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The Poroshiri ophiolite (Hidaka metamorphic belt, Japan) occurs within a crustal‐scale network of high‐temperature, dextral shear zones that accommodated hundreds of kilometres of displacement due to the opening of the Japan Sea in the Neogene. The opholitic rocks comprise ultramafic, mafic and sedimentary protoliths that have been variably hydrated and metamorphosed. This work investigates the mechanisms and timing of fluid influx relative to viscous deformation in metagabbros and amphibolites deformed during exhumation from granulite‐ to amphibolite‐facies conditions. We consider a range of microstructures, from low strain domains and 1–2 mm thick shear bands to mylonites with a thickness of a few meters. Low strain domains of metagabbros exhibit corona textures with symplectites consisting of pargasitic amphibole (Ed0.7) ‐ anorthitic plagioclase (An80–92) ± orthopyroxene ± clinopyroxene forming around olivine and igneous pyroxene and with similar plagioclase–amphibole‐orthopyroxene ± clinopyroxene granoblastic aggregates in micrometric‐thick fractures. These textures result from hydration under low fluid‐rock ratio, with elevated H2O content only occurring locally (H2O > 1–1.2 wt%). Igneous mineral replacement leads to grain size reduction from 1 mm to ~10 μm. Amphibole exhibits a strong core‐rim zonation primarily controlled by the high diffusivity of Fe, Mg and OH and the low diffusivity of Al. Mineral compositional equilibrium is achieved at the scale of 100–200 μm. In mm‐thick localized shear bands and in metric‐scale mylonitic amphibolites, the heterogeneous mineral composition of amphibole (Ed0.2–0.5) and plagioclase (An40−An80) indicates only partial re‐equilibration (at the scale of 200–500 μm) despite higher fluid‐rock ratios and more pervasive fluid percolation than in metagabbros. Plagioclase–amphibole thermometry and equilibrium phase diagrams indicate that the initial fluid infiltration and corona formation occurred at 800–850°C by fracturing and percolation along grain boundaries. This was followed by the main fluid percolation and mylonitization event, which occurred during exhumation and cooling at conditions of 720–580°C, ~4 kbar. Continuous hydration during retrogression was achieved by the influx of dominantly seawater‐derived fluid, as attested by the high chlorine (Cl) contents (>300–400 ppm) of amphiboles in fractures. The heterogeneous distribution of fractures controls the distribution of fluid from the earliest stages of hydration, creating positive feedback where the growth of hydrous minerals as amphibole (up to +300 vol% of amphibole in high strain areas relative to the low‐strain ones) and the formation of fine‐grained mixed domains that led to further localization of viscous strain and mass transfer (variations of ± 30–40% in major elements). The Poroshiri ophiolite therefore represents a good fossil example of a former transpressional plate boundary where coupled metamorphic and deformation processes were triggered by seawater‐derived fluid that percolated to depths of ~15 km. [ABSTRACT FROM AUTHOR]

Linking the timing of allanite growth to metamorphic conditions in metapelites is particularly challenging because of the large variety of allanite textures and chemical compositions. This study focuses on five garnet–biotite metapelites retrogressed to different extents, from the internal domain of the Longmen Shan (eastern Tibet) from which few petrochronological data documenting the metamorphic ages are available. Microstructural observations were combined with whole-rock compositions, detailed mineral characterization, phase equilibria modelling, and in situ U–Th/Pb allanite dating to gain insights into the allanite reactivity relative to other rock-forming minerals. All samples experienced similar peak temperatures of 560–600°C. Allanite is the main rare earth element (REE)-bearing accessory mineral but it exhibits different textures, such as epidote rims with distinct REE contents as well as late inclusion-like dissolution features. Garnet is, along with allanite, critical to reconstruct the REE budget of these rocks. In the two samples where allanite is observed as inclusions in garnet, garnet shows no textural zoning and a low Y content (<90 ppm). This is attributed to early Y fractionation in allanite and epidote rims with no further equilibration with garnet. This allanite is therefore pre-garnet (T  < 520°C). In the sample where allanite is observed only in the matrix, both garnet and allanite–epidote rims exhibit more complex textural and compositional zoning, with a higher Y content in garnet. The Y incorporation in garnet is attributed to a series of reactions involving allanite, interpreted as syn- to post-garnet growth (T  > 520°C). This relative chronology is confirmed by in situ U–Th/Pb allanite dating: pre-garnet allanite shows ages of c. 200 Ma, whereas syn- to post-garnet allanite has ages of c. 180 Ma. The timing of allanite appearance strongly correlates with the biotite-in reaction predicted by the models and observed in microstructures. In the two samples collected close to the major Wenchuan Shear Zone—deformed up to mylonitization and retrogressed under greenschist-facies conditions (3–4 ± 1 kbar, 350–400°C)—allanite still preserves the peak metamorphic ages (c. 200 and c. 180 Ma). Allanite and its epidote rims, however, are fragmented and partly replaced by a retrograde assemblage of Qz + Pb-depleted (<100 ppm) monazite, providing an age <90–100 Ma. Our results allow different metamorphic stages for the studied transect in the central Longmen Shan to be refined: a prograde path at c. 200 Ma, a thermal relaxation and exhumation from c. 180 Ma, and a late greenschist overprint. This study also shows that when allanite is the principal phase in a mineral assemblage datable by U–Th/Pb, detailed observations of allanite textures may be highly informative in constraining the timing of its growth relative to other major mineral phases. [ABSTRACT FROM AUTHOR]

Abstract: Linking ages to metamorphic stages in rocks that have experienced low‐ to medium‐grade metamorphism can be particularly tricky due to the rarity of index minerals and the preservation of mineral or compositional relicts. The timing of metamorphism and the Mesozoic exhumation of the metasedimentary units and crystalline basement that form the internal part of the Longmen Shan (eastern Tibet, Sichuan, China), are, for these reasons, still largely unconstrained, but crucial for understanding the regional tectonic evolution of eastern Tibet. In situ core‐rim 40Ar/39Ar biotite and U–Th/Pb allanite data show that amphibolite facies conditions (~10–11 kbar, 530°C to 6–7 kbar, 580°C) were reached at 210–180 Ma and that biotite records crystallization, rather than cooling, ages. These conditions are mainly recorded in the metasedimentary cover. The 40Ar/39Ar ages obtained from matrix muscovite that partially re‐equilibrated during the post peak‐P metamorphic history comprise a mixture of ages between that of early prograde muscovite relicts and the timing of late muscovite recrystallization at c. 140–120 Ma. This event marks a previously poorly documented greenschist facies metamorphic overprint. This latest stage is also recorded in the crystalline basement, and defines the timing of the greenschist overprint (7 ± 1 kbar, 370 ± 35°C). Numerical models of Ar diffusion show that the difference between 40Ar/39Ar biotite and muscovite ages cannot be explained by a slow and protracted cooling in an open system. The model and petrological results rather suggest that biotite and muscovite experienced different Ar retention and resetting histories. The Ar record in mica of the studied low‐ to medium‐grade rocks seems to be mainly controlled by dissolution–reprecipitation processes rather than by diffusive loss, and by different microstructural positions in the sample. Together, our data show that the metasedimentary cover was thickened and cooled independently from the basement prior to c. 140 Ma (with a relatively fast cooling at 4.5 ± 0.5°C/Ma between 185 and 140 Ma). Since the Lower Cretaceous, the metasedimentary cover and the crystalline basement experienced a coherent history during which both were partially exhumed. The Mesozoic history of the Eastern border of the Tibetan plateau is therefore complex and polyphase, and the basement was actively involved at least since the Early Cretaceous, changing our perspective on the contribution of the Cenozoic geology. [ABSTRACT FROM AUTHOR]

Mutations in the transcription factors encoded by PHOX2B or LBX1 correlate with congenital central hypoventilation disorders. These conditions are typically characterized by pronounced hypoventilation, central apnea, and diminished chemoreflexes, particularly to abnormally high levels of arterial PCO2. The dysfunctional neurons causing these respiratory disorders are largely unknown. Here, we show that distinct, and previously undescribed, sets of medullary neurons coexpressing both transcription factors (dB2 neurons) account for specific respiratory functions and phenotypes seen in congenital hypoventilation. By combining intersectional chemogenetics, intersectional labeling, lineage tracing, and conditional mutagenesis, we uncovered subgroups of dB2 neurons with key functions in (i) respiratory tidal volumes, (ii) the hypercarbic reflex, (iii) neonatal respiratory stability, and (iv) neonatal survival. These data provide functional evidence for the critical role of distinct medullary dB2 neurons in neonatal respiratory physiology. In summary, our work identifies distinct subgroups of dB2 neurons regulating breathing homeostasis, dysfunction of which causes respiratory phenotypes associated with congenital hypoventilation. [ABSTRACT FROM AUTHOR]

The Pyrenean mountain belt experienced a polyphased history from the Variscan to the Alpine orogeny. Deciphering the different phases of metamorphism and deformation in the present structure of the belt is difficult, and particularly challenging in the Axial zone (central Pyrenees), where peak metamorphic conditions remained below 450 °C. This is however crucial to understand the evolution of the Pyrenean belt after pluton emplacement at c. 310-320 Ma. In this study* we characterize the age and conditions of different stages of greenschist-facies metamorphism in the Bielsa Variscan basement massif (below the Gavarnie unit, in the southern Axial Zone) and its Permo-Triassic sedimentary cover. At its northern and southern borders, the massif is extensively metasomatized throughout a vertical section of at least 2000 m depth. Similar alteration features have been observed eastward in the Pyrenees and are therefore probably significant at the scale of the central Axial zone. Field observations near Lake of Urdiceto, in the northern part of the massif, show that the sedimentary cover and its basement are tightly and harmonically folded. Close to the lithological contacts between the basement and the sedimentary cover, mylonitic bands extend ~E-W. Further away from the contact, zones of brittle deformation alternate with undeformed granite and mylonites. In all samples, at microscopic scale, the magmatic mineral assemblage is replaced by greenschist-facies metamorphic minerals, including chlorite, white mica, rutile and titanite. Chlorite-white mica thermobarometry and LA-ICP-MS in-situ U-Pb rutile and titanite dating indicate that the alteration occurred at ~300-350°C, ~2 kbar and 310-280 Ma. In samples with higher strain, secondary chlorite, white mica, monazite and rutile overprint earlier metasomatic events. P-T-t conditions for this event are estimated at ~200-250°C, 3-4 kbar and c. 40 Ma. Mylonitic bands preserve both metamorphic stages. Sediments exhibit two generations of veins (extensional and transtensional) filled with two generations of chlorite and rutile partially reset at Triassic and Eocene times. These results suggest that (1) the sedimentary cover and the basement experienced a similar history since sediment deposition, (2) the Bielsa massif was heterogeneously altered and deformed prior to the Pyrenean orogeny (late-Variscan) without recording the Cretaceous rifting phase preserved in the eastern Pyrenees, (3) Alpine metamorphism is preserved and localized in the most deformed areas of the massif. The spatial distribution of the deformation and metamorphism have been likely controlled by the first late-Variscan alteration, in Bielsa as in the other massifs of the Axial zone: the crust must have been significantly weakened prior to the Pyrenean shortening.*This study is part of the Orogen research project, a tripartite partnership between academy and industry (Total, BRGM, CNRS). [ABSTRACT FROM AUTHOR]

This study uses low-T petro-chronology to establish a link between the rheological effects of low-T fluid-rock interactions and fluid flow. Newly-formed mineral assemblages during fluid-rock interactions are controls of fluid paths and deformation mechanisms during the evolution of orogens, where polyphased deformations are commonplace. In the cold and external domains of mountain belts, these interactions are therefore difficult to distinguish and characterize because they occur at low-T and low-P conditions. For this reason, in the central Pyrenees (Axial zone), the metamorphism associated to the Variscan deformation is hardly distinguishable from the Alpine one. In this study*, we have characterized the different phases of low-grade metamorphism recorded in the Variscan granitic Bielsa massif, underthrusted below the Gavarnie unit during the Cenozoic orogenic evolution. In the studied area (Lake of Urdiceto), undeformed granite alternates with fractured and mylonitic rocks. In all samples, regardless the strain, the original magmatic mineral assemblage of biotite ± amphibole, plagioclase, K-feldspar, quartz and ilmenite is altered to a Fe-rich chlorite (chlorite1) ± white mica, ± prenhite. The titanium content of magmatic biotite and amphibole is incorporated in secondary Ti-bearing phases (rutile1 or titanite). The replacement reactions are pseudomorphic and fluid proceeds through microcraks. In 'discretely' deformed samples, flakes of chlorite and titanite are folded and cross-cut by fractures filled with Fe-poor chlorite (chlorite2), rutile2 ± monazite. In mylonites, rutile2 is the principal accessory mineral. Crystallization temperatures estimated with the multi-equilibrium approach for chlorite1 range between 300 and 350°C, while they range between 200 and 250°C for chlorite2. In-situ U/Pb LA-ICP-MS dating of rutile1, titanite and monazite yields ages of c. 310-280 Ma, while rutile2 and younger monazite domains show ages of c. 40-50 Ma. We conclude that the Bielsa granitic basement was softened and altered at low-T conditions at the late-Variscan, allowing a diffuse deformation. Low-T metamorphism and deformation was then localized during the Pyrenean phase as attested by the occurrence of the youngest rutile2 and monazite in the only mylonites. Furthermore, the occurrence of titanite over rutile is observed to be mainly dependent on the CO2 activity of the fluid phase. Rutile occurs along mylonitic bands, where a localization of the fluid flow accompanying the Pyrenean metamorphism is speculated. The composition of secondary chlorite also varies from rutile-bearing to titanite-bearing samples. Hence, the spatial occurrence of accessory minerals and the chemical variations of chlorite2 allow the composition and circulation paths of the paleofluids to be mapped. *This study is part of the Orogen research project, a tripartite partnership between academy and industry (Total, BRGM, CNRS). [ABSTRACT FROM AUTHOR]

Mutations in the transcription factors encoded by PHOX2B or LBX1 correlate with congenital central hypoventilation disorders. These conditions are typically characterized by pronounced hypoventilation, central apnea, and diminished chemoreflexes, particularly to abnormally high levels of arterial PCO 2 . The dysfunctional neurons causing these respiratory disorders are largely unknown. Here, we show that distinct, and previously undescribed, sets of medullary neurons coexpressing both transcription factors (dB2 neurons) account for specific respiratory functions and phenotypes seen in congenital hypoventilation. By combining intersectional chemogenetics, intersectional labeling, lineage tracing, and conditional mutagenesis, we uncovered subgroups of dB2 neurons with key functions in (i) respiratory tidal volumes, (ii) the hypercarbic reflex, (iii) neonatal respiratory stability, and (iv) neonatal survival. These data provide functional evidence for the critical role of distinct medullary dB2 neurons in neonatal respiratory physiology. In summary, our work identifies distinct subgroups of dB2 neurons regulating breathing homeostasis, dysfunction of which causes respiratory phenotypes associated with congenital hypoventilation.