Home About us Contact
|
Home About us Contact | ||
|
|
||
Sanbagawa Belt (sanbagawa + belt)
Selected AbstractsLarge-scale folding in the Asemi-gawa region of the Sanbagawa Belt, southwest JapanISLAND ARC, Issue 2 2010Hiroshi Mori Abstract The Sanbagawa Belt generally shows higher metamorphic grade at higher structural levels. This inversion can be interpreted as reflecting an original inverted thermal structure typical of subduction zone environments. However, repetitions in the distribution of the metamorphic zones seen in central Shikoku, Japan, clearly show the original thermal structure has been affected by deformation after the peak of metamorphism. This repetition has been attributed to both the action of tectonic discontinuities and regional folding. It is important to distinguish between these two interpretations to determine the extent to which the original subduction zone sequence is preserved. Analyses of lithological and structural data reveal the presence of a large-scale post-metamorphic fold in the central part of the highest grade region. This folding has an axis that coincides with the thermal axis implied by the distribution of the metamorphic zonation, suggesting the repetition of metamorphic zones in this area can be accounted for by folding without the need for major discontinuities. [source] Brief history of petrotectonic research on the Sanbagawa Belt, JapanISLAND ARC, Issue 4 2004Shohei Banno Abstract Petrological study of the Sanbagawa schists was initiated by B. Koto (1856,1935) and extensive petrographic works were performed by J. Suzuki (1896,1970) and Y. Horikosi (1905,1992), who studied in the Besshi area of central Shikoku. Petrological work based on the mineral facies concept of P. Eskola (1883,1964) was initiated in Japan in the 1950s by A. Miyashiro on the low pressure/temperature (P/T) Abukuma complex, and then by Y. Seki and S. Banno on the high P/T Sanbagawa Metamorphic Belt. A unique inverted thermal structure was established by researchers in the 1970s. Therefore, mainly geological and petrographic features of the Sanbagawa Belt were established by the 1990s, and contemporary researchers are now testing the classical images using the new and quantitative viewpoints of geochronological, structural, tectonic, and thermal modeling. [source] Three-dimensional finite strain analysis in the high-grade part of the Sanbagawa Belt using deformed meta-conglomerateISLAND ARC, Issue 2 2002Yoshinori Moriyama Abstract Regional ductile deformation of the Sanbagawa belt is generally thought to be characterized by constrictional strain, based on strain analysis using deformed radiolarians in the low-grade regions. Similar strain analysis could not be carried out in the medium- to high-grade zones, because it is very difficult to identify individual radiolarians after strong recrystallization. However, discovery of the first known meta-conglomerate in the high-grade region of the Sanbagawa Belt allows quantitative 3-D strain to be estimated in this region. Using a development of the Rf-, method, an evaluation of appropriate errors for this estimate can be determined. The principal strain ratios and estimated errors are X/Y = 5.4,6.6 and Y/Z = 3.8,3.9 implying deformation in the flattening field and refuting the idea of uniform constrictional strain. Semi-quantitative markers of the shape of the strain ellipse throughout the high-grade regions suggest that the deformation of the Sanbagawa Belt is dominantly in the flattening field. The difference with the earlier results may be due to late-stage overprinting by upright folding of the main ductile fabric in the low-grade region of western Shikoku. [source] Neoproterozoic high-pressure/low-temperature metamorphic rocks in the Avalon terrane, southern New Brunswick, CanadaJOURNAL OF METAMORPHIC GEOLOGY, Issue 5 2001C. E. White Abstract High -P/low -T metamorphic rocks of the Hammondvale metamorphic suite (HMS) are exposed in an area of 10 km2 on the NW margin of the Caledonian (Avalon) terrane in southern New Brunswick, Canada. The HMS is in faulted contact on the SE with c. 560,550 Ma volcanic and sedimentary rocks and co-magmatic plutonic units of the Caledonian terrane. The HMS consists of albite- and garnet-porphyroblastic mica schist, with minor marble, calc-silicate rocks and quartzite. Pressure and temperature estimates from metamorphic assemblages in the mica schist and calc-silicate rocks using TWQ indicate that peak pressure conditions were 12.4 kbar at 430 °C. Peak temperature conditions were 580 °C at 9.0 kbar. 40Ar/39Ar muscovite ages from three samples range up to 618,615 Ma, a minimum age for high -P/low- T metamorphism in this unit. These ages indicate that the HMS is related to the c. 625,600 Ma subduction-generated volcanic and plutonic units exposed to the SE in the Caledonian terrane. The ages are also similar to those obtained from detrital muscovite in a Neoproterozoic-Cambrian sedimentary sequence in the Caledonian terrane, suggesting that the HMS was exposed by latest Neoproterozoic time and supplied detritus to the sedimentary units. The HMS is interpreted to represent a fragment of an accretionary complex, similar to the Sanbagawa Belt in Japan. It confirms the presence of a major cryptic suture between the Avalon terrane sensu stricto and the now-adjacent Brookville terrane. [source] Parentage of low-grade metasediments in the Sanbagawa belt, eastern Shikoku, Southwest Japan, and its geotectonic implicationsISLAND ARC, Issue 3 2010Kazuo KiminamiArticle first published online: 19 AUG 2010 Abstract This study examines the geology of low-grade (chlorite zone) metamorphic rocks in the Sanbagawa belt and of a Jurassic accretionary complex in the Northern Chichibu belt, eastern Shikoku, Japan. The bulk chemistries of metasandstones and metapelites in the Sanbagawa belt of eastern Shikoku are examined in order to determine their parentage. The Sanbagawa belt can be divided into northern and southern parts based on lithology and geologic structure. Geochemical data indicate that metasediments in the northern and southern parts are the metamorphic equivalents of the KS-II (Coniacian,Campanian) and KS-I (late Albian,early Coniacian) units of the Shimanto belt, respectively. The depositional ages of the parent sediments of low-grade metamorphic rocks found in the Sanbagawa belt and the Jurassic Northern Chichibu belt, indicate a north-younging polarity. In contrast, sedimentological evidence indicates younging to the south. These observations suggest that a tectonic event has resulted in a change from a northerly to southerly dip direction for schistosity and bedding in the Sanbagawa and Northern Chichibu belts of eastern Shikoku. The younging polarity observed in the Sanbagawa and Northern Chichibu belts, together with previously reported data on vitrinite reflectance and geological structure, indicate that the Northern Chichibu belt was part of the overburden formerly lying on top of the Sanbagawa low-grade metamorphic rocks. [source] Three-dimensional finite strain analysis in the high-grade part of the Sanbagawa Belt using deformed meta-conglomerateISLAND ARC, Issue 2 2002Yoshinori Moriyama Abstract Regional ductile deformation of the Sanbagawa belt is generally thought to be characterized by constrictional strain, based on strain analysis using deformed radiolarians in the low-grade regions. Similar strain analysis could not be carried out in the medium- to high-grade zones, because it is very difficult to identify individual radiolarians after strong recrystallization. However, discovery of the first known meta-conglomerate in the high-grade region of the Sanbagawa Belt allows quantitative 3-D strain to be estimated in this region. Using a development of the Rf-, method, an evaluation of appropriate errors for this estimate can be determined. The principal strain ratios and estimated errors are X/Y = 5.4,6.6 and Y/Z = 3.8,3.9 implying deformation in the flattening field and refuting the idea of uniform constrictional strain. Semi-quantitative markers of the shape of the strain ellipse throughout the high-grade regions suggest that the deformation of the Sanbagawa Belt is dominantly in the flattening field. The difference with the earlier results may be due to late-stage overprinting by upright folding of the main ductile fabric in the low-grade region of western Shikoku. [source] Talc-phengite-albite assemblage in piemontite-quartz schist of the Sanbagawa metamorphic belt, central Shikoku, JapanISLAND ARC, Issue 1 2000J. Izadyar Abstract The talc (Tlc) + phengite (Phn) + albite (Ab) assemblage is newly confirmed in MnOtotal -rich (1.65 wt% in average) piemontite-quartz schists from the intermediate- and high-grade part of the Sanbagawa belt, central Shikoku, Japan. Talc is in direct contact with Phn, Ab and chlorite (Chl) with sharp boundaries, suggesting that these four phases mutually coexist. Other primary constituents of the Tlc-bearing piemontite-quartz schist are spessartine, braunite, hematite (Ht), crossite/barroisite and dolomite. Phlogopite (Phl) rarely occurs as a later stage mineral developing along the rim of Phn. The studied piemontite-quartz schist has mg# (= Mg/(Mg + Fe2+)) ~ 1.0, because of its high oxidation state. Schreinemakers' analysis in the KNMASH system and the mineral assemblage in the Sanbagawa belt propose a possible petrogenetic grid, in which the Tlc,Phn assemblage is stable in a P-T field surrounded by the following reactions: lower-pressure limit by Chl + Phl + quartz (Qtz) = Phn + Tlc + H2O as proposed by previous workers; higher-pressure limit by glaucophane + Qtz = Tlc + Ab + H2O; and higher-temperature limit by Tlc + Phn + Ab = Phl + paragonite + Qtz + H2O. Thermodynamic calculation based on the database of Holland & Powell (1998), however, suggests that the Tlc,Phn stability field defined by these reactions is unrealistically limited around 580,600 °C at 11.6,12.0 (± 0.7) kbar. Schreinemakers' analysis in the KNMA-Fe3+ -SH system and the observed mineral assemblage predict that Chl + crossite = Tlc + Ab + Ht + H2O is a preferable Tlc-forming reaction in the intermediate-grade part of the Sanbagawa belt and that excess Ab + hematite narrows the stability field of the Tlc,Phn assemblage. [source] Age and early metamorphic history of the Sanbagawa belt: Lu,Hf and P,T constraints from the Western Iratsu eclogiteJOURNAL OF METAMORPHIC GEOLOGY, Issue 5 2009S. ENDO Abstract Two distinct age estimates for eclogite-facies metamorphism in the Sanbagawa belt have been proposed: (i) c. 120,110 Ma based on a zircon SHRIMP age for the Western Iratsu unit and (ii) c. 88,89 Ma based on a garnet,omphacite Lu,Hf isochron age from the Seba and Kotsu eclogite units. Despite the contrasting estimates of formation ages, petrological studies suggest the formation conditions of the Western Iratsu unit are indistinguishable from those of the other two units,all ,20 kbar and 600,650 °C. Studies of the associated geological structures suggest the Seba and Western Iratsu units are parts of a larger semi-continuous eclogite unit. A combination of geochronological and petrological studies for the Western Iratsu eclogite offers a resolution to this discrepancy in age estimates. New Lu,Hf dating for the Western Iratsu eclogite yields an age of 115.9 ± 0.5 Ma that is compatible with the zircon SHRIMP age. However, petrological studies show that there was significant garnet growth in the Western Iratsu eclogite before eclogite facies metamorphism, and the early core growth is associated with a strong concentration of Lu. Pre-eclogite facies garnet (Grt1) includes epidote,amphibolite facies parageneses equilibrated at 550,650 °C and ,10 kbar, and this is overgrown by prograde eclogite facies garnet (Grt2). The Lu,Hf age of c. 116 Ma is strongly skewed to the isotopic composition of Grt1 and is interpreted to reflect the age of the pre-eclogite phase. The considerable time gap (c. 27 Myr) between the two Lu,Hf ages suggests they may be related to separate tectonic events or distinct phases in the evolution of the Sanbagawa subduction zone. [source] Progress of actinolite-forming reactions in mafic schists during retrograde metamorphism: an example from the Sanbagawa metamorphic belt in central Shikoku, JapanJOURNAL OF METAMORPHIC GEOLOGY, Issue 5 2005A. OKAMOTO Abstract Hydration reactions are direct evidence of fluid,rock interaction during regional metamorphism. In this study, hydration reactions to produce retrograde actinolite in mafic schists are investigated to evaluate the controlling factors on the reaction progress. Mafic schists in the Sanbagawa belt contain amphibole coexisting with epidote, chlorite, plagioclase and quartz. Amphibole typically shows two types of compositional zoning from core to rim: barroisite , hornblende , actinolite in the high-grade zone, and winchite , actinolite in the low-grade zone. Both types indicate that amphibole grew during the exhumation stage of the metamorphic belt. Microstructures of amphibole zoning and mass-balance relations suggest that: (1) the actinolite-forming reactions proceeded at the expense of the preexisting amphibole; and (2) the breakdown reaction of hornblende consumed more H2O fluid than that of winchite, when one mole of preexisting amphibole was reacted. Reaction progress is indicated by the volume fraction of actinolite to total amphibole, Yact, with the following details: (1) reaction proceeded homogeneously in each mafic layer; (2) the extent of the hornblende breakdown reaction is commonly low (Yact < 0.5), but it increases drastically in the high-grade part of the garnet zone (Yact,>,0.7); and (3) the extent of the winchite breakdown reaction is commonly high (Yact,>,0.7). Many microcracks are observed within hornblende, and the extent of hornblende breakdown reaction is correlated with the size reduction of the hornblende core. Brittle fracturing of hornblende may have enhanced retrograde reaction progress by increasing of influx of H2O and the surface area of hornblende. In contrast to high-grade rocks, the winchite breakdown reaction is well advanced in the low-grade rocks, where reaction progress is not associated with brittle fracturing of winchite. The high extent of the reaction in the low-grade rocks may be due to small size of winchite before the reaction. [source] Exhumation rates and age of metamorphism in the Sanbagawa belt: new constraints from zircon fission track analysisJOURNAL OF METAMORPHIC GEOLOGY, Issue 1 2004S. Wallis Abstract Zircon fission track dating and track length analysis in the high-grade part of the Asemigawa region of the Sanbagawa belt demonstrates a simple cooling history passing through the partial annealing zone at 63.2 ± 5.8 (2 ,) Ma. Combining this age with previous results of phengite and amphibole K,Ar and 40Ar/39Ar dating gives a cooling rate of between 6 and 13 °C Myr,1, which can be converted to a maximum exhumation rate of 0.7 mm year,1 using the known shape of the P,T path. This is an order of magnitude lower than the early part of the exhumation history. In contrast, zircon fission track analyses in the low-grade Oboke region show that this area has undergone a complex thermal history probably related to post-orogenic secondary reheating younger than c. 30 Ma. This event may correlate with the widespread igneous activity in south-west Japan around 15 Ma. The age of subduction-related metamorphism in the Oboke area is probably considerably older than the generally accepted range of 77,70 Ma. [source] | ||||||||||