Journal of Metamorphic Geology最新文献

{"title":"A New UHP-HP Tectono-Metamorphic Architecture for the Southern Dora-Maira Massif Nappe Stack (Western Alps) Based on Petrological and Microstructural Evidence","authors":"Chiara Groppo,&nbsp;Simona Ferrando,&nbsp;Fabrizio Tursi,&nbsp;Franco Rolfo","doi":"10.1111/jmg.12812","DOIUrl":"https://doi.org/10.1111/jmg.12812","url":null,"abstract":"<p>The southern Dora-Maira Massif, where coesite was first discovered 40 years ago, is among the most studied and better known example of high/ultra-high-pressure (HP/UHP) terranes. Previous to this study, the Polymetamorphic Basement Complex of the southern Dora-Maira Massif has been defined as a nappe stack consisting of three juxtaposed tectono-metamorphic units: the HP San Chiaffredo Unit at the bottom, the UHP Brossasco-Isasca Unit in the middle and the HP Rocca Solei Unit at the top. The origin of UHP metamorphism in the Brossasco-Isasca Unit is still controversial, due to the difficulties in reconciling the abrupt difference between the UHP conditions recorded by the Brossasco-Isasca Unit (i.e., 700°C–730°C, 4.0–4.3 GPa) and the HP conditions (i.e., ~500°C–520°C, 2.0–2.2 GPa) registered by the adjacent units. Here, we report new petrologic and microstructural evidence supporting the existence of a previously unrecognised UHP unit in the southern Dora-Maira Massif. Our data demonstrate that the tectonic unit overlying the Brossasco-Isasca Unit (i.e., the former Rocca Solei Unit), so far considered a HP unit, is actually divided in two units, one of which (the lowermost Rocca Solei Unit <i>sensu stricto</i>) experienced UHP conditions and the other (the uppermost Grimbassa Unit) reached HP conditions. The newly defined Rocca Solei Unit experienced UHP metamorphism at significantly different P–T conditions (520°C–550°C, 2.7–2.9 GPa) compared to the underlying Brossasco-Isasca Unit, but along a similar ‘cold’ T/P ratio (&lt; 200°C/GPa), markedly lower than that defined in the neighbouring Grimbassa Unit and San Chiaffredo Unit (&gt; 230°C/GPa). After more than 30 years of petrologic investigations, the tectono-metamorphic architecture of the southern Dora-Maira Massif is thus redefined, bridging the gap between the UHP Brossasco-Isasca Unit and the adjacent HP units and opening to new scenarios on its HP–UHP architecture. The results of this study have both regional and petrologic implications: (i) Similarities emerge in the structural position, thickness and metamorphic evolution of the new UHP Rocca Solei Unit in the southern Dora-Maira Massif and those of the Chasterain Unit recently discovered in the northern Dora-Maira Massif, suggesting a common architecture throughout the whole Dora-Maira Massif; (ii) the peculiar quartz microstructure in the metagranites described below represents an exceptional documentation of a ‘frozen’ quartz-to-coesite polymorphic reaction caught in the act and suggests that the availability of fluids was the most crucial factor controlling the progress of the reaction. The metastable persistence of quartz in H₂O-undersaturated lithologies makes even more challenging the identification of UHP units that have only slightly exceeded the quartz–coesite transition and justifies why the newly defined UHP Rocca Solei Unit has remained ‘hidden’ for more than 30 years.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"43 4","pages":"359-383"},"PeriodicalIF":3.5,"publicationDate":"2025-03-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.12812","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143749309","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":"Melt–Rock Interaction Experiments Reveal Rapid Microstructural and Chemical Changes at Lower Crustal Conditions","authors":"Robyn L. Gardner,&nbsp;Nathan R. Daczko,&nbsp;Sandra Piazolo,&nbsp;John Adam,&nbsp;Uvana Meek","doi":"10.1111/jmg.12811","DOIUrl":"https://doi.org/10.1111/jmg.12811","url":null,"abstract":"<p>The reactive flow of melt through the mantle or crust triggers chemical disequilibrium, driving reactions that significantly alter the mineral assemblages and physical properties of host rocks. However, the degrees of chemical difference required to initiate these reactions and their timescale remain poorly understood. In this study, we present piston–cylinder reaction experiments simulating lower crustal conditions, where largely anhydrous lower crustal granoblastic dioritic gneiss interacts with a hydrous mafic melt, created from the same gneiss but modified by the addition of ~6-wt.% H₂O. Remarkably, reactions occurred within just 12 h, producing microstructures that closely resemble those observed in natural, melt-fluxed rocks from the lower arc crust in Fiordland, New Zealand. Melt–rock interactions led to the formation of epitaxial, multilayer symplectic coronae of pargasite + plagioclase or quartz partially replacing pre-existing pyroxene grains. The protolith plagioclase and amphibole are either completely dissolved into the melt or replaced by a modified composition of the same mineral. The melt exhibits compositional variations that correlate with distance from the melt–rock reaction front. Quenched melt chemistry data demonstrate the potential for melt compositions to continuously evolve in response to both crystallisation and melt–rock interactions during reactive flow. Importantly, our findings reveal that melt–rock reactions, initiated by melt not drastically different from the solid rock (protolith), can induce significant changes in rock composition and thus physical properties in a short time. Our findings have broad implications for understanding the compositional evolution of migrating melts and the chemical and mechanical evolution of the Earth's mantle and lower crust in general.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"43 4","pages":"341-358"},"PeriodicalIF":3.5,"publicationDate":"2025-03-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.12811","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143749790","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":"Multi-Stage Growth of Kyanite in Migmatites Interpreted by Integrating Forward Thermodynamic Modelling and Trace Element Signature","authors":"Sara Nerone,&nbsp;Chiara Groppo,&nbsp;Mónica Ágreda-López,&nbsp;Maurizio Petrelli,&nbsp;Franco Rolfo","doi":"10.1111/jmg.12810","DOIUrl":"https://doi.org/10.1111/jmg.12810","url":null,"abstract":"<p>Trace element zoning in kyanite can retain information about its growth history, particularly in anatectic metapelites. There, kyanite can grow (i) at sub-solidus conditions through metamorphic reactions involving other aluminous phases as reactants, (ii) through muscovite dehydration melting reactions, and (iii) during cooling and melt crystallisation either through back-reactions between melt and solid phases (e.g., garnet) or crystallising directly from the melt. Thermodynamic modelling successfully reproduces these reactions, allowing a more robust interpretation of the observed features based on predicted reactants and products. In this study, we interpret the kyanite trace element zoning (particularly of Cr, V, and partly of Fe) observed through cathodoluminescence and quantified through LA-ICP-MS maps, using the forward thermodynamic modelling approach. The studied samples are biotite + kyanite + garnet migmatites from the Lower-Greater Himalayan Sequence of eastern Nepal, which experienced muscovite and incipient biotite dehydration melting. Three main generations of kyanite revealed by trace element zoning have been identified (i.e., Ky1, Ky2, and Ky3), consistent with the three main kyanite-producing reactions predicted by forward thermodynamic modelling, also applying a melt reintegration approach. Ky1 (i.e., sub-solidus kyanite) integrated only minimum amounts of Cr, V and Fe. Ky2 (i.e., peritectic kyanite) incorporates Cr and V released from muscovite during its dehydration melting reaction. Ky3 (i.e., back-reaction overgrowth or magmatic kyanite) is particularly developed in samples where melt segregation has been absent or limited and incorporates lower amounts of Cr and V than Ky2, but is enriched in Fe. The major implications of this study concern the interpretation of the melt segregation processes in anatectic rocks and our understanding of the Cr and V partitioning between minerals and melt. Further methodological considerations are also provided, which could help guide similar studies in the future.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"43 4","pages":"315-339"},"PeriodicalIF":3.5,"publicationDate":"2025-02-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.12810","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143749968","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":"Polyphase Metamorphism of the Northern Rae Craton (Baffin Island, Arctic Canada) and Trace Element Behaviour in Monazite: Insights From Phase Equilibria Modelling and Geochronology","authors":"Diane Skipton,&nbsp;Natasha Wodicka,&nbsp;Owen Weller,&nbsp;Simon Jackson,&nbsp;Marc St-Onge,&nbsp;Benoit Saumur,&nbsp;Duane Petts","doi":"10.1111/jmg.12808","DOIUrl":"https://doi.org/10.1111/jmg.12808","url":null,"abstract":"<p>Integrated field mapping, phase equilibria modelling and in situ U–Pb monazite geochronology from the northern margin of the Rae craton on Baffin Island document three metamorphic events during the Neoarchean to the middle Paleoproterozoic. The Qimivvik area comprises Neoarchean tonalitic gneiss structurally juxtaposed over Neoarchean metasedimentary rocks along the Paleoproterozoic Qimivvik thrust and associated shear zone. High-grade metamorphism at ca. 2.56–2.50 Ga supports a footprint for cryptic late Neoarchean metamorphism over a distance of ∼600 km along the northwestern Rae margin from southern Boothia Peninsula to northern Baffin Island. Thermal peak mineral assemblages in the Qimivvik area equilibrated at ca. 1.9 Ga at conditions of ~710°C–790°C and 4.3–5.5 kbar. The dominant Paleoproterozoic foliation is defined by peak metamorphic phases and is reoriented by folds related to the Qimivvik thrust. Peak metamorphism and associated deformation, including the Qimivvik thrust, are interpreted as a manifestation of the Ellesmere-Inglefield belt of Ellesmere Island and West Greenland, which links with the ca. 1.9 Ga Thelon orogen of western Canada. Partial melting also occurred at ca. 1.8 Ga, possibly resulting from decompression of the Churchill domain following the collisional-accretionary events related to the late stages of amalgamation of Laurentia and supercontinent Nuna. Quantitative trace element maps (acquired using LA-ICP-MS) of monazite reveal distinct trace element signatures associated with each of three growth stages. Ca. 2.5 Ga monazite exhibits complex intragrain compositional zoning, has elevated Y and heavy rare earth elements (HREEs) relative to ca. 1.9 Ga monazite and has higher Th/U overall than both ca. 1.9 Ga and ca. 1.8 Ga monazite. These signatures suggest that ca. 2.5 Ga monazite growth was concomitant with partial melting and preceded the majority of garnet growth. The ca. 1.9 Ga monazite grains are comparatively less zoned and have lower Y + HREE contents than both ca. 2.5 Ga and 1.8 Ga monazite, consistent with the ca. 1.9 Ga monazite forming after most garnet growth. Elevated Y + HREE in the ca. 1.8 Ga monazite imply that it formed after retrograde resorption of garnet rims. In our samples, Y + HREE generally exhibit stronger correlations with monazite age and/or petrographic context than Eu/Eu* and Th/U. As some compositional overlap exists between monazite of different ages and petrographic contexts, quantitative limits (‘cut-offs’) based on trace element concentrations or ratios (e.g., Th/U, Eu/Eu*, La_CN/Yb_CN) are unreliable for distinguishing between monazite populations. In addition to providing important constraints on the early tectonic evolution of northeastern Laurentia, our study offers new insights into trace element behaviour in a key accessory mineral during three metamorphic events occurring over a ~700 Ma time period.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"43 3","pages":"287-314"},"PeriodicalIF":3.5,"publicationDate":"2025-02-14","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.12808","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143530437","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":"Plate Interface Shear Zone in the Sanbagawa Metamorphic Belt, Constrained by RSCM Thermometry, U–Pb Zircon Dating and Phase Equilibria Modelling in the Sarutagawa Region, Central Shikoku, Japan","authors":"Samuele Papeschi,&nbsp;Kenta Kawaguchi,&nbsp;Keishi Okazaki,&nbsp;Yasutaka Hayasaka,&nbsp;Takehiro Hirose","doi":"10.1111/jmg.12807","DOIUrl":"https://doi.org/10.1111/jmg.12807","url":null,"abstract":"<p>Exhumed high-P/low-T complexes are of paramount importance to directly access rocks that experienced subduction zone processes. However, the original tectono-metamorphic fabrics are often partially obliterated by exhumation and later deformation, hindering our understanding of the processes occurring at depth. We show an example of how multiple field- and lab-based analytical techniques may be used to extract information of the pristine fabrics of polydeformed metamorphic rocks. We investigated a natural cross section through the chlorite, garnet and albite-biotite zones of the exhumed Shirataki Unit in the Sanbagawa metamorphic belt, exposed in the Sarutagawa (Saruta River) area of the Central Shikoku, coupling structural-petrographic analysis with Raman Spectroscopy on Carbonaceous Material (RSCM) thermometry, phase equilibrium modelling and U–Pb zircon dating. RSCM thermometry reveals a progressive temperature increase from 350°C–400°C to 500°C–550°C over an ~400 m distance, characterized by condensed metamorphic isograds in the garnet zone. Phase equilibrium modelling indicates slightly decreasing metamorphic pressures through the transect from 0.6–0.9 GPa at low-T to 0.4–0.7 GPa at high-T with preserved blueschist-facies parageneses, documented for the first time in the area, restricted to the ~400°C–450°C range. Hence, rocks developed close to the Sanbagawa subduction gradient are juxtaposed with rocks that experienced significant retrograde heating during exhumation. Moreover, we found that competent lithologies such as quartzite and basic schist along the transect preserve trenchward-directed deformation structures that are obliterated by orogen-parallel stretching in the surrounding, incompetent pelitic schists. U–Pb dating shows progressively older youngest detrital zircon ages and syn-depositional peaks from 79–76 and ~88–80 Ma in the chlorite zone to ~92 and ~100 Ma in the garnet zone, respectively, indicating that the lower grade units were subducted at a later stage and that, hence, different metamorphic grades in the area correspond to different protolith ages. The data discussed above is consistent with the former presence of a regional shear zone that, although partially obliterated by younger deformation, contributed to the exhumation of higher-T rocks of the albite-biotite and oligoclase-biotite zones over subducting rocks of the chlorite and garnet zones, likely exploiting the subduction interface. These results offer a framework to investigate the geological record of subduction in the polyphase metamorphic rocks of the Sanbagawa belt.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"43 3","pages":"257-285"},"PeriodicalIF":3.5,"publicationDate":"2025-02-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.12807","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143530245","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":"Petrochronology of High-Pressure Veins Reveals the Evolution of Fluid Sources in Subducted Oceanic Crust (Rocciavrè Eclogites, W. Alps)","authors":"Clément Herviou,&nbsp;Guillaume Bonnet,&nbsp;Samuel Angiboust,&nbsp;Aitor Cambeses,&nbsp;Tom Raimondo","doi":"10.1111/jmg.12806","DOIUrl":"https://doi.org/10.1111/jmg.12806","url":null,"abstract":"&lt;p&gt;The Rocciavrè massif is a large eclogitized ophiolitic fragment exposed in the Western Alps (Piemonte, Italy) exhibiting an almost complete sequence of the subducted Liguro-Piemont lithosphere. Raman spectroscopy on carbonaceous material in metasediments from Rocciavrè and the juxtaposed Orsiera massif indicates maximum temperatures in the range ~510°C–550°C, whereas thermodynamic modelling in mafic lithologies reveals peak burial metamorphic conditions of 550°C–590°C/2.2–3.0 GPa for both units, suggesting the absence of a metamorphic gap between them. Late Jurassic (ca. 151–158 Ma) zircons extracted from Rocciavrè metagabbros reflect the crystallization age near the seafloor, and no alpine metamorphic rims have been detected. The garnet-omphacite-rutile–dominated Fe-Ti metagabbros are crosscut by a variety of high-pressure vein systems, including garnet-rich, omphacite-rich, omphacite-quartz–rich, glaucophane-quartz–rich and winchite-actinolite-talc veins. Vein textures, mineral assemblages and mineral compositions suggest the formation of garnet-rich and omphacite-rich veins at conditions close to peak burial and the successive formation of omphacite-quartz–rich and glaucophane-quartz–rich types by reopening former omphacite-rich veins at eclogite- to epidote-blueschist-facies conditions along the exhumation path. In contrast, winchite-actinolite-talc veins are interpreted as retrograde greenschist-facies features. In situ U-Pb dating of monazite constrains the age omphacite-quartz–rich veining at 40.4 ± 0.2 Ma. Major and trace element mapping of vein assemblages shows various zoning patterns of omphacite and rutile crystals for a large variety of elements (e.g., Fe, Mg, Mn, Sr, Li, U and Cr). Aqueous primary fluid inclusions trapped in vein-filling and host-rock minerals have intermediate to high salinity values, interpreted to reflect the partial signature of hydrothermal alteration preserved up to eclogite-facies conditions. High fluid inclusion salinity values associated with the presence of N&lt;sub&gt;2&lt;/sub&gt; (± CO&lt;sub&gt;2&lt;/sub&gt;) suggest the presence of fluids produced by local dehydration reactions at peak burial. In contrast, some inclusions from glaucophane-quartz–rich veins contain a low to intermediate salinity CO&lt;sub&gt;2&lt;/sub&gt;-CH&lt;sub&gt;4&lt;/sub&gt;–bearing fluid interpreted as reflecting a sedimentary contribution and a larger scale of fluid circulation. In addition, the mineralogy of winchite-actinolite-talc veins associated with high-salinity values suggests an ultramafic signature. The successive steps of vein formation are interpreted to record the evolution from a closed to open chemical system during exhumation, with late sedimentary and ultramafic fluid contributions that witness the mobility of fluids within the mafic sequence and transport distances likely reaching the kilometre scale. The Rocciavrè massif, which shares a similar metamorphic history to the Monviso Lago Superiore Unit further south, enables a precise characterization of f","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"43 3","pages":"225-256"},"PeriodicalIF":3.5,"publicationDate":"2025-01-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.12806","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143530795","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":"A Cautionary Tale About Not ‘Joining the Dots’ to Infer Anticlockwise \u0000\u0000 P-\u0000\u0000 T Paths","authors":"Owen Weller,&nbsp;Joseph Benson,&nbsp;Alex Copley","doi":"10.1111/jmg.12805","DOIUrl":"https://doi.org/10.1111/jmg.12805","url":null,"abstract":"<p>Uncertainty surrounds the cause and interpretation of anticlockwise pressure-temperature (\u0000<span></span><math>\u0000 <mi>P</mi></math>–\u0000<span></span><math>\u0000 <mi>T</mi></math>) paths in metamorphic terranes. Here, we focus on the viability of a commonly proposed mechanism—magmatic heat transfer during thickening—using a case study of the Roineabhal terrane in Scotland, where such an anticlockwise \u0000<span></span><math>\u0000 <mi>P</mi></math>–\u0000<span></span><math>\u0000 <mi>T</mi></math> path has been proposed. Phase equilibria modelling of new samples, combined with previous \u0000<span></span><math>\u0000 <mi>P</mi></math>–\u0000<span></span><math>\u0000 <mi>T</mi></math> estimates, provides evidence for regional kyanite-grade granulite-facies metamorphism, with an additional spatially localised region of ultrahigh temperature conditions adjacent to an anorthosite intrusion. The spatial geometry of the ultrahigh temperature samples, combined with scaling arguments and thermal modelling of these results, shows that the ultrahigh temperature metamorphism is contact in nature and should not be joined to the regional metamorphism to infer an anticlockwise \u0000<span></span><math>\u0000 <mi>P</mi></math>–\u0000<span></span><math>\u0000 <mi>T</mi></math> path. Rather, the regional metamorphism features hairpin \u0000<span></span><math>\u0000 <mi>P</mi></math>–\u0000<span></span><math>\u0000 <mi>T</mi></math> loops, overlain adjacent to the anorthosite by a short-lived, high-temperature excursion. Because metamorphic rocks typically yield a fragmentary record during fluctuating thermal conditions, due to requiring hydration to maintain equilibrium during down-temperature evolution, it is critical to assess in this manner the thermal viability of the range of \u0000<span></span><math>\u0000 <mi>P</mi></math>–\u0000<span></span><math>\u0000 <mi>T</mi></math> paths that could connect the preserved assemblages. In general, intrusion radii of tens of kilometres, or repeated intrusions of smaller bodies in quick succession (e.g., &lt; 10 kyr for a 1-km radius), would be required for true magmatically driven anticlockwise \u0000<span></span><math>\u0000 <mi>P</mi></math>–\u0000<span></span><math>\u0000 <mi>T</mi></math> paths. Given the unlikely nature of these requirements in most tectonic settings, such anticlockwise \u0000<span></span><math>\u0000 <mi>P</mi></math>–\u0000<span></span><math>\u0000 <mi>T</mi></math> paths are likely to be rarer than reported. For many scenarios, \u0000<span></span><math>\u0000 <mi>P</mi></math>–\u0000<span></span><math>\u0000 <mi>T</mi></math> paths commonly interpreted as anticlockwise \u0000<span></span><math>\u0000 <mi>P</mi></math>–\u0000<span></span><math>\u0000 <mi>T</mi></math> paths are instead likely to take the form described in this study.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"43 5","pages":"407-419"},"PeriodicalIF":3.5,"publicationDate":"2025-01-14","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.12805","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144190961","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":"A Prograde Dual-Segmented Geotherm for (Retro-) Eclogite From Western Dabie and Implications for Maximum Decoupling Depths During Continental Subduction","authors":"Bin Xia,&nbsp;Chunhao Chen,&nbsp;Yuanbao Wu,&nbsp;Wei Wang","doi":"10.1111/jmg.12803","DOIUrl":"https://doi.org/10.1111/jmg.12803","url":null,"abstract":"<div>\u0000 \u0000 <p>Robust quantification of the prograde <i>P–T</i> trajectories of eclogite exhumed from subduction zones is fundamental for deciphering the thermal structure evolution and understanding the geodynamic processes during continental subduction. In this study, we investigate four metabasites located in Hong'an within the western Dabie HP/UHP metamorphic belt. Based on detailed petrographic observations and mineral chemistry analyses, combined with phase equilibrium modelling, average-<i>T</i> calculation and conventional thermobarometry, we quantify the prograde to peak <i>P–T</i> paths for each of the four metabasites and the retrograde <i>P–T</i> conditions for two samples. The results show that three of the four metabasites have similar prograde <i>P–T</i> paths evolving from 15.5–18.5 kbar, 440–485 °C (M0 stage) to 18.5–20.5 kbar, 500–525 °C (M1 stage). On the other hand, although the fourth sample shares a similar <i>P–T</i> evolution for a segment of the late prograde stage from 18.0–19.0 kbar, ~500 °C to 20.0–22.0 kbar, ~550 °C, it attains <i>P</i>_max at a notably higher pressure of ~26.0 kbar at 550–560 °C (M2 stage). During exhumation, we identify an early retrograde stage occurring at 9.0–12.5 kbar, 545–580 °C (M3 stage), followed by a later retrograde stage at 3.5–8.0 kbar, 540–580 °C (M4 stage). In combination with previous studies, we propose a common dual-segmented <i>P–T</i> path for the late prograde evolution of the HP/UHP rocks in western Dabie. The initial segment exhibits a gentle slope with apparent geotherms of 7–8 °C/km, whereas the subsequent segment displays a steeper slope with apparent geotherms of 5–6 °C/km. We interpret the turning point at 20.0–23.0 kbar (corresponding to depths of 70–80 km) as marking the maximum decoupling depths (MDD) between the subducting slab and the overlying mantle wedge. Notably, this prograde dual-segmented geotherm for eclogite in western Dabie and the corresponding MDD are similar to computational geodynamic models.</p>\u0000 </div>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"43 2","pages":"161-190"},"PeriodicalIF":3.5,"publicationDate":"2024-12-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143117876","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":"Cold Deep Over Hot Shallow Crust: A Peculiar Metamorphic Architecture in the Neoarchean Metasedimentary Pontiac Subprovince, Superior Craton (Canada)","authors":"Isaac S. Malta,&nbsp;Carl Guilmette,&nbsp;Antoine Godet,&nbsp;Douglas K. Tinkham,&nbsp;Bruna Coldebella,&nbsp;Georges Beaudoin,&nbsp;Crystal LaFlamme,&nbsp;Taus R. C. Jørgensen,&nbsp;Jeffrey H. Marsh","doi":"10.1111/jmg.12804","DOIUrl":"https://doi.org/10.1111/jmg.12804","url":null,"abstract":"&lt;p&gt;The Neoarchean Era is a key period in Earth's history as it witnessed a significant pulse of crustal formation corresponding to the assembly of several cratons, potentially coeval with a transition in the global tectonic regime. Neoarchean metasedimentary subprovinces of the Superior Craton, the largest unreworked Archean craton on Earth, were formed shortly before its final assembly and cratonization, thus providing valuable insights into the tectonic style, thermal state and architecture prevailing during this key period. Among these, the Pontiac Subprovince is one of the most studied, yet has a largely debated geodynamic history. In its northern extent, metamorphosed turbiditic sequences display a southward succession of index minerals—biotite, garnet, staurolite, kyanite and sillimanite—that may be indicative of a Barrovian-like metamorphic gradient. However, the origin and evolution of this apparent gradient and its link to Neoarchean tectonics remain unclear. New mapping of metamorphic isograds and zones, petrological and microstructural analyses, whole rock and mineral chemistry analyses and phase equilibria modelling are integrated to decipher the Pontiac Subprovince tectonothermal evolution. Our analysis indicates that the peak equilibrium assemblages from the garnet, staurolite and sillimanite/melt zones developed early to late relative to the main regional deformation event (D&lt;sub&gt;2&lt;/sub&gt;) and its associated steeply dipping S&lt;sub&gt;2&lt;/sub&gt; schistosity. Garnet zone rocks recorded a burial-heating path with peak &lt;i&gt;P&lt;/i&gt;–&lt;i&gt;T&lt;/i&gt; conditions at 8.1–8.2 kbar and 582°C–585°C, along a low &lt;i&gt;T&lt;/i&gt;/depth ratio of ~20°C/km. In contrast, staurolite and sillimanite/melt zone rocks followed isobaric heating and isothermal decompression paths with peak &lt;i&gt;P&lt;/i&gt;–&lt;i&gt;T&lt;/i&gt; conditions at 5.9–6.1 kbar and 610°C–625°C and 6 kbar and 700°C, respectively, along a moderate &lt;i&gt;T&lt;/i&gt;/depth ratio of ~30–33°C/km. Since there is clear D&lt;sub&gt;2&lt;/sub&gt; structural continuity across the metamorphic zones over ~12 km and metamorphism occurred pre- to post-D&lt;sub&gt;2&lt;/sub&gt;, we interpret that the diverse &lt;i&gt;P&lt;/i&gt;–&lt;i&gt;T&lt;/i&gt; paths and contrasting &lt;i&gt;T&lt;/i&gt;/depth ratios likely represent a spatially heterogeneous thermal structure developed within a single coherent structural block during and shortly after the formation of the subvertical S&lt;sub&gt;2&lt;/sub&gt; schistosity. Such features are hardly compatible with either modern inverted or continuous Barrovian sequences—known for consistent &lt;i&gt;P&lt;/i&gt;–&lt;i&gt;T&lt;/i&gt; evolution paths and similar &lt;i&gt;T&lt;/i&gt;/depth ratios—or with discontinuous sequences requiring diachronicity. Our findings therefore do not fully reconcile with the existing accretionary/collisional models for the Pontiac Subprovince, given differences in their predicted apparent thermal gradients, metamorphic evolution and structural patterns. Alternatively, our data more closely match predictions for a sagduction-dominated vertical process, where high heat influx at the base of the c","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"43 2","pages":"191-224"},"PeriodicalIF":3.5,"publicationDate":"2024-12-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.12804","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143117875","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":"Evidence of High-Pressure Metamorphism Along the Mahanadi Shear Zone in the Eastern Ghats Province, Eastern India: Implications on Tectonics and Continental Assembly Involving India and East Antarctica","authors":"Shuvankar Karmakar,&nbsp;Sankar Bose,&nbsp;Gautam Ghosh,&nbsp;Kaushik Das,&nbsp;Nilanjana Sorcar,&nbsp;Sneha Mukherjee","doi":"10.1111/jmg.12797","DOIUrl":"https://doi.org/10.1111/jmg.12797","url":null,"abstract":"&lt;div&gt;\u0000 \u0000 &lt;p&gt;A suite of mafic granulite enclaves within mylonitised felsic gneiss occurring along the E-W trending Mahanadi Shear Zone of the Eastern Ghats Province preserves evidence of high-pressure metamorphism. Garnet-clinopyroxene-bearing mafic granulite contains a mineral assemblage of garnet + clinopyroxene + plagioclase + quartz + rutile which was formed after dehydration melting of a hornblende-bearing protolith during M&lt;sub&gt;1&lt;/sub&gt; metamorphism that peaked at 1.1–1.4 GPa, 760°C–840°C. The retrograde stage (M&lt;sub&gt;1R&lt;/sub&gt;) is marked by the formation of hornblende and symplectic intergrowth of clinopyroxene + plagioclase + orthopyroxene after garnet at 0.8–0.9 GPa, 760°C–810°C, suggesting an isothermal decompression type &lt;i&gt;P–T&lt;/i&gt; path. The whole rock trace element and REE characteristics suggest a MORB-OIB protolith for the mafic granulites. The host felsic gneiss has a granitic protolith which was emplaced in an arc setting. The rocks exposed south of the Mahanadi Shear Zone in the Phulbani domain are represented by granulites with contrasting metamorphic characteristics. The garnet-orthopyroxene-bearing mafic granulite within coarse-grained charnockite and the aluminous granulite within felsic gneiss show evidence of biotite dehydration melting. The peak M&lt;sub&gt;1&lt;/sub&gt; assemblage in the aluminous granulite is represented by the assemblage spinel + garnet + quartz + plagioclase + K-feldspar which was stable at 0.70–0.74 GPa, 904°C–935°C. M&lt;sub&gt;1R&lt;/sub&gt; in this rock is characterised by coronas of garnet and sillimanite over spinel and the formation of matrix biotite at 707°C–806°C by near-isobaric cooling. Similar isobaric cooling has been documented from the formation of garnet, clinopyroxene and quartz coronas on orthopyroxene in mafic granulite and garnet and quartz coronas on clinopyroxene, wollastonite and calcite in calc-silicate granulite. The juxtaposition of lower crustal rocks showing clockwise and counterclockwise &lt;i&gt;P–T&lt;/i&gt; paths across the Mahanadi Shear Zone implies a paired metamorphic character in a subduction–collision setting. Zircon U-Pb and monazite U-Th-total Pb data show a complex history of the rock suite. The enclave suite of rocks within the Mahanadi Shear Zone underwent peak M&lt;sub&gt;1&lt;/sub&gt; metamorphism at ca. 980–960 Ma which was followed by decompression to a shallower level by ca. 960 Ma when the host granitic magma crystallised. Rocks occurring in the Phulbani domain (southernly placed crustal domain), on the other hand, underwent ultrahigh temperature metamorphism at shallower crustal levels broadly at the same time. We argue that the southern Phulbani domain of the Eastern Ghats Province, India, collided with the Angul-Prydz domain of the Rayner Province, East Antarctica which eventually caused underthrusting of the former below the latter across the Mahanadi Shear Zone. In the context of the Eastern Ghats-Rayner reconstruction, this indicates the closure of the intervening Mawson Sea. A secon","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"43 2","pages":"123-160"},"PeriodicalIF":3.5,"publicationDate":"2024-11-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143119594","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}