Journal of Metamorphic Geology最新文献

{"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}

{"title":"The Diverse Habitats of Eclogite Formation: Insights From the Mesoproterozoic Glenelg Inlier, Scotland","authors":"Nicholas A. Lucas,&nbsp;Owen M. Weller,&nbsp;Alex Copley,&nbsp;Catherine M. Mottram,&nbsp;Glenn Chapman,&nbsp;William J. McMahon","doi":"10.1111/jmg.12801","DOIUrl":"https://doi.org/10.1111/jmg.12801","url":null,"abstract":"<p>Eclogite-facies rocks provide important constraints on the behaviour of convergent plate boundaries and the geometries of tectonic reconstructions due to the high to ultrahigh pressure conditions at which they form. Many eclogite occurrences are documented near the suture zone of active collisional settings where they are interpreted to mark the approximate location of former ocean basin subduction. Such observations influence tectonic interpretations for older eclogites within more deeply eroded and/or less well-exposed terranes. The eclogitic Glenelg inlier in northwest Scotland is one such example, with c. 1 Ga eclogites having previously been interpreted as marking the trace of a Grenville-aged collisional suture zone that defines a third ‘arm’ to the Grenville orogen alongside well-defined sutures in North America and Scandinavia. Here we use a combination of geochronology, phase equilibrium modelling and accessory-phase thermometry to show that the eclogite-facies assemblages were produced at <span></span><math>\u0000 <semantics>\u0000 <mrow>\u0000 <mo>∼</mo>\u0000 </mrow>\u0000 <annotation>$$ sim $$</annotation>\u0000 </semantics></math>18–19 kbar and 700°C–750°C from c. 1.1 to 1.0 Ga. Accounting for the foreland basin setting of equivalent-aged sedimentary rocks in the region and demonstrating the thermal viability of this setting, we show that eclogite formation occurred in deforming foreland crust adjacent to the Grenville orogen, in a setting broadly analogous to fault-bounded basement uplifts in the forelands of active deformation belts, such as the Himalayas and Andes. Our results demonstrate that eclogite-facies rocks can form in a greater range of tectonic settings than are sometimes considered, with implications for tectonic reconstructions of collisional zones. In this instance, our results remove the need for a third ‘arm’ of the Grenville orogen by placing Glenelg in a foreland setting, reconciling the absence of plentiful Grenville-aged metamorphic rocks in northwest Scotland, the sedimentological record and paleomagnetic data in the wider region.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"43 1","pages":"97-122"},"PeriodicalIF":3.5,"publicationDate":"2024-11-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.12801","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143118779","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":"Archean Polymetamorphism in the Central Dharwar Craton, Southern India","authors":"Jonas Kaempf,&nbsp;Chris Clark,&nbsp;Tim E. Johnson,&nbsp;Mudlappa Jayananda,&nbsp;Julian Alfing,&nbsp;Justin Payne,&nbsp;Krishnan Sajeev,&nbsp;Martin Hand","doi":"10.1111/jmg.12798","DOIUrl":"https://doi.org/10.1111/jmg.12798","url":null,"abstract":"<div>\u0000 \u0000 <p>Petrochronological investigations of granulite-facies metapelitic rocks from the eastern contact of the Closepet granite in the Central Dharwar Craton (CDC), southern India, provide new pressure–temperature–age (<i>P–T–t</i>) constraints on two seemingly discrete Archean metamorphic events during the Neoarchean and late Paleoarchean eras. Phase equilibrium modelling and conventional thermobarometry coupled with in situ monazite and garnet geochronology constrain Neoarchean (ultrahigh-temperature; UHT) metamorphic peak conditions to ~930 °C and ~6.7 kbar at <i>c</i>. 2.63 Ga, then cooling and limited decompression to ~5.7 kbar at ≤810 °C. Monazite inclusions in garnet least affected by Neoarchean recrystallisation have distinct positive Eu anomalies and yield ages of <i>c</i>. 3.2 Ga, whereas garnet cores interpreted to have grown at the same time have slightly younger apparent ages of <i>c</i>. 3.1 Ga. We interpret this age mismatch to be the result of extensive resorption of garnet during Neoarchean UHT metamorphism, which led to widespread modification of the initial Lu–Hf systematics in garnet to produce younger apparent ages. The effect of retention and inward intracrystalline diffusion of Lu on the isotopic composition of garnet is most pronounced close to the resorbed grain margins and decreases towards the core, as reflected by progressively younger apparent single-spot garnet dates from core to rim. Despite extensive overprinting of the sample at <i>c</i>. 2.63 Ga, the trace element composition of Paleoarchean monazite indicates growth in equilibrium with garnet but in the absence of feldspar, which is predicted to occur over a broad stability range at <i>P</i> ≥ 8 kbar and <i>T</i> ≤ 700 °C. Such <i>P–T</i> conditions are uncommon in the metamorphic rock record prior to the Neoarchean, but are typical of Barrovian-type metamorphism, which is considered to be an expression of accretionary-to-collisional orogenesis. Rocks of similar age and metamorphic grade have been reported from the core of the Western Dharwar Craton (WDC) and may reflect regional subduction at the margins of the CDC and WDC since the late Paleoarchean. Neoarchean UHT metamorphism in the Dharwar Craton is coeval with a cluster of other UHT occurrences at 2.7–2.6 Ga, indicating the existence of globally elevated thermal gradients at that time.</p>\u0000 </div>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"43 1","pages":"71-95"},"PeriodicalIF":3.5,"publicationDate":"2024-11-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143117657","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":"Contact Metamorphism of Pelitic Rocks Constrains the Depth of Emplacement of the Re di Castello Intrusion (Adamello Batholith, Italy)","authors":"Bernardo Cesare,&nbsp;Omar Bartoli,&nbsp;Peter Brack,&nbsp;Giorgio Moranduzzo,&nbsp;Arianna Randazzo,&nbsp;Salvatore Iaccarino,&nbsp;Lisa Santello","doi":"10.1111/jmg.12802","DOIUrl":"https://doi.org/10.1111/jmg.12802","url":null,"abstract":"<p>Re di Castello is the southernmost unit of the composite Adamello batholith (central Alps, Italy). It was emplaced between 43 and 40 Ma, yet its depth of emplacement is not well constrained. We have studied the contact metamorphism of pelitic country rocks in two localities: the upper Caffaro Valley and the Borzago Valley in the southern and the northern sub-units of the Re di Castello, respectively. In the Caffaro Valley, the country rocks comprise the Triassic sedimentary Lozio Shale, a carbon-rich slate–siltstone, which was intruded by quartz-dioritic magma. Near the contact, the hornfelses display the mineralogical assemblage Kfs-Crd-Bt-Ms-Pl-Qz-Gr, with rare fibrolitic sillimanite in a single sample. Andalusite was never observed throughout the aureole at this locality. In the Borzago Valley, contact metamorphism developed on schists of the pre-Permian basement with a Variscan, regional metamorphic overprint. The sequence of contact metamorphic mineral assemblages progresses from And-Bt-bearing parageneses to the Sil-Crd-Bt-Kfs-Ms-Pl-Qz that characterizes peak conditions, at which incipient melting is also observed. K-feldspar forms slightly upgrade the first appearance of sillimanite. For samples at both localities, thermodynamic modelling in the NCKFMASHT system failed at predicting a stability field for the sequences of mineral assemblages developed during contact metamorphism. Also the Ti-in biotite thermometer did not constrain temperatures adequately. The P–T conditions at the thermal peak were thus evaluated by an alternative bathograd-like approach, considering phase relationships in the simplified NKASH-C system. To form sillimanite only as product of the incomplete Ms-Qz breakdown—divariant for the presence of Na in Ms and Kfs and shifted to lower temperature due to the presence of graphite (in the Lozio Shale)—an isobaric path typical of contact metamophism must have crossed above the Ms_ss-Qz-Kfs_ss-Sil-And-fluid invariant point. This constrains an emplacement pressure &gt;3.3 kbar in the Caffaro Valley, and &gt;3.2 kbar in the Borzago Valley. Concerning temperature, the same univariant point also constrains the minimum temperature in the Caffaro Valley as &gt;615°C–620°C, consistent with results of RSCM thermometry on graphite from the Lozio Shale. The evidence of incipient melting in some samples from the Borzago Valley indicates higher temperatures, probably approaching 670°C, near the contact to the intrusion. Assuming an average crust density of 2.7 g cm⁻³, the estimated pressures correspond to a minimum paleo-depth of emplacement of about 12 km. These depths are somewhat greater than normally considered and should be regarded as revised constraints on models of the emplacement dynamics of the Adamello batholith and on paleogeographic reconstructions of this part of the Southalpine domain.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"43 1","pages":"47-69"},"PeriodicalIF":3.5,"publicationDate":"2024-11-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.12802","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143117172","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":"Pressure and Temperature Estimation in Greenschist-Facies Metapelites: An Example From the Variscan Belt in Sardinia","authors":"A. Petroccia,&nbsp;J. B. Forshaw,&nbsp;P. Lanari,&nbsp;S. Iaccarino,&nbsp;C. Montomoli,&nbsp;R. Carosi","doi":"10.1111/jmg.12799","DOIUrl":"https://doi.org/10.1111/jmg.12799","url":null,"abstract":"<p>Phase equilibrium modelling is a powerful method for obtaining pressure–temperature (<i>P–T</i>) estimates in metapelites; however, the advantages, disadvantages, and limitations of applying this technique to greenschist-facies pelites have received little attention. This study uses two greenschist-facies metapelites from the Nappe Zone in Sardinia (Italy) to examine differences between currently available thermobarometric techniques. First, this work compares <i>P–T</i> estimates from chlorite–K-white mica–quartz–water and K-white mica–quartz–water multiequilibrium approaches, the latter integrated with previously published <i>T</i>_max results from Raman spectroscopy on carbonaceous material. Consistent <i>P–T</i> conditions are retrieved from these methods for both samples, with <i>P–T</i> estimates suggesting a difference in peak metamorphic conditions between the Internal and External Nappe Zones of Sardinia (peak <i>P–T</i> conditions of 0.6–0.8 GPa, 420°C–450°C and 0.4–0.5 GPa, 370°C–400°C for the Internal and External Nappes, respectively). Second, phase equilibrium modelling was used to assess the ability of six different sets of solution models, alongside their associated thermodynamic datasets, to constrain <i>P–T</i> conditions in the same samples, which are characterized by high-variance mineral assemblages. Significant differences were found between the six thermodynamic databases, and important discrepancies were recognized between the observed and predicted mineral assemblages and compositions (e.g., Si in K-white mica). In only one sample, three databases were able to predict the observed natural assemblage. The compositional isopleths for K-white mica, from the same databases, do not intersect, offering no assistance in constraining <i>P–T</i> conditions. However, one database predicted chlorite's natural composition, yielding a <i>P–T</i> estimate consistent with multiequilibrium methods: ~0.9 GPa, ~350°C–400°C for the Internal Nappe Zone and ~0.5 GPa, ~300°C–350°C for the External Nappe Zone. Although <i>P–T</i> conditions constrained using calculated isopleths from greenschist-facies metapelites and databases optimized for modelling low-grade to medium-grade pelitic systems may be reliable, they should be applied cautiously and compared with estimates from inverse and independent approaches.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"43 1","pages":"21-46"},"PeriodicalIF":3.5,"publicationDate":"2024-11-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.12799","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143117173","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":"Petrogenesis of the Scapolite-Bearing Kongurblak Shear Zone, Chinese Southwestern Tianshan: Implications for Rock-Buffered Fluid Flow Migration During Orogeny","authors":"Zeng Lü,&nbsp;Zhenyu Chen,&nbsp;Andrea Festa","doi":"10.1111/jmg.12800","DOIUrl":"https://doi.org/10.1111/jmg.12800","url":null,"abstract":"<div>\u0000 \u0000 <p>In the lower Kongurblak River, Chinese SW Tianshan, scapolite was first found in a narrow shear zone (the Kongurblak shear zone, KSZ), consisting of variably mylonitized sedimentary rocks, including marble, meta-sandstone and felsic–marly phyllonite. These rocks show varying scapolite textures and modes (from less than 1% to 15%), as a sporadic fine-grained matrix phase, porphyroblasts with abundant inclusions, or blocky aggregates. The Cl content of scapolite (0.90–2.79 wt.%) varies between different rock types, which is higher in meta-sandstone and marble than in felsic–marly phyllonite. Rare patchy lower Cl domains of scapolite and Ba zoning of K-feldspar in meta-sandstone and Cl zoning of porphyroblastic scapolite in marly phyllonite suggest pulsed fluid circulation. P–T estimates, thermodynamic modelling and mineral textures indicate scapolite formation through metasomatizing sodic feldspar and/or metamorphic reactions at amphibolite-facies conditions (470°C°C–600°C and 3–7 kbar). These findings suggest the NaCl–CO₂–H₂O fluids in the KSZ stem from an external source, not from evaporitic horizons, at ~200 Ma by apatite U–Pb dating. The chlorinity and X (CO₂) values of fluids, calculated based on the chemistry of the assemblage scapolite–biotite and T–X (CO₂) diagrams, correlate well with specific rock types they traverse, which show different mineral modes and compositions, prograde reactions, structural permeability and host rock cavities. This indicates that the chemistry of medium-grade infiltrative fluids in shear zones is strongly rock-buffered, with crucial implications for orogenic metallogenesis in specific rock types. Evidence that the lithologies within the KSZ and their tectonometamorphic histories are distinct from those of the two bounding metamorphic units suggests they represent exotic sedimentary blocks transported over long distances within a large-scale mélange, also providing new constraints on the tectonic collage of the Chinese SW Tianshan.</p>\u0000 </div>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"43 1","pages":"1-20"},"PeriodicalIF":3.5,"publicationDate":"2024-11-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143116667","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":"Zircon Coupled Dissolution–Precipitation Replacement During Melt–Rock Interaction Modifies Chemical Signatures Resulting in Misleading Ages","authors":"Nathan R. Daczko,&nbsp;Victoria Elliott,&nbsp;Sandra Piazolo,&nbsp;Kevin Hao","doi":"10.1111/jmg.12796","DOIUrl":"https://doi.org/10.1111/jmg.12796","url":null,"abstract":"<p>Melt migration through Earth's crust drives well-documented melt–rock reactions, locally changing rock assemblage and geochemistry. However, melt–zircon interaction remains understudied. We report on three zircon-melt interaction events from the Pembroke Granulite, New Zealand. Primary zircon from gabbroic gneiss which was subject to minor post-emplacement melt migration and primary zircon from younger dykes exhibit straightforward microstructures, microchemistry, and age data. In contrast, zircon from melt-mediated reaction halos adjacent to the dykes and from melt-fluxed high-strain zones display dissolution modification of grains, micro-porosity and blurred or truncated internal zoning typical of replacement by coupled dissolution-precipitation. Replaced zircon domains show changed rare earth element patterns and redistributed or lost radiogenic Pb that generates ambiguous apparent spot date arrays, smeared over tens of millions of years. We conclude that the metamorphism and three melt–rock interaction events were brief, and the arrays misrepresent the true age and duration of the metamorphism. Pb-loss persisted beyond the metamorphism, with porosity and inclusions formed during coupled dissolution-precipitation making replaced zircon domains more susceptible to subsequent Pb-loss compared to the structurally intact, primary magmatic zircon in the host gabbroic gneiss or dykes. We recommend conducting high-resolution microstructural investigations upon recognition of spot date arrays observed in single samples to rule out the possibility of spurious arrays resulting from coupled dissolution-precipitation.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"42 9","pages":"1197-1214"},"PeriodicalIF":3.5,"publicationDate":"2024-10-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.12796","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142642045","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":"Pressure–Temperature–Time Evolution of a Polymetamorphic Paragneiss With Pseudomorphs After Jadeite From the HP–UHP Gneiss-Eclogite Unit of the Variscan Erzgebirge Crystalline Complex, Germany","authors":"Hans-Joachim Massonne","doi":"10.1111/jmg.12794","DOIUrl":"10.1111/jmg.12794","url":null,"abstract":"<p>A quartz-rich paragneiss from the Variscan Erzgebirge Crystalline Complex (ECC) was studied in detail because of abundant millimetre-sized and clearly oriented pseudomorphs after a sodic mineral interpreted to have been jadeite. This mineral, or pseudomorphs after it, is rarely found in extensive high-pressure (HP)–ultrahigh-pressure (UHP) terranes worldwide despite reported pressure–temperature (P–T) conditions suitable for the formation of jadeite in common paragneisses and orthogneisses. In the studied rock, which contains abundant large and oriented potassic white mica flakes and minor millimetre-sized garnet grains, the pseudomorphs consist of clusters of small albite grains with thin phengitic muscovite flakes in between. X-ray maps for Ca and Mg in garnet demonstrate that an early generation of this mineral (Gt1) was corroded and subsequently overgrown by a Ca-richer generation (Gt2). White mica is phengite with maximum Si contents of 3.42 atoms per formula unit. P–T conditions of 0.85 GPa and 650°C and 1.7 GPa and 660°C were derived for the formation of Gt1 and Gt2 rim + Si-rich phengite, respectively, using pseudosection modelling. The latter conditions representing the pressure peak experienced by the paragneiss are compatible with the original presence of jadeite and possibly paragonite as well. This metamorphic peak occurred at 338.4 ± 2.3 (2σ) Ma based on in situ dating of monazite grains with the electron microprobe. A single monazite age of 386.4 ± 10.5 (2σ) Ma is related to the formation of Gt1. Thus, a Late Devonian metamorphism is suggested here for the first time to have occurred in ECC gneisses before the major HP event in the Early Carboniferous. Furthermore, the study demonstrates that the eclogite-facies gneisses of the Gneiss-Eclogite Unit of the ECC experienced peak pressures of not more than 2 GPa in contrast to recent proposals of an extensive UHP area in this unit. In addition, it is suggested that the localized occurrence of UHP rocks surrounded by other lithologies otherwise lacking evidence for UHP conditions should be interpreted with caution with respect to their regional extent and significance.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"42 9","pages":"1159-1178"},"PeriodicalIF":3.5,"publicationDate":"2024-09-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.12794","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142257946","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}