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The Sabzevar ophiolite is the most extensive and best-exposed section of oceanic lithosphere in NE Iran. In this study, we examined the mantle section of the eastern part of the Sabzevar ophiolite, where mantle peridotites are the most widespread rock type and are crosscut by pyroxenite veins and mafic dykes. Major and trace element compositions (minerals and whole-rock) of lherzolites are akin to those of abyssal peridotites from mid-ocean ridges (MOR), whereas those of harzburgites evidence formation in a supra-subduction zone (SSZ) setting. Rare-earth element (REE) patterns of whole-rock and clinopyroxene of lherzolites and harzburgites suggest polybaric melting, with initial melting in the garnet peridotite field (~5–6%), followed by ~5–15% melting for lherzolites and ~10–20% for harzburgites in the spinel peridotite field. Harzburgites show LREE enrichment incompatible with partial melting models, indicating significant interaction with SSZ-related melts, whereas analysed lherzolites experienced limited melt-rock interaction. Pyroxene dissolution and olivine precipitation in residual peridotites resulting from reaction with island arc tholeiite and/or boninite melts led to the formation of dunites. These melts eventually intruded as dykes and veins in the mantle and crustal sections of the ophiolite. In addition, boninite melts are inferred to be responsible for forming olivine pyroxenite veins within the harzburgites. We conclude that the mantle section of the Sabzevar ophiolite formed in an intra-oceanic arc setting and experienced a geochemical evolution from a MOR-like to SSZ setting. The intra-oceanic subduction of the Neotethys oceanic lithosphere and shifting conditions of melting during the development of subduction beneath the incipient island arc were responsible for the geochemical heterogeneity of mantle peridotites and the generation of SSZ-related magmas within the Sabzevar oceanic lithosphere. [ABSTRACT FROM AUTHOR]

We present an integrated geochemical study of ultra-high pressure (UHP) garnet pyroxenites from the Ronda and Beni Bousera peridotite massifs (Betic-Rif Belt, westernmost Mediterranean). Based on their Sr-Nd-Pb-Hf isotopic systematics, we classify UHP garnet pyroxenites into three groups: Group A pyroxenites (Al 2 O 3 : 15–17.5 wt.%) have low initial 87 Sr/ 86 Sr, relatively high ε Nd , ε Hf and 206 Pb/ 204 Pb ratios, and variable 207 Pb/ 204 Pb and 208 Pb/ 204 Pb. Group B pyroxenites (Al 2 O 3  < 14 wt.%) are characterized by high initial 87 Sr/ 86 Sr and relatively low ε Nd , ε Hf and 206 Pb/ 204 Pb ratios. Group C pyroxenites (Al 2 O 3  ∼ 15 wt.%) have depleted radiogenic signatures with relatively low initial 87 Sr/ 86 Sr and 206 Pb/ 204 Pb, high ε Nd and ε Hf , and their 207 Pb/ 204 Pb and 208 Pb/ 204 Pb ratios are similar to those of Group B pyroxenites. The major and trace element and isotopic compositions of UHP garnet pyroxenites support their derivation from ancient (1.5–3.5 Ga) oceanic crust recycled into the mantle and intimately stirred with peridotites by convection. However, the genesis of these pyroxenites requires also the involvement of recycled continental lower crust with an isotopic composition akin to the lower crustal section of the lithosphere where these UHP garnet pyroxenites now reside in. These oceanic and continental crustal components were stirred in different proportions in the convective mantle, originating pyroxenites with a more marked geochemical imprint of either oceanic (Group A) or continental lower crust (Group B), or hybrid compositions (Group C). The pyroxenite protoliths likely underwent several melting events, one of them related to the formation of the subcontinental lithospheric mantle and continental crust, generating restitic UHP garnet pyroxenites now preserved in the Ronda and Beni Bousera orogenic peridotites. The extent of melting was mostly controlled by the bulk Mg-number (Mg#) of the pyroxenite protoliths, where protoliths with low Mg# experienced higher degrees of partial melting than sources with higher Mg#. Positive Eu and Sr anomalies in bulk rocks, indicative of their origin from cumulitic crustal gabbros, are preserved mostly in high Mg# pyroxenites due to their higher melting temperatures and consequent lower partial melting degrees. The results of this study show that the genesis of UHP garnet pyroxenites in orogenic peridotites requires a new recipe for the marble cake mantle hypothesis, combining significant recycling and stirring of both oceanic and continental lower crust in the Earth’s mantle. Furthermore, this study establishes a firm connection between the isotopic signatures of UHP pyroxenite heterogeneities in the mantle and the continental lower crust. [ABSTRACT FROM AUTHOR]

We present new Sr-Nd-Pb isotope data of the western Alpujárride metamorphic basement and the pre-Miocene Flysch sediments of the Betic Cordillera (southern Spain). Nd model ages are consistent with an increasing detrital input from the Alborán domain to the Flysch Trough in the western Mediterranean during the late Oligocene. The Alpujárride metamorphic crustal rocks derived from Archean-Paleoproterozoic terranes located along the northern margin of Gondwana in the Neoproterozoic. The heterogeneous isotopic signatures of the Alpujárride units indicate that they have different sedimentary protoliths and underwent contrasted Variscan and pre-Variscan tectono-magmatic evolutions. Melts/fluids derived from the western Alpujárride gneisses contaminated the mantle source of the Ronda high-Mg pyroxenite dykes, implying that the Alpujárride lower crust underthrusted the subcontinental lithospheric mantle of the Alborán domain generating subduction-like magmatism in the late Oligocene. The western Alpujárride upper crust is involved in the Neogene volcanism of the Alborán Sea basin, but only contaminated some LREE-enriched calc-alkaline lavas erupted along the continental margins. On the other hand, tholeiitic lavas in the center of the basin show no isotopic evidence of crustal assimilation. This indicates that most of the crust in the central Alborán Sea accreted by Miocene tholeiitic magmatism and that Alpujárride lower crust is absent and likely foundered close to the continental margins of the basin. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

On the basis of their structural and compositional characteristics, mafic rocks in the Ronda peridotite are classified into four groups: group A (garnet-bearing rocks) occurring in the spinel tectonite domain; group B (spinel websterites) along the recrystallization front; group C (Ti-rich spinel pyroxenites) in the granular tectonite and plagioclase tectonite domains; and group D (Cr-rich pyroxenites) as composite layers along the recrystallization front, dykes in the spinel tectonites, and single layers in the other domains. The spinel tectonite domain and group A layers represent the vestiges of an old, veined subcontinental lithosphere. The other mafic groups were formed during a later magmatic event that originated the present petrological zoning of the massif, and, in most cases, have replaced older mafic layers (groups B and D) or peridotites (group C). The magmatic event that led to the diversity and zoning of mafic layers was caused by melting of the base of thinned subcontinental lithosphere by upwelling asthenosphere, followed by infiltration of asthenospheric melts. The different groups of mafic layers record several stages of this event: (1) in the thermal climax, group B was formed at the expense of group A by melt-rock reactions at increasing melt mass. This stage was coeval with partial melting of the host lithospheric peridotites and the development of the recrystallization front; (2) upon cooling of the lithosphere, group C was formed by near-solidus, melt-consuming reactions between melt and peridotite at high melt/rock ratios. Group C parental melts were mixtures of partial melts of lithospheric peridotites and garnet pyroxenites, and alkaline melts infiltrated from the underlying asthenosphere; (3) in the waning stages of this magmatic event, group D was formed by magmatic replacement of group A, B and C layers by small-volume melt fractions that percolated pervasively throughout the massif and had a refractory, calc-alkaline character. Locally, these melts were tapped into cracks forming intrusive dykes. [ABSTRACT FROM PUBLISHER]

The Beni Bousera peridotite contains a diversity of pyroxenite layers. Several studies have postulated that at least some of them represent elongated strips of oceanic lithosphere recycled in the convective mantle. Some pyroxenites were, however, ascribed to igneous crystal segregation or melt–rock reactions. To further constrain the origin of these rocks, we collected 171 samples throughout the massif and examined their variability in relation with the tectono-metamorphic domains. A major finding is that all facies showing clear evidence for a crustal origin are concentrated in a narrow corridor of mylonitized peridotites, along the contact with granulitic country rocks. These peculiar facies were most likely incorporated at the mantle–crust boundary during the orogenic events that culminated in the peridotite exhumation. The other pyroxenites derive from a distinct protolith that was ubiquitous in the massif before its exhumation. They were deeply modified by partial melting and melt–rock reactions associated with lithospheric thinning. [ABSTRACT FROM AUTHOR]

Abstract: We combined microstructural observations and high-resolution crystallographic preferred orientation (CPO) mapping to unravel the active deformation mechanisms in garnet clinopyroxenites, garnet–spinel websterites, and spinel websterites from the Beni Bousera peridotite massif. All pyroxenites display microstructures recording plastic deformation by dislocation creep. Pyroxene CPOs are consistent with dominant slip on [001]{110} in clinopyroxene and on [001](100) or [001](010) in orthopyroxene. Garnet clinopyroxenites have however high recrystallized fractions and finer grain sizes than spinel websterites. Recrystallization mechanisms also differ: subgrain rotation dominates in garnet clinopyroxenites, whereas in spinel websterites nucleation and growth also contribute. Elongated shapes and strong intracrystalline misorientations suggest plastic deformation of garnet, but CPOs are weak. Clinopyroxene porphyroclasts in spinel websterites show deformation twins underlined by orthopyroxene exsolutions. Thermodynamic calculations indicate that garnet clinopyroxenites deformed at 2.0GPa and 950–1000°C and spinel pyroxenites at 1.8GPa and 1100–1150°C. The lower temperatures may explain the faster work rates implied by the finer grained microstructures in garnet clinopyroxenites. Greater stresses may have also reduced the competence contrast between garnet and pyroxene in the garnet pyroxenites and, at the outcrop scale, lowered the competence contrast between pyroxenites and peridotites, favoring mechanical dispersion of pyroxenites in the cooler lithospheric mantle. [Copyright &y& Elsevier]