Journal of Metamorphic Geology最新文献

{"title":"Featured Cover","authors":"Zhen M. G. Li,&nbsp;Fred Gaidies,&nbsp;Yi-Chao Chen,&nbsp;Jia-Shun Hu,&nbsp;Chun-Ming Wu","doi":"10.1111/jmg.70010","DOIUrl":"https://doi.org/10.1111/jmg.70010","url":null,"abstract":"<p>The cover image is based on the Original Article <i>Thermodynamic and Kinetic Controls on the Growth of Metapelitic Garnet in the Danba Dome (SW China): Insights From Microstructure, Element Mapping and Thermodynamic Modelling</i> by Zhen M. G. Li et al., https://doi.org/10.1111/jmg.70005.\u0000 \u0000 <figure>\u0000 <div><picture>\u0000 <source></source></picture><p></p>\u0000 </div>\u0000 </figure></p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"43 8","pages":""},"PeriodicalIF":3.4,"publicationDate":"2025-09-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.70010","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145021953","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":"Tectonothermal Evolution of the NW Wyoming Province: Insights From Coupled Garnet Lu-Hf and Sm-Nd Geochronology","authors":"Tsai-Wei Chen,&nbsp;Jeff Vervoort,&nbsp;Julia Baldwin,&nbsp;Peter Baker","doi":"10.1111/jmg.70006","DOIUrl":"https://doi.org/10.1111/jmg.70006","url":null,"abstract":"<p>Understanding how Paleoproterozoic orogenic processes shaped the assembly of Laurentia remains a critical puzzle in deciphering Earth's ancient tectonic history. To address this challenge, the Montana metasedimentary terrane in the northwest Wyoming Province, which preserves a complex record of multiple metamorphic episodes, provides a unique opportunity. In this study, we integrate garnet composition with coupled Lu-Hf and Sm-Nd geochronology to unravel the polymetamorphic history of this terrane and constrain the timing and mechanisms of orogenic processes. Our new garnet Lu-Hf and Sm-Nd ages reveal three distinct age groups: 2.42, 2.19–2.06 and 1.83–1.70 Ga. Most analysed garnet samples demonstrate typical prograde zoning patterns with enriched Mn and Lu contents in their core, suggesting that the Lu-Hf ages from these samples reflect the timing of prograde metamorphism. The ~30 Myr younger Sm-Nd ages for most samples document the cooling process that followed peak metamorphic conditions during the orogenic cycle. One amphibolite sample shows reverse Lu zoning that likely resulted from mineral breakdown reactions, suggesting complex trace-element incorporation during high-grade metamorphic processes. By integrating our age data with the estimated peak metamorphic conditions reported by previous studies, we identify a spatial and temporal trend of decreasing metamorphic grade that progressed southeastward from the northwestern boundary of the terrane between 1.8 and 1.7 Ga. This pattern reveals a propagation of orogenic activity during the Big Sky orogeny, providing insights into the thermal evolution and incorporation of terranes during Laurentia assembly.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"43 8","pages":"839-862"},"PeriodicalIF":3.4,"publicationDate":"2025-08-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.70006","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145022167","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":"Fluid or Melt? Distinguishing Syn-Deformational Interaction Pathways","authors":"Hindol Ghatak,&nbsp;Nathan R. Daczko,&nbsp;Sandra Piazolo,&nbsp;Tom Raimondo","doi":"10.1111/jmg.70008","DOIUrl":"https://doi.org/10.1111/jmg.70008","url":null,"abstract":"<p>Understanding whether deformation occurred in the presence of aqueous fluid or silicate melt is crucial for interpreting ductile shear zones, impacting their thermal and geochemical evolution, and having rheological consequences. To identify the syn-deformational fluid type, we investigate contrasting shear zones active during the Alice Springs Orogeny in central Australia, focusing on their effects on dry granulite facies gneisses transformed into greenschist–amphibolite facies schists. Shear zones in the north-western part of the orogen (Reynolds–Anmatjira Ranges) exhibit greenschist–lower amphibolite facies muscovite–chlorite assemblages, quartz veins and microstructures indicative of solid-state deformation. These features collectively suggest deformation in the presence of aqueous fluid. In contrast, shear zones in the south-eastern part (Strangways Range) display upper amphibolite facies garnet–biotite–sillimanite assemblages, along with granitic dykes and lenses retaining igneous textures. Microstructures, such as ‘string of bead’ textures and felsic minerals forming films along grain boundaries or exhibiting low apparent dihedral angles, indicate the former presence of melt in high strain rocks. This suggests that hydration in the south-eastern shear zones was driven by externally sourced silicate melt and melt–rock reactions. Differentiating between the two types of shear zones using whole rock major and trace element data is challenging. However, rare earth element (REE) analyses show potential. Limited REE metasomatism is observed where aqueous fluids are inferred, with three samples in a transect displaying consistent patterns. In contrast, where silicate melt is interpreted as the metasomatic agent, REE metasomatism is more variable, exhibiting atypical REE patterns relative to common rock types and considerable variability between samples in a transect. This contrast is attributed to greater mobility of REEs in silicate melt compared to aqueous fluid.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"43 8","pages":"810-838"},"PeriodicalIF":3.4,"publicationDate":"2025-08-14","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.70008","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145022113","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":"Thermodynamic and Kinetic Controls on the Growth of Metapelitic Garnet in the Danba Dome (SW China): Insights From Microstructure, Element Mapping and Thermodynamic Modelling","authors":"Zhen M. G. Li,&nbsp;Fred Gaidies,&nbsp;Yi-Chao Chen,&nbsp;Jia-Shun Hu,&nbsp;Chun-Ming Wu","doi":"10.1111/jmg.70005","DOIUrl":"https://doi.org/10.1111/jmg.70005","url":null,"abstract":"<div>\u0000 \u0000 <p>Understanding the growth history of metamorphic garnet is crucial for revealing metamorphic evolution and distinguishing thermodynamic and kinetic contributions during metamorphism. Garnet crystals in metapelites from a Barrovian-type metamorphic sequence in the Danba dome (SW China) record microstructural and compositional patterns that provide insights into the petrogenetic evolution of the samples during metamorphism. While sector-zoned garnet in graphite-rich layers and garnet crystals with trace element and microstructural evidence of biotite overgrowth are common in the lower grade garnet–staurolite–kyanite zones, these features are absent in the higher grade sillimanite zone. Reactions associated with accessory phases during garnet growth may explain the yttrium (Y) and heavy rare earth element (HREE) annuli observed in garnet of rocks from the garnet and staurolite zones. The absence of these Y and HREE annuli in garnet of rocks from the sillimanite zone is explained by variations in the bulk-rock composition of the samples, which resulted in different pressure–temperature (<i>P–T</i>) conditions of garnet-forming and accessory phase breakdown reactions (e.g., monazite–allanite transition). Whereas isopleth thermobarometry applied to the cores of the largest garnet crystals demonstrates initial garnet growth close to equilibrium in a rock that was collected very close to the garnet isograd, different degrees of driving force (~1.2<i>–</i>2.3 kJ per mole 12-oxygen garnet) were required for garnet nucleation in other rocks, pointing to varying reaction affinities for initial garnet growth. Garnet crystallization modelling for a rock from the garnet zone also predicts the observed mineral assemblages and garnet growth zoning exceptionally well and yields peak <i>P–T</i> conditions that are identical to the results obtained by conventional thermobarometry. Taken together, these results imply that the growth of metapelitic garnet in the Danba dome was controlled by both thermodynamic (e.g., <i>P–T</i> composition (<i>X</i>) relations) and kinetic (e.g., elemental mobility, protolith heterogeneity, reaction history, and chemical driving force for garnet nucleation) factors. This study underscores the specific impacts of these factors on the growth characteristics of metapelitic garnet in a regional metamorphic context.</p>\u0000 </div>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"43 8","pages":"781-809"},"PeriodicalIF":3.4,"publicationDate":"2025-08-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145021785","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":"Calibration, Validation and Evaluation of Machine Learning Thermobarometers in Metamorphic Petrology: An Application to Biotite and Outlook for Future Strategy","authors":"Philip Hartmeier,&nbsp;Jacob B. Forshaw,&nbsp;Pierre Lanari","doi":"10.1111/jmg.70004","DOIUrl":"https://doi.org/10.1111/jmg.70004","url":null,"abstract":"<p>Geothermobarometry provides crucial constraints on the physical conditions of metamorphism, offering insights into petrogenetic processes and providing key information on thermal regimes and metamorphic depths to other geological disciplines. However, calibrating a thermobarometer from the natural record is challenging because independent pressure (<i>P</i>) and temperature (<i>T</i>) estimates are required, and the compositional variation of minerals—governed by multiple metamorphic reactions—must be captured in a complex function. This work calibrates a machine learning thermobarometer for biotite using relative <i>P</i>–<i>T</i> estimates based on mineral assemblage sequences. A neural network is used as a flexible model to fit a high-dimensional thermobarometric regression curve. To address the challenge of sparse training data, a transfer learning strategy is employed, where the model is primarily trained on a large dataset generated with phase equilibrium modelling before refinement with natural data. A general framework for calibrating machine learning thermobarometers is outlined using a neural network thermobarometer for biotite as an example. Selection of the best-performing model is guided by <i>k</i>-fold cross-validation alongside complementary accuracy checks using metamorphic sequences and precision assessments via Monte Carlo error propagation. Evaluation on an independent test dataset, compiled from the literature, indicates that the model is a potential biotite single-crystal thermometer with a root mean square error of ± 45°C, consistent with the estimated uncertainty of Ti-in-Bt thermometry applied to the same data. A potential barometer is affected by systematic underestimation of pressures above 0.6 GPa due to regression to the mean of the natural database, which is biased towards low-pressure metamorphism. This limits its applicability in higher-pressure regimes. This study highlights the potential of using neural networks with transfer learning in petrological applications since they are often constrained by limited natural data.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"43 8","pages":"755-780"},"PeriodicalIF":3.4,"publicationDate":"2025-07-31","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.70004","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145022369","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":"Applications of Quartz-in-Garnet Elastic Geobarometry to Mid-Crustal Barrovian Metamorphic Rocks Demonstrated Using Tensionally Stressed Inclusions From the Grand Canyon","authors":"Suzanne Autrey,&nbsp;Chloe Bonamici,&nbsp;Michael L. Williams","doi":"10.1111/jmg.70002","DOIUrl":"https://doi.org/10.1111/jmg.70002","url":null,"abstract":"<div>\u0000 \u0000 <p>Quartz-in-garnet elastic geobarometry (QuiG) pressures were obtained for inclusions exhumed from low-to-moderate pressure and moderate-to-high temperature Barrovian metamorphic conditions. Exhumation from these conditions causes quartz inclusions to expand less than their garnet hosts and experience tensional stresses. Most of the inclusions analysed in this study had near zero strains and stresses, which can preclude the ability to identify entrapped inclusions. New methods for identifying inclusion strains related to entrapment when the results cluster near zero were applied, revealing a population of tensile inclusions related to entrapment during metamorphism. Three hundred twenty-three inclusions were analysed from rock samples from the Upper Granite Gorge of the Grand Canyon. Inclusion remnant pressures were calculated using (1) shifted Raman modes assuming hydrostatic stress and (2) strains and the phonon-mode Grüneisen tensor of quartz. A range of entrapment pressures was determined for each garnet, but careful inspection of the inclusions and statistical analysis of the data sets successfully identified and removed inclusions that returned spurious results. QuiG yielded robust entrapment pressure estimates using the two different elastic geobarometry approaches. In this study, the refined QuiG technique yielded entrapment pressures that are consistent with the known geology of upper greenschist facies and upper amphibolite facies schists from the base of the Grand Canyon. Results from a zoned garnet grain show that multiple pressure populations can be preserved in a single garnet. The new QuiG results have implications for the robustness of QuiG in Barrovian rocks, where strains in inclusions can be near zero, and the ability of inclusions to retain early pressure information after being subjected to subsequent high-grade metamorphism.</p>\u0000 </div>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"43 7","pages":"734-753"},"PeriodicalIF":3.4,"publicationDate":"2025-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144767665","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":"Comparison of Water Contents Between Ultrahigh-Pressure Oceanic and Continental Eclogites From the North Qaidam Orogen","authors":"Ren-Xu Chen,&nbsp;Zhi-Min Wang,&nbsp;Bing Gong,&nbsp;Xiang-Ping Zha,&nbsp;Yuan Gao","doi":"10.1111/jmg.70003","DOIUrl":"https://doi.org/10.1111/jmg.70003","url":null,"abstract":"<div>\u0000 \u0000 <p>Water storage and transport in eclogite are critical for the water cycle in subduction zones. A combined study of the petrology and water contents of nominally anhydrous minerals (NAMs) was carried out on oceanic and continental eclogites from the North Qaidam orogen. The results are used to compare the presence, abundance and behaviour of free fluids of subducting mafic crust at subarc depths between oceanic and continental subduction zones and to further investigate the water storage and transport in subducting mafic crust with respect to the geothermal gradients of subduction zones. Both oceanic and continental eclogites experienced Alpine-type ultrahigh-pressure (UHP) metamorphism with similar P–T paths. The hydrogen incorporation of OH⁻ modes at 3450–3470 and 3600–3630 cm⁻¹ in omphacite is controlled by the classical mechanism (H⁺+Al³⁺ → Si⁴⁺). The OH⁻ mode at 3510–3540 cm⁻¹ in omphacite is mainly controlled by the Ca and Na contents at the M2 site. The primary structural water content of garnet is positively correlated with temperature, indicating that temperature plays an important role in hydrogen incorporation into garnet during the high-pressure (HP) to UHP metamorphic transition. In contrast, the water content of omphacite does not differ between HP and UHP eclogite-facies conditions. This observation suggests different hydrogen incorporation between garnet and omphacite in response to P–T changes. Garnet and omphacite in oceanic and continental eclogites exhibit similar primary structural water contents under UHP conditions, indicating that the subducting mafic crust in the oceanic and continental subduction zones experienced similar water storage and transport during the HP-to-UHP metamorphic transition despite their different protolith natures. The eclogites that experienced different P–T paths exhibited different water contents under HP and UHP conditions, indicating that water storage and transport in the subducting mafic crust are controlled by the geothermal gradients of subduction zones rather than by the protoliths. An elevated thermal gradient enhances the decomposition of hydrous minerals and the incorporation of hydrogen into NAMs. A large amount of water is released from the subducting mafic crust during prograde Alpine-type HP eclogite-facies metamorphism, resulting in less water left to be released during further Alpine-type HP-to-UHP eclogite-facies transitions.</p>\u0000 </div>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"43 7","pages":"706-733"},"PeriodicalIF":3.4,"publicationDate":"2025-06-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144768052","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":"Coupling Phase Equilibrium Modelling and Apatite-in-Garnet Elastic Barometry to Unravel the P–T Path of the Micaschist Unit From the Zicavo Metamorphic Complex (Corsica, France)","authors":"Lorenzo Dulcetta,&nbsp;Xin Zhong,&nbsp;Gabriele Cruciani,&nbsp;Johannes Vrijmoed,&nbsp;Jan Pleuger,&nbsp;Marcello Franceschelli,&nbsp;Timm John","doi":"10.1111/jmg.70000","DOIUrl":"https://doi.org/10.1111/jmg.70000","url":null,"abstract":"<p>To gain insights into the pressure–temperature evolution of a scarcely studied portion of the Variscan basement of Corsica in southern Europe, we combined phase equilibrium modelling, Zr-in-rutile thermometry and apatite-in-garnet elastic barometry. We investigated Grt-St micaschist rocks containing garnet grains of several millimetres in size hosting chloritoid + rutile inclusions. Garnet was found to be wrapped by a foliated matrix made up of muscovite, staurolite, chlorite, biotite and ilmenite. Intersecting compositional isopleths for garnet core, chloritoid and muscovite yielded <i>P</i>–<i>T</i> conditions for garnet core formation of 1.05–1.35 GPa and 430°C–535°C. These conditions were further constrained by Zr-in-rutile thermometry (470°C) and apatite-in-garnet elastic barometry, which intersect at 1.10 GPa and 455°C. We interpreted these conditions to be the beginning of garnet growth coinciding with the D₁ deformation. After decompression and heating, the thermal peak was reached during the main D₂ deformation, coeval with a dextral shearing; these conditions, constrained through garnet rim and matrix mineral isopleths, were found to be at 0.5–0.9 GPa and 540°C–580°C in the staurolite stability field. After (retrograde) D₃ deformation, isothermal decompression associated with granitoid intrusion caused contact metamorphism at shallower crustal levels. Overall, the combination of this <i>P</i>–<i>T</i> path with available ones from southern Corsica and NE Sardinia allows us to hypothesize a possible new geodynamic scenario. We hypothesize that Corsica and Sardinia were on opposite sides of the Lower Palaeozoic Mid-Variscan Ocean before the Variscan Orogeny occurred, after which they welded, leading to the current configuration of the Corsica–Sardinia block.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"43 7","pages":"681-705"},"PeriodicalIF":3.4,"publicationDate":"2025-06-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.70000","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144768045","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":"Zircon (Re)crystallization During Metamorphism: Zircon Paragenesis in the Papua New Guinea Coesite Eclogite Revisited","authors":"Linus B. Streicher,&nbsp;Fraukje M. Brouwer,&nbsp;Keewook Yi,&nbsp;Suzanne L. Baldwin,&nbsp;Wim van Westrenen","doi":"10.1111/jmg.70001","DOIUrl":"https://doi.org/10.1111/jmg.70001","url":null,"abstract":"<p>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 <i>p</i> = 3.5 ± 0.2 GPa and <i>T</i> = 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.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"43 7","pages":"663-680"},"PeriodicalIF":3.4,"publicationDate":"2025-06-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.70001","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144767932","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":"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}