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Fig. . Geological map of the Poroshiri ophioliteslightly modified from Miyashita1983and Arai and Miyashita1994.
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... Whole-rock major and trace elements were determined by X-ray fluorescence using a Spectris MagiX PRO system with a Rh tube. Additional trace element concentrations, including rare earth elements (REEs), were measured using a Thermo Fisher Scientific X Series II inductively coupled plasma mass spectrometer (ICP-MS) following the method of Miyashita et al. (2007). Sample dissolution and chemical separation procedures for Sr and Nd isotopic analyses followed Pin & Zalduegui (1997), and Pb isotopic analysis followed Kuritani & Nakamura (2002). ...
Volcanic rocks of the Java sector of Sunda arc have a wide range of isotopic compositions that indicate significant addition of subjected sediment. What processes control these geochemical characteristics is a topic of long-standing debate. Here we report Sr-Nd-Pb radiogenic isotope ratios and geochemical data from stratigraphically well-constrained rocks of Sundoro volcano in central Java that represent the volcano’s activity since 34 ka. The rocks range from basalt (51 wt.% SiO2) to andesite (63 wt.% SiO2) and are dominated by basaltic andesite. We divide them into magma types A, B, and C, having low, medium, and high 87Sr/86Sr and Pb isotopic ratios, respectively. According to various differentiation indices, the three magma types have separate, parallel 87Sr/86Sr, Ba/Zr, and La/Yb trends, and disparate Pb isotopic trends. The dominant process of intracrustal differentiation appears to be magma mixing, in which each of the three magma types represents the mixing of a distinct mafic end member and a distinct felsic end member. The distinct geochemical profiles of these magma types indicate that the three mafic end-members are genetically unrelated and that their differences may represent characteristics of their magma sources. On the basis of trace element ratios (Ba/Yb and La/Yb) and Sr-Nd-Pb isotopic compositions, we estimate that magma types A, B, and C represent mantle wedge materials fluxed by ~1%, ~1.5%, and ~2% slab-derived materials containing 50%, 55%, and 65% sediment component, respectively, reflecting increasing proportions of sediments and increasing slab flux. Geochemical data from Merapi volcano, interpreted using the same approach, reveal a similar increase in the slab-derived flux to the magma source, raising the possibility that such short-lived variations in magma genesis, perhaps related to the subduction of bathymetric relief features, characterize the unusual magmatism beneath the volcanic front of the central Java sector of the Sunda arc.
... The geochemistry of the basalt and mineralogy of cumulate and peridotite all suggest that the ophiolite originated from normal oceanic crust formed at an oceanic spreading ridge (Miyashita, Adachi, Tanaka, Nakagawa, & Kimura, 2007;Miyashita et al., 1994). Zircon U-Pb dating of plagiogranite yields an age of 96.7 Ma ±2.6 Ma (Kizaki, 2000) contemporaneous with pelagic sedimentation in the Late Cretaceous unit of the Idonnappu terrane. ...
The Precambrian and lower Paleozoic units of the Japanese basement such as the Hida Oki and South Kitakami terranes have geological affinities with the eastern Asia continent and particularly strong correlation with units of the South China block. There are also indications from units such as the Hitachi metamorphics of the Abukuma terrane and blocks in the Maizuru terrane that some material may have been derived from the North China block. In addition to magmatism, the Japanese region has seen substantial growth due to tectonic accretion. The accreted units dominantly consist of mudstone and sandstone derived from the continental margin with lesser amounts of basaltic rocks associated with siliceous deep ocean sediments and local limestone. Two main phases of accretionary activity and related metamorphism are recorded in the Jurassic Mino–Tanba–Ashio, Chichibu, and North Kitakami terranes and in the Cretaceous to Neogene Shimanto and Sanbagawa terranes. Other accreted material includes ophiolitic sequences, e.g. the Yakuno ophiolite of the Maizuru terrane, the Oeyama ophiolite of the Sangun terrane, and the Hayachine–Miyamori ophiolite of the South Kitakami terrane, and limestone‐capped ocean plateaus such as the Akiyoshi terrane. The ophiolitic units are likely derived from arc and back‐arc basin settings. There has been no continental collision in Japan, meaning the oceanic subduction record is more complete than in convergent orogens seen in intracontinental settings making this a good place to study the geological record of accretion. Hokkaido lacks most of the Paleozoic history recognized in Honshu, Shikoku, Kyushu, and the Ryukyu Islands to the south and its geology reflects the Cenozoic development of two convergent domains with volcanic arcs, their approach, and eventual collision. The Hidaka terrane reveals a cross section through a volcanic arc and the main accretionary complex of the convergent system is represented by the Sorachi–Yezo terrane.
The timing and mechanism of the tectonic transition from an active continental margin to a trench–arc–basin system in NE Asia are debated. In this study we report the pressure–temperature–time (P–T–t) path of this transition based on petrographic observations, phase‐equilibrium modelling, and U–Pb ages of zircon and rutile from pelitic granulites in the Hidaka metamorphic belt (Hokkaido, Japan). The granulites contain an early phase mineral assemblage of staurolite + sillimanite + biotite + plagioclase + quartz + rutile/ilmenite, a peak phase granulite assemblage of garnet + biotite + cordierite + plagioclase + quartz + rutile/ilmenite and a symplectic intergrowth of spinel + cordierite ± sillimanite within garnet porphyroblasts. Phase‐equilibrium modelling indicates two phases of metamorphism with P–T conditions respectively of ~6 kbar/620°C–670°C and ~6 kbar/850°C. A clockwise P–T path was thus reconstructed for the granulites, showing a near‐isobaric temperature increase to the peak conditions and a post‐peak cooling. U–Pb dating of zircon and rutile in the granulites yielded two groupings of metamorphic ages at ca. 37 Ma and 19 Ma, related to early phase amphibolite‐facies and late phase granulite‐facies metamorphism, respectively. The age of magmatism from the previous work at the NE Asian continental margin overlaps with these metamorphic ages, and the two phases of metamorphism in the pelitic granulites is attributed to discrete episodes of supra‐subduction‐zone magmatism (late Eocene, ca. 37 Ma) and back‐arc extension (early Miocene, 24–19 Ma). Consequently, we suggest that the Hidaka metamorphic belt has undergone two phases of metamorphism, which represent two pulsed and separated thermal events. Moreover, we relate the granulites facies metamorphism to the underplating of mafic magma and lithospheric thinning during the opening of the Japan Sea at 24–19 Ma, which is attributed to slab rollback and trench retreat processes in NE Asia.
The Poroshiri ophiolite is exposed on the western side of the Hida-ka metamorphic belt in central Hokkaido, Japan. Here, we present a study of metamorphism in the northern part of the ophiolite, in the Chiroro River area, to determine the relationship between metamor-phism and large-scale folding and faulting. Along the northern E–W transect, the ophiolite succession in the western area forms an isoclinal anticline, whereas the eastern area of the transect shows an open syncline. Along the southern E–W tran-sect, the upper sequence of the ophiolite is exposed in the eastern part of the transect along a NW–SE trending thrust fault. The metamorphic grades in these areas change sharply from greenschist to amphibolite facies at the western margin, then gently increases to °C towards the central fault in the northern transect, whereas the metamorphic temperature to the east of this central fault is around °C, indicating a temperature gap of about °C at the central fault. The folded structure of the ophiolite along the northern transect is discordant with the metamorphic thermal structure , indicating that peak metamorphism occurred after folding. This also indicates that the central fault must have been active after peak metamorphism to cause the temperature gap recorded in the northern transect. The metamorphic temperature of the upper sequence of the ophiolite in the eastern section of the southern transect is about °C, similar to the eastern margin of the western section of the ophiolite within the northern transect. This suggests that the upper sequence along the southern transect was transported from the west after peak metamorphism along the NW–SE trending thrust fault. Amphibole porphyroclasts record pervasive prograde but no ret-rograde zoning, probably resulting from rapid exhumation after peak metamorphism. The metamorphism observed in the Poroshiri ophiolite is vastly different from ordinary regional metamorphism.
