Search

To ensure the presence of coesite and its transformed polymorph, quartz, in ultrahigh-pressure (UHP) rocks and to examine the relic of the phase transformation, crystal structures were analyzed by single-crystal X-ray diffraction (XRD) directly using the rock thin section mounted on a slide glass. The rock sample used is a coesite-bearing eclogite from the Sulu UHP terrain, eastern China. The crystal structures were determined successfully by this new method and the presence of coesite and quartz in UHP rocks was identified for the first time by XRD. The R -factor [ R ( F )] converged to 0.046 for coesite and 0.087 for quartz. The displacement ellipsoids for coesite and quartz are larger than those previously reported for these two phases, which is consistent with expected effects of trapped strain due to the phase transformation from coesite to quartz during exhumation from the Earth’s mantle. This paper is the first report of single-crystal XRD of a rock thin section on a glass slide and establishes the technique, and provides proof-of-concept of the method. Although the mineral species included in a thin section can often be identified by other methods, such as Raman spectroscopy, an advantage of the reported method is that it can be applied to any mineral in a thin section, and not just to the UHP minerals. Moreover, it is applicable to an unknown or new mineral in a thin section, discarding the spots of known minerals and constructing a lattice from the residual spots to find the structure of the unknown phase.

Phase equilibrium relationships for the pelitic schists were examined by "pseudosection" method using the bulk rock composition of Sanbagawa pelitic schist in the garnet zone of central Shikoku, Japan. The obtained P-T pseudosections reproduce the observed sequence of mineral zones in the Sanbagawa pelitic schist: the chlorite, garnet and biotite zones with an increasing order of metamorphic grade. The effect of MnO content on the stability of garnet was examined on P-T pseudosections by changing the MnO content of the bulk rock composition from 0.0 to 0.2 wt%. It follows that the characteristic sequence of the mineral zones observed in the Sanbagawa pelitic schist owes to the higher pressure of metamorphism along with the MnO content. (Communicated by Nobuo MORIMOTO, M.J.A.)

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.

Ultramafic rocks of the Sanbagawa belt include serpentinite, and metamorphosed peridotite, dunite, and associated gabbro, all considered to be parts of an ophiolitic association. This paper presents the hypothesis that parts of the metamorphosed mafic and ultramafic rocks represent the bottom of the mantle wedge that was hydrated by fluid released from the subduction complex. The hydrated mantle material was subsequently scraped off by the down-going slab to be incorporated into the structurally higher portion of the subduction complex where the temperature was higher than the lower parts of the complex. This model explains why the ultramafic rock is metamorphosed at the same grade as the enclosing Sanbagawa schists. Eclogites are widespread remnants of peak metamorphism in the Sanbagawa belt, and reveal an early part of the P-T path of the ultramafic rocks and their maximum depth of burial. P-T conditions of the Sanbagawa subduction complex are (700-800°C, 3-4 GPa), i.e., approaching those of ultrahigh-p...

The frequency of occurrence of minerals in 1876 samples of Sanbagawa pelitic schist in central Shikoku is summarized on the basis of microscopic observation accompanied, in part, by use of an electron microprobe. All samples contain quartz, plagioclase, phengite, chlorite and graphite. More than 90% of samples contain clinozoisite, titanite and apatite. Garnet is present in 95% of samples from the garnet zone, and biotite is present in 64% of samples from the albite-biotite zone. Calcite is found in about 40% of samples of the pelitic schist collected from outcrop, but occurs in 95% of the pelitic schist from drill cores. Calcite was apparently ubiquitous in the pelitic schist during the Sanbagawa metamorphism, but must have been dissolved recently by the action of surface or ground water. The mineral assemblages of the Sanbagawa pelitic schist have to be analyzed in the system with excess calcite, quartz, albite (or oligoclase), clinozoisite, graphite and fluid that is composed mainly of H2O, CO2 and CH4. In the presence of calcite, reactions that produce garnet, rutile, oligoclase, biotite and hornblende, some of which define isograds of the metamorphic belt, should be written as mixed volatile equilibria that tend to take place at lower temperature than the dehydration reactions that have been proposed. The presence of calcite in pelitic schist suggests that fluid composition is a variable as important in determining mineral assemblages as pressure and temperature. Thus Ca-bearing phases must be taken into account to analyze the phase relations of calcite-bearing pelitic schist, even if CaO content of Sanbagawa pelitic schist is low. As calcite is a common phase, the mineral assemblages of the biotite zone pelitic schist may contravene the mineralogical phase rule and warrant further study.

Jadeitic pyroxenes in two Franciscan samples, one, a possible segregation vein in metabasite, and another, a metabasite, both from Ward Creek, Cazadero area, western California, were examined with the electron microprobe. The samples were collected from the area of Type III rocks, the highest-grade portion of the pumpellyite zone. Jadeitic pyroxenes from both samples are very heterogeneous and cannot be ascribed to crystallization under conditions of surface equilibrium with the coexisting phases. Two possibilities for their origin are suggested: one is as tectonic blocks, and the other as the highest-grade rocks from the zone that crystallized close to or within the P-T stability field of pure jadeite + quartz.

Granulitized coesite-bearing eclogite from Weihai, northeastern part of the Shandong peninsula, eastern China was studied in detail to reveal the modification of mineral chemistry during decompression metamorphism. Considerable modification of chemical composition is recorded in clinopyroxene that occurs both as inclusions in garnet and as a matrix mineral. Careful examination of chemical variation with the change in microstructure made it possible to estimate the equilibrium composition of minerals at the coesite eclogite and garnet granulite stages. We were able to define three reference points on the P–T path, namely, coesite eclogite (3 GPa, 660±40°C), granulite (1 GPa, 700±30°C) and amphibolite (0.9 GPa, 600±20°C). The path thus obtained is similar to those obtained by previous workers and supports nearly isothermal decompression of coesite eclogite.

This paper reviews chemical characteristics of common minerals in ultrahigh-pressure (UHP) metamorphic rocks. Garnet in UHP metamorphic rocks belongs to the almandine-pyrope-grossular series and ha...

Petrological and isotopic evidence suggests that the protolith of the Sulu ultrahigh-pressure (UHP) terrane was Precambrian continental crust consisting of granite, granodiorite, gabbro, marble, and basic dikes, with local granulite-facies assemblages. Around 220 Ma this unit of continental rocks was buried to depths up of ∼120 km within the mantle. Structures formed during exhumation suggest highly mobile behavior of acidic rocks, even under conditions of very low water activity. Petrological studies show that the Sulu terrane underwent isothermal decompression, which implies relatively rapid exhumation, and suggests that the role of melting during exhumation may have been underestimated. The later stages of exhumation are associated with NW-SE-directed tectonic transport and the formation of at least one major normal detachment.

The concept of composition space of J.B. Thompson, Jr. is introduced to Japanese readers. It is emphasized that composition space is useful as it is systematic, but it may be too abstract in some cases. It is also emphasized that the concept of the chemical potential of exchange component is particularly useful as it simplifies the expressions of simultaneous chemical equations.

The chemical microstructure of five jadeitic pyroxenes from three Franciscan quartzose metagraywacke samples was investigated by backscattered electron imagery, element mapping, and electron microprobe microanalysis. These clinopyroxenes are neoblastic, subhedral grains or subradial aggregates replacing albite. For each compositionally zoned Cpx grain, a series of up to five parallel, polished sections were cut essentially normal or parallel to the crystallographic c axis. Five investigated grains, single and subradial aggregates, were examined quantitatively and three-dimensional chemical sections of the grains were constructed. Chemical zoning progresses from the Na-Cpx core to rim in four distinct regions (Q, L, Acm, and T). Region Q consists of microcrystalline blebs of quartz and jadeite (chi jd = 0.95) and has a bulk composition of nearly pure albite. Very rarely it contains albite. L is jadeite (chi jd = 0.80) that contains lawsonite inclusions. Acm is acmite-rich Cpx (chi jd = 0.65), and T consists of TiO 2 -bearing jadeite (1-2 wt% TiO 2 , chi jd = 0.95). Isolated Cpx grains and prismatic aggregates display the same chemical architecture regardless of crystallographic orientation. The earliest growth stage, Q, represents a volume-for-volume replacement of pre-existing albite by nearly stoichiometric jadeite plus quartz. As prisms grew, diffusion from the lithic matrix progressively enriched outer Na-Cpx zones in Fe (super 3+) , Ca, and Mg. Terminal stages of high-pressure growth are represented by TiO 2 -bearing jadeitic pyroxenes, possibly reflecting a temperature increase or relatively longterm annealing under the same conditions in the presence of titanite and rutile. The core Q and rim T regions represent a jadeite plus quartz assemblage and confirm our earlier notion that the jadeite zone of Franciscan metamorphic rocks was subducted to depths where the assemblage jadeite and quartz was stable, at pressures of more than 10 kbar.

The pumpellyite–actinolite facies proposed by Hashimoto is defined by the common occurrence of the pumpellyite–actinolite assemblage in basic schists. It can help characterize the paragenesis of basic and intermediate bulk compositions, which are common constituents of various low-grade metamorphic areas. The dataset of mutually consistent thermodynamic properties of minerals gives a positive slope for the boundary between the pumpellyite–actinolite and prehnite–pumpellyite facies in P–T space. In the Sanbagawa belt in Japan, the mineral parageneses of hematite-bearing and -free basic schists, as well as pelitic schists have been well documented. The higher temperature limit of this facies is defined by the disappearance of the pumpellyite+epidote+actinolite+chlorite assemblage in hematite-free basic schists with XFe3+ of epidote around 0.20–0.25 and the appearance of epidote+actinolite+chlorite assemblage with XEpFe3+≤0.20. In hematite-bearing basic schists, there is a continuous change of paragenesis to higher grade, epidote–glaucophane or epidote–blueschist facies. In pelitic schists, the albite+lawsonite+chlorite assemblage does occur but only rarely, and its assemblage cannot be used to determine the regional thermal structure. The lower temperature equivalence of the pumpellyite–actinolite assemblage is not observed in the field. The Mikabu Greenstone complex and the northern margin of the Chichibu complex, which are located to the south of the Sanbagawa belt, are characterized by clinopyroxene+chlorite or lawsonite+actinolite assemblages, which are lower temperature assemblages than the pumpellyite+actinolite assemblage. These three metamorphic complexes belong to the same subduction-metamorphic complex. The pumpellyite–actinolite facies or subfacies can be useful to help reveal the field thermal structure of metamorphic complexes

A garnet-omphacite-kyanite-coesite geothermobarometer is constructed to estimate the peak metamorphic pressure of ultrahigh-pressure (UHP) eclogites. For this we use a quaternary regular solution model for garnet, and propose a new model for sodic pyroxene that assumes double site solid-solution (Na,Ca)M2(Al,Fe3+,Fe2+,Mg)M1Si2O6, where cations in the M2- and the M1-sites mix randomly but with inter-site interactions, not only between nearest neighboring cations but also between next nearest neighboring cations. In the new model, overall excess interaction parameters are quantitatively deduced from the previous experimental data and critical temperature, and the activity of the jadeite component agrees reasonably with that based on the Landau theory. Furthermore, the relations of the calculated order parameter to temperature and composition are consistent with other experimental works. These facts support the usefulness of the new model. Application of our geothermobarometer to kyanite eclogite gave metamorphic conditions around 3.4 GPa and 800 ∘C for both samples from the Dora Maira Massif, Western Alps, and from Rongcheng County, in the Su-Lu region of eastern China.

Coesite eclogite is associated with metagranitoid in a 50×100 m 2 outcrop within the regionally developed amphibolite-facies Su-Lu orthogneiss. Primary intrusive relationships between the metagranitoid and basic rocks and bulk-chemistry analyses show that together they represent a composite igneous body that has subsequently been strongly deformed and metamorphosed. The presence of rutile, sodie pyroxene, corona garnet, and possible pseudomorphs after coesite all suggest very high pressures of metamorphism in the metagranitoid. This is the first documented occurrence of ultrahigh-pressure (UHP) metagranitoid outside of the European Alps. The existence of UHP metagranitoid shows that low density of rocks does not necessarily prevent subduction to mantle depths. Even at peak metamorphic conditions the UHP composite igneous body reported here would have a bulk density less than the mantle. Buoyancy forces may, therefore, have been important in the early exhumation of this unit. Other outcrops of coesite eclogite in the Su-Lu region may also have been originally metamorphosed along with low-density granitoid rocks.

Science does not advance at the same speed on its all facets, and is so in metamorphic petrology of Japan. Rock-forming mineralogy and phase petrology became Japanese tradition. Recognition of widespread partial melting in low-pressure metamorphic complex is helping to understand the evolution of crust. The analysis of chemical texture of rock-forming minerals helped to obtain reasonable estimation of P-T path of metamorphic complexes. Dating by K-Ar and Ar-Ar methods performed on as many samples as done for microprobe, greatly modified Japanese metamorphic geology. However experimental petrology, geochemistry of fluid, stable isotope studies and dating by ion-probe mass-spectroscopy are the fields where far more attention should be focussed.

Recognition of several ultrahigh-pressure (UHP) metamorphic terranes in continental collision belts has revolutionized the concept of geodynamic processes. In order to facilitate better communication and focus among active investigators, the Task Group III-6 of the International Lithosphere Program‘Ultrahigh-Pressure Metamorphism and Geodynamics in Collision-type Orogenic Belts’held the first two day workshop at Stanford University in December, 1994. Petrotectonic settings, mineral paragenesis, geochronoldgy, and geochemical characteristics of UHP rocks from several recognized and suspected UHP terranes were addressed. This special issue presents 11 papers from the more than 50 contributions from the 88 participants representing 15 countries. Many challenging petrotectonic and petrochemical problems remain to be investigated. These include detailed P-T time paths for both the UHP unit and adjacent units, the role of fluids at mantle depths, deep seismic profiles and mechanisms and rate of exhumation of the UHP unit.

Petrological studies of a serpentinized garnet lherzolite body in Rongcheng of the Su-Lu region of eastern China revealed unusually high pressure. Spinel lherzolite probably in a subducting slab was transformed to garnet lherzolite at mantle depth. During exhumation, they were subsequently subjected to the granulite and then amphibolite overprinting and a phase of serpentinization. The peak P–T conditions of the garnet lherzolite estimated after detailed analysis of the metamorphic texture are 4–5 GPa and 820°C or 5–6 GPa and 780°C, depending on the chosen geothermobarometers. The lower dP/dT of the garnet lherzolite can be interpreted as the results of subduction of an old (say 100 Ma older than the time of collision) and cold, slab underneath the margin of the Sino–Korean craton.

In the Su-Lu ultrahigh-P terrane, eastern China, many coesite-bearing eclogite pods and layers within biotite gneiss occur together with interlayered metasediments now represented by garnet-quartz-jadeite rock and kyanite quartzite. In addition to garnet + omphacite + rutile + coesite, other peak-stage minerals in some eclogites include kyanite, phengite, epidote, zoisite, talc, nyboite and high-Al titanite. The garnet-quartz-jadeite rock and kyanite quartzite contain jadeite + quartz + garnet + rutile ± zoisite ± apatite and quartz + kyanite + garnet + epidote + phengite + rutile ± omphacite assemblages, respectively. Coesite and quartz pseudomorphs after coesite occur as inclusions in garnet, omphacite, jadeite, kyanite and epidote from both eclogites and metasediments. Study of major elements indicates that the protolith of the garnet-quartz jadeite rock and the kyanite quartzite was supracrustal sediments. Most eclogites have basaltic composition; some have experienced variable ‘crustal’contamination or metasomatism, and others may have had a basaltic tuff or pyroclastic rock protolith. The Su-Lu ultrahigh-P rocks have been subjected to multi-stage recrystallization and exhibit a clockwise P-T path. Inclusion assemblages within garnet record a pre-eclogite epidote amphibolite facies metamorphic event. Ultrahigh-P peak metamorphism took place at 700–890° C and P>28 kbar at c. 210–230 Ma. The symplectitic assemblage plagioclase + hornblende ± epidote ± biotite + titanite implies amphibolite facies retrogressive metamorphism during exhumation at c. 180–200 Ma. Metasedimentary and metamafic lithologies have similar P-T paths. Several lines of evidence indicate that the supracrustal rocks were subducted to mantle depths and experienced in-situ ultrahigh-P metamorphism during the Triassic collision between the Sino-Korean and Yangtze cratons.

The chemical composition and compositional zoning of garnets from seven Sanbagawa basic schists were studied. The samples were collected from the albite-biotite and oligoclase-biotite zones in the Besshi and Asemi River areas, Central Shikoku. All the examined garnets from the Asemi River area show Mn-bell shaped normal zoning. Mn-content in the core of garnets (up to 30 wt% of MnO) varies from a rock to another, reflecting their bulk composition. In the Besshi area, very Mg-rich garnets as compared with those in the Sanbagawa schist and in the literature were found from basic schists near the epidote-amphibolite masses. Further, reversely zoned garnet in regard to Mn distribution occurs, along with normaly zoned garnets, in two basic schists near the Tonaru epidote-amphibolite mass. These facts imply that higher temperatures than the other Sanbagawa schist were attained at the Besshi area. Disequilibrium crystallization was noticed in the garnet near the amphibolite mass, probably correlating to the emplacement of the amphibolite masses.

Actinolite and hornblende commonly coexist in metabasite of upper greenschist-lower amphibolite facies. Two contradictory interpretations for the coexistence of two calcic amphiboles are suggested, that is, miscibility gap and transitional loop between actinolite and hornblende. We determined the space group of actinolite obtained from a greenschist of the Sanbagawa belt in central Shikoku. The greenschist was taken from lower metamorphic temperature area than that of the hornblende first appearance. Weissenberg and precession photographs indicate that the actinolite have C-lattice, and the existence of the center of symmetry is confirmed by treating the reflected X-ray intensities obtained by 4-circle diffractometer with statistical method (Nz method). The space group of the actinolite is C2/m. Since the actinolite has the same space group as that of hornblende, no crystallographic transformation can be considered in calcic amphibole. Hence, the coexistence of actinolite and hornblende should be explained by the miscibility gap between them.

Deformation in the Sanbagawa Belt is characterized by ductile flow in an east-west direction sub-parallel to its length. The east-west flow (D1) caused large-scale recumbent folding of the metamorphic sequence in central Shikoku, which can explain the inverted thermal structure of this region. Chemical zoning of metamorphic minerals associated with D1 microstructures also suggest that the east-west flow developed under retrograde conditions. D1 is therefore related to exhumation rather than subduction processes. A variety of kinematic indicators show that during the east-west flow, deformation was partitioned into structurally continuous domains with opposed senses of shear. This suggests that bulk deformation was not simple shear but included a component of flattening.

In 1961 Miyashiro presented the concept of paired metamorphic belts based on the mode of occurrences of high-pressure metamorphic belts along the Circum-Pacific orogenic belts, particularly in Japan. According to him the Japanese islands are composed essentially of three paired metamorphic belts: the late Paleozoic Hida-Sangun and late Mesozoic Ryoke-Sanbagawa pairs in Honshu, and the late Mesozoic Kamuikotan­ Hidaka in Hokkaido. At that time he could use only 11 K-Ar mica ages. Detailed petrographic work has continued since that time, and many radiometric ages are now available in the literature. From the late 1970s to the early 1980s, two major breakthroughs were made in Japanese basement geology; the first was in conodont-biostratigraphy followed by radiolaria-biostratigraphy, and the second was due to the increased pro­ ductivity ofK-Ar dating. Finally, the geosyncline concept was abandoned, and has been replaced by plate-tectonics approaches for explaining why particular metamorphic belts are located where they are today. Uyeda & Miyashiro (1974), Saito & Hashimoto (1982), and Maruyama & Seno (1986) pioneered this research. These and later works revealed that many metamorphic areas actually consist of several units.

Granulite in the Iratsu metagabbro of central Shikoku, with color-banding defined by the presence and absence of plagioclase, is surrounded by a narrow epidote amphibolite, which itself is encased in garnet-epidote amphibolite. The epidote amphibolite formed metastably under low PH2O conditions based on thermodynamic considerations. The granulite-epidote amphibolite boundary is formed by the hydration reaction of plagioclase to zoisite, kyanite and quartz, and that of clinopyroxene and orthopyroxene to amphibole. If it is assumed that the temperature range of the Sanbagawa epidote amphibolite facies is 600–650°C, we obtain a lithostatic pressure of 12–15 kbar and a H2O pressure of 1–3 kbar for the hydration conditions. The boundary between epidote amphibolite and garnet-epidote amphibolite is defined by the formation of garnet, which is more stable than the epidote amphibolite assemblages. Pargasitic amphibole occurs at the garnet rim. Up to 0.32 wt.% Cl is present in the amphibole from both of the hydrated facies, but is It is concluded that hydration did not take place in the presence of an excess fluid phase, based on the difference of the Cl contents of amphibole between observed and expected values from a qualitative infiltration metasomatic model and also on the estimated low fluid pressure. The formation of a sharp boundary between the granulite and epidote amphibolite suggests intracrystalline diffusion.

Eclogitic glaucophane schist has been discovered as a boulder (about 4m diameter) from the Yunotani valley in the western Omi area of the late Paleozoic Renge metamorphic belt (Fig.1). The eclogitic glaucophane schist (Fig.2) occurs as a mafic layer (1.2m wide) intercalated within pelitic schist (garnetparagonite-phengite schist), and is characterized by the mineral assemblage garnet (modal volume: 21%)+omphacite (19%)+ glaucophane (37%)+epidote (19%)+rutile+phengite+albite+quartz (Fig.3). This is the first finding of the late Paleozoic eclogite facies metabasite, which is almost devoid of retrogression and preserving textural evolution (Fig.4) and mineral zoning (Fig.5) during progressive metamorphism. This rock provides an evidence for the eclogite facies metamorphism in the late Paleozoic Western-Pacific margin. More detailed description will appear in Tsujimori et al. (in press).

Construction of mathematical models of phase diagrams such as done by Barron (1972 a, b) contributes to the understanding of physicochemical processes in igneous petrogenesis. In this review, the principles and practice of construction of mathematical models are explained in detail for the system Ab-An-Di. Major features of the ternary phase diagram can be predected from the phase diagrams of binary systems, Ab-An, An-Di and Di-Ab. Using the model we have constructed, a computer program was prepared to calculate the composition changes of major and trace elements contents in liquid and crystal phases during the maximum fractionation crystallization. The program makes it possible to calculate the fractionation curves for given starting compositions and relate it to various parameters such as percentage solidified, icophase curves, zonal structure of plagioclase, trace element partition coefficient and so on. The application of the model calculation to the natural igneous processes is also briefly discussed.

The Kurosegawa zone in southwest Japan is a 600 km long serpentinite melange in the Chichibu terrains. It runs generally E-W but is slightly oblique to the subparallel arrangement of the Ryoke, Sanbagawa and Chichibu belts of Southwest Japan. A variety of geological units occurs in the Kurosegawa zone: 1. (1) granodiorite, gneiss and amphibolite of ca. 400 Ma, 2. (2) Siluro-Devonian formations, 3. (3) Upper Carboniferous to Jurassic formations, 4. (4) Upper Jurassic to Lower Cretaceous formations, 5. (5) serpentinite and 6. (6) low- to medium-grade metamorphic rocks of various baric types (ages, 220, 320, 360 and 420 Ma by K-Ar). The most widespread is a high-pressure intermediate group of metamorphic rocks. Serpentinite is emplaced along the faults between and within the constituent units. Rocks of the Kurosegawa zone represent a mature orogenic belt along a continental margin or an island arc. Its original site as constrained by paleomagnetism was near the equatorial area. Here, 400 Ma old paired metamorphism and related magmatism took place. The island arc or microcontinent migrated northward to collide with the Eurasia plate during Late Jurassic, thus consuming the intervening ocean.

The fractionation trend of trace elements with large ionic radii capable of substituting for Ca in plagioclase is calculated for the model system diopside-albite-anorthite. After determining the necessary parameters to reproduce Bowen's (1915) phase diagram of the system, the following assumptions were made on the system. 1. (1) Plagioclase is an ideal solution and diopside has a fixed composition. 2. (2) The thermodynamics of the melt can be described in terms of three “molecules”, diopside, albite and anorthite. Trace elements can substitute Ca and Na in the plagioclase “molecule” of the melt, but not for Ca of the diopside “molecule”. 3. (3) The exchange partition coefficient of a trace element between plagioclase and melt is a constant, independent of temperature and composition of the system. This model leads to the composition and temperature dependence of the “Nernst type” partition coefficient and gives rise to the appearance of maximum trace-element concentration in the melt at an intermediate stage of fractional crystallization. These considerations on the model system allow the qualitative interpretation that during magmatic differentiation Sr concentrates at an intermediate stage of fractionation, while Ba concentrates at the latest stage.

Zonal structure of garnet grown in metamorphism maintaining surface equilibrium was examined on the model system Mn3Al2Si3O12-Fe3Al2Si3O12-H2O. The model assumes that the diffusion of Mn and Fe in both garnet and chlorite is limited, or it is practically negligible in garnet. It is shown that if the rate of temperature change during metamorphism is large in comparison with diffusion rate, a zonal structure, in which the MnO content of garnet decreases from the center of the crystal towards rim and increases again at the rim, giving rise to the minimum MnO content at an intermediate zone, can be formed without accompanying resorption. Two natural examples which support the conclusion on the model system are discussed.

Awaruite was found in serpentinized peridotites from the Mineoka district, southern Boso Peninsula. The mineral occurs invariably in serpentinized parts of the rocks, indicating that it was formed in the course of serpentinization. Heazlewoodite and native copper were formed also in association with awaruite during serpentinization. Chemical analysis of the awaruite yields the formula: Ni2.26 Fe. Native copper associated with heazlewoodite contains about 5 weight percent nickel. The occurrence of awaruite suggests that oxygen fugacity of fluid being responsible to serpentinization was lowered at least locally down to 10-42 bar during serpentinization. The low oxygen fugacity of fluid was resulted from reaction between fluid and fayalite component of olivine producing magnetite and serpentine minerals.

A Caledonian eclogite suite is associated with lower-grade high-pressure rocks in Motalafjella, Oscar II Land, central-western Spitsbergen. A variety of mineral assemblages occur in the suite, depending on the bulk-rock composition; omphacite-paragonite-glaucophane-epidote-garnet in eclogites, jadeite-glaucophane-paragonite-quartz in siliceous schists, and MgFe chloritoid-paragonite-epidote-garnet in pelitic schists. Petrologic and paragenetic studies are presented to characterize this unique set of mineral assemblages in relation to other eclogite suites. Mineralogical data, mainly the distribution of Fe and Mg between coexisting clinopyroxene and garnet, and the existence of impure jadeite+quartz assemblage, give approximately 580–640°C and 18–24 kbar. New criteria for the eclogite classification are derived based on the mineral assemblages and a model petrogenetic grid. The Motalafjella eclogites associated with the glaucophane-epidote assemblage belong to the medium-temperature portion of the eclogite facies.

Paragenetic relations of natural jadeite-diopside solid solution is well explained by a phase diagram proposed by Carpenter (1980). However, his interpretation of free energy-composition relation to give rise the phase diagram is hardly accepted because it requires positive excess energy of mixing in solution with ordered phase. An alternative model giving the same phase diagram is presented. It assumes strong interaction between next neighbours with resultant stabilization of quartet of cations, which is analogous to the stabilization of NaCaMgAl quartet of omphacite. The model favors rather simple ideal solution to approximate the results of synthetic experiments over complex non-ideal model of Holland (1983). An isopleth of univariant composition in P-T is proposed.

In pelitic schists of the Sanbagawa metamorphic belt, sphene, rutile, and ilmenite occur as discrete grains, in composite aggregates, and as inclusions in garnets. Textural relationships, disposition of inclusions in garnet, and the compositions of ilmenites suggest that the titanium-bearing accessories stable at the peak metamorphic conditions were as follows: sphene in the chlorite zone, sphene plus rutile in the garnet zone, and ilmenite in the highest grade of the belt, the biotite zone. Rutile appears in the garnet zone as a consequence of Ca incorporation into garnet and a progressive increase in \(f_{{\text{CO}}_{\text{2}} }\).

SummaryThe lattice types of omphacite-diopside series clinopyroxenes were determined for 9 Japanese, 2 Norwegian, and 1 South African samples. The boundary between the compositional ranges of P and C lattice clinopyroxenes lies at about 14·5 wt. % CaO in Japanese and Norwegian samples, and the compositional gap between them is very small, if it exists. The South African omphacite has C-lattice in the compositional range of P lattice omphacite in metamorphic eclogites. These data combined with the reported electron microscope studies suggest that the omphacite—diopside series clinopyroxene has a C lattice structure above 3 to 400°C and very slow cooling after regional metamorphism is responsible for the transition to the ordered structure with P lattice symmetry.

Most garnets in a small volume of the pelitic Sanbagawa schists nucleated simultaneously, grew, and then ceased with their growth, maintaining near-equilibrium with coexisting minerals such as chlorite and biotite during the prograde metamorphism. Chemical trends of zoned garnets systematically shift from the pyrope-poor to pyrope-rich portions in a ternary MnFeMg system in order of ascending metamorphic grade. A ternary (MnFeMg) model system suggests that the higher-grade garnets started to grow at lower pressures and higher temperatures than the lower-grade ones during prograde metamorphism. It follows that high-grade metamorphism took place at lower pressures in the garnet to oligoclase-biotite zones of the Sanbagawa metamorphic terrain in central Shikoku.

The paragenetic relations of epidote-glaucophane schists are described in terms of the system Al2O3-Fe2O3-Fe2O3-MgO-CaO with excess of quartz, albite and epidote. If alkali-amphibole is free from Ca and AlIV, its composition when associated with epidote is invariant, univariant or divariant at a given pressure and temperature on Miyashiro's (1957) diagram of alkali-amphibole solid solution if it is also associated, respectively, with three, two or one additional minerals in the system. Using a group of epidote-glaucophane schists from the Kotu area of the Sanbagawa metamor-phic belt in Shlkoku, Japan (isophysical compositional),univariant boundary lines were determined for the assemblages that, in addition to the ubiquitous quartz + albite + phengitic mica, contain hematite + chlorite, garnet + chlorite and actinolite + chlorite, respectively. The slopes of the univariant boundary lines obtained from petrographical data are in good agreement with those calculated in a model system. The positions of isophysical univariant boundary lines on the amphibole compositional diagram serve to distinguish the grade of metamorphism among the rocks of the same mineral facies. The hematite-chlorite univariant boundary line can be used to divide the zone of epidote-glaucophane schists of the Sanbagawa metamorphic belt into three, and the garnet-chlorite-paragonite invariant equilibrium can be used to divide the epidote zone of New Caledonia into three.

SummaryCoexisting zoisite and clinozoisite in seventeen specimens from six localities in Japan have been studied with the electron-probe microanalyser. Zoisite and clinozoisite are commonly zoned, but compositional gaps between them are systematic. Referring to the metamorphic grade of the host rocks, a temporary and schematic phase-diagram for the system Ca2Al3Si3O12-(OH)-Ca2Al2Fe3+Si3O12(OH) is presented. With increasing temperature, in the range of low- to medium-grade metamorphism, the compositional gap between the two epidote-group minerals shifts towards higher Fe3+ compositions.