Journal of Metamorphic Geology最新文献

{"title":"Archean ultrahigh temperature event in the northwestern Superior Province, Canada: Metamorphic evolution and tectonic implications of the Pikwitonei Granulite Domain","authors":"Changqing Yin,&nbsp;Xiaoman Wang,&nbsp;Shoufa Lin,&nbsp;Chris G. Couëslan,&nbsp;Christian O. Böhm,&nbsp;Jiahui Qian,&nbsp;Jian Zhang,&nbsp;Yanfei Xia,&nbsp;Wenxuan Li","doi":"10.1111/jmg.12722","DOIUrl":"10.1111/jmg.12722","url":null,"abstract":"<p>The Pikwitonei Granulite Domain (PGD), located in the northwestern Superior Province, is one of the largest Neoarchean high-grade metamorphic domains in the world, and is a key to understanding the Neoarchean crustal evolution of the Superior Province. Here we report results of a study on ultrahigh temperature (UHT) granulites with a sapphirine + quartz-bearing peak assemblage from the Sipiwesk Lake area in the PGD. Three stages of metamorphic assemblage development are recognized based on petrographic observations: Pre-peak stage is marked by garnet, sillimanite, K-feldspar, and biotite inclusions within sapphirine and orthopyroxene. Peak stage is charactered by the typical UHT associations of sapphirine + quartz and sapphirine + orthopyroxene. The retrograde stage is represented by the retrograde formation of cordierite, biotite and sillimanite in the matrix. Phase equilibrium modelling based on the bulk compositions of sapphirine-bearing granulites suggest that the rocks have experienced extensional UHT metamorphism in excess of 1035°C at pressures of 7.9–9.0 kbar, followed by isobaric cooling process. SHRIMP U–Pb dating of metamorphic zircons in sapphirine-bearing granulite record the timing of the UHT event in the PGD with a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 2681 ± 13 Ma. This study in combination with other metamorphic P–T paths and age information reveals that this UHT metamorphism at 2.68 Ga in PGD was generated by upwelling asthenosphere due to slab break-off in collision during the amalgamation of the Superior Province.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"41 7","pages":"899-923"},"PeriodicalIF":3.4,"publicationDate":"2023-04-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43299671","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"The origin and compositions of melt inclusions in an Al2SiO5-free paragneiss from the Namche Barwa Complex in the Eastern Himalayan Syntaxis","authors":"Qiang Liu,&nbsp;Penglei Liu,&nbsp;Xin Li,&nbsp;Junfeng Zhang","doi":"10.1111/jmg.12721","DOIUrl":"10.1111/jmg.12721","url":null,"abstract":"<p>Melt inclusions (MIs) in high-temperature metamorphic rocks provide a unique window into crustal anatexis in collisional orogenic belts and have been widely used to characterize compositions of anatectic melts as well as melting mechanisms. In this study, MIs hosted by peritectic garnet were for the first time identified in an Al₂SiO₅-free graywacke-type paragneiss from the Namche Barwa Complex, the Eastern Himalaya, Southeast Tibet. These MIs occur as nanogranites in the rims of porphyroblastic garnet, exhibit negative crystal shapes with an average diameter of ~12 μm and consist of a mineral assemblage of biotite + quartz + plagioclase + K-feldspar ± muscovite. Re-homogenization experiments of these nanogranites were conducted at a pressure of 1.5 GPa and temperatures of 800°C, 850°C and 900°C and produced homogeneous glasses at 850°C. The homogenized glasses are strongly peraluminous and calc-alkalic in composition, with 66.43–71.31 wt.% SiO₂, 12.64–15.06 wt.% Al₂O₃, high alkaline (5.41–7.22 wt.%) and low ferromagnesian (2.72–4.46 wt.%) contents. They are lower in silica and CaO but higher in K₂O compared with MI produced by fluid-present melting of metasedimentary rocks, thus indicating fluid-absent melting. These glasses are also characterized by enrichment of large ion lithophile elements (particularly Cs and Rb), depletion of Ba and Sr, low contents of light rare earth elements (3.6 to 33.7 ppm), high Rb/Sr ratios (6.19–37.3) and low Nb/Ta ratios (2.55–18.7). In combination with phase equilibrium modelling, these compositional features suggest that a sequential dehydration melting of muscovite and biotite was responsible for the production of MI during prograde metamorphism of the studied paragneiss. By compiling MI data published in the literature, we show that dehydration melting of metasedimentary rocks from the Himalayan orogen can produce initial melts with various peraluminous and granitic compositions.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"41 7","pages":"879-898"},"PeriodicalIF":3.4,"publicationDate":"2023-04-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47036071","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":"Late Neoproterozoic–Cambrian eclogites and high-pressure granulites in the Central Qilian terrane (China) record the earliest subduction of Proto-Tethyan Ocean in the eastern Tethysides","authors":"Hao Wang,&nbsp;Wenjiao Xiao,&nbsp;Rui Li,&nbsp;Hongxu Chen,&nbsp;Zhou Tan,&nbsp;Qigui Mao","doi":"10.1111/jmg.12720","DOIUrl":"10.1111/jmg.12720","url":null,"abstract":"<p>The Central Qilian terrane (CQT) of the northern Tibetan Plateau played a key role in the tectonic evolution of the Proto-Tethyan Ocean in the Tethysides, but its formation and tectonic attribution have been hotly debated. Here, we report the discovery of eclogites and HP mafic granulites in the Beidahe Complex of the western CQT. These occur as blocks of various sizes within a sequence of metavolcanic–sedimentary rocks, exhibiting typical a ‘block-in-matrix’ and thrust imbrication structure. The eclogite facies metamorphic rocks preserve distinct mineral assemblages and textures corresponding to prograde, peak, and retrograde metamorphism. By combining phase equilibrium modelling with SHRIMP and LA-ICPMS U–Pb dating of metamorphic zircon, sphene, and rutile, the Late Neoproterozoic–Cambrian (c. 553–516 Ma) eclogite facies peak with low thermal gradients of 10–14°C/km, Cambrian (c. 515–506 Ma) post-peak decompression and Ordovician (c. 495–455 Ma) cooling histories for these metabasic rocks have been restored. These constitute hairpin-type clockwise pressure–temperature–time (<i>P–T–t</i>) paths depicting in detail the sequence of deep subduction and subsequent exhumation in Central Qilian during the Late Neoproterozoic to Early Palaeozoic. Our new findings suggest that the CQT represents a Japan-type arc-accretionary system that formed as a result of the North Qilian oceanic plate, one of the major branches of the Proto-Tethys Ocean, being subducted southward. Eclogites in the Beidahe Complex in western CQT offer the earliest (c. 553 Ma) metamorphic record of subduction in the Qilian orogen, indicating that the North Qilian Ocean commenced subducting southward prior to the Late Neoproterozoic.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"41 6","pages":"849-878"},"PeriodicalIF":3.4,"publicationDate":"2023-03-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45631065","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"The temporal evolution of subduction initiation in the Samail ophiolite: High-precision U–Pb zircon petrochronology of the metamorphic sole","authors":"Matthew Rioux,&nbsp;Joshua M. Garber,&nbsp;Michael Searle,&nbsp;James L. Crowley,&nbsp;Sally Stevens,&nbsp;Mark Schmitz,&nbsp;Andrew Kylander-Clark,&nbsp;Kayla Leal,&nbsp;Tyler Ambrose,&nbsp;Andrew J. Smye","doi":"10.1111/jmg.12719","DOIUrl":"10.1111/jmg.12719","url":null,"abstract":"&lt;p&gt;High-precision dating of the metamorphic sole of ophiolites can provide insight into the tectonic evolution of ophiolites and subduction zone processes. To understand subduction initiation beneath a young, well-preserved and well-characterized ophiolite, we performed coupled zircon laser-ablation inductively coupled mass spectrometry trace element analyses and high-precision isotope dilution-thermal ionization mass spectrometry U–Pb dating on 25 samples from the metamorphic sole of the Samail ophiolite (Oman-United Arab Emirates). Zircon grains from amphibolite- to granulite-facies (0.8–1.3 GPa, ~700–900°C), garnet- and clinopyroxene-bearing amphibolite samples (&lt;i&gt;n&lt;/i&gt; = 18) show systematic trends of decreasing heavy rare earth element slope (HREE; Yb/Dy) with decreasing Yb concentration, reflecting progressive depletion of the HREE during prograde garnet growth. For half of the garnet-clinopyroxene amphibolite samples, Ti-in-zircon temperatures increase, and U–Pb dates young with decreasing HREE slope, consistent with coupled zircon and garnet growth during prograde metamorphism. In the remaining samples, there is no apparent variation in Ti-in-zircon temperature with decreasing HREE slope, and the combined U–Pb and geochemical data suggest zircon crystallization along either the prograde to peak or prograde to initial retrograde portions of the metamorphic &lt;i&gt;P–T–t&lt;/i&gt; path. The new data bracket the timing of prograde garnet and zircon growth in the highest grade rocks of the metamorphic sole between 96.698 ± 0.094 and 95.161 ± 0.064 Ma, in contrast with previously published geochronology suggesting prograde metamorphism at ~104 Ma. Garnet-free amphibolites and leucocratic pods from lower grade (but still upper amphibolite facies) portions of the sole are uniformly HREE enriched (Yb/Dy &gt; 5) and are ~0.5–1.3 Myr younger than the higher grade rocks from the same localities, constraining the temporal offset between the metamorphism and juxtaposition of the higher and lower grade units. Positive zircon ε&lt;sub&gt;Hf&lt;/sub&gt; (+6.5 to +14.6) for all but one of the dated amphibolites are consistent with an oceanic basalt protolith for the sole. Our new data indicate that prograde sole metamorphism (96.7–95.2 Ma) immediately predated and overlapped growth of the overlying ophiolite crust (96.1–95.2 Ma). The ~600 ky offset between the onset of sole metamorphism in the northern portion of the ophiolite versus the start of ophiolite magmatism is an order of magnitude shorter than previously proposed (~8 Ma) and is consistent with either spontaneous subduction initiation or an abbreviated period of initial thrusting during induced subduction initiation. Taken together, the sole and ophiolite crust preserve a record of the first ~1.5 Myr of subduction. A gradient in the initiation of high-grade metamorphism from the northwest (96.7 Ma) to southeast (96.0–95.7 Ma) may record propagation of the nascent subduction zone and/or variations in subduction rate alo","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"41 6","pages":"817-847"},"PeriodicalIF":3.4,"publicationDate":"2023-03-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.12719","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46013658","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Pressure–temperature paths of tectonic blocks in mélange: Recording thermal evolution of a subduction channel at an initial stage of subduction","authors":"Toru Takeshita,&nbsp;Takeshi Imayama,&nbsp;Mizuho Ando,&nbsp;Yuto Kimura,&nbsp;Marie Python","doi":"10.1111/jmg.12718","DOIUrl":"10.1111/jmg.12718","url":null,"abstract":"<p>We investigated the tectonic evolution of amphibolite and blueschist tectonic blocks in the serpentinite- or pelite-matrix mélange, which are distributed at the highest structural level of the high-<i>P/T</i> type Kamuikotan metamorphic rocks in northern Japan. The tectonic blocks in this study area are divided into six rock types: garnet-epidote amphibolite, epidote amphibolite, amphibolite, plagioclase-poor amphibolite, epidote blueschist and glaucophane-bearing quartz schist. Based on phase equilibrium modelling, garnet-epidote amphibolite and epidote amphibolite experienced peak metamorphism at pressure and temperature conditions of 1.1–1.25 GPa and 550–590°C, and 0.8–1.3 GPa and 475–550°C, respectively, (at an apparent thermal gradient ranging between 13 to 17°C/km). By contrast, although the peak-metamorphic conditions for each one of amphibolite, plagioclase-poor amphibolite, and glaucophane-bearing quartz schist are not well constrained, they may have been originally metamorphosed at amphibolite to epidote-amphibolite facies at thermal gradients exceeding 20°C/km, inferred from the core composition of amphibole (edenite/magnesiohornblende/barroisite). The epidote blueschist experienced peak metamorphism at pressure and temperature conditions of 0.8–1.6 GPa and 360–520°C (most probably 0.8–0.85 GPa and 360–480°C). Although different types of tectonic blocks experienced a variety of peak metamorphism under different <i>P/T</i> conditions, all of them underwent epidote blueschist facies metamorphism at the peak or retrograde stage (as shown by the glaucophane rims of amphibole with different core compositions). The overall <i>P–T</i> paths appear counter-clockwise, which could be interpreted to reflect the cooling history of the subduction channel from the beginning to the steady state of subduction. The geothermal gradient could have changed from 15–17° to ~10°C/km over ~20–25 Myr, as estimated by previously reported radiometric ages. The protoliths to the tectonic blocks could have begun to subduct into the subduction channel at different times (where the thermal structure evolved with time), acquiring different prograde <i>P–T</i> paths. Subsequently, these tectonic blocks were juxtaposed at a certain depth and incorporated into the overlying serpentinite during the subduction stage. Finally, the serpentinite- or pelite-matrix mélange, including these tectonic blocks, were exhumed together with the coherent accretionary units as the former was emplaced over the latter.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"41 6","pages":"787-816"},"PeriodicalIF":3.4,"publicationDate":"2023-03-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45659968","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":"A rutile and titanite record of subduction fluids: Integrated oxygen isotope and trace element analyses in Franciscan high-pressure rocks","authors":"F. Zeb Page,&nbsp;Craig D. Storey,&nbsp;EIMF","doi":"10.1111/jmg.12717","DOIUrl":"10.1111/jmg.12717","url":null,"abstract":"<p>In situ oxygen analysis of garnet in eclogite and related rocks is increasingly being used to probe the composition of subduction fluids. However, in many cases, these samples contain textural signs of both fluid flow and retrograde metamorphism, some of which may take place outside the garnet stability field. In order to test the connection between polymetamorphism and fluid infiltration, rutile rimmed by titanite from high-grade tectonic blocks of the Franciscan Formation (California, USA) was analysed for oxygen isotope ratios and trace element concentrations. Zirconium concentrations in rutile yield temperatures of ~600°C for eclogite and hornblende eclogite from three well-studied localities (Junction School, Tiburon and Ward Creek). Rutile trace element concentrations are generally low and consistent with a mafic protolith. Titanite surrounding rutile has inherited much of its trace element content from rutile, and Zr-in-titanite temperatures are spuriously high. Titanite in rutile-free samples (blueschist and eclogite from Jenner beach) have similar compositions suggesting that they were formed at the expense of rutile as well. Oxygen isotope ratios from rutile and titanite in the same sample are fortuitously similar, indicating disequilibrium between these minerals, which formed at different times and temperatures but in equilibrium with the same oxygen reservoir. Rutile in blocks with garnets zoned in oxygen isotopes are generally in equilibrium with the rims rather than the cores. Slow oxygen diffusion in rutile and the low temperatures of formation require that rutile recrystallized after fluid interaction and before blueschist facies metamorphism. External fluid interaction of Franciscan eclogites took place near the peak of metamorphism.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"41 6","pages":"767-786"},"PeriodicalIF":3.4,"publicationDate":"2023-02-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.12717","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45675663","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":"To be or not to be Alpine: New petrological constraints on the metamorphism of the Chenaillet Ophiolite (Western Alps)","authors":"Alberto Corno,&nbsp;Chiara Groppo,&nbsp;Alessandro Borghi,&nbsp;Pietro Mosca,&nbsp;Marco Gattiglio","doi":"10.1111/jmg.12716","DOIUrl":"10.1111/jmg.12716","url":null,"abstract":"<p>The Chenaillet Ophiolite represents a very well-preserved portion of Ligurian-Piedmont ocean in the Western Alps. It is formed from an oceanic lithospheric succession comprising exhumed mantle, various mafic intrusives (i.e., gabbro sensu <i>lato</i>), and a world-renowned sequence of pillow basalts. Apart from scarce breccias closely related to oceanic lithosphere, no sedimentary cover is exposed. Historically, the Chenaillet Ophiolite has been known for its very low temperature–low pressure Alpine metamorphism, ascribed to obduction processes. However, studies aimed at constraining the peak pressure–temperature (P–T) conditions of Alpine metamorphism are virtually lacking, the general focus having been so far on its high temperature metamorphism and geochemical features. In this paper, we investigate two kinds of rocks: gabbro and albitite/alkali syenite, whose petrographic features shed light on the complex metamorphic history of the Chenaillet Ophiolite. Detailed analyses of mineral assemblages, blastesis/deformation relationships, and mineral chemical data allow two metamorphic events to be distinguished: an earlier, high temperature event (already reported in the literature) and a second, later low temperature, high pressure event, recognized here for the first time. The low temperature, high pressure event is strikingly testified by the occurrence of lawsonite relicts in the gabbro and of interstitial omphacite in the albitite. Thermodynamic modelling (i.e., via isochemical phase diagrams) performed on a gabbro sample suggests for this unit a minimum of 9 kbar and 300°C and a maximum of 15 kbar and 450°C. Overlapping these P–T conditions with those inferred for the albitite based on the observed mineral assemblage allows the Alpine peak metamorphism to be constrained to 10–11 kbar and 340–360°C. These P–T conditions suggest a thickness of the overlying nappe stack of about 35–40 km, which is incompatible with obduction or burial processes, and instead consistent with subduction processes related to the Alpine orogeny. We argue that, opposite to the common belief that the Chenaillet Ophiolite escaped Alpine metamorphism, our new data strongly support the idea that it experienced low temperature-blueschist-facies metamorphism, whose evidence can still be tracked in those (few) rocks that better recorded and preserved it. This finding generates new challenging questions regarding both subduction and exhumation processes in complex orogens such as the Western Alps.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"41 6","pages":"745-765"},"PeriodicalIF":3.4,"publicationDate":"2023-02-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.12716","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43178711","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":"Prograde zircon growth in migmatites","authors":"Chris Yakymchuk","doi":"10.1111/jmg.12715","DOIUrl":"10.1111/jmg.12715","url":null,"abstract":"<p>Empirical studies of zircon in migmatites document features compatible with growth during heating at suprasolidus conditions. However, numerical modelling of zircon behaviour suggests that suprasolidus zircon is expected to grow only during cooling and melt crystallization. Here, phase equilibrium modelling coupled with mineral–melt Zr partitioning is used in an attempt to reconcile the observations from migmatites with the predictions of previous numerical models of zircon behaviour in anatectic systems. In general, an equilibrium-based model that includes Zr partitioning does not allow prograde suprasolidus zircon growth. However, melting of metapelites at temperatures just above the wet solidus may allow limited zircon growth because of the low solubility of zircon in melt coupled with a source of Zr from minor garnet and ilmenite breakdown. Preservation of this zircon requires entrapment in growing peritectic minerals during subsequent heating and further melting. Heating above muscovite exhaustion in metapelites is unlikely to grow zircon because of the progressive increase in zircon solubility as well as an increasing compatibility of Zr in the residual mineral assemblage. The modelled compatibility of Zr in the residue of a metabasite decreases during heating, but an increase in zircon solubility in melt counteracts this; prograde suprasolidus zircon growth in metabasites is unlikely. Infiltration of Zr-rich melt into a migmatite during open-system anatexis provides an additional potential mechanism for prograde suprasolidus zircon growth during high-temperature metamorphism.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"41 5","pages":"719-743"},"PeriodicalIF":3.4,"publicationDate":"2023-02-23","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.12715","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42919509","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":"Carboniferous–Triassic tectonic and thermal evolution of the middle crust section of the Dervio–Olgiasca Zone (Southern Alps)","authors":"Christophe Real,&nbsp;Kathrin Fassmer,&nbsp;Rodolfo Carosi,&nbsp;Nikolaus Froitzheim,&nbsp;Daniela Rubatto,&nbsp;Chiara Groppo,&nbsp;Carsten Münker,&nbsp;Simona Ferrando","doi":"10.1111/jmg.12714","DOIUrl":"10.1111/jmg.12714","url":null,"abstract":"<p>A well-preserved remnant of the middle crust of the former Adriatic passive margin is exposed in the Southern Alps (Italy). The Dervio–Olgiasca Zone is located south of the Insubric Line along the northern part of Como Lake and, because of the lack of Alpine overprint, provides favourable conditions to investigate the pre-Alpine (rift-related) history. We reconstruct the P–T–t–d evolution of the Adria middle crust through petrological (petrography, mineral chemistry, thermobarometry and thermodynamic modelling) and geochronological (Lu/Hf in garnet and U–Pb in monazite) data from pegmatites and host micaschists. These data allow reconstruction of a complex tectono-thermal evolution of the future proximal Adriatic margin at the onset of Alpine rifting. The amphibolite-facies Carboniferous metamorphic basement (7.6–10 kbar and 610–660°C at 318–312 Ma) was affected by pervasive extensional deformation (5.1–7.6 kbar and 580–660°C) in the Middle- to Late-Permian (257.5 ± 3.8 Ma). Pegmatite intruded at 249.8 ± 1.1 Ma in an extensional phase that re-equilibrated rocks of the basement at 3.5–4.5 kbar and 560–600°C. During the Middle- to Late-Triassic (241–235 Ma), the basement experienced static thermal recrystallization (T = 689 ± 41°C and ~5.0 kbar). This Late-Anisian to Early-Carnian thermal event was simultaneous with the emersion of carbonate platforms, volcanism and ore deposition in the future proximal Adriatic margin. The subsequent cooling of the middle crust was synchronous with large-scale extensional detachments developed in the upper crust (e.g., the Lugano-Val Grande Fault), which controlled the formation of the Monte Generoso Basin. This study reveals that the local post-Carboniferous thinning and heating events recorded in the Adriatic middle crust were interconnected to other processes occurring at different crustal levels that were, in turn, induced by crustal stretching in the early stages of the Alpine rifting.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"41 5","pages":"685-718"},"PeriodicalIF":3.4,"publicationDate":"2023-02-23","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45066967","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":"Incompatibility between serpentinization and epidote formation in the lower oceanic crust: Evidence from the Oman Drilling Project","authors":"Toshio Nozaka,&nbsp;Yamato Tateishi","doi":"10.1111/jmg.12713","DOIUrl":"10.1111/jmg.12713","url":null,"abstract":"<p>It is a general tendency that epidote, which is a typical greenschist facies mineral, is scarce in the lower oceanic crust, in spite of the widespread occurrence of the other minerals indicative of similar temperature conditions such as chlorite, actinolite, prehnite and serpentine. To find the cause of this, we carried out petrological analyses of lower crustal rocks of the Oman ophiolite sampled by the Oman Drilling Project of the International Continental Scientific Drilling Program (ICDP). Petrographic observations revealed the tendency, as expected, that the amount of epidote formed by static alteration of plagioclase decreases with depth. Because mineral assemblages indicative of a wide range of temperature conditions from amphibolite to subgreenschist facies occur throughout the cores without systematic variations of abundance, the decrease of epidote amount cannot be explained by the difference of temperature condition of alteration. Petrographic observations also revealed that epidote is absent or rare in rocks containing serpentinized olivine in contrast to prehnite showing a close association with serpentinization of olivine. In an exceptional sample containing both epidote and serpentinized olivine, epidote occurs with chlorite that cuts or replaces plagioclase, mantles adjacent olivine and is connected with chlorite + lizardite veins cutting mesh-forming serpentine veins. The distribution and mode of occurrence of epidote suggest decoupling of its formation with the main stage of serpentinization. Serpentine veins cutting olivine to form mesh texture are typically lizardite with magnetite ribbons at vein centres and have compositions of lizardite–cronstedtite solid solution at vein margins or in magnetite-free veins, suggesting a chemical condition with low silica and low oxygen potentials at an early stage of serpentinization. Thermodynamic modelling for olivine and plagioclase alteration at greenschist facies conditions indicates that silica potential for plagioclase alteration to form prehnite + chlorite and epidote + chlorite could be higher than for olivine serpentinization. On the other hand, oxygen potential for the prehnite + chlorite formation is lower than for the epidote + chlorite formation and is comparable with that for olivine serpentinization. From the observations and analyses, it is concluded that epidote formation is inhibited by olivine serpentinization, which maintains a reducing condition for alteration in the lower oceanic crust.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"41 5","pages":"665-684"},"PeriodicalIF":3.4,"publicationDate":"2023-02-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"49378278","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}