Coesite occurs in garnets from quartz schists and pelitic schists in the Makbal Complex of Kyrgyzian Tien-Shan. The thick quartz schists interbedded with pelitic schists are the transitional facies after coesite schists. Quartz-pseudomorph after coesite also appears widely in pelitic schists and quartz schists, but is rare in eclogitic rocks. The growth zoning of garnets and Na-Ca amphiboles in eclogites and the garnet-omphacite geothermometry indicate that the metamorphic P-T paths of eclogites are different from each. Ultra-high (UHP) pressure metamorphic rocks of the Makbal Complex are mainly composed of pelitic and quartz schists. All eclogite lenses are included in the host UHPM rocks, although most of them do not contain UHP metamorphic (UHPM) evidences. The K-Ar ages of phengite in a host pelitic schist and of paragonite in an eclogite are approximately 500 Ma, while those of biotite and phengite in a biotite-bearing mica schist and of winchite in a winchite schist are approximately 769-717 Ma and 881 Ma, respectively. The evidence indicates that this complex is a tectonic mélange. These geological relationships suggest that the ascending substances must be pelitic and quartz schists. The UHPM rocks in the ascending material capture many exotic blocks as xenoliths during intrusion into the upper crust.
Sr-sulphate, celestine, was newly found in a kyanite-bearing eclogite associated with the Nové Dvory peridotite mass in the Moldanubian Zone of the Bohemian Massif, Czech Republic. Celestine is closely associated with anhydrite, pyrite, pyrrhotite and chalcopyrite, and those minerals occur in the matrix, where fine-grained omphacite aggregate and kelyphites after garnet develop. A common Sr reservoir in eclogite is known to be epidote, but the maximum pressure-temperature (P-T) conditions of the studied eclogite was estimated as about 1050-1150 °C, 4.5-4.9 GPa. In such extremely high P-T conditions, epidote should be unstable. In fact, epidote is absent from most of eclogites in the Moldanubian Zone of Czech Republic. These facts suggest that a Sr-reservoir after the epidote-breakdown in subducting eclogite might be celestine, or its high-P polymorph, although the formation stage of celestine in the study sample and maximum stability limits of celestine are not known. Another idea is that celestine in the study sample was formed at relatively shallow levels after the ascent of the eclogite. In this case, omphacite and apatite would have contained significant amount of Sr under the eclogite-facies conditions. Otherwise, metasomatic infiltration of Sr-rich fluids into the eclogite is necessary for the formation of celestine in order to provide sufficient amount of Sr. A Ba-rich alumino-silicate, probably celsian, was also found as a retrograde product around pyrrhotite in kelyphite and in symplectites mainly composed of augite and plagioclase after omphacite. Several grains of biotite are also present along the margin of garnet and in the symplectites after omphacite. In addition, small amounts of muscovite and amphibole are present in the symplectites after omphacite. These findings suggest that metasomatism with Sr- and Ba-rich fluids may have occurred during decompression of the eclogite and may not be indicative of the UHP history of these rocks.
The Sør Rondane Mountains was situated within the collision zone between the West and East Gondwana during the Pan-African event. The mountains are made up of high-grade metamorphic rocks and various kinds of intrusive rocks. Here, we report the newly found post-kinematic lamprophyre (minette) collected from the southwestern part of Sør Rondane Mountains. The minette intruded along normal faults and has no sings of any metamorphism. The K-Ar dating of biotite separated from the minette gives an age of 563 ± 14 Ma. Considering mode of occurrence, age data and geochemical signatures, the minette intruded in the within-plate tectonic setting and the timing of magma activity would start after the suture event related to the formation of Gondwana supercontinent.
In this study, mafic granulite and related leucosomes from the Song Ma Suture zone in northern Vietnam were investigated. Euhedral garnet appears in both granulites and leucosomes, whereas the former is partly to mostly replaced by orthopyroxene + plagioclase symplectite. Most garnets occurring both in granulite and leucosome contain an Mg-rich and Ca-poor core as well as an Mg-poor and Ca-rich mantle. Garnets in leucosomes have an Mg-rich and Ca-poor thin rim. Amphibole + quartz association is observed only in the core and mantle and not in the rim and matrix. Garnet core is characterized by flat middle to heavy rare earth element (REE) patterns and depletion of light REEs. The rim also contains high concentrations of middle REEs, whereas light and heavy REEs are further depleted in comparison with the core. The REE patterns of the mantle are completely different from core and rim, which show considerable depletion in light to middle REEs and extreme enrichment in heavy REEs. The petrographical features and REE patterns of the associated mineral phases suggest that the complex chemical zonation of garnet might be formed by dehydration melting of amphibole and multiple recrystallization of garnet.
The mode of occurrence of garnet in pelitic schists of the Sanbagawa metamorphic belt along the Asemigara River in central Shikoku is examined. Two different types of garnets, A and B, are identified from the albite-biotite and the oligoclase-biotite zones of the Sanbagawa belt. Type-A garnets are larger than type-B garnets and show concentric normal chemical zoning. There is no evidence of disequilibrium crystal growth in type-A garnets. On the contrary, type-B garnets are small, and some of them occasionally show sector zoning, which is a strong evidence for a disequilibrium crystal growth. The chemical trend observed between the core and the rim of type-B garnets is different from that observed in the case of type-A garnets, although only some of the type-B garnets show chemical sector zoning. Type-B garnets are typically found in clusters in muscovite-rich layers or are included in albite porphyroblasts. These two types of garnets can be distinguished from each other on the basis of grain size, cluster formation, and chemical composition. Type-B grains are found only in the structurally lower part of the oligoclase-biotite zone and in the lower albite-biotite zone. These two types of garnets are thought to represent two stages of garnet growth. Further, in the case of type-A garnets (early garnet), chemical equilibrium is maintained during prograde metamorphism, and hence, these garnets are suitable for the use in thermobarometric studies.
Podiform chromitites have been interpreted as cumulates by harzburgite/melt interaction and related melt mixing at the upper mantle to the Moho transition zone. Recent discovery of diamond and other ultrahigh-pressure (UHP) minerals from some podiform chromitites, especially those from Tibet, however, has raised a question about the depth of their formation. These UHP chromitites are possibly of deep recycling origin; they had been originally formed at the upper mantle before sinking down to deeper mantle, and upwelling again to the shallowest mantle by convection. Diamond is formed by reduction/oxidation of fluidal carbon species (e.g., CO2 or CH4) obtained during the convection history, and has survived oxidation because of strong encapsulation in metal alloys further included by chromian spinel. Exsolved silicates (diopside and coesite) in chromian spinel from some UHP chromitite are possibly derived from hydrous mineral inclusions in chromian spinel formed at low pressures, which have been decomposed/molten during sinking and solved in the UHP chromian spinel phase. Both the UHP chromitites and ordinary low-pressure ones could be present in the upper mantle derived from the mid-oceanic ridge, one of the main ends of the mantle upwelling flow. The mantle recycling issue unraveled through chromitite thus can be one of the targets of deep oceanic mantle drilling including the Mohole.
Strength contrast between plagioclase and olivine was tested utilizing two-layer deformation experiments under hydrous conditions at the pressure and temperature corresponding to the Moho (P = 1.0 GPa and T = 400-800 °C). Deformation microstructures characterized by lattice-preferred orientation and dislocation density indicate that both minerals were plastically deformed via dislocation-controlled creep. Our experimental results show that olivine is weaker than plagioclase at T = 400°C, whereas plagioclase is weaker than olivine at T = 800°C. Consequently, strength contrast between plagioclase and olivine is sensitive to temperature, and olivine may be weaker than plagioclase, or display almost no difference in strength between these materials under the wet continental Moho. This suggests that the “crème brulee” model, in which the upper mantle is weak and the strength is limited to the crust, is expected as a rheological layer model of the water-rich Moho conditions.