No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Amalgamation of the Ryoke and Sanbagawa metamorphic belts at the subduction interface: New insights from the Kashio mylonite along the Median Tectonic Line, Nagano, Japan
Abstract
We present a detailed petrological, structural, and geochronological study of the mylonitic Ryoke metamorphic rocks within the granitic mylonite (Kashio mylonite) and Sanbagawa metamorphic rocks along the Median Tectonic Line (MTL), Japan. Located in the Oshika area of the Chubu district, the Kashio mylonite is one of the few geologic units that can be used to determine detailed pressure–temperature–time–deformation (P–T–t–D) paths during mylonitization because it occurs as many small tectonic blocks of mylonitic metasediment. Detailed petrological analysis coupled with conventional thermobarometry and P–T pseudosection modeling give estimated peak P–T conditions (M1a) of 650–790 °C at 4.6–5.6 kbar for the Ryoke metamorphic rocks. The gneissose Ryoke granitoids were emplaced subhorizontally at around 685–710 °C and 4.6–5.8 kbar, after peak metamorphism. The Kashio shear zone developed immediately after the last igneous activity at ca. 71 Ma, and two stages of mylonitization (stages D1 and D2) can be identified from microstructural observations. The retrograde P–T conditions (M1b) recorded in the Kashio mylonite exhibit a systematic change in temperature from 710 to 450 °C at 5.2–2.6 kbar with decreasing distance from the MTL. By contrast, highly deformed mylonites with zoned garnets demonstrate a striking increase in pressure from 4.0 to 8.3 kbar with decreasing temperature from 590 to 450 °C after low-P/T type metamorphism. Such a temperature range indicating isothermal compression is consistent with deformation temperatures of stage D1 determined from quartz microstructures and quartz c-axis fabric opening-angle deformation thermometer. Moreover, the timing of the two mylonitization episodes during retrograde metamorphism are estimated to be 69–67 Ma and 66–64 Ma, respectively, with a high cooling rate of ~34 °C/Myr using the revised time–temperature relationship of the host Ryoke granitoids. The rapid change in tectonic setting with strain localization occurred during the brief period between 69 and 64 Ma. Our field and petrological observations imply that a thick D1 mylonite zone was formed by rapid subsidence (≥ 10km) with cooling of the hangingwall rocks from the middle crust to the subduction interface. It is considered that the underplating of exhumed high-P/T type metamorphic rocks led to further cooling between hangingwall and footwall rocks, and the formation of a narrow D2 mylonite zone, which served as an old plate boundary. Thus, low and high-P/T type metamorphic belts had already been amalgamated as paired metamorphic belts beneath the brittle–ductile transition of the subduction zone before exhumation. The rapid cooling of hangingwall rocks at the subduction interface is proposed to play an essential role in the thermal overprinting of exhumed high-P/T type metamorphic rocks.
... The Kashio shear zone (Figure 1a) lies within the Cretaceous low-P/high-T Ryoke metamorphic complex along the Median Tectonic Line (MTL) that is the largest onshore strike-slip fault (>1000 km) in central Honshu Island, Japan (e.g., [22][23][24][25][26][27][28][29]). Mylonites within the Kashio shear zone (i.e., Kashio mylonite) in central Honshu Island have been deformed by a sinistral strike-slip or subhorizontal mid-crustal (~10 km depth) shearing (e.g., [22,23,27,28,[30][31][32][33]) and have recently been recognized as a remnant of an old tectonic plate boundary [29]. ...
... The Kashio shear zone (Figure 1a) lies within the Cretaceous low-P/high-T Ryoke metamorphic complex along the Median Tectonic Line (MTL) that is the largest onshore strike-slip fault (>1000 km) in central Honshu Island, Japan (e.g., [22][23][24][25][26][27][28][29]). Mylonites within the Kashio shear zone (i.e., Kashio mylonite) in central Honshu Island have been deformed by a sinistral strike-slip or subhorizontal mid-crustal (~10 km depth) shearing (e.g., [22,23,27,28,[30][31][32][33]) and have recently been recognized as a remnant of an old tectonic plate boundary [29]. In addition, it has been suggested that the Kashio shear zone along the MTL has been bent by collision with the Izu-Bonin-Mariana arc associated with the opening of the Japan Sea [34,35]. ...
... Although this is much smaller than the previous estimate of 37 µm for the steady-state grain size ( [53]), the application of the more recent, sophisticated EBSD method revealed that finer grain sizes of quartz are more common in the Kashio shear zone. For instance, Okudaira and Shigematsu [20] reported mean grain sizes of 2.5-3.1 µm in mylonite and ultramylonite in the Matsusaka-Iitaka area (Mie Prefecture), and Nakamura et al. [29] found the mean grain sizes in mylonite and ultramylonites close to the MTL at Ohshika area (Nagano Prefecture) to be in the range 8.6-11.8 µm. ...
Ultramylonites are among the most extreme fault rocks that commonly occur in the mid-crustal brittle–plastic transition and are mainly characterized by intensely sheared fine-grained microstructures and well-mixed mineral phases. Although the deformation mechanism of ultramylonites is key to understanding the rheological behavior of the mid-crustal shear zone, their microstructural development is still controversial owing to their intensely fine-grained textures. To investigate the possible crustal deformation mechanisms, we studied 13 mylonites obtained from the Kashio shear zone along the Median Tectonic Line that is the largest strike-slip fault in Japan. In particular, we investigated various mixed quartz–plagioclase layers developed within tonalitic mylonite, which are representative of the common mean grain size and crystal fabric of quartz among the studied samples. A high-quality phase-orientation map obtained by electron backscattered diffraction showed not only a wide range of quartz–plagioclase mixing (10%–80% in quartz modal composition) but also revealed a correlation between grain size reduction and crystal fabric weakening in quartz, indicating a change in the deformation mechanism from dislocation creep to grain-size-sensitive creep in the mixed quartz-plagioclase layers. In contrast, plagioclase showed an almost consistent fine grain size and weak to random crystal fabrics regardless of modal composition, indicating that grain size-sensitive creep is dominant. Combined with laboratory-determined flow laws, our results show that the Kashio shear zone could have developed under deformation mechanisms in which the viscosities of quartz and plagioclase are nearly comparable, effectively within 1017–1019 Pa·s, thereby possibly enabling extensive shearing along the Median Tectonic Line.
... The method is rapid and non-destructive, and it is particularly appropriate for analysis of small amounts of CM grains (<10 µm) along grain boundaries in sedimentary rocks. Wider application of micro-Raman spectroscopy has therefore been encouraged as a reliable geothermometer for elucidating the tectonic evolution of metamorphic terrains (Beyssac et al., 2004;Scharf et al., 2013;Groß et al., 2021;Nakamura et al., 2022). On the other hand, Raman intensities are 10 −6 of the intensity required for laser excitation, and the Raman signal easily interferes with strong fluores-cence induced by laser-excitation sources (Panczer et al., 2012). ...
Abstract
A new type of compact deep–UV micro–Raman spectroscopy system was developed with a single monochromator, front–illuminated cooled charge–coupled device, and 266 nm nanosecond pulsed laser to overcome laser–induced fluorescence from surrounding minerals and organic material. Deep–UV micro–Raman spectroscopy is particularly useful in analyzing the fluorescence–free Raman spectra of dispersed low–maturity carbonaceous material and coal, although deep–UV excitation lasers may cause serious degradation and laser–induced heating of the sample surface, especially in microanalysis. The laser–induced damage threshold for fully ordered graphite and coal (VRr = ~ 0.5%) was assessed to facilitate the acquisition of accurate Raman spectra with a spot size of ~ 1 µm. For fully ordered graphite, there was no serious degradation of the sample surface with an energy fluence of 0.10–2.50 J cm−2. Some sample surfaces became black at higher fluences of 1.96–2.50 J cm−2, suggesting irreversible damage by deep–UV lasers. The Raman shift of the G band after measurement involves a downshift of 1.7–7.4 cm−1 relative to other spectra obtained at low laser fluences of <0.34 J cm−2. The G band full width at half maximum (FWHM) also increased with increasing laser fluence. Serious degradation of polished coal surfaces occurs at even lower laser fluences of 0.34–2.50 J cm−2. The degree of change in Raman parameters such as the D and G band FWHM depends on the laser fluence during measurements. Heating and damage by a deep–UV laser is greater than that by visible lasers. Laser fluences of <0.16 and 0.34 J cm−2 are required for accurate Raman analyses of dispersed carbonaceous material in sedimentary rocks and fully ordered graphite in metasediment, respectively.
Paleogene surface tectonics in Japan is not well understood because of the paucity of onshore Paleogene stratigraphic records except for those from accretionary complexes. Paralic Paleogene formations remaining in SW Japan are usually so thin that it is difficult to decipher the tectonics from them. However, the Eocene paralic sedimentary package with a thickness of kilometers indicates syn-depositional tectonic subsidence by a few kilometers in the Amakusa archipelago, west of Kyushu Island. Thus, we made a detailed geological map of the Eocene formations in an area of ~50 square kilometers in the northwestern part of the archipelago. We identified NE-SW and NW-SE trending normal faults, most of which were recognized by previous researchers, and also discovered low-angle faults. NW-SE trending ones are known to be of the Miocene. NE-SW trending and low-angle normal faults are the oldest map-scale structures in the Eocene ones. It is not obvious within the above-mentioned area whether those normal faults are accompanied by growth strata. However, the significant southeastward thickening of the Eocene formations across the Amakusa archipelago suggests that they filled a large half graben with the basin margin fault along the eastern side of the archipelago. This basin model is consistent with the N-S to NW-SE transport directions of the low-angle and NE-SW trending normal faults. Since many NE-SW to EW trending Eocene grabens were formed in the offshore regions west of Kyushu Island and in the East China Sea, the Amakusa region was probably a northeastern branch of the rift system. The geologic structures and depositional ages of the Eocene formations indicate that the Eocene extensional tectonics removed the overlying strata to some extent for the high-P/T Takahama Metamorphic Rocks which crops out to the south of our study area.
We estimated the protolith age and peak metamorphic temperature of the Yokokawagawa metamorphic rocks (YMR) east of the Itoigawa–Shizuoka Tectonic Line using detrital zircon U–Pb dating and Raman carbonaceous material geothermometry, respectively. U–Pb dating of a psammitic rock yielded a youngest age of ∼100 Ma, which corresponds to the protolith age. Raman carbonaceous material geothermometry results for five pelitic rock samples give metamorphic temperatures of ∼350–380 °C. The protolith age is consistent with those of the Sanbagawa metamorphic rocks (SMR), strongly indicating that the YMR are an extension of the SMR. The increase in peak temperature toward intrusive rocks along the eastern margin of the YMR may indicate that the relatively young ages yielded by previous K–Ar dating of the YMR reflect thermal resetting due to contact metamorphism.
Spectroscopy has been widely used in geology since the 1990s because it is non-destructive and easy to analyze. Raman spectroscopy has generally been used to identify mineral phases in geology, but recent studies have proposed new methods to quantitatively estimate the metamorphic pressure (quartz Raman barometry) and peak temperature (Raman carbonaceous material geothermometry). Studies using infrared spectroscopy are also underway to advance our understanding of hydration processes in subduction zones and mantle through the analysis of water in rocks. In addition to the development of new methods using spectroscopy, new tools are also being developed to analyze huge amounts of data through iterative processing, which will enable the extraction of more informative and quantitative results in a shorter time. This paper introduces examples of spectroscopy applications in geology and examines future developments.
The development of a mylonite zone along the Median Tectonic Line (MTL), southwest Japan, which developed in the Cretaceous granitic rocks, was examined to understand strain localization during cooling. The mylonites are E-W trending, north-dipping, and represent the hanging wall of the MTL. The mylonite zone includes both protomylonite and mylonite, which were derived from two distinct protoliths: tonalite (protomylonite) and granite (mylonite) and occur at distances of c. 0–300 m and 300–700 m from the MTL, respectively. We used quartz microstructures and crystallographic preferred orientations (CPOs) and two-feldspar thermometry to infer the spatiotemporal development of deformation conditions in the MTL mylonite zone. The mylonitic granite is sub-divided into southern and northern mylonite zones, which exhibit A- and B-type quartz microstructure formed by subgrain rotation and grain boundary migration recrystallization, respectively. In both the mylonitic granite and protomylonitic tonalite, quartz c-axis CPOs primarily display a moderate-temperature Y-maximum pattern, as well as a less common low-temperature type-I crossed-girdle pattern. In the northern zone, quartz deformation initially occurred at moderate temperatures of 435 °C–490 °C, forming mylonitic granite with B-type quartz microstructure. As temperature decreased to 355 °C–435 °C, the strain localized into the southern zone to form A-type quartz microstructure. The protomylonitic tonalite formed under similar temperature conditions. At temperatures below 355 °C, ductile deformation ceased in the majority of the granite and tonalite, with only a narrow zone of ultramylonite up to 50 m wide, deforming adjacent to the MTL at conditions approximating the brittle–ductile transition in quartz (c. 300 °C). Strain localization did not occur within a single shear zone, but in several deforming strands that formed in distinct protoliths, one of which developed into the narrow ultramylonite zone reworked as the brittle MTL, likely controlled by cooling.
The Median Tectonic Line (MTL) in Southwest Japan, a major east‐west trending arc‐parallel fault, has been defined as the boundary fault between the Cretaceous Sambagawa metamorphic rocks and the Ryoke granitic and metamorphic rocks, which are unconformably covered by the Upper Cretaceous Izumi Group. Based on the detailed fieldwork and microstructural studies of fault rocks, we reconstruct the kinematic history along the MTL during the Paleogene, which can be divided into the Ichinokawa and pre‐Tobe phases. While the Ichinokawa phase is defined by large‐scale, top‐to‐the‐north normal faulting, the pre‐Tobe phase is represented by large‐scale, high‐angle right‐stepping en échelon faults almost parallel to the MTL in the Upper Cretaceous Izumi Group. We found that left‐handed en échelon folds have developed along the right‐stepping faults, which contain 25–60 m wide cataclasite and fault gouge. Both map scale en échelon folds and microstructures (e.g., composite planar structures) in the fault rocks suggest that they were formed by sinistral‐reverse faulting with top‐to‐the‐SW kinematics. Furthermore, based on the new K‐Ar age dating of authigenic illite from the fault gouge along the MTL and right‐stepping faults, it can be concluded that the MTL was activated in two discrete stages at approximately 59 Ma (Ichinokawa phase) and 47–46 Ma (pre‐Tobe phase). Based on these results, we reappraise the kinematic framework of the MTL in the Paleogene, which can be interpreted as the record of the movements of the subducting oceanic plate relative to the continental plate.
Recent studies have debated the timing and spatial configuration of a possible intersection between the Pacific-Izanagi spreading ridge and the northeast Asian continental margin during Cretaceous or early Cenozoic times. Here we examine a newly compiled magmatic catalog of ~900 published Cretaceous to Miocene igneous rock radioisotopic values and ages from the northeast Asian margin for ridge subduction evidence. Our synthesis reveals that a nearsynchronous 56–46 Ma magmatic gap occurred across ~1500 km of the Eurasian continental margin between Japan and Sikhote-Alin, Russian Far East. The magmatic gap separated two distinct phases of igneous activity: (1) an older, Cretaceous to Paleocene pre–56 Ma episode that had relatively lower εNd(t) (–15 to + 2), elevated (87Sr/86Sr)0 (initial ratio, 0.704–0.714), and relatively higher magmatic fluxes (~1090 km2/m.y.); and (2) a younger, late Eocene to Miocene post–46 Ma phase that had relatively elevated εNd(t) (–2 to + 10), lower (87Sr/86Sr)0 (0.702–0.707), and a lower 390 km2/m.y. magmatic flux. The 56–46 Ma magmatic gap links other geological evidence across northeast Asia to constrain an early Cenozoic, low-angle ridge-trench intersection that had profound consequences for the Eurasian continental margin, and possibly led to the ca. 53–47 Ma Pacific plate reorganization.
The belt boundary thrust within the Cretaceous–Neogene accretionary complex of the Shimanto Belt, southwestern Japan, extends for more than ~ 1 000 km along the Japanese islands. A common understanding of the origin of the thrust is that it is an out of sequence thrust as a result of continuous accretion since the late Cretaceous and there is a kinematic reason for its maintaining a critically tapered wedge. The timing of the accretion gap and thrusting, however, coincides with the collision of the Paleocene–early Eocene Izanagi–Pacific spreading ridges with the trench along the western Pacific margin, which has been recently re‐hypothesized as younger than the previous assumption with respect to the Kula‐Pacific ridge subduction during the late Cretaceous. The ridge subduction hypothesis provides a consistent explanation for the cessation of magmatic activity along the continental margin and the presence of an unconformity in the forearc basin. This is not only the case in southwestern Japan, but also along the more northern Asian margin in Hokkaido, Sakhalin, and Sikhote‐Alin. This Paleocene–early Eocene ridge subduction hypothesis is also consistent with recently acquired tomographic images beneath the Asian continent. The timing of the Izanagi–Pacific ridge subduction along the western Pacific margin allows for a revision of the classic hypothesis of a great reorganization of the Pacific Plate motion between ~ 47 Ma and 42 Ma, illustrated by the bend in the Hawaii–Emperor chain, because of the change in subduction torque balance and the Oligocene–Miocene back arc spreading after the ridge subduction in the western Pacific margin.
Molnar and England (1990, https://doi.org/10.1029/JB095iB04p04833) introduced equations using a semianalytical approach that approximate the thermal structure of the forearc regions in subduction zones. A detailed new comparison with high‐resolution finite element models shows that the original equations provide robust predictions and can be improved by a few modifications that follow from the theoretical derivation. The updated approximate equations are shown to be quite accurate for a straight‐dipping slab that is warmed by heat flowing from its base and by shear heating at its top. The approximation of radiogenic heating in the crust of the overriding plate is less accurate but the overall effect of this heating mode is small. It is shown that the previous and updated approximate equations become increasingly inaccurate with decreasing thermal parameter and increasing variability of slab dip. It is also shown that the approximate equations cannot be extrapolated accurately past the brittle‐ductile transition. Conclusions in a recent paper (Kohn et al., 2018, https://doi.org/10.1073/pnas.1809962115) that modest amount of shear heating can explain the thermal conditions of past subduction from the exhumed metamorphic rock record are invalid due to a number of compounding errors in the application of the Molnar and England (1990, https://doi.org/10.1029/JB095iB04p04833) equations past the brittle‐ductile transition. The use of the improved approximate equations is highly recommended provided their limitations are taken into account. For subduction zones with variable dip and/or low thermal parameter finite element modeling is recommended.
Significance
Thermal structure controls numerous aspects of subduction zone metamorphism, rheology, and melting. Many thermal models assume small or negligible coefficients of friction and underpredict pressure–temperature (P–T) conditions recorded by subduction zone metamorphic rocks by hundreds of degrees Celsius. Adding shear heating to thermal models simultaneously reproduces surface heat flow and the P–T conditions of exhumed metamorphic rocks. Hot dry rocks are denser than cold wet rocks, so rocks from young-hot subduction systems are denser and harder to exhume through buoyancy. Thus, the metamorphic record may underrepresent hot-young subduction and overrepresent old-cold subduction.
Recent petrological studies on the Sambagawa metamorphic belt in Shikoku have recognized that the coarse-grained eclogite-bearing lithologies (so-called ‘tectonic blocks’ in earlier studies) in the Besshi area exclusively preserve evidence for the ‘early’ Sambagawa metamorphism, which can be related to onset of the Sambagawa subduction system during Early Cretaceous (c.116Ma). Geological mapping and associated multidisciplinary studies on the regional (spatially widespread) Sambagawa metamorphism (both the eclogite-facies and main metamorphisms) have revealed the tectonic framework of the Late-Cretaceous Sambagawa subduction zone as follows: (i) a spreading ridge was approaching close to the trench; (ii) the subducting slab was coupled with the convective mantle at depth of >65 km; (iii) thickness of the hanging-wall continental crust was 30-35 km; and (iv) the forearc mantle wedge (30-65 km depth) was largely serpentinized. These features allow us to draw a semi-quantitative cross-section of the Sambagawa subduction zone at around 89-85Ma, implying that boundary conditions for thermo-mechanical modeling aiming to simulate exhumation of high-P/T metamorphic rocks are now well constrained. It has also become clear that ultramafic blocks dispersed in the higher-grade part of the Sambagawa belt were derived from the mantle wedge, i.e. the corresponding part of the belt has been re-evaluated as a ‘fossil subduction boundary’ of a relatively warm subduction zone. Field-based petrological studies in the Sambagawa belt can, therefore, have potential to provide invaluable information on material behaviors at the slab-mantle wedge interface including domains of episodic tremor and slip (ETS) in present-day warm subduction zones.
Xenoliths of intensely developed mylonites and metacherts were found from the Tenryukyo granite, which is one of the oldest granitic bodies in the Ryoke high-T/P metamorphic belt, central Japan. The mylonite xenoliths consist of quartz, plagioclase, K-feldspar and biotite with a few accessory minerals. They have typical porphyroclastic microstructures; that is, relatively coarse (1-5mm) feldspar (dominantly plagioclase) porphyroclasts occur in the fine-grained (c50μm) polymineralic matrix. Petrology and geochemistry of the mylonite xenoliths indicate that they were derived from granitic protoliths Combined with geochronological studies, the presence of the mylonite xenoliths indicates that a shear zone existed in the Ryoke belt by c90 Ma before or during the crystallization of the Tenryukyo grainite. Two alternative origins for the xenoliths are possible: 1) the mylonoite xenolith came from an extended part of the porphyroclastic mylonites occuring near the Median Tectonic Line (ie a part of the Kashio shear zone), or 2) the xenoliths came from an another shear zone, such as relics of deformed granite associated with the emplacement of the Tenryukyo granite, iteself. More information is required to identify their origin. An extensive field survey in the Ryoke granites may reveal a wider distribution of mylonite xenoliths.
The International Mineralogical Association's approved amphibole nomenclature has been revised to simplify it, make it more consistent with divisions generally at 50%, define prefixes and modifiers more precisely, and include new amphibole species discovered and named since 1978, when the previous scheme was approved. The same reference axes form the basis of the new scheme and most names are little changed, but compound species names like tremolitic hornblende (now magnesiohornblende) are abolished, as are crossite (now glaucophane or ferroglaucophane or magnesioriebeckite or riebeckite), tirodite (now manganocummingtonite), and dannemorite (now manganogrunerite). The 50% rule has been broken only to retain tremolite and actinolite as in the 1978 scheme; the sodic-calcic amphibole range has therefore been expanded. Alkali amphiboles are now sodic amphiboles. The use of hyphens is defined. New amphibole names approved since 1978 include nyböite, leakeite, kornite, ungarettiite, sadanagaite, and cannilloite. All abandoned names are listed. The formulae and source of the amphibole end-member names are listed and procedures outlined to calculate Fe3+ and Fe2+ where not determined by analysis.
A reorganization centered on the Pacific plate occurred ~53-47 million years ago. A ‘top-down’ plate tectonic mechanism, complete subduction of the Izanagi plate, as opposed to a ‘bottom-up’ mantle flow mechanism, has been proposed as the main driver. Verification based on marine geophysical observations is impossible as most ocean crust recording this event has been subducted. Using a forward modeling approach, which assimilates surface plate velocities and shallow thermal structure of slabs into mantle flow models, we show that complete Izanagi plate subduction and margin-wide slab detachment induced a major change in sub-Pacific mantle flow, from dominantly southward before 60 M to north-northeastward after 50 Ma. Our results agree with onshore geology, mantle tomography, and the inferred motion of the Hawaiian hotspot and are consistent with a plate-tectonic process driving the rapid plate-mantle reorganization in the Pacific hemisphere between 60 and 50 Ma, as reflected in tectonic changes in the Pacific and surrounding ocean basins.
Microtectonics deals with the interpretation of microstructures, small-scale deformation structures in rocks that yield abundant information on the history and type of deformation and metamorphism. The results are used by geologists to obtain data for large-scale geological interpretations. This advanced textbook treats common microstructures such as foliations, porphyroblasts, veins, fringes and shear sense indicators. The book mainly focusses on optical microscopy as a tool to study microstructures, but also describes other techniques such as EBSD and tomography. Many photographs and explanatory drawings clarify the text. The new edition, substantially revised throughout and extended, features two new chapters (primary structures and experimental microstructures), 68 new figures, more than 800 new references. Microtectonics has proven useful for self study of microstructures and as a manual for short- and one-semester courses. Details of the changes in the second edition - Newest developments in microtectonics have been included in all chapters so that al chapters have been revised and updated, e.g. new information on brittle microstructures Two new chapters have been added, on primary structures and experimental microstructures Chapters on veins, shear zones, natural microgauges experimental modelling techniques and alternative techniques have been completely renewed over 800 new references have been added.
Magmatic epidote and zoisite commonly occur in Cretaceous tonalite of the Hazu area in the Ryoke belt, central Japan. The tonalite is mainly composed of amphibole, biotite, plagioclase, quartz, and epidote/zoisite with minor ilmenite, magnetite, pyrite, zircon, and apatite. Small amounts of K-feldspar occur as an interstitial phase between other felsic phases or perthitic patches in plagioclase. Epidote occurs as inclusions in plagioclase, as interstitial phase in the matrix, and as secondary phase in chlorite pseudomorphs after biotite, and in saussuritized plagioclase. The XFe [= Fe3+/(Al + Fe3+)] value of the secondary epidote ranges from 0.27 to 0.39. Epidote inclusions in plagioclase and interstitial grains contain less Fe3+ (XFe = 0.08–0.29), Fe3+-poor epidote with XFeFe value of 0.01–0.07 occurs only as inclusions in plagioclase, and usually has thin lamella-like layers of Fe3+-poor epidote with XFe = 0.09–0.14. The Fe3+-poor epidote with XFe3+-poor epidote and zoisite inclusions and their host plagioclase. Such a reaction texture is not observed at the boundary between Fe3+-richer epidote inclusions with XFe > 0.20 and their host plagioclase. Epidote grains with XFe > 0.20 in plagioclase and the matrix are a magmatic phase that crystallized directly from the tonalite magma. The Fe3+-poor epidote (XFe Keywords: JAPAN; MAGMATIC EPIDOTE; MAGMATIC ZOISITE; RYOKE BELT; TONALITE Document Type: Research Article DOI: http://dx.doi.org/10.1127/0935-1221/2014/0026-2360 Publication date: March 1, 2014 More about this publication? The European Journal of Mineralogy publishes original papers, review articles and letters dealing with the mineralogical sciences s.l. These include primarily mineralogy, petrology, geochemistry, crystallography and ore deposits, as well as environmental, applied and technical mineralogy. Nevertheless, papers in any related field, including cultural heritage, will be considered. Editorial Board Information for Authors Submit a Paper Subscribe to this Title Terms & Conditions ingentaconnect is not responsible for the content or availability of external websites $(document).ready(function() { var shortdescription = $(".originaldescription").text().replace(/\\&/g, '&').replace(/\\, '<').replace(/\\>/g, '>').replace(/\\t/g, ' ').replace(/\\n/g, ''); if (shortdescription.length > 350){ shortdescription = "" + shortdescription.substring(0,250) + "... more"; } $(".descriptionitem").prepend(shortdescription); $(".shortdescription a").click(function() { $(".shortdescription").hide(); $(".originaldescription").slideDown(); return false; }); }); Related content In this: publication By this: publisher In this Subject: Earth and Environmental Sciences , Geology By this author: Masumoto, Yosuke ; Enami, Masaki ; Tsuboi, Motohiro ; Hong, Mei GA_googleFillSlot("Horizontal_banner_bottom");
The Raman spectra of carbonaceous material (CM) from 19 metasediment samples collected from six widely separated areas of Southwest Japan and metamorphosed at temperatures from 165 to 655°C show systematic changes with metamorphic temperature that can be classified into four types: low-grade CM (c. 150–280°C), medium-grade CM (c. 280–400°C), high-grade CM (c. 400–650°C), and well-crystallized graphite (> c. 650°C). The Raman spectra of low-grade CM exhibit features typical of amorphous carbon, in which several disordered bands (D-band) appear in the first-order region. In the Raman spectra of medium-grade CM, the graphite band (G-band) can be recognized and several abrupt changes occur in the trends for several band parameters. The observed changes indicate that CM starts to transform from amorphous carbon to crystallized graphite at around 280°C, and this transformation continues until 400°C. The G-band becomes the most prominent peak at high-grade CM suggesting that the CM structure is close to that of well-crystallized graphite. In the highest temperature sample of 655°C, the Raman spectra of CM show a strong G-band with almost no recognizable D-band, implying the CM grain is well-crystallized graphite. In the Raman spectra of low- to medium-grade CM, comparisons of several band parameters with the known metamorphic temperature show inverse correlations between metamorphic temperature and the full width at half maximum (FWHM) of the D1- and D2-bands. These correlations are calibrated as new Raman CM geothermometers, applicable in the range of c. 150–400°C. Details of the methodology for peak decomposition of Raman spectra from the low to medium temperature range are also discussed with the aim of establishing a robust and user-friendly geothermometer.
The FeMg exchange reaction between coexisting garnet and phengite has been studied by reacting a natural phengite (mg = 67) in the presence of quartz and water at pressures of 20–35 kb and temperatures of 800–1000°C. Compositions of coexisting garnet and mica indicate a linear relation between both the InKD((Fe/Mg) garnet/(Fe/Mg) phengite) and temperature, and InKD and pressure in the above P.T range. This FeMg exchange reaction between garnet and phengite is shown to be dependent on the Ca-content of the garnet, and on the mg number of the bulk composition. These two composition effects have been studied by usin phengitic mica mixes with mg numbers of 23 and 46, and by using a synthetic basaltic composition. The overall results allow broad empirical calibration of separate geothermometers for pelitic and basaltic systems, respectively. However, because of non-ideality in the exchange reaction, this geothermometer should not be used in any practical application outside the composition ranges studied. Also, careful consideration of the presence of Fe3+ in phengite must be made. If the Fe3+ content of the natural phengite is unknown, then the temperatures obtained will be maximum temperatures only.
Magmatic andalusite is found from the metatexite and from the pelitic rock with boudin necks filled with leucosome in the garnet-cordierite (Grt-Crd) zone of the Aoyama area, Ryoke metamorphic belt, SW Japan. In general, andalusite crystals in the pelitic schist without leucosome from the sillimanite-K-feldspar (Sil-Kfs) zone and low-temperature part of the Grt-Crd zone are partly transformed into sillimanite, suggesting that the peak P-T conditions of the Aoyama area have reached to the sillimanite grade. On the other hand, andalusite crystals found from the leucosome of the low-temperature part of the Grt-Crd zone are subhedral, and have not transformed into sillimanite at all. Absence of an evidence for andalusite-sillimanite transition implies that these crystals did not experience the prograde andalusite-sillimanite transition and thus are retrograde products. They characteristically occur in the leucosome containing euhedral plagioclase or subhedral cordierite crystals and show subhedral shape different from the euhedral shape usually observed in the prograde andalusite. The P-T conditions for the low-temperature part of the Grt-Crd zone is sufficiently high for the partial melting of pelitic rocks, and the andalusite-bearing leucosome has appropriate composition as a frozen melt. These observations suggest that the retrograde andalusite is of magmatic origin. Combining effects of the presence of boron in the melt, addition of Al2O3 to the subaluminous Qtz-Ab-Or system to saturate in aluminosilicates, and preferential incorporation of Fe2O3 into andalusite may have favored the crystallization of magmatic andalusite in the Aoyama area. Finding of the magmatic andalusite from the low-temperature part of the Grt-Crd zone suggests that the P-T path of this zone went through the overlapping region of water-saturated solidus of peraluminous granite and andalusite stability field. The P-T path of the low-temperature part of the Grt-Crd zone may, therefore, not be a hairpin shaped one but a clockwise one when the aluminosilicate phase diagram of Holdaway (1971) is used. Non-hairpin shaped nature of the P-T path may be partly responsible for the preservation of the textural evidences of partial melting in the Aoyama area.
We present a new LA–ICP–MS system for zircon fission‐track (FT) and U–Pb double dating, whereby a femtosecond laser combined with galvanometric optics simultaneously ablates multiple spots to measure average surface U contents. The U contents of zircon measured by LA–ICP–MS and standardized with the NIST SRM610 glass are comparable to those measured by the induced fission track method, and have smaller analytical errors. LA–ICP–MS FT dating of seven zircon samples including three IUGS age standards is as accurate as the external detector method, but can give a higher‐precision age depending on the counting statistics of the U content measurement. Double dating of the IUGS age standards gives FT and U–Pb ages that are in agreement. A chip of the Nancy 91,500 zircon has a homogeneous U content of 84 ppm, suggesting the possibility of using this zircon as a matrix‐matched U standard for FT dating. When using the Nancy 91,500 zircon as a U standard, a zeta calibration value of 42–43 yr cm−2 for LA–ICP–MS FT dating is obtained. While this value is strictly only valid for the particular session, it can serve as a reference for other studies.
It is generally accepted that the main strength of subduction boundaries occurs in the shallower region where frictional deformation is dominant. However, estimates of absolute values—commonly expressed as apparent coefficient of friction, μ′—show great variation. Frictional shear heating is closely related to μ′, and estimates of the extra thermal energy supplied by shearing can in principle be used to estimate subduction zone strength. One such approach is based on surface heat flow measurements. However, heat flow in convergent margins shows large local scatter and even in the same area, different studies using this method show large variations in estimates of μ′ indicating large uncertainties. The thermal record of subduction conditions preserved in subduction-type metamorphic rocks is developed over geological time scales that average out local complexities in heat flow and therefore has good potential as an alternative indicator of the amount of shear heating and, hence, shear strength along subduction boundaries. Thermal models that incorporate shear heating were developed for two contrasting and well-known subduction-type metamorphic belts: the relatively warm Sanbagawa belt of SW Japan and the relatively cold Franciscan belt of western USA. High-grade rocks of the Sanbagawa belt show strongly curved P–T paths that display increasing P/T to about 2 GPa. Information on the rate of plate movement and the age of the subducting slab at the time of metamorphism can be combined with modelling results to show that relatively high shear stresses, equivalent to μ′∼0.13 are required to account for the observed curved P–T paths. In contrast, the high-grade rocks of the Franciscan belt show relatively cool P–T conditions that do not allow for strong shear heating with an appropriate upper bound for μ′ of ∼0.03. Modelling suggests subduction rate and lithology are potentially important controls on the development of high- versus low-stress subduction zones. High-stress subduction zones are likely to be associated with high aseismic/seismic slip ratios possibly related to slab roughness.
en We have estimated the timescale of material circulation in the Sanbagawa subduction zone based on U–Pb zircon and K–Ar phengite dating in the Ikeda district, central Shikoku. The Minawa and Koboke units are major constituents of the high‐P Sanbagawa metamorphic complex in Shikoku, southwest Japan. For the Minawa unit, ages of 92–81 Ma for the trench‐fill sediments, are indicated, whereas the age of ductile deformation and metamorphism of garnet and chlorite zones are 74–72 Ma and 65 Ma, respectively. Our results and occurrence of c. 150 Ma Besshi‐type deposits formed at mid‐ocean ridge suggest that the 60‐Myr‐old Izanagi Plate was subducted beneath the Eurasian Plate at c. 90 Ma, and this observation is consistent with recent plate reconstructions. For the Koboke unit, the depositional ages of the trench‐fill sediments and the dates for the termination of ductile deformation and metamorphism are estimated at c. 76–74 and 64–62 Ma, respectively. In the Ikeda district, the depositional ages generally become younger towards lower structural levels in the Sanbagawa metamorphic complex. Our results of U–Pb and K–Ar dating show that the circulation of material from the deposition of the Minawa and Koboke units at the trench through an active high‐P metamorphic domain to the final exhumation from the domain occurred continuously throughout c. 30 Myr (from c. 90 to 60 Ma).
沈みこみ帯における物質循環の時間スケールを検討するため，池田地域の四国三波川変成コンプレックスのジルコンU–Pb年代及びフェンジャイトK–Ar年代測定を行った. 当地域に分布する三縄ユニットと小歩危ユニットは四国三波川コンプレックスの主要構成ユニットである. 構造的上位の三縄ユニットでは，約150 Maの海洋地殻に92–81 Maの海溝充填堆積物が堆積し，74–65 Maまで変成・変形作用が進行したと推定された. この推定は，90 Ma 頃にイザナギプレートがユーラシアプレートへ沈み込んでいたことを支持する. 一方，下位の小歩危ユニットでは， 76–74 Ma の堆積年代と64–62 Maまでの変成・変形作用の進行が見積もられた. 当地域の三波川変成コンプレックス全体では，約3000万年の時間スケールで，海溝での堆積，沈み込み帯深部での変成・変形，及び地殻浅部への上昇が連続して起こっていたと推定される.
The Sanbagawa belt is one of the famous subduction‐related high‐pressure (HP) metamorphic belts in the world. However, spatial distributions of eclogite units in the belt have not yet satisfactorily established, except within the Besshi region, central Shikoku, southwest Japan because most eclogitic rocks were affected by lower‐pressure overprinting during exhumation. In order to better determine the areal distribution of the eclogite units and their metamorphic features, inclusions petrography of garnet porphyroblasts using a combination of electron‐probe microanalyzer (EPMA) and Raman spectroscopy was applied to pelitic and mafic schists from the Asemi‐gawa region, central Shikoku. All pelitic schist samples are highly retrogressed, and include no index HP minerals such as jadeite, omphacite, paragonite or glaucophane in the matrix. Garnet porphyroblasts in pelitic schists occur as subhedral or anhedral crystals, and show compositional zoning with an irregular‐shaped inner segments and an overgrown outer segments, the boundary of which is marked by discontinuous changes in spessartine. This feature suggests that a resorption process of the inner segment occurred prior to the formation of the outer segment, indicating discontinuous crystallization between the two segments. The inner segment of some composite‐zoned garnet grains displays Mn oscillations, implying infiltration of metamorphic fluid during the initial exhumation stage. Evidence for an early eclogite‐facies event was determined from mineral inclusions (e.g., jadeite, paragonite, glaucophane) in the garnet inner segments. Mafic schists include no index HP minerals in the matrix as with pelitic schists. Garnet grains in mafic schists show simple normal zoning, recording no discontinuous growth during crystal formation. There are no index HP mineral inclusions in the garnet, and thus no evidence suggesting eclogite‐facies conditions. Quartz inclusions in garnet of the pelitic and mafic schists show residual pressure values (Δω1) of >8.5 cm−1 and <8.5 cm−1, respectively. The combination of Raman geobarometry and conventional thermodynamic calculations gives peak P–T conditions of 1.6–2.1 GPa at 460–520 °C for the pelitic schists. The Δω1 values of quartz inclusions in mafic schists are converted to a metamorphic pressure of 1.2–1.4 GPa at 466–549 °C based on Raman geothermometry results. These results indicate that a pressure gap definitely exists between the mafic schists and the almost adjacent pelitic schists, which have experienced a different metamorphic history. Furthermore, the peak P–T values of the Asemi‐gawa eclogite unit are compatible with those of Sanbagawa eclogite unit in the Besshi region of central Shikoku, suggesting that these eclogite units share a similar P–T trajectory. The Asemi‐gawa eclogite unit exists in a limited area and is composed of mostly pelitic schists. We infer that these abundant pelitic schists played a key role in buoyancy‐driven exhumation by reducing bulk rock density and strength. This article is protected by copyright. All rights reserved.
The oceanic crust that enters a subduction zone is generally recycled to great depth. In rare and punctuated episodes, however, blueschists and eclogites derived from subducted oceanic crust are exhumed. Compilations of the maximum pressure-temperature conditions in exhumed rocks indicate significantly warmer conditions than those predicted by thermal models. This could be due to preferential exhumation of rocks from hotter conditions that promote greater fluid productivity, mobility, and buoyancy. Alternatively, the models might underestimate the forearc temperatures by neglecting certain heat sources. We compare two sets of global subduction zone thermal models to the rock record. We find that the addition of reasonable amounts of shear heating leads to less than 50 °C heating of the oceanic crust compared to models that exclude this heat source. Models for young oceanic lithosphere tend to agree well with the rock record. We test the hypothesis that certain heat sources may be missing in the models by constructing a global set of models that have high arbitrary heat sources in the forearc. Models that satisfy the rock record in this manner, however, fail to satisfy independent geophysical and geochemical observations. These combined tests show that the average exhumed mafic rock record is systematically warmer than the average thermal structure of mature modern subduction zones. We infer that typical blueschists and eclogites were exhumed preferentially under relatively warm conditions that occurred due to the subduction of young oceanic lithosphere or during the warmer initial stages of subduction.
The spatiotemporal relationship between granitoid intrusions and low-pressure/temperature type regional metamorphism in the Ryoke belt (Mikawa area) is investigated to understand the tectono-thermal evolution of the upper- to middle-crust during a Cretaceous flare-up event at the Eurasian active continental margin. Three plutono-metamorphic stages are recognized; (1) 99–84 Ma: intrusion of granitoids (99–95 Ma pulse) into the upper crust and high-T regional metamorphism reaching sillimanite-grade (97.0 ± 4.4 Ma to 88.5 ± 2.5 Ma) in the middle crust, (2) 81–75 Ma: intrusion of gneissose granitoids (81–75 Ma Ma pulse) into the middle crust at ~19–24 km depth, and (3) 75–69 Ma: voluminous intrusions of massive to weakly-foliated granitoids (75–69 Ma pulse) at ~ 9–13 km depth and formation of contact metamorphic aureoles. Cooling of the highest-grade metamorphic zone below the wet solidus of granitic rocks is estimated at 88.5 ± 2.5 Ma. At ca. 75 Ma, the upper-middle crustal section underwent northward tilting, resulting in the exhumation of regional metamorphic zones to ~ 9–13 km depth. Although the highest-grade metamorphic rocks and the 99–95 Ma pulse granitoids preserve similar U-Pb zircon ages, the absence of spatial association suggests that the regional metamorphic zones were mainly produced by a transient thermal anomaly in the mantle and thermal conduction through the crust, supplemented by localized advection due to granitoid intrusions. The successive emplacement of granitoids into shallow, deep and shallow levels of the crust was probably controlled by the combination of change in thermal structure of the crust and tectonics during granitoid intrusions.
Detrital zircon multi-chronology combined with provenance and low-grade metamorphism analyses enables the reinterpretation of the tectonic evolution of the Cretaceous Shimanto accretionary complex in Southwest Japan. Detrital zircon U–Pb ages and provenance analysis defines the depositional age of trench-fill turbidites associated with igneous activity in provenance. Periods of low igneous activity are recorded by youngest single grain zircon U–Pb ages (YSG) that approximate or are older than the depositional ages obtained from radiolarian fossil-bearing mudstone. Periods of intensive igneous activity recorded by youngest cluster U–Pb ages (YC1σ) that correspond to the younger limits of radiolarian ages. The YC1σ U–Pb ages obtained from sandstones within mélange units provide more accurate younger depositional ages than radiolarian ages derived from mudstone. Determining true depositional ages requires a combination of fossil data, detrital zircon ages, and provenance information. Fission-track ages using zircons estimated YC1σ U–Pb ages are useful for assessing depositional and annealing ages for the low-grade metamorphosed accretionary complex. These new dating presented here indicates the following tectonic history of the accretionary wedge. Evolution of the Shimanto accretionary complex from the Albian to the Turonian was caused by the subduction of the Izanagi plate, a process that supplied sediments via the erosion of Permian and Triassic to Early Jurassic granitic rocks and the eruption of minor amounts of Early Cretaceous intermediate volcanic rocks. The complex subsequently underwent intensive igneous activity from the Coniacian to the early Paleocene as a result of the subduction of a hot and young oceanic slab, such as the Kula–Pacific plate. Finally, the major out-of-sequence thrusts of the Fukase Fault and the Aki Tectonic Line formed after the middle Eocene, and this reactivation of the Shimanto accretionary complex as a result of the subduction of the Pacific plate.
The Ina district of the Ryoke Belt is divided into two mineral zones, based on the mineral parageneses of the pelitic and psammitic rocks at the peak metamorphism. A biotite-muscovite zone (quartz + plagioclase + biotite + muscovite with or without K-feldspar) constitutes the northwestern part, and a biotite-cordierite-K-feldspar zone (quartz + plagioclase + biotite + cordierite + K-feldspar) comprises the central to southern and eastern parts. The isograd reaction between two mineral zones is defined by a divariant reaction: Mg-rich biotite + muscovite + quartz = Fe-rich biotite + cordierite + K-feldspar + H2O (1), which, in the K2O-FeO-MgO-Al2O3-SiO2-H2O (KFMASH) system, occurs at similar to 590 degrees C at 0.2 GPa and 660 degrees C at 0.4 GPa. Fibrolite accompanied by andalusite porphyroblasts in aluminous pelitic rocks of the biotite-muscovite zone and the low-grade part of the biotite-cordierite-K-feldspar zone, suggests that sillimanite was the stable aluminosilicate at the peak metamorphic condition throughout the area. In the high-grade part of the biotite-cordierite-K-feldspar zone, fibrolite mostly occurs as inclusions in cordierite or in plagioclase. The phase relations and the compositional zoning of plagioclase in relation to fibrolite inclusions suggest that fibrolite was formed under relatively high-pressure conditions, and that partial melting took place.
The stress conditions of the ductile-to-brittle regime have been assessed along the Asuke Shear Zone (ASZ), which strikes NE–SW in the Cretaceous Ryoke granite terrain in SW Japan. Along the ASZ, pseudotachylyte and mylonitized pseudotachylyte are locally developed together with cataclasite. The simultaneous operation of dislocation creep and grain-size-sensitive creep, as indicated by the coexistence of the Z-maximum and relatively random c-axis lattice preferred orientations as well as the sizes of dynamically recrystallized quartz grains (6.40–7.79 μm) in the mylonitized pseudotachylyte, suggest differential stresses of 110–130 MPa at ∼300 °C. The e-twin morphology, twinning ratio, and distribution of the glide direction on the e-twin plane of the twinned calcite in the amygdules of the pseudotachylyte suggest the stress conditions of the σ1 and σ3 axes trend 228° and 320° and plunge 55° and 1°, respectively, and indicate differential stresses of 40–80 MPa at 150–200 °C. Based on kinematic indicators in the fault rocks, the stress conditions estimated from calcite twins, and the cooling history of the granitic protolith, the ASZ is inferred to have been activated under a stress state that caused sinistral normal movements before and after pseudotachylyte formation at 70–50 Ma.
The opening-angle of quartz c-axis fabrics (OA) is strongly temperature-dependent and has proven to be a powerful deformation thermometer for natural metamorphic rocks. Previous considerations of empirical data have identified a linear correlation between OA and temperature between 250 and 650 °C, and no correlation above 650 °C. However, possible effects of pressure have not been investigated. We expanded the data set of OA versus temperature, including data from rocks deformed over 300–1050 °C and 2.5–15 kbar. Disregarding possible effects of pressure, the OA-temperature relationship can be described by two linear correlations for the intervals ~ 250–650 °C and ~ 650–1050 °C:
T (°C) = 6.9 OA (degrees) + 48 (250 °C ≤ T ≤ 650 °C and OA ≤ 87°).
T (°C) = 4.6 OA (degrees) + 258 (650 °C ≤ T ≤ 1050 °C and OA ≥ 87°).
The change on the curve slope of the OA-temperature relationship correlates approximately to the low-high quartz transition and to changes in the dynamic recrystallization mechanism from subgrain rotation to grain boundary migration. The available data suggest that pressure has a secondary effect accompanying the major temperature dependence of OA, which is particularly important for temperatures above 650 °C, where the correlation between OA and temperature is less pronounced. For fixed pressures, the OA has logarithmic relationships with temperature over the range 250–1050 °C. The following thermometer equation is formulated from a multiple regression:
T (°C) = 410.44 ln OA (degrees) + 14.22 P (kbar) – 1272.
An uncertainty of ± 50 °C is inherited from the petrological temperature estimates of the natural samples. The data suggest the gradual increasing importance of prism [c] slip relative to < a > slip in quartz with rising temperature. Under conditions of ‘average’ geological strain rate and water weakening, prism [c] slip dominates for deformation above ~ 700 °C.
K-Ar and Rb-Sr age determinations were carried out on rocks along the Median Tectonic Line (MTL) in the Bungui-toge area, central Japan. K-Ar ages for the Ryoke granitic and metamorphic rocks range from 26.2 to 72.3 Ma. Within about 300 m of MTL the ages decrease toward MTL. The cooling rate for the Hiji tonalite and the Katsuma quartz diorite, calculated from K-Ar ages and closure temperatures of minerals, is 28oC/Ma and 27oC/Ma, respectively. The age of emplacement for the 2 granitic masses is estimated to be about 80 Ma, whereas the mylonitization is assumed to have been completed at about 70 Ma. The Sambagawa crystalline schists, which are in direct contact with the Ryoke rocks at MTL, give K-Ar and Rb-Sr muscovite ages of 57.8-65.9 Ma and 57.9-65.9 Ma, respectively. The decrease in muscovite ages of schists is observed only within about 20 m of MTL. Mica ages for rocks of the Ryoke and Sambagawa belts far away from MTL are about 65 Ma and similar to each other. Secondary K- feldspar from veinlets in a pelitic schist near MTL gives a K-Ar age of 20.7 Ma. Seven samples of pelitic rocks give a Rb-Sr whole-rock age of 69.3 + or - 10.3 Ma, which probably represents the final stage of metamorphism.-from English summary
The maximum-pressure PT conditions (Pmax-T) and prograde PT paths of exhumed subduction-related metamorphic rocks are compared to predictions of PT conditions from computational thermal models of subduction systems. While the range of proposed models encompasses most estimated Pmax-T conditions, models predict temperatures that are on average colder than those recorded by exhumed rocks. In general, discrepancies are greatest for Pmax<2 GPa, where only a few of the highest-T model paths overlap petrologic observations and model averages are 100-300 °C colder than average conditions recorded by rocks. Prograde PT paths similarly indicate warmer subduction than typical models. Both petrologic estimates and models have inherent biases. Petrologic analysis may overestimate temperatures at Pmax where overprinting occurs during exhumation, although PT paths suggest that relatively warm conditions are experienced by rocks on the prograde subduction path. Models may underestimate temperatures at depth by neglecting shear heating, hydration reactions and fluid and rock advection. Our compilation and comparison suggest that exhumed high-P rocks provide a more accurate constraint on PT conditions within subduction zones, and that those conditions may closely represent the subduction geotherm. While exhumation processes in subduction zones require closer petrologic scrutiny, the next generation of models should more comprehensively incorporate all sources of heat. Subduction-zone thermal structures from currently available models appear to be inaccurate, and this mismatch has wide-reaching implications for our understanding of global geochemical cycles, the petrologic structure of subduction zones, and fluid-rock interactions and seismicity within subduction zones.
The chlorites are good indicators of rock history because their wide compositional variations are sensitive to the formation conditions, like pressure (P), temperature (T), redox state, fluid composition. Accordingly, many geothermometers based on their composition, either empirically or thermodynamically, have been proposed during the last 30 years, especially in low-temperature contexts (T , 350°C). This paper presents a graphical tool that considerably facilitates the use of two of the most recent chlorite thermometers for low- and very-low-T chlorites. The temperature–composition relationships for low-T chlorites are represented in T–R2+–Si diagrams, allowing chlorite compositions to be predicted as a function of temperature or, conversely, temperature to be estimated from compositional fields of natural chlorites. This graphical projection is based on a comparison of the parameters (ideal chlorite compositions and calculated T) predicted by geothermometers with analyses of natural chlorites for which independent T estimates are available over a range of geological environments. The new T–R2+–Si diagram provides a practical tool for thermometric purposes in the applicability range of the considered models, in particular for diagenesis and low-grade metamorphism.
Geophysical and geological observations document that beneath the submerged forearc, processes of sediment subduction and subduction erosion move large volumes of material toward the mantle. The conveying system is the subduction channel separating the upper plate from the underthrusting ocean plate. Globally, the zero-porosity or solid-volume rate at which continental debris is shuttled toward the mantle is estimated to be similar to 2.5 km(3)/yr. To deliver this volume, the average thickness of the subduction channel is similar to 1.0 km. Some deeply subducted material is returned to the surface of Earth as a component of arc magma or as tracks of high-P/T crustal underplates. But over long periods of time (>50 m.y.), most of the removed material is evidently recycled to the mantle. Applying Cenozoic recycling rates to the past astonishingly implies that since 2.5 Ga a volume of continental crust equal to the standing inventory of similar to 6 x 10(9) km(3) has been removed from the surface of Earth. This minimum estimate does not include crustal material recycled at continental collision zones nor reliable estimates of recycling where large accretionary bodies form. The volume of demolished crust is so large that recycling must have been a major factor determining the areal pattern and age distribution of continental crust. The small areal exposure of Archean rock is thus probably more a consequence of long-term crustal survival than the volume originally produced. Reconstruction of older supercontinents is made difficult if not unachievable by the progressive truncation of continental edges effected by subduction zone recycling, in particular by subduction erosion.
The International Mineralogical Association's approved amphibole nomenclature has been revised in order to simplify it, make it more consistent with divisions generally at 50%, define prefixes and modifiers more precisely and include new amphibole species discovered and named since 1978, when the previous scheme was approved. The same reference axes form the basis of the new scheme and most names are little changed but compound species names like tremolitic hornblende (now magnesiohornblende) are abolished and also crossite (now glaucophane or ferroglaucophane or magnesioriebeckite or riebeckite), tirodite (now manganocummingtonite) and dannemorite (now manganogrunerite). The 50% rule has been broken only to retain tremolite and actinolite as in the 1978 scheme so the sodic calcic amphibole range has therefore been expanded. Alkali amphiboles are now sodic amphiboles. The use of hyphens is defined. New amphibole names approved since 1978 include nyböite, leakeite, kornite, ungarettiite, sadanagaite and cannilloite. All abandoned names are listed. The formulae and source of the amphibole end member names are listed and procedures outlined to calculate Fe ³⁺ and Fe ²⁺ when not determined by analysis.
The metamorphic facies series in regional metamorphism may be classified into the following categories according to an order of increasing rock pressure: (1) andalusite-sillimanite type, (2) low-pressure intermediate group, (3) kyanite-sillimanite type, (4) high-pressure intermediate group, and (5) jadeite-glaucophane type.In Japan and other parts of the circum-Pacific region, a metamorphic belt of the andalusite-sillimanite type and/or low-pressure intermediate group and another metamorphic belt of the jadeite-glaucophane type and/or high-pressure intermediate group run side by side, forming a pair. The latter belt is always on the Pacific Ocean side. They were probably formed in different phases of the same cycle of orogeny. Their origin is discussed.Regional metamorphism under higher rock pressures appears to have taken place in later geological times.The metamorphic facies series of contact metamorphism are briefly discussed.
Cenozoic clastic sediments in the Kuma area, Shikoku, southwest Japan, previously designated as the Kuma Group, are here redefined as the Hiwada-toge Formation and the overlying Kuma Group sensu stricto, considering a significant time-gap between them. The Hiwada-toge Formation is Early Eocene in age on the basis of dinoflagellate cysts, while the lower part of the Kuma Group s.s. (Sagayama Formation) was dated as late Early Miocene by fission-track dating. Organic microfossil assemblages show that the Hiwada-toge Formation contains marine strata and the lower part of the Kuma Group s.s. (Sagayama Formation) is of non-marine origin.These results provide two geochronological constraints critical to the regional tectonic history.The age of the Hiwada-toge Formation indicates that the Sanbagawa metamorphic rocks came under subaerial erosion by Early Eocene in its provenance area. The relationship between the Median Tectonic Line (M.T.L.) and the northern extension of the Miocene Kuma Group s.s. and its contiguous strata shows that a compressional activity of the M.T.L. occurred during a relatively short period in late Early to Middle Miocene.
In models for strain-partitioning at obliquely-convergent plate boundaries, trench-parallel slip occurs on a vertical fault. Trench-parallel slip at the Nankai subduction zone, SW Japan, is mapped along the Median Tectonic Line (MTL) which dips approximately 40°N. To understand its structural context and how the MTL functions in this slip-partitioned system, we collected a set of three seismic profiles in the Kii peninsula south of Osaka, using a multi-scale acquisition strategy that provides increasingly fine resolution. To understand its fault kinematics, we analyzed microseismic activity in two locations on the fault, using source data from Japan's Hi-net monitoring network. Structural details suggest that the MTL functioned as a megathrust during subduction of the Cretaceous Sanbagawa HP metamorphic belt. Its current pattern of microseismicity shows that it behaves as a strike-slip fault with no indication of a vertical fault at or around its surface trace. Thus, trench-parallel slip at the Nankai is now accommodated on an inclined fault plane in an unusual form of partitioning. This system appears to have developed out of a two-phase tectonic history in which a thrust structure that formed under initial-phase compressive stresses has been reactivated as a strike-slip fault under subsequent-phase shear stresses. Its unusual kinematics show that shear failure can occur on an existing non-vertical fault plane at a regional scale in preference to the rupture of a new ideal (vertical) fault plane.
This report describes an investigation of the composite metamorphic history recorded in garnet porphyroblasts of Sambagawa metasediments, and presents discussions of the possible distribution of eclogite facies lithologies in the Besshi region, central Shikoku, southwest Japan. Garnet grains usually show chemically composite zoning with resolved inner and overgrown outer segments, respectively. The inner segment usually contains paragonite as a sodic phase inclusion and rarely contains omphacite or glaucophane, whereas the outer segment rarely includes albite but no paragonite. The inner segment often includes quartz grains that preserve high residual pressures corresponding to the eclogite facies conditions, whereas the outer segment includes quartz grains that preserve lower residual pressures corresponding to the epidote–amphibolite facies conditions. For this composite-zoned garnet, the assemblage of sodic phase inclusions and the values of the residual pressure of quartz inclusions imply the following successive metamorphic path: prograde eclogite facies stage decompression and hydration reaction stage prograde epidote–amphibolite facies stage. Metasediments containing this composite-zoned garnet with such evidence for eclogite facies conditions are more widely distributed than the eclogitic lithologies that contain the assemblage of omphacite + garnet. These results imply that the combination of (i) the chemical composite zoning of garnet, (ii) the nature of sodic phase inclusions, and (iii) the residual pressure of quartz inclusions is useful as criteria to ascertain the true distribution of the lithologies that have experienced eclogite facies metamorphism for the Sambagawa metasediments, which was difficult to recognize in earlier studies.
For quartz-rich tectonites two types of deformation thermometer are currently commonly employed: 1) The quartz c-axis fabric opening-angle thermometer that provides an estimate of deformation temperatures when fabrics were 'locked in' during dislocation creep and dynamic recrystallization. 2) The quartz recrystallization thermometer that indicates a range of likely deformation temperatures based on observed microstructures and inferred mechanisms of dynamic recrystallization. A critically important caveat in applying both thermometers is the assumption that deformation temperature is the primary controlling factor in recrystallization mechanisms and fabric development. However, fabric opening-angles and recrystallization mechanisms are also sensitive to other variables such as strain rate and water weakening. In this paper the development of these thermometers is reviewed, and their potential sensitivities to competing factors such as temperature, strain rate, water weakening and (in the case of opening-angles) 3D strain type are discussed. Examples of the application of these potential thermometers to naturally deformed quartz-rich rocks are given, and case studies of correlations between deformation temperatures estimated by these thermometers and temperatures of synkinematic metamorphism determined by petrology-based thermobarometers are highlighted. In the review, attention is focused on problems associated with applying these thermometers to natural deformation, and examples of such problems are discussed.
The internal structure of the Median Tectonic Line (MTL) fault zone and the processes that prevailed at depths are described based on an analysis of a borehole. The fault plane which defines the boundary between the Ryoke- and Sanbagawa-derived rocks dips at 56° to the north. Immediately beneath the boundary, approximately 40 m thick fractured rocks form the major strand of the MTL fault zone. The hanging wall above the boundary comprises variably deformed Ryoke granitoids, including several mylonite zones and cataclasite zones. The fault zone has evolved through a series of faulting events under temperatures ranging from 400 to 200 °C. The mineral assemblages of the mylonites and cataclasites immediately above the boundary indicate that these fault rocks were formed at temperatures of about 300 °C. These mylonites and cataclasites represent, therefore, fault rocks that formed immediately below and above the brittle–plastic transition, respectively. Development of dissolution seams in these cataclasites suggests that the cataclasite has low strength. The presence of pseudotachylytes in the cataclasite indicates the occurrence of seismicity immediately above the brittle–plastic transition. On the other hand, the very fine grain size of recrystallised quartz in the mylonites indicates high differential stress immediately below the brittle–plastic transition. It is therefore likely that the differential stresses immediately below the brittle–plastic transition are much higher than those immediately above the transition. Formation of laumontite in the major strand of the MTL fault zone occurred at temperatures of around 200 °C. The central slip zone of the major strand is about 30-cm thick, and is surrounded by thick gouge zones. This situation is favourable for thermal pressurisation during earthquake slips.
Since the development of SIMS and LA-ICP-MS technologies in the 1980s and 1990s, single grain U–Pb dating of detrital zircon has quickly become the most popular technique for sedimentary provenance studies. Currently by far the most widespread method for visualising detrital age distributions is the so-called Probability Density Plot (PDP), which is calculated by summing a number of Gaussian distributions whose means and standard deviations correspond to the individual ages and their respective analytical uncertainties. Unfortunately, the PDP lacks a firm theoretical basis and can produce counter-intuitive results when data quantity (number of analyses) and/or quality (precision) is high. As a more robust alternative to the PDP, this paper proposes a standard statistical technique called Kernel Density Estimation (KDE), which also involves summing a set of Gaussian distributions, but does not explicitly take into account the analytical uncertainties. The Java-based DensityPlotter program (http://densityplotter.london-geochron.com) was developed with the aim to facilitate the adoption of KDE plots in the context of detrital geochronology.
The Cretaceous (pre-Japan Sea) Sanbagawa metamorphism affected the Japanese Jurassic complex south of the Median Tectonic Line in the regions now recognized as the Sanbagawa, Mikabu and Chichibu belts. The metamorphic peak (116 Ma) was reached and passed during the tectonic 'D 1' deformation, corresponding to sinistral shear N30°E along the eastern margin of the Asian continent. This was followed by 'D 2' (c. 85 Ma) fold and thrust deformation, the vergence of which is normal to the 'D 1' trend. These deformational events established the present thermal structure. The final regional deformation formed upright 'D 3' folds. The four metamorphic zones based on pelitic assemblages can be enhanced by using basic schists to subdivide the pelitic chlorite zone. Apparent Fe-Mg partition coefficients between chlorite and garnet show an essential regional continuity of metamorphism and that thrust-offsets do not juxtapose elements from different mineral zones. Peak conditions of metamorphism ranging from 250°C and 6 kbars to 600°C and 10 kbars are consistent with simple P-T-t loops which progress at higher pressures and return at lower pressures to the surface.
Application of Raman spectrometry to determine residual pressureretained by quartz grains sealed in garnet reveals significantdifferences between eclogite facies rocks and lower-grade schistoserocks of the Sanbagawa metamorphic belt in central Shikoku,Japan. Garnet in the eclogitic lithologies commonly exhibitstwo chemically distinct growth stages. The inner segment commonlyincludes quartz grains with higher residual pressures than thosein the outer segment, suggesting that the former representsprograde eclogite facies and the latter formed during exhumation.Regional variation of the quartz residual pressure suggeststhat the eclogite unit has a far greater extent than previouslyrecognized, and reveals a large pressure gap between the newlyproposed eclogite and noneclogite units.
Submarine slump folds are common in turbidites of the Izumi Group (Upper Cretaceous), south of Osaka in southwest Japan. Structural analysis of the folds indicates predominantly eastward movement of the slump sheet. However, it is noteworthy that this direction is generally opposite to westward paleocurrents obtained from sole marks, but it coincides with the direction of younging of the strata and the plunge direction of macroscopic synclines in the Izumi Group. The paleoslope inferred from the slump folds is believed to have been caused by the eastward migration of the depocenter of the Izumi sedimentary basin. This slumpi ng is thought to have followed tilting of the beds deposited by the westward current. The migration of the depocenters and tilting are associated with left-slip faulting on the bounding Median tectonic line.
Metasedimentary rocks generally contain carbonaceous material (CM) deriving from the evolution of organic matter originally present in the host sedimentary rock. During metamorphic processes, this organic matter is progressively transformed into graphite s.s. and the degree of organisation of CM is known as a reliable indicator of metamorphic grade. In this study, the degree of organisation of CM was systematically characterised by Raman microspectroscopy across several Mesozoic and Cenozoic reference metamorphic belts. This degree of organisation, including within-sample heterogeneity, was quantified by the relative area of the defect band (R2 ratio). The results from the Schistes Lustrés (Western Alps) and Sanbagawa (Japan) cross-sections show that (1) even through simple visual inspection, changes in the CM Raman spectrum appear sensitive to variations of metamorphic grade, (2) there is an excellent agreement between the R2 values calculated for the two sections when considering samples with an equivalent metamorphic grade, and (3) the evolution of the R2 ratio with metamorphic grade is controlled by temperature (T). Along the Tinos cross-section (Greece), which is characterised by a strong gradient of greenschist facies overprint on eclogite facies rocks, the R2 ratio is nearly constant. Consequently, the degree of organisation of CM is not affected by the retrogression and records peak metamorphic conditions. More generally, analysis of 54 samples representative of high-temperature, low-pressure to high-pressure, low-temperature metamorphic gradients shows that there is a linear correlation between the R2 ratio and the peak temperature [T(°C) = −445 R2 + 641], whatever the metamorphic gradient and, probably, the organic precursor. The Raman spectrum of CM can therefore be used as a geothermometer of the maximum temperature conditions reached during regional metamorphism. Temperature can be estimated to ± 50 °C in the range 330–650 °C. A few technical indications are given for optimal application.
This paper reviews recent progress on the geotectonic evolution of exotic Paleozoic terranes in Southwest Japan, namely the Paleo-Ryoke and Kurosegawa terranes. The Paleo-Ryoke Terrane is composed mainly of Permian granitic rocks with hornfels, mid-Cretaceous high-grade metamorphic rocks associated with granitic rocks, and Upper Cretaceous sedimentary cover. They form nappe structures on the Sambagawa metamorphic rocks. The Permian granitic rocks are correlative with granitic clasts in Permian conglomerates in the South Kitakami Terrane, whereas the mid-Cretaceous rocks are correlative with those in the Abukuma Terrane. This correlation suggests that the elements of Northeast Japan to the northeast of the Tanakura Tectonic Line were connected in between the paired metamorphic belt along the Median Tectonic Line, Southwest Japan. The Kurosegawa Terrane is composed of various Paleozoic rocks with serpentinite and occurs as disrupted bodies bounded by faults in the middle part of the Jurassic Chichibu Terrane accretionary complex. It is correlated with the South Kitakami Terrane in Northeast Japan. The constituents of both terranes are considered to have been originally distributed more closely and overlay the Jurassic accretionary terrane as nappes. The current sporadic occurrence of these terranes can possibly be attributed to the difference in erosion level and later stage depression or transtension along strike-slip faults. The constituents of both exotic terranes, especially the Ordovician granite in the Kurosegawa-South Kitakami Terrane and the Permian granite in the Paleo-Ryoke Terrane provide a significant key to reconstructing these exotic terranes by correlating them with Paleozoic granitoids in the eastern Asia continent.
The Ryoke belt, comprising low-pressure regional metamorphic rocks and low-level granitoids, expresses a deep in the crust of the Cretaceous Eurasian continental margin before the opening of Japan Sea. The petrological P-T estimation of the metamorphic rocks indicates the condition of upper to middle crust level. The zonally arranged areal distribution of the San-yo (shallow facies) to Ryoke (deep facies) granitic provinces from back-arc side to fore-arc side is concordant with the occurrence of the associated low-grade to high-grade regional metamorphic rocks. They are regarded as an upper to middle crustal section of the Cretaceous Eurasian continental margin. These metamorphic rocks are derived mainly from the Jurassic accretionary complex. In the migmatitic high-grade part, the metasedimentary rocks were just being transformed into continental crust.Isotopic ages of these metamorphic and granitic rocks show an along-arc variation of the systematic eastward younging from 100 Ma to 70 Ma for 800 km. It cannot be interpreted with the steady-state subduction model and indicates that the orogeny was caused by an episodic event, presumably the subduction of Kula-Pacific ridge beneath the Eurasian continent during Cretaceous. The present juxtaposition of the Ryoke and the Sanbagawa paired metamorphic belts inevitably needs a secondary displacement from their birthplace. Two possibilities should be examined: along-arc transcurrent movement or across-arc overthrusting.
In the Yanai district, southwest Japan, garnet crystals in pelitic and siliceous rocks are normally zoned, reversely zoned or homogeneous. Normally zoned crystals show a continuous decrease in Mn content from the core towards the margin, whereas reversely zoned crystals show enrichment of Mn at the margin. Normally zoned garnet grew by one of two distinct coarsening mechanisms, either nucleation and growth or impingement and overgrowth.In pelitic rocks, there is a systematic change from normal zoning to homogeneous and reverse zoning with increasing metamorphic grade. This can be ascribed to the enhancement of internal diffusion with increasing metamorphic temperature. In the cordierite-I zone, with an estimated metamorphic temperature of 560 ± 30°, garnet in pelitic rocks with a grain size larger than 0.2 mm preserves its zoning pattern, whereas garnet in siliceous rocks with a similar grain size is homogeneous. This suggests that both rock type and grain size are factors that influence the degree to whole growth zoning of garnet is homogenized. The relative paucity of biotite in siliceous rock can explain the development of fine-grained garnet in this rock type, which is, therefore, easily homogenized. Coarse homogeneous garnet in the siliceous rocks was formed by impingement of small grains during prograde metamorphism. Such aggregates are easily homogenized by both internal and grain-boundary diffusion.
The strain geometry of a Cretaceous low-P/high-T metamorphosed accretionary complex in SW Japan is one of the key geological constraints in understanding the Cretaceous tectonics of the forearc region at the eastern margin of Eurasia. We studied the strain geometry of upper greenschist to lower amphibolite facies metamorphic rocks from the mid-Cretaceous low-P/high-T Ryoke metamorphic belt, which during the mid-Cretaceous were located at mid-crustal depths close to the volcanic front. Strain analysis focused on deformed radiolarian fossils in metachert and sandstone/chert clasts within metapelite. The maximum stretching direction (X-direction) of the strain ellipsoid of the schistosity-forming deformation at the peak metamorphism is oriented E-W, and the XY-plane of the strain ellipsoid developed parallel to schistosity. The metachert and metapelite exhibit plane strain to general flattening strain that probably largely resulted from the schistosity-forming deformation. Based on the results of the previous strain analyses and this study, it reveals that the schistosity-forming deformation occurred under bedding-normal shortening, resulting in general flattening strain throughout the Ryoke metamorphic belt. The X-direction of the strain ellipsoid of the schistosity-forming deformation is oriented parallel to the length of the Ryoke metamorphic belt throughout the SW Japan arc. The schistosity formed parallel to bedding planes that developed horizontally in non-metamorphosed accretionary complexes of SW Japan, indicating horizontal XY-plane of the strain ellipsoid. These observations indicate the occurrence of horizontal shear deformation within the middle crust (∼15km depth) at the time of the low-P/high-T metamorphism. The deep-seated horizontal shear zone and the shallower incipient Median Tectonic Line (MTL) fault system could result in a crustal-scale detachment fault in the forearc region of the mid-Cretaceous SW Japan arc. After the formation of the detachment, the left-lateral pull-apart forearc basins, Onogawa-Izumi Group distributed on the north side of the MTL and the southern marginal mylonite zone might be formed by the sinistral movement along the detachment fault.