Journal of Metamorphic Geology最新文献

{"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}

{"title":"Featured Cover","authors":"Shun Guo,&nbsp;Anping Chen,&nbsp;Xirun Cai,&nbsp;Yi Chen,&nbsp;Pan Tang,&nbsp;Qiuli Li","doi":"10.1111/jmg.12771","DOIUrl":"https://doi.org/10.1111/jmg.12771","url":null,"abstract":"<p>The cover image is based on the Original Article <i>Cold deep subduction of Indian continental crust and release of ultrahigh-pressure fluid during initial exhumation: Insights from coesite-bearing eclogite-vein systems in Kaghan Valley, Pakistan</i> by Shun Guo et al., https://doi.org/10.1111/jmg.12760\u0000 \u0000 <figure>\u0000 <div><picture>\u0000 <source></source></picture><p></p>\u0000 </div>\u0000 </figure></p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"42 3","pages":"i"},"PeriodicalIF":3.4,"publicationDate":"2024-03-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.12771","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140114258","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":"Mechanisms and durations of metamorphic garnet crystallization in the lower nappes of the Caledonian Kalak Nappe Complex, Arctic Norway","authors":"Maria Thereza A. G. Yogi,&nbsp;Fred Gaidies,&nbsp;Olivier K. A. Heldwein,&nbsp;A. Hugh N. Rice","doi":"10.1111/jmg.12766","DOIUrl":"10.1111/jmg.12766","url":null,"abstract":"<p>The 3D microstructure and compositional zoning of garnet populations in micaschists from the Kolvik and Bekkarfjord nappes indicate the quasi-equilibration of their major components across the rock matrices during interface-controlled, size-independent garnet growth. There is microstructural evidence for foliation-parallel, small-scale resorption of garnet rims in the Kolvik Nappe, influencing the metamorphic peak conditions obtained from thermodynamic modelling. The local chemical compositions of rims less affected by resorption indicate a peak temperature of ~630°C, which is ~40°C higher than the temperature obtained from resorbed rims of the largest garnet crystal. Using a diffusion geospeedometry approach that considers the geometry of the compositional zoning of the garnet population, as well as the higher, more realistic peak temperature, a duration of 1 to 4.9 Myr is obtained for garnet growth in the Kolvik Nappe. This is approximately 1 order of magnitude faster than duration estimates obtained when using the apparent, lower temperature estimated from the resorbed garnet rims. Concomitantly to garnet growth in the Kolvik Nappe, garnet overgrowths formed in the Bekkarfjord Nappe for circa 2.5 Myr at metamorphic peak temperatures of ~560°C. The garnet growth durations obtained here are comparable with the uncertainty on the Lu–Hf garnet–whole rock isochron ages of 419.9 ± 2.4 Ma and 423.0 ± 1.9 Ma, previously obtained for these rocks. These results provide new insight into the timescales of repeated Barrovian-type metamorphic events experienced by the lower nappes of the Kalak Nappe Complex during the Caledonian Orogeny in Arctic Norway. This study emphasizes the importance of microstructural and chemical characterization of garnet populations in assessing metamorphic crystallization mechanisms and the extent of equilibration of garnet-forming components during prograde metamorphism. Moreover, our results provide means for reducing the uncertainty on metamorphic durations obtained via diffusion geospeedometry and, so, contributing to our understanding of geological timescales and processes.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"42 5","pages":"637-664"},"PeriodicalIF":3.4,"publicationDate":"2024-03-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.12766","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140107985","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":"Metamorphic evolution and geological significance of Early Palaeozoic high-pressure granulites from the East Kunlun (NW China)","authors":"An-Ping Chen,&nbsp;Hong-Fu Zhang,&nbsp;Ming-Jie Zhang,&nbsp;Xiao-Qi Zhang","doi":"10.1111/jmg.12767","DOIUrl":"10.1111/jmg.12767","url":null,"abstract":"<p>Granulites and eclogites are useful for revealing the thermal and tectonic evolution of orogens. Early Palaeozoic granulites and associated eclogites in the East Kunlun Orogenic Belt (EKOB) display contrasting metamorphic age. Such asynchronous granulite–eclogite associations have rarely been reported, and the geological significance of their existence remains to be further explored. In this study, petrological and geochronological techniques were used to investigate two felsic and two mafic granulites collected from the Qingshuiquan area in the eastern section of the EKOB. These rocks record similar <i>P–T</i> paths, which are characterized by a peak stage within suprasolidus and high-pressure (HP) granulite facies conditions (750–832°C and 10.1–12.0 kbar), followed by an initial decompression and cooling stage to subsolidus conditions (600–748°C and 6.5–8.6 kbar), and then a stage of further retrogression under greenschist facies conditions. The protoliths to these granulites are of volcanic and sedimentary origin and suggested to be a component of the continental basement unit. Metamorphic <i>P–T</i> paths indicate that these rocks experienced peak metamorphism at a depth of ~40 km, then cooling and uplift to a depth of ~25 km, and eventually experienced low-grade retrogression at shallow crustal levels. Cathodoluminescence images and compositional data demonstrate that the zircons in these rocks are of metamorphic origin and they crystallized at or near peak conditions. SIMS U–Pb dating of representative zircon grains yield concordant metamorphic ages of c. 490–520 Ma, with a peak value of 505 Ma on the probability density curve. These ages are similar to other 480–530 Ma ages typically retrieved from EKOB granulites and associated rocks, and are markedly older than the 400–450 Ma ages retrieved from eclogites and their host rocks. The HP granulites and eclogites of the EKOB do not show overprinting relationships. Such asynchronous characteristics imply that the two rock types formed in distinct tectonic settings and at different stages of a protracted subduction–collision process. The studied granulites are suggested to have formed in the root of a continental arc during a stage of Proto-Tethys Ocean subduction. The formation of the eclogites could be attributed to subsequent deep continental subduction.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"42 4","pages":"583-608"},"PeriodicalIF":3.4,"publicationDate":"2024-03-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140070084","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":"Ultrahigh thermal gradient granulites in the Narryer Terrane, Yilgarn Craton, Western Australia, provide a window into the composition and formation of Archean lower crust","authors":"Naomi M. Tucker,&nbsp;Johannes Hammerli,&nbsp;Anthony I. S. Kemp,&nbsp;Matthew L. Rowe,&nbsp;Chris M. Gray,&nbsp;Heejin Jeon,&nbsp;Martin J. Whitehouse,&nbsp;Malcolm P. Roberts","doi":"10.1111/jmg.12752","DOIUrl":"10.1111/jmg.12752","url":null,"abstract":"<p>Granulites from the Narryer Terrane in the northern Yilgarn Craton, Australia, record evidence for high to ultrahigh thermal gradients during the Meso–Neoarchean. U–Pb zircon ages reflect a complex history of high-grade, prolonged and poly-phase metamorphism, with evidence for several thermal pulses at ca. 2745–2725, ca. 2690–2660, and ca. 2650–2610 Ma. Forward phase equilibrium modeling on rocks with varying bulk compositions and mineral assemblages suggests that peak temperatures reached 880–920°C at pressures of 5.5–6 kbar at ca. 2690–2665 Ma, followed by near-isobaric cooling. These new <i>P</i>–<i>T</i> results also indicate that these rocks experienced some of the hottest thermal gradient regimes in the metamorphic record (≥150°C/kbar). Based on <i>P</i>–<i>T</i> models, U–Pb ages, and geochemical constraints, our data suggest that the geodynamic setting for the formation of this unusual thermal regime is ultimately tied to cratonization of the Yilgarn Craton. Previous models have inferred that ultrahigh thermal gradients and coeval large-scale anatexis in the Narryer Terrane were primarily generated by mantle-driven processes, despite most of the lithological, isotopic, and geochemical observations being at odds with the expected geological expression of large-scale mantle upwelling. We re-evaluate the mechanisms for high-grade metamorphism in the Narryer terrane and propose that long-lived high crustal temperatures between ca. 2690 Ma and 2610 Ma were instead facilitated by elevated radiogenic heat production in thickened, highly differentiated ancient crust. Mantle-derived magma input and new crustal addition may not be the only drivers for high- to ultrahigh-temperature metamorphism and stabilization of ancient crustal blocks.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"42 4","pages":"425-470"},"PeriodicalIF":3.4,"publicationDate":"2024-02-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.12752","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139950361","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":"Deformation-induced graphitization and muscovite recrystallization in a ductile fault zone","authors":"M. Rebecca Stokes,&nbsp;Aaron M. Jubb,&nbsp;Ryan J. McAleer,&nbsp;David L. Bish,&nbsp;Robert P. Wintsch","doi":"10.1111/jmg.12763","DOIUrl":"10.1111/jmg.12763","url":null,"abstract":"<p>A suite of slate samples collected along a 2 km transect crossing the Lishan fault in central Taiwan were evaluated to assess the role of ductile deformation in natural graphitization at lower greenschist facies metamorphic conditions. The process of natural aromatization, or graphitization, of an organic precursor is well established as a thermally driven process; however, experimental studies have shown that the energy provided by deformation can substantially reduce the activation energy required for graphitization. This study provides a natural example of deformation-induced graphitization. A strain gradient approaching the Lishan fault was established by scanning electron microscope imaging and X-ray diffraction analysis of phyllosilicates and quartz that showed an increase in the strength of slaty cleavage development via dissolution-precipitation processes. Thermal conditions were constrained to be near isothermal using calcite-dolomite geothermometry. Raman spectroscopic results from carbonaceous material, including D1-full width-at-half-maximum (FWHM), G-FWHM, Raman band separation (RBS), and a lesser-known vibrational mode B_2g-FWHM, showed robust linear trends across the same sampling transect. However, the G-FWHM parameter showed a trend opposite of that expected from thermally driven graphitization. The Raman results are interpreted to reflect a strain-driven reduction in graphite crystallite size (decrease in G-FWHM) but improvement in structural ordering in individual coherent domains. A multiple linear regression with an <i>R</i>² value of 0.92 predicts the graphite D1-FWHM values from the XRD-derived ratio of muscovite populations and muscovite microstrain, demonstrating the concomitant recrystallization of silicates and carbonaceous material across the strain gradient, despite the disparate processes accommodating the deformation. This study demonstrates the role of deformation in natural graphitization and provides a new perspective on the use of graphite as a geothermometer in strongly deformed greenschist facies rocks.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"42 4","pages":"529-550"},"PeriodicalIF":3.4,"publicationDate":"2024-02-14","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.12763","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139778017","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}