Different Stages Of Metamorphism Research Articles

Garnet–omphacite–phengite thermobarometry of eclogites from the coesite-bearing unit of the southern Dora-Maira Massif, Western Alps

Using relevant geothermobarometric methods, P– T-data were collected for the reconstruction of the metamorphic evolution of 34 eclogite samples taken from small lenses and boudins within the ultrahigh-pressure (UHP) metamorphic coesite-bearing Brossasco-Isasca Unit (BIU) of the Dora-Maira Massif. The mineral phases used (clinopyroxene, garnet, phengite), or growth zones thereof, were identified as being coexistent for different stages of metamorphism on the basis of careful petrographic studies. Of several published geothermobarometers, the garnet–clinopyroxene thermometer of Powell [Powell, R., 1985. Regression diagnostics and robust regression in geothermometer/geobarometer calibration: the garnet–clinopyroxene geothermometer revisited. J. Metamorph. Geol., 3, pp. 231–243.] combined with the garnet–clinopyroxene–phengite barometer after Waters and Martin [Waters, D., Martin, H.N., 1993. The garnet–clinopyroxene–phengite barometer. Terra Abstr., 5, pp. 410–411.] was chosen here, because it provided the most reliable results. Nevertheless, the scatter of P– T-data points for the prograde (stage I), peak metamorphic (stage II), and retrograde (stage III) development of the eclogites is still considerable. Among the many possible reasons for this inconsistency discussed, a partial lack of equilibration of some of the eclogites during their metamorphic history should be taken into account. Despite the data scatter, an average P– T-path could be estimated, which includes the following coordinates: for stage I: 15 kbar/500°C; 25 kbar/570°C; 32 kbar/650°C; for stage II: 36 kbar/720°C; and for stage III: 24 kbar/680°C and 14 kbar/620°C. This is in fair agreement with P– T-paths derived earlier for other rock types of the BIU on the basis of other geothermobarometers.

Petrogenesis of sapphirine‐bearing metatroctolites from the Buck Creek ultramafic body, southern Appalachians

The Buck Creek ultramafic body, North Carolina, includes aluminous lenses that have been described as troctolites. These lenses preserve mineral assemblages which record several different stages of metamorphism. The first stage is characterized by anhydrous reactions between olivine and plagioclase to produce coronas of orthopyroxene+ clinopyroxene/spinel symplectite. Thermo barometric results indicate minimum pressures of c. 6 kbar and c. 800 oC. Sapphirine replaces spinel in some clinopyroxene symplectites, and occurs as anhedral grains within amphibole, observations which in combination suggest peak metamorphic conditions of c. 9‐10 kbar and c. 850 oC. Sapphirine‐bearing hydrous assemblages formed at the expense of the coronas, indicating a second metamorphic episode involving deeper burial, deformation and hydration. Schistose rocks from the margins of the lenses are composed of anorthite+amphibole+margarite+corundum, and probably record a later, lower P‐T event. Whole rock analyses for the Buck Creek lenses suggest an accumulate protolith of magnesian olivine and calcic plagioclase. Trace element data for the troctolites are consistent with data for adjacent amphibolites in suggesting that the Buck Creek mafic and ultramafic cumulates crystallized from magmas derived from a mantle source similar to that which produces modern intraplate or rift‐related basalts. We propose that the Buck Creek ultramafics represent basal cumulates(± uppermost mantle) from ocean crust formed in a marginal basin in the latest Precambrian. Subduction‐induced burial to at least 18 km under dry conditions induced corona formation. Collisional events of the Taconic orogeny thrust the Buck Creek rocks into the orogenic pile to at least 30 km depth and hydrated them along zones of weakness, locally producing P‐T ‐PH2O conditions appropriate for formation of sapphirine and hydrated assemblages, but still preserving some dry symplectites.

Multiple stages of phengite formation in Sanbagawa schists

Basic and pelitic schists of the garnet and biotite zones in the Sanbagawa belt in Shikoku, Japan, commonly contain phengite developed at different stages of metamorphism. Textures of the phengite and associated minerals show that Al-rich phengite is a prograde product and Al-poor phengite is a retrograde product.

GEOLOGICAL THERMOMETRY AND BAROMETRY OF METAMORPHIC COMPLEXES: CENTRAL KOLA ARCHEAN GRANULITE-GNEISS REGION

The paper distinguishes two complexes of high-grade meta-morphic rocks in the central portion of the Kola Peninsula (Baltic Shield): 1)the Vyezhe Tundra complex, mainly enderbites and basic granulites with subordinate plagiogneisses, iron formation, and metapyroxenites, and 2)the Kola series (mainly plagiogneisses with subordinate iron formation and basic granulites). Textural and paragenetic relationships point toward multi-cyclic metamorphism in the region. Metamorphic temperature conditions were estimated using a consistent system of geothermometers and geobarometers. Temperature maxima (670, 565 and 500° C) were ob-tained, which correspond to different stages of metamorphism in the re-gion. The results show that metamorphism in the two sequences occurred at similar temperatures; furthermore, no differences were found between unsheared and sheared rocks. The findings support the view that equilibra-tion between minerals during metamorphism is controlled by breakdown-crystallization reactions. The obtai...

Static Dielectric Constant as a Textural Index of Snow

The dielectric constants of alpine snow samples with different stages of metamorphism and with different liquid water saturations have been measured in the frequency range of 10Hz to 50MHz using a plate condenser and network-analyzer. The limiting static dielectric constant, €s’, has been derived from the measured frequency dependence of the complex permittivity of snow by a least-square-fit using the model of Cole-Cole. A strong dependence of e on porosity, liquid water content and on the shape of the snow grains was found. Calculations of shape factors from the measured static permittivity based on the model of Polder and van Santen are given, and are compared to shape factors derived from an analysis of photographs of the snow samples.

Static Dielectric Constant as a Textural Index of Snow

The dielectric constants of alpine snow samples with different stages of metamorphism and with different liquid water saturations have been measured in the frequency range of 10Hz to 50MHz using a plate condenser and network-analyzer. The limiting static dielectric constant, €s’, has been derived from the measured frequency dependence of the complex permittivity of snow by a least-square-fit using the model of Cole-Cole. A strong dependence of e on porosity, liquid water content and on the shape of the snow grains was found. Calculations of shape factors from the measured static permittivity based on the model of Polder and van Santen are given, and are compared to shape factors derived from an analysis of photographs of the snow samples.

Effect of grain geometry on electrical properties of snow at frequencies up to 100 MHz

The complex dielectric constant of snow samples with different stages of metamorphism and different densities and liquid water contents has been measured in the frequency range of up to 100 MHz. Both the static dielectric constant and the loss factor at frequencies between 1 and 100 MHz depend strongly on the snow texture. The shape of the snow grains or clusters has been found to be an important texture parameter; it is particularly suited for snow classification. A significant influence of the size of the grains on the electrical properties has not been observed. Under the condition of a low ionic conductivity, shape factors to classify snow samples can be derived from the static permittivity. Compared to the determination of shape factors from thin snow sections, the dielectric method is most suitable for application to wet snow.

Geological and petrological studies on the Tanzawa mountainland (3),

Three stages metamorphism of the Tanzawa metamorphic rocks are discriminated from the data of the field and microscopic observations, regional metamorphism, contact metamorphism, and hydrothermal metamorphism. The geologic relationships among these different stages of metamorphism are obtained by examining the structure of original rocks and their metamorphism. The conclusions are as follows. 1) Regional metamorphism of the first stage was not followed immediately by the contact metamorphism of the second stage, and the meta-morphosed areas of the two stages do not coinside to each other. The plutonic mass of this district is a composite mass composed of acidic and basic facies, of which the basic plutonic rock is closely related to the regional metamorphism, while the acidic plutonic rock is related to the contact metamorphism. 2) Hydrothermal metamorphism of the third stage gave an effect over-lappling with the effects due to the metamorphism during the preceeding stages. 3) Original rocks are divided into two groupes, i.e., the rocks of the inner zone and the rocks of the outer zone. The metamorphosed area of the first stage coincide to the rocks of the inner zone, while the metamorphosed area of the second stage do not coincide to that and moreover seem to extend to the rocks of the outer zone. 4) Judging from the presence of debris of plutonic rocks in the lower part of the rocks of the outer zone, it is suggested that there might have been some geologic gap between the rocks of the outer zone and those of the inner zone. The gap is compared to that between the two stages of metamorphism, i.e. the first regional metamorphism and the second contact metamorphism.