Petrology, geochemistry and geochronology of highly foliated amphibolites from the ophiolitic mélange beneath the Yarlung Zangbo ophiolites, Xigaze area, Tibet : geodynamical implications

On retrouve localement des amphibolites fortement foliées dans le mélange ophiolitique sous les massifs ophiolitiques de la Zone de Suture du Yarlung Zangbo (ZSYZ). Ces blocs représentent la partie supérieure d’une semelle métamorphique démembrée. La géochimie des amphibolites (La/Yb = 0.65-0.97, Ta/Th = 0.33-0.65) est similaire à celle des roches mafiques provenant de l’ophiolite, suggérant une origine dans le même bassin d’arrière-arc. Le métamorphisme de haut grade (P=14 kbars, T= 800°C) subit par les amphibolites suggère un enfouissement pendant la naissance d’une subduction. Les âges voisins des amphibolites et de la croûte ophiolitique (121-130 vs 120±10 et 126 Ma, respectivement) suggèrent que la naissance de la subduction s’est déroulée dans le bassin arrière-arc Néo-Téthysien. Un tel événement n’avait pas encore été rapporté. La présence de dikes et le métasomatisme tardif responsable de la cristallisation de préhnite pourraient indiquer la subduction d’un centre magmatique. La composition en isotopes stables du fluide responsable confirmerait une telle hypothèse.Blocks of highly foliated amphibolites are locally found within the serpentinite matrix mélange underlying the Yarlung Zangbo ophiolites near Bainang and Buma, Xigaze area, Yarlung Zangbo Suture Zone (YZSZ), Tibet. The mélange is thought to be the result of the tectonic dismemberment of the base of the ophiolitic napes during its obduction over the Indian passive margin, circa 50 Ma. Prior to dismemberment, amphibolites were probably parts of a coherent dynamothermal sole, as observed at the base of many ophiolites. Sampled amphibolites can be subdivided in three groups: garnet, banded and common amphibolites. Medium-grained garnet amphibolites contain the assemblage A) Hb+CPX+Gt+Pl±Rt and B) Gt+Hb+Pl (corona assemblage). Fine to medium-grained banded amphibolites contain the assemblage C) Hb+CPX+Pl+Ep±Sp+Qtz+Ap. Fine-grained common amphibolites contain facies D) Hb+Pl±Ep+Ap+Sp. In all assemblages, plagioclase is pseudomorphosed by an albite-prehnite simplectite. Retrograde cataclastic veins contain the assemblage E) Ab+Pr±Ch+Ep. The geochemistry of the garnet, banded and common amphibolites is very similar to the geochemistry of other mafic blocks in the mélange and of mafic igneous rocks within the ophiolitic massifs. When compared to MORBs, light depletion of LREE (La/Yb = 0.65-0.97) and mild HFSE depletion (Ta/Th = 0.33-0.65) would suggest a mixing between the IAT and MORB sources, as seen in back-arc basins and nascent intra-oceanic arcs. The amphibolites were buried at the inception of a subduction within the back-arc to peak metamorphism conditions of 11-14 kbars and ~800 °C. Ar/Ar analysis of amphiboles revealed a metamorphic age of 121-130 Ma, which is synchronous with ages obtained from the overlying ophiolites. Overlapping in ophiolite-sole age relationship reveals inception of the subduction near or at the spreading center from which originated the ophiolite. Subduction of a buoyant body could explain heterogeneous coronitization of pyrope-rich (up to 35 %) garnet by Al-Tschermakites (Al2O3 up to 21 wt %) at high-pressures. After exhumation, amphibolites were injected by very fine-grained diabasic dykes and were subject to percolation of a prehnite-precipitating fluid. Oxygen stable isotopes suggest that a magmatic fluid is responsible for prehnite precipitation. The magmatic and metamorphic history of the dynamothermal sole and field relationships with adjacent units seem to indicate that most of Neo-Tethys oceanic domain was subducted along this new Late Cretaceous subduction zone

Музейна справа сучасної Франції на прикладі музею Бранлі

The article sums up the results of the first year of operation du quai Braniy. It also sheds light on multicultural image of modern France

Formation of the Xigaze Metamorphic Sole under Tibetan continental lithosphere reveals generic characteristics of subduction initiation

Metamorphic soles found under allochthonous oceanic lithosphere, or ophiolites, are interpreted as derived from lower plate oceanic crust material accreted to upper plate mantle during intraoceanic subduction initiation. Their metamorphic evolution is inferred to reflect the thermal structure at the site of subduction nucleation, with granulite-bearing soles linked to initiation at hot spreading centers. Here we present garnet Lu-Hf geochronology for the granulite-bearing sole of the Xigaze ophiolite in South Tibet, whose oceanic crust formed ∼130 Ma through continental forearc extension. Our study shows that sole metamorphism was ongoing by 144 Ma, implying that north-directed subduction began at least 14 million years before oceanic forearc spreading. The upper plate at the time of subduction initiation was thus continental, not oceanic. Our results demonstrate that metamorphic characteristics of soles are independent of the specific tectonic setting at the subduction nucleation site and rather provide generic constraints on the subduction initiation process

Formation of the Xigaze Metamorphic Sole under Tibetan continental lithosphere reveals generic characteristics of subduction initiation

Metamorphic soles found under allochthonous oceanic lithosphere, or ophiolites, are interpreted as derived from lower plate oceanic crust material accreted to upper plate mantle during intraoceanic subduction initiation. Their metamorphic evolution is inferred to reflect the thermal structure at the site of subduction nucleation, with granulite-bearing soles linked to initiation at hot spreading centers. Here we present garnet Lu-Hf geochronology for the granulite-bearing sole of the Xigaze ophiolite in South Tibet, whose oceanic crust formed ∼130 Ma through continental forearc extension. Our study shows that sole metamorphism was ongoing by 144 Ma, implying that north-directed subduction began at least 14 million years before oceanic forearc spreading. The upper plate at the time of subduction initiation was thus continental, not oceanic. Our results demonstrate that metamorphic characteristics of soles are independent of the specific tectonic setting at the subduction nucleation site and rather provide generic constraints on the subduction initiation process

Re-evaluating monazite as a record of metamorphic reactions

ABSTRACT: This study presents a re-examination of historical specimens (DG136 and DG167) from the Monashee complex in the southeastern Canadian Cordillera that are critical to the current understanding of rare earth element (REE) distribution between garnet and monazite (and other accessory minerals) during metamorphism. Nine-hundred and fifty-one new monazite petrochronology spot analyses on 29 different grains across two specimens outline detailed (re)crystallization histories. Trace element data collected from the same ablated volume, interpreted in the context of new phase equilibria modelling that includes monazite, xenotime and apatite, link ages to specific portions of the pressure-temperature (P-T) paths followed by the specimens. These linkages are further informed by garnet Lu-Hf geochronology and xenotime petrochronology. The clockwise P-T paths indicate prograde metamorphism was ongoing by ca. 80 Ma in both specimens. The structurally deeper specimen, DG136, records peak P-T conditions of similar to 755-770 degrees C and 8.8-10.4 kbar, interpreted to coincide with (re-)crystallization of low Y monazite at similar to 75-70 Ma. Near-rim garnet isopleths from DG167 cross in the observed peak assemblage field at similar to 680 degrees C and 9.3 kbar. These conditions are interpreted to correspond with low Y monazite (re-)crystallisation at similar to 65 Ma. Both specimens record decompression along their retrograde path coincident with high Y 70-55 Ma and 65-55 Ma monazite populations in DG136 and DG167, respectively. These findings broadly agree with those initially reported similar to 20 years ago and confirm early interpretations using trace elements in monazite as generally reliable markers of metamorphic reactions. Modern phase equilibria modelling and in situ petrochronological analysis, however, provide additional insight into monazite behaviour during anatexis and the effects of potential trace element buffering by REE-bearing phases such as apatite. (C) 2021 China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V

A global apparent polar wander path for the last 320 Ma calculated from site-level paleomagnetic data

Apparent polar wander paths (APWPs) calculated from paleomagnetic data describe the motion of tectonic plates relative to the Earth's rotation axis through geological time, providing a quantitative paleogeographic framework for studying the evolution of Earth's interior, surface, and atmosphere. Previous APWPs were typically calculated from collections of paleomagnetic poles, with each pole computed from collections of paleomagnetic sites, and each site representing a spot reading of the paleomagnetic field. It was recently shown that the choice of how sites are distributed over poles strongly determines the confidence region around APWPs and possibly the APWP itself, and that the number of paleomagnetic data used to compute a single paleomagnetic pole varies widely and is essentially arbitrary. Here, we use a recently proposed method to overcome this problem and provide a new global APWP for the last 320 million years that is calculated from simulated site-level paleomagnetic data instead of from paleopoles, in which spatial and temporal uncertainties of the original datasets are incorporated. We provide an updated global paleomagnetic database scrutinized against quantitative, stringent quality criteria, and use an updated global plate motion model. The new global APWP follows the same trend as the most recent pole-based APWP but has smaller uncertainties. This demonstrates that the first-order geometry of the global APWP is robust and reproducible. Moreover, we find that previously identified peaks in APW rate disappear when calculating the APWP from site-level data and correcting for a temporal bias in the underlying data. Finally, we show that a higher-resolution global APWP frame may be determined for time intervals with high data density, but that this is not yet feasible for the entire 320–0 Ma time span. Calculating polar wander from site-level data provides opportunities to significantly improve the quality and resolution of the global APWP by collecting large and well-dated paleomagnetic datasets from stable plate interiors, which may contribute to solving detailed Earth scientific problems that rely on a paleomagnetic reference frame

Green Edge ice camp campaigns : understanding the processes controlling the under-ice Arctic phytoplankton spring bloom

The Green Edge initiative was developed to investigate the processes controlling the primary productivity and fate of organic matter produced during the Arctic phytoplankton spring bloom (PSB) and to determine its role in the ecosystem. Two field campaigns were conducted in 2015 and 2016 at an ice camp located on landfast sea ice southeast of Qikiqtarjuaq Island in Baffin Bay (67.4797∘ N, 63.7895∘ W). During both expeditions, a large suite of physical, chemical and biological variables was measured beneath a consolidated sea-ice cover from the surface to the bottom (at 360 m depth) to better understand the factors driving the PSB. Key variables, such as conservative temperature, absolute salinity, radiance, irradiance, nutrient concentrations, chlorophyll a concentration, bacteria, phytoplankton and zooplankton abundance and taxonomy, and carbon stocks and fluxes were routinely measured at the ice camp. Meteorological and snow-relevant variables were also monitored. Here, we present the results of a joint effort to tidy and standardize the collected datasets, which will facilitate their reuse in other Arctic studies

High-P granulite facies metamorphism from the Tibetan Plateau and the Himalaya : metamorphic history and geochemistry of lower crustal and early subduction metamorphic rocks

Cette thèse porte sur deux suites de roches métamorphiques de haute-pression et haute température provenant de l'Orogène Tibeto-Himalayen. La première suite de roches consiste en des affleurements d'amphibolites à grenat et clinopyroxène se retrouvant sous la forme de blocs dans le mélange ophiolitique à matrice de serpentine sous-jacent aux ophiolites de la Zone de Suture du Yarlung Zangbo, au Sud Tibet. La Zone de Suture du Yarlung Zangbo est un linéament de plus de 2000 km de long situé à la bordure méridionale du plateau Tibétain, au nord de la crête Himalayenne. Elle contient les reliques du vaste océan qui séparait l'Inde du Tibet pendant le Jurassique et le Crétacé : la Téthys. Dans la suture, des fragments de lithosphère océanique ont été préservés sous la forme d'une ceinture ophiolitique discontinue sous laquelle se retrouve un mélange ophiolitique. Les roches documentées dans la première partie de cette thèse ont été échantillonnées dans les occurrences de Bainang et de Angren/Buma, près de Xigaze, et plus à l'ouest sous l'ophiolite de Saga. Les relations de terrain suggèrent que ces roches représentent une semelle sub-ophiolitique démembrée. Sur la base des teneurs en éléments majeurs et traces de ces roches, cette semelle métamorphique aurait une affinité de N-MORB ou de BABB très similaire à celle de la croûte des ophiolites sus-jacentes. La géochronologie en Ar/Ar sur hornblende indique un âge de refroidissement entre 130 et 123 Ma. Considérant les modélisations complétées pour d'autres semelles métamorphiques dans le monde, ces âges peuvent également être considérés comme datant de très près le pic métamorphique. Les conditions du pic métamorphique on été contraintes thermobarométriquement et sont supérieures à 13 kbar et 800°C avec des moyennes dans l'ordre de 15 kbar et 850°C. Les relations de terrain, les données de littérature concernant les unités associées ainsi que la géochimie, la géochronologie et l'histoire métamorphique de la semelle subophiolitique de la Zone de Suture du Yarlung Zangbo supportent le modèle géodynamique suivant. Pendant le Jurassique ou le Crétacé Inférieur, la croûte des ophiolites du Yarlung Zangbo et le protolithe de sa semelle métamorphique sont formés à un centre d'expansion situé dans une zone de supra-subduction comprenant un bassin d'arrière-arc mature. Vers 130 Ma, une perturbation tectonique majeure change la direction relative des plaques et force l'initiation d'une nouvelle subduction localisée sur la ride d'extension du bassin arrière-arc. Le résultat est une ophiolite d'affinité d'arrière-arc piégée en contexte d'avant-arc et sous laquelle se retrouve une semelle métamorphique du faciès des granulites de haute-P et d'affinité d'arrière-arc. La deuxième suite de roches étudiée dans cette thèse consiste en des migmatites alumineuses à kyanite retrouvées dans le coeur de l'Antiforme du Namche Barwa, au sein de la Syntaxie Himalayenne Orientale. Le coeur de l'Antiforme du Namche Barwa est un dôme métamorphique à extrusion très rapide montrant des taux de denudation et d'exhumation extrêmes (~10mm/a). Il est situé à l'extrémité orientale de la chaîne Himalayenne et comporte la gorge la plus profonde de la planète. Les roches étudiées se retrouvent sous la forme de lentilles enrobées dans le gneiss migmatitiques à sillimanite qui forme la majorité du coeur de l'antiforme. Les lentilles migmatitiques à kyanite, d'âge Éocène-Oligocène (Zhang et al. 2010), ont été interprétées comme représentant la croûte inférieure du plateau Tibétain mais leur pic métamorphique dans le faciès des granulites de haute-pression était jusqu'à aujourd'hui contesté. Dans la présente étude, ces roches ont été investiguées quant à leur minéralogie, leur géochimie, les relations texturales entre les minéraux qui les composent et leur chimie minérale. Les résultats ont été interprétés à l'aide de pseudosections. L'interprétation confirme que ces roches représentent des protolithes sédimentaires alumineux ayant été enfouis à des conditions de croûte inférieure de l'ordre de 15 kbar et 850°C où ils ont perdu une proportion de leur liquide anatectique. Cependant, une proportion significative de liquide anatectique est restée piégée dans le réseau cristallin réfractaire, donnant lieu à d'importantes modifications texturales pendant l'exhumation jusqu'à des conditions de l'ordre de 10 kbar et 800°C, correspondant à la solidification finale du liquide piégé. Les résultats des modélisations suggèrent également que le potentiel de fusion par décompression d'une croûte inférieure aussi chaude est très faible puisque la plus grande proportion du liquide anatectique est produite pendant l'enfouissement. Cette étude démontre que la croûte inférieure Tibétaine était déjà fortement épaissie et très chaude peu après la collision initiale Éocène

Petrography, geochemistry and Nd isotope systematics of metaconglomerates and matrix-rich metasedimentary rocks: Implications for the provenance and tectonic setting of the Labrador Trough, Canada

The New Quebec Orogen consists of a supracrustal belt that was reworked when the Superior craton collided with the Core Zone terrane during the Paleoproterozoic Trans-Hudson Orogeny. Within the New Quebec Orogen, the Kaniapiskau Supergroup can be divided into four terrigenous lithotypes metamorphosed at low-grade: one set with greater compositional and textural sedimentary maturity classified as quartz arenites and subarkoses, and another set with lower textural maturity classified as feldspathic wackes and mudrocks. In contrast, the Laporte Group includes homogeneous lithotypes represented by feldspathic and lithic wackes with a range of matrix contents metamorphosed at low to medium-grade. The Kaniapiskau Supergroup rocks have a wide range of SiO2 and Al2O3 contents (SiO2/Al2O3 = 3.7-51) compared to the restricted compositional range of the Laporte Group rocks (SiO2/Al2O3 = 4.4-6.8). In general, the geochemical variations in both formations of the Laporte Group are within the range of the main clast varieties from basal metaconglomerates, although the Deborah Formation (top unit), records higher TiO2, P2O5, MgO and Ni contents and high Cr/Th, Co/Ba, Th/U and Rb/Sr ratios indicating additional mafic sources. Our results support the hypothesis that the Kaniapiskau Supergroup was deposited along an intraplate continental margin with predominantly recycled (ɛNd(1.87Ga) -12) Paleoarchean sources (TDM 3.2 Ga). In contrast, the Laporte Group marks the transition from a continental forearc (Grand Rosoy Fm.) with a typical juvenile source, including granitic clasts (ɛNd(1.83Ga) -0.1 to +3.1), to a wedge-top depozone (Deborah Fm.) in the context of a collisional pro-foreland basin. This syn-collisional sedimentary environment is characterized by the presence of old crustal components (ɛNd(1.83Ga) -4.4 to -9.1).The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author