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TitlePhase equilibria modelling of blueschist and eclogite from the Sanbagawa metamorphic belt of southwest Japan reveals along-strike consistency in tectonothermal architecture
AuthorWeller, O M; Wallis, S R; Aoya, M; Nagaya, T
SourceJournal of Metamorphic Geology vol. 33, issue 6, 2015 p. 579-596,
LinksSupplementary Data - Données supplémentaires
Alt SeriesEarth Sciences Sector, Contribution Series 20150009
Mediapaper; on-line; digital
File formatpdf
AreaShiroku; Kotsu; Besshi; Japan
Lat/Long WENS 133.2833 134.2667 34.5000 33.8333
Subjectstectonics; igneous and metamorphic petrology; tectonic evolution; tectonic environments; subduction zones; mid-ocean ridges; metamorphism; thermal history; deformation; lithology; metamorphic rocks; metamorphic facies; schists; gneisses; eclogites; iron; modelling; phase equilibria; pressure-temperature conditions; petrography; whole rock analyses; bulk composition; fabric analyses; x-ray fluorescence; major element analyses; Upper Cretaceous; Sanbagawa metamorphic belt; Nankai Trough; Phanerozoic; Mesozoic; Cretaceous
Illustrationsgeological sketch maps; phase diagrams; photomicrographs; tables; profiles; plots
ProgramGEM2: Geo-mapping for Energy and Minerals Baffin Bedrock Mapping
Released2015 07 06
AbstractThe Sanbagawa metamorphic belt of southwest Japan is one of the type localities of subduction related high-P metamorphism. However, variable pressure-temperature (P-T) paths and metabasic assemblages have been reported for eclogite units in the region, leading to uncertainty about the subduction zone paleo-thermal structure and associated tectonometamorphic conditions. To analyse this variation, phase equilibria modelling was applied to the three main high-P metabasic rock types documented in the region - glaucophane eclogite, barroisite eclogite and garnet blueschist - with modelling performed over a range of P, T, bulk rock H2O and bulk rock ferric iron conditions using THERMOCALC. All samples are calculated to share a common steep prograde P-T path to similar peak conditions of 16-20 kbar and 560-610 °C. The results establish that regional assemblage variation is systematic, with the alternation in peak amphibole phase due to peak conditions overlapping the glaucophane-barroisite solvus, and bulk composition effects stabilizing blueschist v. eclogite facies assemblages at similar P-T conditions. Furthermore, the results reveal that a steep prograde P-T path is common to all eclogite units in the Sanbagawa belt, indicating that metamorphic conditions were consistent along strike. All localities are compatible with predictions made by a ridge approach model, which attributes eclogite facies metamorphism and exhumation of the Sanbagawa belt to the approach of a spreading ridge.
Summary(Plain Language Summary, not published)
The Sanbagawa metamorphic belt is an 800-km long rock belt situated in south-west Japan. The belt formed during the late Cretaceous (c. 90 Ma) in a subduction zone setting similar to the one offshore Japan today. By studying the rocks exposed in the Sanbagawa belt it is possible to learn about subduction zone processes. This contribution applies ¿thermobarometric analysis¿ to a suite of samples from the region. Thermobarometric analysis uses the composition of minerals in a rock sample to calculate the pressure-temperature (P-T) history that it has experienced. We applied the latest form of thermobarometry, known as phase equilibria modeling, to provide the advanced insight. Previous studies of the belt described many different kinds of metamorphic rocks and P-T histories. The results of this work systematically account for this variation, and show that P-T conditions were in fact uniform along strike, such that a single model can now describe the formation of the entire belt.

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