Journal of Metamorphic Geology最新文献

{"title":"Redistribution of heat-producing elements during melting of Archean crust","authors":"Carson Kinney,&nbsp;Jillian Kendrick,&nbsp;Manuel Duguet,&nbsp;Chris Yakymchuk","doi":"10.1111/jmg.12751","DOIUrl":"10.1111/jmg.12751","url":null,"abstract":"<p>Heat generated from the decay of K, Th, and U plays a fundamental role in the differentiation and stabilization of Earth's continental crust. This is particularly important in the construction of Archean cratons that form the nuclei of Earth's continents. The Kapuskasing uplift is a rare exposure of an Archean-age crustal cross-section that provides a snapshot of crustal melting, differentiation, and compositional stratification. We integrate field observations, whole-rock compositions, thermodynamic equilibrium and accessory mineral modelling with heat production and latency time modelling to provide insights into the partitioning of heat-producing elements between residue and melt during anatexis of metabasites as well as the resulting effects on metamorphic timescales and the production of tonalite–trondhjemite–granodiorite (TTG) suites. We model six metabasite compositions ranging from relatively fertile greenschist facies metabasites to melt-depleted residual mafic (upper-)amphibolites to granulites. Heat-producing elements are modelled to be partitioned between melt and residue; the dominant minerals in the residue that host these elements are apatite, hornblende, K-feldspar, and epidote. At 800–850°C epidote is no longer stable, and the melt fraction is predicted to contain roughly half of the heat production capacity for the system. Apatite and melt are expected to be the dominant repositories for Th and U during anatexis; zircon is predicted to be completely consumed by 850°C, whereas apatite persists to higher temperatures and allanite is expected only in minor modal abundances at high-P, low-T conditions. The partitioning of heat-producing elements into relatively low-density melt decreases the heat production of the residual system during anatexis. Due to their high density and affinity for U and Th, epidote and apatite retain heat production capacity in the residue during metabasite melting. Thermal latency modelling of metamorphism suggests that enriched metabasite compositions require 38–46 My to increase the temperature from ~650 to 850°C (solidus temperature to peak metamorphic temperature of the Kapuskasing uplift), whereas estimates are considerably shorter for depleted compositions (7–25 My). Four of the six samples modelled require 60–70 My to reach 1000°C from the solidus. Our modelling of heat-producing element partitioning and predicted proportions of melt suggest that enriched basaltic compositions are the most reasonable source of TTG magmas and our heating time modelling indicates the mantle as an equal to dominant source of heat for metabasite anatexis compared with radiogenic heat production.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"42 2","pages":"197-224"},"PeriodicalIF":3.4,"publicationDate":"2023-12-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.12751","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138573731","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":"Assessing the importance of H2O content in the tectono-metamorphic evolution of shear zones: A case study from the Dora-Maira Massif (Western Alps)","authors":"Sara Nerone,&nbsp;Alessandro Petroccia,&nbsp;Fabiola Caso,&nbsp;Davide Dana,&nbsp;Andrea Maffeis","doi":"10.1111/jmg.12750","DOIUrl":"10.1111/jmg.12750","url":null,"abstract":"<p>Metamorphic reactions are commonly driven to completion within shear zones thanks to fluid circulation, making the re-equilibration of the mineral assemblage one of the dominant processes. Despite the important role of H₂O in such processes, forward thermodynamic modelling calculations commonly assume either H₂O-saturated conditions or only fluid loss during prograde evolution to peak conditions. These assumptions influence the understanding of shear zones during the retrograde evolution. Here, we investigate the <i>P–T–M</i>H₂O retrograde evolution of the Mt. Bracco Shear Zone (MBSZ), an Alpine ductile tectonic contact which marks the boundary between two H<i>P</i> units in the Dora-Maira Massif (Western Alps, Italy). After the eclogite-facies peak (at 500–520°C and 1.8–2.2 GPa), the subsequent mylonitic event is constrained at amphibolite-facies conditions, continuing its evolution at decreasing pressure and temperature during rock exhumation, from ~590°C, 1.0 GPa down to ~520°C, 0.7 GPa. The <i>P</i>/<i>T</i>–<i>M</i>H₂O forward modelling highlights different behaviour for the two analysed samples. After reaching a minimum H₂O content at the transition from eclogite- to amphibolite-facies conditions, a significant fluid gain is modelled for only one of the two analysed samples just before the mylonitic event. The MBSZ then evolves towards H₂O-undersaturated conditions. This work thus underlines the necessity of investigating the H₂O evolution within shear zones, as the H₂O content is susceptible to change through the <i>P–T</i> path, due to dehydration reactions or fluid infiltration events. Furthermore, lithological heterogeneities influence possible different fluid circulation regimes in shear zones, resulting in externally or internally derived fluid gain.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"42 2","pages":"171-196"},"PeriodicalIF":3.4,"publicationDate":"2023-12-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.12750","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138515091","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"The origin of calcite in calc-silicate rocks from the Kokchetav ultrahigh-pressure metamorphic complex","authors":"Anastasia O. Mikhno,&nbsp;Anton F. Shatskiy,&nbsp;Andrey V. Korsakov,&nbsp;Yulia G. Vinogradova,&nbsp;Jasper Berndt,&nbsp;Stephan Klemme,&nbsp;Sergey V. Rashchenko","doi":"10.1111/jmg.12749","DOIUrl":"10.1111/jmg.12749","url":null,"abstract":"<p>Understanding calcite genesis in ultrahigh-pressure crustal rocks is a key to the reconstruction of the evolution of ultrahigh-pressure metacarbonate rocks. Here, we present new data and a new model on the genesis and the <i>P–T</i> conditions of the formation of calcite found in the ultrahigh-pressure calc-silicate rocks from the Kokchetav massif. In the studied sample aragonite inclusions coexist with Type A calcite inclusions (previously interpreted as mineral inclusions) and the inclusions of Type B calcite (previously interpreted as derived from the crystallization of carbonatitic melt) in cores of garnet porphyroblasts. The most Mg-rich calcite from Type A inclusions coexisting with aragonite inclusions in one garnet growth zone shows X_Ca = 0.935 implying their crystallization during a retrograde metamorphic stage at <i>P</i> ~ 2.3 GPa and <i>T</i> ~ 870°C along the <i>P–T</i> path. Type A calcite and aragonite inclusions were also found coexisting in one growth zone with K-bearing clinopyroxene inclusion (ω[K₂O] = 0.5 wt.%). Such a high K₂O-content in clinopyroxene testify that the pressure of inclusion capture exceeded 3.5 GPa, which contradicts the <i>P–T</i> conditions estimated by X_Ca in magnesian calcite. Thus, Type A calcite inclusions were initially captured as an aggregate of aragonite+ magnesian calcite at ultrahigh pressure metamorphic stage (<i>P</i> ≥ 3.5 GPa, <i>T</i> = 900–1,000°C) and then re-equilibrated at lower conditions (<i>P</i> ≤ 2.3 GPa and <i>T</i> ≤ 870°C). The trace element composition of aragonite and Type A and Type B calcite from inclusions was also studied to clarify calcite genesis in these inclusions. Aragonite shows high LREE (5–57 ppm) and Sr-content (600–800 ppm). Calcite from Type A inclusions shows low LREE (2.9–19.8 ppm) and Sr-content (490–670 ppm). Calcite from Type B inclusions forms two groups according to the LREE and Sr content distribution (Type B1 and Type B2). Trace element distribution in Type B1 calcite is identical to that of Type A calcite, while Type B2 calcite shows high LREE (6.8–64.9 ppm) concentrations along with low Sr-content (180–340 ppm). Type A and Type B1 calcite is interpreted to have been re-equilibrated. Type B2 calcite inclusions crystallized from the hydrous carbonatitic melt.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"42 2","pages":"143-170"},"PeriodicalIF":3.4,"publicationDate":"2023-11-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135272098","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":"Eclogite thermobarometry: The consistency between conventional thermobarometry and forward phase-equilibrium modelling","authors":"David Hernández-Uribe,&nbsp;Robert M. Holder,&nbsp;Juan D. Hernández-Montenegro","doi":"10.1111/jmg.12747","DOIUrl":"10.1111/jmg.12747","url":null,"abstract":"&lt;p&gt;Eclogite thermobarometry is crucial for constraining the depths and temperatures to which oceanic and continental crust subduct. However, obtaining the pressure and temperature (&lt;i&gt;P–T&lt;/i&gt;) conditions of eclogites is complex as they commonly display high-variance mineral assemblages, and the mineral compositions only vary slightly with &lt;i&gt;P–T&lt;/i&gt;. In this contribution, we present a comparison between two independent and commonly used thermobarometric approaches for eclogites: conventional thermobarometry and forward phase-equilibrium modelling. We assess how consistent the thermobarometric calculations are using the garnet–clinopyroxene–phengite barometer and garnet–clinopyroxene thermometer with predictions from forward modelling (i.e. comparing the relative differences between approaches). Our results show that the overall mismatch in methods is typically ±0.2–0.3 GPa and ±29–42°C although differences as large as 80°C and 0.7 GPa are possible for a few narrow ranges of &lt;i&gt;P–T&lt;/i&gt; conditions in the forward models. Such mismatch is interpreted as the relative differences among methods, and not as absolute uncertainties or accuracies for either method. For most of the investigated &lt;i&gt;P–T&lt;/i&gt; conditions, the relatively minor differences between methods means that the choice in thermobarometric method itself is less important for geological interpretation than careful sample characterization and petrographic interpretation for deriving &lt;i&gt;P–T&lt;/i&gt; from eclogites. Although thermobarometry is known to be sensitive to the assumed &lt;i&gt;X&lt;/i&gt;&lt;sub&gt;Fe&lt;/sub&gt;&lt;sup&gt;3+&lt;/sup&gt; of a rock (or mineral), the &lt;i&gt;relative&lt;/i&gt; differences between methods are not particularly sensitive to the choice of bulk-rock &lt;i&gt;X&lt;/i&gt;&lt;sub&gt;Fe&lt;/sub&gt;&lt;sup&gt;3+&lt;/sup&gt;, except at high temperatures (&gt;650°C, amphibole absent) and for very large differences in assumed &lt;i&gt;X&lt;/i&gt;&lt;sub&gt;Fe&lt;/sub&gt;&lt;sup&gt;3+&lt;/sup&gt; (0–0.5). We find that the most important difference between approaches is the activity–composition (&lt;i&gt;a–x&lt;/i&gt;) relations, as opposed to the end-member thermodynamic data or other aspects of experimental calibration. When equivalent &lt;i&gt;a–x&lt;/i&gt; relations are used in the conventional barometer, &lt;i&gt;P&lt;/i&gt; calculations are nearly identical to phase-equilibrium models (Δ&lt;i&gt;P&lt;/i&gt; &lt; 0.1). To further assess the implications of these results for real rocks, we also evaluate common mathematical optimizations of reaction constants used for obtaining the maximum &lt;i&gt;P–T&lt;/i&gt; with conventional thermobarometric approaches (e.g. using the highest &lt;i&gt;a&lt;/i&gt;Grs&lt;sup&gt;2&lt;/sup&gt; × &lt;i&gt;a&lt;/i&gt;Prp in garnet and Si content in phengite, and the lowest &lt;i&gt;a&lt;/i&gt;Di in clinopyroxene). These approaches should be used with caution, because they may not represent the compositions of equilibrium mineral assemblages at eclogite facies conditions and therefore systematically bias &lt;i&gt;P–T&lt;/i&gt; calculations. Assuming method accuracy, geological meaningful &lt;i&gt;P&lt;/i&gt;&lt;sub&gt;max&lt;/sub&gt; at a typical eclogite facies temperature of ~660°C will be obtained ","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"42 1","pages":"89-108"},"PeriodicalIF":3.4,"publicationDate":"2023-10-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.12747","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135779815","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":"Constraints on the Palaeoproterozoic tectono-metamorphic evolution of the Lewisian Gneiss Complex, NW Scotland: Implications for Nuna assembly","authors":"Silvia Volante,&nbsp;Annika Dziggel,&nbsp;Jesse B. Walters,&nbsp;Noreen J. Evans,&nbsp;Maximilian Herbst,&nbsp;Richard Albert Roper","doi":"10.1111/jmg.12748","DOIUrl":"10.1111/jmg.12748","url":null,"abstract":"<p>Despite extensive investigation, the tectono-thermal evolution of the Archean crust in the Lewisian Gneiss Complex in NW Scotland (LGC) is debated. Most U–Pb zircon geochronological and metamorphic studies have focused on rocks from the central region of the mainland LGC, where granulite facies assemblages associated with the oldest (Badcallian) tectono-metamorphic event at c. 2.75 Ga are overprinted by younger amphibolite facies assemblages related to the Inverian (c. 2.5 Ga) and subsequent Laxfordian (c. 1.9–1.65 Ga) tectono-thermal events. In the southern and northern regions of the mainland LGC, deformation and metamorphism associated with the Laxfordian event are pervasive, although the timing and conditions are poorly constrained. Here, we present new field, petrographic and structural data, U–Pb zircon and titanite geochronology and phase equilibrium modelling of amphibolite samples from the northern and southern regions. Our field observations show that in both regions, pre-Laxfordian structures are significantly reworked by steep NW-striking fabrics that are themselves pervasively overprinted by co-axial deformation and amphibolite facies metamorphism related to the Laxfordian event. In situ U–Pb titanite geochronology yields Laxfordian ages of 1853 ± 20 Ma in the southern region (P = 6–8 kbar and T = 640–690°C) and 1750 ± 20 Ma and 1776 ± 10 Ma in the northern region (P = 6–7.5 kbar and T = 740–760°C). While U–Pb dating of zircon rims from felsic gneisses in the central region shows a dominant Inverian metamorphic overprint at c. 2500 Ma, zircon rims in felsic gneisses from the northern and southern regions commonly yield Laxfordian dates as young as c. 1800 Ma. Combined, the results support the idea that, during the Palaeoproterozoic, the central region of the LGC acted as low-strain domain, in which intense deformation and metamorphism were restricted to crustal-scale shear zones. By contrast, in the southern and northern regions, early (c. 1.85 Ga) and late (c. 1.75 Ga) Laxfordian deformation and fluid-mediated metamorphism were much more pervasive and at higher P–T conditions than previously proposed. The diachronous Laxfordian evolution of the southern and northern regions indicate that they reflect early and late snapshots of collisional to transpressional tectonics in the mainland LGC. The long-lasting Laxfordian evolution documents the collision of the Rae and North Atlantic cratons during the Palaeoproterozoic amalgamation of the supercontinent Nuna, with implications for the palaeogeographic configuration of NW Scotland during Palaeoproterozoic Nuna.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"42 1","pages":"109-142"},"PeriodicalIF":3.4,"publicationDate":"2023-10-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.12748","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135779948","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":"Chromium in minerals as tracer of the polycyclic evolution of eclogite and related metabasite from the Pohorje Mountains, Slovenian Eastern Alps","authors":"Botao Li,&nbsp;Hans-Joachim Massonne","doi":"10.1111/jmg.12746","DOIUrl":"10.1111/jmg.12746","url":null,"abstract":"<p>Significantly different peak pressure–temperature (P–T) conditions (18–26 kbar and 630–760°C versus 29–37 kbar and 750–940°C) have previously been published for eclogite and related metabasites from the south-eastern flank of the Pohorje Mountains in Slovenia. These rocks can show a bimodal distribution of chromium in the rock-forming minerals, particularly garnet, the role of which in their metamorphic evolution is unclear. Therefore, we studied an eclogite and a related rock with clinopyroxene containing only 17 mol% jadeite + acmite (sample 18Ca35a). KαCr intensity maps of garnet particularly in sample 18Ca35a show a sharp irregular boundary between the core (Gt1) and the mantle (Gt2). Gt1 of millimetre-sized garnet in this rock is nearly Cr-free and unzoned, whereas Gt2 is of different composition (0.22 wt.% Cr₂O₃) and slightly zoned. Nearly Cr-free amphibole, (clino)zoisite, kyanite and staurolite inclusions are present in Gt1. The matrix consists of garnet and Cr-bearing clinopyroxene, (clino)zoisite and amphibole. Thermodynamic modelling suggests peak P–T conditions of 22.5 ± 2 kbar at 710 ± 25°C (Gt1) and 23 ± 2 kbar at 700 ± 25°C (Gt2) in both samples. We interpret these findings to suggest that olivine- and hornblende-bearing gabbros with some chromite experienced early metamorphism in the eclogite facies, when Gt1 formed. The rock was subsequently exhumed and cooled leading to significant garnet corrosion. A second stage of metamorphism, recognized by mappable Cr contents in garnet, led to the growth of Gt2 and other Cr-bearing minerals at the expense of chromite relics, which survived stage I. The peak P–T conditions of stage II are compatible with those previously derived by same authors and support the view that probably no ultrahigh-pressure eclogite exists in the Pohorje Mountains. We relate the two metamorphic events to the Cretaceous and Palaeogene high-pressure events recently reported from micaschists of the Pohorje Mountains.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"42 1","pages":"63-88"},"PeriodicalIF":3.4,"publicationDate":"2023-10-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.12746","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"136212314","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":"Polymetamorphism during the Grenvillian Orogeny in SE Ontario: Results from trace element mapping, in situ geochronology, and diffusion geospeedometry","authors":"F. Gaidies,&nbsp;T. Mccarron,&nbsp;A. D. Simpson,&nbsp;R. M. Easton,&nbsp;S. Glorie,&nbsp;B. Putlitz,&nbsp;K. Trebus","doi":"10.1111/jmg.12745","DOIUrl":"10.1111/jmg.12745","url":null,"abstract":"&lt;p&gt;The Flinton Group is a metasedimentary succession of the Grenville Province in SE Ontario, potentially allowing insight into the tectono-thermal evolution of continental crust during the Mesoproterozoic. At its Green Bay locality, Flinton Group metapelites of the staurolite zone contain abundant, post-kinematic garnet porphyroblasts. Whereas the larger garnet crystals are typically impinged, smaller crystals are isolated from each other, occasionally exhibiting elongated shapes with apparently trigonal morphology. Central sections of the garnet population of a representative sample reveal that garnet is composed of different compositional and microstructural domains. In the largest crystals of the population, garnet contains rectangular to rhombic domains, marked by sharp increases in the concentrations of Nb, V, Ti, and Cr. These domains are associated with irregularly shaped patches, characterized by spatially heterogenous enrichments of Ca and LREE, and depletions in the contents of P, Y, MREE, and HREE, accompanied by increased densities of comparatively coarse-grained quartz inclusions hosting apatite. Microstructural relationships indicate that these domains correspond to portions of garnet that pseudomorphed biotite, with the enrichments of Nb, V, Ti, and Cr outlining the original biotite shapes. The compositional patterns formed by Ca, P, Y, and REE indicate that apatite participated in the grain-fluid interactions that operated during the metasomatic replacement of biotite by garnet. The statistical analyses of the garnet number and size distributions confirm that garnet initially nucleated on biotite, controlled by the kinetics of attachment and detachment processes at the garnet/biotite interface, resulting in the typical impingement habit. In situ Lu–Hf garnet geochronology applied to garnet that did not pseudomorph biotite, and hence is enriched in HREE, points to a first metamorphic event at c. 1080 \u0000&lt;math&gt;\u0000 &lt;mo&gt;±&lt;/mo&gt;&lt;/math&gt; 31 Ma. Subsequent pseudomorphism of staurolite by white mica in a Al&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt;- and FeO-mobile system resulted in the concomitant crystallization of a new garnet generation, forming overgrowths on the first garnet generation and nuclei in the fine-grained matrix. Garnet that nucleated during this event grew to isolated and elongated crystals with apparently trigonal morphology, aligned in a direction c. perpendicular to the rock matrix foliation. The open-system behaviour during this event limits the use of whole-rock-based geochronological and thermobarometrical applications. However, previously published in situ U–Pb ages of monazite included in the rims of the garnet crystals and in the rock matrix indicate that this event took place at c. 976 \u0000&lt;math&gt;\u0000 &lt;mo&gt;±&lt;/mo&gt;&lt;/math&gt; 4 Ma, likely associated with a period of increased hydrothermal activity late in the metamorphic history of the Grenvillian Orogeny. Diffusion geospeedometry calculations indicate that garnet grow","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"42 1","pages":"35-61"},"PeriodicalIF":3.4,"publicationDate":"2023-09-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.12745","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135306776","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":"Formation of low-pressure reaction textures during near-isothermal exhumation of hot orogenic crust (Bohemian Massif, Austria)","authors":"Dominik Sorger,&nbsp;Christoph A. Hauzenberger,&nbsp;Fritz Finger,&nbsp;Manfred Linner,&nbsp;Etienne Skrzypek,&nbsp;Simon Schorn","doi":"10.1111/jmg.12744","DOIUrl":"10.1111/jmg.12744","url":null,"abstract":"<p>Two types of aluminous paragneiss from the Loosdorf complex (Bohemian Massif, NE Austria) contain coarse-grained granulite assemblages and retrograde reaction textures that are investigated to constrain the post-peak history of the Gföhl unit in the southern Bohemian Massif. Both types have a peak assemblage garnet–biotite–sillimanite–plagioclase–K-feldspar–quartz–granitic melt ± kyanite ± ilmenite ± rutile, recording peak metamorphic conditions of \u0000<math>\u0000 <mo>∼</mo></math>0.9–1.1 GPa and \u0000<math>\u0000 <mo>∼</mo></math>780–820°C estimated by isochemical phase equilibrium modelling. The first sample type (Ysper paragneiss) developed (i) cordierite coronae around garnet and (ii) cordierite–spinel and cordierite–quartz reaction textures at former garnet–sillimanite interfaces. Calculated chemical potential relationships indicate that the textures formed in the course of a post-peak near-isothermal decompression path reaching \u0000<math>\u0000 <mo>∼</mo></math>0.4 GPa. Texture formation follows a two-step process. Initially, cordierite coronae grow between garnet and sillimanite. As these coronae thicken, they facilitate the development of local compositional domains, leading to the formation of cordierite–spinel and cordierite–quartz symplectites. The second sample type (Pielach paragneiss) exhibits only discontinuous cordierite coronae around garnet porphyroblasts but lacks symplectites. The formation of cordierite there also indicates near-isothermal decompression to 0.4–0.5 GPa and 750–800°C. This relatively hot decompression path is explained by the contemporaneous exhumation of a large HP–UHT granulite body now underlying the Loosdorf complex. The timing of regional metamorphism in the granulites and the southern Bohemian Massif in general is well constrained and has its peak at \u0000<math>\u0000 <mo>∼</mo></math>340 Ma. Monazite from Loosdorf paragneiss samples yield a slightly younger age of \u0000<math>\u0000 <mo>∼</mo></math>335 Ma. Although the ages overlap within error, they are interpreted to reflect near-isothermal decompression and exhumation resulting in the formation of the observed reaction textures.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"42 1","pages":"3-34"},"PeriodicalIF":3.4,"publicationDate":"2023-09-14","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.12744","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"134910600","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":"Origin of Erzgebirge ultrahigh-pressure garnetite: Formation from a basaltic protolith by serpentinization-assisted metasomatism?","authors":"Esther Schmädicke,&nbsp;Thomas M. Will","doi":"10.1111/jmg.12742","DOIUrl":"10.1111/jmg.12742","url":null,"abstract":"&lt;p&gt;Erzgebirge ultrahigh-pressure (UHP) garnet peridotite includes scarce layers of garnet pyroxenite, nodules of garnetite and, very rarely, of eclogite. Peridotite-hosted eclogite shows the same subalkali-basaltic bulk rock composition, mineral assemblage and peak conditions as gneiss-hosted eclogite present in the same UHP unit. Garnetite has considerably more Mg, moderately enhanced Ca and Fe and significantly lower contents of Na, Ti, P, K and Si than eclogite, whereas Al is very similar. In addition, the compatible trace elements (Ni, Co, Cr, V) are elevated and most incompatible elements (Zr, Hf, Y, Sr, Rb and rare Earth elements [REE]) are depleted in garnetite relative to eclogite. In contrast to other large ion lithophile elements (LILEs), Pb (+121%) and Ba (+83%) are strongly enriched. The REE patterns of garnetite are characterized by depletion of light and heavy REE and a medium REE hump indicative of metasomatism, features being absent in eclogite. An exceptional garnetite sample shows an REE distribution similar to that of eclogite. Garnetite is interpreted to have formed from the same, but metasomatically altered, igneous protolith as eclogite. Except for Ba and Pb, the chemical signature of garnetite is explained best by metasomatic changes of its basaltic protolith caused by serpentinization of the host peridotite. Garnetite is chemically similar to basaltic rodingite/metarodingite. Although rodingite is commonly more enriched in Ca, there are also examples with moderately enhanced Ca matching the composition of Erzgebirge garnetite. Limited Ca metasomatism is attributed to the preservation of Ca in peridotite during hydrous alteration. This can be explained by incomplete serpentinization favouring metastable survival of the original clinopyroxene. In this case, most Ca is retained in peridotite and not available for infiltration and metasomatism of the garnetite protolith. This inescapable consequence is supported by the fact that clinopyroxene is part of the garnet peridotite UHP assemblage, which would not be the case if Ca had been removed from the protolith prior to high-pressure metamorphism. The enrichment of compatible elements in garnetite is attributed to decomposition of peridotitic olivine (Ni, Co) and spinel (Cr, V) during serpentinization. Enrichment of Ba and Pb contrasts the behaviour of other LILEs and is ascribed to dehydration of the serpentinized peridotite (deserpentinization). This requires two separate stages of metasomatism: (1) intense chemical alteration of the basaltic garnetite precursor, together with serpentinization of peridotite at the ocean floor or during incipient subduction; and (2) prograde metamorphism and dehydration of serpentinite during continued subduction, thereby releasing Pb–Ba-rich fluids that reacted with associated metabasalt. Finally, subduction to &gt;100 km and UHP metamorphism of all lithologies led to formation of garnetite, eclogite and garnet pyroxenite hosted by co-facial g","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"41 9","pages":"1237-1259"},"PeriodicalIF":3.4,"publicationDate":"2023-09-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43728042","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":"Long-lived high-grade metamorphism in southern India: Constraints from charnockites and sapphirine-bearing semipelitic granulites from the Madurai Block","authors":"Ashish Kumar Tiwari,&nbsp;Tapabrato Sarkar,&nbsp;Sourav Karmakar,&nbsp;Nilanjana Sorcar,&nbsp;Sneha Mukherjee","doi":"10.1111/jmg.12743","DOIUrl":"10.1111/jmg.12743","url":null,"abstract":"&lt;p&gt;The Granulite Terrane of Southern India is a collage of Mesoarchean–Neoproterozoic crustal blocks that underwent high-grade metamorphism associated with the final assembly of the Gondwana supercontinent during late Neoproterozoic–Cambrian. Here, we investigate the charnockites and associated sapphirine-bearing semipelitic granulites from the eastern part of the Madurai Block (MB). We present new petrographic, mineral chemistry, and geochronological data to constrain the &lt;i&gt;P&lt;/i&gt;–&lt;i&gt;T&lt;/i&gt;–&lt;i&gt;t&lt;/i&gt; evolution of the block and unravel the timescale and source of heat for the ultrahigh-temperature metamorphism. Both the rock types contain coarse-grained porphyroblastic garnet and orthopyroxene, yielding peak &lt;i&gt;P&lt;/i&gt;–&lt;i&gt;T&lt;/i&gt; conditions of 950 ± 30°C at 10.5 ± 0.8 kbar and 970 ± 40°C at 10 ± 0.5 kbar for semipelite and charnockite, respectively, using conventional thermobarometry. Peak ultrahigh temperatures are further supported by high Al content in the orthopyroxene (8.78 wt% Al&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt;) coexisting with garnet (&lt;i&gt;X&lt;/i&gt;&lt;sub&gt;Mg&lt;/sub&gt;: up to 0.57) and feldspar thermometry of the mesoperthites and antiperthites in the semipelite, yielding 950–980°C at 10 kbar. Subsequent decompression has led to the formation of coronal orthopyroxene3 + plagioclase3 in the charnockite and symplectic orthopyroxene3 + cordierite ± sapphirine ± plagioclase3 in the semipelite, yielding &lt;i&gt;P&lt;/i&gt;–&lt;i&gt;T&lt;/i&gt; range of 950–850°C and 9.5–6.8 kbar for semipelites and 950–820°C and 8–6.5 kbar for charnockite. Based on the obtained &lt;i&gt;P&lt;/i&gt;–&lt;i&gt;T&lt;/i&gt; estimates, preserved reaction textures, and phase equilibria modelling in the MnNCKFMASHTO system, a clockwise &lt;i&gt;P&lt;/i&gt;–&lt;i&gt;T&lt;/i&gt; evolution with isothermal decompression followed by cooling is inferred for both the rock types.&lt;/p&gt;&lt;p&gt;Texturally constrained in situ monazite dating and rare earth element (REE) patterns show that the core of matrix monazite having low-Th, Y, and extreme heavy rare earth element (HREE) depletion, yielding weighted mean ages of 582 ± 12 and 590 ± 22 Ma for semipelite and charnockite, respectively, dates the prograde evolution. The mantle of the matrix monazite in semipelite and comparable rim in charnockite, having relative Th-enrichment compared to the core, yielding weighted mean ages of 552 ± 9 and 557 ± 13 Ma, respectively, dates extensive dissolution–reprecipitation from the melt at the peak stage. The relatively Th- and Y-rich and moderately HREE-depleted rim of matrix monazite in the semipelite, yielding weighted age of 516 ± 6 Ma, date initial garnet breakdown during post-peak melt crystallization. By contrast, compositionally homogenous HREE + Y-enriched monazite in the symplectite and retrograde monazites yielding weighted mean ages of 487 ± 47 Ma for semipelites and 508 ± 19 Ma for charnockites dates extensive garnet breakdown during final stages of melt crystallization and subsequent cooling. Our findings point to collision initiation at ~590 Ma, with the peak conditions attained","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"41 9","pages":"1261-1297"},"PeriodicalIF":3.4,"publicationDate":"2023-08-31","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"48583028","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}