Conventional Thermobarometry Research Articles

Polymetamorphic evolution of the granulite-facies Paleoproterozoic basement of the Kabul Block, Afghanistan

The Kabul Block is an elongate crustal fragment which cuts across the Afghan Central Blocks, adjoining the Indian and Eurasian continents. Bounded by major strike slip faults and ophiolitic material thrust onto either side, the block contains a strongly metamorphosed basement consisting of some of the only quantifiably Proterozoic rocks south of the Herat-Panjshir Suture Zone. The basement rocks crop-out extensively in the vicinity of Kabul City and consist predominantly of migmatites, gneisses, schists and small amounts of higher-grade granulite-facies rocks. Granulite-facies assemblages were identified in felsic and mafic siliceous rocks as well as impure carbonates. Granulite-facies conditions are recorded by the presence of orthopyroxene overgrowing biotite in felsic rocks; by orthopyroxene overgrowing amphibole in mafic rocks and by the presence of olivine and clinohumite in the marbles. The granulite-facies assemblages are overprinted by a younger amphibolite-facies event that is characterized by the growth of garnet at the expense of the granulite-facies phases. Pressure-temperature (P-T) conditions for the granulite-facies event of around 850 °C and up to 7 kbar were calculated through conventional thermobarometry and phase equilibria modeling. The younger, amphibolite-facies event shows moderately higher pressures of up to 8.5 kbar at around 600 °C. This metamorphism likely corresponds to the dominant metamorphic event within the basement of the Kabul Block. The results of this work are combined with the litho-stratigraphic relations and recent geochronological dating to analyze envisaged Paleoproterozoic and Neoproterozoic metamorphic events in the Kabul Block.

P-T evolution of elusive UHP eclogites from the Luotian dome (North Dabie Zone, China): How far can the thermodynamic modeling lead us?

In those ultrahigh pressure (UHP) terranes that experienced protracted high/ultrahigh temperature (HT/UHT) exhumation histories, slow exhumation rates and a widespread anatexis, the UHP metamorphism is often elusive and difficult to be constrained. In the Dabie-Sulu orogenic belt of central-eastern China, which is the largest UHP terrane in the world, the migmatitic North Dabie complex Zone (NDZ) stands out for the widespread anatexis that widely overprinted the traces of eclogite-facies metamorphism, hampering a precise reconstruction of its P-T-(t) evolution.Different peak P-T conditions, varying between non-eclogitic to UHP conditions, have been proposed so far for various high-grade metamorphic rocks from the NDZ. Few attempts were also made to reconstruct its P-T evolution. Most of the proposed P-T paths, based on conventional thermobarometry, follow a clockwise trajectory at relatively HT (>750°C) and almost none of them infer the prograde portion.In this paper we present a detailed petrologic study on two eclogites from the Luotian dome of the NDZ: sample 11-7c2 shows a well preserved eclogitic assemblage (Grt+Cpx+Rt), whereas sample 11-9c1 is a Qtz-Ky-bearing eclogite pervasively retrogressed under granulite-facies conditions. The results of the thermodynamic forward modelling allowed to constrain, for the first time, the prograde portion of the NDZ P-T evolution; the decompression evolution at granulite-facies conditions has been also precisely constrained. However, due to the HT overprinting and to poorly reactive bulk compositions, this method alone is not sufficient to reconstruct the whole P-T trajectory of the NDZ: more specifically, unambiguous evidence of the attainment of UHP conditions have not been found. Different “unconventional” thermobarometric methods (such as those based on trace element and textural characterization of zircons) might be more suitable to decipher the HP/UHP history of this “really hot and slow” UHP terrane.

Taking the temperature of Earth's hottest crust

The limitations of conventional thermobarometry and petrogenetic grids for determining the peak P–T conditions of granulites are well known. These limitations have been overcome during the past decade with the calibration of single mineral thermometers, particularly Al-in-orthopyroxene, Zr-in-rutile and Ti-in-zircon, and the increased use of P–T pseudosection thermobarometry. Most recent studies of ultrahigh temperature (UHT) granulites (those formed at >900 °C) have used one or other of these methods to argue for peak metamorphic temperatures up to or beyond 1000 °C. Since models for the thermal evolution of orogens generally do not predict such extreme temperatures it is important to confirm their veracity. Here we combine in a single study single mineral thermometry with P–T pseudosection thermobarometry to provide a robust determination of peak temperature and tight constraints on the retrograde P–T path for one UHT granulite locality in the Eastern Ghats Province. This is the first study to apply the most recent update of the internally consistent thermodynamic dataset of Holland and Powell (2011) and the re-parameterized a–x models of White et al. (2014) and Wheller and Powell (2014) to UHT granulites. For two samples, we report Zr-in-rutile temperatures of >1000 °C and Ti-in-zircon temperatures of ∼900 °C, supported by Al-in-orthopyroxene temperatures of ∼900 °C, that correspond closely to those estimated using P–T pseudosections for conditions at the thermal peak and at the solidus on the retrograde P–T path, respectively. The P–T path is counter-clockwise in common with other UHT granulite localities in the Eastern Ghats Province. By demonstrating that UHT metamorphism at T>1000°C is real we provide a robust constraint that must be met by geodynamic models for the development of ultrahot orogens.

Microstructures and petrology of melt inclusions in the anatectic sequence of Jubrique (Betic Cordillera, S Spain): Implications for crustal anatexis

Abstract We report a new occurrence of melt inclusions in polymetamorphic granulitic gneisses of the Jubrique unit, a complete though strongly thinned crustal section located above the Ronda peridotite slab (Betic Cordillera, S Spain). The gneissic sequence is composed of mylonitic gneisses at the bottom and in contact with the peridotites, and porphyroblastic gneisses on top. Mylonitic gneisses are strongly deformed rocks with abundant garnet and rare biotite. Except for the presence of melt inclusions, microstructures indicating the former presence of melt are rare or absent. Upwards in the sequence, garnet decreases whereas biotite increases in modal proportion. Melt inclusions are present from cores to rims of garnets throughout the entire sequence. Most of the former melt inclusions are now totally crystallized and correspond to nanogranites, whereas some of them are partially made of glass or, more rarely, are totally glassy. They show negative crystal shapes and range in size from ≈ 5 to 200 μm, with a mean size of ≈ 30–40 μm. Daughter phases in nanogranites and partially crystallized melt inclusions include quartz, feldspars, biotite and muscovite; accidental minerals include kyanite, graphite, zircon, monazite, rutile and ilmenite; glass has a granitic composition. Melt inclusions are mostly similar throughout all the gneissic sequence. Some fluid inclusions, of possible primary origin, are spatially associated with melt inclusions, indicating that at some point during the suprasolidus history of these rocks granitic melt and fluid coexisted. Thermodynamic modeling and conventional thermobarometry of mylonitic gneisses provide peak conditions of ≈ 850 °C and 12–14 kbar, corresponding to cores of large garnets with inclusions of kyanite and rutile. Post-peak conditions of ≈ 800–850 °C and 5–6 kbar are represented by rim regions of large garnets with inclusions of sillimanite and ilmenite, cordierite-quartz-biotite coronas replacing garnet rims, and the matrix with oriented sillimanite. Previous conventional petrologic studies on these strongly deformed rocks have proposed that anatexis started during decompression from peak to post-peak conditions and in the field of sillimanite. The study of melt inclusions shows, however, that melt was already present in the system at peak conditions, and that most garnet grew in the presence of melt.

Banded iron formations of Um Nar, Eastern Desert of Egypt: P–T–X conditions of metamorphism and tectonic implications

Banded iron formations (BIF) in Um Nar, central eastern desert of Egypt, occur intercalated with schists of volcaniclastic and epiclastic origins within “ophiolitic–island arc rocks” of the Arabo–Nubian Shield. The BIF and its host rocks were affected by folding, thrusting, and regional metamorphism during the Pan-African Orogeny resulting in the development of north-verging overturned folds and E–W striking, S-dipping thrusts. Following the intrusion of granitoids, the entire sequence was refolded into south-plunging folds with NW–SE trending fold axes. Peak mineral assemblages of hornblende+plagioclase, and garnet+biotite+plagioclase+quartz in the host rocks, and andradite-rich garnet+epidote+hematite+magnetite+quartz in the BIF indicate metamorphism under epidote amphibolite facies conditions. Using the multiequilibrium approach of Thermocalc, and conventional thermobarometry, peak P–T conditions of metamorphism are estimated at 520±30°C, 5±2kbar. Fluids attending peak conditions in the oxide facies layers of the BIF were characterized by XCO2~0.03 and log fO2~−40. Textural and mineral chemical criteria suggest that, following peak conditions, the rocks underwent a stage of near-isobaric cooling or cooling and compression characteristic of a counter-clockwise P–T path.These results are consistent with a model in which the BIFs formed by hydrothermal activity from off-axis submarine vents in several pulses during a protracted event of oceanic crust generation in an inter-arc basin. Concomitant arc volcanism supplied the basin with pyroclastic material that imposed suboxic conditions on the basin leading to increased concentrations of soluble Fe2+. During periods of arc quiescence, Fe2+ was likely oxidized, leading to deposition of several layers of BIF. During the Pan-African Orogeny, the BIFs, tuffs, oceanic crust and lithospheric mantle were deformed and metamorphosed while being emplaced onto the continental margin.

Calculated phase equilibria for phengite-bearing eclogites from NW Spitsbergen, Svalbard Caledonides

Abstract Phengite-bearing eclogites occur in the Richarddalen Complex of NW Spitsbergen, Arctic Caledonides. Phase equilibrium modelling and conventional geothermobarometry have been used to constrain the metamorphic evolution of these eclogites. Pseudosections are calculated for the peak-pressure assemblage garnet+omphacite+phengite+amphibole+dolomite quartz+rutile. Compositional isopleths for garnet and phengite constrain the pressure–temperature ( P–T ) conditions to 1.9–2.0 GPa and 720–730 °C, in good agreement with the results obtained from conventional thermobarometry (720–740 °C and 2.4–2.5 GPa). Further P–T pseudosection modelling of clinopyroxene+plagioclase±amphibole±clinozoisite symplectites after omphacite suggests that decompression to c. 1.2 GPa occurred along a steep exhumation path. The eclogite-bearing Richarddalen Complex constitutes the uppermost unit of a simple stack of thrust sheets where the metamorphic grade is increasing structurally upwards in the pile. Thrusting is the favoured uplift mechanism for the initial syn-orogenic exhumation to lower crustal levels. Constrictional north–south stretching in a transpressional regime is interpreted to be responsible for the final exhumation of the assembled stack of thrust sheets. Late Silurian–Early Devonian conglomerates were deposited directly on the eclogite-bearing gneisses of the Richarddalen Complex, and mark the end of exhumation of the nappe stack.

High-grade metamorphism of ironstones in the Mesoarchaean of southwest Swaziland

A sequence of high-grade rocks collectively referred to as Mkhondo Valley Metamorphic Suite is exposed around the Nhlangano Gneiss Dome in southwest Swaziland and also less extensively further north. Meta-ironstones are part of this sequence and have been studied specifically with two main objectives in mind, firstly, to test the hypothesis of the high-grade metamorphic sequence correlating with lower-grade Pongola-age rocks nearby, and secondly, to assess the potential of these meta-ironstones for retrieving P-T conditions of metamorphism. The studied ironstones contain substantial Al due to terrigenous clastic input and also include Mn-rich varieties. Three compositional groups of meta-ironstones can be distinguished: garnet-orthopyroxene-bearing ones, garnet-bearing, orthopyroxene-free ones, and a garnet-poor to garnet-free type. A comparison of bulk rock compositions between these high-grade meta-ironstones and low-grade Mn-poor to Mn-rich ironstones from the Mozaan Group supports a stratigraphic correlation. Pseudosection modelling demonstrates that aluminous meta-ironstones provide useful P-T constraints at high peak temperatures as they do not tend to produce significant amounts of melt, if any. The presence of Al is critical for introducing garnet to amphibolite and granulite-facies assemblages, which also allows to apply conventional thermobarometers. The combination of pseudosection modelling and conventional thermobarometry yielded peak metamorphic conditions of 850–890 °C at 7 to 8 kbar. These data are compatible with the ones obtained from associated metapelitic gneisses, except that the latter indicate decompression to lower pressure granulite-facies conditions. The petrological, structural and geochronological data from southwest Swaziland confirm major orogenic activity in the eastern Kaapvaal Craton during the Neo-Archaean.

Trace element composition of rutile and the application of Zr-in-rutile thermometry to UHT metamorphism (Epupa Complex, NW Namibia)

The trace element composition of rutile and Zr-in-rutile temperatures were determined for ultra-high temperature (UHT) metamorphic rocks from the Epembe Unit of the Epupa Complex in NW-Namibia. Ortho- and paragneisses record Mesoproterozoic peak metamorphic UHT conditions of 970 ± 40 °C at 9.5 ± 2 kbar, as estimated from conventional thermobarometry and constraints from pseudosection modeling. Rutile exhibits superchondritic concentrations of V, Zr, Nb, Hf, Ta, Th and U while rare earth elements (REE) are far less enriched. Zr and Hf correlate positively with two distinct trends. Nb and Ta as well as Cr and V show a positive correlation although with less clear trends. Only Hf correlates with Zr, suggesting a decoupling of Zr and Hf from the other high field-strength elements (HFSE) probably during retrogression. In general, the non-homogeneous HFSE distribution in rutile indicates that equilibrium trace element distribution achieved during UHT peak metamorphic conditions was either almost completely erased or had never been achieved as a common feature of all rutile grains. The retrograde metamorphic evolution of the UHT rocks is interpreted to be responsible for trace element redistribution under equilibrium conditions restricted to small domains. This has affected the trace element composition of the rutile grains investigated here thereby disturbing their UHT signature, which may cause problems for provenance studies involving such ‘disturbed’ grains. A systematic comparison of all available Zr-in-rutile thermometer calibrations shows that beside of one, all give similar temperature estimates for the studied samples. No systematic differences regarding the Zr content were observed between rutile grains in different textural positions (i.e. matrix grains, those shielded by host minerals or post-peak grains). However, calculations revealed a broad range of temperatures between < 400 °C and > 1000 °C. The large spread of calculated temperatures is interpreted to result from intergrain diffusion and trace element exchange by fluid-mediated recrystallization during the retrograde metamorphic evolution. This interpretation is supported by the presence of extensively formed retrograde reaction textures involving hydrous phases such as cordierite and biotite in the studied samples. In addition, tiny (≤ 5 μm) Zr-silica-rich phase separations which occur either homogeneously or heterogeneously distributed in single rutile grains may cause intergrain Zr variations.

Timing of eclogite facies metamorphism in the North Qinling by U–Pb and Lu–Hf geochronology

Although petrologic data from the North Qinling orogen in central China have provided essential information on the geodynamic history of the orogenic cycle, the timing of the continental collision between the North China block and the South China block in the North Qinling orogen remains essentially unconstrained. To refine existing geodynamic models, two eclogites from the North Qinling were dated by U–Pb and Lu–Hf geochronology. The upper limit of the pressure during the eclogite facies metamorphism was restricted within the quartz stability field at 2.6–2.8GPa within a temperature range of 660–710°C by combining conventional thermobarometry and pseudosection estimates. Mineral–whole rock dating for multi-garnet-(omphacite) fractions from two localities yielded ages of 516.4±5.8Ma (2σ) and 494.3±2.7Ma (2σ) with corresponding weighted mean zircon 206Pb/238U ages of 507.0±8.7Ma (2σ) and 490.0±4.1 (2σ) Ma, respectively. Garnet porphyroblasts from the two samples bore evidence of prograde elemental zonation, but the major portion of the garnet grains grew exclusively under eclogite facies conditions as indicated by the occurrence of omphacite inclusions from core to rim. Therefore, these Lu–Hf ages were interpreted to reflect high-pressure eclogite facies metamorphism. The difference between the two Lu–Hf ages likely represents the minimum duration of garnet growth and thus brackets the onset and the closure of the subduction. These results allow for more detailed insight into the subduction history of the North Qinling orogen. These new U–Pb and Lu–Hf ages mark the timing of the onset of the continental collision between the North China block and the South China block in the Qinling belt, suggesting that the entry of the leading edge of the South China block continent into the North China block trench occurred from ca. 516Ma to 490Ma in the North Qinling orogen.

Dating low-temperature deformation by 40Ar/39Ar on white mica, insights from the Argentera-Mercantour Massif (SW Alps)

In order to date low-temperature deformation, intensely strained muscovite porphyroclasts and neocrystallized shear band phengite from greenschist-facies shear zones have been dated by 40Ar/39Ar method in the Argentera-Mercantour massif. Shear zones are featured by gradual mylonitization of a Variscan granite, gneiss and Permian pelite protolith (300–315Ma) during the Alpine orogenic event. Mineralogical and textural observations indicate that phengites and chlorites developed from biotite and plagioclase in fluid system during deformation following dissolution–transport–precipitation reactions of the type biotite + plagioclase + aqueous fluid = chlorite + albite + phengite + quartz + titanite + K-bearing fluid in the granite-gneiss mylonite. Contrariwise, phengite developed at the expense of clays following substitution reaction in pelite mylonite. Based on conventional thermobarometry on phengite and chlorite and Pressure–Temperature-aqueous fluid (P–T-MH2O) pseudosections calculated with shear zone bulk compositions, the conditions during shear deformation were estimated at 375±30°C and 4.8–7±1kbar in an H2O-satured system. In this low temperature environment, 40Ar/39Ar analysis of the Variscan muscovite for various grades of ductile strain intensity shows a limited 40Ar/39Ar isotopic resetting, all ages scattering between 296 and 315Ma. Under conditions of intense ductile deformation and large-scale fluid circulation, muscovite grains formed during the Variscan retain their much older ages. 40Ar/39Ar dating of very fine grained synkinematic phengite grains, neoformed during the Alpine history, give consistent plateau ages (34–20Ma) for each shear zone. In detail, 40Ar excess can be detected in the pelite mylonitic sample where phengites crystallized by substitution process while the other mylonitic samples where phengites grow from fluid-induced reactions do not evidence any 40Ar excess. These results demonstrate that the 40Ar/39Ar dating of neocrystallized synkinematic white mica allows the determination of precise ages of deformation and fluid activity. Together with precise thermobarometry undertaken on the basis of mineral chemistry and whole-rock composition, 40Ar/39Ar dating of white mica leads to the reconstitution of precise depth-deformation history of low-grade (<400°C) metamorphic units. At the Argentera-Mercantour massif scale, several stages of shear zone development at 15–21km depth are dated between 33 and 20Ma. In the SE part of the massif shear zone ages are well constrained to be either (1) 33.6±0.6Ma or in the range (2) 26.8±0.7Ma–26.3±0.7Ma. In the West of the massif, younger shear zone ages range between (3) 22.2±0.3Ma and (4) 20.5±0.3Ma.

Permo-Triassic thermal events in the lower Variscan continental crust section of the Northern Calabrian Arc, Southern Italy: Insights from petrological data and in situ U–Pb zircon geochronology on gabbros

The Calabride nappe is the uppermost structural element of the Calabria–Peloritani Arc, it consists of a Variscan continental crust section intruded by late-Variscan granitoids, on which a Mesozoic syn-rift sedimentary cover was deposited. The deepest part of the Variscan continental section crops out in Northern Catena Costiera. The transition from crustal to mantle-derived rocks is often marked by gabbros. P–T estimates for gabbro emplacement have been calculated applying conventional thermobarometry and a mean crystallization temperature of about 800 °C at pressures between ~ 0.46 and 0.65 GPa has been determined. Gabbro emplacement occurs at the base of the crust, during a low-pressure granulitic stage that affected the country rocks at the end of an isothermal decompression. The gabbros can be classified as tholeiites with MORB affinity. In situ LA-ICP-MS U–Pb analyses have been performed on zircons from representative samples of gabbro. U–Pb concordia ages range from 224 ± 4 Ma to 296 ± 6 Ma. Two age populations of middle–early Permian and middle Triassic age, respectively, seem to document the magmatic activity responsible for gabbros intrusion. According to cathodoluminescent images and REE pattern of zircon, the early Permian age refers to a metamorphic inherited core. Lithospheric thinning and a positive thermal anomaly, related to the exhumation of the deepest portion of the continental lithosphere, can explain the post-Variscan metamorphic evolution of the Calabride lithosphere and the underplating gabbroic magmatism. Since the major episode of gabbro emplacement is Triassic, this event could be related to the continental rifting that led to the opening of the Jurassic Tethys and Alpine cycle.

Modelling grain‐recycling zoning during metamorphism

AbstractChemical zoning, recorded by grain growth during metamorphism, is a key source of information about P–T–t paths. Interpretation of these data must be carried out using appropriate models and recognizing their inherent assumptions. To assist with defining how zoned minerals form, a set of geometric criteria for three types of chemical zoning developed in minerals (diffusion, growth and grain recycling) is outlined. Re‐equilibration of minerals by lattice diffusion causes zoning if the re‐equilibration is incomplete. Growth of porphyroblasts is commonly considered in pelites, but in metagranitoids, large monophase domains undergo coarsening by recycling of material from one grain to another as grain boundaries migrate driven by surface energy. This type of grain size increase is termed here ‘grain recycling’. Zoning developed during grain recycling due to equilibration of the recycled material with grain‐boundary chemistry is termed ‘grain‐recycling zoning’. Furthermore, short lattice diffusion lengths relative to grain sizes cause metamorphic fractionation because material in the grain cores is not in communication thermodynamically with the rest of the rock. A new model is derived for this sort of grain size increase coupled with metamorphic reactions using Theriak–Domino. An example is given of plagioclase undergoing an increase in anorthite content as epidote breaks down during amphibolite facies metamorphism of a metagranitoid. Agreement between naturally occurring zoning profiles and those derived from modelled P–T–t paths shows that this model can be used to extract metamorphic conditions from rocks which are not accessible using conventional thermobarometry.

Fluid-assisted retrogression of garnet and P–T history of metapelites from HP/UHP metamorphic terrane (Pohorje Mountains, Eastern Alps)

Fluid inclusions in garnet combined with element X-ray mapping, phase equilibrium modelling and conventional thermobarometry have been used to constrain the metamorphic evolution of metapelitic gneiss from the HP/UHP metamorphic terrane of Pohorje Mountains in the Eastern Alps, Slovenia. Retrograde P–T trajectory from ~2.75 GPa and 780°C is constrained by the composition of matrix phengite (6.66 apfu Si) coexisting with garnet cores, kyanite and quartz. The intersection of the XPrp = 0.25 isopleth for the garnet with the upper stability boundary for K-feldspar in the matrix indicates near-isothermal decompression to ~0.9 GPa at 720°C. Temperatures over 650°C during this stage are corroborated by the high degree of ordering of graphite inclusions associated with Zn, Mg-rich staurolite and phlogopite in the Mg-rich (XPrp = 0.22–0.25) garnet cores. Majority of garnet porphyroblasts are depleted in Mg (down to XPrp = 0.09) and enriched in Mn (up to XSps = 0.12) along cracks and at their margins. The associated retrograde mineral assemblage comprises Zn, Mg-poor staurolite, muscovite, biotite–siderophyllite, sillimanite and quartz. The onset of the retrogression and the compositional modification of the garnet porphyroblasts were accompanied by the addition of fluid-deposited graphite around older graphite inclusions, probably due to removal of water from a graphite-buffered COH fluid by dissolution in partial silicic melt. Instantaneous expulsion of water near the melt solidus (640°C, max. 0.45 GPa) caused dissolution of the graphite at redox conditions corresponding to 0.25–1.25 logfO2 units below the QFM buffer, giving rise to a H2O–CO2–CH4 fluid trapped in primary inclusions in Mn-rich, Mg-poor, almandine garnet that reprecipitated within the retrogressed domains. The absence of re-equilibration textures and consistent densities of the fluid inclusions reflect a near-isochoric cooling postdating the near-isothermal decompression. Bulk water content in the metapelite attained 2 wt% during this stage. The low-degree partial melting and extensive hydration due to the release of the internally derived, low-pressure aqueous fluids led to the reset of peak-pressure mineral assemblage.

Garnet exsolution in pyroxene from clinopyroxenites in the Moldanubian zone: constraining the early pre‐convergence history of ultramafic rocks in the Variscan orogen

AbstractExsolution lamellae of garnet in clinopyroxene and orthopyroxene porphyroclasts from garnet pyroxenites in the Moldanubian zone were studied to elucidate the pressure–temperature conditions of the exsolution process and to reconstruct the burial and exhumation path of ultramafic rocks in the Variscan orogen. The porphyroclasts occur in a fine‐grained matrix with metamorphic fabrics, which consists of clinopyroxene and small amounts of garnet, orthopyroxene and amphibole. The clinopyroxene porphyroclasts contain garnet + orthopyroxene lamellae as well as ilmenite rods that have orientation parallel to (100) planes of the porphyroclasts. Orthopyroxene porphyroclasts host garnet and clinopyroxene lamellae, which show the same lattice preferred orientation. In both cases, lamellar orthopyroxene, clinopyroxene and garnet were partially replaced by secondary amphibole. Composition of exsolution phases and that of host pyroxene were reintegrated according to measured modal proportions and demonstrate that the primary pyroxene was enriched in Al and contained 8–11 mol.% Tschermak components. Conventional thermobarometry and thermodynamic modelling on the reintegrated pyroxene indicate that primary clinopyroxene and orthopyroxene megacrysts crystallized at 1300–1400 °C and 2.2–2.5 GPa. Unmixing and exsolution of garnet and a second pyroxene phase occurred in response to cooling and pressure increase before the peak pressure of 4.5–5.0 GPa was reached at ∼1100 °C. This scenario is consistent with a burial of hot upper‐mantle ultramafics into a cold subcratonic environment and subsequent exhumation through 900 °C and 2.2–3.3 GPa, when the pyroxenites would have partially recrystallized during tectonic incorporation into eclogites and felsic granulites.

Petrologic modeling of chloritoid–glaucophane schists from the NW Iberian Massif

Two P– T pseudosections computed with Thermocalc and Perplex software for high-pressure pelitic Ceán Schists from the NW Iberian Massif show that the paragenesis chloritoid + glaucophane, preserved as relict micro-inclusions in garnet porphyroblasts, is stable above 17–18 kbar, indicating a first stage of subduction with 65–70 km of burial. The subsequent growth of biotite and albite porphyroblasts, according to their stability fields in a MnNCKFMASH P– T phase diagram, indicates strong decompression accompanied by slight heating to reach a metamorphic peak at ∼ 500 °C. This mineral association, described in many subduction-related terranes around the world as one of the high-pressure indicators for metapelites, has not been reported previously in the NW Iberian Massif. The schists contains an initial high-pressure assemblage formed by chloritoid + garnet (Alm 0.58 Prp0 0.03 Grs 0.38 Sps 0.09) ± glaucophane + phengite (3.5–3.4 Si p.f.u.) + paragonite + chlorite + epidote + rutile + ilmenite + quartz, preserved as micro-inclusions in garnet, chloritoid and albite porphyroblasts defining an S 1 internal fabric. The matrix foliation (S 2) additionally contains a high-pressure association formed by garnet (Alm 0.68 Prp 0.04 Grs 0.25 Sps 0.03) + phengite (3.4–3.3 Si p.f.u.) + paragonite + winchite + barroisite + hornblende + chloritoid + chlorite + epidote + rutile + ilmenite + albite + quartz ± biotite. An initial pseudosection calculated in the KFMASH system with Thermocalc 3.26 gives pressure estimates through Si-content in phengite barometry that are in agreement with conventional thermobarometry and Thermocalc average P– T calculations, but is unable to describe the full complexity of the mineral assemblages of the schists. For this reason, a more complete P– T pseudosection in the model system MnNCKFMASH was calculated with Perplex 07. This provides a reliable succession of mineral assemblages that are consistent with the petrographic observations and allows a mineralogical sequence to be set for each metamorphic event in the P– T evolution of the high- P pelitic Ceán Schists.

Fluid-present melting of meta-igneous rocks and the generation of leucogranites — Constraints from garnet major- and trace element data, Lu–Hf whole rock–garnet ages and whole rock Nd–Sr–Hf–O isotope data

Pan-African high-grade metamorphism in the Proterozoic Damara orogen (Namibia) led to formation of garnet-bearing leucosomes in potassic meta-igneous gneisses producing a meta-igneous migmatite. In addition, the migmatite (gneiss (mesosome) plus leucosome) was intruded by small-scale leucogranitic melts with a high amount of accumulated biotite and garnet. U–Pb zircon ages obtained on the mesosome and the leucosome indicate late Proterozoic (ca. 850 Ma) concordia upper intercept ages which are interpreted as minimum ages of the precursor rock of the migmatite. U–Pb monazite ages obtained on the leucogranite give a concordant age of 512 ± 1 Ma and two reversely discordant ages with 207Pb/ 235U ages of 544 ± 1 and 534 ± 1 Ma, indicating the growth of monazite before or close to the age of high-grade metamorphism in the Damara orogen. High precision Lu–Hf garnet–whole rock dating gave ages of 492.6 ± 1.7 Ma for the mesosome, 497.6 ±1.7 Ma for the leucosome and 494.0 ± 1.7 Ma for the garnet- and biotite-bearing leucogranite indicating that the growth of garnet postdates the growth of monazite during high-grade metamorphism. In addition, it is suggested that melting and intrusion was coeval and occurred probably shortly after the main peak of metamorphism which occurred at c. 512 Ma. P– T estimates obtained by conventional thermobarometry (c. 690–720 °C) and accessory mineral dissolution thermometry on the leucogranite (c. 730 °C) suggest that partial melting occurred through limited fluid present melting of biotite via the reaction: bt + kfs + plg + qtz +H 2O ⇔ grt + melt. Outcrop evidence (diffuse relationship between the gneiss domain and the leucosomes, similar size of the leucosomes, homogeneous distribution of leucosomes on the sample scale) suggests that minor melt segregation had occurred. Whole rock Sr, Nd, Hf and O isotope data of the mesosome indicate that it belongs to basement rocks from this area. Geochemical and isotope data obtained on the leucosomes argue for derivation by in-situ melting of the mesosome. Both, leucosome and leucogranite originated from the same source rock but the leucogranite represents an accumulated melt that was able to segregate and to intrude the gneiss domain. The similar isotope features of the mesosome, leucosome and leucogranite indicate a direct relationship for the gneiss and the melts. Chemical and mineral data favour a derivation of both types of melt through fluid-present melting of isotopically and chemically comparable biotite + plagioclase + K-feldspar +quartz-bearing gneisses.

P-T conditions and the origin of quartzo-feldspathic veins in metasyenites from the Central Zone of the Limpopo Belt, South Africa

In this study, we present petrological data from metasyenites in the Central Zone of the Limpopo Belt, South Africa. Results from conventional thermobarometry and pseudosection modelling in the chemical system Na2O-CaO-K2O-FeO-MgO-Al2O3-H2O-TiO2-Fe2O3 (NCKFMASHTO) indicate the rocks underwent a metamorphic evolution that was characterised by a maximum pressure of 8 kbar and temperatures of ~770°C. Subsequent retrogression involved a simultaneous P-T decrease via decompression-cooling to 4 kbar at temperatures <550°C. The inferred P-T path, in combination with previous petrological data from Venetia and Messina can be interpreted to support the existence of a metamorphic field gradient that was developed during a single metamorphic event. The metasyenitic body is cross-cut by a number of quartzo-feldspathic veins that probably represent mobilised melt from the surrounding Alldays gneiss. An in-situ partial melt origin for the quartzo-feldspathic veins is ruled-out on the basis of modelling which suggests the measured compositions could not have been derived from the partial melting of the metasyenite. Modelling suggests the metasyenite could theoretically melt at the P-T conditions attained. However, due to a lack of available H2O, melting did not proceed.

Formation of clinopyroxene + spinel and amphibole + spinel symplectites in coronitic gabbros from the Sierra de San Luis (Argentina): a key to post‐magmatic evolution

AbstractThe El Arenal metagabbros preserve coronitic shells of orthopyroxene ± Fe‐oxide around olivine, as well as three different types of symplectite consisting of amphibole + spinel, clinopyroxene + spinel and, more rarely, orthopyroxene + spinel. The textural features of the metagabbros can be explained by the breakdown of the olivine + plagioclase pair, producing orthopyroxene coronas and clinopyroxene + spinel symplectites, followed by the formation of amphibole + spinel symplectites, reflecting a decrease in temperature and, possibly, an increase in water activity with respect to the previous stage. The metagabbros underwent a complex P–T history consisting of an igneous stage followed by cooling in granulite, amphibolite and greenschist facies conditions. Although the P–T conditions of emplacement of the igneous protolith are still doubtful, the magmatic assemblage suggests that igneous crystallization occurred at a pressure lower than 6 kbar and at 900–1100 °C. Granulitic P–T conditions have been estimated at about 900 °C and 7–8 kbar combining conventional thermobarometry and pseudosection analysis. Pseudosection calculation has also shown that the formation of the amphibole + spinel symplectite could have been favoured by an increase in water activity during the amphibolite stage, as the temperature of formation of this symplectite strongly depends on aH2O (&lt;740 °C for aH2O = 0.5; &lt;790 °C for aH2O = 1). Furthermore, but not pervasive, re‐equilibration under greenschist facies P–T conditions is documented by retrograde epidote and chlorite. The resulting counterclockwise P–T path consists of progressive, nearly isobaric cooling from the igneous stage down to the granulite, amphibolite and greenschist stage.

Counterclockwise P– T evolution of the Aghil Range: Metamorphic record of an accretionary melange between Kunlun and Karakorum (SW Sinkiang, China)

This paper describes the metamorphic evolution and the tectonic significance of the Aghil Range, a poorly known terrane located between Kunlun and Karakorum north of K2 in the framework of western Tibet. The Aghil Range consists of different units separated by syn- to late-metamorphic thrusts and post-metamorphic faults of similar attitude; among other units, the Surukwat Complex is a composite sequence of thrust sheets trending WNW–ESE and steeply dipping SSW showing a general increase of metamorphic grade from lower to higher structural levels. P– T pseudosections and conventional thermobarometry of metapelites at the top of the Surukwat Complex tightly constrain the P– T path of this highest-grade metamorphic portion of the Aghil Range. The prograde path is characterized by an early increase in both P and T and by a later, nearly isothermal, increase in P, from 500 < T < 530 °C and 0.25 < P < 0.40 GPa to 580 < T < 600 °C and 0.80 < P < 0.90 GPa. The peak metamorphic event is constrained at 550 < T < 590 °C and 0.77 < P < 0.91 GPa. The retrograde path is characterized by decompression associated to a slight cooling to T ~ 500 °C and P < 0.5 GPa. Altogether, the petrology of the studied rocks suggests a P– T path with a narrow counterclockwise shape. The studied sequence could represent the result of the early subduction of an accretionary complex, interpreted further eastward according to a melange underthrusting model. As concerning the tectonic setting of this terrane, a number of geologic and petrologic similarities link the Aghil Range and the central Qiangtang metamorphic belt, suggesting that the Aghil Range is the possible NW extension of the Qiangtang microplate separating Kunlun from Karakorum.

On thermobarometry

AbstractThermobarometry, the estimation of the pressure–temperature (P–T) conditions of metamorphism, can be undertaken by using pseudosection calculations as well as by conventional methods. Conventional thermobarometry uses only the equilibrium thermodynamics of balanced reactions between end‐members of minerals, combined with the observed compositions of minerals. In contrast, pseudosections involve a forward calculation of mineral equilibria for a given rock composition. When related to observed rock data such as mineral assemblages, mineral proportions and mineral compositions, pseudosections have the power to provide valuable additional thermobarometric information. This is because the rock composition provides added constraints on P–T, unavailable in conventional thermobarometry, such as when minerals in the mineral assemblage are no longer stable, or when additional minerals join the mineral assemblage. Considering both conventional and pseudosection thermobarometry, a minimum requirement is that they use the same thermodynamic data and activity–composition models for the minerals involved. A new thermocalc facility is introduced that allows pseudosection datafile coding to be used for conventional thermobarometry. Guidelines are given and pitfalls discussed relating to pseudosection modelling and conventional thermobarometry. We argue that, commonly, pseudosection modelling provides the most powerful thermobarometric tools.