Journal of Metamorphic Geology最新文献

This study presents a comprehensive examination of exceptionally preserved eclogites from the Baie Verte Peninsula, Newfoundland Appalachians, which display a high-temperature overprint, thus offering insights into the metamorphic evolution and exhumation mechanisms of such terrains. Through an integrated approach combining field observations, petrographic analysis and thermodynamic modelling, we unravel the tectonometamorphic history of the Taconic eclogites within the East Pond Metamorphic Suite. The eclogites record a complex multistage metamorphic path characterized by initial A-subduction at ~2.7 GPa at ~640°C, followed by near-isothermal decompression to ~2 GPa at ~660°C, and significant heating during exhumation to the metamorphic peak (~1.5 GPa, ~800°C). We highlight the pivotal role of fluids in facilitating metamorphic reactions and influencing the rheology and buoyancy of subducting slabs. We propose a two-stage exhumation model for these eclogites: (1) initial ascent driven by buoyancy forces within a low-density and low-viscosity mantle wedge and (2) subsequent exhumation to shallower crustal levels, aided by external tectonic forces, such as shear zone displacement, erosion or extension. The relationship between the hydration history and the reconstructed pressure–temperature path, featuring a β-shaped trajectory, underscores the significance of thermal perturbations and fluid migration in the exhumation history of HP–UHP terrains in subduction–collision zones.

The pressure–temperature–time (P–T–t) evolution of a metapelitic ultrahigh-temperature (UHT) granulite from Rundvågshetta (Lützow–Holm Complex, East Antarctica) and chemical evolution of partial melt during the prograde metamorphism up to UHT metamorphism are studied in detail. The presence of different phosphorus (P) concentration zones in garnet is used to distinguish four phases of garnet growth. The P–T conditions for the P-poor garnet core, P-rich mantle and two P-poor rim growths are estimated, respectively, at ~840°C–920°C/7.7–12.5 kbar, ~920°C–1015°C/12.5–14.3 kbar, ~950°C–1000°C/7 kbar and ~800°C/5 kbar using Al₂SiO₅ inclusions, Zr-in-rutile thermometry, P–T grid and pseudosection analyses. The glassy inclusions in the P-poor core of garnet plot approximately on the Qz-Or cotectic line for 10 kbar in the CIPW normative Qz-Ab-Or diagram, representing prograde to UHT melt formed through the dehydration melting of biotite + sillimanite. Furthermore, the earlier prograde P–T–t–melting information was constrained from inclusions in zircon. The inner mantle of zircon dated at 564 ± 10 Ma includes prograde inclusions of muscovite + quartz + nanogranitoids (NIs) that predate the garnet growth. The NIs in zircon remelted by piston-cylinder experiments plot approximately on the Qz-Or cotectic line for 5 kbar, representing the early melt formed through dehydration melting of muscovite at ~700°C/5 kbar. The P-rich garnet mantle and the CL-bright inner rim of zircon dated at 532 ± 5 Ma were in equilibrium at 900°C–1100°C, based on the REE distribution between them. This suggests that the peak UHT metamorphism occurred at 532 ± 5 Ma, with the prograde metamorphic period lasting ~30 Myr and overall anatectic period exceeding ~40 Myr. Negligible Pb diffusion between zircon zones possibly indicates that peak UHT was short lived, lasting less than 10 Myr. The systematic compositional change of the above-mentioned two stages of melt inclusions is consistent with compositional evolutions in published melting experiments and thus reflects the near-equilibrium compositional evolution of partial melts as the P–T conditions change over ~30 Myr of prograde metamorphism. Therefore, the UHT metamorphism in Rundvågshetta was probably caused by radiogenic self-heating in the thickened crust during the continental collision.

Carbonate rocks react with infiltrating hydrothermal fluids to produce zoned calcsilicate assemblages in contact aureoles. Petrogenetic grids provide valuable insights into phase relations, metamorphic temperature (T) and the fluid composition (X) of the metacarbonate systems, as well as semi-quantification of the prograde decarbonation at convergent boundaries. In this study, we constructed T-X_CO2 (composition of H₂O–CO₂ binary fluid) grids in the system CFMASHc (CaO–FeO–MgO–Al₂O₃–SiO₂–H₂O–CO₂), supplemented with Fe₂O₃ or TiO₂, and its subsystems (CMASHc, CMSHc, CFSHc and CASHc). The grids were constructed to encompass upper crustal conditions, with temperatures ranging from 300°C to 1000°C at 2 kbar and 4 kbar, and X_CO2 from 0 to 0.8 (0 = pure water). We adopted internally consistent thermodynamic datasets and compatible activity–composition models for solid solutions. The grids illustrate the index minerals and field gradients observed in classical aureoles. Typical calcsilicate assemblages in these contact aureoles appear along a heating trajectory at a relatively low X_CO2, in the sequence of talc, tremolite, diopside (±olivine), garnet and wollastonite. The grids in the CASHc, CMSHc and CMASHc subsystems are sufficient to cover important reactions that lead to the formation and decomposition of these minerals. The grids with an additional TiO₂ component help interpret phase relations involving rutile, titanite and ilmenite. In addition, we note that phase relations calculated with endmember carbonates are practically similar to those calculated for a complete ternary solid-solution model at low-to-mid temperatures (< 600 °C). In this study, we recalculated reactions in subsystem grids from previous studies across various P-T-X_CO2 conditions within a consistent framework. These results are contextualized with natural assemblages and applied to constrain the field gradient of a representative contact aureole. By incorporating additional components, the grids accommodate a broader range of assemblages observed in metacarbonate rocks. Together, these expanded grids provide a robust framework for future studies of contact metamorphism in metacarbonate systems. The calculated phase equilibria were specifically applied to a contact aureole in southern Tibet, with temperature estimations derived from the phase equilibria aligning closely with a conduction model based on the timescales from diffusion speedometry.

Fluid–rock interactions play a key role in the formation, evolution and recycling of the Earth's crust. For fluids to infiltrate rocks and enable and sustain fluid-mediated mineral transformations, fluid pathways are required. In this study, we examined the potential mechanisms of formation of such pathways via detailed mineralogical, petrophysical and thermodynamic analysis of a dry, essentially ‘non-porous’ gabbro that was hydrated and transformed into an amphibolite under amphibolite-facies conditions. During a previous regional HP eclogite-facies metamorphism, the gabbro did not equilibrate and preserved almost entirely its igneous textures and magmatic minerals. Rock transformation during amphibolitization was triggered by fluid infiltration through a newly opened N–S striking fracture network. An equally spaced fracture network formed by mode I opening related to the formation of an E–W striking shear zone at the northern and southern borders of the gabbro body. The amphibolitization process allowed the fluid to pervasively infiltrate the rock from the fracture into the pristine gabbro. The essentially fully amphibolitized sample exhibits some unaffected gabbroic mineral relicts. Even though the amphibolitization process led to the formation of ~70 vol.% hydrous phases, it was accompanied by densification and related porosity formation. The modes and compositions of minerals within partly amphibolitized rocks indicate that besides the uptake of H₂O, no significant mass exchanges were necessary for this transformation, at least on the thin section scale. Thermodynamic modelling and petrological data show that the transition from gabbro to amphibolite favours porosity formation. In the model, the reaction front proceeded as soon as the gabbro at the reactive interfaces of the affected minerals was sufficiently transformed. At this point, fluid was not consumed further but remained as a free fluid phase, which progressed through the newly formed pore space and advanced amphibolitization. Once the gabbro was almost entirely amphibolitized, its mineral content and mineral chemistry no longer changed, so the progress of amphibolitization progress was controlled by fluid availability. This case study shows that fluid–rock interaction leading to hydration of a rock can be efficiently maintained in almost non-permeable, dry and mafic crust and, therefore, strongly affects the petrophysical properties of the Earth's crust.

The southern Dora-Maira Massif, where coesite was first discovered 40 years ago, is among the most studied and better known example of high/ultra-high-pressure (HP/UHP) terranes. Previous to this study, the Polymetamorphic Basement Complex of the southern Dora-Maira Massif has been defined as a nappe stack consisting of three juxtaposed tectono-metamorphic units: the HP San Chiaffredo Unit at the bottom, the UHP Brossasco-Isasca Unit in the middle and the HP Rocca Solei Unit at the top. The origin of UHP metamorphism in the Brossasco-Isasca Unit is still controversial, due to the difficulties in reconciling the abrupt difference between the UHP conditions recorded by the Brossasco-Isasca Unit (i.e., 700°C–730°C, 4.0–4.3 GPa) and the HP conditions (i.e., ~500°C–520°C, 2.0–2.2 GPa) registered by the adjacent units. Here, we report new petrologic and microstructural evidence supporting the existence of a previously unrecognised UHP unit in the southern Dora-Maira Massif. Our data demonstrate that the tectonic unit overlying the Brossasco-Isasca Unit (i.e., the former Rocca Solei Unit), so far considered a HP unit, is actually divided in two units, one of which (the lowermost Rocca Solei Unit sensu stricto) experienced UHP conditions and the other (the uppermost Grimbassa Unit) reached HP conditions. The newly defined Rocca Solei Unit experienced UHP metamorphism at significantly different P–T conditions (520°C–550°C, 2.7–2.9 GPa) compared to the underlying Brossasco-Isasca Unit, but along a similar ‘cold’ T/P ratio (< 200°C/GPa), markedly lower than that defined in the neighbouring Grimbassa Unit and San Chiaffredo Unit (> 230°C/GPa). After more than 30 years of petrologic investigations, the tectono-metamorphic architecture of the southern Dora-Maira Massif is thus redefined, bridging the gap between the UHP Brossasco-Isasca Unit and the adjacent HP units and opening to new scenarios on its HP–UHP architecture. The results of this study have both regional and petrologic implications: (i) Similarities emerge in the structural position, thickness and metamorphic evolution of the new UHP Rocca Solei Unit in the southern Dora-Maira Massif and those of the Chasterain Unit recently discovered in the northern Dora-Maira Massif, suggesting a common architecture throughout the whole Dora-Maira Massif; (ii) the peculiar quartz microstructure in the metagranites described below represents an exceptional documentation of a ‘frozen’ quartz-to-coesite polymorphic reaction caught in the act and suggests that the availability of fluids was the most crucial factor controlling the progress of the reaction. The metastable persistence of quartz in H₂O-undersaturated lithologies makes even more challenging the identification of UHP units that have only slightly exceeded the quartz–coesite transition and justifies why the newly defined UHP Rocca Solei Unit has remained ‘hidden’ for more than 30 years.

The reactive flow of melt through the mantle or crust triggers chemical disequilibrium, driving reactions that significantly alter the mineral assemblages and physical properties of host rocks. However, the degrees of chemical difference required to initiate these reactions and their timescale remain poorly understood. In this study, we present piston–cylinder reaction experiments simulating lower crustal conditions, where largely anhydrous lower crustal granoblastic dioritic gneiss interacts with a hydrous mafic melt, created from the same gneiss but modified by the addition of ~6-wt.% H₂O. Remarkably, reactions occurred within just 12 h, producing microstructures that closely resemble those observed in natural, melt-fluxed rocks from the lower arc crust in Fiordland, New Zealand. Melt–rock interactions led to the formation of epitaxial, multilayer symplectic coronae of pargasite + plagioclase or quartz partially replacing pre-existing pyroxene grains. The protolith plagioclase and amphibole are either completely dissolved into the melt or replaced by a modified composition of the same mineral. The melt exhibits compositional variations that correlate with distance from the melt–rock reaction front. Quenched melt chemistry data demonstrate the potential for melt compositions to continuously evolve in response to both crystallisation and melt–rock interactions during reactive flow. Importantly, our findings reveal that melt–rock reactions, initiated by melt not drastically different from the solid rock (protolith), can induce significant changes in rock composition and thus physical properties in a short time. Our findings have broad implications for understanding the compositional evolution of migrating melts and the chemical and mechanical evolution of the Earth's mantle and lower crust in general.

Trace element zoning in kyanite can retain information about its growth history, particularly in anatectic metapelites. There, kyanite can grow (i) at sub-solidus conditions through metamorphic reactions involving other aluminous phases as reactants, (ii) through muscovite dehydration melting reactions, and (iii) during cooling and melt crystallisation either through back-reactions between melt and solid phases (e.g., garnet) or crystallising directly from the melt. Thermodynamic modelling successfully reproduces these reactions, allowing a more robust interpretation of the observed features based on predicted reactants and products. In this study, we interpret the kyanite trace element zoning (particularly of Cr, V, and partly of Fe) observed through cathodoluminescence and quantified through LA-ICP-MS maps, using the forward thermodynamic modelling approach. The studied samples are biotite + kyanite + garnet migmatites from the Lower-Greater Himalayan Sequence of eastern Nepal, which experienced muscovite and incipient biotite dehydration melting. Three main generations of kyanite revealed by trace element zoning have been identified (i.e., Ky1, Ky2, and Ky3), consistent with the three main kyanite-producing reactions predicted by forward thermodynamic modelling, also applying a melt reintegration approach. Ky1 (i.e., sub-solidus kyanite) integrated only minimum amounts of Cr, V and Fe. Ky2 (i.e., peritectic kyanite) incorporates Cr and V released from muscovite during its dehydration melting reaction. Ky3 (i.e., back-reaction overgrowth or magmatic kyanite) is particularly developed in samples where melt segregation has been absent or limited and incorporates lower amounts of Cr and V than Ky2, but is enriched in Fe. The major implications of this study concern the interpretation of the melt segregation processes in anatectic rocks and our understanding of the Cr and V partitioning between minerals and melt. Further methodological considerations are also provided, which could help guide similar studies in the future.