Pseudosection Modelling Research Articles

Metamorphic evolution of lawsonite eclogites from the southern Motagua fault zone, Guatemala: insights from phase equilibria and Raman spectroscopy

AbstractLawsonite eclogite (metabasalt and metadolerite) and associated metasedimentary rocks in a serpentinite mélange from an area just south of the Motagua fault zone (SMFZ), Guatemala, represent excellent natural records of the forearc slab–mantle interface. Pseudosection modelling of pristine lawsonite eclogite reproduces the observed predominant mineral assemblages, and garnet compositional isopleths intersect within the phase fields, yielding a progradeP–Tpath that evolves from 20 kbar, 470 °C (M1) to 25 kbar, 520 °C (M2). The dominant penetrative foliation within the eclogite blocks is defined by minerals developed during the prograde evolution, and the associated deformation, therefore, took place during subduction. Thermometry using Raman spectra of carbonaceous material in metasedimentary rocks associated with the SMFZ eclogites gives estimates of peak‐Tof ∼520 °C. Barometry using Raman spectroscopy shows unfractured quartz inclusions in garnet rims retain overpressures of up to ∼10 kbar, implying these inclusions were trapped at conditions just below the quartz/coesite transition, in agreement with the results of phase equilibrium analysis. Additional growth of Ca‐rich garnet indicates initial isothermal decompression to 20 kbar (M3) followed by hydration and substantial cooling to the lawsonite–blueschist facies (M4). Further decompression of the hydrated eclogite blocks to the pumpellyite–actinolite facies (3–5 kbar, 230–250 °C) is associated with dehydration and veining (M5). The presence of eclogite as m‐ to 10 m‐sized blocks in a serpentinite matrix, lack of widespread deformation developed during exhumation and derived progradeP–Tpath associated with substantial dehydration of metabasites within the antigorite stability field suggest that the SMFZ eclogites represent the uppermost part of the forearc slab crust sampled by an ascending serpentinite diapir in an active, moderate‐Tsubduction zone.

P–T Alpine metamorphic evolution of the Monte Rosa nappe along the Piedmont Zone boundary (Gressoney Valley, NW Italy)

The pseudosection modelling of two chemical systems (both metabasic and metapelitic) allowed to reconstruct the exhumation P–T path followed by the southern slope of the Monte Rosa nappe (Upper Gressoney Valley) during the Alpine orogenesis. The metamorphic evolution of the polymetamorphic basement complex from the Monte Rosa nappe is marked by texturally distinct mineral assemblages, defining four Alpine metamorphic stages (M1 to M4) reflecting a sequence of different P–T conditions. Well preserved eclogitic boudins (M1 and M2 assemblages) were investigated to reconstruct the HP history related to the subduction phase, while re-equilibrated metapelites allowed to infer the P–T conditions attained during the development of the regional foliation (M3 and M4 assemblages). The HP stage (M1) occurred at 550–570°C and 24–27kbar and is characterised by the assemblage Omp+Grt+Lws+Phe+Qtz+Gln±Tlc in the eclogites. The M2 metamorphic stage consists of the assemblage Omp+Grt+Barr+Zo+Phe+Pg+Qtz and developed at 590–630°C and 14.5–16.5kbar suggesting a T increasing during decompression. After a further decompressional stage associated with a T decrease, the M3 tectono-metamorphic stage developed syn-kinematically with the main regional foliation S1. It is marked by the assemblage Phe+Pg+Grt+Chl+Ab/Olig+Hbl+Qtz+Rt/Ilm±Bt in metapelites and by Cam (blue-green)+Chl+Ab+Bt+Qtz+Rt in the re-equilibrated metabasite boudins. S1 developed during increasing T (from 550 to ca. 600°C) and sligthly increasing P (from 7 to 9kbar). Finally, the M4 assemblage grew as rims over the M3 minerals or overgrew the S1 regional foliation (albite porphyroblasts overgrowing the S1 foliation already defined by albite) and therefore can be considered as the final stage of the M3 stage. The P–T path proposed for the Monte Rosa nappe differs from previous works. In particular, we propose post-eclogitic decompression trajectory up to 7kbar, followed by a P–T increase that can be related to a second burial–exhumation cycle during Alpine orogenesis, as recently reported for other Alpine units.

Petrology and P–T path of the Yishui mafic granulites: Implications for tectonothermal evolution of the Western Shandong Complex in the Eastern Block of the North China Craton

Mafic granulites from the Yishui Group of the Western Shandong Complex in the Eastern Block of the North China Craton occur as enclaves or boudins within Late Archean TTG gneisses, and are composed mainly of garnet, clinopyroxene, orthopyroxene, plagioclase, hornblende, and minor quartz, ilmenite, and magnetite. Petrographic examination has revealed three distinct metamorphic mineral assemblages: the pre-peak prograde assemblage (M1) of hornblende+plagioclase+quartz+ilmenite+magnetite occurring as inclusions within garnet and pyroxene grains, peak assemblage (M2) of orthopyroxene+clinopyroxene+plagioclase+garnet+hornblende+quartz+ilmenite+magnetite, and post-peak assemblage (M3) represented by garnet+quartz and garnet+ilmenite/magnetite symplectites. Pseudosection modeling using THERMOCALC in the NCFMASHTO model system for a representative sample constrains the P–T conditions of M1, M2 and M3 stages at 660–730°C/<6.6kbar, 800–820°C/8.0–8.5kbar and 686–710°C/7.6–8.6kbar, respectively. The results of petrology and quantitative P–T pseudosection modeling define an anticlockwise P–T path involving near-isobaric cooling following the peak medium-pressure granulite-facies metamorphism, suggesting that the metamorphism of the Yishui Group was most likely related to the intrusion and underplating of mantle-derived magmas. Although the underplating of voluminous mantle-derived magmas leading to granulite-facies metamorphism with an anticlockwise P–T path involving isobaric cooling may occur in continental magmatic arc regions, above hot spots driven by mantle plumes, or in continental rift environments, a mantle plume model is favored because this model can reasonably interpret many other geological features of Late Archean basement rocks from the Western Shandong Complex in the Eastern Block of the North China Craton as well as their anticlockwise P–T paths involving isobaric cooling. The relatively cooler mantle-plume head heated the crust initially, causing amphibolite-facies metamorphism (M1). Subsequently, the relatively hotter mantle-plume tail heated the crust, causing granulite-facies metamorphism (M2). Finally, a near-isobaric cooling process (M3) occurred when the mantle plume ceased to heat the crust.

Late Neoproterozoic P-T evolution of HP-UHT Granulites from the Palni Hills (South India): New Constraints from Phase Diagram Modelling, LA-ICP-MS Zircon Dating and in-situ EMP Monazite Dating

Late Neoproterozoic (c. 555 Ma) high-pressure-ultrahigh-temperature (HP-UHT) metamorphism has been documented for MgAl-rich migmatitic granulites from the Palni Hills in the Southern Granulite Terrane (South India). Conspicuous reaction textures indicate a clockwise P-T evolution, which is constrained through P-T pseudosection modelling and thermobarometry. The transformation of sillimanite to kyanite, which coexisted with orthopyroxene and/or garnet, records an early stage of loading. During subsequent heating to UHT conditions at deep-crustal levels (c. 1000 degrees C, 13 center dot 0 kbar) kyanite was transformed to sillimanite, and distinct peak-temperature assemblages (orthopyroxene + sillimanite + mesoperthite + rutile +/- garnet +/- quartz +/- sapphirine, garnet + biotite + sillimanite + spinel + corundum + rutile + plagioclase and garnet + orthopyroxene + rutile + plagioclase +/- quartz) formed in specific bulk compositions through biotite-dehydration-melting reactions. A sequence of corona and sapphirine-bearing symplectite textures records subsequent isothermal decompression of the order of c. 6 kbar at persistent extreme temperatures (1010-920 degrees C). UHT decompression is consistent with the uniformly high Al contents of porphyroblastic, coronitic and symplectitic orthopyroxene (up to 10 center dot 4 wt % Al(2)O(3)). Regrowth of garnet and biotite documents post-decompressional cooling to subsolidus conditions of < 800 degrees C at mid-crustal levels (c. 6 kbar). HP-UHT metamorphism and the clockwise P-T path of the Palni Hills granulites is attributed to a single late Neoproterozoic tectono-metamorphic event, which has been consistently dated at c. 555 Ma through laser ablation inductively coupled plasma mass spectrometry U-Pb analyses of zircon and in situ electron microprobe U-Th-total Pb analyses of monazite. The MgAl-rich granulites occur as enclaves in enderbitic orthogneiss. The intrusion of the orthogneiss in the late Archean (2534 +/- 28 Ma) marks the beginning of voluminous granitoid emplacement in the Southern Granulite Terrane between 2530 and 2440 Ma, which presumably caused a first high-grade metamorphic event in the early Paleoproterozoic (2469 +/- 13 Ma), recorded by zircon cores in the MgAl-rich granulites. The clockwise P-T-t evolution indicates that HP-UHT metamorphism in the central part of the Southern Granulite Terrane is related to collisional tectonics during the final assembly of Gondwana in the late Neoproterozoic. Extreme heating is ascribed to upwelling of the asthenosphere during delamination of the thickened lithospheric mantle. Fast uplift of the rocks followed by mid-crustal isobaric cooling reflects extension of the hot overthickened crust and its subsequent cooling to a normal geotherm.

Evidence for palaeo-Tethyan oceanic subduction within central Qiangtang, northern Tibet

The mechanism of formation of blueschist–eclogite belts and their space-time distribution are important in understanding the tectonics associated with convergent plate boundaries. Here we investigate the garnet-bearing blueschists from Rongma area of central Qiangtang in northern Tibet. The mineral assemblage in these rocks is characterized by porphyroblastic garnet set within a matrix of fine-grained amphibole, white mica, epidote, chlorite, albite and quartz with accessory rutile, titanite and apatite. The garnet porphyroblasts exhibit core and rim portions, and the cores carry abundant inclusions of Na amphibole, quartz and rutile, as well as rhomb-shaped inclusions of paragonite and epidote which are interpreted as pseudomorphs after lawsonite. The rims are characterized by coarse-grained inclusions of epidote as well as the absence of paragonite and epidote aggregates, clearly suggesting that the transition from garnet core to rim marks a metamorphic transformation from lawsonite- to epidote-stability field. The Mn content of the garnet porphyroblasts decreases from core to rim, whereas the Fe and Mg contents show an increasing trend. In the matrix, we identify two stages of Na amphibole rimmed by Na-Ca amphibole and albite. Retrograde chlorite is rimmed by fine-grained biotite. Based on microstructural observations and pseudosection modelling, we trace the P–T path for the Rongma garnet blueschist from 1.92GPa and 490°C (lawsonite eclogite field) to about 1.68GPa and 535°C (epidote eclogite field), marking an initial increase in temperature and decrease in pressure. This stage is followed by a decrease of pressure through the blueschist facies down to P–T conditions of about 0.6GPa and 530°C. In combination with previous work including the available isotopic age data, the P–T path obtained in the present study suggests the deep subduction of palaeo-Tethyan oceanic crust between southern and northern Qiangtang blocks, supporting the model that the blueschist belt defines the location of the palaeo-Tethyan suture zone within northern Tibet. T-X(O) pseudosection modelling further reveals that the transformation of lawsonite to epidote occurred under high O contents. Our results provide important clues to better understand the subduction-related late Permian to early Triassic magmas in northern Tibet.

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.

Crustal growth during back-arc closure: Cretaceous exhumation history of Cordillera Darwin, southern Patagonia

The Cordillera Darwin metamorphic complex is unique in the Andes in exposing kyanite–staurolite schist north of the Beagle Channel in southern Patagonia. Garnet in amphibolite facies pelitic schists from Bahia Pia has patchy textures whereby some grains consist of clear, grossular-rich garnet with fine-grained S1 inclusion trails truncated by regions of turbid spessartine–pyrope-rich garnet with biotite, muscovite, plagioclase and quartz inclusions. Micron-scale aqueous inclusions in turbid garnet are consistent with recrystallization facilitated by fluid ingress; S2 inclusion trails indicate this was broadly contemporary with the growth of kyanite and staurolite in the matrix. Pseudosection modelling in Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 (NCKFMASHTO) is used to infer a P–T path dominated by decompression from 12 to 9 kbar at T ≈ 620 °C, coupled with garnet mode decreasing from ∼5% to <1%. U–Th–Pb in situ dating of S2 monazite indicates that staurolite and kyanite growth and thus exhumation was underway before 72.6 ± 1.1 Ma. Contact aureoles developed adjacent to late granite intrusions include sillimanite-bearing migmatites formed at P ≈ 6 kbar after 72 Ma. Metamorphism of southern Cordillera Darwin induced by continental underthrusting beneath the arc, related to closure of the Rocas Verdes back-arc basin, was terminated by thrusting-controlled exhumation, with the rocks at P ≈ 9 kbar by c. 73 Ma and 6 kbar by c. 70 Ma.

Lithostratigraphic setting and P-T metamorphic evolution for the Dora Maira Massif along the Piedmont Zone boundary (middle Susa Valley, NW Alps)

A new structural and petrological study, associated with a detailed geological mapping, allowed to better understand the tectonic relationships between the Dora Maira Massif and the Piedmont Zone in the middle Susa Valley. In the study area, a thick sequence of calcschists, previously attributed to the stratigraphic cover of the Dora Maira, is interpreted as belonging to the Piedmont Zone. Four deformation phases were identified: D1 represents the eclogite facies stage, D2 developed under greenschist facies conditions, transposed the early foliation and was responsible for the development of the regional schistosity S2. D3 is characterized by close to open folds with N dipping axial surfaces and finally, D4 developed macro-scale folds with E-dipping axial planes. Structural analyses allow to infer a relative timing for the tectonic contact between the two nappes which were coupled after the eclogite facies metamorphism but before the development of the S2 foliation under greenschist facies conditions. Petrographic investigation into metapelite samples permitted to identify two main metamorphic assemblages within the Dora Maira polymetamorphic basement: M1 (Phe + Pg + Cld + Grt + ChlI + Qtz + Rt) assemblage defines the S1 relict foliation, while M2 (Ms + Pg + Ab + ChlII + Qtz + Ilm ± Bt) assemblage is related to the regional foliation S2. PERPLE_X pseudosection modelling allowed to reconstruct a P-T path for a garnet–chloritoid-bearing micaschist from the northern part of the Dora Maira Massif, which reached eclogite facies conditions at 18–20 kbar and 515–525°C (M1/S1 event) and then was exhumed during increasing T (10–11 kbar and 555–565°C). The M2 assemblage defining the S2 regional foliation developed at P < 7 kbar and T < 575°C. According to the structural evolution, the tectonic coupling between Dora Maira and Piedmont Zone took place during exhumation along the subduction channel.

Metamorphic evolution of sapphirine- and orthoamphibole-cordierite-bearing gneiss, Okanogan dome, Washington, USA

Gneiss domes are commonly cored by quartzofeldspathic rocks that provide little information about the pressure–temperature–fluid history of the domes. Three northern Cordilleran migmatite domes (Thor-Odin and Valhalla/Passmore, British Columbia, Canada; Okanogan, Washington, USA), however, contain Mg–Al-rich orthoamphibole-cordierite gneiss as layers and lenses that record metamorphic conditions and pressure–temperature (P–T) path information not preserved in the host migmatite. These Mg–Al-rich rocks are therefore a valuable archive of metamorphic conditions during dome evolution, although refractory rocks such as these commonly contain reaction textures that may complicate the calculation of metamorphic conditions. In the Okanogan dome, Mg–Al-rich layers are part of the Tunk Creek unit, which occurs at the periphery of an underlying migmatite domain. Bulk compositional layers (mm- to m-scale) consist of gedrite-dominated, hornblende-dominated and biotite-bearing layers that contain variable amounts of gedrite, hornblende, anorthite, cordierite, spinel, sapphirine, corundum, kyanite, biotite and/or staurolite. The presence of different compositional layers (some with reaction textures, some without) allows systematic analysis of metamorphic history by a combined petrographic and phase equilibrium analysis. Gedrite-dominated layers containing relict kyanite preserve evidence of the highest-P conditions; symplectitic and coronal reaction textures around kyanite indicate decompression at high temperature. Gedrite-dominated layers lacking these reaction textures contain layers of sapphirine and spinel in apparent textural equilibrium and record a later high-T–low-P part of the path. Phase equilibria (pseudosection) analysis for layers that lack reaction textures indicates metamorphic conditions of 720–750 °C at a range of pressures (>8 to <4 kbar) following decompression. Elevated crustal temperatures and concordant structural fabrics in the Tunk Creek unit and underlying migmatite domain suggest that the calculated P–T conditions recorded in Tunk Creek rocks were coeval with anatexis, extension, and dome formation in Palaeocene–Eocene time. In contrast to orthoamphibole-cordierite gneiss in the other Cordilleran domes, the Tunk Creek unit occurs as a discontinuous km-scale layer rather than as smaller (m-scale) pods, is more calcic, and lacks garnet. In addition, kyanite did not transform to sillimanite, and spinel commonly occurs as a blocky matrix phase in addition to vermicules in symplectite. These differences, along with the compositional layering, allow an analysis of bulk composition v. tectonic (P–T path) controls on mineral assemblages and textures. Pseudosection modelling of different layers in the Tunk Creek unit provides a basis for understanding the metamorphic history of these texturally complex, refractory rocks and their host gneiss domes, and other such rocks in similar tectonic settings.

The structural, metamorphic and temporal evolution of the country rocks surrounding Venetia Mine, Limpopo Belt, South Africa: Evidence for a single palaeoproterozoic tectono-metamorphic event with implications for a tectonic model

This paper presents the results of a detailed and fully integrated pressure–temperature–time-deformation (P–T–t-D) study of the country rocks around Venetia Mine in the Central Zone of the Limpopo Belt, South Africa. Detailed structural mapping around Venetia Mine delineates four deformation events (D1–D4). Relict S1 comprises quartzofeldspathic bands in biotite gneiss and amphibolite. S2, defined by biotite in biotite schist, gneissic banding in biotite gneiss, long axes of quartzofeldspathic augen, and the long axes of amphibolite lenses/boudins, is axial planar to F2. F3 occurs predominantly at the contacts between biotite gneiss and biotite schist, forming open to closed, upright to inclined, E-W- to ENE-trending, shallowly plunging folds. D4, which was constrictional-prolate in nature, refolded S1, S2, F2 and F3. F4 folds and an L4 mineral lineation, defined predominantly by sillimanite in metapelitic schist, plunge moderately NE to NNE and overprint all previous fabrics. Poles to refolded S2 foliations show a characteristic great circle distribution. In turn, the pole to this great circle coincides with the orientation of L4. Such patterns are also found in the Avoca, Bellevue and Ha-Tshansi sheath folds in the Central Zone, albeit that the plunge of the lineations at Venetia are predominantly NE-wards, rather than SW-wards. The moderately NE- to NNE-plunging F4 sheath folds and L4 lineations accord with the NE-SW trend of sheath folds and associated mineral elongation lineations observed elsewhere in the Central Zone. However, PbSL dating of syntectonic garnets constrain the minimum and maximum age of D2 and D4 structures respectively to c. 2037–2040 +/− 22Ma. The metamorphism developed in metapelitic lithologies is characterized by peak amphibolite facies conditions of ∼6.5kbar and 680°C. Pseudosection modelling of growth zoning in garnet provides evidence of a prograde pressure and temperature increase from ∼5kbar at 600°C to ∼6–6.5kbar at 650–680°C, which is interpreted to be a consequence of tectonic thickening. Collectively, the integrated P–T–t-D data unequivocally demonstrates that the country rocks around Venetia experienced a structural–metamorphic event in the palaeoproterozoic. Coupled with published data we suggest a simple two-fold tectonic model, which involves the Central Zone as separate terrane docking with the Kaapvaal during the Neoarchean and later, during the Palaeoproterozoic this Central Zone-Kaapvaal amalgam collides with the Zimbabwe Craton.

Prograde P–T Evolution of a Lawsonite Eclogite from the Monviso Meta-ophiolite (Western Alps): Dehydration and Redox Reactions during Subduction of Oceanic FeTi-oxide Gabbro

Lawsonite eclogites represent fossil records of processes occurring during subduction of cold- and fast-subducting slabs, and provide an opportunity to investigate the thermal and metamorphic evolution of palaeosubduction zones. Occurrences of lawsonite eclogites are rare because lawsonite is often replaced during exhumation. We report here, for the first time, the occurrence of a lawsonite eclogite from the Monviso meta-ophiolite (Western Alps), representing the product of the Alpine metamorphism of a FeTi-oxide gabbro. The prograde metamorphic evolution of this lawsonite eclogite has been investigated using the petrological approach of pseudosections. The reconstruction of its prograde P-T evolution allowed us to monitor changes in chemical and physical properties (mineral assemblages and compositions, density, H2O content, oxygen fugacity) during subduction. The pseudosection modelling suggests peak metamorphic conditions of T epsilon 550 degrees C, P = 25-26 kbar, and a decrease in the thermal gradient during subduction from about 9 degrees C km(-1) to less than 7 degrees C km(-1), which may be interpreted as related to an increase in the subduction rate. During its prograde evolution, at a depth of about 65-70 km, the eclogite-facies metagabbro experienced a significant dehydration passing from the Grt(1) + Omp(1) + Lws + Chl + Qtz + Rt assemblage (stage I) to the Grt(2) + Omp(2) + Tlc + Qtz + Rt assemblage (stage II). The breakdown of lawsonite and chlorite caused the release of up to 3 wt % of H2O and also of oxygen, as modelled by the redox equilibrium Lws + Qtz + Chl + Omp(1) = Grt + Omp(2) + H2O + O-2. This redox-equilibrium represents the boundary between an earlier, more oxidized assemblage (stage I), stable at lower T, and a later, more reduced assemblage (stage II), stable at higher T. These results have possible implications for the understanding of the complex interactions between crust and mantle in subduction zones, especially in clarifying the processes locally involved in the oxidation of the mantle wedge overlying the subducting slab.

Initial water budget: The key to detaching large volumes of eclogitized oceanic crust along the subduction channel?

We herein investigate the extent to which extensive hydration of the oceanic lithosphere influences the preservation and exhumation of large-scale ophiolite bodies from subduction zones. The Zermatt–Saas ophiolite (ZS, W. Alps), which was subducted during the late stages of oceanic subduction, preserves a complete section of Mesozoic Tethys oceanic lithosphere and particularly fresh eclogites, and represents, so far, the largest and deepest known portion of exhumed oceanic lithosphere. Pervasive hydrothermal processes and seafloor alteration led to the incorporation of large amounts of fluid bound in the hydrated upper layers of the oceanic crust (now as lawsonite eclogites, glaucophanites, and chloritoschists) and in associated ultramafic rocks. Internally, the ZS ophiolite is made up of a series of tectonic slices of oceanic crust (150–300 m thick) which are systematically separated by a 5 to 100 m thick layer of serpentinite. This stack of slices is separated from the underlying eclogitized continental crust (e.g., Monte Rosa) by a thick (~ 500 m) serpentinite sole. Field observations, textural relationships and pseudosection modelling reveal that lawsonite was abundant and widespread in mafic eclogites when the ophiolite detached from the slab at around 550 °C and 24 kbar. Comparison between fresh eclogitic samples and pseudosection modelling shows that (i) water remained in excess from burial to eclogitic peak conditions, (ii) the lightest eclogitized metabasalts correspond to the portions of oceanic crust where metasomatism was the strongest, (iii) crystallization of widespread hydrated parageneses (such as lawsonite, glaucophane and phengite) instead of garnet and omphacite decreased by 5 to 10% the rock density and subsequently enhanced its buoyancy. We propose that this density decrease acted as a ‘float’ which prevented the slices from an irreversible sinking in the mantle. These slices were subsequently detached from the downgoing slab and stacked in the serpentinized subduction channel at pressures between 15 and 20 kbar, in the epidote blueschist facies. Exhumation of the underlying, positively buoyant continental crust dragged this “frozen” nappe-stack from the subduction channel towards the surface.

Corundum-leucosome-bearing aluminous gneiss from Ayyarmalai, Southern Granulite Terrain, India: A textbook example of vapor phase-absent muscovite-melting in silica-undersaturated aluminous rocks

An aluminous gneissic rock associated with high-pressure mafic and felsic granulites in the Palghat-Cauvery Shear Zone of southern India provides a classic example of quartz-absent muscovite melting. The anatectic gneiss shows a conspicuous migmatitic structure defined by closely spaced centimeter to decimeter sized, corundum-bearing leucosomes developed in a weakly foliated mesosome of plagioclase (An 21 Ab 77 Or 2 ) and biotite (4.9 wt% TiO 2 , X Mg = 0.51–0.47). The boundaries between leucosome and mesosome domains are sharp, and no melanosome selvages are developed at the interface. Corundum occurs as euhedral crystals up to 2 cm in diameter, typically centered in the leucosome matrix of coarse-grained perthitic alkali feldspar (integrated composition: An 2 Ab 35 Or 63 ), minor relict biotite (4.2–5.1 wt% TiO 2 , X Mg = 0.48–0.46) and plagioclase (An 21 Ab 78 Or 1 ). In some domains, the mesosomes also contain elongate clusters of similarly oriented smaller corundum plates that are intergrown with perthitic alkali feldspar, presumably replacing former kyanite blades. The textural and mineralogical characteristics and petrogenetic grid considerations indicate breakdown of muscovite through two successive dehydration-melting reactions: (1) formation of corundum+K-feldspar-clusters via the reaction muscovite+aluminosilicate → corundum+liquid at the sites of kyanite/sillimanite, and (2) development of corundum-bearing leucosomes through the reaction muscovite → corundum+K-feldspar+liquid, focused around the sites of nucleation and growth of peritectic corundum. P - T pseudosection modeling in the Na 2 O-CaO-K 2 O-FeO-MgO-Al 2 O 3 -SiO 2 -H 2 O-TiO 2 system locates the onset and completion of the muscovite-melting reaction 2 in the steep narrow quadrivariant field Ms+Bt+Pl+Kfs+Crn+Liq, which extends from ~6 kbar, 720 °C to higher pressures. Biotite remained stable and was not involved in the melting reactions. Two-feldspar thermometry gives peak-temperatures of 800 ± 50 °C. Combined with P - T estimates for metapelitic granulites in the area, these P - T constraints appear to be consistent with a clockwise P - T evolution of the eastern Palghat Cauvery shear zone with peak P - T conditions not exceeding ca. 800 °C and 10–12 kbar. The timing of partial melting and HT-metamorphism is constrained at ~529 Ma by U-Pb spot dating of oscillatory zoned individual grains and overgrowths on detrital zircon cores included in peritectic corundum of leucosome domains. The zircon cores indicate a Paleoproterozoic (2.5–2.0 Ga) provenance of the sedimentary protolith.

The tectono‐metamorphic evolution of the Balcooma Metamorphic Group, north‐eastern Australia: a multidisciplinary approach

AbstractThe sequential growth of biotite, garnet, staurolite, kyanite, andalusite, cordierite and fibrolitic sillimanite, their microstructural relationships, foliation intersection axes preserved in porphyroblasts (FIAs), geochronology, P–T pseudosection (MnNCKFMASH system) modelling and geothermobarometry provide evidence for a P–T–t–D path that changes from clockwise to anticlockwise with time for the Balcooma Metamorphic Group. Growth of garnet at ∼530 °C and 4.6 kbar during the N–S‐shortening event that formed FIA 1 was followed by staurolite, plagioclase and kyanite growth. The inclusions of garnet in staurolite porphyroblasts that formed during the development of FIAs 2 and 3 plus kyanite growth during FIA 3 reflect continuous crustal thickening from c. 443 to 425 Ma during an Early Silurian Benambran Orogenic event. The temperature and pressure increased during this time from ∼530 °C and 4.6 kbar to ∼630 °C and 6.2 kbar. The overprinting of garnet‐, staurolite‐ and kyanite‐bearing mineral assemblages by low‐pressure andalusite and cordierite assemblages implies ∼4‐kbar decompression during Early Devonian exhumation of the Greenvale Province.

Anti‐clockwise P–T paths in the lower crust: an example from a kyanite‐bearing regional aureole, George Sound, New Zealand

AbstractThe George Sound Paragneiss (GSP) represents a rare Permo‐Triassic unit in Fiordland that occurs as a km‐scale pillar to gabbroic and dioritic gneiss of c. 120 Ma Western Fiordland Orthogneiss (WFO). It is distinguished from Palaeozoic paragneiss common in western Fiordland (Deep Cove Gneiss) by SHRIMP and laser‐ablation U–Pb ages as young as c. 190 Ma and 176Hf/177Lu &gt;0.2828 for detrital zircon grains. The Mesozoic age of the GSP circumvents common ambiguity in the interpretation of Cretaceous v. Palaeozoic metamorphic assemblages in the Deep Cove Gneiss. A shallowly dipping S1 foliation is preserved in the GSP distal to the WFO, cut by 100 m scale migmatite contact zones. All units preserve a steeply dipping S2 foliation. S1 staurolite and sillimanite inclusions in the cores of metapelitic garnet grains distal to the WFO preserve evidence for prograde conditions of T &lt; 650 °C and P &lt; 8 kbar. Contact aureole and S2 assemblages include Mg‐rich, Ca‐poor cores to garnet grains in metapelitic schist that reflect WFO emplacement at ≈760 °C and ≈6.5 kbar. S2 kyanite‐bearing matrix assemblages and Ca‐enriched garnet rims reflect ≈650 °C and ≈11 kbar. Poorly oriented muscovite–biotite intergrowths and rare paragonite reflect post‐S2 high‐P retrogression and cooling. Pseudosection modelling in NCKFMASH defines a high‐P anti‐clockwise P–T history for the GSP involving: (i) mid‐P amphibolite facies conditions; preceding (ii) thermal metamorphism adjacent to the WFO; followed by (iii) burial to high‐P and (iv) high‐P cooling induced by tectonic juxtaposition of cooler country rock.

Metamorphic evolution of the Gran Paradiso Massif: A case study of an eclogitic metagabbro and a polymetamorphic glaucophane–garnet micaschist

Sub-continental type gabbro preserving an eclogite facies assemblage and glaucophane –garnet bearing micaschist from the eastern border of the Gran Paradiso Massif (Western Alps) have been investigated to constrain its tectono-metamorphic evolution. Glaucophane-bearing metapelites were never reported before while metagabbros are only found in the southern part of the Massif. Growth zoning of multistage garnet in metapelites and the investigation of two different chemical systems (metapelitic and basic), allowed to reconstruct the P– T path followed by Gran Paradiso Massif during the late Variscan stage and during the Alpine orogeny. Micro-structural investigations allowed to identify different assemblages related to different tectono-metamorphic stages. Pseudosection modelling with the software PERPLE_X of the stable assemblages and of the mineral compositions has allowed to infer the P– T conditions for these different metamorphic stages. Variscan metamorphism reached 620–630 °C at 8–9 kbar. The Alpine HP stage peaked at 580–600 °C and 26–27 kbar and the first decompression stage was at 580–610 °C and 21–22 kbar with no T increase before the development of the S 1 regional foliation. S 1 represents the main tectono-metamorphic event recorded by Gran Paradiso Massif and developed at 600–625 °C and 9.5–11.5 kbar in the low T and high P amphibolite facies. From these data a different P– T path is proposed respect to previous works. The exhumation from peak eclogite facies conditions to amphibolite facies was nearly isothermal and then was followed by considerable cooling suggesting a two-stage exhumation process controlled by different mechanisms.

Pseudosection modelling for a selected eclogite body from the Koralpe (Hohl), Eastern Alps

In the Kor-, Saualpe and Pohorje regions of the Eastern Alps eclogite bodies occur within metapelitic gneisses. The bodies are between 1 meter and several hundreds of meters in size and some of them were defined by Hauy (1822) as the type locality for the rock type “eclogite”. A growing body of petrological work has documented the metamorphic evolution of the metapelites surrounding the eclogites. However, few phase diagrams have been constructed for the eclogite bodies themselves. Here we use recently available activity models for amphiboles to present new thermodynamic pseudosections for the Hohl locality of the Koralpe eclogites. We show that this eclogite reached peak conditions in a narrow PT field obliquely oriented in PT space between 16.5 and 20.5 kbar and 620°C to 720°C and that its metamorphic evolution was likely to have occurred under water saturated conditions. We conclude that eclogite and the surrounding metapelites have certainly undergone the same metamorphic peak in Eo-alpine time. Comparison of our results with different PT estimates on the eclogite from Pohorje, suggest that a PT gradient from Koralpe to Pohorje is likely.

Granulite facies anatexis in the Ancient Gneiss Complex, Swaziland, at 2.73 Ga: Mid-crustal metamorphic evidence for mantle heating of the Kaapvaal craton during Ventersdorp magmatism

The 3.7–3.2 Ga Ancient Gneiss Complex (AGC) of Swaziland, located S and SE of the Barberton Greenstone Belt (BGB), is a complex geological terrane where few studies have focussed on the timing and conditions of deformation and metamorphism, despite the fact that minor metasedimentary units within the AGC provide excellent opportunities for constraining metamorphic evolution. One of these, the Mkhondo Valley Metamorphic Suite consists of migmatitic metapelitic granulites, within which the peak metamorphic assemblage garnet + cordierite + biotite + plagioclase + quartz ± K-feldspar + melt is dominant. NCKFMASHTO pseudosection modelling of this anatectic assemblage constrains peak metamorphic conditions to 850–830 °C and 4.4 kbars, followed by near isobaric cooling of the terrane to 680 °C and 3.9 kbars. These conditions indicate a period of anomalously high heat flow within the crust due to the direct addition of mantle heat. U–Pb SHRIMP dating of zircons from an in situ leucosome indicates an age of ca. 2.73 Ga for peak granulite facies metamorphism, approximating the age of Ventersdorp Supergroup flood basalt volcanism on the craton and widespread intracratonic granitic magmatism. The current study suggests that substantial heat input to the Kaapvaal cratonic crust during underplating by Ventersdorp related magmas, possibly related to the presence of a ∼2.7 Ga mantle plume marginal to the craton, was responsible for high-grade metamorphism in the AGC and a period associated with regional crustal anatexis. The resultant mid-crustal ductility and weakening may have initiated mobilisation of the mid-crust as gneissic domes, producing basement uplift and sedimentary recycling of auriferous reefs of the Central Rand Group of the Witwatersrand Supergroup.

Metamorphism of High/ultrahigh-pressure Pelitic-Felsic Schist in the South Tianshan Orogen, NW China: Phase Equilibria and P-T Path

The South Tianshan high/ultrahigh-pressure (HP^UHP) metamorphic belt, NW China is characterized by extensive pelitic^felsic schists with numerous blocks of eclogite, ultramafic rock and marble.The pelitic^felsic schists preserve two generations of garnetbearing mineral assemblages: (1) an early generation that involves garnet, phengite, glaucophane, and quartz/coesite with or without paragonite; (2) a late generation that involves albite, hornblendic amphibole, quartz and grossular-rich, pyrope-poor garnet. Early generation garnet has growth zoning involving core to rim increases in pyrope content, coupled with increased or constant grossular content. Pseudosection modeling of these growth zoning textures in three samples (T311,T314, andT316) reflects prograde paths and peak conditions of c. 32 kbar at 550^5708C, c. 22.5 kbar at 5508C, and c. 22^23 kbar at 540^5508C, respectively.These P^T estimates are consistent with those recovered from eclogite blocks hosted by the schists. Phase equilibrium modeling predicts that the early garnet would have mainly grown in mineral assemblages involving lawsonite, jadeite and chloritoid, with or without coesite or carpholite, distinct from the assemblages now in the matrix. The post-peak decompression of the pelitic^felsic schists is inferred to have involved two stages.The early stage decompression is characterized by dehydration reactions involving lawsonite and carpholite at P420 kbar, coupled with mode and compositional changes of garnet, glaucophane and phengite. The late-stage decompression after lawsonite disappearance led to the rocks being fluid-absent. Under such fluid-absent conditions, the solid transition of jadeite to albite occurs at P 14 kbar, and hornblendic amphibole forms at 12^13 kbar. Most garnet grains are somewhat changed in composition, to produce a pyrope-poor and grossular-rich outer rim. In contrast to above-solidus conditions, decompression of the pelitic^felsic schists at subsolidus conditions does not destabilize phengitic muscovite and tends to preserve their peak mineral assemblages if there is no intensive fluid infiltration. In the SouthTianshan belt, buoyancy of subducted metasediments with respect to mantle rocks could be one of the major reasons for fast exhumation of the HP^UHP rocks.

Timing and rate of granulite facies metamorphism and cooling from multi-mineral chronology on migmatitic gneisses, Sierras de La Huerta and Valle Fértil, NW Argentina

Migmatitic paragneisses of the Valle Fértil–La Huerta Ranges at the Western margin of the Sierras Pampeanas are composed of garnet–cordierite–plagioclase–biotite–quartz-bearing units that experienced peak metamorphic conditions of ca. 800 °C at 6–7 kbar. Based on petrological studies, pseudosection modeling and petrographic observations, an anticlockwise P– T path with a small pressure increment is proposed. Rare earth element LA-ICP-MS patterns acquired from rutile bearing garnets suggest a single stage of garnet growth at high- T at pressures above the ilmenite–rutile transition. U–Pb dating of zircon rims from the migmatites indicates two distinct metamorphic U–Pb ages of 525 ± 9 Ma and 478 ± 9 Ma. The older age is suggested to record an amphibolite facies event of the Pampean orogeny. The younger metamorphic age is contemporary with igneous zircons from metatonalites and pegmatites that yield 478 ± 4 Ma. We suggest that the prograde high- T metamorphic Famatinian event is associated with the emplacement of large magmatic bodies in which large-scale magmatic activity gave rise to an increased geothermal gradient of about 35 °C/km. Sm–Nd garnet ages of 447 ± 3 Ma indicate a time span of around 30 Ma for which temperatures above the garnet closure temperature prevailed. Using U–Pb, Sm–Nd and Rb–Sr isotope systems, a cooling rate of 3 to 6 °C/Myr is inferred.