Abstract The convex form of subduction‐stage pressure–temperature ( P–T ) paths up to c. 2.0 GPa implies the Sambagawa high‐ P metamorphic belt, Japan, formed a few million years before ridge subduction. Additional compilation of P–T conditions for higher‐ P Sambagawa rocks ( c. 2.0–2.5 GPa) reveals that the thermal profile along the slab surface shows a remarkable high‐ T ‐ward warping at c. 2.0 GPa ( c. 65 km). Previous thermal models indicate that this warping corresponds to the onset of induced mantle flow towards the subducting slab. If a normal thickness continental crust of c. 30 km was present, this implies the hangingwall region between 30 and 65 km depth was occupied by serpentinized wedge mantle isolated from large‐scale mantle flow. Subsequent arrival of the spreading ridge, reheating and dehydration of the serpentinized wedge probably supplied the water necessary for causing granitic magmatism in the Ryoke high‐ T metamorphic belt, which is paired with the Sambagawa belt.
Abstract The R aman spectra of carbonaceous material ( CM ) from 19 metasediment samples collected from six widely separated areas of S outhwest J apan 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 R aman 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 R aman spectra of CM show a strong G ‐band with almost no recognizable D ‐band, implying the CM grain is well‐crystallized graphite. In the R aman 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 D 1‐ and D 2‐bands. These correlations are calibrated as new R aman CM geothermometers, applicable in the range of c. 150–400°C. Details of the methodology for peak decomposition of R aman spectra from the low to medium temperature range are also discussed with the aim of establishing a robust and user‐friendly geothermometer.
Abstract Two contrasting results of strain analyses, constriction and flattening, are recognized in the Sambagawa high‐pressure/temperature metamorphic belt, SW Japan. An unverified proposal to account for this situation is that the constrictional strain ellipsoids develop only in areas where there is strong overprinting by a secondary Du‐phase folding after a penetrative Ds‐phase deformation. Field studies in the Hibihara district, central Shikoku, which is located between a southern constrictional region and a northern flattening region, reveal there is a map‐scale contrast in the effect of Du: outcrop‐scale Du upright folds are common in the southern region while they are rare in the northern region. Field measurements show that overall orientation of Ds strain is characterized by E–W stretching and vertical shortening, while that of Du strain is characterized by N–S shortening and vertical extension. The shortening caused by Du in the southern high‐Du‐strain regions estimated by the fold‐curve tracing method are down to about 70%–56% and by removing this Du shortening most of the constrictional strain ellipsoids are restored back to the flattening field with only a few exceptions, suggesting that pre‐Du strain states of the Sambagawa metamorphic rocks were dominantly of the flattening type. These results support the previous ideas that invoke differences in the strength of Du overprinting for the two different types of strain ellipsoids observed in the Sambagawa belt. From a tectonic point of view, the semi‐penetrative occurrence of Du folds throughout the Sambagawa belt, which stretches about 800 km in SW Japan, suggests that the Du phase can be related to some ancient plate movement. The significant shortening subnormal to the orogen that characterizes Du may reflect a shift to orogen‐subnormal subduction of the Pacific plate beneath the Eurasian plate at around 60 Ma.
Research Article| November 01, 2005 North-south extension in the Tibetan crust triggered by granite emplacement Mutsuki Aoya; Mutsuki Aoya 1Institute of Geology and Geoinformation, National Institute of Advanced Industrial Science and Technology, Central 7, Tsukuba 305-8567, Japan Search for other works by this author on: GSW Google Scholar Simon R. Wallis; Simon R. Wallis 2Department of Earth and Planetary Sciences, Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8602, Japan Search for other works by this author on: GSW Google Scholar Kentaro Terada; Kentaro Terada 3Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University, Hiroshima 739-8526, Japan Search for other works by this author on: GSW Google Scholar Jeffrey Lee; Jeffrey Lee 4Department of Geological Sciences, Central Washington University, Ellensburg, Washington 98926, USA Search for other works by this author on: GSW Google Scholar Tetsuo Kawakami; Tetsuo Kawakami 5Department of Earth Sciences, Faculty of Education, Okayama University, Okayama 700-8530, Japan Search for other works by this author on: GSW Google Scholar Yu Wang; Yu Wang 6Department of Geology, China University of Geosciences, Beijing 100083, China Search for other works by this author on: GSW Google Scholar Matt Heizler Matt Heizler 7New Mexico Bureau of Mines and Mineral Resources, New Mexico Tech, 801 Leroy Place, Socorro, New Mexico 87801-4796, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Mutsuki Aoya 1Institute of Geology and Geoinformation, National Institute of Advanced Industrial Science and Technology, Central 7, Tsukuba 305-8567, Japan Simon R. Wallis 2Department of Earth and Planetary Sciences, Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8602, Japan Kentaro Terada 3Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University, Hiroshima 739-8526, Japan Jeffrey Lee 4Department of Geological Sciences, Central Washington University, Ellensburg, Washington 98926, USA Tetsuo Kawakami 5Department of Earth Sciences, Faculty of Education, Okayama University, Okayama 700-8530, Japan Yu Wang 6Department of Geology, China University of Geosciences, Beijing 100083, China Matt Heizler 7New Mexico Bureau of Mines and Mineral Resources, New Mexico Tech, 801 Leroy Place, Socorro, New Mexico 87801-4796, USA Publisher: Geological Society of America Received: 20 Apr 2005 Revision Received: 04 Jul 2005 Accepted: 06 Jul 2005 First Online: 02 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (2005) 33 (11): 853–856. https://doi.org/10.1130/G21806.1 Article history Received: 20 Apr 2005 Revision Received: 04 Jul 2005 Accepted: 06 Jul 2005 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation Mutsuki Aoya, Simon R. Wallis, Kentaro Terada, Jeffrey Lee, Tetsuo Kawakami, Yu Wang, Matt Heizler; North-south extension in the Tibetan crust triggered by granite emplacement. Geology 2005;; 33 (11): 853–856. doi: https://doi.org/10.1130/G21806.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract We combine zircon sensitive high-resolution ion microprobe U-Pb spot dating and mica 40Ar-39Ar plateau ages with field-geological and geochemical constraints from the Mala shan area of Southern Tibet to show that the deformed granite core of the North Himalayan metamorphic domes in this area is not Indian basement, but was intruded and deformed during the Himalayan orogeny. Microstructural observations reveal that a transition from top-to-the-south thrust-related to top-to-the-north extension-related deformation occurred during granite intrusion and related metamorphism. This suggests that intrusion triggered the onset of extensional tectonics in the Tibetan middle to upper crust. Expected positive feedback mechanisms between decompression melting leading to more intrusion and more extensional deformation suggest that this mechanism may have been important on a regional scale. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
Both structural and petrological data can be used to constrain the P–T path of an eclogitic schist unit (the Seba basic schist) in the Sambagawa belt of SW Japan. The relationships between these two sets of data are well defined by porphyroblastic and other microstructures. The derived P–T path for the Seba basic schist has an overall clockwise trajectory with the decompression, or exhumation-related, path taking place under a lower P/T gradient than the burial, or subduction-related, path. The clockwise nature of the P–T path is qualitatively supported by chemical zoning of amphibole coexisting with eclogitic minerals. The significant feature of the P–T path is the presence of two temperature maxima, the first in the eclogite facies and the second in the epidote-amphibolite facies. The existence of two temperature maxima gives a simple explanation for the observation that metamorphic zonal boundaries postdating the eclogite facies metamorphism cross-cut the distribution of the main eclogite bodies in the Sambagawa belt. Estimates of metamorphic pressure using the jadeite content of clinopyroxene in the Seba area demonstrate the existence of a tectonic discontinuity between the eclogitic schist and surrounding non-eclogitic schist. Structural studies show that although these two units have experienced very different peak metamorphic conditions, they became juxtaposed during a single ductile deformation affecting both units. This deformation is related to exhumation of the eclogitic schist and subduction of the non-eclogitic schist, indicating that both were formed during the same subduction event. The presence of a major tectonic boundary between two units with a similar origin as subducted and accreted material, but contrasting metamorphic histories, can be interpreted in terms of nappe tectonics, and the existence of an 'eclogite nappe', the third nappe of the Sambagawa belt, is proposed.
Abstract Prograde P–T paths and thermal modelling suggest metamorphism in the Sanbagawa belt represents unusually warm conditions for subduction‐type metamorphic belts, and these likely reflect conditions of a convergent margin a few million years before the arrival of an active spreading ridge. Radiometric age data and kinematic indicators of ductile deformation suggest the Sanbagawa belt formed in a Cretaceous convergent margin associated with a plate movement vector that had a large sinistral oblique component with respect to the belt, the East Asian margin. Plate reconstructions for the Cretaceous to Tertiary for this region show that the only plausible plate compatible with such motion at this time is the Izanagi plate. These reconstructions also show that progressively younger sections of the Izanagi plate were subducted beneath eastern Asia, i.e. a spreading ridge approached, until 85–83 Ma when the Izanagi Plate ceased to exist as an independent plate. The major reorganization of plates and associated movements around this time is likely to be the age of major interaction between the ridge and convergent margin. The ridge‐approach model for the Sanbagawa metamorphism, therefore, predicts that peak metamorphism is a few million years older than this age range. New Lu–Hf dating of eclogite in the Sanbagawa belt gives ages of 89–88 Ma, in excellent agreement with the prediction. Combining this estimate for the peak age of metamorphism with published P–T‐t results implies vertical exhumation rates of greater than 2.5 cm yr −1 . This high rate of exhumation can explain the lack of a significant thermal overprint in the Sanbagawa belt during subduction of the ridge.
Abstract Carbonaceous material (CM) undergoes progressive changes that reflect its thermal history. These changes are in general irreversible and provide valuable information for understanding diagenetic and metamorphic processes of crustal rocks. Among various approaches to quantify these changes, the R2 ratio, area ratio of specific peaks in CM Raman spectra, is widely used to estimate the maximum temperature of intermediate- to moderately high-grade metamorphism. The calculation of the R2 ratio requires peak deconvolution of the original spectrum, and the results depend on the details of how this is carried out. However, a clear protocol for selecting appropriate initial conditions has not been established and obtaining a reliable temperature estimate depends at least in part on the experience and skill of the operator. In this study, we developed a Python code that automatically calculates the R2 ratio from CM Raman spectra. Our code produces R2 ratios that are generally in good agreement with those of Aoya et al. (J Metamorph Geol 28:895–914, 2010, https://doi.org/10.1111/j.1525-1314.2010.00896.x ) for the same Raman data, with much less time and effort than was the case in the previous studies. We have confirmed that the code is also applicable to other previous datasets from both contact and regional metamorphic regions. The overall trend of the recalculated data indicates that samples with R2 greater than ~ 0.7 are not sensitive to the changes in CM maturity and thus should not be used for the calibration of an R2-based geothermometer. We propose a modified geothermometer for contact metamorphism that is strictly applicable to samples with R2 from 0.023 to 0.516, with the proviso that a laser with a wavelength of 532 nm should be used. A slight extrapolation of the newly proposed geothermometer up to R2 of 0.57 provides a temperature estimate that is consistent with the geothermometer of Kaneki and Kouketsu (Island Arc 31:e12467, 2022; https://doi.org/10.1111/iar.12467 ); the boundary between the two geothermometers corresponds to a temperature of 391 °C.
