Journal of Metamorphic Geology最新文献

{"title":"Metamorphic Reactivity Influences Amphibole Creep Behaviour","authors":"Alix Osinchuk,&nbsp;Brendan V. Dyck,&nbsp;Jesse B. Walters","doi":"10.1111/jmg.70051","DOIUrl":"https://doi.org/10.1111/jmg.70051","url":null,"abstract":"<p>The rheological behaviour of solid-solution minerals may play an important role in determining how metamorphic crust accommodates deformation. Whether or not a mineral's deformation is diffusion or dislocation-mediated depends on a range of extrinsic variables such as stress, strain rate and temperature, as well as intrinsic variables such as diffusivity and water activity. However, it remains unknown if the local gradients in chemical potential that develop between metamorphic phases with evolving pressures and temperatures can also impact the deformation mechanics of metamorphic rocks. Here, we pair electron-backscatter diffraction (EBSD) data with phase equilibria modelling of high-variance, low-variance and compositionally layered blueschist mineral assemblages to demonstrate that the local chemical driving force may directly influence the relative prevalence of dislocation creep and diffusion creep in amphibole. In high-variance assemblages with glaucophanite domains (<i>F</i> = 7), glaucophane displays subgrain boundaries with a consistent misorientation axes distribution indicative of dislocation creep. In domains with rare (meta stable) omphacite, fewer subgrains are observed in glaucophane and the distribution of subgrain boundary misorientation axes is more diffuse. Whereas in neighbouring rocks with low-variance assemblages (<i>F</i> = 1–3), glaucophane subgrain boundaries are rare and the distribution of low-angle misorientation axes is random. The amphibole microstructures in the low-variance assemblages are consistent with diffusion creep having been the dominant deformation mechanism. The increased reactivity associated with low-variance assemblages enhances the chemical driving force for intracrystalline diffusion, thereby accelerating mass transfer and favouring diffusion-mediated deformation. In contrast, higher variance assemblages exhibit a reduced contribution from chemically driven diffusion and accommodate strain primarily by dislocation creep.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"44 8","pages":"611-632"},"PeriodicalIF":3.4,"publicationDate":"2026-09-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.70051","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"148878526","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":"Phase Relation Modelling of Mg-Metasomatism: Application to a Granodiorite From the Tauern Window (Central Alps, Italy)","authors":"Manon Amiot,&nbsp;Philippe Goncalves,&nbsp;Flavien Choulet,&nbsp;Guillaume Siron","doi":"10.1111/jmg.70052","DOIUrl":"https://doi.org/10.1111/jmg.70052","url":null,"abstract":"<div>\u0000 \u0000 <p>Fluid-assisted Mg metasomatism profoundly alters crustal rocks, producing phyllosilicate-rich assemblages, with major implications for rheology as well as petrophysical properties such as permeability. However, the effects such processes remain poorly constrained due to limitations in our ability to model the sequence of mineral reactions and bulk composition changes during fluid–rock interactions. Here we address this challenge through a phase equilibrium analysis of Mg–Ca/Na–Si metasomatism of a granodiorite infiltrated by an ultramafic-derived fluid, using chemical potential phase projections and sections of mobile elements. We investigated a shear zone, in a granodiorite located in the Zillertal massif (Neves area, Tauern window in Eastern Alps, Italy), where Alpine amphibolite-facies metasomatism and deformation (550°C—0.55 GPa) transformed the late-Variscan granodiorite into amphibole-bearing schists (garbenschiefer) and ultimately into a chlorite–muscovite schist. Mass balance calculations show strong MgO enrichment (up to 370%) and H₂O gain, coupled with substantial CaO, Na₂O and SiO₂ loss (−92%, −86% and −13%, respectively). Chemical potential phase diagrams of these most mobile elements computed at constant pressure and temperature demonstrate that amphibole is a transient phase stabilized by MgO and CaO gain at the expense of quartz, before it breaks down into a CaO-free chlorite–muscovite assemblage. Although absent in the studied area, our modelling indicates that greater MgO gain and SiO₂ loss could stabilize that talc-bearing assemblage: the so-called whiteschist in the Alps. We infer that the metasomatizing fluid is externally derived from the dehydration of nearby ultramafic bodies, consistent with processes described in the Greiner shear zone, located to the north of the studied area. Finally, we demonstrate that our modelling approach can be applied to model metasomatic zones at contacts between chemically contrasting lithologies, such as ultramafic and sediments, like in subduction mélange zones.</p>\u0000 </div>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"44 8","pages":"633-661"},"PeriodicalIF":3.4,"publicationDate":"2026-09-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"148878245","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":"Quantifying Melting Reactions in Granitic Veins in Eclogite Exhumed From Ultrahigh-Pressure","authors":"Peng Feng,&nbsp;Lu Wang,&nbsp;Michael Brown,&nbsp;Tim E. Johnson","doi":"10.1111/jmg.70048","DOIUrl":"https://doi.org/10.1111/jmg.70048","url":null,"abstract":"<div>\u0000 \u0000 <p>Recent studies have suggested various mechanisms to generate melt during exhumation of deeply subducted crust, including dehydroxylation of omphacite, fluid-absent omphacite- and/or phengite-breakdown melting and fluid-present melting. However, the space–time relationships among these mechanisms remain uncertain. Here, we report microstructural evidence of the reactions responsible for, and compositions of, low-volume melts frozen in situ (as leucosome pockets) within weakly deformed granitic veins that cross-cut foliated UHP eclogite boudins in gneisses from the central Sulu belt, China. Phengite in the granitic veins records crystallization pressures of 3.4–2.7 GPa, and the granitic veins have whole-rock Sr–Nd isotope compositions and trace element patterns consistent with derivation from the eclogite. The granitic veins likely crystallized from a solute-rich supercritical fluid or hydrous melt generated by dehydroxylation of nominally anhydrous minerals during the early stage of exhumation (decompression). Subsequently, leucosome pockets and grain-boundary films of melt formed in the granitic veins by the successive breakdown of omphacite and phengite. Based on the mineral modes and chemical compositions of 55 leucosome pockets (cf. microgranitoids), we distinguish those with (1) high Na/K ratios mainly composed of plagioclase and euhedral amphibole with skeletal omphacite and (2) low Na/K ratios predominantly composed of K-feldspar and plagioclase and which contain phengite with corroded margins and fine-grained biotite. High Na/K leucosome pockets (HLPs) are consistent with a melting reaction involving mostly omphacite-breakdown, whereas low Na/K leucosome pockets (LLPs) are inferred to have formed by a reaction consuming variable proportions of phengite and omphacite. We argue that the reactions to form HHP then LLP initiated at approximately 1.5 and 1.1 GPa, respectively, and document two different mechanisms to generate melt during exhumation of deeply subducted crust in which breakdown of omphacite occurs before (deeper than) phengite. At higher temperatures, these melt-producing reactions would generate a larger volume of melt that potentially could facilitate exhumation and increase crust–mantle interactions, thereby increasing the compositional heterogeneity of orogenic mantle.</p>\u0000 </div>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"44 8","pages":"569-584"},"PeriodicalIF":3.4,"publicationDate":"2026-09-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"148878455","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":"(Ultra)high-Pressure Metamorphism in Sedimentary Versus Basaltic Rocks: Insights From a Variscan Mélange in the South Carpathians","authors":"Otavio S. G. Silva,&nbsp;Bruno V. Ribeiro,&nbsp;Gavril Săbău,&nbsp;Fabricio Caxito,&nbsp;Paulo A. de Paiva-Silva,&nbsp;Pierre Lanari,&nbsp;George Luvizotto,&nbsp;Mahyra Tedeschi","doi":"10.1111/jmg.70049","DOIUrl":"https://doi.org/10.1111/jmg.70049","url":null,"abstract":"<p>Reconstructing the pressure–temperature–time (<i>P–T–t</i>) evolution of (ultra)high-pressure rocks is essential for constraining subduction and collisional dynamics. Subduction zones are often investigated using basic eclogites, whereas nonbasic high-pressure rocks remain comparatively understudied, potentially hindering a comprehensive understanding of complex subduction systems. The Bughea Complex is a modern-style mélange within the pre-Alpine basement of the South Carpathians (Romania) that hosts a wide variety of eclogitic rocks, providing an opportunity to investigate their response to subduction processes. Here, we integrate phase equilibrium modelling, rutile thermometry, zircon and apatite U–Pb dating coupled with trace element analysis and garnet Lu-Hf dating to reconstruct the <i>P–T–t</i> evolution of three different eclogitic rocks from the Bughea Complex: a kyanite eclogite, a basic eclogite and an eclogitic schist. The kyanite eclogite is interpreted to derive from a continental sedimentary protolith based on its continental crust-like trace-element patterns and detrital-like zircon U–Pb age spectrum (ranging from Archean to Cambrian). It records the highest peak pressure conditions (~3.2–3.6 GPa at 700°C–820°C), consistent with ultrahigh-pressure metamorphism, followed by decompression and cooling to &lt; 1.6 GPa and &lt; 675°C. In contrast, the basic eclogite, interpreted to derive from altered oceanic crust based on its MORB-like trace-element patterns, and the eclogitic schist, which exhibits trace-element signatures similar to trench sediments in volcanic-arc settings, record lower peak pressures (~2.0–2.3 GPa at 580°C –610°C and ~1.9–2.4 GPa at 530°C–600°C, respectively). The former records near-isothermal decompression (~1.3 GPa at 585°C–615°C), whereas the latter records decompression and minimal heating (~1.0–1.2 GPa at 600°C–615°C). Distinct garnet Lu–Hf ages (469 ± 96 to 345 ± 21 Ma) suggest diachronous metamorphism during a prolonged Variscan subduction, whereas apatite U–Pb age (ca. 340 Ma) likely records late-stage cooling and exhumation. Differences in <i>P–T–t</i> records may reflect mélange rheology and variations in subduction dynamics, whereas limited heating during retrogression and compositional buoyancy appear critical for preserving peak conditions, particularly in nonbasic rocks. Overall, these findings demonstrate that continental-derived metasedimentary rocks can preserve evidence of extreme pressure conditions not always recorded by associated basic eclogites, highlighting the importance of integrating nonbasic high-pressure rocks into <i>P</i>–<i>T</i>–<i>t</i> reconstructions of subduction-related mélanges to achieve a more complete understanding of complex collisional systems.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"44 8","pages":"585-610"},"PeriodicalIF":3.4,"publicationDate":"2026-09-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.70049","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"148878467","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 Preservation of Intragranular and Intergranular Coesite in Metasedimentary Rocks: Insights From a TEM-Based Structure Analysis","authors":"Jinyu Zheng,&nbsp;Tao Chen,&nbsp;Junfeng Zhang,&nbsp;Penglei Liu,&nbsp;Wenlong Liu,&nbsp;Feng Shi,&nbsp;Mingxing Gong,&nbsp;Zhe Gong","doi":"10.1111/jmg.70046","DOIUrl":"https://doi.org/10.1111/jmg.70046","url":null,"abstract":"<div>\u0000 \u0000 <p>Coesite is a key indicator mineral of ultrahigh-pressure (UHP) metamorphism. A “dry” environment is widely recognized as a prerequisite for coesite preservation during exhumation. However, the deformation behaviour of coesite, its retrograde replacement by quartz and their host and surrounding minerals remain poorly constrained at the micro- to nanoscale, especially for coesite included in or surrounded by nonrigid minerals. Here, we employed electron backscatter diffraction (EBSD), focused ion beam (FIB) and transmission electron microscopy (TEM) to characterize the micro-nanostructure of intragranular coesite (within dolomite) and intergranular coesite (surrounded by dolomite and apatite) in a UHP metasedimentary rock from the Ganjialing area, Dabie Shan region in east-central China. Our observations reveal that both intragranular and intergranular coesite grains are essentially undeformed and dislocation-free, whereas the palisade quartz rim enveloping coesite exhibits abundant dislocations and tangles. Based on the dislocation densities within these quartz rims, the differential stress is estimated at 347–663 MPa for intragranular coesite and 320–741 MPa for intergranular coesite grains, respectively. Combining this differential stress estimate with the regional P–T–t path constrains the corresponding metamorphic conditions to 715°C–720°C and 1.7–2.0 GPa. Furthermore, dolomite and apatite domains adjacent to coesite show higher dislocation densities than those distal to coesite. Collectively, the palisade quartz rim, dolomite and apatite form a composite buffer zone that not only maintains a closed, dry system but also accommodates the high differential stress generated by phase transformation. This buffer system effectively suppresses the replacement of coesite by quartz during decompression. Our findings demonstrate that transformation-induced differential stress plays a pivotal role in the preservation of coesite within the mechanically weak, nonrigid matrix of UHP metasedimentary rocks during exhumation.</p>\u0000 </div>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"44 7","pages":"493-509"},"PeriodicalIF":3.4,"publicationDate":"2026-08-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"148662069","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":"Subduction Initiation and Early Subduction Zone Development: Insights From the P–T–t Path of the Bay of Islands Metamorphic Sole","authors":"François Fournier-Roy,&nbsp;Carl Guilmette,&nbsp;Mark Coleman,&nbsp;Kyle Larson,&nbsp;Douglas K. Tinkham,&nbsp;Alana M. Hinchey,&nbsp;John Wakabayashi,&nbsp;Jean H. Bédard,&nbsp;Louise Mérit,&nbsp;Philippe Agard,&nbsp;Loïc Labrousse,&nbsp;Antoine Godet","doi":"10.1111/jmg.70045","DOIUrl":"https://doi.org/10.1111/jmg.70045","url":null,"abstract":"<p>The processes involved during subduction initiation (SI) remain poorly constrained, in part due to the rarity of geological archives. Ophiolites and their underlying metamorphic soles are thought to form during subduction zone infancy and may thus provide critical constraints. Metamorphic soles are derived from oceanic crust of the lower plate accreted to the base of the upper plate following SI and are exhumed during upper plate extension and ophiolitic crust generation. Their metamorphic evolution therefore provides insights into the thermal conditions at the nascent subduction plate interface and the trajectory of the lower plate during subduction infancy. In this study, we revisit the Bay of Islands complex (BOIC) metamorphic sole in the Newfoundland Appalachian Orogen and constrain P–T–t paths using phase equilibria modelling, melt reintegration and in situ U–Pb titanite geochronology. Our findings reveal generic SI and early subduction processes that align with geochronologic and metamorphic constraints reported for other soles as well as with the proposed early development of some modern subduction zones. The BOIC sole exhibits an inverted metamorphic gradient, with granulite and amphibolite facies metabasites at upper and middle structural levels, and greenschist facies metasedimentary and meta-igneous rocks in the lowest level. Our results are consistent with burial of garnet–clinopyroxene–amphibolite sole rocks to peak pressure conditions of ~14 kbar and 750°C–850°C, followed by decompression to peak temperature conditions of 880°C–940°C and 10.5–12 kbar. The immediately underlying plagioclase–hornblende amphibolites equilibrated at ~740°C and ~8.5 kbar. Sole metamorphism produced melts and fluids that may have contributed to arc magmatism during early subduction. Titanite geochronology of granulite facies samples yields dates of 512 ± 11 Ma and 501 ± 11 Ma, interpreted to constrain prograde metamorphism. Our results indicate that the sole formed along a clockwise P–T–t path, with initial burial occurring along a relatively cold 18°C/km metamorphic gradient, followed by heating of the subduction interface during decompression and eventually exhumation to a 26°C–27°C/km gradient. Peak temperature conditions in the sole were coeval with <i>c</i>. 484–488 Ma ophiolitic crust formation. We propose that subduction initiated from far-field forces and evolved to a self-sustained state as the slab pull force increased. Upwelling of hot asthenospheric mantle during rollback can explain decompressional heating and ensuing exhumation along an increasing thermal gradient, coeval with upper plate extension.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"44 7","pages":"510-536"},"PeriodicalIF":3.4,"publicationDate":"2026-08-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.70045","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"148662161","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":"Subduction Initiation and Early Subduction Zone Development: Insights From the P–T–t Path of the Bay of Islands Metamorphic Sole","authors":"François Fournier-Roy,&nbsp;Carl Guilmette,&nbsp;Mark Coleman,&nbsp;Kyle Larson,&nbsp;Douglas K. Tinkham,&nbsp;Alana M. Hinchey,&nbsp;John Wakabayashi,&nbsp;Jean H. Bédard,&nbsp;Louise Mérit,&nbsp;Philippe Agard,&nbsp;Loïc Labrousse,&nbsp;Antoine Godet","doi":"10.1111/jmg.70045","DOIUrl":"https://doi.org/10.1111/jmg.70045","url":null,"abstract":"<p>The processes involved during subduction initiation (SI) remain poorly constrained, in part due to the rarity of geological archives. Ophiolites and their underlying metamorphic soles are thought to form during subduction zone infancy and may thus provide critical constraints. Metamorphic soles are derived from oceanic crust of the lower plate accreted to the base of the upper plate following SI and are exhumed during upper plate extension and ophiolitic crust generation. Their metamorphic evolution therefore provides insights into the thermal conditions at the nascent subduction plate interface and the trajectory of the lower plate during subduction infancy. In this study, we revisit the Bay of Islands complex (BOIC) metamorphic sole in the Newfoundland Appalachian Orogen and constrain P–T–t paths using phase equilibria modelling, melt reintegration and in situ U–Pb titanite geochronology. Our findings reveal generic SI and early subduction processes that align with geochronologic and metamorphic constraints reported for other soles as well as with the proposed early development of some modern subduction zones. The BOIC sole exhibits an inverted metamorphic gradient, with granulite and amphibolite facies metabasites at upper and middle structural levels, and greenschist facies metasedimentary and meta-igneous rocks in the lowest level. Our results are consistent with burial of garnet–clinopyroxene–amphibolite sole rocks to peak pressure conditions of ~14 kbar and 750°C–850°C, followed by decompression to peak temperature conditions of 880°C–940°C and 10.5–12 kbar. The immediately underlying plagioclase–hornblende amphibolites equilibrated at ~740°C and ~8.5 kbar. Sole metamorphism produced melts and fluids that may have contributed to arc magmatism during early subduction. Titanite geochronology of granulite facies samples yields dates of 512 ± 11 Ma and 501 ± 11 Ma, interpreted to constrain prograde metamorphism. Our results indicate that the sole formed along a clockwise P–T–t path, with initial burial occurring along a relatively cold 18°C/km metamorphic gradient, followed by heating of the subduction interface during decompression and eventually exhumation to a 26°C–27°C/km gradient. Peak temperature conditions in the sole were coeval with <i>c</i>. 484–488 Ma ophiolitic crust formation. We propose that subduction initiated from far-field forces and evolved to a self-sustained state as the slab pull force increased. Upwelling of hot asthenospheric mantle during rollback can explain decompressional heating and ensuing exhumation along an increasing thermal gradient, coeval with upper plate extension.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"44 7","pages":"510-536"},"PeriodicalIF":3.4,"publicationDate":"2026-08-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.70045","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"148662162","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":"Carboniferous to Permian Tectonic Transition of the North Tianshan Belt (NW China): Insights From an Inverted HT-LP Metamorphic Sequence in the Harlik Range","authors":"Xinghua Ni,&nbsp;Bo Wang,&nbsp;Mathieu Soret,&nbsp;Dominique Cluzel,&nbsp;Yan Chen,&nbsp;Tingting Cao,&nbsp;Chengzao Jia,&nbsp;Dong Jia,&nbsp;Michel Faure,&nbsp;Jiashuo Liu,&nbsp;Yuxin Sun,&nbsp;Yingying Li,&nbsp;Mohamed Abu Anbar","doi":"10.1111/jmg.70047","DOIUrl":"https://doi.org/10.1111/jmg.70047","url":null,"abstract":"<div>\u0000 \u0000 <p>The North Tianshan Belt in the southwestern Central Asian Orogenic Belt (CAOB) provides unique insights into crustal evolution and geodynamic processes of accretionary orogeny. Here, we present new structural, thermobarometric and geochronological data from the Xiaopu Metamorphic Complex (XMC) in the eastern North Tianshan. Structural analysis indicates that an initial shortening phase (D1), which produced NW-SE-striking folds, was followed by a sinistral transtension (D2). The latter is characterized by syn-kinematic crystallization of peak mineral assemblages, suggesting an inverted Buchan-type metamorphic sequence. Subsequent strike-perpendicular extension (D3) likely facilitated the exhumation of the XMC. Phase modelling and geothermobarometry show a progressive increase in peak temperature from the andalusite–staurolite zone (~580°C) to the sillimanite–migmatite zone (~690°C) at nearly constant pressure (~4 kbar), indicating a high-temperature and low-pressure (HT-LP) thermal regime consistent with an extensional setting. U–Pb dating on zircon, monazite, and apatite indicates that D2 occurred at ~330–310 Ma, coeval with the HT-LP metamorphism (320–310 Ma), and that D3 occurred at ~285 Ma. Biotite ⁴⁰Ar/³⁹Ar dating confirms syn-/post-D3 cooling (298–275 Ma). We suggest that magmatic intrusion controlled by transtension at the mid-crustal level and asymmetric exhumation likely formed the inverted Buchan-type metamorphic sequence. Our new results, together with regional data, reveal the late Palaeozoic tectono-metamorphic evolution of the southwestern CAOB, involving arc–arc accretion, back-arc transtension, and post-orogenic extension. These processes governed the tectonic transition from shortening to extension in the mid-crust of arc systems and induced magmatic intrusion and crustal exhumation during the final stages of accretionary orogeny.</p>\u0000 </div>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"44 7","pages":"537-568"},"PeriodicalIF":3.4,"publicationDate":"2026-08-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"148662661","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":"Carboniferous to Permian Tectonic Transition of the North Tianshan Belt (NW China): Insights From an Inverted HT-LP Metamorphic Sequence in the Harlik Range","authors":"Xinghua Ni,&nbsp;Bo Wang,&nbsp;Mathieu Soret,&nbsp;Dominique Cluzel,&nbsp;Yan Chen,&nbsp;Tingting Cao,&nbsp;Chengzao Jia,&nbsp;Dong Jia,&nbsp;Michel Faure,&nbsp;Jiashuo Liu,&nbsp;Yuxin Sun,&nbsp;Yingying Li,&nbsp;Mohamed Abu Anbar","doi":"10.1111/jmg.70047","DOIUrl":"https://doi.org/10.1111/jmg.70047","url":null,"abstract":"<div>\u0000 \u0000 <p>The North Tianshan Belt in the southwestern Central Asian Orogenic Belt (CAOB) provides unique insights into crustal evolution and geodynamic processes of accretionary orogeny. Here, we present new structural, thermobarometric and geochronological data from the Xiaopu Metamorphic Complex (XMC) in the eastern North Tianshan. Structural analysis indicates that an initial shortening phase (D1), which produced NW-SE-striking folds, was followed by a sinistral transtension (D2). The latter is characterized by syn-kinematic crystallization of peak mineral assemblages, suggesting an inverted Buchan-type metamorphic sequence. Subsequent strike-perpendicular extension (D3) likely facilitated the exhumation of the XMC. Phase modelling and geothermobarometry show a progressive increase in peak temperature from the andalusite–staurolite zone (~580°C) to the sillimanite–migmatite zone (~690°C) at nearly constant pressure (~4 kbar), indicating a high-temperature and low-pressure (HT-LP) thermal regime consistent with an extensional setting. U–Pb dating on zircon, monazite, and apatite indicates that D2 occurred at ~330–310 Ma, coeval with the HT-LP metamorphism (320–310 Ma), and that D3 occurred at ~285 Ma. Biotite ⁴⁰Ar/³⁹Ar dating confirms syn-/post-D3 cooling (298–275 Ma). We suggest that magmatic intrusion controlled by transtension at the mid-crustal level and asymmetric exhumation likely formed the inverted Buchan-type metamorphic sequence. Our new results, together with regional data, reveal the late Palaeozoic tectono-metamorphic evolution of the southwestern CAOB, involving arc–arc accretion, back-arc transtension, and post-orogenic extension. These processes governed the tectonic transition from shortening to extension in the mid-crust of arc systems and induced magmatic intrusion and crustal exhumation during the final stages of accretionary orogeny.</p>\u0000 </div>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"44 7","pages":"537-568"},"PeriodicalIF":3.4,"publicationDate":"2026-08-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"148662699","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":"The Preservation of Intragranular and Intergranular Coesite in Metasedimentary Rocks: Insights From a TEM-Based Structure Analysis","authors":"Jinyu Zheng,&nbsp;Tao Chen,&nbsp;Junfeng Zhang,&nbsp;Penglei Liu,&nbsp;Wenlong Liu,&nbsp;Feng Shi,&nbsp;Mingxing Gong,&nbsp;Zhe Gong","doi":"10.1111/jmg.70046","DOIUrl":"https://doi.org/10.1111/jmg.70046","url":null,"abstract":"<div>\u0000 \u0000 <p>Coesite is a key indicator mineral of ultrahigh-pressure (UHP) metamorphism. A “dry” environment is widely recognized as a prerequisite for coesite preservation during exhumation. However, the deformation behaviour of coesite, its retrograde replacement by quartz and their host and surrounding minerals remain poorly constrained at the micro- to nanoscale, especially for coesite included in or surrounded by nonrigid minerals. Here, we employed electron backscatter diffraction (EBSD), focused ion beam (FIB) and transmission electron microscopy (TEM) to characterize the micro-nanostructure of intragranular coesite (within dolomite) and intergranular coesite (surrounded by dolomite and apatite) in a UHP metasedimentary rock from the Ganjialing area, Dabie Shan region in east-central China. Our observations reveal that both intragranular and intergranular coesite grains are essentially undeformed and dislocation-free, whereas the palisade quartz rim enveloping coesite exhibits abundant dislocations and tangles. Based on the dislocation densities within these quartz rims, the differential stress is estimated at 347–663 MPa for intragranular coesite and 320–741 MPa for intergranular coesite grains, respectively. Combining this differential stress estimate with the regional P–T–t path constrains the corresponding metamorphic conditions to 715°C–720°C and 1.7–2.0 GPa. Furthermore, dolomite and apatite domains adjacent to coesite show higher dislocation densities than those distal to coesite. Collectively, the palisade quartz rim, dolomite and apatite form a composite buffer zone that not only maintains a closed, dry system but also accommodates the high differential stress generated by phase transformation. This buffer system effectively suppresses the replacement of coesite by quartz during decompression. Our findings demonstrate that transformation-induced differential stress plays a pivotal role in the preservation of coesite within the mechanically weak, nonrigid matrix of UHP metasedimentary rocks during exhumation.</p>\u0000 </div>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"44 7","pages":"493-509"},"PeriodicalIF":3.4,"publicationDate":"2026-08-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"148662064","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}