Éditeur(s) : HAL CCSD Wiley-Blackwell
Résumé : International audience
Although lawsonite-bearing rocks are rare in exhumed high-pressure (HP) terranes, they are considered to exert a primary role in subduction dynamics. Recent observations in natural settings have shown that fluid–rock interaction at HP conditions, including metasomatism, may lead to unusually high lawsonite amounts even in rocks that originally contained little or no lawsonite. This process may therefore bear important implications for element recycling in subduction zones. A detailed characterization of the geochemical fingerprints associated with lawsonite metasomatism is presented in this contribution. The studied rocks belong to the HP terranes of Alpine Corsica (France), which is the largest documented exposure for lawsonite metasomatism. Metasomatic lawsonite displays complex compositional zoning, including high trace element, Cr and Ti content. The trace element content is much higher compared with the average of non-metasomatic lawsonite, and is in line with the re-incorporation of large amounts of trace elements (e.g. REE, Sr, Pb, Th) in the rock during metasomatism, as shown by mass transfer calculations. Our data suggest that serpentinites represented the main fluid source for the metasomatism, with concurrent contribution of other, possibly Ca-rich lithologies, such as mafic or meta-sedimentary rocks. We propose that the breakdown of metasomatic lawsonite may contribute to the genesis of magmas and their characteristic geochemical signatures.
ISSN: 0263-4929

hal-01107395
https://hal.archives-ouvertes.fr/hal-01107395
DOI : 10.1111/jmg.12074

Éditeur(s) : HAL CCSD Elsevier
Résumé : Kyanite and zoisite eclogites from the Limousin area in the Variscan French Massif Central (FMC) have been formed by deep subduction to 100 km depth at peak pressure-temperature conditions around 660 degrees C and 2.9 GPa. They belong to a thin tectonic unit comprising ophiolitic bodies devoid of HP-UHP markers and forming a transported suture zone now separating the two main lithotectonic units of the FMC with contrasted tectono-metamorphic features (namely the lower and upper allochthons). Composition of kyanite-eclogites is similar to supra-subduction-zone basalts found in back-arc basins or island arcs (LREE depleted, negative Nb-Ta anomalies, epsilon Nd: +6 to + 9). Zoisite eclogites have the major-element fingerprint of ultrabasic iron-rich plagioclase cumulates common in continental intrusions or forming Fe-Ti oceanic gabbros. The distribution of immobile trace-elements is however more comparable to alkaline Si-undersaturated rocks, but the Sr-Nd isotopic composition (Sr-87/Sr-86: 0.706; epsilon Nd: -3 to -5) suggests the intervention of an old radiogenic crustal component during their petrogenesis. The chemical composition of hydrous zoisite eclogites does not correspond closely to a common magmatic precursor. It is thus suggested that, by analogy with well exposed worldwide HP-UHP terranes, at least a part of the trace-element signature and the isotopic compositions are controlled by channelled fluid-rock interaction at UHP conditions, leading to localised segregation of zoisite that hosts nearly all Sr and LREE budgets of the bulk eclogite. Sr-Nd isotopic composition of these samples is thus controlled by zoisite and probably represents the isotopic composition of the fluid phase present during UHP recrystallisation. Zircons extracted from a zoisite eclogite were dated in-situ by LA-ICP-MS. Crystallisation of the magmatic precursor is bracketed between 489 and 475 Ma and the UHP event is dated at 412 +/- 10 Ma, in agreement with published ages for the Eo-Variscan HP stage in the FMC. A late resetting of the U-Pb isotopic system at 382 +/- 7 Ma is related to an anatectic high pressure-medium pressure event, well known in the Limousin area. The oceanic eclogites dated in this study are tentatively linked to the Galicia-South Brittany Lower Paleozoic Ocean; they show evidence of the existence of an intra-oceanic subduction zone at the Lower Ordovician and of partial closure of this oceanic domain by ocean-continent subduction during Early Devonian.
ISSN: 0024-4937

hal-00512835
https://hal.archives-ouvertes.fr/hal-00512835
DOI : 10.1016/j.lithos.2010.05.013

Éditeur(s) : HAL CCSD Société géologique de France
Résumé : International audience
– The 300 km long allochthonous sheet of oceanic mantle forming the New Caledonia ophiolite displays three specific characters: 1) the ophiolite pile lacks concordant sheeted dykes and pillow basalt layers; 2) the ophiolite, refered to as the Peridotite nappe, is thrusted over the basaltic formations of the Poya terrane which are classicaly thought to originate from a different oceanic environment; 3) The basal contact of the ultramafic sheet is remarkably flat all along New-Caledonia and the Peridotite nappe has not been thickened during obduction, rather it experienced significant extension. This suggests that the peridotites have not been emplaced by a tectonic force applied to the rear. New petrological and geochemical results obtained from mantle rocks finally show that the Poya terrane may originate from the same oceanic basin as the peridotites. In this article, we consider such possible cogenetic links and we propose a simple model for the obduction of the New Caledonia ophiolite in which the Poya basalts represent the original cover of the Peridotite nappe. We infer that continuous uplift of the subducted units buried beneath the oceanic lithosphere in the northern part of New Caledonia drove passive uplift of the ophiolite and led to erosion and to initiation of sliding of the basaltic layer. During the Priabonian (latest Eocene), products of the erosion of the basaltic layer were deposited together with sediments derived from the Norfolk passive oceanic margin. These sediments are involved as tectonic slices into an accretionary wedge formed in response to plate convergence. The volcaniclastic sedimentation ends up with the emplacement of large slided blocks of basalts and rafted mafic units that progressively filled up the basin. Obduction process ended with the gravity sliding of the oceanic mantle sheet, previously scalped from its mafic cover. This process is contemporaneous with the exhumation of the HP-LT units of Pouebo and Diahot. Gravity sliding was facilitated by the occurrence of a continuous serpentine sole resulting from metasomatic hydratation of mantle rocks, which developed during the uplift of the Norfolk basement and overlying Diahot and Pouébo units. Progressive emersion of the obducted lithosphere allowed subsequent weathering under subaerial, tropical conditions. Obduction passive et mise en place gravitaire des grandes nappes ophiolitiques : les ophiolites de Nouvelle-Calédonie (SW Pacifique) sont-elles un cas d'école ? Résumé. – La nappe de manteau de Nouvelle-Calédonie, s'étendant sur 300 km de long montre trois caractéristiques im-portantes: 1) La succession ophiolitique ne contient ni dykes, ni basaltes en coussin, 2) l'ophiolite est charriée sur des formations basaltiques (nappe de Poya) que l'on considère classiquement comme originaires d'un bassin océanique dif-férent de celui d'où est issu le manteau, et 3) le contact basal de la nappe ultrabasique est remarquablement plat tout le long de l'île et celle-ci n'a pas été épaissie durant l'obduction. Cela suggère que les péridotites n'ont pas été mises en place en réponse à une poussée tectonique arrière. De nouveaux résultats pétrologiques et géochimiques obtenus sur l'ophiolite montrent en réalité que les basaltes de Poya peuvent être extraits de la fusion du manteau de l'ophiolite. Nous proposons donc un nouveau modèle pour l'obduction, dans lequel les basaltes de Poya forment la couver-ture initiale primitive du manteau de la nappe des péridotites. La surrection progressive des unités profondes enfouies lors de la subduction de la lithosphère du bassin sud-Loyauté à l'Eocène, a permis le soulèvement passif de l'ophiolite et l'érosion puis le glissement des unités basaltiques. Durant le Priabonien, l'accumulation des produits de l'érosion de la couche des basaltes et de la marge passive de Norfolk ont formé les séquences de flysch comprenant localement des débris basaltiques grossiers. Ces sédiments font partie d'un prisme d'accrétion tectonique construit en réponse à la convergence au front de la subduction. La sédimentation détritique se termine par la mise en place de grandes unités ba-saltiques glissées qui comblent le bassin. Le processus de l'obduction se termine par le glissement gravitaire de l'écaille de manteau océanique sur les basaltes. Cet événement est contemporain de l'exhumation des unités HP-BT de Pouébo et du Diahot. Le glissement gravitaire a été facilité par la présence sous l'ophiolite, d'une semelle continue de serpentines résultant de l'interaction du manteau avec des fluides métasomatiques remontant durant l'exhumation des unités en-fouies pendant la subduction continentale. L'émersion progressive de la lithosphère obduite a placé le manteau dans des conditions d'altération supergène tropicale.
EISSN: 1777-5817

hal-01257702
https://hal.archives-ouvertes.fr/hal-01257702
https://hal.archives-ouvertes.fr/hal-01257702/document
https://hal.archives-ouvertes.fr/hal-01257702/file/Lagabrielleetal-BSGF-2012-revised-light.pdf
DOI : 10.2113/gssgfbull.184.6.545