Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
Mantle-derived xenoliths hosted by melilitite lavas from In Teria (Ahaggar, SE Algeria) include garnet and spinel peridotites, pyroxenite and phlogopite megacrysts. The spinel and garnet peridotites record an early deformation event, which formed porphyroclastic microstructures and olivine crystal preferred orientations, followed by static infiltration of hydrous alkaline melts. This metasomatic stage (stage 1) is characterized by the crystallization of phlogopite in the garnet and spinel peridotites, amphibole in the spinel peridotites and clinopyroxene in the garnet peridotite, which record chemical equilibration with an alkaline silicate melt. These early events were largely overprinted by carbonatitic metasomatism (stage 2), which is observed only in the spinel peridotites. Spinel peridotite major and trace element compositions, as well as the compositions of newly formed minerals, are characteristic of interaction with carbonate melt, associated with strong enrichment in incompatible trace elements in clinopyroxene. This second stage was followed by crystallization of pyroxenites (stage 3) in vein conduits, probably segregated from alkaline melts. We propose a scenario in which the different metasomatic imprints record successive stages of interaction between lithospheric mantle and sublithospheric melts throughout the Cenozoic. In Sr–Nd isotope space, the host melilitites and several xenoliths are clustered and plot close to the HIMU mantle end-member. However, some peridotite xenoliths are shifted towards more radiogenic 87Sr/86Sr values. In 207Pb/204Pb–206Pb/204Pb and 208Pb/204Pb–06Pb/204Pb space the In Teria samples define a relatively large domain characterized by high 206Pb/204Pb and 208Pb/204Pb, consistent with a contribution of an HIMU component, considered to represent a sublithospheric signature. The highest 87Sr/86Sr values are comparable with those ascribed to the EM1 mantle end-member, representing the signature of the lower continental lithosphere, and are probably inherited from the pre-metasomatic lithospheric mantle beneath In Teria. Numerical modelling of porous percolation of melt of sublithospheric origin through an EM1-like lithospheric mantle protolith reproduces the In Teria peridotite compositions, using moderately sub-chondritic Sr/Nd values for the peridotite (e.g. In Teria garnet peridotite) and moderately super-chondritic Sr/Nd values in the melt (approximately ocean island basalt values). A few spinel peridotites require a component characterized by a 143Nd/144Nd signature higher than both the EM1 end-member and the local Ahaggar basalts; the 208Pb/204Pb compositions of several samples point to a component with a depleted mid-ocean ridge basalt (MORB) mantle (DMM) signature. Thus the lithospheric mantle beneath In Teria probably did not have a uniform EM1 signature before the onset of metasomatism; it included a DMM peridotite component as well as some peridotites with elevated 143Nd/144Nd values recording long-term LREE depletion.
ISSN: 0022-3530

hal-01356781
https://hal.archives-ouvertes.fr/hal-01356781
DOI : 10.1093/petrology/egw009

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 : National audience
Late Triassic submarine alkali basalts and hawaiites were collected from two superimposed tectonic slices belonging to the Kara Dere – Sayrun unit of the Middle Antalya nappes, southwestern Turkey. New determinations on conodont faunas allow to date this sequence to the Lower Carnian (Julian). The volcanic rocks show rather homogeneous compositions, with high TiO2 and relatively low MgO and Ni contents which suggest olivine fractionation. Their primitive mantle-normalised multi-elements plots show Nb and Ta enrichments relative to La, Pb negative anomalies and heavy rare earth element and Y depletions typical of intraplate ocean island basalts. These characteristics are consistent with the major and trace element compositions of their primary clinopyroxene phenocrysts, which do not show any feature ascribable to crustal contamination. The studied lavas display a restricted range of {varepsilon}Nd (+4.6 to +5.2) which falls within the range of ocean island basalts. Their initial (143Nd/144Nd)i ratios are too low to be explained by a simple mixing line between depleted MORB mantle (DMM) and HIMU components. Their Pb and Nd isotopic compositions plot along a mixing line between HIMU component and an enriched mantle, the composition of which could be the result of the addition of about 5 to 8% of an EM2 component (recycled marine sediments) to DMM. The lack of evidence for any continental crustal component in their genesis could be consistent with their emplacement in an intra-oceanic setting.
EISSN: 1777-5817

insu-00338829
https://hal-insu.archives-ouvertes.fr/insu-00338829
DOI : 10.2113/gssgfbull.179.4.397

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Almost all Red Sea deeps contain metal-rich sediments covered by brine pools. It is generally agreed that these metal-rich deposits precipitated from overlying metal-rich brines that originated from migrating hydrothermal fluids. No brine pool has ever been reported in Thetis Deep, inciting us to evaluate if such a brine layer ever occurred in the deep during the past. In order to address that questioning, a study combining mineralogical, geochemical (major-, minor-, rare-earth elements) and isotopic (Sr. Nd. Pb) approaches was completed on cored sediments and extracted interstitial waters from inside and outside the deep.;The sediments have an overall hydrothermal origin, as shown by the REE concentrations and patterns, metal contents, and Pb-Nd isotopic data, all pointing to a mantle signature. The intensity of the hydrothermal activity varied with time in the deep; the most intense episode resulting in an almost pure Fe-oxi-hydroxide layer. Varied chemical arguments, especially the Zr and REE data of the sediments, favor the fact that the whole sedimentation in Thetis Deep occurred in the absence of a stable, salt-rich and mineralized brine pool, and that no brine layer ever existed. This conclusion is supported by the constant Sr isotope composition of the sediment and its interstitial waters that are almost identical to that of the Red Sea seawater. The study also suggests that hydrothermal activity monitored fluid supplies that interacted differently with seawater in the different Red Sea deeps, resulting in an overall formation of metal-rich sediments, but along varied local conditions. (C) 2009 Elsevier B.V. All rights reserved.
ISSN: 0375-6742

hal-00475646
https://hal.archives-ouvertes.fr/hal-00475646
DOI : 10.1016/j.gexplo.2009.10.001

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
This study presents new geochemical data on 26 mafic lavas from the Middle Atlas and Central Morocco volcanic provinces, including Miocene nephelinites and Pliocene-Quaternary (3.9-0.6Ma) nephelinites, basanites, alkali and subalkaline basalts. Most of them represent near-primary magmas, although some alkali basalts were derived from the minor fractionation of olivine and diopside phenocrysts. These evolved samples and the subalkaline basalt display higher 207Pb/204Pb and Zr/Nb ratios and lower εNd consistent with their contamination by lower crustal granulites during an open fractionation process. The progressive enrichment in incompatible elements observed from alkali basalts to nephelinites suggests their derivation from decreasing partial melting degrees of an enriched mantle source located at the garnet-spinel transition zone. The strong negative spikes observed for K in multielement patterns indicate that this source contained a residual pargasitic amphibole. We propose that partial melting occurred at around 2GPa, i.e. near the lithosphere-asthenosphere boundary beneath the Middle Atlas (60-80km).The trace element and isotopic Sr-Nd-Pb-Hf signature of the uncontaminated lavas displays a geochemical flavour intermediate between those of high μ (HIMU), "C", and enriched mantle components. It is very similar to that of abundant metasomatic amphibole- and clinopyroxene-rich lithospheric peridotites and pyroxenites carried by Middle Atlas lavas, which likely represent an analog of the source of these lavas. It is therefore not necessary to postulate the contribution of a "fresh" asthenospheric mantle to their genesis. We propose that they resulted from the partial melting of the base of a veined lithospheric mantle metasomatised during the late Cretaceous by alkaline melts from the Central Atlantic plume, the ancestor of the Canary plume. Melting was probably triggered by the flux of a hot mantle within a regional SW-NE sub-lithospheric channel, in response to either the Alboran slab retreat or edge-driven convection around the West African craton.
ISSN: 0024-4937

insu-01056523
https://hal-insu.archives-ouvertes.fr/insu-01056523
DOI : 10.1016/j.lithos.2014.07.009

Éditeur(s) : Université de Bretagne Occidentale
Résumé : ln Java island, a number of quaternary volcanoes are Iocated in a back-arc position relative to the Sunda arc volcanie axis. These volcanie centers show an important variability in the petrologic composition of their products (potassic calc-alkali, shoshonitie, alkali potassie and alkali u/trapotassie leucitic series).
L'île de Java comporte, à l'arrière de l'axe volcanique de l'arc de la Sonde, un grand nombre de volcans quaternaires dont les laves présentent une grande diversité pétrologique (séries calco-alcalines potassiques, shoshonitiques, alcalines potassiques, alcalines ultrapotassiques à leucite). Ces édifices arrière-arc, mis en place à la faveur de fractures majeures obliques par rapport à l'axe de l'arc, se situent à des distances très variables de la fosse (profondeurs du plan de Benioff comprises entre 200 et 650 km). Le but de ce travail est la caractérisation géochimique (éléments en trace et isotopes Sr-Nd-Pb) des laves arrière-arc de cette région, leur comparaison avec celles des magmas de l'axe de l'arc, et la discussion de l'origine des variations observées.

Droits : UBO
http://archimer.ifremer.fr/doc/00034/14540/11815.pdf
http://archimer.ifremer.fr/doc/00034/14540/

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The Maghrebides, Betics and some parts of the Calabrian, NE Sicilian and Tuscan allochtons constitute dismembered fragments of the Alpine belt that resulted from the Cenozoic collision between Africa and Eurasia and the opening of the Western Mediterranean basin. Mineral and whole-rock geochemical analyses have been performed on three distinct outcrops of mantle material from the Edough Massif of NE Algeria, namely the Bou Maiza (BM) gabbros, the La Voile Noire (LVN) amphibolites and the Sidi Mohamed (SM) peridotites. In all samples, Sr isotopes are largely affected by seawater alteration (87Sr/86Sract. > 0.70384 and up to 0.70888) and cannot be used to evaluate the nature of the source reservoirs. SM peridotites display variable depleted mantle Nd isotopic signatures (εNdact. from + 7.0 to + 12.2) and geochemical features suggesting no significant chemical depletion as a result of partial melting and melt extraction (Mg# < 90; slightly LREE-depleted patterns with La/YbN = 0.33-0.39). These rocks are interpreted as parts of the subcontinental lithospheric mantle incorporated into the crustal units of the Edough Massif during the early stages of opening of the Algerian basin. BM gabbros and LVN amphibolites show geochemical signatures indicating derivation from a common Depleted Mantle reservoir (εNd > + 7.9) and are likely cogenetic, but without filiation with the SM peridotites. Pb isotopes indicate a contribution of sediments in the source reservoir, which is attributed to contamination solely by hydrous fluids released from a sedimentary component. This observation, together with a LILE-enrichment, suggests a back-arc basin environment. These results indicate that BM and LVN units were obducted onto the North African margin and subsequently fragmented, probably during doming and exhumation of the lower continental crust of the Edough massif. Doming resulted in opposite movements of the overlying oceanic units, southward for the BM units and northward for LVN amphibolites.
ISSN: 0040-1951

hal-01122777
https://hal.archives-ouvertes.fr/hal-01122777
DOI : 10.1016/j.tecto.2014.03.031

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
In pre-drift reconstructions, the central and southern parts of the Borborema Province, northeastern Brazil, belong to a large Brasiliano-Pan-African orogenic realm situated to the north of the São Francisco-Congo Craton. In order to better understand the timing and geodynamic setting under which this orogenic system developed, a structural, geochemical and geochronological study was conducted across the east Pernambuco shear zone (EPSZ) system, which separates the Pernambuco-Alagoas Domain (PEAL) from the Central Domain. A sample of the Pinhões orthogneiss (GE-1), in the Central Domain, one sample of a syenitic orthogneiss (CA-34) wrapped by the EPSZ, and one sample of orthogneiss named Altinho (CA-40), in the northern portion of the PEAL, were dated by LA-ICP-MS. The Pinhões orthogneiss yielded an age of 869 ± 9 Ma, interpreted as the emplacement age of the protolith during a late Tonian magmatic episode. Samples CA-40 and CA-34 yielded 206Pb/238U weighted mean ages of 652 ± 6 Ma and 636 ± 3 Ma, respectively, which are interpreted as dating emplacement and crystallization of the magmatic protoliths. However, it is also possible that these rocks were formed during the same magmatic event in view of the identical ages of 646 ± 13 Ma and 646 ± 11 Ma, respectively, given by the less precise upper intercept of the discordia lines. The metaluminous and magnesian nature of the Altinho orthogneiss is akin to the calc-alkalic suite. However, some samples plot in the intraplate field in tectonic discrimination diagrams and the Nd TDM model age of 1.36 Ga is unlike that of juvenile magmas in convergent settings. The Altinho orthogneiss is quite similar in terms of trace elements geochemistry to the syenitic orthogneiss, which has a clearer intraplate affinity, and the dated samples have identical initial Sr isotope ratios (0.7047). Therefore, emplacement in an extensional setting is preferred over a convergent one. Two samples of paragneisses (SB-1 and BB-9) from the PEAL were also dated. The ages of the youngest zircon grains in sample BB-9 (655–642 Ma) overlap the crystallization age of the Altinho orthogneiss, implying that sedimentation is younger than or, at best, synchronous with magmatism. The age of low Th/U grains in samples CA-34 (615 ± 8 Ma) and SB-1 (587 ± 12 Ma) are related to a subsequent metamorphic overprint, which is loosely constrained between 580 and 620 Ma. These observations, combined with evidence provided by previous studies, suggest that the change from an extensional to a contracional setting occurred at ca. 640–630 Ma. In contrast with most collisional orogens, where a long period of oceanic subduction precedes collision, the inferred tectonic evolution suggests that the Brasiliano Orogeny resulted from inversion of continental and/or proto oceanic rifts.
ISSN: 0895-9811

hal-01174177
https://hal.archives-ouvertes.fr/hal-01174177
DOI : 10.1016/j.jsames.2014.09.007

Éditeur(s) : Oxford Univ Press
Résumé : In subduction zones, melting and dehydration of the subducted slab introduce material into the mantle wedge and modify its chemical and isotopic composition. As a consequence, island arc lavas differ significantly from mid-ocean ridge basalts and ocean island basalts. In some arcs, the composition of lavas is strongly influenced by the sedimentary material introduced with the slab; in others, magma composition is mainly affected by aqueous fluids released by the slab. The Lesser Antilles arc is known for its extreme continental-crust-like signature but for some Lesser Antilles lavas subducted sediments are barely involved and enrichment in fluid-mobile elements (Ba, U, Sr, Pb, etc.) is the dominant feature. Here we evaluate whether La/Sm is a quantitative proxy of sediment involvement in volcanic arcs, and we relate dehydration and melting processes to the temperature and pressure conditions of the slab. We use Martinique as a case study because in this island both dehydration and sediment melting fingerprints coexist. We measured major and trace elements for about 130 age-constrained samples, carefully chosen to cover all volcanic phases of Martinique (25 Ma to present). Using these results we demonstrate that: (1) weathering does not modify the La/Sm ratio; (2) fractional crystallization of amphibole and/or garnet does not increase La/Sm by more than 20%; (3) rare earth element transfer from wall-rock to magma during fractionation is not significant; (4) melting of the mantle source increases La/Sm by only about 20%. As a consequence, we show that the proportion of slab sediment incorporated in the mantle wedge controls the La/Sm ratio of the source. The observed correlations between La/Sm and Nd and Hf isotopic compositions indicate that the effect of sediment addition is the overwhelming factor: La/Sm is a good proxy for slab sediment proportion in Martinique. We observe a geographical gradient between slab dehydration and sediment melting on the island. Whereas lavas located on the western side of the island display a clear sedimentary input in their source, lavas located on the eastern side of the island, closer to the trench, are clearly influenced by dehydration of the subducted slab. In addition, the aqueous fluids clearly come from the subducted basalt and they did not interact with the overlying sediments. The influence of sediment added to the source of the magmas increases from the eastern part to the western part of the island. We relate this geographical change to the pressure and temperature conditions at the slab surface. Sediments probably cross their solidus under Martinique and hydrous melting is triggered. Finally, we show that under all volcanic arcs where the signature of sediments overwhelms the signature of fluids, the slab surface reaches P-T conditions that allow the subducted sediments to melt. Inversely, under most volcanic arcs where the signal of aqueous fluids dominates over sediment melts, the subducted slab is not hot enough for the sedimentary pile to melt.
Journal Of Petrology (0022-3530) (Oxford Univ Press), 2012-12 , Vol. 53 , N. 12 , P. 2441-2464

Droits : info:eu-repo/semantics/openAccess
http://archimer.ifremer.fr/doc/00113/22392/20234.pdf
DOI:10.1093/petrology/egs055
http://archimer.ifremer.fr/doc/00113/22392/

Éditeur(s) : HAL CCSD Wiley-Blackwell
Résumé : International audience
The Early Palaeozoic Orogen of SE China consists of three litho-tectonic elements, from top to bottom: a sedimentary Upper Unit, a metamorphic Lower Unit and a gneissic basement. The boundaries between these units are flat lying, south directed, ductile decollements. The lower one is coeval with an amphibolite facies metamorphism (M1). The belt is reworked by migmatite-granite domes, high-temperature metamorphism (M2) and granitic plutons related to post-orogenic crustal melting. We date here the syn-M1 ductile shearing at 453 +/- 7 Ma by U-Th/Pb method on monazite. Previous ages and our new
ISSN: 0954-4879

hal-00420901
https://hal.archives-ouvertes.fr/hal-00420901
https://hal.archives-ouvertes.fr/hal-00420901/document
https://hal.archives-ouvertes.fr/hal-00420901/file/Faureetal-TerraNova-2009.pdf
DOI : 10.1111/j.1365-3121.2009.00888.x

Éditeur(s) : HAL CCSD
Résumé : International audience
We present a chronological (K-Ar), petrologic and geochemical study (major and trace elements, Sr-Nd isotopes) of Middle Miocene (ca. 16-14 Ma) calc-alkaline rocks (basalts to andesites) extruded in the present-day back-arc region of Central Patagonia (Zeballos Volcanic Sequence (ZVS), 47 S). This magmatism started shortly after mafic plutonism ceased in the arc region (ca. 16 Ma, 200 km west), and ended ca. 2 My before the onset of voluminous slab tear-related back-arc alkaline basaltic magmatism (ca. 12 to Pliocene). The studied calc-alkaline rocks have a typical subduction-related signature (high LILE/HFSE ratios, depletion in Nb, Ta and Ti; Ba/La >20; Ta/Hf <1.5; (Sr-87/Sr-86)(o)=0.70366-0.70402, epsilon(Nd)=+0.1-+3.8). Major and trace elements contents are consistent with their evolution by closed system fractional crystallization of a presumed parental liquid similar in composition to the most basic rock of the suite. Moreover, a strong subducted sediment imprint is recognized (increasing Th/HFSE and decreasing Ce/Pb during differentiation). However, these rocks show striking similarities with volcanic complexes emplaced above areas where a gently dipping slab occurs (high K contents; similar LREE/HREE, Nb/Zr, Ba/Nb; Th/Hf; Th/Ta, Ta/Hf<0.3), particularly the present-day Andean flat-slab region and the Neuquen Basin during the Late Miocene. A comprehensive tectono-magmatic model is here presented to explain the generation and extrusion of these calc-alkaline magmas during the Middle Miocene. The development of a transient low-angle subduction and the resulting eastward migration of the volcanic front are then proposed. Mixing between stored remnants of calc-alkaline magmatism and later primitive alkaline melts is envisioned as the most likely process accounting for the transitional signature (i.e., intermediate between calc-alkaline and alkaline, La/Nb >I; TiO2 <2 wt%) of some basalts extruded synchronously with genuine alkaline lavas in the Neogene Patagonian Plateau Lavas province.
Volcanismo calcoalcalino durante el Mioceno Medio en Patogonia Central (47°S): petrogenesis e implicaciones en la dinamica de placas. En este trabajo se presenta un estudio cronologico (K-Ar), petrologico y geoquimico (elementos mayores y trazas, isotopos de Sr-Nd) de rocas calcoalcalinas (basaltos a andesitas) del Mioceno Medio (16-14 Ma) que ocurren en una posicion de trasarco en Patagonia Central (47°S; Secuencia Volcanica Zeballos, SVZ). Este magmatismo comenzo poco despees de que el plutonismo mafico cesara en la region bajo el arco (ca. 16 Ma, 200 km al oeste), y termino ca. 2 My antes del comienzo de un voluminoso evento magmatico basaltico alcalino desarrollado en el trasarco, relacionado con ventanas astenosfericas, que continuo hasta el Pleistoceno. Las rocas calcoalcalinas estudiadas tienen una quimica tipicamente relacionada con procesos de subduccion (altas razones LILE/HFSE, empobrecimientos en Nb, Ta y Ti; Ba/La>20; Ta/Hf l,5; (87Sr/86Sr)0=0,70366-0,70402, End~O,1±3,8); y sus contenidos de elementos mayores y trazas son consistentes con una evolucion por cristalizacion fraccionada, en un sistema cerrado, desde un liquido parental de composicion similar a la de la roca mas basica del conjunto analizado. Se reconoce, ademas, la influencia de sedimentos subductados en la composicion de estos magmas (aumento de la razon Tb/HFSE y disminucion de la razon Ce/Pb durante la diferenciacion). Sin embargo, estas rocas muestran importantes similitudes composicionales con las de complejos volcanicos emplazados sobre areas donde ocurre subduccion de bajo angulo (altos contenidos de K, similares razones LREF/HREE, Nb/Zr, Ba/Nb; TblHf; TblTa, Ta/Hf O,3), particularmente rocas de la actual region de subduccion plana en Chile central-Argentina y de la cuenca de Neuquen durante el Mioceno Superior. Se propone aqui un modelo tectono-magmatico que explica la generacion y extension de magmas calcoalcalinos en una posicion de trasarco durante el Mioceno Medio en Patagonia mediante el desarrollo de un fenomeno transiente de subduccion de bajo angulo y el consiguiente desplazamiento hacia el este del frente volcanico. Ademas, la mezcla entre remanentes de este magmatismo calcoalcalino almacenados en la corteza y fundidos alcalinos posteriores, provenientes de la astenosfera, se presenta como el proceso responsable de la generacion de la signatura transicional (i.e., intermedia entre calcoalcalina y alcalina, La/Nb>1; TiO2 2 wt%) reconocida en algunos basaltos que ocurren contemporaneamente con otros puramente alcalinos en la provincia de las Lavas Neogenas de Plateau de la Patagonia.
ANDEAN GEOLOGY

insu-00561888
https://hal-insu.archives-ouvertes.fr/insu-00561888
DOI : 10.5027/andgeo2010-678

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Spinel (± plagioclase) peridotite xenoliths from the Tallante and Los Perez volcanic centres in the eastern Betics (SE Spain) range from depleted (clinopyroxene-poor) harzburgites to fertile (clinopyroxene-rich) lherzolites and orthopyroxene-free wehrlites. Significantly, only one harzburgite, which is depleted in heavy rare earth elements (HREE), retains the imprint of ca. 20% ancient melting of an original garnet lherzolite source. In contrast, REE abundances of other harzburgites and lherzolites from the eastern Betics have been increased by melt-rock reaction. The whole-rock and mineral compositions of these mantle rocks are largely controlled by three types of modal metasomatism: 1) common clinopyroxene-orthopyroxene addition and olivine consumption which increased FeOt, SiO2 and Al2O3, and decreased MgO compared to the refractory melting products; 2) subordinate orthopyroxene dissolution and precipitation of clinopyroxene and olivine, which led to higher FeOt and MgO and lower SiO2 than in common (orthopyroxene-rich) lherzolites; and 3) rare orthopyroxene consumption and olivine addition that caused higher FeOt and lower SiO2 compared to the original melting residues. These mineral modal and major element variations have been produced mostly by interactions with relatively FeOt-rich/SiO2-poor melts, likely derived from a peridotite-pyroxenite lithospheric mantle with a highly heterogeneous isotopic composition. Melting of the lithospheric mantle in the western Mediterranean was triggered by upwelling of the asthenosphere induced by back-arc extension in the Late Oligocene-Early Miocene. Trapping of small fractions of exotic melts in whole-rocks — likely the parental magmas of Miocene back-arc dykes that intruded the Betic crust — caused local disequilibrium between the trace element signatures and Pb isotopic compositions of clinopyroxene and whole-rock. Subsequent interaction with SiO2-undersaturated magmas, similar to the parental melts of the Pliocene alkali basalts that host the xenoliths, promoted orthopyroxene consumption and clinopyroxene-olivine enrichment at locations close to magma conduits, and finally generated orthopyroxene-free wehrlites. This event constitutes the last episode of the Cenozoic magmatic evolution of the westernmost Mediterranean which is recorded in the mantle xenoliths from the eastern Betics.
ISSN: 0024-4937

hal-01553092
https://hal.archives-ouvertes.fr/hal-01553092
DOI : 10.1016/j.lithos.2016.12.011

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The eastern part of the Cordillera Occidental of Ecuador comprises thick buoyant oceanic plateaus associated with island-arc tholeiites and subduction-related calc-alkaline series, accreted to the Ecuadorian Continental Margin from Late Cretaceous to Eocene times. One of these plateau sequences, the Guaranda Oceanic Plateau is considered as remnant of the Caribbean–Colombian Oceanic Province (CCOP) accreted to the Ecuadorian Margin in the Maastrichtien. Samples studied in this paper were taken from four cross-sections through two arc-sequences in the northern part of the Cordillera Occidental of Ecuador, dated as (Río Cala) or ascribed to (Macuchi) the Late Cretaceous and one arc-like sequence in the Chogòn-Colonche Cordillera (Las Orquídeas). These three island-arcs can clearly be identified and rest conformably on the CCOP. In all four localities, basalts with abundant large clinopyroxene phenocrysts can be found, mimicking a picritic or ankaramitic facies. This mineralogical particularity, although not uncommon in island arc lavas, hints at a contribution of the CCOP in the genesis of these island arc rocks. The complete petrological and geochemical study of these rocks reveals that some have a primitive island-arc nature (MgO values range from 6 to 11 wt.%). Studied samples display marked Nb, Ta and Ti negative anomalies relative to the adjacent elements in the spidergrams characteristic of subduction-related magmatism. These rocks are LREE-enriched and their clinopyroxenes show a tholeiitic affinity (FeOT–TiO2 enrichment and CaO depletion from core to rim within a single crystal). The four sampled cross-sections through the island-arc sequences display homogeneous initial Nd, and Pb isotope ratios that suggest a unique mantellic source for these rocks resulting from the mixing of three components: an East-Pacific MORB end-member, an enriched pelagic sediment component, and a HIMU component carried by the CCOP. Indeed, the ankaramite and Mg-basalt sequences that form part of the Caribbean-Colombian Oceanic Plateau are radiogenically enriched in 206Pb/204Pb and 207Pb/204Pb and contain a HIMU component similar to that observed in the Gorgona basalts and Galápagos lavas. The subduction zone that generated the Late Cretaceous arcs occurred far from the continental margin, in an oceanic environment. This implies that no terrigenous detrital sediments interacted with the source at this period. Thus, the enriched component can only result from the melting of subducted pelagic sediments. We have thus defined the East-Pacific MORB, enriched (cherts, pelagic sediments) and HIMU components in an attempt to constrain and model the genesis of the studied island-arc magmatism, using a compilation of carefully selected isotopic data from literature according to rock age and paleogeographic location at the time of arc edification. Tripolar mixing models reveal that proportions of 12–15 wt.% of the HIMU component, 7–15 wt.% of the pelagic sediment end-member and 70–75 wt.% of an East-pacific MORB end-member are needed to explain the measured isotope ratios. These surprisingly high proportions of the HIMU/CCOP component could be explained by the young age of the oceanic plateau (5–15 Ma) during the Late Cretaceous arc emplacement. The CCOP, basement of these arc sequences, was probably still hot and easily assimilated at the island-arc lava source.
ISSN: 0895-9811

insu-00353692
https://hal-insu.archives-ouvertes.fr/insu-00353692
DOI : 10.1016/j.jsames.2008.06.003

Éditeur(s) : HAL CCSD University of Chicago Press
Résumé : International audience
Contrasting tectonic reconstructions of the westernmost Mediterranean have been proposed to explain the origin of the Alboran marine basin contemporaneously with Cenozoic convergence between the African and European plates. Cr-rich pyroxenites in the Ronda massif record the geochemical processes occurring in the subcontinental mantle of the Alboran domain in the Late Oligocene, thus constraining the geodynamic scenario of Cenozoic extension in the western Mediterranean lithosphere. Clinopyroxene in intrusive Cr-rich websterite dikes crosscutting the Ronda peridotite is strongly depleted in Nb-Ta and enriched in light rare earth elements, as typically observed in arc magmas, and is in trace element equilibrium with Neogene subduction-related lavas from the western and central Mediterranean. Sr-Nd-Pb radiogenic isotopes indicate that the mantle source of the Ronda pyroxenite dikes was contaminated by a subduction component released by detrital sediments likely deposited in passive continental margins. Rather than convective removal or delamination of the lithospheric root, our data strongly support Alboran geodynamic models that envisage slab rollback as the tectonic mechanism responsible for the Miocene lithospheric thinning. The Ronda Cr-rich pyroxenite dikes represent the earliest unambiguous manifestation of subduction-related magmatism in the western Mediterranean and testify to the involvement of terrigenous sediments in the primitive stages of subduction.
ISSN: 0022-1376

hal-00745853
https://hal.archives-ouvertes.fr/hal-00745853
DOI : 10.1086/663875

Éditeur(s) : HAL CCSD
Résumé : Subduction is one of the major geodynamical process active during the Earth evolution. The characterization of arc magmas, their evolution through space and time, and the relationship between arc magmatism and continental crust forming processes, constitute the main research topics in petrology and geochemistry since the last three decades. This study focuses on an exhumed portion of the Kohistan Arc (N Pakistan), which is an exceptional ‘in situ' laboratory to study and follow through time the processes active on subduction zones. Based on a pluri-geochemical approach, this study allows unraveling two major issues : 1/ The lack of a cogenetic relationship between the arc root (i.e. ultramafic rocks) and the overlying gabbroic crustal section. The Jijal ultramafic section does not bear a crustal origin and was emplaced at ~117 Ma via a magma-rock reaction between REE-depleted ‘boninite-like' melts and the pre-existing lithospheric mantle with Indian-MORB affinities; 2/ A three-stage geodynamical model covering the arc evolution through ~30 Ma. This model starts with the onset of the subduction followed by the building of the volcanic arc (1st stage). The 2nd stage corresponds to a major thermal event, characterized by abundant magma underplating and by granulitisation of the arc base. Recycling of the residual/cumulative lower crust into the convective asthenospheric mantle was efficient during this stage, and probably related to thermo-mechanical erosion processes. The last stage, between ~95 and 85 Ma, seals the end of the intra-oceanic subduction and corresponds to an “amagmatic” period. This step is followed by a voluminous, but short, magmatic pulse at ~85 Ma, occurring just before the arc-continent collision. At last, this study emphasizes the more mafic composition of the investigated crustal arc section when compared to the bulk continental crust. Nevertheless the arc section composition nicely fits with the lower continental crust average.
Les zones de subduction représentent des zones d'évolution clés dans la compréhension des processus géologiques majeurs actifs sur notre planète. En particulier, la caractérisation et l'évolution des magmas générés dans les zones d'arcs océaniques et la relation entre leur genèse et les processus de croissance crustale sont des axes primordiaux. Cette étude est focalisée sur la portion d'arc océanique exhumée du Kohistan (Nord Pakistan) qui constitue un laboratoire exceptionnel pour appréhender cette problématique. Basée sur une approche géochimique multi-méthodes (éléments majeurs et en trace, isotopes), cette étude a permis d'établir : 1/ l'absence de relation génétique directe entre les roches ultrabasiques de la racine de l'arc et la section crustale sus-jacente. La formation de la séquence ultrabasique de Jijal se produit à ~117 Ma via une réaction de type magma-roche entre des liquides de type boninitique (appauvris en terres rares), et le manteau lithosphérique de type MORB-Indien ; 2/ un modèle géodynamique en trois stades majeurs résumant l'évolution de l'arc océanique et de la subduction sur une période d'environ 30 Ma. Ce modèle débute par l'initiation de la subduction et la formation de l'arc volcanique s.s. (1er stade) ; le 2ème stade correspond à un évènement thermique majeur. Il est représenté par un sous-placage important de magmas et une granulitisation intense de la base de l'arc. Le recyclage de la croûte inférieure cumulative et résiduelle, dans le manteau sous-jacent, se produit durant cette étape vraisemblablement suite à des processus d'érosion thermo-mécanique. Le dernier stade, entre 95 Ma et 85 Ma, scelle la fin du fonctionnement de la zone de subduction et correspond à une période amagmatique suivie par une brève reprise du magmatisme avant la collision de l'arc après 85 Ma. Enfin, une modélisation numérique comparative avec la croûte continentale globale met en évidence la composition significativement plus basique de la croûte de l'arc insulaire du Kohistan. De fortes similitudes sont en revanche observées entre la section d'arc étudiée et la croûte continentale inférieure.
https://tel.archives-ouvertes.fr/tel-00437770

tel-00437770
https://tel.archives-ouvertes.fr/tel-00437770
https://tel.archives-ouvertes.fr/tel-00437770/document
https://tel.archives-ouvertes.fr/tel-00437770/file/thesedhuime.pdf

Éditeur(s) : HAL CCSD Springer Verlag
Résumé : International audience
The age of Ultra-high pressure metamorphism and early cooling remains controversial in the Alps. In a new approach to the problem, samples from the undeformed Hercynian metagranite, Brossasco, were studied using an ultra-violet laser ablation microprobe technique for 40Ar-39Ar dating. The results demonstrate the frequent occurence of excess argon with high 40Ar/36Ar ratios (1000-10000) and a strong relationship between apparent ages and metamorphic textures. The highest excess argon ratios are always associated with high closure temperature minerals or large diffusion domains within single mineral phases. The best interpretation of this relationship seems to be that excess argon was incorporated in all phases during the high pressure event, then mixed with an atmospheric component during rapid cooling and retrogression, producing a wide range of argon concentrations and 40Ar/36Ar ratios. In the present case, some ages in the range 60-110 Ma could be explained by the presence of excess argon incorporated around 40-50 Ma ago. Similar results found in other high-pressure terrains in the Alps may reconcile the argon geochronometer with other systems such as Rb/Sr, U/Pb or Sm/Nd. This study therefore calls for an increasing use of high resolution in-situ sampling techniques to clarify the meaning of 40Ar/39Ar ages in many high pressure terrains.
ISSN: 0010-7999

hal-00197563
https://hal.archives-ouvertes.fr/hal-00197563
https://hal.archives-ouvertes.fr/hal-00197563/document
https://hal.archives-ouvertes.fr/hal-00197563/file/Excess_UV_Arnaud_and_Kelley_GCA_Pub_preprint.pdf