The structural history of the Mont Blanc massif with regard to models for its recent exhumation

The tectonic evolution of the Mont Blanc range with regard to its cooling and exhumation history has been discussed and debated over many years and is still controversial. Recently, several low-temperature thermochronology studies have determined the cooling history of the massif in considerable detail and various tectonic models proposed to explain the young and fast exhumation signal established from these studies. Here we present detailed field data from the wider Mont Blanc area and assess possible exhumation processes in terms of these field constraints. Our observations indicate that none of the major faults or shear zones around the Mont Blanc massif (i.e. Mont Blanc shear zone, Mont Blanc back-thrust, Penninic thrust) was active in Late Neogene times and that young exhumation is therefore not controlled by movements along these structures. We demonstrate that the position of Mont Blanc in the bend of the western Alps plays an important role in its tectonic history and that simple 2D models are insufficient to explain its evolution. Interference between NW–SE compression and orogen-parallel extension along the Rhône-Simplon fault system resulted in a complex regional structural pattern, with strike-slip movements on both sides of the Mont Blanc massif. Young brittle faults are predominantly strike slip without significant vertical offset. The young (<2 Ma) rapid exhumation of Mont Blanc is more broadly distributed and cannot be directly linked to discrete faults bounding the massif. The mechanisms driving this recent accelerated exhumation must similarly be of broader scale.     

矿物-腐植酸-微生物体系对重金属吸附研究

设施农业中土壤重金属污染问题日趋严重.由于土壤中矿物、腐植酸、微生物等多相组分之间存在交互作用,重金属与土壤单组分体系中所获得的结合机制并不能真实有效地评价其在自然条件下的转化与归趋.本研究以蒙脱石(Mont)和高岭石(Kao)为辽宁蔬菜大棚及农田土壤层状硅酸盐代表矿物,选取胡敏酸(HA)为有机质代表,土著微生物革兰氏...     

Mineral Chemistry of Mildly Alkalic Basalts from the 25 Ma Mont Crozier Section, Kerguelen Archipelago: Constraints on Phenocryst Crystallization Environments

Phenocryst compositions and mineral–melt equilibria inthe mildly alkalic basalts from the 25 Ma Mont Crozier sectionon the Kerguelen Archipelago are used to estimate the depthsat which magmas stalled and crystallized and to constrain therole of crustal structure in the evolution of magmas producedby the Kerguelen mantle plume. The Crozier section, of nearly1000 m height, consists of variably porphyritic flows (up to21 vol. % phenocrysts), dominated by plagioclase ± clinopyroxene± olivine ± Fe–Ti oxides. Feldspars showan extreme range of compositions from high-Ca plagioclase (An₈₈)to sanidine and variable textures that are related to extensivefractionation, degassing, and mixing in relatively low-pressure(sub-volcanic) magma chambers. Although clinopyroxene is a minorphenocryst type (0–3 vol. %), its non-quadrilateral components,principally Al (1·9–8·6 wt % Al₂O₃), varywidely. The results of clinopyroxene–liquid thermobarometryand clinopyroxene structural barometry indicate that the Croziermagmas crystallized at pressures ranging from     

Permian magmatism and metamorphism in the Dent Blanche nappe: constraints from field observations and geochronology

In the Dent Blanche Tectonic System, the Mont Morion biotite-bearing granite is a km-scale intrusion preserved in a low-strain volume. Zircon saturation thermometry suggests that it crystallised from a melt that reached about 800 °C. U–Pb zircon and allanite geochronology indicates crystallization of the magma in the Permian (290 ± 3 Ma; 280 ± 8 Ma, respectively). Migmatitic biotite-gneiss and amphibolite are found as xenoliths within the Mont Morion granite and constitute its country-rocks. In two samples of migmatitic biotite-gneiss zircon has metamorphic overgrowths that yield U–Pb ages of 285 ± 3 Ma and 281 ± 4 Ma, and are thus contemporaneous with the intrusion of the granite. The Mont Morion granite with its country-rocks of migmatitic biotite-bearing gneiss and amphibolite was thus emplaced at middle crustal levels while amphibolite facies metamorphism affected its country rocks. The magmatic and metamorphic record in the Mont Morion area reflects the high-temperature regime and lithospheric thinning of the Adriatic continental margin during Permian.     

Mineralogy,geochemistry, and genesis of high-alumina fenites of the Mont Saint-Hilaire alkaline pluton,Québec,Canada

High-alumina fenites in the Mont Saint-Hilaire alkaline complex, Québec, Canada, form bodies at the contact of peralkaline nepheline syenite. Fenites are subdivided into four types: corundum-spessartine-biotite-feldspar, muscovite-corundum-hercynite-biotite-feldspar, carbonated muscovite-biotite-hercynite-feldspar, and spessartine-hercynite-feldspar. Accessory minerals of the ilmenite-pyrophanite series, columbites, zircon, thorite, pyrrhotite, Fe, Mn, Mg, Ca, Ba, and REE carbonates, uedaite-(Ce), etc. are identified. Three stages are suggested in the formation of these rocks. In mineralogy and geochemistry, the Mont Saint-Hilaire high-alumina fenites are similar to Al-rich fenites replacing xenoliths in the Khibiny alkaline complex, Russia. In both cases, fenites are related to peralkaline rocks and replace high-alumina protoliths: granite at Mont Saint-Hilaire and metapelites in the Khibiny Mountains. These fenites are regarded as a specific type of fenites with rock-forming Mg-depleted hercynite.     

On the age of mylonites within the Mont Blanc massif

Abstract

Three generations of mylonites occur in the Mont Blanc Massif :

— the first one has produced mylonitic schists trending N20°E, of regional extension; the late Variscan Mont Blanc granite intruded these mylonites;

— the second one has affected large areas in the Mont Blanc granite, with the same N20°E direction;

— the third one is located along rather narrow bands N45°E, in the granite as well as in the older mylonitic schists.

All theses mylonites have been generated under (Bi) - greenschist facies conditions. They exhibit steeply plunging schistosity and lineation. They are all ante-Alpine; paradoxically, Alpine structures within the massif do not exhibit a striking appearence.     

The restoration of thrust systems and displacement continuity around the Mont Blanc massif,NW external Alpine thrust belt

Foreland-propagating external thrust belts may be considered as essentially plane strain phenomena so that displacements can be correlated throughout their linked, three-dimensional fault geometry. This approach has been applied to part of the northwest external French-Swiss Alps, around the Mont Blanc basement massif. Imbricates of basement and cover sequences on the SW margin of this massif restore to a width in excess of 77 km with an implicit shortening of at least 67 km. These displacements can be correlated with those in the neighbouring Helvetic nappes by transferring movements, via lateral branch lines, onto the Mont Blanc thrust. By reappraising thrust geometries, the Helvetic/Ultrahelvetic nappe complex has been restored to a width of 114 km to the ESE of the Aiguilles Rouges basement massif. Displacements on the internal (SE) margin of the Mont Blanc massif, estimated by balanced sections and a restoration of the Ultrahelvetic klippen in the sub-alps, exceed 59 km. Thrust continuity, incorporating the restorations of nappes and imbricate geometries around the Mont Blanc massif, is illustrated on a crude, restored branch-line map which also serves as a preliminary palaeogeographic reconstruction. External thrust systems, to the east of the external Belledonne/Aiguilles Rouges massif, restore to a width of at least 140 km in the footwall to the Frontal Pennine thrust.     

不同分子质量聚乙二醇在蒙脱石层间的吸附作用

用蒙脱石(Mont)吸附水溶液中不同分子质量的聚乙二醇(PEG)，制备PEG／Mont复合物，采用XRD、FTIR和DTA／TG表征PEG／Mont复合物的结构。结果表明：PEG通过置换部分层间水合离子进入蒙脱石层间通道内；PEG以单层或双层平卧于蒙脱石层间；大分子质量的PEG容易形成双层复合物，但双层复合物的热稳定性较差。     

Laboratory and In Situ Simulation Tests of the Excavation Damaged Zone Around Galleries in Opalinus Clay

In the context of nuclear waste disposal in clay formations, laboratory and in situ simulation experiments were performed to study at reduced scale the excavation damaged zone (EDZ) around tunnels in the indurated Opalinus Clay at Mont Terri, Switzerland. In the laboratory, thick-walled hollow cylindrical specimens were subjected to a mechanical unloading mimicking a gallery excavation. In samples cored parallel to bedding, cracks sub-parallel to the bedding planes open and lead to a buckling failure in two regions that extend from the borehole in the direction normal to bedding. The behaviour is clearly anisotropic. On the other hand, in experiments performed on specimens cored perpendicular to bedding, there is no indication of failure around the hole and the response of the hollow cylinder sample is mainly isotropic. The in situ experiment at Mont Terri which consisted in the overcoring of a resin-injected borehole that follows the bedding strike of the Opalinus Clay showed a striking similarity between the induced damaged zone and the fracture pattern observed in the hollow cylinder tests on samples cored parallel to bedding and such a bedding controlled “Excavation” Damaged Zone is as well consistent with the distinct fracture patterns observed at Mont Terri depending on the orientation of holes/galleries with respect to the bedding planes. Interestingly, the damaged zone observed in the hollow cylinder tests on samples cored parallel to bedding and in situ around URL galleries is found to develop in reverse directions in Boom Clay (Mol) and in Opalinus Clay (Mont Terri). This most probably results from different failure mechanisms, i.e. shear failure along conjugated planes in the plastic Boom Clay, but bedding plane splitting and buckling in the indurated Opalinus Clay.     

Balanced cross-sections and their implications for the deep structure of the northwest Alps: reply

The structure of the northwest external French Alps, around the internal Belledonne and Mont Blanc massifs in the Beaufortain area, can be explained by foreland (WNW) propagating thrust tectonics. The internal Belledonne massif is interpreted as having developed by imbrication of an originally thin ‘skin’ of basement with a Mesozoic cover. Above this massif all thrusts tend to climb lateral sections to the south to converge with the frontal Pennine thrust: the southern termination of the Mont Blanc massif is merely a spectacular example of such a lateral ramp.     

Fluid inclusion, stable isotope and Ar-Ar evidence for the age and origin of gold-bearing quartz veins at Mont Chemin, Switzerland

Summary A new Swiss gold occurrence at Mont Chemin, comprising gold-bearing quartz veins, displays many characteristics that are typical of mesothermal gold deposits within the Alps and globally. The most notable of these features are: i) the presence of NaCl-H₂O-CO₂-bearing fluid with an XCO₂ of approximately 0.016 and NaCl equivalents in the range 4.6 to 10.6 weight percent, ii) greenschist formational temperatures and pressures in the range 265-285 °C and 700-1400 bars; and iii) the proximity of the occurrence to the Rhone-Simplon Line, a deep crustal structure in the Swiss Alps.Corrected Ar-Ar data for hydrothermal adularia, considered to be contemporaneous with mineral deposition from the gold-bearing fluid, yields an age of 9.9 ±1.0 Ma. Geothermal gradients and uplift rates derived from the Ar-Ar age data and the geothermometry are in agreement with existing data for this region, and indicate that the hydrothermal activity at the Mont Chemin gold occurrence records one of the last Alpine metamorphic events in the northeastern Mont Blanc massif.Temperature estimates from fluid-muscovite-quartz-feldspar equilibrium and oxygen isotope thermometry of coexisting adularia and quartz are combined with the fluid inclusion isochores to derive depositional pressures. These data yield geothermal gradients on the order of 50 °C/km and uplift rates of 0.44 mm/a for the NE portion of the Mont Blanc massif.

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Evidenz aus Flüssigkeitseinschluß-, stabilen Isotopen- und Ar- Ar Daten fü r das Alter und den Ursprung Gold führender Quarzgänge am Mont Chemin, Schweiz

Zusammenfassung Ein neues Schweizer Goldvorkommen am Mont Chernin, es handelt sich um Goldführende Quarzgänge, zeigt viele Charakteristika, die für mesothermale Goldlagerstätten der Alpen und weltweit typisch sind: i) Die Anwesenheit von NaCl-H₂O-CO₂ Fluiden mit einem XCO₂ von ca. 0.016 und NaCl zwischen 4.6 und 10.6 Gew. % Äquiv.ii) Grünschieferfazielle Bildungstemperaturen und -drucke von 265-285°C bzw. 7001400bar. iii) Die Nähe der Vorkommen zur Rhone-Simplon Linie, einer tiefgreifenden Struktur in der Kruste der Schweizer Alpen.Korrigierte Ar-Ar Daten von hydrothermalem Adular, der als zeitgleich mit den Minerallagerstätten gebildet, angesehen wird, ergaben ein Alter von 9.9 ± 1.0 Ma. Die aus aus den Ar-Ar Daten bestimmten geothermalen Gradienten und Hebungsraten und die Ergebnisse der Geothermometrie stimmen mit bisher existierenden Daten aus dieser Region überein und zeigen, daß die hydrothermale Aktivität in den Goldvorkommen des Monte Chemin eines der letzten alpidischen metamorphen Ereignisse im nordöstlichen Mont Blanc Massiv darstellt.Temperaturabschätzungen aus Fluid-Muscovit-Quarz-Feldspat Gleichgewichten und Sauerstoffisotopen-Thermometrie an koexistierendem Quarz und Adular werden mit den Isochoren der Flüssigkeitseinschlüsse kombiniert, um die Bildungsdrucke abzuleiten. Diese Daten ergeben geothermische Gradienten in der Größenordnung von ca. 50 °C/km und Hebungsraten von 0.44 mm/Jahr für den Nordostteil des Mont Blanc Massives.

     

Flood Basalt from Mont Tourmente in the Central Kerguelen Archipelago: the Change from Transitional to Alkalic Basalt at ~25 Ma

The surface of the Cenozoic Kerguelen Archipelago, constructedon the Kerguelen Plateau in the southern Indian Ocean, is dominantlyflood basalt. With the objective of understanding the Cenozoichistory of the Kerguelen mantle plume, the age and geochemicalcharacteristics of this flood basalt province are being determinedby studying stratigraphic sections of basalt flows at severallocations. Sections from the NW, north–central, east andSE parts of the archipelago have been studied. Here we reportresults for a 597 m succession of lavas from Mont Tourmentefrom the Plateau Central, a region of the archipelago that hasnot been studied in detail. Mont Tourmente lavas range from     

A Variscan pressure-temperature-time path for the N-E Mont Blanc massif

The northeastern portion of the Mont Blanc massif in western Switzerland is predominantly comprised of the granitic rocks of the Mont Blanc intrusive suite and the Mont Blanc basement gneisses. Within these metamorphic rocks are a variety of sub-economic Fe skarns. The mineral assemblages and fluid inclusions from these rocks have been used to derive age, pressure, temperature and fluid composition constraints for two Variscan events. Metamorphic hornblendes within the assemblages from the basement amphibolites and iron skarns have been dated using ⁴⁰Ar/³⁹Ar, and indicate that these metamorphic events have a minimum age of approximately 334 Ma. Garnet-hornblende-plagioclase thermobarometry and stable isotope data obtained from the basement amphibolites are consistent with metamorphic temperatures in the range 515 to 580 °C, and pressures ranging from 5 to 8 kbar. Garnet-hornblende-magnetite thermobarometry and fluid inclusion studies indicate that the iron skarns formed at slightly lower temperatures, ranging from 400 to 500 °C in the presence of saline fluids at formational pressures similar to those experienced by the basement amphibolites. Late Paleozoic minimum uplift rates and geothermal gradients calculated using these data and the presence of Ladinien ichnofossils are on the order of 0.32 mm/year and 20 °C/km respectively. These uplift rates and geothermal gradients differ from those obtained from the neighbouring Aiguilles Rouges massif and indicate that these two massifs experienced different metamorphic conditions during the Carboniferous and Permian periods. During the early to late Carboniferous period the relative depths of the two massifs were reversed with the Aiguilles Rouges being initially unroofed at a much greater rate than the Mont Blanc, but experiencing relatively slower uplift rates near the termination of the Variscan orogeny. Received: 23 April 1996 / Accepted: 27 August 1996     

Flood Basalts and Lunar Petrogenesis

The popular interpretation of lunar maria, as sequences of primarypicritic flood basalts derived by remelting a mantle that hadaccumulated from a global magma ocean, has many unsatisfactoryaspects. The expertise of Keith Cox would have been valuablein their interpretation. KEY WORDS: europium anomaly; Io; magma ocean; mare basalt; picrite; quench crystal; regolith     

The importance of local mean time in remote sensing for mapping megatidal zones

The local mean time is rarely debated in applied remote sensing, although it is a key parameter, especially for the geological mapping of coastal areas with high tidal ranges. French test sites are used to illustrate the role of this parameter. Depending from location and for an average time of acquisition of 11:00 UTC, the exposed tidal flat ranges from 55% (Baie des Veys), 80% (Mont Saint-Michel) to 100% (Cap-Ferret). Eight multisource satellite images were used in the Mont Saint-Michel Bay to draw the limit of the sea and evaluate the corresponding tidal flat's surfaces for various times of acquisition. The relation emphasizes some geometrical properties of the lower part of the intertidal zone. A NW–SE profile allows one to identify a distal part with a slope of 0.2% and a proximal part with a slope of only 0.06%. JERS 1-OPS and ALOS-AVNIR-2 data have been also compared to evaluate the progression of the halophytic vegetation, covering more than 6 km² between 1992 and 2007. From a methodological point of view, the ALOS-AVNIR-2 acquired on October 2007 is the first satellite data covering the Mont Saint-Michel Bay with a water elevation of only 2.56 m, i.e. a tidal flat free of water over 80% of its surface.     

Silicate Liquid Immiscibility in Tholeiitic Basalts

Immiscible silicate liquids, preserved as fresh glasses, constituteat least 32 per cent of a tholeiitic basalt in the Triassic-Jurassicbasin of Southbury, Connecticut. This apparently uncommon abundanceof immiscible liquids cannot be attributed to an unusual magmacomposition, for the rock is similar to many quartz-normativetholeiites associated with other basins of similar age in easternNorth America. Nor can peculiar conditions of fractional crystallizationbe involved, for experiments carried out at one atmosphere totalpressure reveal that two silicate liquids coexist over a widetemperature interval during crystallization, and that this intervalis increased by raising the oxygen fugacity. Experiments onthe standard diabase, W-1, which differs only slightly in compositionfrom the Connecticut basalt, indicates the presence of immiscibleliquids during the crystallization of this magma compositionas well. It is concluded that liquid immiscibility is to beexpected during the crystallization of many tholeiitic magmas,and at a sufficiently early stage of solidification to playa role in igneous differentiation.     

Laser‐cut Rb–Sr microsampling dating of deformational events in the Mont Blanc‐Aiguilles Rouges region (European Alps)

Spatial control for in situ dating of mineral phases in fine‐grained rocks is a significant challenge in geochronology, and the precision of microsampling is a crucial factor in obtaining accurate results. In this study, a new microsampling approach to Rb–Sr geochronology has been applied to greenschist facies mylonitic shear zones in the Mont Blanc‐Aiguille Rouges region of the western European Alps. Using a laser‐ablation system for microsampling by laser cutting followed by conventional TIMS Rb–Sr isotopic analysis of μg‐sized samples provides an improved workflow for texturally controlled, quasi in situ dating of mineral phases. The automated cutting process minimizes material loss and the risk of handling errors, while facilitating sampling of complex shapes of almost any size, a significant improvement over earlier microscope‐mounted microdrills. The new Rb–Sr white mica–calcite ages of between 27 and 30 Ma indicate Oligocene deformation in Alpine shear zones from two specific areas in the Mont Blanc‐Aiguilles Rouges region.     

Balanced cross-sections and their implications for the deep structure of the northwest Alps

The Mont Blanc massif is one of a chain of basement culminations which crop out along the external French Alps. Its southwestern margin is interpreted as being a major thrust belt which propagated in a piggy-back sequence towards the foreland. These imbricates have developed in the footwall of the high-level Valais thrust. The depth to the floor thrust and shortening within imbricates above this thrust are estimated by a series of partially balanced cross-sections drawn between the ‘synclinal median’ and the Valais thrust. These sections restore to a pre-thrust length of at least 50 km, probably exceeding 100 km, above a floor thrust never deeper than 1 km below the sub-Triassic unconformity. All this thrust displacement is transferred via a series of lateral branch lines onto the Mont Blanc thrust in the Chamonix area. A corollary of this is that the Aiguilles Rouges and the main part of the Mont Blanc massif were separated by probably as much as 100 km prior to Alpine thrusting. Such large shortening estimates imply a hitherto unsuspected Dauphinois stratigraphic consistency in both thickness and lithology.To achieve a balance a restored crustal cross-section must show an equal length of both lower and upper crust. Thus a high-level basal detachment which floors large thrust displacements must overlie a long, undeformed lower crustal wedge. A restored section 100 km long requires such a lower crustal wedge to exist beneath the entire Alpine internal zones. Perrier & Vialon's crustal velocity profile through the western Alps is reinterpreted in these terms. The Ivrea body is considered to be a portion of an external lower crustal wedge which has been uplifted by thrusts after most of the displacement on the external thrust belt.