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M P Searle - One of the best experts on this subject based on the ideXlab platform.

  • the geochemical and temporal evolution of the continental lithosphere and its relationship to continental scale faulting the karakoram fault eastern karakoram nw himalayas
    Geochemistry Geophysics Geosystems, 2013
    Co-Authors: R J Phillips, M P Searle, Randall R Parrish
    Abstract:

    [1] New laser ablation multicollector–inductively coupled plasma–mass spectrometry and isotope dilution-thermal ionization mass spectrometry U-Pb ages, coupled with Sm-Nd isotope and geochemical analysis, define the temporal and geochemical evolution of the continental lithosphere in the eastern Karakoram, India, NW Himalaya. Our analysis demonstrates that magmatism occurred between ~108 and 69 Ma and ~22 and 13 Ma. The new age data, coupled with geochemical examination of the granitoids, confirm a parallel evolution with the western Karakoram in Pakistan and supports a model of regional continental crustal thickening and related metamorphism. Middle to Late Cretaceous magmatism immediately adjacent to the Karakoram fault suggests that crustal melting and associated metamorphism are unrelated to shearing along the fault. Miocene Leucogranite magmatism occurred almost exactly concomitant with the emplacement of the Baltoro batholith in Pakistan. These trans-Karakoram Leucogranites also display similar geochemical evolution trends. Our new data clearly link the Leucogranites along the fault to the regional Baltoro batholith and related metamorphic complexes to the west. This supports previous work suggesting that magmatism and metamorphism were not syn-kinematic with continental-scale faulting. The data demonstrate that the Karakoram fault could not have accommodated lateral offset in this region prior to ~16 Ma, limiting the long-term averaged slip rate to a maximum of ~10 mm/yr.

  • metamorphism melting and channel flow in the greater himalayan sequence and makalu Leucogranite constraints from thermobarometry metamorphic modeling and u pb geochronology
    Tectonics, 2010
    Co-Authors: M J Streule, M P Searle, D J Waters, Matthew S A Horstwood
    Abstract:

    [1] The Makalu Leucogranite in the eastern Nepal Himalaya is a multiphase intrusion forming the structurally highest foliation-parallel sheets along the top of the Greater Himalayan Sequence. It is part of a chain of Miocene granites seen continuously along the length of the Himalaya and is composed of Grt + Tur + Ms ± Bt Leucogranites but, unlike most other Himalayan granites, also locally contains coarse-grained cordierite. The cordierite-bearing Leucogranite intrudes through and overlies lower sheets of “normal” tourmaline granites and represents the most recent phase of magmatism. Cross-cutting feeder dykes channelled magma up from the source region within the sillimanite grade Barun gneiss to the upper sheet. Petrology shows evidence for muscovite dehydration melting (∼700°C) in the upper part of the Barun gneiss of the Greater Himalayan Sequence, which retains biotite, indicating that melting temperatures did not exceed 800°C. Secondary cordierite around garnet in these gneisses and the presence of cordierite in Leucogranites record the last low-pressure decompression phase of melting. P-T determinations detail peak sillimanite grade metamorphism at 713°C/5.9 kbar, with a secondary cordierite overprint at 618°C/2.1 kbar; this P-T transition lies wholly within the modeled melt field. Monazite, zircon, and xenotime geochronology links the metamorphism and the different Leucogranites. The main phase of Leucogranite production occurred from 24 to 21 Ma, while the most recent melting occurred in the cordierite Leucogranite and the migmatitic Barun gneisses at 15.6 ± 0.2 and 16.0 ± 0.6 Ma, respectively. Pseudosections for the migmatitic Barun gneiss and cordierite Leucogranite show conditions of final cordierite bearing melt crystallization at approximately 4 kbar and 700°C and two main phases of melting: one associated with muscovite dehydration melting and one associated with formation of cordierite. These data support the channel flow model for the Greater Himalaya where decompression melting was coeval with southward ductile extrusion of a partially molten layer of middle crust during the Early and Middle Miocene.

  • structural constraints on the timing of left lateral shear along the red river shear zone in the ailao shan and diancang shan ranges yunnan sw china
    Geosphere, 2010
    Co-Authors: M P Searle, Meng Wan Yeh, Te Hsien Lin, Sunlin Chung
    Abstract:

    The >1000-km-long Oligocene—Miocene left-lateral Red River shear zone (RRSZ) and metamorphic belt and the Pliocene—active right-lateral Red River fault (RRF), stretching from SE Tibet to the South China Sea, has been cited as one of the primary examples of a lithospheric scale strike-slip fault that has resulted in syn-kinematic metamorphism and partial melting and accommodated several hundred to a thousand kilometers of horizontal motion as a result of the indentation of India into Asia. Alternatively we interpret the metamorphic complexes along the RRSZ as exhumed metamorphic core complexes of older rocks, subsequently affected by Oligocene–Early Miocene left-lateral shear and localized partial melting (Leucogranite dykes), Miocene low-angle normal faulting along margins (Range Front faults), and Pliocene active dextral strike-slip faulting (RRF). Along the Ailao Shan (ALS) and Diancang Shan (DCS) ranges in Yunnan, SW China, early amphibolite facies metamorphic rocks were intruded by K-feldspar orthogneisses of Triassic age (Indosinian). LA-ICP-MS U-Pb zircon dating reveals a complex history with zircon cores showing evidence of Indosinian (∼239–243 Ma) to Neoproterozoic magmatism. Zircon rims show an Oligocene (∼26 Ma) magmatic or metamorphic overprint. Biotite granodiorites and syenites of mantle origin intruded the gneisses during the Oligocene (∼35 Ma). Later biotite Leucogranites intruded the orthogneisses and migmatite host rocks before a significant phase of tight to isoclinal folding. Ductile, left-lateral strike-slip shear fabrics were superimposed on all lithologies at high temperature (∼500–550 °C) for the ALS and lower temperatures (∼250–150 °C) after peak metamorphism and after granite intrusion. A few very small biotite (±Grt ± Tur) Leucogranite veins and dykes crosscut the ductile strike-slip shear fabrics at Yuanjiang, in the Ailao Shan. Low-angle normal faulting along the margins of the metamorphic massif accommodated final exhumation of the Red River gneisses. Using published U-Th-Pb and 40 Ar/ 39 Ar ages of granites along the shear zone, the age of left-lateral ductile shearing along the RRSZ can be constrained as between the earlier deformed Leucogranites (31.9–24.2 Ma) and the later crosscutting dykes (21.7 Ma) with exhumation-related cooling continuing until ∼17 Ma.

  • melting and exhumation of the upper structural levels of the greater himalaya sequence and makalu granite constraints from thermobarometry metamorphic modeling and u pb geochronology
    Himalayan Journal of Sciences, 2008
    Co-Authors: M J Streule, M P Searle, Matthew S A Horstwood, David J Waters
    Abstract:

    The Makalu massif of Eastern Nepal displays a complex suite of Leucogranites and host sillimanite grade gneisses. These Leucogranites are linked to the intrusions at the base of the Everest-Lhotse-Nuptse massif immediately to the west (Searle et al 2003) The Makalu intrusion is multiphase and forms the structurally highest foliationparallel sheets of Leucogranite along the top of the Greater Himalayan Sequence on the Nepal-Tibet border. It is comprised of massive Grt + Tur + Ms ± Bt Leucogranites that also occasionally contain large cordierite crystals. The abundance of cordierite in the upper granite sheet is unlike other granites in the Nepalese Himalaya. The cordierite bearing Leucogranite overlies lower sheets of ‘normal’ Himalayan granites intruded into black sillimanite gneisses and is thought to be the most recent phase of magmatism. A few crosscutting feeder dykes mapped adjacent to the upper Barun glacier have channeled magma to the upper sheet. Petrology shows evidence for muscovite dehydration melting (~<700°C) in the upper part of the Barun gneiss which is a likely mechanism by which to produce the Makalu granite melts. Host gneisses retain biotite and so melting temperatures did not exceed 800°C (White et al. 2001). Secondary cordierite rims around garnets in these gneisses and the presence of cordierite in Leucogranites record the last low pressure phase of melting. We use these field and petrographic observations for the basis of detailed metamorphic modeling of decompression and geochronology work for the upper parts of the GHS. P-T determinations (THERMOCALCv.3.30) detail peak

  • the structural geometry metamorphic and magmatic evolution of the everest massif high himalaya of nepal south tibet
    Web Science, 2003
    Co-Authors: M P Searle, R L Simpson, Richard D Law, Randall R Parrish, D J Waters
    Abstract:

    This paper presents a new geological map together with cross-sections and lateral sections of the Everest massif. We combine field relations, structural geology, petrology, thermobarometry and geochronology to interpret the tectonic evolution of the Everest Himalaya. Lithospheric convergence of India and Asia since collision at c. 50 Ma. resulted in horizontal shortening, crustal thickening and regional metamorphism in the Himalaya and beneath southern Tibet. High temperatures (>620 °C) during sillimanite grade metamorphism were maintained for 15 million years from 32 to 16.9 ± 0.5 Ma along the top of the Greater Himalayan slab. This implies that crustal thickening must also have been active during this time, which in turn suggests high topography during the Oligocene–early Miocene. Two low-angle normal faults cut the Everest massif at the top of the Greater Himalayan slab. The earlier, lower Lhotse detachment bounds the upper limit of massive Leucogranite sills and sillimanite–cordierite gneisses, and has been locally folded. Ductile motion along the top of the Greater Himalayan slab was active from 18 to 16.9 Ma. The upper Qomolangma detachment is exposed in the summit pyramid of Everest and dips north at angles of less than 15°. Brittle faulting along the Qomolangma detachment, which cuts all Leucogranites in the footwall, was post-16 Ma. Footwall sillimanite gneisses and Leucogranites are exposed along the Kharta valley up to 57 km north of the Qomolangma detachment exposure near the summit of Everest. The amount of extrusion of footwall gneisses and Leucogranites must have been around 200 km southwards, from an origin at shallow levels (12–18 km depth) beneath Tibet, supporting models of ductile extrusion of the Greater Himalayan slab. The Everest–Lhotse–Nuptse massif contains a massive ballooning sill of garnet + muscovite + tourmaline Leucogranite up to 3000 m thick, which reaches 7800 m on the Kangshung face of Everest and on the south face of Nuptse, and is mainly responsible for the extreme altitude of both mountains. The middle crust beneath southern Tibet is inferred to be a weak, ductile-deforming zone of high heat and low friction separating a brittle deforming upper crust above from a strong (?granulite facies) lower crust with a rheologically strong upper mantle. Field evidence, thermobarometry and U–Pb geochronological data from the Everest Himalaya support the general shear extrusive flow of a mid-crustal channel from beneath the Tibetan plateau. The ending of high temperature metamorphism in the Himalaya and of ductile shearing along both the Main Central Thrust and the South Tibetan Detachment normal faults roughly coincides with initiation of strike-slip faulting and east–west extension in south Tibet (

Michel Pichavant - One of the best experts on this subject based on the ideXlab platform.

  • Mica-liquid trace elements partitioning and the granite-pegmatite connection: The St-Sylvestre complex (Western French Massif Central)
    Chemical Geology, 2019
    Co-Authors: Arnaud Villaros, Michel Pichavant
    Abstract:

    We constrain the genetic relation between granite and pegmatite parental melts from the Variscan Saint Sylvestre Leucogranite complex and associated pegmatite bodies (Massif Central, France) through compositions of micas. Using mica trace element concentrations and available partition coefficients for Li, Rb, Ba, Cs and F, we calculated the trace element contents of granitic and pegmatitic melts at equilibrium with micas. Biotite in Leucogranites and pegmatites ranges mostly from Fe-biotite to siderophyllite. More evolved facies contain protolithionite and zinnwaldite and lepidolite occurs in the most fractionated pegmatite. White micas have homogeneous compositions, from muscovite to Li-phengite. In granites, biotite and muscovite trace element distributions are clustered. In pegmatites, mica trace element contents globally follow differentiation from the least to the most evolved body. The reconstructed pegmatitic and granitic melts present strong compositional similarities such as enrichments in Li, Rb, Cs and depletion in Ba that suggest a similar origin. However, micas are shown to have selectively equilibrated with the last melt (or fluid) in contact and so trace element concentrations of early magmatic liquids are rarely preserved. Inversion of the mica data constrains the composition of parental melts and the trace element evolutions during crystallization. Leucogranite and pegmatite melts show mutual incompatible element evolutions inconsistent with a parent-daughter genetic relation. The data and the modelling suggest that they represent non-cogenetic melts generated by discrete episodes of partial melting. Differentiation is thus inherited from source processes rather than being the consequence of fractional crystallization of a common parental melt/magma. We suggest that both the Leucogranites and the pegmatites originate from partial melting of a heterogeneous source rather than pegmatites being the product of granite crystallization.

Ballouard Christophe - One of the best experts on this subject based on the ideXlab platform.

  • Multiple crust reworking in the French Armorican Variscan belt: implication for the genesis of uranium-fertile Leucogranites
    'Springer Science and Business Media LLC', 2018
    Co-Authors: Ballouard Christophe, Poujol Marc, Zeh Armin
    Abstract:

    International audienceIn the French Armorican Variscan belt, most of theeconomically significant hydrothermal U deposits are spatiallyassociated with peraluminous Leucogranites emplacedalong the south Armorican shear zone (SASZ), a dextral lithosphericscale wrench fault that recorded ductile deformationfrom ca. 315 to 300 Ma. In the Pontivy-Rostrenen complex, acomposite intrusion, the U mineralization is spatially associatedwith brittle structures related to deformation along theSASZ. In contrast to monzogranite and quartz monzodiorite(3 3), the Leucogranite samples arecharacterized by highly variable U contents (~ 3 to 27 ppm)and Th/U ratios (~ 0.1 to 5) suggesting that the crystallizationof magmatic uranium oxide in the more evolved facies wasfollowed by uranium oxide leaching during hydrothermal alterationand/or surface weathering. U-Pb dating of uraniumoxides from the deposits reveals that they mostly formed betweenca. 300 and 270 Ma. In monzogranite and quartzmonzodiorite, apatite grains display magmatic textures andprovide U-Pb ages of ca. 315 Ma reflecting the time of emplacementof the intrusions. In contrast, apatite grains from theLeucogranite display textural, geochemical, and geochronologicalevidences for interaction with U-rich oxidized hydrothermalfluids contemporaneously with U mineralizingevents. From 300 to 270 Ma, infiltration of surface-derivedoxidized fluids leached magmatic uranium oxide from fertileLeucogranite and formed U deposits. This phenomenon wassustained by brittle deformation and by the persistence ofthermal anomalies associated with U-rich granitic bodies

  • Uranium metallogenesis of the peraluminous Leucogranite from the Pontivy-Rostrenen magmatic complex (French Armorican Variscan belt): the result of long-term oxidized hydrothermal alteration during strike-slip deformation
    'Springer Science and Business Media LLC', 2018
    Co-Authors: Ballouard Christophe, Poujol Marc, Mercadier Julien, Deloule Etienne, Boulvais Philippe, Cuney Michel, Cathelineau Michel, Baele Jean-marc
    Abstract:

    International audienceIn the French Armorican Variscan belt, most of theeconomically significant hydrothermal U deposits are spatiallyassociated with peraluminous Leucogranites emplacedalong the south Armorican shear zone (SASZ), a dextral lithosphericscale wrench fault that recorded ductile deformationfrom ca. 315 to 300 Ma. In the Pontivy-Rostrenen complex, acomposite intrusion, the U mineralization is spatially associatedwith brittle structures related to deformation along theSASZ. In contrast to monzogranite and quartz monzodiorite(3 3), the Leucogranite samples arecharacterized by highly variable U contents (~ 3 to 27 ppm)and Th/U ratios (~ 0.1 to 5) suggesting that the crystallizationof magmatic uranium oxide in the more evolved facies wasfollowed by uranium oxide leaching during hydrothermal alterationand/or surface weathering. U-Pb dating of uraniumoxides from the deposits reveals that they mostly formed betweenca. 300 and 270 Ma. In monzogranite and quartzmonzodiorite, apatite grains display magmatic textures andprovide U-Pb ages of ca. 315 Ma reflecting the time of emplacementof the intrusions. In contrast, apatite grains from theLeucogranite display textural, geochemical, and geochronologicalevidences for interaction with U-rich oxidized hydrothermalfluids contemporaneously with U mineralizingevents. From 300 to 270 Ma, infiltration of surface-derivedoxidized fluids leached magmatic uranium oxide from fertileLeucogranite and formed U deposits. This phenomenon wassustained by brittle deformation and by the persistence ofthermal anomalies associated with U-rich granitic bodies

  • Magmatic and hydrothermal behavior of uranium in syntectonic Leucogranites: The uranium mineralization associated with the Hercynian Grande granite (Armorican Massif, France)
    'Elsevier BV', 2017
    Co-Authors: Ballouard Christophe, Poujol Marc, Mercadier Julien, Deloule Etienne, Boulvais Philippe, Tartese Romain, Vennemann Torsten, Jolivet Marc, Kéré Inoussa, Cathelineau Michel
    Abstract:

    International audienceMost of the hydrothermal uranium (U) deposits from the European Hercynian belt (EHB) are spatially associated with Carboniferous peraluminous Leucogranites. In the southern part of the Armorican Massif (French part of the EHB), the Guérande peraluminous Leucogranite was emplaced in an extensional deformation zone at ca. 310 Ma and is spatially associated with several U deposits and occurrences. The apical zone of the intrusion is structurally located below the Pen Ar Ran U deposit, a perigranitic vein-type deposit where mineralization occurs at the contact between black shales and Ordovician acid metavolcanics. In the Métairie-Neuve intragranitic deposit, uranium oxide-quartz veins crosscut the granite and a metasedimentary enclave.Airborne radiometric data and published trace element analyses on the Guérande Leucogranitesuggest significant uranium leaching at the apical zone of the intrusion. The primary U enrichment inthe apical zone of the granite likely occurred during both fractional crystallization and the interactionwith magmatic fluids. The low Th/U values (< 2) measured on the Guérande Leucogranite likelyfavored the crystallization of magmatic uranium oxides. The oxygen isotope compositions of theGuérande Leucogranite (δ18Owhole rock = 9.7 – 11.6 ‰ for deformed samples and δ18Owhole rock = 12.2 - 13.6 ‰ for other samples) indicate that the deformed facies of the apical zone underwent subsolidusalteration at depth with oxidizing meteoric fluids. Fluid inclusion analyses on a quartz combfrom a uranium oxide-quartz vein of the Pen Ar Ran deposit show evidence of low-salinity fluids (1-6wt.% NaCl eq.), in good agreement with the contribution of meteoric fluids. Fluid trappingtemperatures in the range of 250-350°C suggest an elevated geothermal gradient, probably relatedto regional extension and the occurrence of magmatic activity in the environment close to thedeposit at the time of its formation. U-Pb dating on uranium oxides from the Pen Ar Ran andMétairie-Neuve deposits reveal three different mineralizing events. The first event at 296.6 ± 2.6 Ma(Pen Ar Ran) is sub-synchronous with hydrothermal circulations and the emplacement of lateleucogranitic dykes in the Guérande Leucogranite. The two last mineralizing events occur at 286.6 ±1.0 Ma (Métairie-Neuve) and 274.6 ± 0.9 Ma (Pen Ar Ran), respectively. Backscattered uranium oxideimaging combined with major elements and REE geochemistry suggest similar conditions ofmineralization during the two Pen Ar Ran mineralizing events at ca. 300 Ma and ca. 275 Ma, arguingfor different hydrothermal circulation phases in the granite and deposits. Apatite fission track datingreveals that the Guérande granite was still at depth and above 120°C when these mineralizing eventsoccurred, in agreement with the results obtained on fluid inclusions at Pen Ar Ran.Based on this comprehensive data set, we propose that the Guérande Leucogranite is the mainsource for uranium in the Pen Ar Ran and Métairie-Neuve deposits. Sub-solidus alteration viasurface-derived low-salinity oxidizing fluids likely promoted uranium leaching from magmaticuranium oxides within the Leucogranite. The leached out uranium may then have been precipitated in the reducing environment represented by the surrounding black shales or graphitic quartzites. Assimilar mineralizing events occurred subsequently until ca. 275 Ma, meteoric oxidizing fluids likelypercolated during the time when the Guérande Leucogranite was still at depth. The age of the Umineralizing events in the Guérande region (300 – 275 Ma) are consistent with those obtained onother U deposits in the EHB and could suggest a similar mineralization condition, with long-termupper to middle crustal infiltration of meteoric fluids likely to have mobilized U from fertileperaluminous Leucogranites during the Late Carboniferous to Permian crustal extension events

  • Origin, evolution and exhumation of the peraluminous Leucogranites from the Armorican Hercynian belt : implication for uranium metallogenesis
    2016
    Co-Authors: Ballouard Christophe
    Abstract:

    Les granites peralumineux sont les acteurs principaux de la différentiation de la croûte continentale et représentent un enjeu sociétal important car ils sont associés à de nombreux gisements métallifères. Dans la chaîne hercynienne européenne, la majorité des gisements hydrothermaux d'uranium (filons ou épisyenites) sont associés à des Leucogranites peralumineux d'âge tardi-carbonifère. Ainsi dans le Massif armoricain, 20000 t d'uranium (U) (~20% de la production historique française), ont été extraites des gisements associés aux Leucogranites de Mortagne, Pontivy et Guérande. L'objectif de ce travail est de mieux comprendre le cycle de l'uranium dans la chaîne hercynienne armoricaine depuis la source des Leucogranites, leur évolution et leur mise en place dans la croûte supérieure jusqu'à leur lessivage par des fluides, la formation des gisements puis leur exhumation en sub-surface. Dans ce but, des données pétro-géochimiques, géochronologiques et thermochronologiques ont été obtenues sur les Leucogranites de Guérande, Pontivy et leurs gisements d'uranium associés. Les Leucogranites de Guérande et de Pontivy se sont mis en place, respectivement, à ca. 310 Ma dans une zone de déformation extensive dans le domaine interne de la chaîne et ca. 315 Ma dans le domaine externe le long du cisaillement sud armoricain (CSA), une faille décrochante d'échelle lithosphérique. Les deux Leucogranites sont issus d'un faible taux de fusion partielle de métasédiments détritiques et d'orthogneiss peralumineux, la fusion de ces derniers ayant vraisemblablement joué un rôle majeur dans la richesse en uranium des Leucogranites. La fusion de la croûte continentale dans la zone interne de la chaîne a été induite par l'extension tardi-orogénique alors que la fusion de la croûte mais aussi du manteau dans la zone externe était probablement contrôlée par une déformation décrochante diffuse. La cristallisation d'oxydes d'uranium magmatiques dans les facies les plus évolués des Leucogranites au moment de leur mise en place a été vraisemblablement rendue possible grâce à l'action combinée de la cristallisation fractionnée et d'une activité magmatique-hydrothermale diffuse. De ca. 300 Ma à 270 Ma, une activité tectonique fragile le long du CSA et des détachements a permis l'infiltration de fluides météoriques oxydants en profondeur induisant la mise en solution des oxydes d'uranium des Leucogranites. Ensuite, les fluides ont précipité leur U dans des failles ou des fentes de tension à proximité du contact avec des lithologies sédimentaires avec un caractère réducteur variable. Les Leucogranites étaient toujours en profondeur à des températures supérieures à 120°C au moment de la formation des gisements et leur exhumation en sub-surface n'est pas enregistrée avant le Trias ou le Jurassique. Ce modèle métallogénique n'est probablement pas exclusif au Massif armoricain car la période de formation des gisements d'U dans la région entre 300 et 270 Ma est la même que dans l'ensemble de la chaîne hercynienne européenne.Peraluminous Leucogranites are the principal actors for the differentiation of the continental crust and play an important economic role because they are commonly associated with significant metalliferous deposits. Most hydrothermal uranium (U) deposits (vein or episyenite types) from the European Hercynian belt are spatially associated with Carboniferous peraluminous Leucogranites and in the French Armorican Massif (western part of the European Hercynian belt) 20000 t of U (~20 % of the French production) were extracted from the deposits associated with the Mortagne, Pontivy and Guérande Leucogranites. The objective of this work is to improve our knowledge about the U cycle in the Armorican Hercynian Belt from the Leucogranites sources, their evolution and emplacement in the upper crust to U leaching, deposit formation and Leucogranites exhumation at the subsurface level. For that purpose, petro-geochemical, geochronological and thermochronological data were obtained on the Guérande and Pontivy Leucogranites as well as their spatially associated U deposits. The Guérande Leucogranite was emplaced ca. 310 Ma ago in an extensional deformation zone in the internal domain of the belt whereas the Pontivy Leucogranite was emplaced ca. 315 Ma ago in the external domain along the South Armorican Shear Zone (SASZ), a lithospheric scale wrench fault. Both Leucogranites were formed by a low degree of partial melting of detrital metasediments and peraluminous orthogneisses; the fusion of the latter probably played a major role in the generation of U rich Leucogranites. Partial melting of the crust in the internal zone of the belt was triggered by late orogenic extension whereas partial melting of the crust but also the mantle in the external zone was likely controlled by pervasive wrenching. The crystallization of magmatic uranium oxides in the most evolved leucogranitic facies was induced by fractional crystallization and probably enhanced by magmatic-hydrothermal processes. From ca. 300 to 270 Ma, a fragile tectonic activity along detachments and the SASZ, allowed for the infiltration at depth of meteoric oxidizing fluids, able to dissolve magmatic uranium oxides in the Leucogranites. These fluids have then precipitated their U in faults or tension gashes close to the contact with sediments having a variable reducing character. The Leucogranites were at depth above 120°c during the formation of U deposits and the exhumation of these intrusions did not occur before the Trias or the Jurassic. The proposed metallogenic model is likely not exclusive to the Armorican Massif as the timing of U deposits formation in the region from ca. 300 to 270 Ma is similar to the main U mineralizing event in the whole European Hercynian belt

  • Origine, évolution et exhumation des Leucogranites peralumineux de la chaîne hercynienne armoricaine : implication sur la métallogénie de l'uranium
    HAL CCSD, 2016
    Co-Authors: Ballouard Christophe
    Abstract:

    Peraluminous Leucogranites are the principal actors for the differentiation of the continental crust and play an important economic role because they are commonly associated with significant metalliferous deposits. Most hydrothermal uranium (U) deposits (vein or episyenite types) from the European Hercynian belt are spatially associated with Carboniferous peraluminous Leucogranites and in the French Armorican Massif (western part of the European Hercynian belt) 20000 t of U (~20 % of the French production) were extracted from the deposits associated with the Mortagne, Pontivy and Guérande Leucogranites. The objective of this work is to improve our knowledge about the U cycle in the Armorican Hercynian Belt from the Leucogranites sources, their evolution and emplacement in the upper crust to U leaching, deposit formation and Leucogranites exhumation at the subsurface level. For that purpose, petro-geochemical, geochronological and thermochronological data were obtained on the Guérande and Pontivy Leucogranites as well as their spatially associated U deposits. The Guérande Leucogranite was emplaced ca. 310 Ma ago in an extensional deformation zone in the internal domain of the belt whereas the Pontivy Leucogranite was emplaced ca. 315 Ma ago in the external domain along the South Armorican Shear Zone (SASZ), a lithospheric scale wrench fault. Both Leucogranites were formed by a low degree of partial melting of detrital metasediments and peraluminous orthogneisses; the fusion of the latter probably played a major role in the generation of U rich Leucogranites. Partial melting of the crust in the internal zone of the belt was triggered by late orogenic extension whereas partial melting of the crust but also the mantle in the external zone was likely controlled by pervasive wrenching. The crystallization of magmatic uranium oxides in the most evolved leucogranitic facies was induced by fractional crystallization and probably enhanced by magmatic-hydrothermal processes. From ca. 300 to 270 Ma, a fragile tectonic activity along detachments and the SASZ, allowed for the infiltration at depth of meteoric oxidizing fluids, able to dissolve magmatic uranium oxides in the Leucogranites. These fluids have then precipitated their U in faults or tension gashes close to the contact with sediments having a variable reducing character. The Leucogranites were at depth above 120°c during the formation of U deposits and the exhumation of these intrusions did not occur before the Trias or the Jurassic. The proposed metallogenic model is likely not exclusive to the Armorican Massif as the timing of U deposits formation in the region from ca. 300 to 270 Ma is similar to the main U mineralizing event in the whole European Hercynian belt.Les granites peralumineux sont les acteurs principaux de la différentiation de la croûte continentale et représentent un enjeu sociétal important car ils sont associés à de nombreux gisements métallifères. Dans la chaîne hercynienne européenne, la majorité des gisements hydrothermaux d'uranium (filons ou épisyenites) sont associés à des Leucogranites peralumineux d'âge tardi-carbonifère. Ainsi dans le Massif armoricain, 20000 t d'uranium (U) (~20% de la production historique française), ont été extraites des gisements associés aux Leucogranites de Mortagne, Pontivy et Guérande. L'objectif de ce travail est de mieux comprendre le cycle de l'uranium dans la chaîne hercynienne armoricaine depuis la source des Leucogranites, leur évolution et leur mise en place dans la croûte supérieure jusqu'à leur lessivage par des fluides, la formation des gisements puis leur exhumation en sub-surface. Dans ce but, des données pétro-géochimiques, géochronologiques et thermochronologiques ont été obtenues sur les Leucogranites de Guérande, Pontivy et leurs gisements d'uranium associés. Les Leucogranites de Guérande et de Pontivy se sont mis en place, respectivement, à ca. 310 Ma dans une zone de déformation extensive dans le domaine interne de la chaîne et ca. 315 Ma dans le domaine externe le long du cisaillement sud armoricain (CSA), une faille décrochante d'échelle lithosphérique. Les deux Leucogranites sont issus d'un faible taux de fusion partielle de métasédiments détritiques et d'orthogneiss peralumineux, la fusion de ces derniers ayant vraisemblablement joué un rôle majeur dans la richesse en uranium des Leucogranites. La fusion de la croûte continentale dans la zone interne de la chaîne a été induite par l'extension tardi-orogénique alors que la fusion de la croûte mais aussi du manteau dans la zone externe était probablement contrôlée par une déformation décrochante diffuse. La cristallisation d'oxydes d'uranium magmatiques dans les facies les plus évolués des Leucogranites au moment de leur mise en place a été vraisemblablement rendue possible grâce à l'action combinée de la cristallisation fractionnée et d'une activité magmatique-hydrothermale diffuse. De ca. 300 Ma à 270 Ma, une activité tectonique fragile le long du CSA et des détachements a permis l'infiltration de fluides météoriques oxydants en profondeur induisant la mise en solution des oxydes d'uranium des Leucogranites. Ensuite, les fluides ont précipité leur U dans des failles ou des fentes de tension à proximité du contact avec des lithologies sédimentaires avec un caractère réducteur variable. Les Leucogranites étaient toujours en profondeur à des températures supérieures à 120°C au moment de la formation des gisements et leur exhumation en sub-surface n'est pas enregistrée avant le Trias ou le Jurassique. Ce modèle métallogénique n'est probablement pas exclusif au Massif armoricain car la période de formation des gisements d'U dans la région entre 300 et 270 Ma est la même que dans l'ensemble de la chaîne hercynienne européenne

L. J. Wang - One of the best experts on this subject based on the ideXlab platform.

  • petrogenesis of ca 1 95 ga meta Leucogranites from the jining complex in the khondalite belt north china craton water fluxed melting of metasedimentary rocks
    Precambrian Research, 2017
    Co-Authors: L. J. Wang, Peng Peng
    Abstract:

    Abstract Anatexis occurs in response to changing P-T -fluid conditions during orogenesis, is thus crucial for understanding the tectonic evolution of orogenic belts. Here we report an integrated petrological, geochemical, whole-rock oxygen isotopic and zircon U-Pb-Hf-O isotopic study of meta-Leucogranites from the Jining Complex, North China Craton, to unravel its petrogenesis and constrain the tectonic evolution of the Paleoproterozoic Khondalite Belt. The meta-Leucogranites are weakly peraluminous with homogeneous minimum-melt compositions, and are characterized by low Rb contents and Rb/Sr ratios, high Ba and Sr contents and Sr/Y ratios, and moderately fractionated REE patterns without obvious Eu anomalies. These trace-element characteristics differ from that of Leucogranites derived from fluid-absent melting. Considering their low whole-rock Zr saturation temperatures (746–780 °C) and comparable compositions with experimentally generated melts produced by water-fluxed melting, we infer that the meta-Leucogranites are low-temperature granites, and were generated by water-fluxed melting of metasedimentary rocks at 6–8 kbar. SIMS zircon U-Pb analyses suggest that the meta-Leucogranites were emplaced at ∼1.95 Ga or slightly earlier, and were subsequently metamorphosed and cut by a pegmatite dike at ∼1.92 Ga. The abundant inherited zircons yield nearly concordant 207 Pb/ 206 Pb ages ranging from 2.24 to 1.98 Ga, which are consistent with the detrital zircon age spectra of the metasedimentary rocks from the Khondalite Belt. The Hf isotopic compositions of inherited and magmatic zircons (e Hf(t)  = +0.2 to +7.7) closely overlap with the time evolved e Hf range of detrital zircons from metasediments in the Khondalite Belt. Different types of zircons (inherited, magmatic and metamorphic zircons) have similar δ 18 O values of 7.2–9.2‰, which is lower than that of metamorphic zircons (10.0–13.6‰) from surrounding metasediments. Thus, we propose that the meta-Leucogranites were formed by water-fluxed melting of metasedimentary rocks from the Khondalite belt at ∼1.95 Ga during the prograde upper amphibolite-facies metamorphism.

Freire, David M. - One of the best experts on this subject based on the ideXlab platform.

  • Comparación de propiedades petrográficas y petrofísicas de tres fragmentos escultóricos para determinar su pertenencia al sepulcro de Nuno Freire de Andrade II (siglo XIV)
    2018
    Co-Authors: Freire, David M.
    Abstract:

    [Resumen] Tres fragmentos de piedra que representan una cabeza, un cuerpo y una tapa de sarcófago atribuidos a la lauda de Nuno Freire de Andrade II, Gran Maestre de la Orden de Cristo del siglo XIV, fueron analizados para determinar sus propiedades petrográficas, velocidad de propagación de ondas de ultrasonido y color. Los resultados indican que las piedras corresponden a tres tipos litológicos con distinta petrografía, velocidad de propagación de ondas P, color y variación de color entre secas y húmedas. El fragmento de la cabeza corresponde a un ortogneis, el del cuerpo a una granodiorita y el de la tapa de sarcófago a un leucogranito. A partir de estos datos se infiere que ninguno de los tres fragmentos ha formado parte de la misma lauda, ya que las tapas de sarcófagos medievales gallegas talladas en granito estaban hechas en un único bloque de piedra[Abstract] Three fragments of stone representing a head, a body and a sarcophagus lid, and attributed to the grave of Nuno Freire de Andrade II, Grand Master of the Order of Christ (fourteenth century), were analysed to determine their petrographic, ultrasonic P-wave pulse velocity, and colour properties. The results indicate that the stones correspond to three lithological types (ortogneis, granodiorite and Leucogranite) with different ultrasonic P-wave pulse velocity, colour and colour variation between dry and wet. From these data it is inferred that none of the three fragments have been part of the same sarcophagus lid, since the granite lids of medieval Galician sarcophagus were carved in a single block of stone