Journal of Metamorphic Geology最新文献

{"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}

{"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}