indicating that the events of pyro- Photo.43 Felsite (FS) is intruded by the black tuffisite clastic eruptions were occurred at least twice from the (T2) (the Moritanidani valley in Tobiishi district). The dikes. boundary between the felsite and black tuffisite is irregular and the felsite is fractured to angular fragments. These 2-8. Explanations for photographs of the Younger occurrences suggest that the gas coring by the mixture of cauldron and related volcanic rocks and tuff and volcanic gas contributed to form this composite intrusions (Photo.35 to 56) dike. (1) Kunimidake rhyolitic pyroclastic flow deposit Photo.44 Contact between the tuffisite (T) and sand- Photo.35 Densely welded tuff of the Kunimidake rhyo- stone of Shimanto super-group (arrow) (the Moritanidani litic pyroclastic flow deposit (KRT) (the eastern flank of valley in Tobiishi district). The contact is irregular, sug- Mt.Kunimidake, at 800m in altitude). The KRT is com- gesting that the wall rocks were eroded by the gas-coring posed of the two different flow and cooling units. of tuffs. Photo.36 Columnar cooling joints of the densely welded Photo.45 Felsite (FS) is intruded by the tuffisite (T) tuff of Kunimidake rhyolitic pyroclastic flow deposit (the and included as fragments (the Moritanidani valley in To- upper reaches of Kawachigawa river, at 780m in altitude). biishi district). The boundary is irregular and somewhat Photo.37 Fragment of aphyric andesite in the densely rounded, indicating that the felsite is rounded owing to welded tuff of the Kunimidake rhyolitic pyroclastic flow the abrasion by gas-coring of tuffs. deposit (the upper reaches of Kawachigawa river, at 770m Photo.46 Fragments of felsite are included in the in altitude). tuffisite (the Moritanidani valley in Tobiishi district). Photo.38 Fragment of densely welded tuff of the Kun- Photo.47 Irregular contact between the felsites and imidake rhyolitic pyroclastic flow deposit is included in tuffisite (the Moritanidani valley in Tobiishi district). the densely welded tuff of Kunimidake rhyolitic pyroclas- Photo.48 Tuffisite invades the felsite (FS) through frac-

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tures probably by the gas coring (the Moritanidani valley block stoping is a plausible mechanism for emplacement. in Tobiishi district). Photo.49 View of the ring dike of granite porphyry (3) Contact migmatite (Mt,Mukabakisan(830m)). The contact migmatite is developed in the septa of wall Photo.50 View of the ring dike of granite porphyry (Mt. rocks comprising the sandstone and mudstone of Shiman- Hieizan (right)(918m) and Mt.Yahazudake (left)(666m)) to super-group between the batholithic granite and pre- Photo.51 Contact boundary between the granite por- ceding older granitoids I consisting of hypersthene biotite phyry (GP) and sandstone of Shimanto super-group (the granodiorite (Photos. 62 to 69). The septa of sandstone Kamiaka). The contact boundary is clean-cut. and mudstone are metamorphosed to the pyroxene horn- Photo.52 Contact boundary between the granite por- fels (granulite) facies. The contact migmatites are com- phyry (GP) and sandstone of Shimanto super-group (the posed of venite and nebulite, which was probably the lower reaches of Hinokagegawa river near Togawa). The product of partial melting of sedimentary rocks caused by contact boundary is clean-cut. the repeated intrusion of granitic magmas with high tem- Photo.53 Alignment of phenocrystic K-feldspar showing perature. the vertical magmatic flow in dike (the lower reaches of Tsunanosegawa river, near Mt.Hieizan). (4) Granitic surge Photo.54 Xenolith of the sandstone of Shimanto super- The surge is the secondary flow in the granitic magma group in the granite porphyry dike (the lower reaches of chamber or the intrusion unit successively emplaced be- Tsunanosegawa river, near Mt.Hieizan). fore the complete solidification of preceded granitic mag- Photo.55 Inclusion of metamorphic sedeimentary rock mas (Cobbing and Pitcher, 1972)．The surges of various in the granite porphyry dike (the lower reaches of Tsuna- scales are observed in the Okueyama granitic batholith nosegawa river, near Mt.Hieizan). (Photos. 100 to 103). The large-scale surge of biotite gran- Photo.56 Inclusions of pelitic to psamitic metamorphic ite intruded the hornblende biotite granodiorite in the rocks with gneissose texture (the Kamiaka). southeastern area of Okueyama granitic pluton (the OKG1 is intruded by the OKG2). 2-9. Batholithic granite The batholithic granite intruded finally both the coun- (5) Inclusions try rocks and preceded volcanic rocks and thermally The inclusions are abundant in the batholithic granite metamorphosed them to hornfels and contact migmatite. (Photos. 104 to 125). Different types of inclusions are present in the batholithic granite; they are (a) mafic mag- (1) Three dimensional shape of the batholithic granitic matic inclusions (MMI), (b) xenoliths of country sedi- pluton. mentary rocks, (c) xenoliths of high-grade metamorphic The batholithic granites are exposed as independent rocks (gneiss), (d) Composite mafic inclusions (double masses, such as OKG1-2, FKG, HNG1-2, OMG, OBG1 enclaves) consisting of igneous rocks including metamor- and UHG, but they are connected to form a continuous phic rocks of sedimentary origin and/or mafic inclusions. large batholithic granitic mass beneath the surface The inclusions are more abundant in the lower horizon (Fig.2). The granitic batholith with 20×30km in area has than in the upper horizon of granitic pluton (OKG1). a flat roof and steep wall, forming a box-shaped body (Photos. 70 to 82) (6) Aplite and pegmatite complex The aplite and pegmatite are the crystallization prod-

(2) Contacts of granitic pluton ucts of ﬁnal residual granitic melt enriched in H2O. The The contact of batholithic granite to country rocks is occurrence of aplite and pegmatite are as follows (Photos. discordant and clean cut, indicating that the emplacement 126 to 143); (1) aplite and pegmatite complex just beneath mechanism of granitic magma was the fracture filling the roof, (2) thick aplite and pegmatite sheet complex de- (Photos. 83 to 99). The underground cauldron or the veloped in the middle horizon of OKG1, (3) aplite and

（ 160 ） ─ 204 ─ Subsurface structure of Miocene large-scale caldera cluster: Illustrated descriptions of geology of the Okueyama volcano-plutonic complex, Southwest Japan pegmatite horizontal veins, (4) massive aplite with grada- the sedimentary rocks of Shimanto super-group. The dot- tional contact to surrounding granite, (5) aplite and peg- ted line denotes the horizontal and flat roof contact of the matite dikes, intruding both the batholithic granite and batholithic granite, the altitude of which is about 1,300m. country rocks. Photo.72 View of the batholithic granite (the Horiga- wa). The flat and horizontal roofs are well preserved near 2-10. Explanation for photographs of the batholithic the summit of mountains. The roof rocks are composed of granite (Photo.62 to 143) sedimentary rocks of the Shimanto super-group. The dot- (1) Contact migmatite ted line denotes the horizontal and flat roof contact of the Photo.62 Outcrop of contact migmatite along the batholithic granite, the altitude of which is about 1,300m. Horigawa river (the Horigawai, at 350m in altitude). An The vertical distance from the foot of mountains along the arrow indicates the location of outcrops of the contact Horigawa river to the roof contact is about 1,000m at max- migmatite. imum. Photo.63 Boundary between the contact migmatite and Photo.73 View of the roof rocks (the Horigawa). The the batholithic granite (Gr) at the wall along Horigawa riv- Mt.Happongiyama (1,408m) comprises the welded tuff of er (the Horigawa, at 350m in altitude). Sobosan dacitic pyroclastic flow deposit and Katamukiya- Photo.64 Clean cut contact between the contact migma- ma dacitic pyroclastic flow deposit which overlie the con- tite and batholithic granite (Gr) (the Horigawa, at 350m in glomerate of Mitate formation and the sedimentary rocks altitude). of Shimanto super-group. The steep cliff consists of the Photo.65 Contact migmatite is fragmented by brittle batholithic granite beneath the roof rocks. The dotted fractures and included into the batholithic granite (the lines denote the horizontal and flat roof contacts. Horigawa, at 350m in altitude). Photo.74 View of the batholithic granite (the upper Photo.66 Sandstone of the Shimanto super-group was reaches of Horigawa river). partially melted to form contact migmatite. Leucosome Photo.75 Cliff of the Takehiradani valley comprising the (white patch) is the partially melted zone (the Horigawa, batholithic granite (the Horigawa). at 350m in altitude). Photo.76 Summit of the Mt. Kozumiiwa consisting of Photo.67 Boundary between the contact migmatite and batholithic granite (the upper reaches of Horigawa river). the batholithic granite (Gr) (the Horigawa, at 350m in alti- Photo.77 View of the batholithic granite (the Kamishi- tude). The foliation of the contact migmatite is cut by the shikawa). The summit area of mountains are composed of contact boundary. The formation of the contact migmatite sedimentary rocks of the Shimanto super-group and Mi- was preceded by the final intrusion of batholithic granite. tate formation. Dotted lines denote horizontal and flat roof Photo.68 Leucosome (partially melted band) in the con- contacts. tact migmatite originated from the clastic sedimentary Photo.78 View of the batholithic granite (the Kamishi- rocks of Shimanto super-group (the Horigawa, at 350m in shikawa). The distant ragged mountain range is a ring altitude). dike of granite porphyry. Photo.69 Contact migmatite (venite) composed of Photo.79 View of the batholithic granite (the Kamishi- bands of leucosome and melanosome (the Horigawa, at shikawa). Dotted lines denote the horizontal and flat roof 350m in altitude). contacts. Photo.80 View of the batholithic granite (the Kamishi- (2) Three dimensional shape of the batholithic pluton shikawa). The summit of mountain is the Mt. Okueyama. Photo.70 Mt.Okueyama (1,643m) and the batholithic Dotted lines denote the horizontal and flat roof contacts. granite viewed from the Horigawa. The steep cliffs of Photo.81 View of the batholithic granite (the Kamishi- flank of mountains consist of the batholithic granite. shikawa). The distant ragged mountain range is a ring Photo.71 View of the batholithic granite (the Horiga- dike of granite porphyry. wa). The flat and horizontal roofs are well preserved near Photo.82 Summit of the Mt. Hokodaki comprising the summit of mountains. The roof rocks are composed of batholithic granite (the Kamishishikawa).

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(3) Intrusion contact rock is the sandstone of Shimanto super-group, which is Photo.83 Horizontal and flat roof contact of the batho- thermally metamorphosed (the Kanodani valley in Ka- lithic granite (the Kamishishikawa). The roof rocks con- mishishikawa district, at 1230m in altitude) sist of sedimentary rocks of the Shimanto super-group Photo.93 Pegmatite and aplite complex beneath the and Mitate formation. The roof contact of cliff is about roof contact (the Utouchikotani valley in Kamishishikawa 1,300m in altitude and the road at the foot of cliff is about district, at 1300m in altitude). 1,000m in altitude. Photo.94 Sheet of granite (Gr) in the roof sandstone of Photo.84 Closed view of the horizontal and flat roof con- the Shimanto super-group (S) (the Utouchihondani valley tact between the pegmatite of batholithic granite (P) and in Kamishishikawa district, at 1150m in altitude). conglomerate of the Mitate formation (M) (the Kamishi- Photo.95 Sheet of granite(Gr) in the roof sandstone of shikawa, at 1,300m in altitude). The sedimentary rocks of Mitate formation (M) (the Kuwazurudani valley in Mitate Shimanto super-group beneath the Mitate formation are district, at 660m in altitude). completely lacking and must have subsided into the mag- Photo.96 Sheet of granite (Gr) in the felsite of roof (FS) ma chamber of batholithic granite. (the Obira, at 780m in altitude). Photo.85 Wall contact between the batholithic granite Photo.97 Block of sandstone of the Shimanto super- and sandstone of the Shimanto super-group (the Kamishi- group in the roof is dislodged and the open cracks are shikawa, at 400m in altitude). The wall contact is clean- filled with granite (the Kamishishikawa at 1,300m in alti- cut. The aplite vein intrudes both the granite and wall tude). rocks. Photo.98 Block of sandstone of the Shimanto super- Photo.86 Wall contact between the batholithic granite group in the roof (S) is dislodged and the open cracks are and sandstone of the Shimanto super-group (the Kamishi- filled with granite (Gr) (the Kamishishikawa district, at shikawa, at 400m in altitude). The aplite and pegmatite 1,300m in altitude). dike intrudes both the granite and sandstone of the Shi- Photo.99 Roof sandstone of the Shimanto super-group manto super-group. (S) is fractured to pieces and the open cracks are filled Photo.87 Closed view of the wall contact between the with granite (the Utouchihondani in Kamishishikawa dis- batholithic granite and sandstone of the Shimanto super- trict, at 1,150m in altitude). group. The aplite vein intrudes both the granite and sandstone of the Shimanto super-group (the Kamishishikawa, (4) Granitic surge at 400m in altitude). Photo.100 Granodiorite (GD) of the batholithic granite Photo.88 Wall contact between the batholithic granite (OKG1) is intruded by the surge of granite (G) (OKG2) and sandstone of the Shimanto super-group (the Kamishi- (the Horigawa, at 400m in altitude). shikawa, at 400m in altitude). The wall contact is clean- Photo.101 Granodiorite (GD) of the batholithic granite cut. The aplite veins intrude both the granite and wall (OKG1) is intruded by the surge of granite(G) (OKG2) rocks. (the Horigawa, at 450m in altitude) Photo.89 Horizontal and flat roof contact between the Photo.102 Contact between the hornblende monzonite sandstone of Shimanto super-group (S) and the batholith- (M) and granodiorite (GD) (the Kamishishikawa, at 650m ic granite (the Kamishishikawa, at 1,300m in altitude). in altitude). Photo.90 Roof contact between the sandstone of Shi- Photo.103 Closed view of the boundary (arrow) be- manto super-group and the batholithic granite (the Ka- tween the hornblende monzonite (M) and granodiorite mishishikawa, at 1,300m in altitude). (GD) (the Kamishishikawa, at 650m in altitude). The Photo.91 Horizontal and flat roof contact between the boundary is not clean-cut but rather ambiguous. sandstone of Shimanto super-group (S) and the batholithic granite (Gr) (the Utouchihondani valley in Kamishi- (5) Inclusions shikawa district, at 1150m in altitude). Photo.104 Inclusions in the upper horizon of Okueyama Photo.92 Pegmatite beneath the roof contact. The roof granitic pluton (OKG1) higher than 1,000m in altitude.

（ 162 ） ─ 206 ─ Subsurface structure of Miocene large-scale caldera cluster: Illustrated descriptions of geology of the Okueyama volcano-plutonic complex, Southwest Japan

The density of numbers of inclusion is small (less than 1/ tite) (the Horigawa, at 440m in altitude). m2) (the Kamishishikawa, at 1020m in altitude)． Photo.119 Composite mafic inclusion (the Horigawa, at Photo.105 Closed view of the same outcrop as Pho- 710m in altitude). Metasedimentary rocks are included in to.104. the inclusion. A leucoclatic band is developed between the Photo.106 Xenoliths of the country sedimentary rocks composite mafic inclusion and host granitic rocks. of Shimanto super-group in the upper horizon of Okueya- Photo.120 Composite mafic inclusions (the Horigawa, ma granitic pluton (OKG1) (the Kamishishikawa, at at 800m in altitude) 1,020m in altitude). Photo.121 Xenolith of metasedimentary rock is includ- Photo.107 Large composite inclusions in the upper ho- ed in the composite mafic inclusion (the middle reaches rizon of Okueyama granitic pluton (OKG1) (the Kamishi- of Hinokagegawa river, at 320m in altitude). An aplite vein shikawa, at 1,010m in altitude) intrudes both the composite inclusion and host granite. Photo.108 Inclusions in the lower horizon of Okueyama Photo.122 Xenolith of metasedimentary rock (migma- granitic pluton (OKG1) lower than 1,000m in altitude. The tite) is included in the composite mafic inclusion (the density of numbers of inclusion is large (3/m2-4/m2) (the middle reaches of Hinokagegawa river, at 320m in altitude). Kamishishikawa, at 570m in altitude). Photo.123 Cluster of mafic inclusions and composite Photo.109 Inclusions in the lower horizon of Okueyama mafic inclusions (the middle reaches of Hinokagegawa granitic pluton (OKG1) lower than 1,000m in altitude. The river, at 320m in altitude ). density of numbers of inclusion is large (3/m2-4/m2) (the Photo.124 Large composite mafic inclusions are nested Horigawa, at 440m in altitude). together (the middle reaches of Hinokagegawa river, at Photo.110 Inclusions in the lower horizon of Okueyama 320m in altitude ) . granitic pluton (OKG1) lower than 1,000m in altitude. The Photo.125 Horizontal layer of inclusions (the middle density of numbers of inclusion is large (3/m2-4/m2) (the reaches of Hinokagegawa river, at 340m in altitude ). The Kamishishikawa, at 550m in altitude). layer of inclusions is reversely zoned. Photo.111 Mafic magmatic inclusion (MMI) with flame structure. The flame structure at the margin of MMI indi- (6) Aplite and pegmatite cates that the MMI was viscous liquid when it was en- Photo.126 Miarolitic cavity in the aplite and pegmatite trained into the surrounding granitic magma (the complex just beneath the roof contact (the Kanodani val- Horigawa, at 440m in altitude). ley in Kamishishikawa district, at 1,130m in altitude) Photo.112 Xenolith of country sedimentary rock (sand- Photo.127 Pegmatite complex comprising muscovite, stone) of the Shimanto super-group (the Horigawa, at tourmaline and quartz under the roof (the Kamishishika- 440m in altitude). wa, at 1,300m in altitude). GS: greisen consists of musco- Photo.113 Xenolith of sedimentary rock is reacted with vite and quartz; P: pegmatite of quartz (white) and surrouding granitic magma (the Horigawa, at 440m in alti- tourmaline (black). tude ). Photo.128 Pegmatite comsists of quratz, tourmaline and Photo.114 Degree of reaction of a xenolith of sedimen- muscovite developed under the roof (the Kamishishikawa, tary rock with surrounding granitic magma increases to at 1,300m in altitude). form reaction zone (the Horigawa, at 440m in altitude) Photo.129 Boundary between the aplite sheet (Ap) and Photo.115 Reacted and assimilated xenolith of sedimen- granite (Gr). The contact boundary is rather sharp. tary rock (the Horigawa, at 440m in altitude ). Photo.130 Horizontal pegmatite pod consists of quartz Photo.116 Extremely reacted and assimilated xenolith and feldspar (the Horigawa, at 1,200m in altitude). of sedimentary rock (the Horigawa, at 440m in altitude). Photo.131 Horizontal pegmatite pod consists of quartz Photo.117 Composite mafic inclusion (double enclave). and feldspar (the Kamishishikawa, at 1,250m in altitude). Metasedimentary rocks (migmatite) are included in the A melanocratic band above the pegmatite pod is a schriel- mafic inclusion (the Horigawa, at 950m inaltitude). en composed of biotite. Photo.118 Xenolith of metasedimentary rock (migma- Photo.132 Massive aplite (Ap) with gradational contact

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to surrounding granite (Gr) (the Kamishishikawa, at (3) The lapilli tuff of KDT is a densely welded pyroclastic 1,020m in altitude). flow deposit. Photo.133 Massive aplite (Ap) with gradational contact (4) The SACV is a cluster of composite volcanoes com- to surrounding granite (Gr) (the Horigawa, at 600m in al- prising flat piles of lavas and pyroclastic to volcanicla- titude). stic rocks, which filled the caldera depression. Photo.134 Horizontal pod of pegmatite (the Horigawa, (5) The volcaniclastic tuffaceous rocks are inferred to be at 890m in altitude) the lake sediments and some of pyroclastic rocks are Photo.135 Horizontal sheets of aplite and pegmatite (the subaqueous autobrecciated lavas and/or hyaloclas- Kamishishikawa, at 900m in altitude). tites. These evidences show that the small lakes were Photo.136 Thick sheet of the aplite and pegmatite com- present in the caldera depression during the eruption plex (Ap) (the Horigawa, at 820m in altitude). The thick- of composite volcanoes. ness of the sheet is about 10m. The aplite and pegmatite (6) The older granitoids II (OBG2) are probably the vent veins are derived from the thick main sheet. Gr: host of SACV. The OBG2 is a zoned pluton, in which the granite. marginal quartz monzodiorite is intruded by the cen- Photo.137 Thick sheet of the aplite and pegmatite com- tral granophyre with sharp contact. plex (Ap) (the Horigawa, at 800m in altitude). Arrows in- (7) The KRT is densely welded tuffs with many flow and dicate the upper and lower boundaries of a sheet of aplite cooling units and not lava flows. and pegmatite complex with surrounding granite (Gr). (8) The ring dike and associated dikes are composite Photo.138 Layering in the aplite and pegmatite complex dikes comprising felsite, tuffisite and granite porphy- sheet (the Horigawa, at 930m in altitude). ry. The order of intrusion is as follows: Photo.139 Closed view of the layering (the Horigawa, at felsite→ tuffisite→ granite porphyry. The tuffisite is 930m in altitude). The layer is expressed by the differ- probably the feeder of KRT. ence of grain size of constituent minerals and concentra- (9) The batholithic granite is a pluton with a flat and hori- tion of biotite zontal roof and steep walls. Photo.140 Horizontal segregation vein of aplite and peg- (10) The contact is clean-cut and fractured in brittle mode, matite (the Horigawa, at 580m in altitude). suggesting the emplacement mechanism of granitic Photo.141 Aplite dike (Ap) intruding the batholithic batholith is fracture filling. The underground caul- granite (Gr), the contact of which is straight and clean-cut dron or the block stoping is a plausible mechanism of (the Kamishishikawa, at 490m in altitude). emplacement. Photo.142 Horizontal aplite and pegmatite veins in the (11) Contact migmatite, partially melted sediments of the batholithic granite (the Horigawa, at 650m in altitude) Shimanto super-group, is present in the septa be- Photo.143 Aplitic granite and pegmatite dike intruding tween the batholithic granite and hyperthene biotite the wall rocks (thermally metamorphosed sandstone of granodiorite of older granitoids I (OKG3). The wall the Shimanto super-group), which is derived from the rocks around migmatite are thermally metamor- neighbouring granite (the upper reaches of Tsunanosega- phosed to the pyroxene hornfels (granulite) facies. wa river near Kamishishikawa, at 390m in altitude). (12) Surges of granite are present in the batholithic pluton. The contact between the surge and host granite 5. Summary and conclusion is either sharp or gradual. The important points of the field occurrence of Okue- (13) The batholithic granites include various types of in- yama volcano-plutonic complex are summarized as fol- clusions. They are (a) mafic inclusion, (b) xenolith of lows. country sedimentary rocks, (c) xenolith of high- (1) The basal part of SDT shows that it was terrestrial grade metamorphic rocks (migmatite), (d) composite and densely welded. mafic inclusion (2) The fluidal structure of KRL shows that it is a thick (14) The inclusions are more abundant in the lower hori- lava flow. zon of batholithic granitic pluton.

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(15) The aplite and pegmatite are well developed in the vein of aplite and pegmatite, (d) massive aplite with batholithic granite, especially just beneath the roof gradational contact, (e) aplite and pegmatite dikes and middle horizon of granitic pluton. with clean-cut contact (16) The aplite and pegmatite comprise various types such as (a) aplite and pegmatite complex just be- Acknowledegement neath the roof, (b) thick aplite and pegmatite sheet I am grateful for Profs. Aramaki,S., Ishihara, S. and Ikeda,Y. for their discussions and encouragement during the study. complex in the middle horizon, (c) horizontal pod or

References

Cobbing, E.J. and Pitcher, W.S. (1972) : The coastal batholith of Jap.145p (in Japanese) Central Peru. J.Geol.Soc.London, 128, 421-460 Shibata, K. (1978) : Contemporaneity of Tertiary granites in the Matsumoto, Y. and Miyahisa, M. (1973) : Caldera type depres- Outer Zone of Southwest Japan. Bull.Geol.Surv.Jpn, 29, sions structure in the Sobosan volcanic body, Kyushu, Ja- 551-554 (in Japanese) pan. J. Geol. Soc. Jpn, 79, 99-101 (in Japanese) Takahashi, M.(1986) : Anatomy of a middle Miocene Valles- Murao, S. and Matsumoto, T. (1991) K-Ar datings of Sobosan type caldera cluster: Geology of the Okueyama volcano- volcanic rocks in the Obira metalogenic province. Bull. plutonic complex, Southwest Japan. J.Volcanol.Geotherm. Geol. Surv. Jpn., 42, 497-502 (in Japanese) Res., 29, 33-70. Okumura, K., Sakai, A., Takahashi, M., Miyazaki, K. and Takahashi, M., Tono, K. and Kanamaru,T. (2014 in press); Hoshizumi, H. (1998) : Geology of the Kumata District. Whole-rock major element chemistry for igneous rocks of GeolSurv.Jpn. 100p (in Japanese) the Okueyama volcano-plutonic complex, Kyushu, South- Ono, K., Matsumoto,Y., Miyahisa, M., Teraoka,Y. and Kanbe,N. west Japan: Summary of 271 analytical data. Proc. Inst. (1977) : Geology of the Takeda District. Geol.Surv. Nat. Sci., CHS, Nihon Univ.

─ 209 ─ （ 165 ） Masaki TAKAHASHI

Mt. Katamukiyama

KDT 3

SDT KRL

SDT １ 2

SDT M

（ 166 ） ─ 210 ─ Subsurface structure of Miocene large-scale caldera cluster: Illustrated descriptions of geology of the Okueyama volcano-plutonic complex, Southwest Japan

7 8

9 10

11 12 13

─ 211 ─ （ 167 ） Masaki TAKAHASHI

Mt. Katamukiyama PA

Katamukiyama Cauldron PC

15 16 AA

17 AA 18

19 20

21 22

（ 168 ） ─ 212 ─ Subsurface structure of Miocene large-scale caldera cluster: Illustrated descriptions of geology of the Okueyama volcano-plutonic complex, Southwest Japan

23 24

25 26

Mt.Sobosan

27 28 QMD

Mt.Sobosan

29 30

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Mt.Sobosan

QMD

OG GRP

31 32 33 34

35 36

（ 170 ） ─ 214 ─ Subsurface structure of Miocene large-scale caldera cluster: Illustrated descriptions of geology of the Okueyama volcano-plutonic complex, Southwest Japan

38 39 40

Shimanto FS super-group

Gr 41

T2 Shimanto super-group T1

T2 FS FS 42

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44 Shimanto super-group 45 T

T 46

（ 172 ） ─ 216 ─ Subsurface structure of Miocene large-scale caldera cluster: Illustrated descriptions of geology of the Okueyama volcano-plutonic complex, Southwest Japan

Mt.Mukabakisan

Shimanto super-group

Mt.Hieizan Mt.Yahazudake GP

50 53

Shimanto Super-group GP

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Gr OG 56 57

58 59

60 Opx 61

（ 174 ） ─ 218 ─ Subsurface structure of Miocene large-scale caldera cluster: Illustrated descriptions of geology of the Okueyama volcano-plutonic complex, Southwest Japan

Migmataite

Gr 63

Migmatite

64 63

66 Gr

68 69

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Mt.Okueyama

Shimanto super group Mt.Okueyama

Mt.Okueyama

Mt.Kiyamauchidake

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Mt.Happongiyama

73 74

75 76

Shimanto Mitate formation super-group

Ring dike of granite porphyry

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Mt.Okueyama

79 80

81 82 Mitate firmation

Shimanto super-group

P 84 75 76

（ 178 ） ─ 222 ─ Subsurface structure of Miocene large-scale caldera cluster: Illustrated descriptions of geology of the Okueyama volcano-plutonic complex, Southwest Japan

85 S

88 91

89 S

Shimanto super group

─ 223 ─ （ 179 ） Masaki TAKAHASHI

93 S S

Gr 92 Gr 95 S

M Gr

Gr 94

S 97

S Gr

S 98 99

（ 180 ） ─ 224 ─ Subsurface structure of Miocene large-scale caldera cluster: Illustrated descriptions of geology of the Okueyama volcano-plutonic complex, Southwest Japan

G 100 101 102 M GD

GD 103 93

94104 GD

M GD

10695 107

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108

109

110 111 113 112

101 102

103 114 104 115

（ 182 ） ─ 226 ─ Subsurface structure of Miocene large-scale caldera cluster: Illustrated descriptions of geology of the Okueyama volcano-plutonic complex, Southwest Japan

116 117 118

119

121

109 120 110 123

122111 112

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125

124

114

126

P GS

116

127

117 118 128

（ 184 ） ─ 228 ─ Subsurface structure of Miocene large-scale caldera cluster: Illustrated descriptions of geology of the Okueyama volcano-plutonic complex, Southwest Japan

130

129 131

Gr Ap

Gr 133 132 123

122

134 124135

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136 137 Gr Gr

138 139

141

140

143

S 142 132

（ 186 ） ─ 230 ─