Journal of Metamorphic Geology最新文献

{"title":"Kinetics and pulses of zircon growth in migmatites beneath a volcanic arc: An example from the high-T Ryoke Complex, southwest Japan","authors":"Kazuhiro Miyazaki,&nbsp;Takeshi Ikeda,&nbsp;Hideki Iwano,&nbsp;Takafumi Hirata,&nbsp;Tohru Danhara","doi":"10.1111/jmg.12711","DOIUrl":"10.1111/jmg.12711","url":null,"abstract":"<p>We present an analysis of kinetics and pulses of zircon growth in migmatites formed at middle to lower crustal depths beneath a volcanic arc. Migmatites in high-<i>T</i> metamorphic complexes at active continental margins, such as in the Ryoke Complex of southwest Japan, are thought to have been produced beneath volcanic arcs. Thermal models suggest that melt advection supplies the heat to form such high-<i>T</i> complexes. We found that zircons in the migmatites of the Ryoke Complex grew in multiple discrete stages by rapid diffusion-controlled growth. The individual growth pulses can be distinguished using a Gaussian mixture model when the duration of each growth pulse is shorter than 1σ of the analytical error of the zircon age dating, and where the interval between each growth pulse is larger than 2σ of them. This method allows extraction of the growth pulses even when the zircon exhibits incomplete textural evidence for multiple stages of growth. Application of the method to the Ryoke Complex revealed three and four pulses of zircon growth with 3–10 Myr intervals for two migmatite samples respectively in the Mikawa area and three pulses with 2–4 Myr intervals from one migmatite sample in the Yanai area. The detected zircon growth pulses are consistent with previously reported pulses of plutonic activity in the two areas, with the exception of the oldest growth pulse in the Mikawa area. Therefore, the growth pulses are interpreted to be the result of thermal pulses because of melt flux events at lower to middle crustal levels. The inferred intervals of pulsed melt fluxes are similar to those of individual caldera formation in coeval caldera clusters at the surface.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"41 5","pages":"639-664"},"PeriodicalIF":3.4,"publicationDate":"2023-01-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"49305060","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":"Repeated Caledonian burial and ultrafast cooling and exhumation of high-pressure granulite facies rocks from the Blåhø Nappe on the island of Fjørtoft, Western Gneiss Region, Norway","authors":"Penglei Liu,&nbsp;Hans-Joachim Massonne","doi":"10.1111/jmg.12710","DOIUrl":"10.1111/jmg.12710","url":null,"abstract":"<p>The allochthonous Blåhø Nappe in the Nordøyane ultra high pressure (UHP) domain, Western Gneiss Region in Norway, acts as a window to examine geological processes occurring in continent–continent collisional zones, but many aspects regarding its tectonometamorphic evolution remain debated and elusive. In this contribution, an integrated study including major- and trace-element zoning in garnet, phase equilibrium modelling and the simulation of cation diffusion in garnet was conducted on two high-pressure (HP) granulite facies rocks from the Blåhø Nappe on the island of Fjørtoft. The results shed new light on the complex geodynamic processes that act in continent–continent collisional zones and finally shape collisional orogens. Phengite, biotite, amphibole, zoisite-allanite and low-Zr rutile enclosed in garnet likely attest to a prograde eclogite facies metamorphism for the studied rocks. Pressure–temperature (P–T) conditions of ~1.5–1.6 GPa and 615–670°C were retrieved for this stage. An extensive re-equilibration under peak HP granulite facies conditions of ~1.5 GPa and 925 ± 50°C followed. Subsequently, the rocks were cooled and reburied to eclogite facies conditions of ~1.8–1.9 GPa and 805–825°C. This was followed by a final stage of decompression and cooling to amphibolite facies conditions of ~650–780°C and 0.5–1.0 GPa. Cooling and exhumation rates of &gt;400°C/Ma and &gt;75 km/Ma, respectively, indicating an ultrafast temperature and pressure decrease are estimated for this stage from simulations of cation diffusion in garnet. The anticlockwise P–T path obtained here is relatively complete and compatible with a repeated burial history during the Caledonian orogeny but not with UHP conditions proposed for the Blåhø Nappe. Our model proposes that the rocks later forming the Blåhø Nappe were buried to lower crustal depths of approximately 55 km equating to a geothermal gradient of ~13°C/km during the early Caledonian orogeny. Subsequent heating of these rocks to HP granulite facies conditions was likely driven by slab break-off and hot mantle upwelling. Baltica underthrusting during the Scandian continent–continent collision cooled and transported the Blåhø Nappe to greater depths. The obtained cooling and exhumation rates indicate ultrafast exhumation, presumably in an exhumation channel.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"41 5","pages":"603-638"},"PeriodicalIF":3.4,"publicationDate":"2023-01-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44204641","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":"Pattern and source of unusually high-temperature metamorphism in an Eocene forearc recorded by the Pacific Rim Terrane, British Columbia, Canada","authors":"Alexander C. Geen,&nbsp;Dante Canil","doi":"10.1111/jmg.12709","DOIUrl":"10.1111/jmg.12709","url":null,"abstract":"<p>The Pacific Rim Terrane is of forearc affinity and one of the most recent crustal elements accreted to the North American Cordillera in western Canada. Two units, the Leech River Complex and Pandora Peak Unit, within the terrane were subject to high-temperature, medium-pressure metamorphism. Biotite, garnet and staurolite isograds occur concentrically in the Leech River Complex, centred on the Leech River shear zone at its southern boundary. A local thermal overprint in the Pandora Peak Unit is characterized by replacement of prehnite-pumpellyite and lawsonite-bearing assemblages with muscovite + chlorite. Pseudosection models (Perple_X), and thermometry using garnet-biotite Fe-Mg exchange and Raman spectroscopy of carbonaceous material (RSCM) show a thermal gradient at ~3.8 kbar from ~230°C in the north to ~600°C in the south. Isotherms are continuous across the Leech River–Pandora Peak boundary. The small-volume, interfoliated intrusions of Eocene age occurring throughout the terrane show no spatial relation to the isotherms. Elevated forearc metamorphism is due to the subcretion at ~51 Ma of nascent oceanic crust (and related spreading ridge or hotspot) of the underlying Siletz-Crescent terrane along the south-bounding Leech River shear zone. Our re-evaluation of the metamorphic history requires revision of the role of magmatism as a source of heat transport in forearc metamorphism and the tectonic assembly in this setting.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"41 4","pages":"583-602"},"PeriodicalIF":3.4,"publicationDate":"2023-01-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45091123","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":"Protracted eclogite-facies metamorphism of the Dulan area, North Qaidam ultrahigh-pressure terrane: Insights on zircon growth during continental subduction and collision","authors":"David Hernández-Uribe,&nbsp;Chris G. Mattinson,&nbsp;Megan E. Regel,&nbsp;Jianxin Zhang,&nbsp;Katie A. Stubbs,&nbsp;Andrew R. C. Kylander-Clark","doi":"10.1111/jmg.12708","DOIUrl":"10.1111/jmg.12708","url":null,"abstract":"<p>Continental subduction and collision are recorded by ultrahigh-pressure (UHP) terranes; UHP terranes that form at early stages of an orogeny tend to be small and experience short residence at eclogite-facies depths, whereas terranes that form at mature stages of an orogeny tend to be larger and experience longer residence at these depths, but accurately determining eclogite-facies residence time requires a large geochronologic dataset tied to metamorphic conditions (via trace elements and/or inclusions). In the Dulan area, North Qaidam UHP terrane, China, it remains unclear whether the terrane experienced a long residence at eclogite-facies depths, marking the mature stage of an orogeny or two distinct (ultra)high pressure ([U]HP) events (with short residence times), interpreted as the transition from oceanic subduction to continental collision, where one (U)HP event is related to the former and second (U)HP event to the latter. To address this issue, we report new zircon U–Pb ages and trace-element data from eclogite and host paragneiss from the Dulan area and show that this terrane records ~42 Myr of eclogite-facies metamorphism at (U)HP conditions, similar to other large UHP terranes. Zircon from 11 eclogite and 2 gneiss samples yields weighted mean ages of 463–425 Ma, flat heavy rare earth element (HREE) patterns without negative Eu anomalies, and eclogitic mineral inclusions, indicating eclogite-facies conditions. Paragneiss metamorphic ages overlap with ages from eclogite but are generally younger, suggesting that a lack of internally generated fluids may have inhibited zircon growth and/or recrystallization until early decompression and white mica consumption in felsic gneiss generated fluids; thus, we interpret that these felsic rocks record the later stages of continental collision. Dataset patterns from all new and previously published analyses for the Dulan area (34 eclogite and 14 gneiss) suggest that metamorphic zircon in eclogite records prograde, peak and possibly early retrograde conditions, in contrast to the prediction from mass balance models that metamorphic zircon should only grow during exhumation and cooling. We reconcile our observations with these model predictions by recognizing that differential solubility can lead to grain-scale zircon growth or recrystallization over a large segment of the pressure–temperature (<i>P–T</i>) path even where zircon abundance decreases at the whole-rock scale.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"41 4","pages":"557-581"},"PeriodicalIF":3.4,"publicationDate":"2023-01-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45285177","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":"Metamorphic and chronological constraints on the early Paleozoic tectono-thermal evolution of the Olkhon Terrane, southern Siberia","authors":"Zhiyong Li,&nbsp;Yingde Jiang,&nbsp;Stephen Collett,&nbsp;Pavla Štípská,&nbsp;Karel Schulmann,&nbsp;Sheng Wang,&nbsp;Vasiliy Sukhorukov","doi":"10.1111/jmg.12706","DOIUrl":"10.1111/jmg.12706","url":null,"abstract":"<p>Terranes accreted to the southeastern margin of the Siberian Craton record an important early Paleozoic tectono-thermal event (known as the Baikal orogenic cycle) in the evolution of the Central Asian Orogenic Belt (CAOB). However, the precise metamorphic conditions and relative timing of this event and its linkage to the wider CAOB remain far poorly constrained. The best exposed of these terranes is the Olkhon Terrane on the western bank of Lake Baikal. Here, late Neoproterozoic through early Paleozoic island arc and back-arc assemblages were metamorphosed to form a thin granulite facies belt cropping out adjacent to the Siberian Craton and lower temperature/pressure paragneiss and migmatite towards the southeast. Phase equilibria modelling suggests that the granulite facies belt preserved moderate pressure (c. 0.80 GPa) and high temperature (up to 900°C) conditions while the paragneiss and migmatites in the southeast have peak metamorphic conditions around 700–770°C at 0.60–0.80 GPa. New geochronological data (zircon U–Pb in granulite and monazite U–Pb in paragneiss/migmatite) in combination with phase equilibria modelling and petro-structural analysis suggest that the tectono-metamorphic evolution of the Olkhon Terrane was controlled by a long-lasting (535–450 Ma) and pervasive thermal anomaly. Discrete maxima in the zircon and monazite U–Pb ages at c. 535, 500, and 450 Ma are linked to different stages of a semi-continuous high-temperature metamorphic evolution. Based on existing geological data of the region, a generalized geodynamic model for the Baikal orogenic cycle involving switching between compressional and extensional regimes during the early Paleozoic accretion of ‘exotic’ CAOB-derived material to the southern margin of Siberia is proposed. The tectono-metamorphic evolution of the Olkhon Terrane may represent a world-class example of polyphase shortening of a long-lived hot intra-continental arc–back-arc system during its collision with cratonic blocks.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"41 4","pages":"525-556"},"PeriodicalIF":3.4,"publicationDate":"2022-12-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"48101515","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 tectonic reworking of serpentinites and chlorite-tremolite-talc rocks (SW Spain) from the subduction forearc to intracontinental emplacement","authors":"Benito Ábalos,&nbsp;Pablo Puelles,&nbsp;José Ignacio Gil Ibarguchi","doi":"10.1111/jmg.12704","DOIUrl":"10.1111/jmg.12704","url":null,"abstract":"<p>The petrostructural and geochronological study of a poorly known ultramafic unit from SW Spain (Badajoz–Córdoba belt) combined with previous structural data permits disclosure of a history of metasomatism, tectono-metamorphism, reworking and isotopic resetting related to a poly-orogenic evolution in different geodynamic scenarios. The heterogeneous ultramafic unit studied contains antigorite-serpentinites and metasomatized ultramafic rocks (chlorite-talc schists, tremolite-talc-chlorite rocks and magnesio-hornblende-chlorite rocks). Mantle-wedge serpentinization was followed by Si and Al pre- to syn-metamorphic/tectonic metasomatism in a subduction realm. Petrofabrics of selected lithologies reveal variable syn-metamorphic crystal-plastic deformation and recrystallization (assisted by other mechanisms) under relative high pressure, concomitant with the conditions recorded by neighbouring tectonic units that were later intruded by Ordovician granites. The resultant ensemble was reworked and isotopically reset much later in an intracontinental ductile shear zone. Syn- to late-tectonic apatite from chlorite-talc schists provides an anchored Tera–Wassenburg isochron radiometric age of 342.8 ± 12.2 Ma that provides evidence for the decoupling between isotopic systems and microstructures. The results are discussed from a twofold perspective: with regard to the likely tectonic context of this ophiolite (the current analogue of the Mariana forearc) and with regard to regional geological implications.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"41 4","pages":"491-523"},"PeriodicalIF":3.4,"publicationDate":"2022-12-14","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.12704","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"48181883","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":"Discrimination of thermodynamic and kinetic contributions to the heavy rare earth element patterns in metamorphic garnet","authors":"Matthias Konrad-Schmolke,&nbsp;Ralf Halama,&nbsp;David Chew,&nbsp;Céline Heuzé,&nbsp;Jan De Hoog,&nbsp;Hana Ditterova","doi":"10.1111/jmg.12703","DOIUrl":"10.1111/jmg.12703","url":null,"abstract":"<p>Variations of rare earth element (REE) concentrations in metamorphic garnet are an important source of information of geodynamic and geochemical processes in the deeper Earth. In order to extract this information, the thermodynamic equilibrium and kinetic contributions of the REE uptake in garnet must be distinguished and quantified. Utilizing high-resolution trace element and μ-Raman mapping together with combined thermodynamic–geochemical–diffusion models, we demonstrate that the equilibrium and kinetic aspects of the REE uptake in metamorphic garnet can be discriminated by interpreting 2D trace element mapping in a single sample. The heavy (H) REE (Tb to Lu) zoning in the investigated garnet from a high-pressure blueschist comprises an inner part with an overall decrease from core to inner rim, followed by a concentric zone of HREE enrichment and a drastic HREE decrease towards the outermost rim. The central peak in the garnet core decreases in intensity with decreasing atomic number of the REE. The broad overall shape of this pattern resembles those often observed in metamorphic garnet from different rock types and tectonic settings. Superimposed on this trend is a concentric pattern of minor recurring fluctuations in the HREE concentrations with at least six regularly spaced sets of peaks and troughs along the entire garnet radius. Comparison of the observed inclusion suite, the trace element maps and thermodynamic–geochemical models show that the inner part with decreasing HREE concentrations results from fractional garnet growth in an unchanged mineral assemblage, whereas the REE enrichment zone is caused by the breakdown of titanite. We suggest that the width of the central peak is controlled by the bulk permeability of the interconnected transport matrix and the fraction of matrix minerals that the garnet equilibrates with. The superimposed REE fluctuations result from changing element transport properties of the host rock and mark recurring changes from equilibrium REE uptake to transport-limited REE uptake in garnet. Such fluctuating element transport properties can be best explained by pulse-like fluid fluxes that rhythmically change the interconnectivity of the intercrystalline transport matrix. Increasing numbers of published spatially highly resolved REE analyses show that such trace element fluctuations are common in metamorphic garnet indicating that recurring changes in rock permeabilities due to pulsed fluid fluxes are a common phenomenon during metamorphism.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"41 4","pages":"465-490"},"PeriodicalIF":3.4,"publicationDate":"2022-12-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.12703","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"49095283","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":"Repeated metamorphism in the pelitic granulites of the Hidaka metamorphic belt, Hokkaido, Japan: Implications for the formation of the present-day trench-arc-basin system in NE Asia","authors":"Jinrui Zhang,&nbsp;Wenliang Xu,&nbsp;Yu Dong,&nbsp;Zheng Ji,&nbsp;Chunjing Wei,&nbsp;Shuang Tang,&nbsp;Yibing Li,&nbsp;Kiyoaki Niida","doi":"10.1111/jmg.12701","DOIUrl":"10.1111/jmg.12701","url":null,"abstract":"<p>The timing and mechanism of the tectonic transition from an active continental margin to a trench-arc-basin system in NE Asia are debated. In this study, we report the pressure–temperature–time (<i>P</i>–<i>T</i>–<i>t</i>) path of this transition based on petrographic observations, phase-equilibrium modelling, and U–Pb ages of zircon and rutile from pelitic granulites in the Hidaka metamorphic belt (Hokkaido, Japan). The granulites contain an early phase mineral assemblage of staurolite + sillimanite + biotite + plagioclase + quartz + rutile/ilmenite, a peak phase granulite assemblage of garnet + biotite + cordierite + plagioclase + quartz + rutile/ilmenite and a symplectic intergrowth of spinel + cordierite ± sillimanite within garnet porphyroblasts. Phase-equilibrium modelling indicates two phases of metamorphism with <i>P</i>–<i>T</i> conditions, respectively, of ~6 kbar/620–670°C and ~6 kbar/850°C. A clockwise <i>P</i>–<i>T</i> path was thus reconstructed for the granulites, showing a near-isobaric temperature increase to the peak conditions and a post-peak cooling. U–Pb dating of zircon and rutile in the granulites yielded two groupings of metamorphic ages at c. 37 Ma and 19 Ma, related to early phase amphibolite facies and late phase granulite facies metamorphism, respectively. The age of magmatism from the previous work at the NE Asian continental margin overlaps with these metamorphic ages, and the two phases of metamorphism in the pelitic granulites is attributed to discrete episodes of supra-subduction-zone magmatism (late Eocene, c. 37 Ma) and back-arc extension (early Miocene, 24–19 Ma). Consequently, we suggest that the Hidaka metamorphic belt has undergone two phases of metamorphism, which represent two pulsed and separated thermal events. Moreover, we relate the granulites facies metamorphism to the underplating of mafic magma and lithospheric thinning during the opening of the Japan Sea at 24–19 Ma, which is attributed to slab rollback and trench retreat processes in NE Asia.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"41 3","pages":"425-448"},"PeriodicalIF":3.4,"publicationDate":"2022-11-23","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"41618861","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":"Petrochronology of polygenetic white micas (Naxos, Greece)","authors":"Igor M. Villa,&nbsp;Johannes Glodny,&nbsp;Alexandre Peillod,&nbsp;Alasdair Skelton,&nbsp;Uwe Ring","doi":"10.1111/jmg.12700","DOIUrl":"10.1111/jmg.12700","url":null,"abstract":"<p>Naxos in the Greek Cyclades preserves a type example of polymetamorphism. The southern and northern parts of the island record different Tertiary <i>P–T</i> histories between Eocene and Miocene times, including a blueschist facies event, one or more amphibolite/greenschist facies overprint(s) and contact metamorphism. Age attributions for these events are inconsistent in the literature. Here, we propose a new approach that combines electron probe microanalyzer (EPMA) characterization of the white mica (WM) with ³⁹Ar-⁴⁰Ar–Rb-Sr multichronometry. Textural–petrographic–compositional observations reveal that the polygenetic WM consists of five different generations: pre-Eocene relicts, paragonite, high-Si phengite, low-Si phengite and muscovite. EPMA mapping of four WM samples, previously analysed by Rb-Sr, reveals major element compositions heterogeneous down to the μm scale. Each WM consists of chemically distinct generations, documenting submicron-scale retrogression of high-pressure (HP) phengite grains to muscovite. Four WM samples from a N-S traverse across the island were analysed by ³⁹Ar-⁴⁰Ar stepheating, comparing coarse and fine sieve size fractions to obtain overdetermined K-Ar systematics. Fine sieve fractions are richer in Cl than coarse ones. Linear arrays in Cl/K-age isotope correlation diagrams show two predominant WM generations (one Cl-poor at ca. 38 Ma and one Cl-rich at &lt;20 Ma). A lower-grade sample from southern Naxos was less pervasively recrystallized, provides older ages and preserves at least three WM generations, including a relict WM with a pre-Palaeocene K-Ar age, consistent with the high Ar retentivity of WM in the absence of complete recrystallization. The age of the Cl-poor end-member WM approximates the age of the HP event, 38 Ma. Ar inheritance in Cretaceous mica relicts is heterogeneous at the single-grain scale. Comparing the degassing rates of the WM fractions rules out ‘multidomain’ diffusion. As no sample is monomineralic, the degassing rate of each polygenetic mica is instead controlled by the mass balanced sum of the unrelated rate constants of its constituent minerals. Given the commonness of zoned and composite micas, the approach detailed here is potentially useful for reconstructing polyphase metamorphic histories worldwide.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"41 3","pages":"401-423"},"PeriodicalIF":3.4,"publicationDate":"2022-11-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.12700","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46153390","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":"Partial melting and reaction along deformation features in plagioclase","authors":"Sarah Incel,&nbsp;Marie Baïsset,&nbsp;Loïc Labrousse,&nbsp;Alexandre Schubnel","doi":"10.1111/jmg.12702","DOIUrl":"10.1111/jmg.12702","url":null,"abstract":"<p>Geological processes involving deformation and/or reactions are highly influenced by the rock grain size, especially if diffusion-controlled processes take place such as metamorphic reactions and diffusion creep. Although many processes, inducing grain-size reduction, are documented and understood at relatively high stresses and low temperatures (e.g., cataclasis) as well as at lower stress and higher temperature conditions (e.g., bulging and subgrain rotation), deformation twinning, a plastic deformation mechanism active in various minerals at lower temperatures, has been neglected as nucleation site for melting and reaction and thus as a cause for grain-size reduction so far. We conducted experiments on natural plagioclase-bearing aggregates at 2.5 to 3 GPa confining pressure and temperatures of 700°C to 950°C using two different deformation apparatus, a deformation multianvil apparatus (DDIA) and a Griggs press, as well as a piston-cylinder apparatus. Regardless of the apparatus type, we observe the breakdown of plagioclase into an eclogite-facies paragenesis, which is associated with partial melting in the high temperature domain of the eclogite facies. Partial melting mostly takes place along the grain and interphase boundaries. However, several melt patches or plagioclase decomposition products coincide with the occurrence of deformation twins and grain-scale microcracking in plagioclase indicating intracrystalline melting and reaction in addition to melting and reaction along grain and interphase boundaries. In the present study, we demonstrate how the interplay between brittle microcracking and plastic deformation twinning can cause intracrystalline melting and/or reaction, which has the potential to lower the effective grain size of plagioclase-rich rocks and thus impacts their reactivity and deformation behaviour.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"41 3","pages":"449-464"},"PeriodicalIF":3.4,"publicationDate":"2022-11-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.12702","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44881211","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}