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Volume 113, Issue 7
Displaying 1-9 of 9 articles from this issue
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Review
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Daisuke Miura, Yutaka Wada2007 Volume 113 Issue 7 Pages 283-295
Published: 2007
Released on J-STAGE: March 29, 2008
JOURNAL FREE ACCESSWe reviewed and synthesized middle Miocene caldera volcanoes at the frontal margin of southwest Japan in order to reveal possible relationship between the caldera formation and the compressive tectonics. Cumulative frequency diagram for fault-slip can be used for determining the deformation style of caldera floor. The deformation style is defined as the ratio of caldera collapse energy to the total eruptive energy. The changing of style is to modify the ratio. Mechanical collapse energy for the piston model is rather reduced than the piecemeal deformation or the funnel explosion. This character is suitable to the larger caldera formation.
Long-term eruption rate of the Miocene calderas indicates larger rate for the compressive stress regime. The ellipticity, eruptive volume and roof aspect ratio of calderas in relation to distance from subduction trench was used to reveal an effect of strong compression. Both ellipticity and eruptive volume increase toward the trench area, and the upper limit of roof aspect ratio decreases toward there. These suggest that near the trench region, only the piston collapse caldera is capable to erupt against the strong compression.
Orientation of dyke swarm, caldera shape, and magma chamber were used to infer the orientation of paleostress at the time of caldera formation. The optimal paleostress orientation of horizontal compressive stress (σHmax) lies on NNE-SSW. The sinistral (left-wrenching) fault-slip of Median Tectonic Line (MTL) had occurred in the same period and is concordant to the NNE-SSW orientation of σHmax. The clockwise rotation of Japan-Sea Opening, however, occurred in the same period and it might have modified the original orientation into the NNE-SSW. In the southwest Japan, the migration of volcanic front took place backward after middle Miocene period. We concluded that the backward migration could be induced by the decrease of magma supply rate and increase of compressive stress at frontal margin after middle Miocene.
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Articles
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Yuu Kawakami, Hiroyuki Hoshi2007 Volume 113 Issue 7 Pages 296-309
Published: 2007
Released on J-STAGE: March 29, 2008
JOURNAL FREE ACCESSWe have investigated the geology of the Kumano Acidic Rocks (KARs), a major Miocene volcano-plutonic complex in Japan, and in this paper we present a geologic map, geologic profiles, and lithologic and structural descriptions of the KARs in the Owase-Kumano area. The KARs in the mapped area consist of rhyolite lavas (the Konogi Rhyolite), rhyolitic pyroclastics occurring as both ash-flow deposits (the Owase-Shirahama Pyroclastic Rocks) and pyroclastic vent fills, granite porphyry intrusions (the Kumano Granite Porphyry) penetrating the rhyolite and the pyroclastics, and a stock of fine-grained granite (the Fudodani Granite) that intrudes the granite porphyry. Among them, the Kumano Granite Porphyry, comprising most of the KARs, occurs as a ring dike and a subhorizontal sheet intrusion. The ring dike, some 2 km or less wide and approximately 10×15 km in size, is found at the northern end of the mapped area. The subhorizontal sheet intrusion approximately 1 km or more in the maximum thickness, the eastern half of which is hidden in the sea. The roof of the sheet intrusion is observed in the northwestern part of the mapped area. Detailed mapping reveals a cauldron structure and indicates that a part of the sheet intrusion branches off from the ring dike at depth. Our observations argue against the previous interpretation that the Kumano Granite Porphyry formed as a gigantic lava lake body, instead we suggest a new view in which voluminous magma intrusions in the form of a ring dike-subhorizontal sheet interconnection are likely to be an intriguing feature of the volcano-plutonic complex at depth.
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Hironao Shinjoe, Yuji Orihashi, Yutaka Wada, Tomoaki Sumii, Shun'ichi ...2007 Volume 113 Issue 7 Pages 310-325
Published: 2007
Released on J-STAGE: March 29, 2008
JOURNAL FREE ACCESSWe present newly obtained whole rock major and trace element compositions of the Miocene felsic igneous rocks in the Kii Peninsula. The felsic igneous rocks include I-and S-type granitic rocks in the Outer Zone and felsic member of the Setouchi Volcanic Rocks. Regional variation of whole rock composition discussed for data set including previously reported one suggests following observations: (1) Granitic rocks in the Outer Zone are characterized by relatively constant chondrite-normalized middle to heavy REE abundance, whereas rhyolitic rocks of the Setouchi Volcanic Rocks show strongly HREE-fractionated patterns. (2) Distribution of S-type and I-type felsic rocks forms trench parallel zones and runs SW-NE direction, which is almost parallel to those of the high-Mg andesites in the Outer Zone. This direction coincides with the strikes of the contour lines of Wadati-Benioff Zone beneath eastern Kii peninsula. S-type felsic rocks in the Outer Zone and rhyolitic rocks of Setouchi Volcanic Rocks are peraluminous with high SiO2 and K2O contents, which are expected to be produced by melting of sedimentary rocks. REE abundances of sediment melt were modeled based on the melt fractions and residual mineralogies of Pati–o Douce and Beard's (1995) melting experiments. Differences in the REE compositions of these two rock types are explained by simply the difference of the melting depth where S-type felsic rocks were formed at shallower level with plagioclase-rich residue, and rhyolitic rocks of the Setouchi Volcanic Rocks at deeper level with garnet-rich residue. REE abundance model based on the melt fraction and residual mineralogy of Beard and Lofgren's (1990) melting experiments of basaltic rocks suggests that source rock of the I-type granitic magma should be enriched in light REE.
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Hideki Iwano, Tohru Danhara, Hiroyuki Hoshi, Yuu Kawakami, Tomoaki Sum ...2007 Volume 113 Issue 7 Pages 326-339
Published: 2007
Released on J-STAGE: March 29, 2008
JOURNAL FREE ACCESSFission track dating was carried out for 52 samples from the Muro Pyroclastic Flow Deposit, the Sekibutsu Tuff and the Furudera Tuff in the northern region of the Kii Peninsula, and from fine-grained granitic bodies of the Omine Granitic Rocks (the Dorogawa, Shirakura, Kose, Asahi, Tenguyama, and Shiratani bodies) and various rock facies of the Kumano Acidic Rocks (the Konogi Rhyolite, the Owase-Shirahama Pyroclastic Rocks, the Kumano Granite Porphyry, and the Kozagawa Dike) in the central to southeastern region of the Kii Peninsula. The resulting ages of all these samples and previously reported ages of the Tamateyama Tuff, the Nakaoku Tuffite Dike and the Shionomisaki Igneous Complex in the peninsula were clustering at 15 Ma. Most of the above igneous bodies contain a mixture of reddish and colorless zircons. Characteristic of the mixed zircons from the Muro Pyroclastic Flow Deposit, Sekibutsu Tuff, Tamateyama Tuff and the north and south units of the Kumano Granite Porphyry is similar in terms of a ratio of reddish and colorless zircons being 2 : 3, and dominant crystal planes {100} for reddish and {110} for colorless. Based on the simultaneity and similarity of the mixed zircons, it is strongly suggested that the Muro Pyroclastic Flow Deposit and the adjacent tuffs in the northern Kii Peninsula are correlated with each other as a large-scale pyroclastic flow deposit at 15 Ma, and that its possible source is the Kumano Acidic Rocks in the southeastern Kii Peninsula.
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Tohru Yamashita, Tohru Danhara, Hideki Iwano, Hiroyuki Hoshi, Yuu Kawa ...2007 Volume 113 Issue 7 Pages 340-352
Published: 2007
Released on J-STAGE: March 29, 2008
JOURNAL FREE ACCESSMode analyses using refractive index of light minerals were performed for samples from the Muro Pyroclastic Flow Deposit (MPFD) and the adjacent felsic tuffs (the Sekibutsu Tuff, the Furudera Tuff and the Tamateyama Tuff) in northern Kii Peninsula, and Miocene felsic rocks in southern part of the peninsula. We have confirmed that plagioclase phenocryst of the MPFD and the adjacent three felsic deposits are principally composed of oligoclase to labradorite, and that these four Miocene deposits are correlative each other. The MPFD is also correlative to a certain horizon of the felsic pyroclastic facies of the Kumano Acidic Rocks (KAR) on the southern end of the peninsula. We thus conclude that the MPFD and correlative tephras are an identical large pyroclastic flow deposit sourced from ancient (ca. 15 Ma) caldera volcanoes in the Kumano area. Some parts of the felsic pyroclastic flow deposits of the KAR contain plagioclase phenocryst of principally albite-composition, and could have derived from a different caldera, now represented by an arcuate pyroclastic intrusion (Nakaoku tuffite dike swarm) in the middle part of the peninsula.
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Yutaka Wada, Chinatsu Fujita, Hironao Shinjoe2007 Volume 113 Issue 7 Pages 353-365
Published: 2007
Released on J-STAGE: March 29, 2008
JOURNAL FREE ACCESSWe reexamined the Miocene Miyataki dike exposed at central Kii Peninsula, southwest Japan to infer the emplacement process of the dike. Although the dike has been regarded as of pyroclastic origin, our detailed field and petrographical investigations reveal that the Miyataki dike is composed of andesite mingled with rhyolite. In addition, granite enclaves also occur both in andesite and rhyolite, and those in rhyolite are often mantled by andesite. Shapes of rhyolite, andesite and granite enclaves are irregular and amoeboidal, indicative of ductile deformation during dike emplacement. These evidences show that andesite and rhyolite magmas and granite body could mingle with one another before dike emplacement, and that after the interaction, mingled magma rose to form the Miyataki dike. Whole rock compositions also support this mingling model for the Miyataki dike. Therefore, the facies and structures observed in the Miyataki dike are considered to be formed by a physical mingling processes in dike conduit.
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Yuji Orihashi, Hideki Iwano, Takafumi Hirata, Tohru Danhara, Hironao S ...2007 Volume 113 Issue 7 Pages 366-383
Published: 2007
Released on J-STAGE: March 29, 2008
JOURNAL FREE ACCESSIn order to understand compositional characteristics of zircon crystals with various appearance and to relate them with evolution of S-type, felsic magmatism of the Kumano Acidic Rocks in the Outer Zone of southwest Japan, U-Pb ages and trace element compositions of colorless and reddish zircons were measured using LA-ICPMS technique. Sorted U-Pb ages for these zircons range from 15.5 Ma to 16.7 Ma and ca. 1 m.y. older than fission track ages, implying slightly longer time elapsed between generation and cooling of the felsic melt. The REE patterns for both colorless and reddish zircons have a steep increase in LREE contents from La to Lu with significant positive Ce and negative Eu anomalies and show a common feature with those in granitoids. Scrutiny in the obtained data revealed that the slopes of HREE region of the reddish zircons are gentler than those of colorless zircons. REE composition of coexisting felsic melt estimated from the colorless zircons are relatively similar to the bulk rock composition of the evolved host rocks. Whereas the melt composition estimated from the reddish zircons has an adakitic signature. These suggest existence of roughly two different felsic melts for the origin of the Kumano Acidic Rocks. Large amounts of plagioclase fractionation from the parental magmas are required due to deep Eu troughs in the host melts. The production rate of the adakitic melts might be significantly smaller due to low population of reddish zircon. This compositional variation may reflect variable degree of partial melting of accretionary prism sediments that comprise the basement of the Kumano area. The partial melting of the sediments would have been triggered either by underplating of tholeiitic basalt magma derived from Shikoku basin or felsic magma derived from slab melting of the subducted plate, or melting directly of the subducted sediment.
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Tohru Danhara, Hiroyuki Hoshi, Hideki Iwano, Tohru Yamashita, Isao Mit ...2007 Volume 113 Issue 7 Pages 384-389
Published: 2007
Released on J-STAGE: March 29, 2008
JOURNAL FREE ACCESSWidespread correlation of early Middle Miocene (15 Ma) tephras is proposed. Mineralogical and geochronological analyses suggest that a felsic tephra on the Boso Peninsula (central Japan) is correlative to an ash-flow deposit on the Kii Peninsula (southwestern Japan), some 400 km distant. The felsic tephra, the Kn-1 tuff in the sedimentary Kinone Formation, is approximately 16 m thick and contains minerals and volcanic glasses that show close affinities with those from the Muro Pyroclastic Flow Deposit and correlative ash-flow units (the Tamateyama and Sekibutsu Tuffs) on the northern Kii Peninsula. Our zircon fission track ages for the Kn-1 tuff are about 15 Ma and indistinguishable from those for the ash-flow deposits. Because the Tamateyama-Sekibutsu-Muro ash-flow deposit originates from a caldera-forming pyroclastic eruption in the Outer Zone (Pacific-side) igneous province including the Kumano and Omine Granitic Rocks, the Kn-1 tuff also comes from that eruption in the Outer Zone (probably at Kumano) of the Kii Peninsula.
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Pictorial
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Tohru Danhara, Hideki Iwano, Hiroyuki Hoshi2007 Volume 113 Issue 7 Pages IX-X
Published: July 15, 2007
Released on J-STAGE: March 23, 2013
JOURNAL FREE ACCESSDownload PDF (1121K)
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