Journal of Metamorphic Geology最新文献

The youngest known ultrahigh-pressure (UHP) eclogite from Tumagabuna Island, a small island of the D'Entrecasteaux Islands chain in Papua New Guinea, exhibits zircon U–Pb age variations spanning a few million years. This age variation complicates the interpretation of the evolution of this high and ultrahigh-pressure ([U]HP) terrane. We investigate the discrepancies in zircon ages using SHRIMP U–Pb dating and same-spot REE abundance data. Petrographic observations reveal a main mineral assemblage of garnet, omphacite, amphibole, coesite, phengite, rutile and zircon in the eclogite. The formation of the main mineral assemblage is constrained by P–T pseudosection modelling. The main mineral assemblage likely crystallized from a melt at UHP conditions of p = 3.5 ± 0.2 GPa and T = 690°C ± 40°C. Two distinct generations of zircon are identified: an older group with ages ranging between 7.0 ± 0.2 and 7.9 ± 0.3 Ma and a younger group with ages ranging between 4.4 ± 0.3 Ma and 5.5 ± 0.4 Ma. These zircon populations differ in internal zoning and MREE and HREE contents. The initial zircon growth phase corresponds to the formation of the main mineral assemblage at UHP conditions, whereas the younger zircon generation likely resulted from Zr release during exhumation-related breakdown of the Zr-bearing UHP phases garnet, rutile and possibly omphacite.

Two samples of garnet + glaucophane + chloritoid + phengite + paragonite + quartz + epidote + rutile schist from the Cycladic Blueschist Unit of Syros, Greece, have been examined in detail in order to constrain the paragenetic sequence and to infer the processes and time scales for metamorphic recrystallization. Petrographic evidence clearly demonstrates the sequence of porphyroblast growth is garnet ➔ glaucophane ➔ chloritoid in contrast to the sequence predicted by equilibrium calculations that is glaucophane ➔ garnet ➔ chloritoid. Most importantly, both garnet and glaucophane do not appear in the assemblage at conditions near the equilibrium phase-in reaction boundary and require considerable overstepping before nucleation occurs. These results reveal that equilibrium thermodynamic forward modelling (e.g., pseudosections) does not accurately predict or explain the observed paragenesis. Growth of glaucophane following nucleation has been modelled using a model of local equilibrium driven by excess reaction affinity and is shown to drive additional reactions among phases not directly in contact with the glaucophane. These reactions have resulted in the consumption of garnet in some locations and the simultaneous production of garnet in other locations, which is used to explain the formation of high-Mn overgrowths on many garnet crystals in these samples. Diffusion modelling of these high-Mn overgrowths suggests very rapid cooling, which must require extremely rapid exhumation subsequent to the rocks having reached peak metamorphic conditions.

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.