Journal of Metamorphic Geology最新文献

{"title":"Orogen-scale uniformity of recorded granulite facies conditions due to thermal buffering and melt retention","authors":"Johann F. A. Diener,&nbsp;Paul H. Macey","doi":"10.1111/jmg.12778","DOIUrl":"10.1111/jmg.12778","url":null,"abstract":"<p>Granulite facies metapelitic gneisses collected over a \u0000<span></span><math>\u0000 <mn>200</mn>\u0000 <mo>×</mo>\u0000 <mn>120</mn></math> km exposed area of the Kakamas Domain of the Namaqua–Natal Metamorphic Province in southern Namibia all contain similar garnet–sillimanite–cordierite–biotite–quartz–K-feldspar–ilmenite \u0000<span></span><math>\u0000 <mo>±</mo></math> plagioclase \u0000<span></span><math>\u0000 <mo>±</mo></math> magnetite mineral assemblages. These assemblages are interpreted to have equilibrated at suprasolidus retrograde conditions, and most samples contain distinct biotite- or sillimanite-free peak assemblages. Pseudosection modelling constrains extremely uniform residuum solidus conditions of \u0000<span></span><math>\u0000 <mn>5</mn>\u0000 <mo>.</mo>\u0000 <mn>5</mn>\u0000 <mo>±</mo>\u0000 <mn>1</mn></math> kbar and \u0000<span></span><math>\u0000 <mn>790</mn>\u0000 <mo>±</mo>\u0000 <mn>30</mn></math>°C for the entire Kakamas Domain. Estimated peak metamorphic conditions overlap with these but are more smeared out at between 4 and 7 kbar at 760°C to potentially more than 900°C. The uniformity of residuum solidus conditions is not coincidental, but is a consequence of retrograde re-equilibration due to minor melt retention after peak metamorphism. Re-equilibration could only stop once all retained melt had crystallized, which required the concomitant growth of a hydrous mineral to account for its H₂O component. Biotite is the most stable hydrous mineral in these rocks, such that the residuum \u0000<span></span><math>\u0000 <mi>P</mi></math>–\u0000<span></span><math>\u0000 <mi>T</mi></math> conditions in the Kakamas Domain reflect the upper-\u0000<span></span><math>\u0000 <mi>T</mi></math> stability of biotite, and also corresponds to the intersection of the well-known biotite–sillimanite melting reaction that consumed all biotite during prograde metamorphism. The calculated melt fertility of the sample suite indicates that the variable amounts of heat consumed to overcome the latent heat of fusion could have caused a 25°C spread in the peak temperature achieved by the most and least fertile samples. Peak temperature in the Kakamas Domain may have been as much as 100°C higher than residuum solidus conditions for specific samples but cannot be confidently constrained as it is obscured by the effects of both thermal buffering during prograde metamorphism and melt retention during retrograde metamorphism. Both processes are an inescapable part of the evolution of all granulite facies rocks, but their effects are most pronounced in fertile rocks like metapelites that are traditionally the preferred lithology for quantifying the \u0000<span></span><math>\u0000 <mi>P</mi></math>–\u0000<span></span><math>\u0000 <mi>T</mi></math> history of exhumed terranes.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"42 7","pages":"909-931"},"PeriodicalIF":3.5,"publicationDate":"2024-06-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.12778","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141268749","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":"Heat sources for Variscan high-temperature–low-pressure metamorphism: Petrochronological constraints from the Trois Seigneurs massif, French Pyrenees","authors":"Charlotte H. Connop,&nbsp;Andrew J. Smye,&nbsp;Joshua M. Garber,&nbsp;Tushar Mittal","doi":"10.1111/jmg.12775","DOIUrl":"10.1111/jmg.12775","url":null,"abstract":"&lt;p&gt;High-temperature–low-pressure metamorphism is commonly associated with intermediate to felsic magmatism in continental orogenic belts. The heat budgets and transfer mechanisms responsible for such elevated temperatures and partial melting of the upper crust are uncertain. The Trois Seigneurs massif, French Pyrenees, preserves a structurally continuous record of Variscan high-temperature–low-pressure metamorphism through a sequence of upper-to-mid-crustal Paleozoic metasedimentary rocks. Conventional thermobarometry and phase equilibria calculations show that metamorphic conditions span ~2.5 kbar, 575°C to suprasolidus conditions of ~6 kbar, 700°C. Peak temperatures depend strongly on depth: temperature gradients of 50–60°C/km are present through the uppermost 12 km of the section; deeper portions (12–20 km) define restricted temperature conditions of ~650–700°C. The lowest-grade metamorphic rocks preserve the largest spread in monazite &lt;sup&gt;206&lt;/sup&gt;Pb*/&lt;sup&gt;238&lt;/sup&gt;U dates, from c. 325–285 Ma, while the spread in dates is restricted to c. 305–290 Ma in the highest-grade rocks. Within this spread, each sample yields a well-defined population of monazite &lt;sup&gt;206&lt;/sup&gt;Pb*/&lt;sup&gt;238&lt;/sup&gt;U dates with peaks at c. 305 Ma in the andalusite schists, 295 Ma in the sillimanite schists, and 300 Ma in the migmatite sample. Monazite trace-element compositions capture a systematic change with decreasing date and increasing metamorphic grade, including a more negative Eu-anomaly and decreasing Sr concentrations, consistent with co-crystallizing feldspar; increasing HREE and Y contents, consistent with xenotime breakdown; and decreasing Th/U, reflecting increasing U content during breakdown of inherited zircon. Zircon rims from a granite unit that formed via partial melting of the Paleozoic sedimentary package yields a &lt;sup&gt;206&lt;/sup&gt;Pb/&lt;sup&gt;238&lt;/sup&gt;U-&lt;sup&gt;207&lt;/sup&gt;Pb/&lt;sup&gt;235&lt;/sup&gt;U concordia age of 304.1 ± 3.73 Ma. These rims have trace-element compositions reflecting cogenetic apatite and zircon growth during granite formation. Zircon from a calc-alkaline granodiorite intrusion preserves a 40 Ma record of melt-related activity in the lower crust that preceded the regional thermal climax. We interpret these petrochronological data to show that the Trois Seigneurs field gradient including andalusite schist and biotite granite samples represents a genuine geotherm through Variscan orogenic crust during the regional thermal climax at 305 Ma. When combined with constraints from other Pyrenean massifs, the form of the geotherm is consistent with a thermal scenario in which heat is advected to the upper crust by intermediate-composition magmas generated in the lower crust. A simple thermal model for this process indicates that anatexis in the upper crust may plausibly occur within 10 Ma of the initiation of the lower-crustal melting. Such a thermal scenario, however, requires focusing of melt through a fertile lower crust and an elevated Moho heat flux. We sugg","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"42 6","pages":"867-907"},"PeriodicalIF":3.5,"publicationDate":"2024-05-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.12775","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141187824","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":"Coeval formation and exhumation of metamorphic sole and ophiolite in the Saga ophiolitic mélange: Insights into subduction initiation of the Neo-Tethys","authors":"Jie Shen,&nbsp;Jingen Dai,&nbsp;Kai Yang,&nbsp;Lingling Zhao,&nbsp;Wencang Zhang,&nbsp;Nadia Malaspina's,&nbsp;Pietro Sternai's","doi":"10.1111/jmg.12776","DOIUrl":"10.1111/jmg.12776","url":null,"abstract":"<p>Subduction initiation is recorded by upper plate magmatism and lower plate metamorphism, that is, supra-subduction zone (SSZ) ophiolite–metamorphic sole pair. Here, we report geochemical and geochronological data as well as P–T calculations of amphibolites (metamorphic sole) and hornblende gabbros (SSZ ophiolite) from the Saga ophiolitic mélange in Tibetan Plateau. Amphibolites show trace element contents compatible with normal-mid-ocean ridge basalt (N-MORB), indicating that the protolith of amphibolite formed in a MOR setting. Instead, hornblende gabbros show significant high field strength elements (HFSEs) negative anomalies, enriched large ion lithophile elements (LILEs) and high zircon ε_Hf(t) values, suggesting they formed by fluid-induced partial melting of a depleted mantle. Thermobarometry and phase equilibrium modelling suggest two stages of metamorphism for garnet–clinopyroxene amphibolites: (I) a peak metamorphic stage (~1.9 GPa and 1000°C) and (II) a retrograde metamorphic stage (1.1–1.6 GPa and 800–1000°C). Zircon U–Pb ages of amphibolite and hornblende gabbro are 128.8 ± 5.1 Ma and 128.1 ± 1.5 Ma, respectively, suggesting subduction initiation within the eastern Neo-Tethys occurred no later than 128 Ma and SSZ ophiolite formed at ~128 Ma. Apatite U–Pb ages of amphibolite and hornblende gabbro are 121.8 ± 2.1 Ma and 117.5 ± 4.5 Ma, respectively. Titanite U–Pb age of amphibolite is 122.2 ± 1.5 Ma. Overall, our data suggest that the metamorphic sole and SSZ ophiolite were exhumed since 128–118 Ma, and finally exhumed into the ophiolitic mélange.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"42 6","pages":"843-865"},"PeriodicalIF":3.5,"publicationDate":"2024-05-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140976728","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":"Partial melting mechanisms of peraluminous felsic magmatism in a collisional orogen: An example from the Khondalite belt, North China craton","authors":"Guangyu Huang,&nbsp;Hao Liu,&nbsp;Jinghui Guo,&nbsp;Richard M. Palin,&nbsp;Lei Zou,&nbsp;Weilong Cui","doi":"10.1111/jmg.12774","DOIUrl":"10.1111/jmg.12774","url":null,"abstract":"<p>Sedimentary-derived (S-type) granites are an important product of orogenic metamorphism, and a range of subtypes can be recognized by differences in field occurrence, mineralogy and geochemistry. These subtypes can reflect variations of initial protolith composition, partial melting reactions, pressure and temperature of anatexis, or magmatic processes that occur during ascent through the crust (e.g. mineral fractional crystallization or crustal assimilation). Together, these diverse factors complicate geological interpretation of the history of peraluminous felsic melt fractions in orogenic settings. To assess the influence of these factors, we performed integrated field investigation, petrology, geochemistry, geochronology and phase equilibrium modelling on a series of leucosomes within migmatite associated with different S-type granites within the Khondalite belt, North China craton (NCC), which is an archetypal collisional orogen. Three types of leucosome are recognized in the east Khondalite belt: leucogranitic leucosome, K-feldspar (Kfs)-rich granitic leucosome and garnet (Grt)-rich granitic leucosome. Phase equilibrium modelling of partial melting and fractional crystallization processes indicate that the leucogranitic leucosomes were mostly produced through fluid-present melting, Kfs-rich granitic leucosomes are produced through muscovite dehydration melting with 3 vol.% garnet fractional crystallization, and Grt-rich granitic leucosomes are produced through biotite dehydration melting with 20–40 vol.% K-feldspar fractional crystallization and up to 20 vol.% peritectic garnet entrainment. Mineral fractional crystallization and peritectic mineral entrainment occur in the source during melting, and play equally important roles in partial melting mechanisms in terms of affecting the geochemical compositions of granitic melts. Thus, we suggest that peraluminous felsic magmas preserved in collisional orogens are dominantly produced by fluid-absent melting in the middle to deep continental crust, although extraction of low-volume melt fractions from an anatectic source region at shallower depths during fluid-present melting can also generate small amounts of S-type granites that subsequently crystallize at high structural levels in the crust.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"42 6","pages":"817-841"},"PeriodicalIF":3.5,"publicationDate":"2024-04-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140653227","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":"Metasomatism of the continental crust and its impact on surface uplift: Insights from reactive-transport modelling","authors":"James R. Worthington,&nbsp;Emily J. Chin,&nbsp;Richard M. Palin","doi":"10.1111/jmg.12772","DOIUrl":"10.1111/jmg.12772","url":null,"abstract":"<p>High-elevation, low-relief continental plateaus are major topographic features and profoundly influence atmospheric circulation, sediment transport and storage, and biodiversity. Although orogenic surface-uplift mechanisms for modern continental plateaus near known plate margins like Tibet are well-characterized, they cannot account for examples in intracontinental settings like the Colorado Plateau. In contrast to canonical plate-tectonic uplift mechanisms, broad-scale hydration-induced metasomatism of the lower crust has been suggested to reduce its density and increase its buoyancy sufficiently to contribute to isostatic uplift. However, the relationships between key petrophysical properties in these environments are not fully quantified, which limits application of this model. Here, we develop a series of petrological models that describe the petrological and topographic effects of fluid–rock interaction in non-deforming continental crust of varying composition. We apply an open-system petrological modelling framework that utilizes reactive-transport calculations to determine the spatial and temporal scales over which mineralogic transformations take place compared with the magnitude of infiltration of aqueous fluids derived from devolatilization of subducting oceanic lithosphere. The buoyancy effect of hydration-induced de-densification is most significant for metabasic lower crust, intermediate for metapelitic crust, and minimal for granodioritic crust. We apply these results to a case study of the ~2 km-high Colorado Plateau and demonstrate that under ideal conditions, hydration of its lower–middle crust by infiltrating aqueous fluids released by the Farallon slab during Cenozoic low-angle subduction could have uplifted the plateau surface by a maximum of ~1 km over 16 Myr. However, realistically, although hydration likely has a measurable effect on surface tectonics, the uplift of orogenic plateaus is likely dominantly controlled by other factors, such as lithospheric delamination.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"42 6","pages":"789-815"},"PeriodicalIF":3.5,"publicationDate":"2024-04-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.12772","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140624088","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":"Allanite U–Pb dating places new constraints on the high-pressure to high-temperature evolution of the deep Himalayan crust","authors":"Eleni Wood,&nbsp;Clare J. Warren,&nbsp;Barbara E. Kunz,&nbsp;Tom W. Argles,&nbsp;Anna Bidgood,&nbsp;Alison Halton,&nbsp;Samantha J. Hammond,&nbsp;Ian L. Millar,&nbsp;Nick M. W. Roberts","doi":"10.1111/jmg.12773","DOIUrl":"10.1111/jmg.12773","url":null,"abstract":"<p>During continental collision, crustal rocks are buried, deformed, transformed and exhumed. The rates, timescales and tectonic implications of these processes are constrained through the sequence and conditions of metamorphic reactions in major and accessory phases. Petrographic, isotopic and elemental data from metabasite samples in NW Bhutan, eastern Himalaya, suggest initial equilibration under high-pressure (plagioclase-absent and rutile-present) conditions, followed by decompression to lower pressure conditions at high-temperatures that stabilized plagioclase, orthopyroxene and ilmenite. Field observations and chemical indicators suggest equilibration under the lower pressure conditions is likely linked to the infiltration of melt from the host metasedimentary rocks. The metabasites preserve two metamorphic growth stages of chemically-and petrographically distinct allanite that temporally overlap two stages of zircon growth. Allanite cores and zircon mantles grew at c. 19 ± 2 and 17–15.5 Ma respectively, linked texturally and chemically to the high-pressure evolution. Symplectitic rims on embayed allanite cores, wholly symplectized Aln–Ilm and Aln–Cpx grains, and high U zircon rims grew at c. 15.5–14.5 Ma, linked chemically to the presence of melt and lower pressure, high-temperature conditions. A single garnet Lu–Hf date is interpreted as geologically meaningless, with the bulk rock composition modified by melt infiltration after garnet formation. The open system evolution of these rocks precludes precise determination of the reactive bulk composition during metamorphic evolution and thus absolute conditions, especially during the early high-pressure evolution. Despite these limitations, we show that combined geochemical and petrographic datasets are still able to provide insights into the rates and timescales of deep orogenic processes. The data suggest a younger and shallower evolution for the NW Bhutan metabasites compared to similar rocks in the central and eastern Himalayas.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"42 6","pages":"767-788"},"PeriodicalIF":3.5,"publicationDate":"2024-04-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.12773","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140568700","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":"Devonian Andean-type convergence in the southern Dunhuang block (NW China): Petro-structural, metamorphic P–T and geochronological constraints","authors":"Jérémie Soldner,&nbsp;Yingde Jiang,&nbsp;Pavla Štípská,&nbsp;Karel Schulmann,&nbsp;Chao Yuan,&nbsp;Zongying Huang,&nbsp;Yunying Zhang","doi":"10.1111/jmg.12768","DOIUrl":"10.1111/jmg.12768","url":null,"abstract":"&lt;p&gt;Archean to Palaeoproterozoic basement rocks exposed in the Dunhuang block in NW China were affected by Palaeozoic crustal reworking, as constrained by previous zircon U–Pb geochronological investigations. However, relationships between the Palaeozoic metamorphic ages, &lt;i&gt;P–T&lt;/i&gt; evolution and deformational history of the region remain ambiguous. In order to address this issue, &lt;i&gt;P–T–t–D&lt;/i&gt; paths of paragneisses from the basement of the Hongliuxia belt in the southern Dunhuang block were investigated. Inclusions in garnet and kyanite from the paragneisses are considered as vestiges of Palaeozoic M1 metamorphism corresponding to initiation of the prograde evolution. The earliest continuous metamorphic fabric is an originally steep N–S striking foliation S2. This fabric was reworked by vertical folds F3 associated with the development of a ubiquitous steep, mainly south-dipping, E-W striking axial planar foliation S3. The S2 foliation in paragneisses is mainly associated with Grt–St–Ky–Sil–Bt–Ms–Pl–Qz–Rt assemblages in samples from the western domain and with Grt–Ky–Sil–Bt–Kfs–Pl–Qz–Rt assemblages in samples from the northeastern domain of the Hongliuxia belt. The S3 foliation is associated with Grt–Sil–St–Bt–Ms–Pl–Qz–Ilm assemblages in the western domain and with Grt–Sil–Bt–Ms–Pl–Qz–Kfs–Ilm assemblages in the northeastern domain, followed by growth of chlorite in both domains. Early prograde stage (M1) from 4.0–6.5 kbar and 540–560°C to metamorphic peak (M2a) at 9–10 kbar and ~650–675°C is mainly recorded by paragneisses from the western domain. Subsequent decompression is initially accompanied by heating (M2b) constrained to 6.5–7 kbar and 675–710°C in the western domain, and to 6–6.5 kbar and ~730°C in the northeastern domain, followed by cooling (M3) through 4–6.5 kbar and 550–650°C till late chloritization (late M3). In situ U–Pb dating of monazite combined with monazite trace-element compositions suggests that prograde evolution (M1) most likely started at c. 406 Ma, peak-&lt;i&gt;P&lt;/i&gt; conditions (M2a) were reached at 400–394 Ma, decompression associated with heating (M2b) took place at 393–391 Ma, and cooling (M3) during exhumation probably lasted from 380 to 354 Ma. The prograde metamorphism probably reflects burial during underthrusting of neighbouring continental basement (the Alxa block or an equivalent) below the Dunhuang block. This event culminated in pure shear thickening (D2a) of the whole supra-subduction margin followed by minor heating and exhumation (D2b). The D3-M3 event is interpreted as reflecting exhumation during orthogonal shortening of the system, possibly in response to an independent orogenic cycle. Combined with the available regional data, this study reveals the existence of a complex tectono-metamorphic evolution for the Dunhuang block characterized by two distinct orogenic phases with (i) the thickening of a previously thinned arc-back-arc crust recorded in the northern and central belts at 420–410 Ma in the pro-wedg","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"42 5","pages":"665-702"},"PeriodicalIF":3.4,"publicationDate":"2024-04-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140568699","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":"Dehydration-driven deformation of eclogite: Interplay between fluid discharge and rheology","authors":"Michał Bukała,&nbsp;Károly Hidas,&nbsp;Iwona Klonowska,&nbsp;Christopher J. Barnes,&nbsp;Kathrin Fassmer,&nbsp;Jarosław Majka","doi":"10.1111/jmg.12765","DOIUrl":"10.1111/jmg.12765","url":null,"abstract":"<p>Aqueous fluids released during dehydration of a subducting slab have a large effect on the rheology of the subduction interface. While high-pressure experiments and natural-case studies link deformation with critical dehydration reactions during eclogitization, the exact interplay between these processes remains ambiguous. To investigate fluid–rock interaction and associated deformation at high-pressure, we studied a suite of eclogites from the Tsäkkok Lens of the Scandinavian Caledonides that record prograde metamorphism within an Early Palaeozoic cold subduction zone. Our results show that in-situ dehydration during the blueschist to eclogite facies transition produces fluid fluxes leading to rheological weakening and densification, consequently promoting ductile-brittle deformation. Petrographic evidence, supported by thermodynamic modelling and thermobarometry, attest to a prograde passage from lawsonite-blueschist to peak eclogite facies of ~2.5 GPa and ~620°C. Phengite-bearing eclogites imply interaction with an externally-derived fluid, whereas rare phengite-free, kyanite-eclogites only record internally-derived fluid production. Models predict that prograde breakdown of chlorite, lawsonite and amphibole between 500 and 610°C lead to progressive dehydration and release of up to 4.6 wt.% of aqueous fluid. Microstructural data reveal elongated shapes of highly strained omphacite porphyroblasts, displaying minor yet gradual changes in misorientation towards the grain boundaries. Occasionally, these intragranular structures form subgrain cells that have similar sizes to those of neoblasts in the rock matrix. These observations point to the potential onset of dynamic recrystallization processes via dislocation creep. Moreover, the omphacite neoblasts and rutile show non-random crystallographic preferred orientations (CPOs), which are characterized by the subparallel alignment of point-like maxima in rutile [001] and [100] axes to those of [001] and (010) of omphacite neoblasts, respectively. Additionally, the [001] axes of these minerals are also subparallel to the weak stretching mineral lineation, and the (100) of rutile and the (010) of omphacite neoblasts are distributed in the plane of the foliation. This suggests that the development of their CPOs was coeval and structurally controlled. Garnet microfractures normal to the foliation are dilated and sealed predominantly by omphacite. The lack of obliquity between CPO and foliation plane, as well as the systematic orientation of garnet microfracture orientations, are consistent with coaxial deformation at peak-pressure conditions. Unlike other studies, we show that neither an external fluid source nor channelized fluid flow is needed to facilitate a ductile-brittle deformation of eclogite in a subduction setting.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"42 5","pages":"609-636"},"PeriodicalIF":3.4,"publicationDate":"2024-03-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.12765","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140155283","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":"Retentiveness of rare earth elements in garnet with implications for garnet Lu-Hf chronology","authors":"Matthijs A. Smit,&nbsp;Johannes C. Vrijmoed,&nbsp;Erik E. Scherer,&nbsp;Klaus Mezger,&nbsp;Ellen Kooijman,&nbsp;Melanie Schmitt-Kielman,&nbsp;Lorraine Tual,&nbsp;Carl Guilmette,&nbsp;Lothar Ratschbacher","doi":"10.1111/jmg.12769","DOIUrl":"10.1111/jmg.12769","url":null,"abstract":"<p>Incorporation of rare earth elements (REE) in garnet enables garnet chronology (Sm-Nd, Lu-Hf), and imparts a garnet-stable signature on cogenetic phases, which allows petrochronology and general petrogenetic tracing of garnet stability in minerals and melts. Constraints on the uptake and redistribution mechanisms, as well as on the diffusive behaviour of REE in garnet are required for allowing accurate interpretation of REE signatures and ages. Garnet REE profiles are often measured to gain insight into the nature and cause of REE zoning. Interpretation of such profiles is nevertheless complicated by poor constraints on the extent of diffusive relaxation. This is especially relevant for Lu, which, according to experiments, has a relatively high diffusivity and thus may re-equilibrate with possible consequences for Lu-Hf chronology. To provide new insight into the REE systematics of garnet, we applied quantitative trace-element mapping of garnet grains from metamorphic rocks that record peak temperatures above 750°C and cooling rates as low as 1.5°C Ma⁻¹. Garnet in all samples preserves Rayleigh-type or oscillatory growth zoning with sharply defined interfacial angles that match the garnet habit. Re-equilibration of REE compositions appears restricted to domains with nebulous and patchy zoning, which likely form by interface-coupled dissolution and re-precipitation reactions mediated by fluids or melts, rather than REE volume diffusion. The possible effect of Lu diffusion in the analysed grains was investigated by comparing the observations to the results from 2D numerical modelling using Lu diffusivities from recent diffusion experiments. This test indicates that Lu diffuses significantly slower in natural garnet than experiments predict. The retentiveness of REE in garnet demonstrates the reliability of REE signatures in magmatic tracing and petrochronology and establishes Lu-Hf chronology as a robust means of dating garnet growth and recrystallization in metamorphic rocks, including those that underwent high- or ultrahigh-temperature conditions.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"42 5","pages":"703-727"},"PeriodicalIF":3.4,"publicationDate":"2024-03-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.12769","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140128100","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 metamorphic footprint of western Laurentia preserved in subducted rocks from southern Australia","authors":"Dillon A. Brown,&nbsp;Laura J. Morrissey,&nbsp;Martin Hand,&nbsp;Jacob A. Mulder,&nbsp;Benjamin Wade,&nbsp;Vitor Barrote","doi":"10.1111/jmg.12770","DOIUrl":"10.1111/jmg.12770","url":null,"abstract":"<p>Polymetamorphic metapelitic rocks in central-west Tasmania, southern Australia, contain high-pressure mineral assemblages that formed during Cambrian-aged subduction and relict garnet with published Lu–Hf ages of c. 1285–1240 Ma. These garnet ages, along with published detrital zircon data from throughout western Tasmania and western North America, have been used to propose the presence of Mesoproterozoic Laurentian crust in western Tasmania. In this study, we combine zircon petrochronology with compositional information from the inclusion assemblages in relict garnet to extract Mesoproterozoic pressure–temperature data from subduction-overprinted rocks, which effectively constitute an interpreted remnant of Laurentian crust now residing in central-west Tasmania. The new data suggest Mesoproterozoic metamorphism involved two stages. The first event is recorded by c. 1480–1235 Ma zircon that formed in a garnet-absent, plagioclase-present, high-thermal gradient environment at pressures no greater than ~5–5.5 kbar. The second event recorded by c. 1285–1240 Ma relict garnet was characterized by the development of a moderate-pressure kyanite–plagioclase–biotite-bearing mineral assemblage, which formed at ~8.5 kbar and ~590–680°C. These pressure–temperature constraints are attributed to extension within a deep basin system associated with the cryptic East Kootenay Orogeny in North America, which coincides with the final stages of c. 1450–1370 Ma upper Belt-Purcell Basin sedimentation. Taking into account new detrital zircon U–Pb–Hf isotopic data from central-west Tasmania in this study and existing zircon provenance data from throughout western Tasmania and the Belt-Purcell Basin, our results strengthen the hypothesis of a Laurentian footprint that potentially encompasses much of western Tasmania and relates to both Nuna and Rodinian tectonism.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"42 5","pages":"729-765"},"PeriodicalIF":3.4,"publicationDate":"2024-03-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.12770","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140105075","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}