Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The formation age of the Siberian cratonic mantle is not well established as yet. Re–Os data on various mantle-derived materials have shown that it contains Archaean components, but the reported Re-depletion ages show a broad variation range (3.4 to 1 Ga) and are commonly ≤ 2 Ga for peridotite xenoliths. We report Hf and Nd isotope data for cpx and garnet separated from nine refractory spinel and garnet peridotite xenoliths from the Udachnaya kimberlite. The cpx from low-opx spinel harzburgites show extremely high εHf values, from + 607 to + 2084, which testify to long-term evolution of these rocks with high Lu/Hf ratios, consistent with their residual origin and near absence of post-melting enrichments in the Lu–Hf system. Such high εHf values are unusual for cpx from other cratonic peridotites and are higher than those reported for depleted cpx in off-cratonic peridotites. The clinopyroxenes from low-opx spinel harzburgites yield Hf model ages from 1.9 to 1.7 Ga while the cpx from high-opx spinel harzburgites yield Hf model ages from 3.0 to 1.9 Ga. When plotted together, they define a Lu/Hf isochron with an age of 1.80 Ga, which we consider as a robust estimate of the formation age (melt extraction event) because it is obtained on residual rocks that show no evidence for HREE and Hf enrichments and because the model ages for three out of four individual samples are similar to each other. The cpx have high εNd of + 94 to + 123, which are inconsistent with their low Sm/NdPM of < 1 and yield no meaningful age estimates. The consistently high, positive εNd in these cpxs can be interpreted in terms of long-term evolution of refractory peridotites with high Sm/Nd, followed by relatively recent LREE enrichments.We infer that a significant part of the lithospheric mantle in the central Siberian craton may have been formed during a major event (or a series of events) at around 1.8 Ga. Older ages reported for the central Siberian craton may refer to less common materials from cratonic or other domains formed in the Archaean that were later incorporated into the cratonic roots. The transition from the “Archaean” to “modern” tectonic regimes in Siberia and possibly elsewhere may have taken place at 1.8–1.9 Ga rather than at ~ 2.5 Ga, i.e. in the second half of the Paleoproterozoic rather than at the Archaean–Proterozoic boundary, at which time the asthenospheric mantle became generally too cold to experience high-degree melting on a large scale. The ~ 1.8 Ga formation age of the Siberian cratonic mantle is coeval with that for a major part of the ancient continental crust in the central Siberian craton. The temporal crust–mantle links may be explained either by the generation of the initial source materials for continental crust in the same melting event that formed the residual peridotites or, alternatively, by subduction and melting of pre-existing proto-lithosphere destabilized by a major mantle upwelling that formed the residual mantle.
ISSN: 0009-2541

hal-01115519
https://hal.archives-ouvertes.fr/hal-01115519
DOI : 10.1016/j.chemgeo.2014.10.018

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The Ukrainian shield hosts two Palaeoproterozoic anorthosite-mangerite-charnockite-granite (AMCG) complexes (the Korosten and Korsun-Novomyrhorod complexes) that intruded Palaeoproterozoic continental crust in north-western and central parts of the shield, respectively. We report results of U-Pb zircon and baddeleyite dating of 16 samples from the Korosten plutonic complex (KPC), and 6 samples from the Korsun-Novomyrhorod plutonic complex (KNPC). Fifteen zircon samples from both complexes were also analysed for Hf isotopes. These new, together with previously published data indicate that the formation of the KPC started at c. 1815 Ma and continued until 1743 Ma with two main phases of magma emplacement at 1800–1780 and 1770–1758 Ma. Each of the main phases of magmatic activity included both basic and silicic members. The emplacement history of the KNPC is different from that of the KPC. The vast majority of the KNPC basic and silicic rocks were emplaced between c. 1757 and 1750 Ma; the youngest stages of the complex are represented by monzonites and syenites that were formed between 1748 and 1744 Ma. Both Ukrainian AMCG complexes are closely associated in space and time with mantle-derived mafic and ultramafic dykes. The Hf isotope ratios in the zircons indicate a predominantly crustal source for the initial melts with some input of juvenile Hf from mantle-derived tholeiite melts.The preferred model for the formation of the Ukrainian AMCG complexes involves the emplacement of large volumes of hot mantle-derived tholeiitic magma into the lower crust. This resulted in partial melting of mafic lower-crustal material, mixing of lower crustal and tholeiitic melts, and formation of ferromonzodioritic magmas. Further fractional crystallization of the ferromonzodioritic melts produced the spectrum of basic rocks in the AMCG complexes. Emplacement of the ferromonzodioritic and tholeiitic melts into the middle crust and their partial crystallization caused abundant melting of the ambient crust and formation of the large volumes of granitic rocks present in the complexes.
ISSN: 0301-9268

hal-01622715
https://hal.archives-ouvertes.fr/hal-01622715
DOI : 10.1016/j.precamres.2017.02.009

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Sedimentary rocks and detrital minerals sample large areas of the continental crust, and they are increasingly seen as a reliable archive for its global evolution. This study presents two approaches to model the growth of the continental crust through the sedimentary archive. The first builds on the variations in U-Pb, Hf and O isotopes in global databases of detrital zircons. We show that uncertainty in the Hf isotope composition of the mantle reservoir from which new crust separated, in the 176Lu/177Hf ratio of that new crust, and in the contribution in the databases of zircons that experienced ancient Pb loss(es), adds some uncertainty to the individual Hf model ages, but not to the overall shape of the calculated continental growth curves. The second approach is based on the variation of Nd isotopes in 645 worldwide fine-grained continental sedimentary rocks with different deposition ages, which requires a correction of the bias induced by preferential erosion of younger rocks through an erosion parameter referred to as K. This dimensionless parameter relates the proportions of younger to older source rocks in the sediment, to the proportions of younger to older source rocks present in the crust from which the sediment was derived. We suggest that a Hadean/Archaean value of K = 1 (i.e., no preferential erosion), and that post-Archaean values of K = 4–6, may be reasonable for the global Earth system. Models built on the detrital zircon and the fine-grained sediment records independently suggest that at least 65% of the present volume of continental crust was established by 3 Ga. The continental crust has been generated continuously, but with a marked decrease in the growth rate at ~ 3 Ga. The period from > 4 Ga to ~ 3 Ga is characterised by relatively high net rates of continental growth (2.9–3.4 km3 yr− 1 on average), which are similar to the rates at which new crust is generated (and destroyed) at the present time. Net growth rates are much lower since 3 Ga (0.6–0.9 km3 yr− 1 on average), which can be attributed to higher rates of destruction of continental crust. The change in slope in the continental growth curve at ~ 3 Ga is taken to indicate a global change in the way bulk crust was generated and preserved, and this change has been linked to the onset of subduction-driven plate tectonics. At least 100% of the present volume of the continental crust has been destroyed and recycled back into the mantle since ~ 3 Ga, and this time marks a transition in the average composition of new continental crust. Continental crust generated before 3 Ga was on average mafic, dense, relatively thin (< 20 km) and therefore different from the calc-alkaline andesitic crust that dominates the continental record today. Continental crust that formed after 3 Ga gradually became more intermediate in composition, buoyant and thicker. The increase in crustal thickness is accompanied by increasing rates of crustal reworking and increasing input of sediment to the ocean. These changes may have been accommodated by a change in lithospheric strength at around 3 Ga, as it became strong enough to support high-relief crust. This time period therefore indicates when significant volumes of continental crust started to become emergent and were available for erosion and weathering, thus impacting on the composition of the atmosphere and the oceans.
ISSN: 0037-0738

hal-01622696
https://hal.archives-ouvertes.fr/hal-01622696
DOI : 10.1016/j.sedgeo.2017.06.001

Éditeur(s) : HAL CCSD
Résumé : Volatile-rich, silica-undersaturated alkaline lavas record the important role of fluids during fluid-rock and magma-rock interactions in the mantle, which are key processes to understand the dynamics of the convective mantle and lithosphere-asthenosphere interactions in intracontinental settings. The aim of this thesis is to bring new constraints on the genesis of alkaline magmas by characterizing the crystallization conditions, the source and the partial melting processes taking part in the genesis of olivine nephelinites, pyroxene nephelinites and basanites from the Jbel Saghro volcanic field in the Moroccan Anti Atlas.The petrological and geochemical study of rocks and minerals coupled with the analysis of fluid inclusions constrains the pre-eruptive conditions of Saghro nephelinites to 1.7–2.2 GPa and ~1350 °C. Minerals show that nephelinitic magmas are rich in volatile elements (Cl, F, S), and fluid inclusions indicate that magmas were saturated with a CO2-rich fluid at pressures > 590 MPa. The various mineralogical assemblages and the presence of peridotite xenoliths suggest a rapid ascent for olivine nephelinites and more complex processes at depth for pyroxene nephelinites.Fractional crystallization and partial melting modelling of Saghro mafic lavas indicate that they are low-degree melts (0.6–2.5 %) of an amphibole-bearing carbonated peridotite enriched in incompatible elements, at the garnet-spinel transition (~80–85 km). Saghro nephelinites display a temporal evolution with a slight increase of the degree of melting and a decrease of the amount of residual amphibole from the oldest (olivine nephelinites, 9.6 Ma) to the most recent (pyroxene nephelinites, 2.9 Ma). Basanites form a system that is independent from nephelinites and are slightly higher-degree melts. Important variations in their chemical composition suggest variable amounts of fractional crystallization during ascent.The peculiar characteristics of Saghro nephelinites and basanites (enrichment in incompatible elements, negative anomalies in K, Zr, Hf and Ti, high Ca/Al and Zr/Hf ratios) indicate that their source was affected by carbonatitic metasomatism. The influence of this metasomatism is stronger for pyroxene nephelinites than for olivine nephelinites. These results suggest fluid-rock interactions beneath the Northwest African Craton, leading to the formation of a metasomatized mantle by CO2-rich carbonatitic components at the lithosphere-asthenosphere transition. The origin of the metasomatism inducing source enrichment and the formation of amphibole veins could be attributed to the melting of relict subducted oceanic lithosphere. The relatively low melting temperatures (< 1350 °C) suggest the absence of a thermal anomaly beneath the Jbel Saghro, and thus support a lithosphere delamination model as precursor of Saghro volca0,3nism. However, the increasing degree of partial melting over time, also observed in the Middle Atlas, together with the isotopic and geochemical similarities with Canary Islands alkaline lavas does not allow us to discard the influence of a deviation of the Canary mantle plume beneath northwest Africa.
Les laves alcalines sous-saturées riches en éléments volatils sont les marqueurs du rôle important des fluides dans le manteau et des interactions fluide-roche et magma-roche, processus clés pour comprendre la dynamique du manteau convectif et les interactions asthénosphère-lithosphère en domaine intracontinental. L’objectif de cette thèse est d’apporter de nouvelles contraintes sur la genèse des magmas alcalins en caractérisant les conditions de cristallisation, la source et les processus de fusion partielle à l’origine des néphélinites à olivine, des néphélinites à pyroxène et des basanites du champ volcanique du Jbel Saghro dans l’Anti-Atlas marocain.L’étude pétrologique et géochimique des roches et des minéraux, couplée à l’analyse des inclusions fluides a permis de contraindre les conditions pré-éruptives des néphélinites de Saghro à 1.7–2.2 GPa et ~1350 °C. Les minéraux montrent que les magmas néphélinitiques sont riches en éléments volatils (Cl, F, S), et les inclusions fluides indiquent que les magmas étaient saturés en fluide riche en CO2 à des pressions > 590 MPa. Les différents assemblages minéralogiques des néphélinites et la présence de xénolites péridotitiques suggèrent une ascension rapide des néphélinites à olivine et des processus plus complexes en profondeur pour les néphélinites à pyroxène.La modélisation des processus de cristallisation fractionnée et de fusion partielle des laves mafiques de Saghro a permis de déterminer qu’elles sont issues de faibles taux de fusion partielle (0.6–2.5 %) d’une péridotite carbonatée enrichie en éléments incompatibles, au niveau de la transition grenat–spinelle (~80–85 km) et en présence d’amphibole. Les néphélinites de Saghro montrent une évolution temporelle avec une légère augmentation du taux de fusion et une diminution de la quantité d’amphibole au résidu des plus anciennes (néphélinites à olivine, 9.6 Ma) aux plus récentes (néphélinites à pyroxène, 2.9 Ma). Les basanites forment un système indépendant des néphélinites et sont issues de taux de fusion plus élevés. Les fortes variations dans leur composition chimique suggèrent qu’elles ont subi de la cristallisation fractionnée lors de leur ascension.Les caractéristiques particulières des néphélinites et basanites de Saghro (enrichissement en éléments incompatibles, anomalies négatives en K, Zr, Hf et Ti, rapports Ca/Al et Zr/Hf élevés) indiquent que leur source a subi un métasomatisme principalement carbonatitique. L’influence de ce métasomatisme est plus forte pour les néphélinites à pyroxène que pour les néphélinites à olivine, impliquant une évolution temporelle de l’intensité du métasomatisme. Ces résultats suggèrent des interactions fluide-roche sous le craton Nord-Ouest Africain, entraînant la formation d'un manteau métasomatisé par des composants carbonatitiques riches en CO2 au niveau de la transition lithosphère-asthénosphère. L’origine du métasomatisme provoquant l’enrichissement de la source et la formation de veines d’amphibole pourrait être liée à la fusion de reliquats de croûte océanique subductée. Les températures de fusion relativement faibles (< 1350 °C) suggèrent l’absence d’anomalie thermique sous le Jbel Saghro, et favorisent donc un modèle de délamination de la lithosphère comme initiateur du volcanisme. Cependant, l’augmentation du taux de fusion partielle au cours du temps, également observée dans le Moyen Atlas, et les similitudes isotopiques et géochimiques avec les laves alcalines des îles Canaries ne permettent pas d’exclure une influence du panache des Canaries sur la source du volcanisme alcalin du Jbel Saghro.
https://tel.archives-ouvertes.fr/tel-01615888

NNT : 2015MONTS070
tel-01615888
https://tel.archives-ouvertes.fr/tel-01615888
https://tel.archives-ouvertes.fr/tel-01615888/document
https://tel.archives-ouvertes.fr/tel-01615888/file/53752_CHAMBOREDON_2015_archivage_cor.pdf

Éditeur(s) : Elsevier Science Bv
Résumé : Here we present a new geochemical study of Lopevi volcano, one the most active volcanoes in the Vanuatu island arc. We focus on the temporally well-defined sequence of lava flows emitted since 1960, and for the first time, on pre-1960 volcanic products, including high-MgO basalts and felsic andesites, the most evolved lavas sampled so far on this island. This work reports the first Pb and Hf isotopic study of lavas from Lopevi island. These lavas display correlations between differentiation indexes such as SiO2 content and isotopic ratios. The felsic andesites extend the known correlations with both the least (Sr-Pb) and the most (Nd-Hf) radiogenic isotopic compositions on the island. Our results confirm that the rising magma interacted with the sub-arc crust. Assimilation-Fractional Crystallization (AFC) quantitative modeling of trace element ratios and isotopic compositions requires 1% and 10% of assimilated partial melts of a mafic oceanic crust to account for the pre- and post-1960 lavas, respectively. The post-1960 lavas differ from the former lavas emitted similar to 20 years earlier by enrichments in fluid mobile elements (K, Ba, Rb ... ), Th, and Light Rare Earth Elements (LREE). We ascribe these features to slight variations in the metasomatic agent added to the sub-arc mantle and ultimately derived from the subducted lithosphere. However, the contrasting time scales involved in subducted lithosphere dehydration and magma genesis, relative to the time elapsed between eruptions of the two lava series, suggest that two different portions of mantle which have undergone slightly different metasomatism, gave birth to the Lopevi lavas. These distinct magmas are still present beneath the volcano.
Journal Of Volcanology And Geothermal Research (0377-0273) (Elsevier Science Bv), 2013-08 , Vol. 264 , P. 72-84

Droits : 2013 Elsevier B.V. All rights reserved.
http://archimer.ifremer.fr/doc/00167/27801/26223.pdf
DOI:10.1016/j.jvolgeores.2013.07.005
http://archimer.ifremer.fr/doc/00167/27801/

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Most peridotite massifs and mantle xenoliths show a wide range of isotopic variations, often involving significant decoupling between Hf, Nd and Sr isotopes. These variations are generally ascribed either to mingling of individual components of contrasted isotopic compositions or to time integration of parent-element enrichment by percolating melts/fluids, superimposed onto previous depletion event(s). However, strong isotopic decoupling may also arise during porous flow as a result of daughter-elements fractionation during solid-liquid interaction. Although porous flow is recognized as an important process in mantle rocks, its effects on mantle isotopic variability have been barely investigated so far. The peridotites of the Lherz massif (French Pyrenees) display a frozen melt percolation front separating highly refractory harzburgites from refertilized lherzolites. Isotopic signatures observed at the melt percolation front show a strong decoupling of Hf from Nd and Sr isotopes that cannot be accounted for by simple mixing involving the harzburgite protolith and the percolating melt. Using one dimensional percolation-diffusion and percolation-reaction modeling, we show that these signatures represent transient isotopic compositions generated by porous flow. These signatures are governed by a few critical parameters such as daughter element concentrations in melt and peridotite, element diffusivity, and efficiency of isotopic homogenization rather than by the chromatographic effect of melt transport and the refertilization reaction. Subtle variations in these parameters may generate significant inter-isotopic decoupling and wide isotopic variations in mantle rocks. (C) 2009 Elsevier B.V. All rights reserved.
ISSN: 0012-821X

hal-00413135
https://hal.archives-ouvertes.fr/hal-00413135
DOI : 10.1016/j.epsl.2008.12.033

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
High-grade migmatitic and mylonitic gneisses from the Day Nui Con Voi massif in northern Vietnam record temperatures of 760-810 °C at pressures of 6-10 kbars. High temperature conditions have resulted in the development of major element diffusional profiles in garnet. Laser ablation ICP-MS analyses of trace elements indicate that REE and Hf closely followed Rayleigh-like fractionation trends but underwent significant post-crystallization modification. Light REE and to a lesser extent, HREE rim-to-rim zonation profiles show progressive flattening with the decreasing garnet size. Nd and Sm are completely homogenized in the crystals smaller than 1.5 mm, while Lu always preserves variable degrees of core-to-rim concentration gradient. The observed REE patterns are interpreted as resulting from the combination of protracted garnet growth of progressively smaller crystals and intracrystalline diffusion. This had profound influence on Sm-Nd geochronology and resulted in isochron ages ranging from 50 to 32 Ma. The youngest age was obtained for a sample, where all garnet crystals are smaller than 2 mm in which light REE profiles are completely or nearly completely homogenized. Thus, only the youngest age represents geologically meaningful event, and 31.7 ± 0.9 Ma age is interpreted as the best estimate of the resetting episode due to high temperature diffusional homogenization of light REE during early Oligocene metamorphism. Older Sm-Nd ages reflect mixed analyses of variably reset individual garnet crystals. Lu-Hf isotopic analyses of bulk garnet fractions, despite yielding high parent/daughter isotopic ratios appeared very scattered and did not allow defining isochron ages. Instead, apparent ages defined by whole rock and individual garnet fractions range from c. 80 to 160 Ma. Very old apparent ages are interpreted as being the consequence of intracrystalline Lu diffusion and preservation of the original Hf distribution, which leads to lower 176Lu/177Hf ratios, and thus steeper (older) isochrons. Back diffusion of Lu during commonly observed resorption played a subordinate role in modifying isotope systematics.
ISSN: 0009-2541

hal-00813949
https://hal.archives-ouvertes.fr/hal-00813949
DOI : 10.1016/j.chemgeo.2012.04.024

Éditeur(s) : Elsevier
Résumé : The combined use of Lu-Hf and Sm-Nd isotope systems potentially offers a unique perspective for investigating continental erosion, but little is known about whether, and to what extent, the Hf-Nd isotope composition of sediments is related to silicate weathering intensity. In this study, Hf and Nd elemental and isotope data are reported for marine muds, leached Fe-oxide fractions and zircon-rich turbidite sands collected off the Congo River mouth, and from other parts of the SE Atlantic Ocean. All studied samples from the Congo fan (muds, Fe-hydroxides, sands) exhibit indistinguishable Nd isotopic composition (epsilon(Nd)similar to - 16), indicating that Fe-hydroxides leached from these sediments correspond to continental oxides precipitated within the Congo basin. In marked contrast, Hf isotope compositions for the same samples exhibit significant variations. Leached Fe-hydroxide fractions are characterized by epsilon(Hf) values (from -1.1 to +13) far more radiogenic than associated sediments (from -7.1 to -12.0) and turbidite sands (from -27.2 to -31.6). epsilon(Hf) values for Congo fan sediments correlate very well with Al/K (i.e. a well-known index for the intensity of chemical weathering in Central Africa). Taken together, these results indicate that (1) silicate weathering on continents leads to erosion products having very distinctive Hf isotope signatures, and (2) a direct relationship exists between epsilon(Hf) of secondary clay minerals and chemical weathering intensity. These results combined with data from the literature have global implications for understanding the Hf-Nd isotope variability in marine precipitates and sediments. Leached Fe-hydroxides from Congo fan sediments plot remarkably well on an extension of the 'seawater array' (i.e. the correlation defined by deep-sea Fe-Mn precipitates), providing additional support to the suggestion that the ocean Hf budget is dominated by continental inputs. Fine-grained sediments define a diffuse trend, between that for igneous rocks and the,seawater array', which we refer to as the 'zircon-free sediment array' (epsilon(Hf)=0-91 epsilon(Nd)+3.10). Finally, we show that the Hf-Nd arrays for seawater, unweathered igneous rocks, zircon-free and zircon-bearing sediments (epsilon(Hf)= 1.80 epsilon(Nd)+2.35) can all be reconciled, using Monte Carlo simulations, with a simple weathering model of the continental crust. (C) 2008 Elsevier B.V. All rights reserved.
Earth and Planetary Science Letters (0012-821X) (Elsevier), 2009 , Vol. 277 , N. 3-4 , P. 318-326

Droits : 2009 Elsevier B.V. All rights reserved.
http://archimer.ifremer.fr/doc/2009/publication-6451.pdf
DOI:10.1016/j.epsl.2008.10.028
http://archimer.ifremer.fr/doc/00000/6451/

Éditeur(s) : HAL CCSD Elsevier
Résumé : Pb, Hf, Nd and Sr isotopes of basaltic lavas from the two Reunion Island volcanoes are reported in order to examine the origin of the sources feeding these volcanoes and to detect possible changes through time. Samples, chosen to cover the whole lifetime of the two volcanoes (from 2 Ma to present), yield a chemically restricted (compared to 0113 lavas) but complex distribution. Reunion plume isotopic characteristics have been defined on the basis of the composition of uncontaminated shield-building lavas from the Piton de la Fournaise volcano. The average epsilon(Nd), epsilon(Hf), Sr-87/Sr-86 and Pb-206/Pb-204, Pb-207/Pb-204, and Pb-208/Pb-204 isotope ratios calculated for this component are +4.4, +9.1, 0.70411, 18.97, 15.59 and 39.03, respectively. In Pb-Pb isotope space, each volcano defines a distinct linear trend but slight variation are also detected within the various volcanic sequences. The Piton des Neiges volcano yields a distinct and significantly more scattered isotopic distribution than Piton de la Fournaise for both Pb, Hf and Nd isotope tracers. A principal component analysis of the Pb isotope data from Piton de la Fournaise reveals a major contribution of the C and EM-1 components (with a clear Dupal flavor) as main components for the modem Reunion plume. The same components have been identified for Piton des Neiges but with a stronger participation of a depleted mantle component and a weaker EM-1 contribution. The compositional change of the lavas erupted by the Piton des Neiges and Piton de la Fournaise volcanoes is attributed to the impingement of two small-scale blobs of plume material at the base of the Reunion lithosphere. Compared to other hot-spots worldwide, in particular Hawaii and Kerguelen, magmas beneath Reunion are generated from a considerably more homogeneous, compositionally more primitive plume higher in Pb-206. Although shallow-level contamination processes have been locally detected they did not alter significantly the composition of the plume magmas. This is tentatively attributed to mantle dynamics producing small, high-velocity blobs that ascend rapidly through the lithosphere, and to the lack of a well-developed magma chamber at depth in the lithosphere.
ISSN: 0012-821X

hal-00412196
https://hal.archives-ouvertes.fr/hal-00412196
DOI : 10.1016/j.epsl.2007.11.018

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Cratons represent the oldest preserved lithospheric domains. Their lithosphere (lithospheric mantle welded to overlying Precambrian crystalline basement) is considered to be particularly robust and long-lived due to the protecting presence of buoyant and rigid “keels” made up of residual harzburgites. Although the cratons are mostly assumed to form in the Archaean, the timing of their formation remains poorly constrained. In particular, there are very few datasets describing concurrently the age of both the crustal and mantle portions of the lithosphere. In this study, we report new U–Pb ages and Hf isotope compositions for zircons in crustal xenoliths from the Udachnaya kimberlite in the central Siberian craton; this dataset includes samples from both the upper and lower portions of the crust. The zircon ages agree well with model melt-extraction Re–Os ages on refractory peridotite xenoliths from the same pipe; taken together they allow an integrated view of lithosphere formation. Our data reveal that the present day upper crust is Archaean, whereas both the lower crust and the lithospheric mantle yield Paleoproterozoic ages. We infer that the deep lithosphere beneath the Siberian craton was not formed in a single Archaean event, but grew in at least two distinct events, one in the late Archaean and the other in the Paleoproterozoic. Importantly, a complete or large-scale delamination and rejuvenation of the Archaean lower lithosphere (lower crust and lithospheric mantle) took place in the Paleoproterozoic. This further demonstrates that craton formation can be a protracted, multi-stage process, and that the present day crust and mantle may not represent complementary reservoirs formed through the same tectono-magmatic event. Further, deep cratonic lithosphere may be less robust and long living than often assumed, with rejuvenation and replacement events throughout its history.
ISSN: 0012-821X

hal-01467648
https://hal.archives-ouvertes.fr/hal-01467648
DOI : 10.1016/j.epsl.2016.09.046

Éditeur(s) : HAL CCSD Annual Reviews
Résumé : International audience
The record of the continental lithosphere is patchy and incomplete; no known rock is older than 4.02 Ga, and less than 5% of the rocks preserved are older than 3 Ga. In addition, there is no recognizable mantle lithosphere from before 3 Ga. We infer that there was lithosphere before 3 Ga and that ∼3 Ga marks the stabilization of blocks of continental lithosphere that have since survived. This was linked to plate tectonics emerging as the dominant tectonic regime in response to thermal cooling, the development of a more rigid lithosphere, and the recycling of water, which may in turn have facilitated plate tectonics. A number of models, using different approaches, suggest that at 3 Ga the volume of continental crust was ∼70% of its present-day volume and that this may be a minimum value. The continental crust before 3 Ga was on average more mafic than that generated subsequently, and this pre-3 Ga mafic new crust had fractionated Lu/Hf and Sm/Nd ratios as inferred for the sources of tonalite-trondhjemite-granodiorite and later granites. The more intermediate composition of new crust generated since 3 Ga is indicated by its higher Rb/Sr ratios. This change in composition was associated with an increase in crustal thickness, which resulted in more emergent crust available for weathering and erosion. This in turn led to an increase in the Sr isotope ratios of seawater and in the drawdown of CO2. Since 3 Ga, the preserved record of the continental crust is marked by global cycles of peaks and troughs of U-Pb crystallization ages, with the peaks of ages appearing to match periods of supercontinent assembly. There is increasing evidence that the peaks of ages represent enhanced preservation of magmatic rocks in periods leading up to and including continental collision in the assembly of supercontinents. These are times of increased crustal growth because more of the crust that is generated is retained within the crust. The rates of generation of continental crust and mantle lithosphere may have remained relatively constant at least since 3 Ga, yet the rates of destruction of continental crust have changed with time. Only relatively small volumes of rock are preserved from before 3 Ga, and so it remains difficult to establish which of these are representative of global processes and the extent to which the rock record before 3 Ga is distorted by particular biases.
ISSN: 0084-6597

hal-01622692
https://hal.archives-ouvertes.fr/hal-01622692
DOI : 10.1146/annurev-earth-063016-020525

Éditeur(s) : HAL CCSD Taylor & Francis
Résumé : Modern geochronology has moved beyond the acquisition of dates: the goal is to understand the significance of these numbers for the geodynamic evolution of Earth at all scales. The coupling of the laser-ablation microprobe (LAM) to inductively coupled plasma mass spectrometers (ICPMS, multicollector (MC)-ICPMS) has revolutionised geochronology and geochemistry over the last 10 years. These systems enable the rapid and precise in situ analysis of trace-element patterns and isotopic systems, while adding information related to microstructural context and major-element composition. The integration of these multiple sources of data is crucial in constraining the origin of the sample and the processes leading to its formation, so that we can understand the meaning of a date in terms of geological events. LAM-ICPMS measurement of U-Pb ages and trace-element patterns in zircon, coupled with LAM-MC-ICPMS analysis of Hf isotopes in the same grains, gives new insights into the processes of magma genesis. Applied to detrital zircons from modern drainages or sedimentary rocks (the TerraneChron approach), it becomes a powerful tool to investigate problems of crustal evolution on scales ranging from single terranes to continents. The in situ analysis (LAM-MC-ICPMS) of Re-Os systematics in single grains of sulfides in mantle-derived peridotites has demonstrated that most mantle rocks contain several generations of Os-bearing sulfides; whole-rock analyses are mixtures reflecting multiple melting and metasomatic events in the lithospheric mantle. These deep-seated events are commonly mirrored in the crust; Os model-age spectra from xenolith suites show age 'peaks' that correspond to the ages of thermal/tectonic events in the overlying crust, suggesting strong linkages between crust and mantle. Integrated studies of the timing and nature of crustal and mantle events, using these techniques, will be important for understanding the large-scale dynamics of the Earth.
ISSN: 0812-0099

hal-00411328
https://hal.archives-ouvertes.fr/hal-00411328
DOI : 10.1080/08120090802097450

Éditeur(s) : HAL CCSD
Résumé : Differentiation of the lithospheric mantle occurred principally through partial melting and extraction of melts. Harzburgites are generally considered as melting residues whereas lherzolites are regarded as pristine mantle weakly affected by melting. However, some orogenic peridotites show evidence of igneous refertilization. In this context, this work re-investigates the nature of the Lherz lherzolites (Pyrenees), type-locality of lherzolites, described as a piece of preserved fertile mantle. Structural and geochemical data show that these lherzolites are not pristine but formed through a refertilization reaction between MORB-like melts and refractory lithosphere. Moreover, the Lherz peridotites were partly used to infer the composition of the primitive upper mantle and these results may have important implications for the nature of the late veneer. Additionally, crystal-preferred orientations of minerals (CPO) highlight a strong feedback between melt percolation and finite strain in the percolated rocks. CPO variations are ruled by a subtle balance between instantaneous melt fraction and local strain rate. This work also investigated the effect of melt percolation on Hf, Nd and Sr isotopes. Isotope systematics in Lherz shows that strong isotopic decoupling may arise in a percolation front. The modelling suggests that decoupled isotopic signatures are generated during porous flow and governed by the melt/matrix elements concentrations, chemical diffusivities or efficiency of isotopic homogenization. Melt-rock interactions can generate “intraplate-like” isotopic signatures. This suggests that a part of isotopic signatures of mantle-related rocks could be generated by diffusional processes associated with melt transport.
La différenciation du manteau lithosphérique s'est principalement effectuée par fusion partielle et extraction de magmas. Les harzburgites sont généralement considérées comme des résidus de fusion tandis que les lherzolites représentent un manteau originel peu affecté par la fusion. Cependant, certaines péridotites orogéniques montrent des évidences de refertilisation magmatique. Dans ce contexte, ce travail remet en cause la nature primitive des lherzolites de Lherz (Pyrénées), lithotype de la lherzolite. Les données structurales et géochimiques montrent que ces lherzolites ne représentent pas du manteau préservé mais qu'elles se sont formées par une réaction de refertilisation entre un liquide de type MORB et une lithosphère réfractaire. De plus, les orientations préférentielles de réseau (OPR) des minéraux montrent une forte interdépendance entre la percolation magmatique et la déformation finie observée dans les roches percolées. Les variations d'OPR semblent contrôlées par un équilibre subtil entre fraction de liquide instantanée et taux de déformation local. Ce travail a également abordé les effets de la percolation magmatique sur la variabilité isotopique des roches mantelliques. Les résultats montrent qu'un fort découplage isotopique peut s'observer dans un front de percolation. Les modèles suggèrent que le découplage des signatures isotopiques se produit pendant la percolation, gouverné par le rapport des concentrations en éléments (magma/matrice), la diffusivité et le processus d'homogénéisation isotopique. Les réactions magma/roche peuvent générer des signatures isotopiques de type intraplaque. Ceci suggère qu'une part des signatures isotopiques des roches ou laves dérivées du manteau pourrait être produite par des processus diffusionnels associés à du transport de magma.
https://tel.archives-ouvertes.fr/tel-00431325

tel-00431325
https://tel.archives-ouvertes.fr/tel-00431325
https://tel.archives-ouvertes.fr/tel-00431325/document
https://tel.archives-ouvertes.fr/tel-00431325/file/theseleroux08.pdf

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
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) : 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 Elsevier
Résumé : International audience
The crust of the Paleoproterozoic West African Craton in northern Ghana is dominantly made of Tonalite-Trondhjemite-Granodiorite (TTG) suites, low-K to high-K calc-alkaline granites, with minor LILE-enriched diorites, and chemically related lavas. Results of zircon U-Pb dating indicate that all these different rock types were formed together over a prolonged period of ca. 100 Ma, between 2.21 and 2.11 Ga. Zircon Hf isotope compositions of five samples are supra-chondritic (εHft from +1 to +6) indicating that the granitoids derive from the reworking of juvenile crustal components as old as 2.60 Ga. Geochemical data show that the TTGs were derived from the reworking of a low-K mafic crust, while biotite granites formed by the reworking of older felsic crustal rocks, and diorites derived from the melting of LILE-enriched mantle sources.The combined datasets support a three-step model for the formation of the crust of the West African craton in northern Ghana: (1) a juvenile, mafic proto-crust was extracted from the depleted mantle between 2.60 and 2.30 Ga, in an oceanic environment remote from any pre-existing continental nucleus; (2) the mafic proto-crust was reworked in several primitive magmatic arcs or in accreted oceanic plateaux. Crustal reworking initially produced minor felsic magmatism between 2.35 and 2.21 Ga, followed by the 2.21–2.15 Ga emplacement of low- to medium-P TTG and K-rich calc-alkaline granitoid suites, and was coeval with the formation of an enriched mantle source. (3) Various proto-continental terranes collided during the Eburnean orogeny between 2.14 and 2.11 Ga, triggering intense reworking of the composite crust and mantle reservoirs formed in the previous two steps, i.e. remnants of the juvenile mafic crust, older TTGs and biotite granites, and previously enriched mantle sources. The emplacement of high-P TTGs, granites and LILE-enriched diorites resulted from lower crustal heating in response to the delamination of the lower crust and lithospheric mantle. The Eburnean collision therefore marks the final stages of a 200–500 Ma long period of juvenile continental crust formation and stabilisation. The datasets further reveal that the Paleoproterozoic West African Craton comprises a minimum of two distinct crustal blocks, with north-western Ghana lying at their interface.
ISSN: 0301-9268

hal-01467999
https://hal.archives-ouvertes.fr/hal-01467999
DOI : 10.1016/j.precamres.2016.10.011

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Melting experiments were performed on a silica-rich peridotite composition at 10-17 GPa to determine majoritic garnet-melt partition coefficients (D) for major and trace elements. Our results show that D for many elements, including Na, Sc, Y and rare earth elements (REE), varies significantly with increasing pressure or proportion of majorite component. Lu and Sc become incompatible at 17 GPa, with D decreasing from 1.5 at 10 GPa to 0.9 at 17 GPa. As predicted from lattice strain, log D for isovalent cations entering the large site of majoritic garnet exhibits a near-parabolic dependence on ionic radius. Our data are used to refine a previously published predictive model for garnet-melt partitioning of trivalent cations, which suffered from a lack of calibration in the 10-20 GPa range. Our results suggest that Archean Al-depleted komatiites from Barberton (South Africa) may have been generated by partial melting of dry peridotite at depths between 200 and 400 km. We also speculate that transition zone diamonds from Kankan (Guinea), which contain inclusions of majoritic garnet, may have formed from the partial reduction of CO2-rich magmas that subsequently transported them to the surface. This hypothesis would provide an explanation for the REE patterns of majoritic garnet trapped within these diamonds, including Eu anomalies. Finally, we show that segregation of majoritic garnet-bearing cumulates during crystallisation of a deep Martian magma ocean could lead to a variety of Lu/Hf and Sm/Nd ratios depending on pressure, leading to a range of ε143Nd and ε176Hf isotope signatures for potential mantle sources of Martian rocks.
ISSN: 0024-4937

hal-00767567
https://hal.archives-ouvertes.fr/hal-00767567
DOI : 10.1016/j.lithos.2012.06.013

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The Dahomey belt in Togo and Benin is an important segment of the larger West Gondwana Orogen. Here, we review the geodynamic evolution of the Dahomey belt and discuss new U–Pb and Lu–Hf zircon data in light of similar data previously acquired on the geologically related Northern Borborema Province, in NE Brazil. Eighteen samples from different tectonic settings and regions within the belt were collected for zircon isotopic investigation. Passive margin deposits of the Atacora Structural Unit and lower units of the Volta Basin have detrital zircon signatures compatible with the flanking West Africa Craton. The arc-related magmatism resulted from the east-dipping subduction of the Goiás-Pharusian oceanic lithosphere and is represented by a variety of granitoids emplaced in the Benino-Nigerian Shield between 670 and 610 Ma. These granitoids were mainly sourced from crustal reservoirs with subordinate juvenile input. Detrital zircon ages from syn-orogenic deposits in Benino-Nigerian Shield suggest that arc development could have started as early as 780 Ma. The main period of melting in the internal part of the belt, the Benino-Nigerian Shield, is related to crustal thickening and occurs only ca. 30 m.y. after initiation of the continental collision, marked by the ca. 610 Ma ultra-high pressure (UHP) metamorphism recorded at Lato Hills. Foreland development represented by the upper units of the Volta basin developed soon after continental collision and persisted with the development of the west-verging thrust front synchronously with the main period of crustal melting due to collision at ca. 580 Ma. The subvertical Transbrasiliano Lineament in South America, that corresponds to the Kandi Lineament in Africa, provides a present-day fit between NW Africa and NE Brazil. Restoration of the movement of the Transbrasiliano-Kandi Lineament (strike-slip plate boundary) places the Dahomey belt and Borborema Province (NE Brazil) along the same section of the West Gondwana Orogen. This configuration would explain some of the misfits previously discussed in the literature and aligns the UHP eclogites in Togo and NE Brazil.
ISSN: 0301-9268

hal-01358584
https://hal.archives-ouvertes.fr/hal-01358584
DOI : 10.1016/j.precamres.2016.01.032

Éditeur(s) : HAL CCSD Geological Society of America
Résumé : International audience
his study describes the structural setting, petrogenesis, and geochronology of a suite of acidic magmatic rocks that are intruded in the metamorphic core of the Tertiary ophiolitic suture zone of the Sabzevar Range, NE central Iran. These intrusive bodies show tabular geometries with solid-state fabrics documenting syntectonic emplacement during crustal shortening. In the total alkalis-silica (TAS) diagram, their compositions define a medium-K calc-alkaline suite, spanning from basaltic andesite to the dacite and rhyolite fields. They show characteristic low MgO (0.15–0.60 wt%) and Ni (<20 ppm), high Sr contents, a negligible Eu anomaly, and extremely fractionated rare earth element (REEs), with high La/Yb and Sr/Y (up to 900) ratios, but very low Yb and Y contents. They also show zircon Hf isotope compositions compatible with a mid-ocean-ridge basalt (MORB)–type oceanic crust. Inverse and forward thermobarometry constrains conditions of magma crystallization in the upper-pressure field of the amphibolite facies (ca. 1.2–1.5 GPa and 750 °C). Integrated U-Pb zircon and 40Ar/39Ar white mica and amphibole geochronology constrains the Sabzevar magmatism to the late Paleocene (at ca. 58 Ma). Genesis of the Sabzevar magmatic suite is interpreted in terms of prograde, wet amphibolite melting during oceanic subduction, within a pressure-temperature range between a plagioclase-out and a hornblende-out boundary. Magma differentiation and high-pressure amphibole fractionation of pristine slab melts are proposed as the dominant factors that imparted the adakite signature in the Sabzevar structural zone. Implications in terms of the regional tectonic scenario are discussed and framed within the advancing and retreating evolution of the Neotethyan subduction during the Mesozoic–Tertiary time span.
ISSN: 0016-7606

hal-01122824
https://hal.archives-ouvertes.fr/hal-01122824
DOI : 10.1130/B30913.1