Different Stages Of Metamorphism Research Articles

Trace Elements of Multi‐stage Minerals and Titanite U‐Pb Dating for the Gneisses from Liansan Island, Sulu UHPM Belt

AbstractGneisses with anatectic characteristics from the Liansan island in the Sulu UHPM (ultra‐high pressure metamorphic) belt were studied for petrography, titanite U‐Pb dating and mineral geochemistry. Three origins of garnets are distinguished: metamorphic garnet, peritectic garnet and anatectic garnet, which are formed in the stages of peak metamorphism, retrograde anatexis and melt crystallization, respectively. The euhedral titanite has a high content of REE and high Th/U ratios, which is interpreted as indicating that it was newly‐formed from an anatectic melt. The LA‐ICP‐MS titanite U‐Pb dating yields 214–217 Ma ages for the titanite (melt) crystallization. The distribution of trace elements varies in response to the different host minerals at different stages. At the peak metamorphic stage, Y and HREE are mainly hosted by garnet, Ba and Rb by phengite, Sr, Nb, Ta, Pb, Th, U and LREE by allanite and Y, U and HREE by zircon. During partial melting, Y, Pb, Th, U and REE are released into the melt, which causes a dramatic decline of these element contents in the retrograde minerals. Finally, titanite absorbs most of the Nb, U, LREE and HREE from the melt. Therefore, the different stages of metamorphism have different mineral assemblages, which host different trace elements.

Metamorphic evolution of the UHT granulites from Mandikota, Eastern Ghats Mobile Belt, India: Constraints from phase equilibria modelling, fluid inclusions and monazite U-Th-total Pb geochronology

The Eastern Ghats Mobile Belt (EGMB) had been subjected to extreme thermal metamorphic history at various sections of its terrane. One such terrane (Mandikota) located near the western part of the EGMB has been speculated based on asserted reaction textures, conventional geothermobarometry, geochronology, fluid inclusions, and pseudosection modelling. The present integrated study aims to understand the intricacies of the diversified and controversial tectono-metamorphic evolution of these polycyclic granulite terranes. The stable occurrence of sapphirine + quartz in contact is rare in its own and is a diagnostic feature of Ultra-High Temperature (UHT) metamorphism. Conventional geo-thermobarometeric tools applied, suggests an ordered series of reactions for the different stages of metamorphism experienced by the investigated granulites. Also, the P-T estimated by conventional methods lies in sound correlation with the peak P-T condition (in excess of 950 °C at nearly 9 kbar) attained in the constrained pseudosection model. The exhumation trajectory of the examined UHT terrane reflects P-T path responsible for the thrusting mechanism with isothermal decompression of 3 kbar followed by late-stage isobaric cooling. This study also presents the primary report for the systematic fluid inclusion study of the UHT granulites and its correlation with the evolved P-T trajectory. In addition, monazite dating using Electron Micro Probe Analysis (EPMA) is procured to assess the time frame in understanding the P-T-t evolution of the high-grade terrane. The geochronological study further revealed a new age (∼600 Ma) associated with the Visakhapatnam Domain which adheres to the Pan-African orogeny and marks the reworking phase of the widely reported the Grenvillian event in the EGMB province. As such, the results from the studied fluid inclusions displaying monophase inclusions of CO2-rich carbonic fluid, and its corroboration with the P-T pseudosection model interpreting the phase equilibrium conditions prevailing during the Neoproterozoic time (∼900 Ma).

Metamorphic history of mafic granulite and garnet‐bearing amphibolite from the Oki‐Dogo Island, Japan

AbstractOki metamorphic rocks have long been considered as a constituent of the Hida Belt based on their geographic proximity, lithology, and Permo‐Triassic metamorphism. However, recent geochronological studies have demonstrated that both para‐ and ortho‐gneisses in the Oki‐Dogo Island display Paleoproterozoic protolith formation and two separate phases of metamorphism at 1.85 Ga and 250–230 Ma. Consequently, the Oki metamorphic rocks are closely connected to the Paleoproterozoic massifs in the Korean Peninsula, although little is known about their pressure (P)–temperature (T) history. Here, we provide petrological data on mafic metamorphic rocks in the Oki‐Dogo Island. The mafic lithologies are classified into mafic granulite, amphibolitized granulite and amphibolite. In addition, we first discover a garnet‐bearing variety of amphibolite from the Oki‐Dogo Island. The texture and composition of Ca amphibole suggest these rock types share a common P–T history but the dominant mineral assemblage in each rock type records different stages of metamorphism. The inferred P–T history includes two distinct events. The first event includes a low‐P granulite facies stage (~900°C, 0.7–0.8 GPa) and subsequent amphibolite facies retrogression. This event is linked to the continuous compositional change of Ca amphibole from Ti‐rich pargasite to hornblende/actinolite. The second event is prograde amphibolite facies metamorphism, which is associated with the formation of tschermakitic hornblende and calcic plagioclase. In high Fe/(Mg + Fe) rocks, garnet was formed at ~550–580°C, 0.45–0.50 GPa in this stage. Depending on the age of the first event, the low‐P granulite facies metamorphism is likely to have occurred in a similar tectonic setting as the Paleoproterozoic crustal metamorphism in the Yeongnam Massif or the Permo‐Triassic ultrahigh‐T metamorphism in the northern Gyeonggi Massif.

Analysis of metamorphism and tendency of coal seams and their hazardous properties

The object of this study is coals of different stages of metamorphism. Currently, a situation has arisen when indicators developed to establish the consumer qualities of coal are used to predict the manifestation of hazardous properties of mine seams during mining operations. The need to consider the fuel for its working condition is due to different end goals between the establishment of consumer qualities of coal and the manifestation of the hazardous properties of mine seams. The condition and quality of coal after its preparation for use is significantly different from the condition in the mining area. Appropriate sample preparation changes the physical and chemical properties of coals, which determine the manifestation of the hazardous properties of mine layers during mining. To eliminate such a discrepancy, the quality indicators of coals were recalculated for their working condition, taking into account the yield of ash and moisture content for the mined mine seams, followed by an analysis of changes in the correlations be-tween the indicators. The indicators of the manifestation of the hazardous proper-ties of mine layers are borrowed from the characteristics of the quality of the fuel, reduced to a dry, ash-free state. Using these indicators, additional errors are introduced in advance into the accuracy of the prediction of the manifestation of hazardous properties during mining operations. The values of the indicators of the organic (combustible) part of the fuel serve as general reliable characteristics of its quality for the entire set of mine seams, but they cannot be used to predict the hazardous properties of a particular mine seam due to a decrease in the accuracy of their determination due to the unpredictable content of mineral impurities and moisture. The initial experimental data, which have been accumulated over several decades based on the experience of using coal for industrial purposes, are analyzed. On the basis of the conducted researches the peculiarities of the choice of indicators of metamorphic transformations of coal, which are used in parallel respectively to establish the quality of fuel and forecast the dangerous properties of coal seams, have been established. The discrepancies between the indicators of the degree of metamorphism used in the current regulatory framework for the safe conduct of mining operations, the state of fuel during mining operations in underground conditions.

CALIBRATION COEFFICIENTS OF CONNECTION BETWEEN THE LANGMUIR ADSORPTION EQUATION AND V.N. OPARINA’S KINEMATIC EXPRESSION FOR PENDULUM-TYPE WAVES ACCORDING TO LOCAL MEASUREMENTS AT THE COAL DEPOSITS OF KUZBASS

The article shows that at present there are experimental and theoretical prerequisites for establishing formalized relationships between deformation-wave and physicochemical processes occurring in real multiphase rock massifs. The calibration coefficients of the relationship in the correspondence operator between the adsorption equation and the kinematic expression for pendulum waves were established, verified in comparison with the available measurements in laboratory and field experiments for coals of the entire metamorphic series: quantitative values of the physicochemical invariant were determined for coals of different stages of metamorphism and structural parameter of the geomedium, as well as the velocity of propagation of a longitudinal wave in geoblocks-carriers of pendulum waves and translational movement of "colliding" geoblocks for different stages of metamorphism. It is shown that the values of the obtained dispersion allow one to take into account all the variety of the main stages of metamorphism of Kuzbass coal deposits in the theory and forecast of outburst hazard, i.e. take into account the real qualities of coal seams.

Study of the kinetic dependences of the glow and determination of the threshold characteristics of ignition of coals with different stages of metamorphism under the action of microsecond laser pulses

In this work, the study of the kinetic dependences of the glow and determination of the threshold characteristics of the ignition of coal of grades long-flame, semi-lean caking, weakly caking, lean and anthracite under the action of microsecond laser pulses was carried out. For the aforementioned coals, three successive stages of ignition were found with characteristic Hcr thresholds. The first stage of ignition is associated with the heating of the surface of coal particles and the ignition of microprotrusions; the duration of the glow practically repeats the duration of the laser pulse. The second stage of ignition is associated with the release and ignition of volatiles in a millisecond time interval; the third-with the ignition of a non-volatile residue, the glow duration reaches 120 ms. With an increase in the degree of coalification, almost constant values of the first ignition threshold Hcr(1) are found. For the studied coals, with an increase in the degree of coalification, a decrease in the second threshold Hcr<2) and an increase in the third ignition threshold Hcr (3)are observed.

Study of the porous structure of carbon sorbents based on Kuzbass coal at various stages of metamorphism

The work is devoted to the investigation of the porous structure of carbon sorbents prepared on the basis of long-flame, gas, low-caking and lean coal of Kuzbass. Carbon sorbents were obtained by alkaline activation with potassium hydroxide at 800 °C and a KOH/coal (RKOH) ratio of 0.5 g/g. Using the method of low-temperature nitrogen adsorption (analyzer ASAP-2400), the values of the textural characteristics of carbon sorbents based on coal of different stages of metamorphism were determined: pore volume, micropore volume and specific surface area. It is shown that the carbon sorbents obtained in this work are predominantly microporous (more than 80% by volume). A carbon sorbent based on long-flame coal has the greatest texture characteristics; its specific surface area is 1220 m2/g and a pore volume of 0.54 cm3/g.

Indentation of bituminous coals: Fracture, crushing and dust formation

Bituminous coals are still the main source of energy in the world. However, these brittle porous materials are prone to crushing under action of industrial tools. Our early depth-sensing nanoindentation tests of bituminous coals showed that even if the depth of indentation is within the nanoscale, these brittle coals are no longer continuous elastic media within the indentation zone but rather fine powders of crushed particles irrespectively to the coal maceral. Dust formation of the materials is a problem of great practical importance for the mining industry. Indeed, the powders of coal particles formed during crushing may cause not only explosions in mines but they also contaminate the environment around the roads of the coal transportation. In this study, the crushing of coals due to action of rigid conical indenters and formation of small coal particles is investigated. The studies are based on development of the Galanov-Grigoriev (GG) model of crushing of brittle porous materials and the model adjustment to specific features of coal fracture. Using the indentation tests, we estimate the size distribution of the dust particles formed within the region of fully crushed material. Results are presented for coals of three different stages of metamorphism.

Analysis of metamorphism and tendency of black coals to spontaneous combustion

The object of this study is coals of different stages of metamorphism and the volatile products of their thermal decomposition. Currently, based on the basic genetic signs of metamorphism, there is no reliable regulatory framework for determining the hazardous properties of mine plastics, including the propensity of coal for spontaneous combustion. Difficulties in systematization consist in the absence of at least one classification parameter, which determines the need for an additional analysis of the physicochemical properties of coals of different stages of metamorphism and the volatile products of their thermal decomposition as an object of study.Thanks to the obtained functional dependences characterizing the elemental composition of fossil fuels in the entire range of a series of metamorphism, it is possible to evaluate the classification indices of fossil fuels. The result shows that changes in the properties of coal as a result of transformations of the internal structure can take maximum or minimum values. It is impossible to judge the change in the properties of coals from the elemental content of these components by the monotonous and one-sided changes in the components С0, О0, and N0. The nature of the dependence of Н0 on Vdaf and specific gravity (Kd) suggests that coals acquire new properties after Vdaf decreases by less than 30–25 %, and specific gravity at Kdg 1.3It is noted that coals with the same properties can in some cases be characterized by different values of the classification indicators (Vdaf Cdaf, Kd), in others–- coals with different values of the classification indicators have the same properties. The changes in the physicomechanical and calorific value of coal from Vdaf and Сdaf are complex and ambiguous. This is indirect evidence of changes in the internal structure of coals in the process of their geological transformation. The restructuring of the internal structure of coal led to a change in their electromagnetic characteristics.It is proposed, when establishing the propensity of coal for spontaneous combustion by genetic and technological parameters, to use modern knowledge in the field of geology, historical geology and paleontology, physics, chemistry, thermodynamics, as well as experience in the industrial use of coal.

Structure and metamorphism of a subducted seamount (Zagros suture, Southern Iran)

Abstract Millions of seamounts on modern and past seafloor end up being subducted, and only small pieces are recovered in suture zones. How they are metamorphosed and deformed is, however, critical to understand how seamount subduction can impact subduction zone geometry, fluid circulation or seismogenic conditions, and more generally to trace physical conditions along the subduction boundary. Since geophysical studies mostly reach the shallowest subducted seamounts and miss internal structures due to low resolution, there is a high need for fossil seamount exposures. We herein report on a fully exposed, 3D example of seamount that we discovered in the Siah Kuh massif, Southern Iran. Through a series of sections across the whole massif and the combination of magmatic-metamorphic-sedimentary petrological data, we document several distinct stages associated with seamount build-up on the seafloor and with subduction. In particular, we constrain different stages of metamorphism and associated mineralogy, with precise conditions for subduction-related metamorphism around 250 °C and 0.7 GPa, in the middle of the seismogenic zone. Extensive examination of the seismogenic potential of the Siah Kuh seamount reveals that it was not a large earthquake asperity (despite the report of a rare example of cm-scale, high-pressure pseudotachylyte in this study), and that it possibly behaved as a barrier to earthquake propagation. Finally, we discuss the nature of high-pressure fluid circulation preserved in this seamount.

Effect of Thermal Memory in Acoustic Emission in Fossil Coal after Pre-Disintegration by Cryogenic Treatment

Acoustic emission response of fossil coals being at different stages of metamorphism to cyclic variation of effective thermal stresses is experimentally investigated. The equipment and procedure used in the experiments are described. The features of the response are revealed and analyzed in the samples of anthracite, lignite and bituminous coal with different damage extent governed by the preliminary cyclic freezing and thawing, as well as by water saturation. It is shown that the signature of such features is a thermal analog of the Felicity effect which appears in each cycle of temperature action. The regularities of this effect are found, and their physical explanation is given based on the analysis of defect formation in coals at different stages of thermal treatment. The methodical approaches are proposed and substantiated, which allow structural damage, thermal resistance, oxidation and proneness to frost weathering of coal to be estimated by the Felicity effect in the acoustic emission response of coal to cyclic thermal forces. Possibility of using the found features to predict structural changes in coal products which are in long-term storage under specific climatic conditions, as well as for forecasting risk of self-heating and spontaneous combustion of coal products is discussed.

Geochemistry and boron isotopic compositions of tourmaline from the Paleoproterozoic amphibolites, NE China: Implications for the origin of borate deposit

The Paleoproterozoic metasediments and metavolcanics in the Jiao-Liao-Ji belt, northeastern China, host a large number of borate deposits. To trace the source of boron in these borate deposits, we carried out in situ chemical and boron isotopic studies on tourmalines of amphibolites from the Lieryu Formation of the Liaohe Group in the Jiao-Liao-Ji belt. The petrology and the chemical compositions of tourmalines of amphibolites show that these tourmalines formed by influx of B-rich fluids during different stages of metamorphism. The δ11B values of tourmaline continue to decline during the successive metamorphism. Tourmalines that formed during the prograde metamorphism show higher δ11B values (+9.52∼+10.18‰) than those formed during the peak metamorphism of amphibolite facies (+7.22∼+9.43‰). Subsequently, tourmalines with lower δ11B values formed during the retrograde metamorphism of greenschist facies, which have slightly different boron isotopic compositions in cores (+4.18∼+5.50‰) and rims (+2.66∼+4.66‰), mainly due to the temperature effect. The boron isotopic variations of tourmalines of amphibolites match well with our modeling study of boron isotope. The results suggest that tourmalines may be precipitated from fluids released from dehydration of marine sediments with relative high δ11B values (+9‰) during prograde metamorphism. In the same way, the borate deposits formed from the liberated B-rich fluids that interacted with the Mg-rich carbonates and/or silicates.

The Effect of water–glycerol mixtures on rheological properties of coal slurries

The rheological behavior of slurries of coal at different stages of metamorphism has been studied as depending on glycerol content in an aqueous dispersion medium. Increased viscosity and reduced polarity of water–glycerol solutions have been found to increase the shear stress and viscosity of coal slurries. The effective viscosity of the slurries has been revealed to decline at the same weight content of the dispersed phase with a rise in the stage of coal metamorphism (i.e., when passing from brown coal to anthracite).

Precipitation of humic substances from aqueous and alkaline solutions under the action of electrolytes: A review

The precipitation of humic substances by electrolytes from aqueous and alkaline solutions occurs in two stages: the first is the transformation of true solutions into colloidal solutions, and the second is the decrease in the aggregative stability, the adhesion of particles, and the formation of sediment. The term coagulation, which is commonly accepted in colloid chemistry, is most suitable for the characterization of the above processes because the formation of the sediments of humic substances is of colloid-chemical nature. Methods for the determination of a coagulation threshold are not standardized; therefore, its values for the same materials determined by different researchers were different. The coagulation thresholds of the humic acids of solid fuels are very typical for different stages of metamorphism, and they adequately reflect the nature of fossil fuels.

Quantitative study of the macromolecular structures of tectonically deformed coal using high-resolution transmission electron microscopy

Tectonic deformations damage the macrostructure and impact the macromolecular structures of coal. In this study, high-resolution transmission electron microscopy (HRTEM) was applied to observe the macromolecular structures of six different types of tectonically deformed coal, and quantitative analyses were conducted by using image analysis algorithms to extract the images of lattice fringes to obtain the following three structural parameters: fringe length, fringe tortuosity and fringe separation. The results show that coal rank has an important influence on the macromolecular structures of coal, and this effect varies with different stages of metamorphism. For weak tectonically deformed coal, increases in coal rank indicate an increase in fringe length and decrease in fringe separation and fringe tortuosity. The increments in the fringe separation and fringe length first increase and then decrease, and the maximum values occur during the evolution from mid metamorphic stage to high metamorphic stage. Fringe tortuosity shows a gradually decreasing trend. Tectonic deformation also has an important influence on the macromolecular structure of coal, and this effect varies with different stages of coal rank. With decreases of tectonic deformation, fringe length is increased and fringe separation and tortuosity are decreased; however, the impact of tectonic deformation on the macromolecular parameters of low-rank coal is stronger than that of the mid-rank coal. The impacts of tectonic deformation on the nanoscale pores, coal methane adsorption capacity and coalbed methane contents of low-rank coal are higher than those of the mid-rank coal.

A Laser Based Fluorination (BrF5) System for the Extraction of Oxygen (O2) from Silicate Rocks of Himalaya and δ18O Measurements: Method Establishment and Implications

Oxygen forms the majority of minerals constituting the terrestrial and planetary objects. Its chemical and isotopic compositions often bear signatures of the processes occurring within these objects on different time scales. Silicates are the common rocks in the Himalaya, the major feature on the Earth, which has witnessed different stages of metamorphism and got reworked during its evolution. Therefore, oxygen isotopes in silicate rocks can be used to reveal the processes pertaining to the Himalayan orogeny. These processes include petrogenesis, metamorphism, subduction and associated magmatic events, etc. Oxygen isotopic composition (δ18O) is needed to be measured in silicate rocks which require special arrangements. Towards this, a vacuum line of stainless steel coupled with a computer controlled CO2 laser system was fabricated, for the first time in India, to extract oxygen (O2) from rock forming mineral in the BrF5 environment. The line is equipped with suitable valves and traps for the efficient recovery of oxygen from the silicate minerals and reliable handling of hazardous oxidants (i.e., BrF5, etc.) that are used as fluorination agents to break Si=O bonds. This extraction system is interfaced with a continuous flow stable isotope ratio mass spectrometer and is being used successfully for the oxygen isotopic (δ18O) measurements in silicate and oxide minerals. Here we report the δ18O results on a set of silicate rocks and standard reference materials including Norway olivine, Gee Whiz Garnet standard from University of New Mexico, olivine from Norway Fan (Lab Standard), silicate rocks from North West Himalaya (India) and NBS-28 quartz a standard reference material from International Atomic Energy Agency, Vienna. The average δ18O values (relative to Vienna standard mean ocean water) and 2SD are 6.4 ± 0.3 ‰ (N = 17) for Norway olivine, 12.7 ± 0.2 ‰ (N = 10) for Gee Whiz Garnet and 9.5 ± 0.3 ‰ (N = 12) for NBS-28 quartz. In addition, some of the quartz vein and quartzites from the Garhwal Himalaya were also measured to establish the measurement protocols and procedures. Silicate rocks from the Himalaya near the main central thrust zone were tried to fix the end-member composition for oxygen isotopes (δ18O). Such information is helpful to infer about the origin of quartzite and their relationship with possible protoliths in the region. Further, this technique finds applications in the analysis of biogenic silica (diatoms and phytoliths, etc.) for paleoclimatic reconstruction.

Metamorphic history of glaucophane‐paragonite‐zoisite eclogites from the Shanderman area, northern Iran

AbstractThe Shanderman eclogites and related metamorphosed oceanic rocks mark the site of closure of the Palaeotethys ocean in northern Iran. The protolith of the eclogites was an oceanic tholeiitic basalt with MORB composition. Eclogite occurs within a serpentinite matrix, accompanied by mafic rocks resembling a dismembered ophiolite. The eclogitic mafic rocks record different stages of metamorphism during subduction and exhumation. Minerals formed during the prograde stages are preserved as inclusions in peak metamorphic garnet and omphacite. The rocks experienced blueschist facies metamorphism on their prograde path and were metamorphosed in eclogite facies at the peak of metamorphism. The peak metamorphic mineral paragenesis of the rocks is omphacite, garnet (pyrope‐rich), glaucophane, paragonite, zoisite and rutile. Based on textural relations, post‐peak stages can be divided into amphibolite and greenschist facies. Pressure and temperature estimates for eclogite facies minerals (peak of metamorphism) indicate 15–20 kbar at ~600 °C. The pre‐peak blueschist facies assemblage yields &lt;11 kbar and 400–460 °C. The average pressure and temperature of the post‐peak amphibolite stage was 5–6 kbar, ~470 °C. The Shanderman eclogites were formed by subduction of Palaeotethys oceanic crust to a depth of no more than 75 km. Subduction was followed by collision between the Central Iran and Turan blocks, and then exhumation of the high pressure rocks in northern Iran.

The role of amphiboles in the metamorphic evolution of the UHP rocks: a case study from the Tso Morari Complex, northwest Himalayas

Amphiboles represent a crucial phase of the ultra-high-pressure (UHP) metamorphic rocks as their solid solution behavior reflects both bulk compositional and P–T changes. Three different types of amphibole have been reported from the UHP metamafic rocks of the Tso Morari Crystalline Complex, NW Himalayas: Na-rich (glaucophane); Na–Ca-rich (barroisite, taramite, winchite) and Ca-rich (tremolite, magnesio-hornblende, pargasite). The Na-amphibole is presented as a core of the zoned amphibole with Na–Ca-rich rim; Na–Ca-amphibole is presented as inclusion in garnets as well as in matrix, and Ca-amphibole is generally found in the matrix. The Na–Ca-amphibole is observed at two different stages of metamorphism. The first is pre-UHP, and the second is post-garnet–omphacite assemblage though with a significant difference in composition. The pressure–temperature estimations of the formation of these two sets of Na–Ca-amphiboles corroborate their textural associations. Ca-rich amphiboles are generally present in the matrix either as symplectite with plagioclase or as a pseudomorph after garnet along with other secondary minerals like chlorite and biotite. Two different types of zoning have been observed in the amphibole grains: (1) core is Na-rich followed by Na–Ca rim and (2) core of Na–Ca-amphibole is followed by Ca-rich rim. The pre-UHP (or the prograde P–T path) and post-UHP stages (or the retrograde P–T path) of Tso Morari eclogites are defined by characteristic amphibole compositions, viz. Na/Na–Ca-amphibole, Na–Ca-amphibole and Ca-amphibole and thus indicate their utility in inferring crustal evolution of this UHP terrain.

Pressure–temperature-fluid evolutionary history of orthopyroxene-bearing quartzofeldspathic and mafic granulites from northern parts of the Eastern Ghats Belt, India: implications for Indo-Antarctic correlation

Mafic and felsic orthogneisses form an integral component of the rock association of the Eastern Ghats Belt (EGB). Such rocks show similar P– T-fluid evolutionary history at two different localities of the EGB from where UHT metamorphism has already been recorded. Both mafic and felsic orthogneisses show contrasting petrological characters and are classified according to structural and mineralogical characteristics. All the varieties of mafic granulite and the gneissic enderbite preserve the strong regional foliation (S 2/S 3). The pegmatoidal enderbite is post deformational and hybrid in nature containing xenocrysts derived from associated rocks. Textural and thermobarometric data reveal different stages of metamorphism. Peak metamorphism (M 1) occurs at 8–9 kbar pressure and temperature in excess of 950 °C. Emplacement of mafic magma could be the principal cause of this metamorphism. This was followed by retrograde R 1 stage when the rocks suffered a near isobaric cooling up to 700–750 °C at 7–7.5 kbar. At the terminal retrograde stage (R 2) fluid–rock interaction took place at 6–6.5 kbar, 600–650 °C. The fluid composition is calculated to be poor in H 2O and CO 2. Textural data predict that K-rich brine could be responsible for such composition. The observed P– T-fluid characteristics show similarity with other terranes belonging to erstwhile Gondwanaland.

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.