Journal of Metamorphic Geology最新文献

{"title":"Reaction-Driven Development of Fine-Grained Plagioclase Networks Influences the Rheology of the Crust in the Bergen Arcs, Norway","authors":"Lorena H. Filiberto,&nbsp;Håkon Austrheim,&nbsp;José Alberto Padrón-Navarta,&nbsp;Andrew Putnis,&nbsp;Christine V. Putnis","doi":"10.1111/jmg.70044","DOIUrl":"https://doi.org/10.1111/jmg.70044","url":null,"abstract":"<p>In the middle to lower continental crust, mineral reactions can play a key role in controlling rheological behaviour by generating fine-grained, mechanically weak domains. In granulite-facies rocks from Krossøy (Bergen Arcs, Norway), we document how garnet breakdown produced interconnected networks of orthopyroxene + plagioclase ± spinel that formed along grain boundaries and reaction fronts. We interpret the reaction as being initiated during early Caledonian decompression and cooling, which changed from Grenvillian high-pressure granulite- to lower pressure granulite-facies. The networks are characterized by fine-grained Ca-enriched plagioclase (relative to the surrounding coarse-grained plagioclase) and orthopyroxene symplectite (10–50 μm) surrounded by larger feldspar grains (1–2 mm). Phase-equilibrium modelling indicates that the formation of fine-grained Ca-enriched plagioclase networks occurred under lower pressure granulite-facies conditions and predates the main amphibolite-facies overprint. These reaction products form mm- to cm-scale bands that align under stress and are reactivated during amphibolite-facies conditions. Our observations show that reaction-driven grain size reduction, even when limited to 5%–10% of the rock volume, produces mechanically weak plagioclase networks that are potential sites for later strain localization. This suggests that the early formation of fine-grained reaction products can locally weaken the crust, forming distinct rheological domains and controlling where deformation and later metamorphic evolution are concentrated.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"44 6","pages":"470-491"},"PeriodicalIF":3.4,"publicationDate":"2026-07-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.70044","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"148389086","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":"Variable Sr Diffusion and Implications for Rb–Sr Biotite Geochronology","authors":"Riley Rohrer,&nbsp;Christopher L. Kirkland,&nbsp;Hugo Olierook,&nbsp;Tim Johnson,&nbsp;Katy Evans","doi":"10.1111/jmg.70040","DOIUrl":"https://doi.org/10.1111/jmg.70040","url":null,"abstract":"<p>The rubidium–strontium (Rb–Sr) system has a long pedigree as a geochronological tool that relies on the application of an isochron approach. In particular, biotite contains strongly radiogenic Rb–Sr isotopes, and thus can yield meaningful crystallisation or cooling ages via this approach. However, the response of the Rb–Sr system in biotite to multiple tectonothermal events, and interpretation of the resulting apparent ages, requires a nuanced understanding of the degree to which isotopic equilibration has been attained. Here, we investigate biotite in metapelitic rocks from the Fraser Zone, Albany–Fraser Orogen, Western Australia, a fault-bound block of Mesoproterozoic crust. Using a combination of phase equilibrium modelling, in situ laser ablation Rb–Sr analyses of biotite and U–Pb monazite dating, we show that these rocks reached peak metamorphic conditions of 830°C–875°C and 7.8–11.3 kbar at 1280 ± 5 Ma, during late Stage I of the Albany Fraser Orogeny. Rb–Sr biotite model ages, ages calculated from single analyses and a set initial ⁸⁷Sr/⁸⁶Sr_<i>i</i>, reveal a direct relationship to their petrographic setting; grains associated with quartz–K-feldspar segregations have lower ⁸⁷Sr content and younger model ages than grains within biotite- and sillimanite-rich bands. The model ages imply that biotite closed to Sr isotope exchange at different times depending on petrological context. Ages of c. 1200 Ma associated with biotite–sillimanite matrix bands likely record cooling through 300°C–400°C during Stage II of the Albany Fraser Orogeny, whereas younger, c. 1100 Ma Rb–Sr biotite ages are associated with leucosome segregations and were influenced by later Sr loss. The presence of variable apparent ages from the same fabric within single samples implies differential Sr equilibration controlled by those minerals in contact with biotite.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"44 6","pages":"417-439"},"PeriodicalIF":3.4,"publicationDate":"2026-07-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.70040","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"148389144","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":"Correction to ‘Thermal Buffering, Heat Advection and Crustal Thinning in the Ryoke Metamorphic Complex, Yanai, Southwest Japan’","authors":"","doi":"10.1111/jmg.70043","DOIUrl":"https://doi.org/10.1111/jmg.70043","url":null,"abstract":"<p>\u0000 <span>Ikeda, T.</span>, <span>Miyazaki, K.</span>, <span>Sugawara, Y.</span>, and <span>Hayashi, R.</span> <span>Thermal Buffering, Heat Advection and Crustal Thinning in the Ryoke Metamorphic Complex, Yanai, Southwest Japan</span>. <i>Journal of Metamorphic Geology</i>, <span>2026</span>; <span>44</span> (<span>4</span>) <span>287</span>–<span>310</span>.</p><p>In the Introduction section, in the first paragraph, the text ‘a relatively static heat supply’ was incorrect. This should have read as ‘heat supplied to the nearly static crust’.</p><p>Also, Figure 5 was incorrect, and the sentence “except three plagioclase-free assemblages at &gt; 810°C and &lt; 500 MPa, which are shown as ‘pl’ in red” should be removed from its caption.</p><p>The following are the correct figure and caption:</p><p>We apologize for this error.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"44 6","pages":""},"PeriodicalIF":3.4,"publicationDate":"2026-07-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.70043","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"148389141","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":"P–T Modelling Constrains the Depth of Emplacement of the Porto Azzurro Pluton and Implies Minor Exhumation Caused by the Zuccale Fault (Island of Elba, Italy)","authors":"Samuele Papeschi,&nbsp;Marco Bonini,&nbsp;Matteo Lupi,&nbsp;Chiara Del Ventisette,&nbsp;Domenico Montanari","doi":"10.1111/jmg.70041","DOIUrl":"https://doi.org/10.1111/jmg.70041","url":null,"abstract":"<p>Understanding the pressure of emplacement of granitic intrusions is crucial to understanding the exhumation history of plutons and constraining the tectonic setting of magma emplacement. However, P–T and geochronological constraints from exhumed plutons are often characterized by large uncertainties, especially in shallow crustal settings with <i>p</i> &lt; 2–3 kbar, where minimal changes in estimated pressure can drastically change the exhumation model. We show an example from a typical upper crustal intrusion, the Porto Azzurro Pluton on the Island of Elba (Italy). This pluton emplaced at ~6.5 Ma at less than 7–8-km depth and is believed to have been exhumed under a regional low-angle normal fault, known as the Zuccale Fault. Phase equilibrium modelling of garnet ± cordierite ± andalusite-bearing parageneses in hornfels shows that the metamorphic pressure registered by the contact aureole rocks is lower than previously thought, constraining the Porto Azzurro Pluton to ~3.8–4.6-km depth (<i>p</i> = 1.0–1.2 kbar). Our P–T data constrain, for the first time, the minimum and maximum metamorphic pressure of the Zuccale Fault footwall and hanging wall blocks. P estimates overlap in the footwall (<i>p</i> = 0.3–1.8 kbar) and hanging wall (<i>p</i> = 0.4–1.2 kbar) blocks of the Zuccale Fault. Moreover, both fault blocks show a similar thermal evolution, which indicates that the fault contributed little, if at all, to the exhumation of the intrusion.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"44 6","pages":"440-469"},"PeriodicalIF":3.4,"publicationDate":"2026-07-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.70041","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"148389275","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":"Two-Stage Garnet Growth and Polyphase Metamorphism in Eclogites From the Serkout Area (Central Hoggar, Algeria): Constraints From Petrology and Thermodynamic Modelling","authors":"Sid Ali Doukkari,&nbsp;Gaston Godard,&nbsp;Khadidja Ouzegane,&nbsp;Amar Arab,&nbsp;Chris Clark","doi":"10.1111/jmg.70038","DOIUrl":"https://doi.org/10.1111/jmg.70038","url":null,"abstract":"<div>\u0000 \u0000 <p>Garnet in eclogites is routinely analysed to reconstruct detailed <i>P–T</i> paths, as its chemical zoning records multiple metamorphic stages. In this aim, eclogites from the Serkout area (Aleksod terrane, Central Hoggar, Southern Algeria) have been petrologically investigated and thermodynamically modelled to trace their <i>P–T</i> history. Our study reveals that the samples have preserved the eclogite-facies paragenesis, characterised by garnet, omphacite, quartz, clinozoisite and amphibole, indicative of prograde to peak metamorphism. However, these minerals have been partially replaced during retrogression by clinopyroxene–plagioclase and amphibole–plagioclase symplectites. Coarse garnet crystals have highly heterogeneous, almandine-dominated compositions (<i>X</i>_<i>Alm</i> ranging from 0.30 to 0.56), whereas omphacite has a maximum jadeite content (<i>X</i>_<i>Jd</i>) of 0.31. Coarse amphibole, assumed to be of eclogite facies, is magnesio-hornblende with Na_M4 = 0.31–0.47. Secondary plagioclase shows a wide range of compositions (<i>X</i>_<i>An</i> = 0.20–0.99), from albite-rich in plagioclase–diopside symplectites after omphacite to anorthite-rich in plagioclase coronae around clinozoisite. The garnet grains in the Serkout eclogites display clear core–rim zoning, which, together with the evolution of mineral inclusions, allows the garnet growth to be correlated with different metamorphic events. Garnet cores formed under low-pressure, relatively high-temperature conditions (first M1 event), whereas garnet inner to outer rims developed during the prograde path from M1 to M2 stages. The late symplectites and coronas are related to retrogression (M3 stage). The combination of detailed mineralogical analysis, Ti-in-amphibole thermometry and thermodynamic modelling (<i>P–T-M</i>_<i>H2O</i> pseudosections), using THERMOCALC-345, has revealed the existence of two distinct events: (a) an older high-temperature event (718°C) most likely Paleoproterozoic in age, and (b) a younger eclogitic event characterised by (i) a prograde <i>P–T</i> path starting at 9 ± 1 kbar and 610 ± 50°C under H₂O-undersaturated conditions, progressing to peak conditions of 21.5 ± 1 kbar and 720 ± 50°C under H₂O-saturated conditions, and (ii) a retrograde path characterised by nearly isothermal decompression towards 5.5 kbar and 685°C.</p>\u0000 </div>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"44 5","pages":"355-375"},"PeriodicalIF":3.4,"publicationDate":"2026-06-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"148147201","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":"Jadeite in Continental Subduction: Its Preservation, Breakdown and Pseudomorphic Replacement in Erzgebirge UHP Gneisses","authors":"Ondrej Lexa,&nbsp;Martin Racek,&nbsp;Petr Jeřábek,&nbsp;Benjamin Paddon","doi":"10.1111/jmg.70039","DOIUrl":"https://doi.org/10.1111/jmg.70039","url":null,"abstract":"&lt;p&gt;The absence of jadeite in regions where subduction of continental crust has been documented by other high-pressure (HP) or ultrahigh-pressure (UHP) minerals is striking. This study presents only the second discovery of jadeite in the crystalline rocks of the Erzgebirge, a region known for its occurrences of microdiamond, coesite and omphacite in eclogite, felsic granulite and other quartzofeldspathic rocks involved in continental subduction. The studied quartzofeldspathic gneiss is composed of quartz, garnet, plagioclase, K-feldspar, muscovite, biotite, rutile and kyanite. Garnet occurs mainly as large porphyroblastic grains of Grt1 overgrown by thin discontinuous rim of Grt2. Jadeite occurs as inclusions in kyanite and Grt1 garnet, the latter also containing coesite. Domains up to 3 mm in size of fine-grained plagioclase-muscovite symplectite, surrounded by plagioclase-muscovite mosaic and occasionally associated with small garnet grains of Grt2, are interpreted as pseudomorphic replacement of jadeite. The pressure-temperature (\u0000&lt;span&gt;&lt;/span&gt;&lt;math&gt;\u0000 &lt;mi&gt;P&lt;/mi&gt;\u0000 &lt;mo&gt;−&lt;/mo&gt;\u0000 &lt;mi&gt;T&lt;/mi&gt;&lt;/math&gt;) evolution of the samples was reconstructed by comparing the composition and proportion of the observed mineral assemblage with pseudosections constructed in THERMOCALC. Prograde metamorphism is constrained by the presence of garnet and jadeite inclusions in kyanite. Peak \u0000&lt;span&gt;&lt;/span&gt;&lt;math&gt;\u0000 &lt;mi&gt;P&lt;/mi&gt;\u0000 &lt;mo&gt;−&lt;/mo&gt;\u0000 &lt;mi&gt;T&lt;/mi&gt;&lt;/math&gt; conditions reaching &gt; 28 kbar and \u0000&lt;span&gt;&lt;/span&gt;&lt;math&gt;\u0000 &lt;mo&gt;∼&lt;/mo&gt;&lt;/math&gt;600°C–800°C are constrained by the composition of porphyroblastic garnet Grt1 containing coesite. Early exhumation is constrained by the rim composition of Grt1 containing jadeite. Modelling results suggest that exhumation was accompanied by an influx of H&lt;sub&gt;2&lt;/sub&gt;O, leading to a significant increase in the proportion of Ti-bearing muscovite. The subsequent exhumation and cooling are associated with a second generation of garnet Grt2, which partly preceded the breakdown of jadeite into plagioclase-muscovite symplectite and intergrowths. The late-stage \u0000&lt;span&gt;&lt;/span&gt;&lt;math&gt;\u0000 &lt;mi&gt;P&lt;/mi&gt;\u0000 &lt;mo&gt;−&lt;/mo&gt;\u0000 &lt;mi&gt;T&lt;/mi&gt;&lt;/math&gt; conditions, constrained by the composition of albitic plagioclase and small garnet Grt2 grains in the pseudomorphs, are estimated at \u0000&lt;span&gt;&lt;/span&gt;&lt;math&gt;\u0000 &lt;mo&gt;∼&lt;/mo&gt;&lt;/math&gt;9–13 kbar and \u0000&lt;span&gt;&lt;/span&gt;&lt;math&gt;\u0000 &lt;mo&gt;∼&lt;/mo&gt;&lt;/math&gt;420°C–530°C. The balanced reaction of jadeite breakdown fits the observed mineral compositions and the plagioclase-muscovite proportions in the pseudomorphs. Furthermore, the reaction balance reveals that while K and Na are highly mobile, the Al/Si ratio in the former jadeite remains unchanged in the resulting plagioclase-muscovite pseudomorphs. The jadeite- and coesite-bearing rocks reported in this study broaden the known extent of UHP metamorphism in","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"44 5","pages":"376-393"},"PeriodicalIF":3.4,"publicationDate":"2026-06-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.70039","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"148147215","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":"Tracking Dehydration Reactions and Fossil Fluid Flux in Shear Zones Using Garnet Microstructures","authors":"Alessandro Petroccia,&nbsp;Francesco Giuntoli,&nbsp;Alissa Kotowski,&nbsp;Gianmarco Buono,&nbsp;Alireza Chogani,&nbsp;Eric Hellebrand,&nbsp;Lucia Pappalardo,&nbsp;Ivan Callegari","doi":"10.1111/jmg.70042","DOIUrl":"https://doi.org/10.1111/jmg.70042","url":null,"abstract":"<p>Shear zones act as preferential fluid pathways during prograde and retrograde stages of metamorphism. Nonetheless, we still have limited knowledge of the drainage and permeability of natural settings. The preserved signature of fluid in exhumed rocks provides insights into fluid flow during burial and exhumation. This study investigates fluid flow processes recorded by garnet in quartz-schists from the As Sheik shear zone (Saih Hatat window, NE Oman). We observed garnet as equant, oblate and honeycomb (i.e., skeletal) shapes that document distinct fluid-related growth stages from peak-pressure to early exhumation, both occurring at eclogite facies conditions. Garnet nucleated at 2.0–2.2 GPa and 500°C–550°C after the chloritoid-out dehydration reaction. Subsequent decompression and heating (1.3–1.5 GPa, 600°C–650°C) promoted further fluid release and honeycomb garnet growth. We infer that dissolution, transport and precipitation rates primarily influenced whether garnets grew as oblate grains (i.e., as pseudomorphs on peak-pressure chloritoid grains) or newly nucleated equant grains. Honeycomb garnet was interpreted to show the permeability network that records migration and drainage of fluid. This internally produced fluid escaped from the shear zone using pre-existing grain boundaries and reaction-formed pathways. Assuming 1 km of shear zone thickness, we computed a time-integrated fluid flux of ~34 m³ m⁻² for the entire duration of garnet growth. This study highlights garnet morphology as a tracer of transient fluid pathways during a burial–exhumation cycle of an eclogitic shear zone. The close connection between garnet morphology and fluids calls for a re-evaluation of similar microstructures in different tectonic settings.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"44 5","pages":"394-414"},"PeriodicalIF":3.4,"publicationDate":"2026-06-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.70042","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"148147167","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":"Early Palaeozoic Barrovian Metamorphism in the Northern Central Qilian Block, NW China: Structure and Evolution of a Continental Arc in the Proto-Tethys Tectonic Domain","authors":"Zhuoyang Li,&nbsp;Yilong Li,&nbsp;Xiujuan Bai,&nbsp;Huaning Qiu,&nbsp;Fraukje M. Brouwer","doi":"10.1111/jmg.70037","DOIUrl":"https://doi.org/10.1111/jmg.70037","url":null,"abstract":"<div>\u0000 \u0000 <p>Regional metamorphism within the Central Qilian Block provides critical constraints on the tectonic evolution and geodynamic processes in the Qilian orogenic system. Combined phase equilibrium modelling and multiple isotope-dating methods reveal the P–T–t evolution of the Barrovian metamorphic zones preserved in the Huangyuan Group in the northern Central Qilian Block. A metamorphic zircon U–Pb age of 466 ± 2 Ma from a paragneiss, a muscovite ⁴⁰Ar/³⁹Ar age of 434.8 ± 3.9 Ma and a biotite ⁴⁰Ar/³⁹Ar age of 365.6 ± 1.2 Ma from a micaschist and a biotite ⁴⁰Ar/³⁹Ar age of 372.4 ± 0.9 Ma from an amphibolite in the group record Palaeozoic polyphase metamorphism. Phase equilibrium modelling indicates clockwise P–T paths with peak conditions of 6.1–6.8 kbar and 575°C–625°C in the garnet zone, 6.2–7.2 kbar and 610°C–670°C in the staurolite zone and 6.0–7.3 kbar and 750°C–780°C in the sillimanite zone. The Barrovian metamorphism is inferred to result from crustal thickening and the resulting elevated geothermal gradient in the northern Central Qilian Block. A Palaeozoic thickened continental arc resulting from southward subduction of the North Qilian Ocean at ~466 Ma is proposed for the northern Central Qilian Block. Southward subduction of the North Qilian Ocean beneath the Central Qilian Block lasted until 457–428 Ma. With the waning of arc-related magmatism, a decrease in the regional geothermal gradient led to greenschist-facies to amphibolite-facies retrograde metamorphism in the Huangyuan Group at ~435 Ma. After the closure of the South Qilian Ocean, the Quanji block collided with the Hualong and Central Qilian Blocks. Post-collisional extensional collapse at 400–350 Ma affected the Central Qilian Block, resulting in hydrothermal activity in the Huangyuan Group at 372–366 Ma.</p>\u0000 </div>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"44 5","pages":"337-354"},"PeriodicalIF":3.4,"publicationDate":"2026-06-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"148147221","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":"Garnet–Clinopyroxene Double-Layered Coronae in a Metagabbronorite From the High-Grade Metamorphic Gföhl Unit, Moldanubian Zone","authors":"Rene Asenbaum,&nbsp;Tereza Nemeškalová,&nbsp;Martin Racek,&nbsp;Vojtěch Janoušek,&nbsp;Julian Portenkirchner,&nbsp;Rainer Abart","doi":"10.1111/jmg.70036","DOIUrl":"10.1111/jmg.70036","url":null,"abstract":"<p>This study investigates garnet–clinopyroxene double-layered coronae in a metagabbronorite from the high-grade metamorphic Gföhl Unit (Bohemian Massif). The coronae formed at the interfaces between relic magmatic orthopyroxene, still preserved in the cores of the coronae, and the plagioclase-rich rock matrix. They comprise an inner clinopyroxene layer and an outer garnet layer, both of which are polycrystalline. The coronae record a polyphase metamorphic evolution. The first metamorphic stage occurred under high-pressure–high-temperature (H\u0000<span></span><math>\u0000 <mi>P</mi></math>–H\u0000<span></span><math>\u0000 <mi>T</mi></math>) conditions and produced the garnet–clinopyroxene coronae. The garnet layer exhibits a pronounced overall compositional zoning, with inward-decreasing grossular and outward-decreasing pyrope contents, reflecting chemical potential gradients that drove the necessary chemical mass transfer during corona growth. Subsequent decompression led to a granulite-facies overprint, which is manifest from discontinuous layers of secondary orthopyroxene and plagioclase along the interface between the stage-I garnet and clinopyroxene layers. In addition, a secondary compositional zoning developed on the scale of individual garnet grains, which is characterised by a rimward decrease in the grossular content and a concomitant increase in the pyrope and almandine contents. The resulting complex compositional zoning of garnet allows distinguishing between the features generated during the early H\u0000<span></span><math>\u0000 <mi>P</mi></math>–H\u0000<span></span><math>\u0000 <mi>T</mi></math> evolution and those associated with decompression. Thermodynamic modelling yields pressure–temperature conditions indicating formation of the primary garnet–clinopyroxene double-layered coronae at conditions corresponding to a geothermal gradient of approximately 15°C/km, followed by isothermal decompression and, finally, by rapid cooling. The inferred conditions for the H\u0000<span></span><math>\u0000 <mi>P</mi></math>–H\u0000<span></span><math>\u0000 <mi>T</mi></math> stage likely reflect a transitional geodynamic setting involving deceleration of a subducting slab and associated thermal relaxation.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"44 4","pages":"311-336"},"PeriodicalIF":3.4,"publicationDate":"2026-04-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.70036","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"147714995","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":"Timing of Metamorphism and Deformation in the Neoarchean Quetico Metasedimentary Belt, Superior Craton, Constrained by Garnet Trace-Element Mapping and Lu–Hf Geochronology","authors":"Adrian G. Rehm,&nbsp;Antoine Godet,&nbsp;Doug Tinkham,&nbsp;Carl Guilmette,&nbsp;Matthijs A. Smit","doi":"10.1111/jmg.70034","DOIUrl":"10.1111/jmg.70034","url":null,"abstract":"<p>The Quetico subprovince of the Superior Province is among the largest metasedimentary belts globally and provides evidence of tectonic processes at play during the final stages of Archean cratonization. Although previous studies have shown that deformation and metamorphism of the 2.7-Ga turbiditic protolith resulted in regional subvertical tectonic fabrics and symmetrical N–S field gradients with the highest conditions in the east-central part of the belt, constraints on the spatiotemporal evolution of metamorphism and deformation in the Quetico belt are underdeveloped. To obtain new insights into the tectonometamorphic history of the belt, 12 samples from garnet-in to orthopyroxene-in were subjected to garnet Lu–Hf chronology and trace-element mapping. Garnet Lu–Hf ages from the garnet, staurolite–andalusite, and cordierite zones in the northwestern and southwestern Quetico overlap between 2664 and 2657 Ma, demonstrating that subsolidus metamorphism was coeval in these parts of the belt. In the eastern Quetico, ages are younger in the staurolite–andalusite, sillimanite, melt, K-feldspar, and orthopyroxene zones (2651–2643 Ma). Subsolidus garnet features Cr, V, Ti, Y, and HREE zoning showing a ghost phyllosilicate fabric that is continuous and parallel, or slightly rotated relative to the external foliation, implying syn to post-tectonic garnet growth. Suprasolidus garnet features more homogenized Cr, V, Ti, Y, and HREE distributions and higher Zr and P concentrations indicative of prograde suprasolidus growth. Prominent HREE-rich annuli around embayed garnet boundaries in the orthopyroxene zone are attributed to garnet resorption near peak temperatures in the belt. These observations indicate a single, protracted tectonothermal event that extended across the Quetico, rather than a series of discrete events involving syn-tectonic medium-<i>P</i>/<i>T</i> metamorphism followed by low-<i>P</i>/<i>T</i> overprinting and plutonism. The data are interpreted to record melt-enhanced advective heat transport and exhumation within the upper crust during transpressive shortening, characteristic of ultra-hot orogens (UHO). East–west diachroneity in garnet growth and differences in peak metamorphic grade indicate two loci where the sequence of heating and extrusion occurred at different times. This model provides a testable hypothesis for the tectonic evolution of UHO belts in Archean terranes.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"44 4","pages":"265-286"},"PeriodicalIF":3.4,"publicationDate":"2026-04-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.70034","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"147714983","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}