関東山地三波川変成帯の岩石学的研究

京都大学0048新制・課程博士理学博士甲第3247号理博第905号新制||理||491(附属図書館)UT51-60-G336京都大学大学院理学研究科地質学鉱物学専攻(主査)教授 坂野 昇平, 教授 森本 信男, 教授 亀井 節夫学位規則第5条第1項該当Kyoto UniversityDFA

Abukuma and Sanbagawa metamorphic belts in the Kanto district

'29th IGC field trip guide book' vol.5, C08: [Editors] Hirokazu Kato, Harufumi Noro「IGC巡検ガイドブック」 第5巻: [編者] 加藤 碵一, 野呂 春

Progressive evolution of whole-rock composition during metamorphism revealed by multivariate statistical analyses

The geochemical evolution of metamorphic rocks during subduction‐related metamorphism is described on the basis of multivariate statistical analyses. The studied data set comprises a series of mapped metamorphic rocks collected from the Sanbagawa metamorphic belt in central Shikoku, Japan, where metamorphic conditions range from the pumpellyite–actinolite to epidote–amphibolite facies. Recent progress in computational and information science provides a number of algorithms capable of revealing structures in large data sets. This study applies k‐means cluster analysis (KCA) and non‐negative matrix factorization (NMF) to a series of metapelites, which is the main lithotype of the Sanbagawa metamorphic belt. KCA describes the structures of the high‐dimensional data, while NMF provides end‐member decomposition which can be useful for evaluating the spatial distribution of continuous compositional trends. The analysed data set, derived from previously published work, contains 296 samples for which 14 elements (Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, P, Rb, Sr, Zr and Ba) have been analysed. The KCA and NMF analyses indicate five clusters and four end‐members, respectively, successfully explaining compositional variations within the data set. KCA indicates that the chemical compositions of metapelite samples from the western (Besshi) part of the sampled area differ significantly from those in the east (Asemigawa). In the west, clusters show a good correlation with the metamorphic grade. With increasing metamorphic grade, there are decreases in SiO₂ and Na₂O and increases in other components. However, the compositional change with metamorphic grade is less obvious in the eastern area. End‐member decomposition using NMF revealed that the evolutional change of whole‐rock composition, as correlated with metamorphic grade, approximates a stoichiometric increase of a garnet‐like component in the whole‐rock composition, possibly due to the precipitation of garnet and effusion of other components during progressive dehydration. Thermodynamic modelling of the evolution of the whole‐rock composition yielded the following results: (1) the whole‐rock composition at lower metamorphic grade favours the preferential crystallization of garnet under the conditions of the garnet zone, with biotite becoming stable together with garnet in higher‐grade rock compositions under the same P–T conditions; (2) with higher‐grade whole‐rock compositions, more H₂O is retained. These results provide insight into the mechanism suppressing dehydration under high‐P metamorphic conditions. This mechanism should be considered in forward modelling of the fluid cycle in subduction zones, although such a quantitative model has yet to be developed

Partitioning of trace elements and rare earth elements between titanite and major rock-forming minerals in eclogite and clinopyroxenite from the northeastern Shandong Peninsula, eastern China

第2回極域科学シンポジウム/第31回極域地学シンポジウム 11月17日（木） 国立極地研究所　2階大会議

Schreinemakers' nets for Group B and C Eclogites in a Model 4-Component and 8-Phase System

Petrogenetic grids for Group B and C eclogites, which were formed in the jadeite-quartz stability field, are constructed in a model 4-component and 8-phase system, with excess Fe-rich garnet. 8 geometrically independent nets are derived from this model system which treats univariant curves as straight lines. Varying the composition of the excess garnet causes topological changes in the nets and three topologically similar groups can be distinguished: Set I with the most Fe-rich garnet, Set II with intermediate garnet composition, and Set III with garnet richer in Mg than usual group C eclogites. All possible nets for Sets I and II will be presented and the most plausible net will be chosen by comparing them with petrographic and synthetic evidence

Preservation and retrogression of ultra high pressure (UHP) rocks: case studies of UHP metagranitoids in Western Alps and Su-Lu region

The in situ association of ultra high pressure (UHP) rocks and country gneiss, and possible mechanisms to obliterate the UHP metamorphic minerals during exhumation will be discussed for giving a new insight to a debate between in situ vs. external origin of UHP rocks. The UHP rocks associated with UHP metagranitoids found in southern Dora Maira massif, western Alps and the Yangkou unit in the Su-Lu region, eastern China are distinct examples of the in situ origin. The deformation and accompanied fluid infiltration at lowpressure conditions pervasively erased the precursor UHP evidence without a significant heating in these areas. Granulite facies overprinting on UHP/HP metamorphic rocks is reported in some UHP terranes. Annealing at medium temperature and low-pressure conditions could be another candidate for obliterating the UHP metamorphic minerals pervasively

Additional evidence of pre-Silurian high-pressure metamorphic rocks in Spitsbergen

It has been previously suggested that the high-pressure metamorphic rock complcx of Motalafjclla. central-western Spitsbergen, is older than Lower Silurian. An unconformity has been discovered at the base of a limestone which contains fossils similar to those reported as Lower Silurian from the same massif. This proves that the high-pressure metamorphics represent an older rock complex. The unconformity surfacc is invcrted in the area, and large recumbent folds involving the Silurian Bulltindcn Formation havc been mapped. These folds are considered to be prc-Carboniferous

High- and ultrahigh-pressure metamorphism from microscopic to orogenic scale

International audienceThis volume includes contributions presented during the 10th International Eclogite Conference held in Courmayeur, Italy, September 2nd-10th, 201

Fossilized Melts in Mantle Wedge Peridotites

The shallow oxidized asthenosphere may contain a small fraction of potassic silicate melts that are enriched in incompatible trace elements and volatiles. Here, to determine the chemical composition of such melt, we analysed fossilized melt inclusions, preserved as multiphase solid inclusions, from an orogenic garnet peridotite in the Bohemian Massif. Garnet-poor (2 vol.%) peridotite preserves inclusions of carbonated potassic silicate melt within Zn-poor chromite ( 1180 °C at 3 GPa). The carbonated potassic silicate melt, which has a major element oxide chemical composition of K₂O= 5.2 wt.%, CaO = 17 wt.%, MgO = 18 wt.%, CO₂ = 22 wt.%, and SiO₂ = 20 wt.%, contains extremely high concentrations of large ion lithophile elements, similar to kimberlite melts. Peridotites cooled down to ≅800 °C during Stage 2, resulted in the growth of garnet relatively poor in pyrope content, molar Mg/(Mg + Fe + Ca + Mn), (ca. 67 mol.%). This garnet displays a sinusoidal REE pattern that formed in equilibrium with carbonatitic fluid. Subsequently, subduction of the peridotite resulted in the formation of garnet with a slightly higher pyrope content (70 mol.%) during the Variscan subduction Stage 3 (950 °C, 2.9 GPa). These data suggest the following scenario for the generation of melt in the mantle wedge. Primarily, infiltration of sediment-derived potassic carbonatite melt into the deep mantle wedge resulted in the growth of phlogopite and carbonate/diamond. Formation of volatile-bearing minerals lowered the density and strength of the peridotite. Finally, phlogopite-bearing carbonated peridotite rose as diapirs in the mantle wedge to form carbonated potassic silicate melts at the base of the overriding lithosphere