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.     

Micro-XANES determination of ferric iron and its application in thermobarometry

Micro-X-ray absorption near-edge structure (XANES) analysis was employed to determine the content of ferric iron in minerals formed in ultrahigh-pressure (UHP) eclogites. It is observed that omphacite and phengite contain significant amounts of Fe³⁺/Fe_tot (0.2–0.6), whereas only very low contents are present in garnet (Fe³⁺/Fe_tot=0.0–0.03), the latter being consistent with results from stoichiometric charge-balance calculations. Furthermore, considerable variations in the Fe³⁺/Fe_tot ratios of omphacite and phengite are observed depending on the textural sites and local bulk chemistry (eclogite and calc-silicate matrix) within one thin section. The oxidation state of isofacial minerals is thus likely to depend on the local fluid composition, which, in the studied case, is controlled by calcareous and meta-basic mineral compositions. These first in-situ measurements of ferric iron in an eclogite sample from the Dabie Shan, E China, are used to recalculate geothermobarometric data. Calculations demonstrate that the temperature during UHP metamorphism was as high as 780 °C, about 80–100 °C higher than previously estimated. Temperatures based on charge balance calculations often give erroneous results. Pressure estimates are in good agreement with former results and confirm metamorphism in the stability field of diamond (43.7 kbar at 750 °C). These P–T data result in a geothermal gradient of ca. 6 °C/km during UHP metamorphism in the Dabie Shan. However, accounting for ferric iron contents in geothermobarometry creates new difficulties inasmuch as calibrations of geothermometers may not be correctable for Fe³⁺ and the actual effect on Mg–Fe²⁺ partitioning is unknown. The present study further shows that micro-XANES is a promising technique for the in situ determination of ferric iron contents without destroying the textural context of the sample: a clear advantage compared to bulk methods.     

Fluid inclusions in granulites, granulitized eclogites and garnet clinopyroxenites from the Dabie–Sulu terranes, eastern China

Minor granulites (believed to be pre-Triassic), surrounded by abundant amphibolite-facies orthogneiss, occur in the same region as the well-documented Triassic high- and ultrahigh-pressure (HP and UHP) eclogites in the Dabie–Sulu terranes, eastern China. Moreover, some eclogites and garnet clinopyroxenites have been metamorphosed at granulite- to amphibolite-facies conditions during exhumation. Granulitized HP eclogites/garnet clinopyroxenites at Huangweihe and Baizhangyan record estimated eclogite-facies metamorphic conditions of 775–805 °C and ≥15 kbar, followed by granulite- to amphibolite-facies overprint of ca. 750–800 °C and 6–11 kbar. The presence of (Na, Ca, Ba, Sr)-feldspars in garnet and omphacite corresponds to amphibolite-facies conditions. Metamorphic mineral assemblages and P–T estimates for felsic granulite at Huangtuling and mafic granulite at Huilanshan indicate peak conditions of 850 °C and 12 kbar for the granulite-facies metamorphism and 700 °C and 6 kbar for amphibolite-facies retrograde metamorphism. Cordierite–orthopyroxene and ferropargasite–plagioclase coronas and symplectites around garnet record a strong, rapid decompression, possibly contemporaneous with the uplift of neighbouring HP/UHP eclogites.

Carbonic fluid (CO₂-rich) inclusions are predominant in both HP granulites and granulitized HP/UHP eclogites/garnet clinopyroxenites. They have low densities, having been reset during decompression. Minor amounts of CH₄ and/or N₂ as well as carbonate are present. In the granulitized HP/UHP eclogites/garnet clinopyroxenites, early fluids are high-salinity brines with minor N₂, whereas low-salinity fluids formed during retrogression. Syn-granulite-facies carbonic fluid inclusions occur either in quartz rods in clinopyroxene (granulitized HP garnet clinopyxeronite) or in quartz blebs in garnet and quartz matrices (UHP eclogite). For HP granulites, a limited number of primary CO₂ and mixed H₂O–CO₂(liquid) inclusions have also been observed in undeformed quartz inclusions within garnet, orthopyroxene, and plagioclase which contain abundant, low-density CO₂±carbonate inclusions. It is suggested that the primary fluid in the HP granulites was high-density CO₂, mixed with a significant quantity of water. The water was consumed by retrograde metamorphic mineral reactions and may also have been responsible for metasomatic reactions (“giant myrmekites”) occurring at quartz–feldspar boundaries. Compared with the UHP eclogites in this region, the granulites were exhumed in the presence of massive, externally derived carbonic fluids and subsequently limited low-salinity aqueous fluids, probably derived from the surrounding gneisses.     

Coesite inclusions and prograde compositional zonation of garnet in whiteschist of the HP-UHPM Kokchetav massif, Kazakhstan: a record of progressive UHP metamorphism

Coarse-grained whiteschist, containing the assemblage: garnet+kyanite+phengite+talc+quartz/coesite, is an abundant constituent of the ultrahigh-pressure metamorphic (UHPM) belt in the Kulet region of the Kokchetav massif of Kazakhstan.

Garnet displays prograde compositional zonation, with decreasing spessartine and increasing pyrope components, from core to rim. Cores were recrystallized at T=380°C (inner) to 580°C (outer) at P<10 kbar (garnet–ilmenite geothermometry, margarite+quartz stability), and mantles at T=720–760°C and P_H20=34–36 kbar (coesite+graphite stability, phengite geobarometer, KFMASH system reaction equilibria). Textural evidence indicates that rims grew during decompression and cooling, within the Qtz-stability field.

Silica inclusions (quartz and/or coesite) of various textural types within garnets display a systematic zonal distribution. Cores contain abundant inclusions of euhedral quartz (type 1 inclusions). Inner mantle regions contain inclusions of polycrystalline quartz pseudomorphs after coesite (type 2), with minute dusty micro-inclusions of chlorite, and more rarely, talc and kyanite in their cores; intense radial and concentric fractures are well developed in the garnet. Intermediate mantle regions contain bimineralic inclusions with coesite cores and palisade quartz rims (type 3), which are also surrounded by radial fractures. Subhedral inclusions of pure coesite without quartz overgrowths or radial fractures (type 4) occur in the outer part of the mantle. Garnet rims are silica-inclusion-free.

Type 1 inclusions in garnet cores represent the low-P, low-T precursor stage to UHPM recrystallization, and attest to the persistence of low-P assemblages in the coesite-stability field. Coesites in inclusion types 2, 3, and 4 are interpreted to have sequentially crystallized by net transfer reaction (kyanite+talc=garnet+coesite+H₂O), and were sequestered within the garnet with progressively decreasing amounts of intragranular aqueous fluid.

During the retrograde evolution of the rock, all three inclusion types diverged from the host garnet P–T path at the coesite–quartz equilibrium, and followed a trajectory parallel to the equilibrium boundary resulting in inclusion overpressure. Coesite in type 2 inclusions suffered rapid intragranular H₂O-catalysed transformation to quartz, and ruptured the host garnet at about 600°C (when inclusion P27 kbar, garnet host P9 kbar). Instantaneous decompression to the host garnet P–T path, passed through the kyanite+talc=chlorite+quartz reaction equilibrium, resulting in the dusty micro-assemblage in inclusion cores. Type 3 inclusions suffered a lower volumetric proportion transformation to quartz at the coesite–quartz equilibrium, and finally underwent rupture and decompression when T<400°C, facilitating coesite preservation. Type 4 coesite inclusions are interpreted to have suffered minimal transformation to quartz and proceeded to surface temperature conditions along or near the coesite–quartz equilibrium boundary.     

Petrology of ultrahigh-pressure eclogites from the ZK703 drillhole in the Donghai, eastern China

The 558-m-deep ZK703 drillhole located near Donghai in the southern part of the Sulu ultrahigh-pressure metamorphic belt, eastern China, penetrates alternating layers of eclogites, gneisses, jadeite quartzites, garnet peridotites, phengite–quartz schists, and kyanite quartzites. The preservation of ultrahigh-pressure metamorphic minerals and their relics, together with the contact relationship and protolith types of the various rocks indicates that these are metamorphic supracrustal rocks and mafic-ultramafic rock assemblages that have experienced in-situ ultrahigh-pressure metamorphism. The eclogites can be divided into five types based on accessory minerals: rutile eclogite, phengite eclogite, kyanite–phengite eclogite, quartz eclogite, and common eclogite with rare minor minerals. Rutile eclogite forms a thick layer in the drillhole that contains sufficient rutile for potential mining. Two retrograde assemblages are observed in the eclogites: the first stage is characterized by the formation of sodic plagioclase+amphibole symplectite or symplectitic coronas after omphacite and garnet, plagioclase+biotite after garnet or phengite, and plagioclase coronas after kyanite; the second stage involved total replacement of omphacite and garnet by amphibole+albite+epidote+quartz. Peak metamorphic P–T conditions of the eclogites were around 32 to 40 kbar and 720°C to 880°C. The retrograde P–T path of the eclogites is characterized by rapidly decreasing pressure with slightly decreasing temperature. Micro-textures and compositional variations in symplectitic minerals suggest that the decompression breakdown of ultrahigh-pressure minerals is a domainal equilibrium reaction or disequilibrium reaction. The composition of the original minerals and the diffusion rate of elements involved in these reactions controlled the symplectitic mineral compositions.     

Kyanite eclogite xenolith from the orthogneiss terrane of the Tisza Megaunit, Jánoshalma area, crystalline basement of southern Hungary

New evidence for high-pressure, eclogite facies metamorphism in the crystalline basement of the Tisza Megaunit (southern Hungary) is reported. The retrogressed mafic eclogite forms a small lens in the orthogneiss and it was found in the borehole near Jánoshalma. The carbonated eclogite contains the peak metamorphic assemblage omphacite + garnet + phengite + kyanite + clinozoizite + rutile + K-feldspar + quartz. Omphacite (Xjd_0.40–0.41Xdio_0.52–0.53Xhd_0.05Xaug_1.55–2.85) occurs in the matrix and as inclusions in garnet (Xpy_0.37–0.38Xgrs_0.21–0.22Xalm_0.39–0.40Xsps_0–0.01Xadr_0–0.01) and kyanite. Thermobarometry based on net-transfer reactions between garnet, omphacite, kyanite and phengite yields P–T conditions of 710 ± 10 °C and 2.6 ± 0.75 GPa. Retrogression during decompression is manifested by formation of symplectites; the most typical are diopside + plagioclase after omphacite, corundum + spinel + plagioclase after kyanite and biotite + plagioclase after phengite. Carbonatization along the veins of the retrogressed eclogite was probably coeval with formation of these symplectites. At places where carbonate is absent the rock was completely hydrated and retrogressed down to the greenschist facies with the development of actinolite. Similar eclogites together with abundant orthogneisses occur mainly in the eastern parts of the Tisza Megaunit, suggesting the existence of an ancient (possibly Variscan) subduction/accretionary complex.     

Excess Ar-free phengite in ultrahigh-pressure metamorphic rocks from the Lago di Cignana area, Western Alps

Ar/Ar analyses of phengites and paragonites from the ultrahigh-pressure metamorphic rocks (zoisite–clinozoisite schist, garnet–phengite schist and piemontite schist) in the Lago di Cignana area, Western Alps were carried out with a laser probe step-heating method using single crystals and a spot dating method on thin sections. Eight phengite and two paragonite crystals give the plateau ages of 37–42 Ma with 96–100% of ³⁹Ar released. Each rock type also contains mica crystals showing discordant age spectra with age fractions (20–35 Ma) significantly younger than the plateau ages. Phengite inclusions in garnet give ages of 43.2 ± 1.1 Ma and 44.4 ± 1.5 Ma, which are significantly older than the spot age (36.4 ± 1.4 Ma) from the matrix phengites, and the plateau ages from the step-heating analyses. Inclusion ages (43 and 44 Ma) are consistent with a zircon SHRIMP age (44 ± 1 Ma) in this area. These results suggest that the oceanic materials that underwent a simple subduction related UHPM, form excess ⁴⁰Ar-free phengite and that the peak metamorphism is ca. 44 Ma or little older. We suggest that matrix phengites experienced a retrogression reaction changing their chemistry contemporaneously with deformation related to the exhumation of rocks releasing significant radiogenic ⁴⁰Ar from the crystals. This has lead to the apparent ages of the matrix phengites that are significantly younger than the inclusion age.     

Effect of Fe on garnet–melt trace element partitioning: experiments in FCMAS and quantification of crystal-chemical controls in natural systems

Garnet–melt trace element partitioning experiments were performed in the system FeO–CaO–MgO–Al₂O₃–SiO₂ (FCMAS) at 3 GPa and 1540°C, aimed specifically at studying the effect of garnet Fe²⁺ content on partition coefficients (D^Grt/Melt). D^Grt/Melt, measured by SIMS, for trivalent elements entering the garnet X-site show a small but significant dependence on garnet almandine content. This dependence is rationalised using the lattice strain model of Blundy and Wood [Blundy, J.D., Wood, B.J., 1994. Prediction of crystal–melt partition coefficients from elastic moduli. Nature 372, 452–454], which describes partitioning of an element i with radius r_i and valency Z in terms of three parameters: the effective radius of the site r₀(Z), the strain-free partition coefficient D₀(Z) for a cation with radius r₀(Z), and the apparent compressibility of the garnet X-site given by its Young's modulus E_X(Z). Combination of these results with data in Fe-free systems [Van Westrenen, W., Blundy, J.D., Wood, B.J., 1999. Crystal-chemical controls on trace element partitioning between garnet and anhydrous silicate melt. Am. Mineral. 84, 838–847] and crystal structure data for spessartine, andradite, and uvarovite, leads to the following equations for r₀(3+) and E_X(3+) as a function of garnet composition (X) and pressure (P):

r₀(3+) [Å]=0.930X_Py+0.993X_Gr+0.916X_Alm+0.946X_Spes+1.05(X_And+X_Uv)−0.005(P [GPa]−3.0)(±0.005 Å)

E_X(3+) [GPa]=3.5×10¹²(1.38+r₀(3+) [Å])^−26.7(±30 GPa)

Accuracy of these equations is shown by application to the existing garnet–melt partitioning database, covering a wide range of P and T conditions (1.8 GPa<P<5.0 GPa; 975°C<T<1640°C). D^Grt/Melt for all 3+ elements entering the X-site (REE, Sc and Y) are predicted to within 10–40% at given P, T, and X, when D^Grt/Melt for just one of these elements is known. In the absence of such knowledge, relative element fractionation (e.g. D_Sm^Grt/Melt/D_Nd^Grt/Melt) can be predicted. As an example, we predict that during partial melting of garnet peridotite, group A eclogite, and garnet pyroxenite, r₀(3+) for garnets ranges from 0.939±0.005 to 0.953±0.009 Å. These values are consistently smaller than the ionic radius of the heaviest REE, Lu. The above equations quantify the crystal-chemical controls on garnet–melt partitioning for the REE, Y and Sc. As such, they represent a major advance en route to predicting D^Grt/Melt for these elements as a function of P, T and X.     

Geothermobarometry in albite-garnet orthogneisses: A case study from the Gran Paradiso nappe (Western Alps)

The aim of this paper is to estimate syntectonic P-T conditions within albite- and garnet-bearing orthogneisses. These rocks are generally characterized by the assemblage quartz + albite + biotite + phengite + CaFe-garnet + epidote + titanite. Garnet contains up to 55 mole per cent of grossular. K-feldspar is a relict magmatic phase.

P-T conditions are estimated using several independent methods. First, it is shown that exchange reactions based on the Fe---Mg partitioning between garnet and biotite or garnet and phengite cannot be used to estimate temperatures in these rocks, due to the high grossular content of garnet. Second, maximum and minimum pressures are constrained, respectively, by the occurrence of albite instead of jadeite + quartz and by the assemblage phengite + biotite + quartz. Third, phase equilibria in albite- and garnet-bearing metagranites are modelled in the system K₂O---CaO---FeO---Al₂O₃---SiO₂---H₂O. Equilibrium curves are calculated for the observed phase compositions. Uncertainties in P-T estimates mainly result from the choice of appropriate non-ideal solution models for the garnet.

An application is developed for granites from the Gran Paradiso nappe (Western Alps). These granites show an heterogeneous deformation of Alpine age expressed by mylonitic shear zones cutting across weakly deformed domains. Estimated P-T conditions for the synkinematic assemblages are 10–16 kbar at 550±50°C.     

Cretaceous isochron ages from K–Ar and Ar / Ar dating of eclogitic rocks in the Tso Morari Complex, western Himalaya, India

The Tso Morari Complex, which is thought to be originally the margin of the Indian continent, is composed of pelitic gneisses and schists including mafic rock lenses (eclogites and basic schists). Eclogites studied here have the mineral assemblage Grt + Omp + Ca-Amp + Zo + Phn + Pg + Qtz + Rt. They also have coesite pseudomorph in garnet and quartz rods in omphacite, suggesting a record of ultrahigh-pressure metamorphism. They occur only in the cores of meter-scale mafic rock lenses intercalated with the pelitic schists. Small mafic lenses and the rim parts of large lenses have been strongly deformed to form the foliation parallel to that of the pelitic schists and show the mineral assemblages of upper greenschist to amphibolite facies metamorphism. The garnet–omphacite thermometry and the univariant reaction relations for jadeite formation give 13–21 kbar at 600 °C and 16–18 kbar at 750 °C for the eclogite formation using the jadeite content of clinopyroxene (X_Jd = 0.48).

Phengites in pelitic schists show variable Si / Al and Na / K ratios among grains as well as within single grains, and give K–Ar ages of 50–87 Ma. The pelitic schist with paragonite and phengite yielded K–Ar ages of 83.5 Ma (K = 4.9 wt.%) for paragonite–phengite mixture and 85.3 Ma (K = 7.8 wt.%) for phengite and an isochron age of 91 ± 13 Ma from the two dataset. The eclogite gives a plateau age of 132 Ma in Ar/Ar step-heating analyses using single phengite grain and an inverse isochron age of 130 ± 39 Ma with an initial ⁴⁰Ar / ³⁶Ar ratio of 434 ± 90 in Ar/Ar spot analyses of phengites and paragonites. The Cretaceous isochron ages are interpreted to represent the timing of early stage of exhumation of the eclogitic rocks assuming revised high closure temperature (500 °C) for phengite K–Ar system. The phengites in pelitic schists have experienced retrograde reaction which modified their chemistry during intense deformation associated with the exhumation of these rocks with the release of significant radiogenic ⁴⁰Ar from the crystals. The argon release took place in the schists that experienced the retrogression to upper greenschist facies metamorphisms from the eclogite facies conditions.     

Mineral inclusions in zircons of para- and orthogneiss from pre-pilot drillhole CCSD-PP1, Chinese Continental Scientific Drilling Project

The pre-pilot drillhole CCSD-PP1, Chinese Continental Scientific Drilling Project (CCSD), with depth of 432 m, is located in the Donghai area in the southwestern Sulu terrane. The core samples are mainly comprised of paragneiss, orthogneiss and ultramafic rock with minor intercalated layers of eclogite and phengite-bearing kyanite quartzite. All analyzed paragneiss and orthogneiss samples were overprinted on amphibolite facies retrograde metamorphism. Coesite and coesite-bearing ultrahigh-pressure (UHP) mineral assemblages were identified by Raman spectroscopy and electron microprobe analysis as inclusions in zircons separated from paragneiss, eclogite and phengite-bearing kyanite quartzite samples. In the paragneiss samples, UHP mineral inclusion assemblages mainly consist of Coe+Omp+Grt+Phe, Coe+Jd+Phe+Ap preserved in the mantles (M) and rims (R) of zircons. These UHP mineral inclusion assemblages yield temperatures of 814–852 °C and pressures of ≥28 kbar, presenting the P–T condition of UHP peak metamorphism of these country rocks. According to the mineral inclusions and cathodoluminescence images of zircons, the orthogneisses can be divided into two types: UHP (OG1) and non-UHP (OG2). In OG1 orthogneisses, low-pressure mineral inclusion assemblage, mainly consisting of Qtz+Phe+Ab+Ksp+Ap, were identified in zircon cores (C), while coesite or coesite-bearing UHP mineral inclusions were identified in the mantles (M) and rims (R) of the same zircons. These features suggest that the OG1 orthogneisses, together with the paragneisses, phengite-bearing kyanite quartzite and eclogite experienced widespread UHP metamorphism in the Sulu terrane. However, in the zircons of OG2 orthogneiss samples, no UHP mineral inclusions were found. Inclusions mainly comprised Qtz+Phe+Ap and were identified in cores (C), mantles (M) and rims (R) of OG2 zircons; the cathdoluminescence images of all analyzed zircons showed clear zonings from cores to rims. These features indicate that the OG2 orthogneisses in pre-pilot drillhole CCSD-PP1 did not experience UHP metamorphism. Therefore, we should not rule out the possibility that some orthogneisses in Sulu terrane might represent relatively low-pressure granitic intrusives emplaced after the UHP event.     

Conditions and timing of high-pressure Variscan metamorphism in the South Carpathians, Romania

High-pressure (HP) metamorphic rocks, including garnet peridotite, eclogite, HP granulite, and HP amphibolite, are important constituents of several tectonostratigraphic units in the pre-Alpine nappe stack of the Getic–Supragetic (GS) basement in the South Carpathians. A Variscan age for HP metamorphism is firmly established by Sm–Nd mineral–whole-rock isochrons for garnet amphibolite, 358±10 Ma, two samples of eclogite, 341±8 and 344±7 Ma, and garnet peridotite, 316±4 Ma.

A prograde history for many HP metamorphic rocks is documented by the presence of lower pressure mineral inclusions and compositional zoning in garnet. Application of commonly accepted thermobarometers to eclogite (grt+cpx±ky±phn±pg±zo) yields a range in “peak” pressures and temperatures of 10.8–22.3 kbar and 545–745 °C, depending on tectonostratigraphic unit and locality. Zoisite equilibria indicate that activity of H₂O in some samples was substantially reduced, ca. 0.1–0.4. HP granulite (grt+cpx+hb+pl) and HP amphibolite (grt+hbl+pl) may have formed by retrogression of eclogites during high-temperature decompression. Two types of garnet peridotite have been recognized, one forming from spinel peridotite at ca. 1150–1300 °C, 25.8–29.0 kbar, and another from plagioclase peridotite at 560 °C, 16.1 kbar.

The Variscan evolution of the pre-Mesozoic basement in the South Carpathians is similar to that in other segments of the European Variscides, including widespread HP metamorphism, in which P–T–t characteristics are specific to individual tectonostratigraphic units, the presence of diverse types of garnet peridotite, diachronous subduction and accretion, nappe assembly in pre-Westphalian time due to collision of Laurussia, Gondwana, and amalgamated terranes, and finally, rapid exhumation, cooling, and deposition of eroded debris in Westphalian to Permian sedimentary basins.     

Experimental evidence for diamond-facies metamorphism in the Dora-Maira massif

The peak metamorphic conditions of subducted continental crust in the Dora-Maira massif (Western Alps) have been revised by combining experimental results in the KCMASH system with petrologic information from whiteschists. Textural observations in whiteschists suggest that the peak metamorphic assemblage garnet+phengite+kyanite+coesite±talc originates from the reaction kyanite+talc↔garnet+coesite+liquid. In the experimentally determined petrogenetic grid, this reaction occurs above 45 kbar at 730 °C. At lower pressures, talc reacts either to orthopyroxene and coesite or, together with phengite, to biotite, coesite and kyanite. The liberated liquid contains probably similar amounts of H₂O and dissolved granitic components. The composition of the liquid in the whiteschists at peak metamorphic conditions, a major unknown in earlier studies, was probably very similar to the liquid composition produced in the experiments. Therefore, the experimentally determined petrogenetic grid represents a good model for the estimation of the peak metamorphic conditions in whiteschists. Experimentally determined Si-isopleths for phengite further constrain peak pressures to 43 kbar for the measured Si=3.60 of phengite in the natural whiteschists. All these data provide evidence that the whiteschists reached diamond-facies conditions.

The fluid-absent equilibrium 4 kyanite+3 CELADONITE=4 coesite+3 muscovite+pyrope has been calibrated on the basis of garnet and phengite compositions in the experiments and serves as a geothermobarometer for ultra-high-pressure (UHP) metapelites. For graphite-bearing metapelites and kyanite–phengite eclogites, forming the country rocks of the whiteschists, peak metamorphic pressures of about 44±3 kbar were calculated from this barometer for temperatures of 750 °C estimated from garnet–phengite thermometry. Therefore, the whole ultra-high-pressure unit of the Dora-Maira massif most likely experienced peak metamorphic conditions in the diamond stability field. While graphite is common in the metapelites, diamond has not been found so far. The absence of metamorphic microdiamonds might be explained by the low temperature of metamorphism, the absence of a free fluid phase in the metapelites and a short residence time in diamond-facies conditions resulting in kinetic problems in the conversion of graphite to diamond.     

Reactions and textures in grossular–wollastonite–scapolite calc–silicate granulites from Maligawila, Sri Lanka: evidence for high-temperature isobaric cooling in the meta-sediments of the Highland Complex

Grossular–wollastonite–scapolite calc–silicate granulites from Maligawila in the Buttala klippe, which form part of the overthrusted rocks of the Highland Complex of Sri Lanka, preserve a number of spectacular coronas and replacement textures that could be effectively used to infer their P–T–fluid history. These textures include coronas of garnet, garnet–quartz, and garnet–quartz–calcite at the grain boundaries of wollastonite, scapolite, and calcite as well as calcite–plagioclase and calcite–quartz symplectites or finer grains after scapolite and wollastonite respectively. Other textures include a double rind of coronal scapolite and coronal garnet between matrix garnet and calcite. The reactions that produced these coronas and replacement textures, except those involving clinopyroxene, are modelled in the CaO–Al₂O₃–SiO₂–CO₂ system using the reduced activities. Calculated examples of T–X_CO2 and P–X_CO2 projections indicate that the peak metamorphic temperature of about 900–875 °C at a pressure of 9 kbar and the peak metamorphic fluid composition is constrained to be low in X_CO2 (0.1<X_CO2<0.30). Interpretation of the textural features on the basis of the partial grids revealed that the calc–silicate granulites underwent high-temperature isobaric cooling, from about 900–875 °C to a temperature below 675 °C, following the peak metamorphism. The late-stage cooling was accompanied by an influx of hydrous fluids. The calc–silicate granulites provide evidence for high-temperature isobaric cooling in the meta-sediments of the Highland Complex, earlier considered by some workers to be confined exclusively to the meta-igneous rocks. The coronal scapolite may have formed under open-system metasomatism.     

The Homestead kimberlite, central Montana, USA: mineralogy, xenocrysts, and upper-mantle xenoliths

The Homestead kimberlite was emplaced in lower Cretaceous marine shale and siltstone in the Grassrange area of central Montana. The Grassrange area includes aillikite, alnoite, carbonatite, kimberlite, and monchiquite and is situated within the Archean Wyoming craton. The kimberlite contains 25–30 modal% olivine as xenocrysts and phenocrysts in a matrix of phlogopite, monticellite, diopside, serpentine, chlorite, hydrous Ca–Al–Na silicates, perovskite, and spinel. The rock is kimberlite based on mineralogy, the presence of atoll-textured groundmass spinels, and kimberlitic core-rim zoning of groundmass spinels and groundmass phlogopites.

Garnet xenocrysts are mainly Cr-pyropes, of which 2–12% are G10 compositions, crustal almandines are rare and eclogitic garnets are absent. Spinel xenocrysts have MgO and Cr₂O₃ contents ranging into the diamond inclusion field. Mg-ilmenite xenocrysts contain 7–11 wt.% MgO and 0.8–1.9 wt.% Cr₂O₃, with (Fe⁺³/Fe^tot) from 0.17–0.31. Olivine is the only obvious megacryst mineral present. One microdiamond was recovered from caustic fusion of a 45-kg sample.

Upper-mantle xenoliths up to 70 cm size are abundant and are some of the largest known garnet peridotite xenoliths in North America. The xenolith suite is dominated by dunites, and harzburgites containing garnet and/or spinel. Granulites are rare and eclogites are absent. Among 153 xenoliths, 7% are lherzolites, 61% are harzburgites, 31% are dunites, and 1% are orthopyroxenites. Three of 30 peridotite xenoliths that were analysed are low-Ca garnet–spinel harzburgites containing G10 garnets. Xenolith textures are mainly coarse granular, and only 5% are porphyroclastic.

Xenolith modal mineralogy and mineral compositions indicate ancient major-element depletion as observed in other Wyoming craton xenolith assemblages, followed by younger enrichment events evidenced by tectonized or undeformed veins of orthopyroxenite, clinopyroxenite, websterite, and the presence of phlogopite-bearing veins and disseminated phlogopite. Phlogopite-bearing veins may represent kimberlite-related addition and/or earlier K-metasomatism.

Xenolith thermobarometry using published two-pyroxene and Al-in-opx methods suggest that garnet–spinel peridotites are derived from 1180 to 1390 °C and 3.6 to 4.7 GPa, close to the diamond–graphite boundary and above a 38 mW/m² shield geotherm. Low-Ca garnet–spinel harzburgites with G10 garnets fall in about the same T and P range. Most spinel peridotites with assumed 2.0 GPa pressure are in the same T range, possibly indicating heating of the shallow mantle. Four of 79 Cr diopside xenocrysts have P–T estimates in the diamond stability field using published single-pyroxene P–T calculation methods.     

Mineral and fluid inclusions in zircon of UHP metamorphic rocks from the CCSD-main drill hole: A record of metamorphism and fluid activity

The Chinese Continental Scientific Drilling (CCSD) main drill hole (0–3000 m) in Donghai, southern Sulu orogen, consists of eclogite, paragneiss, orthogneiss, schist and garnet peridotite. Detailed investigations of Raman, cathodoluminescence, and microprobe analyses show that zircons from most eclogites, gneisses and schists have oscillatory zoned magmatic cores with low-pressure mineral inclusions of Qtz, Pl, Kf and Ap, and a metamorphic rim with relatively uniform luminescence and eclogite-facies mineral inclusions of Grt, Omp, Phn, Coe and Rt. The chemical compositions of the UHP metamorphic mineral inclusions in zircon are similar to those from the matrix of the host rocks. Similar UHP metamorphic P–T conditions of about 770 °C and 32 kbar were estimated from coexisting minerals in zircon and in the matrix. These observations suggest that all investigated lithologies experienced a joint in situ UHP metamorphism during continental deep subduction. In rare cases, magmatic cores of zircon contain coesite and omphacite inclusions and show patchy and irregular luminescence, implying that the cores have been largely altered possibly by fluid–mineral interaction during UHP metamorphism.

Abundant H₂O–CO₂, H₂O- or CO₂-dominated fluid inclusions with low to medium salinities occur isolated or clustered in the magmatic cores of some zircons, coexisting with low-P mineral inclusions. These fluid inclusions should have been trapped during magmatic crystallization and thus as primary. Only few H₂O- and/or CO₂-dominated fluid inclusions were found to occur together with UHP mineral inclusions in zircons of metamorphic origin, indicating that UHP metamorphism occurred under relatively dry conditions. The diversity in fluid inclusion populations in UHP rocks from different depths suggests a closed fluid system, without large-scale fluid migration during subduction and exhumation.     

Retention of Precambrian Rb/Sr phlogopite ages through Caledonian eclogite facies metamorphism, Bergen Arc Complex, W-Norway

The Lindås Nappe, Caledonides W-Norway was affected by two major tectonometamorphic events. A Precambrian granulite facies event at T=800–900°C, P<10 kbar was followed by localized Caledonian eclogite facies (T=650–700°C and P>15 kbar) and localized amphibolite facies reworking. During the granulite–eclogite facies transition, anorthositic rocks were converted from garnet granulites to kyanite eclogites, while phlogopite-bearing spinel lherzolite reacted to garnet lherzolite. The eclogite and amphibolite facies reequilibration took place along shear zones and fluid pathways. In the unhydrated and undeformed parts, the minerals preserved their granulite facies composition with constant Fe/Mg ratios from core to rim, suggesting diffusional reequilibration. Rb/Sr age dating was carried out on relict granulite facies minerals from three lenses of ultramafites (Alvfjellet, Hundskjeften and Kvamsfjellet). Phlogopite from phlogopite lherzolite at Alvfjellet give 857±9 Ma, while clinopyroxene, amphibole, phlogopite and whole rock from a lherzolite at Hundskjeften yield an age of 842±12 Ma (MSWD=1.9). Clinopyroxene, feldspar, orthopyroxene phlogopite and whole rock from websterite, Kvamsfjellet, yield an age of 835±7 Ma (MSWD<1), while clinopyroxene, phlogopite and whole rock from a lherzolite from the same lens gives a result of 882±9 Ma. These results are interpreted as minimum ages for the granulite facies event and only slightly younger than, or overlap with previous U–Pb zircon ages (929±1 Ma) and Sm–Nd garnet–pyroxene ages (890–923 Ma) interpreted to date the end of the granulite facies event. By contrast, ages obtained for the eclogite and amphibolite facies range from 460 (U–Pb, sphene), 440 (Ar–Ar), 419 (U–Pb, zircon) to 410 Ma (Rb/Sr mineral ages).

These results demonstrate that the reopening temperature for the Rb/Sr system in phlogopite–biotite under dry and static high-pressure conditions is, in the given mineral assemblages, at least 650°C, considerably higher than the 300–400°C assumed as the closure temperature of this system. We ascribe this elevated reopening temperature to fluid absent conditions that prevented element transport and rehomogenization.     

Distribution of Fe and Mg between coexisting garnet and hornblende in synthetic and natural systems: an empirical calibration of the garnet–hornblende Fe–Mg geothermometer

Multiple regression analysis of a compilation of the Fe²⁺–Mg distribution between garnet and hornblende from experimental runs on basaltic to intermediate compositions (n=22) and coexisting garnet–clinopyroxene–hornblende from natural (intermediate to basaltic) rocks (n=43) has been performed to define ln K_{D(Fe²⁺/Mg)}^Grt–Hbl as a function of temperature and garnet composition. The regression of data covering a large span in pressure (5–16 kbar), temperature (515–1025°C) and composition yields the ln K_{D(Fe²⁺/Mg)}^Grt–Hbl–P–T compositional relationship (r²=0.93):

where

Application of this expression to natural garnet–hornblende pairs in intermediate to basaltic and semipelitic rock types from various settings gives temperatures that are consistent with other methods.     

P-T evolution of metasediments from the Eclogite Zone, south-central Tauern Window, Austria

Petrologic data on the paragenesis of (I) kyanite-zoisite marbles and (II) garnet-chloritoid quartz-mica schists are presented with the goal of providing constraints on the pressure-temperature evolution of the Eclogite Zone, Tauern Window, Austria. The peak metamorphic assemblages in the two rock types are: (I) kyanite + zoisite + dolomite + quartz; zoisite + muscovite + dolomite + calcite + quartz; and (II) garnet + chloritoid + kyanite + muscovite + quartz + epidote ± dolomite ± Zn-staurolite. The estimated peak metamorphic conditions are 19 ± 2 kbar, 590 ± 20°C.

Secondary alteration of the kyanite-zoisite marbles was accomplished in two stages. The early stage resulted in the production of margarite, paragonite, secondary muscovite and chlorite and the later stage resulted in the formation of sudoite (a di/trioctahedral Mg---Al layer silicate) and kaolinite. The early alteration is bracketed at conditions between 3 and 10 kbar, 450–550°C and the later alteration between 200 and 350°C, P 3 kbar.

The P-T path is characterized by maximum burial to approximately 19 kbar (60–70 km) (at≈590°C), followed by nearly isothermal decompression to approximately 10 kbar (30 km), and then more gradual decompression with cooling to approximately 3 kbar (10 km). Alteration was apparently accomplished by the influx of H₂O-rich fluids, with the composition of the fluid locally buffered by the mineral assemblage.     

P-T evolution of eclogites from the Agualada Unit (Ordenes Complex, northwest Iberian Massif, Spain): Implications for crustal subduction

Eclogite lenses in the Agualada Unit (western Ordenes Complex, Spain) contain the peak mineral assemblage garnet (prograde rim: Alm = 48 mol%, Prp = 30 mol%), omphacite (Jd max = 36 mol%), quartz, rutile and rare zoisite, which equilibrated at T = 700 °C and P > 12–14 kbar. Garnet shows discontinuous growth zoning, with a pyrope-poor intermediate zone (Alm = 51 mol%, Prp = 10 mol%) between a core zone where pyrope is slightly higher (Alm = 46 mol%, Prp = 16 mol%) and areas just inward from the rims where the maximum pyrope contents (Alm = 48 mol%, Prp = 30 mol%) are recorded. In atoll interiors, garnet contains inclusions of a first generation of omphacite (Jd max = 40 mol%). This omphacite is replaced in the matrix by a second generation (Jd max = 36 mol%) with higher Fe/Fe + Mg ratio. The compositions of garnet and omphacite suggest a complex syneclogitic tectonothermal evolution for the Agualada Unit, characterized by: (1) eclogite-facies metamorphism (T = 585 °C, P > 12–13 kbar), followed by (2) cooling during a slight decompression (T = 500 °C, P > 11–12 kbar), and (3) a final increase in P and T to form the garnet rim-matrix omphacite mineral assemblage. The Agualada Unit is part of a subduction complex which affected the Gondwana margin at the beginning of the Variscan cycle. The P-T evolution of the Agualada eclogites is closely related to the structural evolution of the accretionary complex and the whole orogenic wedge. The cooling event recorded by the Agualada eclogites may have resulted from the accretion of a new colder crustal slice under the unit, whereas the final progradation reflects the emplacement of the Agualada Unit directly under the mantle wedge. This evolution fits well with previously presented the retical models, both for the tectonothermal evolution of accretionary complexes and for the dynamic evolution of orogenic wedges. P-T paths such as the one for the Agualada Unit eclogites, probably reflect a prolonged structural evolution. Although evidently rarely preserved, such paths are probably the rule rather than the exception during plate convergence.