Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
We report gabbroic dikes in the plagioclase tectonite domains of the Ojén and Ronda massifs (Betic Cordillera, southern Spain), which record crystallization at low-pressure syn-, or slightly postkinematic to the late ductile history of the Betic Peridotite in the westernmost Mediterranean. We present mineral major and trace element compositional data of discordant gabbroic dikes in the Ojén massif and gabbroic patches in the Ronda massif, complemented by the whole rock and electron backscattered diffraction (EBSD) data of the Ojén occurrence. In the Ojén massif, gabbro occurs as 1–3 centimeter wide discordant dikes that crosscut the plagioclase tectonite foliation at high angle. These dikes are composed of cm-scale igneous plagioclase and clinopyroxene crystals that show shape preferred orientations subparallel to the lineation of the host peridotite and oblique to the trend of the dike. Intrusion of Ojén gabbro dikes is coherent with the stress field that formed the high temperature, ductile plagioclase tectonite foliation and then attests for a mantle igneous event prior to the intracrustal emplacement of the massif. In the Ronda massif, gabbroic rocks crystallized in subcentimeter wide anastomozing veins, or as interstitial patches in the host dunite. They are mostly composed of plagioclase and clinopyroxene. Plagioclase composition is bytownitic in the Ojén, and andesinic in the Ronda massif. Clinopyroxene in both places shows identical, light Rare-Earth Element (LREE) depleted trace element patterns. The calculated trace element composition of melts in exchange equilibrium with the studied igneous clinopyroxenes reflects LREE-enriched character coupled with negative Eu anomaly, and indicates that gabbro-forming melts in Ronda and Ojén share a common melt source with an island arc tholeiitic affinity. Geothermobarometric data and liquidus mineralogy indicate that gabbro crystallization occurred at shallow depths (0.2–0.5 GPa) in a 7–16 km thick lithospheric section. These data suggest that gabbro-forming melts in the Betic Peridotite record a mantle igneous event at very shallow depths and provide evidence for the hyperextension of the continental lithosphere compatible with extreme backarc basin extension induced by the slab rollback of the Cenozoic subduction system in the westernmost Mediterranean.
ISSN: 0040-1951

hal-01172246
https://hal.archives-ouvertes.fr/hal-01172246
DOI : 10.1016/j.tecto.2015.03.011

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
We report structural evidence of ductile strain localization in mantle pyroxenite from the spinel to plagioclase websterite transition in the Ronda Peridotite (southern Spain). Mapping shows that, in this domain, small-scale shear zones occurring at the base of the lithospheric section are systematically located within thin pyroxenite layers, suggesting that the pyroxenite was locally weaker than the host peridotite. Strain localization is associated with a sudden decrease of grain size and increasing volume fractions of plagioclase and amphibole as a result of a spinel to plagioclase phase transformation reaction during decompression. This reaction also fostered hydrogen extraction ('dehydroxylation') from clinopyroxene producing effective fluid saturation that catalyzed the synkinematic net-transfer reaction. This reaction produced fine-grained olivine and plagioclase, allowing the onset of grain-size sensitive creep and further strain localization in these pyroxenite bands. The strain localization in the pyroxenites is thus explained by their more fertile composition, which allowed earlier onset of the phase transition reactions. Geothermobarometry undertaken on compositionally zoned constituent minerals suggests that this positive feedback between reactions and deformation is associated with cooling from at least 1000°C to 700°C and decompression from 1*0 to 0*5 GPa.
ISSN: 0022-3530

hal-00941293
https://hal.archives-ouvertes.fr/hal-00941293
DOI : 10.1093/petrology/egt039

Éditeur(s) : HAL CCSD Elsevier
Résumé : We studied the evolution of the microstructure of ice-Ih during static recrystallization by stepwise annealing experiments. We alternated thermal annealing and electron backscatter diffraction (EBSD) analyses on polycrystalline columnar ice pre-deformed in uniaxial compression at temperature of −7 °C to macroscopic strains of 3.0–5.2. Annealing experiments were carried out at −5 °C and −2 °C up to a maximum of 3.25 days, typically in 5–6 steps. EBSD crystal orientation maps obtained after each annealing step permit the description of microstructural changes. Decrease in average intragranular misorientation at the sample scale and modification of the misorientation across subgrain boundaries provide evidence for recovery from the earliest stages of annealing. This initial evolution is similar for all studied samples irrespective of their initial strain or annealing temperature. After an incubation period ≥1.5 h, recovery is accompanied by recrystallization (nucleation and grain boundary migration). Grain growth proceeds at the expense of domains with high intragranular misorientations, consuming first the most misorientated parts of primary grains. Grain growth kinetics fits the parabolic growth law with grain growth exponents in the range of 2.4–4.0. Deformation-induced tilt boundaries and kink bands may slow down grain boundary migration. They are stable features during early stages of static recrystallization, only erased by normal growth, which starts after >24 h of annealing.
ISSN: 0191-8141

hal-01685644
https://hal.archives-ouvertes.fr/hal-01685644
DOI : 10.1016/j.jsg.2017.05.001

Éditeur(s) : HAL CCSD American Geophysical Union (AGU)
Résumé : Mantle xenoliths in Pliocene alkali basalts of the eastern Betics (SE Iberia, Spain) are spinel ± plagioclase lherzolite, with minor harzburgite and wehrlite, displaying porphyroclastic or equigranular textures. Equigranular peridotites have olivine crystal preferred orientation (CPO) patterns similar to those of porphyroclastic xenoliths but slightly more dispersed. Olivine CPO shows [100]-fiber patterns characterized by strong alignment of [100]-axes subparallel to the stretching lineation and a girdle distribution of [010]-axes normal to it. This pattern is consistent with simple shear or transtensional deformation accommodated by dislocation creep. One xenolith provides evidence for synkinematic reactive percolation of subduction-related Si-rich melts/fluids that resulted in oriented crystallization of orthopyroxene. Despite a seemingly undeformed microstructure, the CPO in orthopyroxenite veins in composite xenoliths is identical to those of pyroxenes in the host peridotite, suggesting late-kinematic crystallization. Based on these observations, we propose that the annealing producing the equigranular microstructures was triggered by melt percolation in the shallow subcontinental lithospheric mantle coeval to the late Neogene formation of veins in composite xenoliths. Calculated seismic properties are characterized by fast propagation of P waves and polarization of fast S waves parallel to olivine [100]-axis (stretching lineation). These data are compatible with present-day seismic anisotropy observations in SE Iberia if the foliations in the lithospheric mantle are steeply dipping and lineations are subhorizontal with ENE strike, implying dominantly horizontal mantle flow in the ENE-WSW direction within vertical planes, that is, subparallel to the paleo-Iberian margin. The measured anisotropy could thus reflect a lithospheric fabric due to strike-slip deformation in the late Miocene in the context of WSW tearing of the subducted south Iberian margin lithosphere.
ISSN: 0278-7407

Droits : info:eu-repo/semantics/OpenAccess
hal-01685809
https://hal.archives-ouvertes.fr/hal-01685809
https://hal.archives-ouvertes.fr/hal-01685809/document
https://hal.archives-ouvertes.fr/hal-01685809/file/Hidas_et_al-2016-Tectonics.pdf
DOI : 10.1002/2016TC004165

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
We report microstructural evidence for fluid-assisted ductile strain localization in a ≤ 50 m-wide mylonitic–ultramylonitic shear zone in the Ronda Peridotite massif, Southern Spain. Strain localization occurred at relatively low pressure (< 0.8 GPa) and moderate temperature (750–1000 °C). Initial deformation by dislocation creep resulted in formation of mylonites. Focusing of aqueous fluids in the shear zone favored the activation of dissolution–precipitation creep, resulting in further strain localization. This process is recorded by two generations of ultramylonitic bands composed of fine-grained, well-mixed olivine–orthopyroxene aggregates. Microstructural observations in the ultramylonites suggest alternating dissolution and precipitation of olivine and orthopyroxene, which may be explained by local changes in silica molality of the percolating fluid (disequilibrium and mass transfer at scales > mm). In the mylonites, olivine shows a crystal preferred orientation (CPO) coherent with dominant (001)[100] glide, probably due to the presence of interstitial fluids during deformation. In the ultramylonites, olivine CPO is weak to very weak, consistently with a decreasing contribution of dislocation creep to deformation. In contrast, fine-grained orthopyroxene in both mylonites and ultramylonites displays a clear CPO characterized by a [001] maximum normal to the foliation, which is not consistent with dislocation glide in any known slip system for orthopyroxene. We interpret this CPO as formed by oriented crystallization during dissolution–precipitation. In the present study, dissolution–precipitation creep predominates only in small-scale ultramylonite bands due to limited fluid availability and localized dynamic permeability. However, this process may be important in intermediate temperature domains of subduction zones, where it may lead to a feedback between strain localization and fluid transport.
ISSN: 0024-4937

hal-01395122
https://hal.archives-ouvertes.fr/hal-01395122
DOI : 10.1016/j.lithos.2016.07.038

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 Elsevier
Résumé : International audience
Geochemical studies of primitive basalts have documented the presence of crustal-derived garnet pyroxenite in their mantle sources. The processes whereby melts with the signature of garnet pyroxenite are produced in the mantle are, however, poorly understood and somewhat controversial. Here we investigate a natural example of the interaction between melts of garnet pyroxenite derived from recycled plagioclase-rich crust and surrounding mantle in the Ronda peridotite massif. Melting of garnet pyroxenite at ∼1.5 GPa generated spinel websterite residues with MREE/HREE fractionation and preserved the positive Eu anomaly of their garnet pyroxenite precursor in whole-rock and clinopyroxene. Reaction of melts from garnet pyroxenite with depleted surrounding peridotite generated secondary fertile spinel lherzolite. These secondary lherzolites differ from common spinel lherzolite from Ronda and elsewhere by their lower-Mg# in clinopyroxene, orthopyroxene and olivine, lower-Cr# in spinel and higher whole-rock Al2O3, CaO, Sm/Yb and FeO⁎ at a given SiO2. Remarkably, secondary spinel lherzolite shows the geochemical signature of ghost plagioclase in the form of positive Eu and Sr anomalies in whole-rock and clinopyroxene, reflecting the transfer of a low-pressure crustal imprint from recycled pyroxenite to hybridized peridotite. Garnet pyroxenite melting and melt-peridotite interaction, as shown in the Ronda massif, may explain how the signature of subducted or delaminated crust is transferred to the mantle and how a garnet pyroxenite component is introduced into the source region of basalts. The efficiency of these processes in conveying the geochemical imprint of crustal-derived garnet pyroxenite to extruded lavas depends on the reactivity of pyroxenite melt with peridotite and the mantle permeability, which may be controlled by prior refertilization reactions similar to those documented in the Ronda massif. Highly fertile heterogeneities produced by pyroxenite-peridotite interaction, such as secondary spinel lherzolite in Ronda, may nucleate magmatic channels that remain chemically isolated from the ambient mantle and act as preferential pathways for melts with the signature of recycled crust.
ISSN: 0012-821X

hal-00814010
https://hal.archives-ouvertes.fr/hal-00814010
DOI : 10.1016/j.epsl.2012.11.047

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