Journal of Metamorphic Geology最新文献

{"title":"Out-of-Sequence Nucleation in a Blueschist From Syros, Greece","authors":"Frank S. Spear,&nbsp;Megan M. Koch","doi":"10.1111/jmg.12821","DOIUrl":"https://doi.org/10.1111/jmg.12821","url":null,"abstract":"<p>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.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"43 6","pages":"641-659"},"PeriodicalIF":3.5,"publicationDate":"2025-05-23","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.12821","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144525135","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"The Distribution of Major and Trace Elements Across a Garnet Population From the Kalak Nappe Complex (Finnmark, Scandinavian Caledonides): Evidence for Size-Dependent Growth and Compositional Equilibration in the Garnet Zone","authors":"M. T. A. G. Yogi,&nbsp;F. Gaidies,&nbsp;O. K. A. Heldwein,&nbsp;A. H. N. Rice","doi":"10.1111/jmg.12820","DOIUrl":"https://doi.org/10.1111/jmg.12820","url":null,"abstract":"&lt;p&gt;A garnet population from the lower Kalak Nappe Complex in Finnmark (Arctic Norway) was characterized using high-resolution X-ray micro-computed tomography, electron probe micro-analysis and laser ablation inductively coupled plasma mass spectrometry mapping to assess the extent of compositional equilibration and the controlling crystallization mechanisms during garnet growth. The obtained petrological dataset includes the spatial relationships of garnet crystals and the rock matrix fabrics, as well as the two-dimensional distributions of major and trace elements in differently-sized garnet crystals. Our results indicate that the observed elongated shape and clustered distribution of garnet resulted from crystallization in a texturally and chemically differentiated matrix, evidenced by the preferred distribution of biotite porphyroblasts. The major component (Fe, Mn, Mg, Ca) zoning presents systematic variations across differently sized garnet crystals, indicative of progressive nucleation and growth of garnet in equilibrium with an evolving matrix composition at increasing \u0000&lt;span&gt;&lt;/span&gt;&lt;math&gt;\u0000 &lt;mi&gt;P&lt;/mi&gt;&lt;/math&gt;-\u0000&lt;span&gt;&lt;/span&gt;&lt;math&gt;\u0000 &lt;mi&gt;T&lt;/mi&gt;&lt;/math&gt; conditions. Annular features with the same Ca concentration in the analysed garnet crystals are used as markers of the contemporaneous growth of specific segments in crystals of different sizes. The slopes of compositional gradients correlate with crystal size, with smaller crystals showing steeper gradients for equivalent segments in the largest crystals of the rock. The chemical signature and microstructural properties of garnet suggest that growth rates were anisotropic, interface-controlled and size-dependent. Since similar concentrations and distribution patterns are observed for Sc, Ti, V, Co, Y and rare earth elements (Gd to Lu) across the differently sized crystals at the positions of the markers defined by the low-Ca annuli in the crystals, the quasi-equilibration of these elements at the centimetre scale across the intergranular medium can be inferred. A possible explanation for the observed trace element distribution across the garnet population is a sufficiently slow heating rate during prograde metamorphism, which provided the time required for the efficient transport of trace elements in the intergranular medium during garnet growth. Crystallization simulations using equilibrium thermodynamics indicate garnet growth over an interval of approximately 60°C and 1 kbar until peak conditions of approximately 570°C and 4.5 kbar. Previously published Lu-Hf garnet-whole rock ages coupled with our \u0000&lt;span&gt;&lt;/span&gt;&lt;math&gt;\u0000 &lt;mi&gt;P&lt;/mi&gt;&lt;/math&gt;-\u0000&lt;span&gt;&lt;/span&gt;&lt;math&gt;\u0000 &lt;mi&gt;T&lt;/mi&gt;&lt;/math&gt; constraints indicate that heating rates could have been as slow as 1.3°C/Myr, suggesting that interface-controlled, size-dependent growth is not restricted to metamorphic garnet that crystallized rapidly and at fast heating rates (\u0000&lt;span&gt;&lt;/span&gt;&lt;math&gt;\u0000 ","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"43 6","pages":"613-640"},"PeriodicalIF":3.5,"publicationDate":"2025-04-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.12820","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144525144","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Identification of Neoarchean UHT (Ultrahigh-Temperature) Metamorphism From Intermediate Migmatitic Granulites in the Yinshan Block, North China Craton","authors":"Zizhen Yang,&nbsp;Chunjing Wei,&nbsp;Bin Wang,&nbsp;Shuguang Song,&nbsp;Wei Tian","doi":"10.1111/jmg.12819","DOIUrl":"https://doi.org/10.1111/jmg.12819","url":null,"abstract":"<div>\u0000 \u0000 <p>Identifying UHT metamorphism in intermediate granulites is challenging due to the lack of diagnostic mineral assemblages. The Yinshan Block from the North China Craton (NCC) experienced granulite-facies metamorphism during the Neoarchean, but the <i>P–T</i> paths and tectonic regimes remain debated. In this paper, we conduct a comprehensive study of petrography, mineral chemistry, phase equilibrium modelling, and zircon dating for two intermediate migmatitic granulites from the Xiwulanbulang area, and for the first time, report UHT metamorphism from the Yinshan Block. The migmatitic granulites include mesosomes and leucosomes. The mesosomes contain two-pyroxene granulite-facies assemblages (Opx + Cpx + Pl + Amp + Qz) and can further be subdivided into amphibole-bearing residues and small former melt pockets. Multiple thermobarometers and phase equilibrium modelling suggest that the two samples record UHT peak conditions of 1000–1050 °C/8–9 kbar, with possible anticlockwise <i>P–T</i> paths dominated by the post-peak near-isobaric cooling. Phase equilibria modelling also suggests that the isopleths of the Al^T (Cpx) (tetrahedral Al in clinopyroxene), <i>X</i>_An (Pl) (anorthite content in plagioclase), and Ti (Amp) (Ti content in amphibole) in <i>P–T</i> pseudosections are potentially useful for defining the peak temperatures of two-pyroxene granulites. However, the Al^T (Cpx) isopleths are highly dependent on the bulk-rock O (Fe₂O₃) content in the two-pyroxene granulite-facies assemblages without amphibole, and care should be taken in using them to determine temperatures. Moreover, <i>T–X</i>_Melt pseudosections indicate that the amphibole-bearing residues and melt pockets preserve distinct aspects of the metamorphic evolution. The residues tend to record UHT solidi at 960–1000 °C, where amphibole can be stable above 1000 °C, making Ti (Amp) a reliable indicator for constraining the lower limit of the peak temperature; while the melt pockets record HT solidi likely at ~850 °C, with amphibole stable just above the solidi. Zircon dating yields a continuous metamorphic age spectrum of 2.55–2.47 Ga, approximately coeval with the regional magmatism. Combining other geologic data from the Yinshan Block, we prefer a sagduction model of vertical tectonics to account for the UHT metamorphism.</p>\u0000 </div>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"43 6","pages":"591-612"},"PeriodicalIF":3.5,"publicationDate":"2025-04-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144525114","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Polyphase Deformation During Prolonged High-Temperature, Low-Pressure Metamorphism: An Example From the Namibfontein-Vergenoeg Migmatite Domes, Central Zone, Damara Belt, Namibia","authors":"Robyn J. MacRoberts,&nbsp;Pavlína Hasalová,&nbsp;Marlina A. Elburg,&nbsp;Jérémie Lehmann","doi":"10.1111/jmg.12818","DOIUrl":"https://doi.org/10.1111/jmg.12818","url":null,"abstract":"<div>\u0000 \u0000 <p>The Pan-African Damara Belt in Namibia is considered a polydeformed and polymetamorphic terrain, with a long history of magmatism (~100 m.y.). However, the timing, duration and tectonic significance of high-temperature, low-pressure metamorphic phases in relation to deformation in the Central Zone remain unclear. A combined pressure–temperature–time deformation (<i>P</i>–<i>T</i>–<i>t</i>-<i>d</i>) approach applied to rocks of the Namibfontein-Vergenoeg domes, two contiguous migmatitic domes in the southern Central Zone of the Damara Belt, suggests that these rocks underwent multiple deformation phases at suprasolidus HTLP conditions, over a prolonged period of time. Monazite petrochronology of structurally controlled leucogranite and leucosome reveals that early E–W shortening (D1) linked to the Kaoko Belt formation operated from &lt; 558 to ~535 Ma. At ~535–532 Ma, a tectonic switch from E–W to NNW–SSE shortening and progressive D2 and D3 occurred, linked to the formation of the Damara Belt. Lastly, enigmatic localised NE–SW shortening (D4), parallel to the main structural trend of the Damara Belt, was active from ~523 to ~494 Ma. Monazite U–Pb geochronology records at least ~50 m.y. (~540 to ~494 Ma) of monazite crystallisation and recrystallisation at suprasolidus conditions. Petrography and pseudosection modelling of Damara Supergroup metapelite indicate that all deformation occurred at similar, anatectic, HTLP conditions of 740°C–780°C and ~5 kbar. This tectono-metamorphic evolution is recorded in a shallow dP/dT PT path where the three regional shortening events (D1, D2–D3 and D4) operated in a thermally driven system with little change in pressure. These results suggest that deformation is compatible with a model of distributed strain at the scale of the orogen and a lack of pronounced crustal thickening. Long-lasting, high-temperature suprasolidus conditions raise questions regarding possible heat sources for sustained HTLP metamorphism in the Central Zone. Furthermore, these data call into question the importance of crustal thickening and subsequent exhumation of the Central Zone.</p>\u0000 </div>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"43 6","pages":"555-589"},"PeriodicalIF":3.5,"publicationDate":"2025-04-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144525112","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Integrated Petrology, Geochronology and 3D Microstructural Analysis of Garnetiferous Micaschists: Insights Into the Tectono-Metamorphic Evolution of the Betic Cordillera","authors":"Alejandro Ruiz-Fuentes,&nbsp;Domingo G. A. M. Aerden,&nbsp;Delphine Bosch,&nbsp;Olivier Bruguier,&nbsp;Michel Corsini,&nbsp;Jean-Marc Lardeaux,&nbsp;Philippe Münch","doi":"10.1111/jmg.12816","DOIUrl":"https://doi.org/10.1111/jmg.12816","url":null,"abstract":"<div>\u0000 \u0000 <p>Integration of microstructural, petrological and geochronological techniques has allowed detailed characterization of the timing and metamorphic features of deformation events in the Betic Cordillera. Phase equilibrium modelling, thermobarometric estimations, in situ U–Pb monazite geochronology and Ar–Ar geochronology (amphibole and micas) have been applied to key samples containing well-constrained deformation fabrics and garnet porphyroblasts of different timing that can be distinguished based on their specifically orientated inclusion trails. Our study helps constrain the timing and kinematics of initial crustal thickening stages in the Alpujárride complex, followed by orogenic collapse and renewed continental collision around 17 Ma. In the underlying Nevado-Filábride complex, multiple garnet generations coexisting in a single investigated sample record a similar history ending with garnet growth under low-pressure/high-temperature conditions around 13 Ma. New age constraints for microstructural trends fossilized within porphyroblasts refine the sequence of changing shortening directions previously linked to the plate-motion interplay between the Alborán Domain, Iberia and Africa from Eocene to Middle Miocene.</p>\u0000 </div>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"43 6","pages":"523-554"},"PeriodicalIF":3.5,"publicationDate":"2025-04-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144524885","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Exhumation Mechanisms of High-Pressure Rocks With High-Temperature Overprinting: Insights From Eclogites of the Baie Verte Peninsula, Newfoundland","authors":"Ludovico G. Scorsolini,&nbsp;Cees van Staal,&nbsp;Chris Yakymchuk,&nbsp;John M. Hanchar,&nbsp;Sabastien Dyer","doi":"10.1111/jmg.12817","DOIUrl":"https://doi.org/10.1111/jmg.12817","url":null,"abstract":"<p>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.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"43 5","pages":"497-521"},"PeriodicalIF":3.5,"publicationDate":"2025-04-14","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.12817","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144191059","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Tectonic Setting and Evolution of Anatectic Melt Composition During Prograde Metamorphism up to UHT Metamorphism: Constraints From P–T–t–Melting Path From Rundvågshetta, Lützow–Holm Complex, East Antarctica","authors":"Kota Suzuki,&nbsp;Tetsuo Kawakami,&nbsp;Tetsu Kogiso,&nbsp;Shuhei Sakata,&nbsp;Fumiko Higashino,&nbsp;Masanori Yokoi,&nbsp;Shumpei Kudo","doi":"10.1111/jmg.12815","DOIUrl":"https://doi.org/10.1111/jmg.12815","url":null,"abstract":"<p><span>The p</span>ressure–temperature–time (<i>P–T–t</i>) 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 <i>P–T</i> 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, <i>P–T</i> 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 <i>P–T–t</i>–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 <i>P</i>–<i>T</i> 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.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"43 5","pages":"467-495"},"PeriodicalIF":3.5,"publicationDate":"2025-04-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.12815","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144191127","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"The Structure and P–T–t–d Evolution of the Saxothuringian/Brunovistulian Variscan Boundary Zone in the Bohemian Massif (Czech Republic, Poland)","authors":"Marek Śliwiński,&nbsp;Mirosław Jastrzębski,&nbsp;Jiří Sláma,&nbsp;Gabriela A. Kozub-Budzyń,&nbsp;Aleksandra Jaźwa","doi":"10.1111/jmg.12813","DOIUrl":"https://doi.org/10.1111/jmg.12813","url":null,"abstract":"<div>\u0000 \u0000 <p>The Variscan suture zone between the Saxothuringian and Brunovistulian terranes of the Bohemian Massif in Central Europe is characterized by a highly tectonized zone: the Stáre Město Belt (SMB). Despite extensive previous research, better understanding of some essential aspects regarding the metamorphic conditions and timing of the tectonic events within the SMB are needed to understand the structural architecture and the Variscan evolution of the Saxothuringian/Brunovistulian suture zone. The present study present systematic data from thermodynamic modelling and U–Pb LA-ICPMS dating of monazite, titanite and zircon on mica schists, felsic and mafic metavolcanic rocks, and leucocratic veins exposed in the SMB. The results indicate that progressive metamorphism related to the collision of the Saxothuringian margin and the rigid Brunovistulian crust started at c. 370–366 Ma (D1 stage). The mostly sedimentary outer units and the mostly volcanic middle unit of the SMB were buried to middle and lower crustal depths, respectively. The continued collision resulted in further burial of the outer units from 4–6 kbar at 510°C–550°C to c. 6.5–7.5 kbar at 650°C with coeval extrusion of the middle unit from 10.5 at 670°C to 6 kbar at 680°C. As a result of the juxtaposition of the units with slightly different prograde P–T histories, portions of the lower crust that were extruded in the middle of the SMB and portions of the middle crust forming the outer parts of the SMB, were transported to the same crustal level (6–7 kbar) at 640°C–680°C (D2 stage), together with formation of anatectic leucosomes at c. 342 Ma. The structure of the SMB was modified by D3 stage dextral transpression during which the former S2 planes were reactivated to form deep-seated, ductile strike-slip faults, with the coeval intrusion of tonalitic magmas along the SMB axis. The predominantly c. 340–330 Ma monazite U–Pb ages obtained from the metamorphic rocks of the SMB are interpreted as the time of the subsequent cooling due to exhumation. This study emphasizes that the long-lasting metamorphic history of the border zone between Saxothuringian and Brunovistulian terranes started at c. 370 Ma and continued to c. 330 Ma.</p>\u0000 </div>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"43 5","pages":"421-444"},"PeriodicalIF":3.5,"publicationDate":"2025-03-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144190737","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Phase Equilibria Analysis for Metacarbonate With Applications to Zoned Calc-Silicate Aureoles","authors":"Zhenhao Zhou,&nbsp;Xu Chu","doi":"10.1111/jmg.12814","DOIUrl":"https://doi.org/10.1111/jmg.12814","url":null,"abstract":"<p>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 (<i>T</i>) and the fluid composition (<i>X</i>) of the metacarbonate systems, as well as semi-quantification of the prograde decarbonation at convergent boundaries. In this study, we constructed <i>T</i>-<i>X</i>_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 <i>X</i>_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 <i>X</i>_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 (&lt; 600 °C). In this study, we recalculated reactions in subsystem grids from previous studies across various <i>P</i>-<i>T</i>-<i>X</i>_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.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"43 5","pages":"445-465"},"PeriodicalIF":3.5,"publicationDate":"2025-03-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.12814","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144190738","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"A Mechanistic Look at the Amphibolitization of Mafic Crust: Insights From the Kråkeneset Gabbro Body, Western Gneiss Region, Norway","authors":"Saskia Grund,&nbsp;Timm John,&nbsp;Johannes C. Vrijmoed,&nbsp;Håkon Austrheim,&nbsp;Torgeir B. Andersen","doi":"10.1111/jmg.12809","DOIUrl":"https://doi.org/10.1111/jmg.12809","url":null,"abstract":"<p>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.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"43 4","pages":"385-405"},"PeriodicalIF":3.5,"publicationDate":"2025-03-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.12809","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143749988","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}