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.     

Evolution of fluids associated with metasedimentary sequences from Chaves (North Portugal)

In order to identify and characterise fluids associated with metamorphic rocks from the Chaves region (North Portugal), fluid inclusions were studied in quartz veinlets, concordant with the main foliation, in graphitic-rich and nongraphitic-rich lithologies from areas with distinct metamorphic grade. The study indicates multiple fluid circulation events with a variety of compositions, broadly within the C–H–O–N–salt system. Primary fluid inclusions in quartz contain low salinity aqueous–carbonic, H₂O–CH₄–N₂–NaCl fluids that were trapped near the peak of regional metamorphism, which occurred during or immediately after D2. The calculated P–T conditions for the western area of Chaves (CW) is P=300–350 MPa and T500 °C, and for the eastern area (CE), P=200–250 MPa and T=400–450 °C. A first generation of secondary fluid inclusions is restricted to discrete cracks at the grain boundaries of quartz and consists of low salinity aqueous–carbonic, H₂O–CO₂–CH₄–N₂–NaCl fluids. P–T conditions from the fluid inclusions indicate that they were trapped during a thermal event, probably related with the emplacement of the two-mica granites.

A second generation of secondary inclusions occurs in intergranular fractures and is characterised by two types of aqueous inclusions. One type is a low salinity, H₂O–NaCl fluid and the second consists of a high salinity, H₂O–NaCl–CaCl₂ fluid. These fluid inclusions are not related to the metamorphic process and have been trapped after D3 at relatively low P (hydrostatic)–T conditions (P<100 MPa and T<300 °C).

Both the early H₂O–CH₄–N₂–NaCl fluids in quartz from the graphitic-rich lithologies and the later H₂O–CO₂–CH₄–N₂–NaCl carbonic fluid in quartz from graphitic-rich and nongraphitic-rich lithologies seem to have a common origin and evolution. They have low salinity, probably resulting from connate waters that were diluted by the water released from mineral dehydration during metamorphism. Their main component is water, but the early H₂O–CH₄–N₂–NaCl fluids are enriched in CH₄ due to interaction with the C-rich host rocks.

From the early H₂O–CH₄–N₂–NaCl to the later aqueous–carbonic H₂O–CO₂–CH₄–N₂–NaCl fluids, there is an enrichment in CO₂ that is more significant for the fluids associated with nongraphitic-rich lithologies.

The aqueous–carbonic fluids, enriched in H₂O and CH₄, are primarily associated with graphitic-rich lithologies. However, the aqueous–carbonic CO₂-rich fluids were found in both graphitic and nongraphitic-rich units from both the CW and CE studied areas, which are of medium and low metamorphic grade, respectively.     

Review of the systematics of CO2–H2O fluid inclusions

Aqueous solutions that contain volatile (gas) components are one of the most important types of fluid in the Earth's crust. The record that such fluids have left in the form of fluid inclusions in minerals provides a wealth of insight into the geochemical and petrologic processes in which the fluids participated. This article reviews the systematics of CO₂–H₂O fluid inclusions as a starting point for interpreting the chemically more complex systems. The phase relations of the binary are described with respect to a qualitative P–T–X model, and isoplethic–isochoric paths through this model are used to explain the equilibrium and non-equilibrium behaviour of fluid inclusions during microthermometric heating and cooling. The P–T–X framework is then used to discuss the various modes of fluid inclusion entrapment, and how the resulting assemblage textures can be used to interpret the P–T conditions, phase states, and evolution paths of the parent solutions. Finally, quantitative methods are reviewed by which bulk molar volume and composition of CO₂–H₂O fluid inclusions can be determined from microthermometric observations of phase transitions.     

Tectonothermal history of the basement rocks in the western zone of the North China Craton and its tectonic implications

The basement of the North China Craton can be divided into the eastern, central and western zones, based on lithological, structural, metamorphic and geochronological data. The western zone comprises two different petrotectonic units: Archaean tonalitic–trondhjemitic–granodioritic (TTG) grey gneisses and metamorphic mafic rocks, and Palaeoproterozoic khondalite series. The former is characterized by isobaric cooling (IBC)-type anticlockwise P–T paths in the north-northwestern part of the zone and near-isothermal decompression (ITD)-type clockwise P–T paths in the eastern part, adjacent to the central zone. On the other hand, the tectonothermal evolution of Palaeoproterozoic khondalite series rocks is characterized exclusively by nearly isothermal decompression following the peak of metamorphism and then cooling, defining clockwise P–T paths. The Archaean TTG gneisses and associated mafic rocks with anticlockwise metamorphic P–T paths reflects an origin related to underplating and intrusion of mantle-derived magmas which may be derived from mantle plumes. They represent a late Archaean continental block in the western part of the North China Craton. The Palaeoproterozoic khondalite series rocks represent passive continental margin deposits. They were metamorphosed and deformed in the late Palaeoproterozoic during the amalgamation of the western continental block with another continental block in the east part of the North China Craton. The ITD-type clockwise P–T–t paths of the Palaeoproterozoic khondalite series rocks record the tectonothermal histories of the collision of the western and eastern continental blocks which resulted in the final assembly of the North China Craton at c. 1800 Ma.     

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.     

The preservation of high-pressure rocks during exhumation: metagranites and metapelites

Metagranites in the NKFMASH system require external hydration during prograde high-pressure metamorphism in order to equilibrate to ambient HP conditions by producing more siliceous muscovite. Any lack of external fluid or the disappearance of biotite stops re-equilibration and thus prevents recording of high-pressure conditions. The same hydration reactions cause dehydration during exhumation. Orthogneiss from shear zones or adjacent to metapelites and metabasites will take up external fluid during subduction and record the highest P–T conditions, but will also be the first to dehydrate upon exhumation, now hydrating other lithologies and probably refuelling shearzones.

The (de)hydration behavior of Ca-bearing metagranitoids is similar to that in the Ca-free system. However, the anorthite component of plagioclase decomposes with increasing pressure to form either (clino)zoisite or a grossular-rich garnet. Both reactions are fluid-consuming. If H₂O is supplied from an external source, the garnet-bearing assemblage can record P–T conditions up to very high pressures, but dehydrates again during heating and/or decompression to form a more Fe-rich garnet and Al-rich mica(s). The garnet compositions observed in natural HP-metagranites are mostly too Fe-rich to be formed in the presence of an H₂O-rich fluid.

N(C)KFMASH metapelites generally have a more complex mineralogy and succession of mineral assemblages along a P–T path. The H₂O contained in hydrous silicates like chlorite and chloritoid is only partly released, but partly transferred to other minerals like paragonite, glaucophane or phengite during subduction and further dehydration during exhumation is common. The mineral assemblage preserved by the rock may then record P–T conditions way below those of the actual pressure and temperature peak of the path. Contouring of the P–T pseudosection of a specific metapelite composition with mode isopleths for H₂O shows which P–T conditions along a given path are the ones most likely recorded by the rock.     

Petrogenesis of UHP metamorphic rocks from Qinglongshan, southern Sulu, east-central China

Five kinds of UHP metamorphic rocks, including eclogite, orthogneiss, paragneiss, schist and quartzite are exposed in the Qinglongshan roadcut, southern Sulu orogenic belt of eastern central China. They comprise metamorphic supracrustal rocks with bimodal volcanic characteristics and continental affinity, and granitic intrusive associations. The preservation of coesite inclusions and/or its pseudomorphs in eclogite and other rocks indicate that they have been subjected to in-situ UHP metamorphism. Four stages of metamorphism were recognized by combining petrographic observations and compositions of minerals from various UHP rocks. Prograde epidote-amphibolite facies, UHP coesite–eclogite facies, post UHP quartz–eclogite facies, and retrograde amphibolite facies assemblages delineate an inferred P–T path with a clockwise trajectory and a retrograde event characterized by the coupling of decompression with a temperature decrease. Garnet porphyroblasts in UHP eclogites display a complex growth zoning and mineral distribution, and record a crucial segment of the prograde and retrograde metamorphic evolution. The preservation of growth zoning in eclogitic and gneissic garnets suggests that the UHP rocks had a short residence time before retrograde metamorphism and a very high uplift rate in order to preserve the prograde growth zoning.     

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.     

Ultra high temperature-metamorphism and its significance in the Central Indian Tectonic Zone

In the present study from the southern margin of the Central Indian Tectonic Zone, it is demonstrated how the metamorphic P–T path of ultrahigh-temperature granulite terranes can be reconstructed using the metamorphic transition in corundum granulites from early biotite melting to later FMAS solid–solid reaction. The extreme metamorphism in these rocks caused two-stage biotite melting, resulting in initial porphyroblastic garnet₁ and later sapphirine–spinel₁ incongruent solid mineral assemblages. During this process, the leucocratic and melanocratic layers in the corundum granulites evolved from an initial silica-oversaturated to a later silica-undersaturated domain. In the melanocratic layer, this allowed localized concentration of sapphirine-spinel₁ and residual sillimanite₁, producing an extremely restitic assemblage, at the culmination of peak metamorphism, BM₁. BM₁ is constrained at  1000 °C at relatively deep crustal levels (P  9 kbar) from the stability of ferroaugite in a co-metamorphosed Iron Formation granulite. During subsequent metamorphism (BM₂), the reaction path and history in the corundum granulites shifted to the restitic domain allowing reacting sapphirine, spinel₁ and sillimanite to produce coronal garnet₂–corundum assemblage via a FMAS univariant reaction. In the final stages of reaction history, biotite₂–sillimanite₂–spinel₂ assemblage was produced after garnet₂–corundum due to localized melt–crystal interaction. The metamorphic sequence, when interpreted with the help of a newly constructed, qualitative KFMASH petrogenetic grid, reveals successive stages of heating, increasing pressure and cooling around the KFMASH invariant point, [Opx,Crd], which is consistent with a counterclockwise metamorphic P–T path. The near isobaric nature of post-peak cooling (ΔT  250–300 °C) is also evident from multistage pyroxene exsolution and by the appearance of lamellar and coronal garnets in the Iron Formation granulites. This study provides the first tight constraint for ultrahigh-T metamorphism along a counter clockwise P–T trajectory in the Central Indian Tectonic zone, and has important bearing for terrane correlations in this part of East Gondwanaland. In addition, the new KFMASH grid allows evaluation of metamorphic phase relations in ultrahigh-T, corundum-bearing and corundum-absent aluminous granulites.     

Tracing the protolith, UHP metamorphism, and exhumation ages of orthogneiss from the SW Sulu terrane (eastern China): SHRIMP U–Pb dating of mineral inclusion-bearing zircons

Orthogneisses are the major country rocks hosting eclogites in the Sulu UHP terrane, eastern China. All of the analyzed orthogneiss cores from the main drilling hole of the Chinese Continental Scientific Drilling Project (CCSD-MH) have similar major and trace element compositions and a granite protolith. These rocks have relatively high LREE/HREE ratios, strong negative Eu anomalies (Eu/Eu*=0.20–0.39), and negative Ba anomalies (Ba/Ba*=0.25–0.64). Coesite and coesite-bearing UHP mineral assemblages are common inclusions in zircons separated from orthogneiss, paragneiss, amphibolite, and (retrograded) eclogite of the CCSD-MH. This suggests that the eclogite, together with its country rocks, experienced in situ ultrahigh-pressure (UHP) metamorphism. Laser Raman spectroscopy and cathodoluminescence (CL) images show that zircons from the orthogneisses are zoned and that they have distinct mineral inclusions in the different zones. Most zircons retain early magmatic cores with abundant low-pressure mineral inclusions, which are mantled with metamorphic zircon-containing inclusions of coesite and other UHP minerals. The outermost rims on these grains contain low-pressure mineral inclusions, such as quartz and albite. SHRIMP U–Pb dating of the zoned zircons gives three discrete and meaningful groups of ages: Proterozoic ages for the protolith, 227±2 Ma for the coesite-bearing mantles, and 209±3 Ma for the amphibolite facies retrograde rims. The widespread occurrence of UHP mineral inclusions in zircons from the Sulu metamorphic belt dated at about 227 Ma suggests that voluminous continental crust experienced late Triassic subduction to depths of at least 120 km and perhaps more than 200 km. Eighteen million years later, the terrane was rapidly exhumed to midcrustal levels, and the UHP rocks were overprinted by amphibolite facies metamorphism. The exhumation rate deduced from the zircon age data and previously obtained metamorphic P–T data is estimated to be 5.6–11.0 km/Ma. Such rapid exhumation of the Sulu UHP terrane may be due to the buoyancy forces produced by subduction of low-density continental material into the deep mantle.     

Preservation/exhumation of ultrahigh-pressure subduction complexes

Ultrahigh-pressure (UHP) metamorphic terranes reflect subduction of continental crust to depths of 90–140 km in Phanerozoic contractional orogens. Rocks are intensely overprinted by lower pressure mineral assemblages; traces of relict UHP phases are preserved only under kinetically inhibiting circumstances. Most UHP complexes present in the upper crust are thin, imbricate sheets consisting chiefly of felsic units ± serpentinites; dense mafic and peridotitic rocks make up less than  10% of each exhumed subduction complex. Roundtrip prograde–retrograde P–T paths are completed in 10–20 Myr, and rates of ascent to mid-crustal levels approximate descent velocities. Late-stage domical uplifts typify many UHP complexes.

Sialic crust may be deeply subducted, reflecting profound underflow of an oceanic plate prior to collisional suturing. Exhumation involves decompression through the P–T stability fields of lower pressure metamorphic facies. Scattered UHP relics are retained in strong, refractory, watertight host minerals (e.g., zircon, pyroxene, garnet) typified by low rates of intracrystalline diffusion. Isolation of such inclusions from the recrystallizing rock matrix impedes back reaction. Thin-aspect ratio, ductile-deformed nappes are formed in the subduction zone; heat is conducted away from UHP complexes as they rise along the subduction channel. The low aggregate density of continental crust is much less than that of the mantle it displaces during underflow; its rapid ascent to mid-crustal levels is driven by buoyancy. Return to shallow levels does not require removal of the overlying mantle wedge. Late-stage underplating, structural contraction, tectonic aneurysms and/or plate shallowing convey mid-crustal UHP décollements surfaceward in domical uplifts where they are exposed by erosion. Unless these situations are mutually satisfied, UHP complexes are completely transformed to low-pressure assemblages, obliterating all evidence of profound subduction.     

A possible UHP-eclogite in the Leaota Mts. (South Carpathians) and its history from high-pressure melting to retrograde inclusion in a subduction melange

An unusual eclogite pod containing impure jadeite to omphacite pyroxene with anti-phase domain (APD) visible under the optical microscope is hosted in a medium-grade matrix in the Leaota Massif. Textural relationships support the hypothesis of garnet formation by peritectic reactions involving high-jadeite clinopyroxene, while inclusions entrapped in garnet, mainly a wide range of amphibole compositions, seemingly crystallised from a fluid phase in disequilibrium with the garnet host. Thermobarometric estimates indicate UHP, HT peak conditions (2.8–3.2 GPa, 780–840°C) inconsistent with those of the associated rocks, followed by a complicated exhumation path. The P–T-array of the retrograde path suggests syn-subduction exhumation and tectonic inclusion in a melange with a matrix derived from the accretionary wedge. Partial melting of the subducting slab, which induces tectonic weakening, may be an effective mechanism in detaching and enclosing small eclogite pods in metamorphic terrains.     

Thermodynamic modelling of C–O–H fluids

H₂O, CO₂, CH₄, CO, H₂ and O₂ are the most important species in crustal fluids. The composition of these C–O–H fluids can be calculated if the pressure, temperature, carbon activity, and either the oxygen fugacity or the atomic H/O ratio of the fluid is known. The calculation methods are discussed and calculation results are illustrated with isobaric T–X_i, P–T, and isobaric–isothermal ternary C–O–H diagrams. Fluid inclusion compositions, in particular, the X_CO2/(X_CO2+X_CH4) ratio, can be used for C–O–H model calculations. However, care should be taken about possible post-entrapment changes, which may have modified the chemical composition of the fluid inclusion.     

Emplacement at granulite facies conditions of the Sesia–Lanzo metagabbros: an early record of Permian rifting?

The evolution of the pre-Alpine Corio and Monastero metagabbros points to strong chemical and mineralogical similarities with that of other Permian gabbro bodies of the Alps, which are concentrated in the Southalpine and Austroalpine domains. The structural and metamorphic pre-Alpine evolution of these gabbros records a re-equilibration following the emplacement in the deep crust (P=0.6–0.9 GPa and T=850±70 °C), exhumation through amphibolite facies conditions (P=0.5–0.35 GPa and T=570–670 °C), followed by a greenschist facies imprint (0.25≤P≤0.35 GPa and T<550 °C). This retrograde P–T evolution suggests that the exhumation occurred in a high thermal gradient regime, such as that induced by upwelling of an asthenospheric plume during continental rifting. This would be consistent with the crustal thinning known to have occurred in both the Southalpine and Austroalpine domains during Permian times. The gabbros and their country acid granulites are spatially associated with the serpentinised subcontinental mantle of the Lanzo Massif. This lithologic association and the metamorphic evolution is similar to that of the Fedoz gabbro (Austroalpine Domain of the Central Alps) and completely different from that observed in passive margins, where no remnants of the lower crust occur and the upper granitic crust directly overlies the serpentinized lherzolites. The location of Permian gabbro bodies in the Austroalpine and Southalpine domains and their absence in the Helvetic domain is evidence for asymmetric rifting.     

Metamorphic evolution, mineral chemistry and thermobarometry of schists and orthogneisses hosting ultra-high pressure eclogites in the Dabieshan of central China

As is typical of ultra-high pressure (UHP) terrains, the regional extent of the UHP terrain in the Dabieshan of central China is highly speculative, since the volume of eclogites and paragneisses preserving unequivocal evidence of coesite and/or diamond stability is very small. By contrast, the common garnet (X_Mn=0.18–0.45)–phengite (Si=3.2–3.35)–zoned epidote (Ps_38–97)–biotite–titanite–two feldspars–quartz assemblages in the more extensive orthogneisses have been previously thought to have formed under low P–T conditions of ca. 400±50°C at 4 kbar. However, certain orthogneiss samples preserve garnets with X_Ca up to 0.50, rutile inclusions within titanite or epidote and relict phengite inclusions within epidote with Si contents p.f.u. of up to 3.49 — overlapping with the highest values (3.49–3.62) recorded for phengites in samples of undoubted UHP schists. These and other mineral composition features (such as A-site deficiencies in the highest Si phengites, Na in garnets linked to Y+Yb substitution and Al F Ti₋₁ O₋₁ substitution in titanites) are taken to be pointers towards the orthogneisses having experienced a similar metamorphic evolution to the associated UHP schists and eclogites. Re-evaluated garnet–phengite and garnet–biotite Fe/Mg exchange thermometry and calculated 5 rutile+3 grossular+2SiO₂+H₂O=5 titanite+2 zoisite equilibria indicate that the orthogneisses may indeed have followed a common subduction-related clockwise P–T path with the UHP paragneisses and eclogites through conditions of P_max at ca. 690°C–715°C and 36 kbar to T_max at ca. 710°C–755°C and 18 kbar, prior to extensive re-crystallisation and re-equilibration of these ductile orthogneisses at ca. 400°C–450°C and 6 kbar. The consequential conclusion, that it is no longer necessary to resort to models of tectonic juxtapositioning to explain the spatial association of these Dabieshan orthogneisses with undoubted UHP lithologies, has far-reaching implications for the interpretation of controversial gneiss–eclogite relationships in other UHP metamorphic terrains.     

P–T paths reconstruction of a collisional event: The example of the Thiviers-Payzac Unit in the Variscan French Massif Central

Because of late metamorphic and tectonic overprints, the reconstruction of prograde parts of P–T paths is often difficult. In the SW Variscan French Massif Central, the Thiviers-Payzac Unit (TPU) is the uppermost allochthon emplaced above underlying units. The TPU experienced a Barrovian metamorphism coeval with a top-to-the-NW ductile shearing (D2 event) in Early Carboniferous times (ca. 360–350 Ma). The tectonic setting of the D2 event, compression or synconvergence extension, remains unclear. Using the THERMOCALC software and the model system MnNCKFMASH, the peak P–T conditions are estimated from garnet rims and matrix minerals and the prograde evolution is deduced from garnet core compositions. The combination of these two approaches demonstrates that the TPU experienced pressure and temperature increases before reaching peak conditions at 6.6–9.0 +/− 1.2 kbar and 615–655 +/− 35 °C. This kind of P–T path shows that the regional D2 event corresponds to crustal thickening.     

Transition of metamorphic series from the Kyanite- to andalusite-types in the Altai orogen, Xinjiang, China: Evidence from petrography and calculated KMnFMASH and KFMASH phase relations

Metamorphic zones in the Chinese Altai orogen have previously been separated into the kyanite- and andalusite-types, the andalusite-type being spatially more extensive. The kyanite-type involves a zonal sequence of biotite, garnet, staurolite, kyanite, sillimanite and, locally, garnet–cordierite zones. The andalusite-type zonal sequence is similar: it includes biotite, garnet and staurolite zones at lower-T conditions and sillimanite and garnet–cordierite zones at higher-T conditions, but additionally contains staurolite–andalusite and andalusite–sillimanite zones at intermediate-T conditions. As relic kyanite-bearing assemblages commonly persist in the staurolite–andalusite, andalusite–sillimanite and sillimanite zones, it is not clear that the distinction is valid. On the basis of a reevaluation of phase relations modelled in KMnFMASH and KFMASH pseudosections, kyanite and andalusite-bearing rocks of the Chinese Altai orogen record, respectively, the typical burial and exhumation history of the terrane. Mineral assemblages distributed through the various zones reflect a mix of portions of the ambient P–T array and the effects of evolving P–T conditions. The comparatively low-T biotite, garnet and staurolite zones mostly preserve kyanite-type peak assemblages that only experienced minor changes during exhumation. Rocks in the comparatively high-T sillimanite and garnet–cordierite zones are dominated by mineral assemblages of a transitional sillimanite type, having formed by the extensive modification of earlier higher pressure assemblages during exhumation. Only rocks in the intermediate-T kyanite and probably some lower sillimanite zones were clearly recrystallized by late stage andalusite metamorphism, producing the staurolite–andalusite and andalusite–sillimanite zones. This andalusite metamorphism could not reach an equilibrium state because of limited fluid availability.     

CO2-Brine immiscibility at high temperatures,evidence from calcareous metasedimentary rocks

Study of fluid inclusions in quartz segregations and in the rock matrix of a calcareous psammite and a carbonate schist suggests that brines containing 23–24 weight percent salt (NaCl equivalent) are immiscible with CO₂ at the metamorphic conditions of approximately 600° and 6.5 Kb. The presence of a high temperature solvus between saline brine and CO₂ is supported by other fluid inclusion studies as well as experimental measurements from the literature. As saline brines are common in metamorphic and hydrothermal systems, CO₂-brine immiscibility should play an important role in petrogenesis. The fluid inclusions preserved in the quartz segregations probably represent the fluids generated by prograde metamorphic reactions, whereas the compositions of the fluids trapped in the rock matrix quartz suggest they have reequilibrated with the matrix minerals during incipient retrograde reactions. The isochores from the densest inclusions observed in this study pass close to the inferred peak metamorphic conditions; other isochores suggest an episode of deformation and recrystallization at 275° C and 1.4 Kb. Using the density information preserved in all the inclusions, a convex-downward uplift path on a P-T diagram is inferred for these rocks.     

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.     

Hornblende-bearing leucosome development during syn-orogenic crustal extension in the Tormes Gneiss Dome, NW Iberian Massif, Spain

The high-grade Lower Unit of the Tormes Gneissic Dome (TGD; NW sector of the Iberian Massif) contains tonalitic orthogneissic bodies that were migmatized during a heterogeneous ductile shearing, related to a major episode of syn-orogenic extensional deformation (D2). Both peak and retrograde P–T conditions were deduced from the analysis of reaction textures related to superimposed S2 fabrics developed during exhumation, analysis of mineral zoning and thermobarometric calculations in CaKFMASH and KFMASH systems. Syn-thermal peak partial melting of the tonalitic orthogneisses, via a biotite-dehydration melting reaction yielding hornblende, K-feldspar and melt, produced stromatic migmatites with leucosomes parallel to the S2 foliation of the mesosome as well as discordant patchy pegmatitic migmatites. The deduced post-thermal peak P–T path comprises an initial phase of decompression combined with cooling from 5.5–6 kbar at 750°C to ca. 4–4.5 kbar at 675°C, consistent with the sequence of initial retrograde mineral assemblages present in high-T mylonitic S2 fabrics. The later part of the syn-D2 P–T path indicates a significant cooling and is recorded by lower amphibolite to greenschist assemblages related to the later mylonitic S2 fabrics. The proposed melt reaction generating hornblende-bearing leucosomes may be important for crustal differentiation processes at mid-crustal levels during syn-orogenic extension, as well as for the production of certain types of high-level granites. The necessary conditions are large volumes of source material of intermediate composition and the production of a sufficient melt fraction.