Beni Bousera Research Articles

Structure and peridotite of Gibraltar arc southern bloc: gravimetric and aeromagnetic evidences

The southern Gibraltar Arc’s bloc (Rif’s belt) has a complex geological setting including peridotites outcropping in the Rif and Betic. They are among the largest infracontinental mantle rock outcrops worldwide. In spite of several geological studies, the deep structure of this area remains poorly defined. The aim of this work is giving a reliable geometry of the major geological structures of the Rif, especially the peridotite bodies and faults or geological contacts. The results obtained from a multiscale analysis of geological contacts applied to gravity and aeromagnetic data were used to generate a structural map of the studied area. This method in association with the Euler method allowed estimating the strike direction, shape, dip, and depth of contacts. The map gives a model of the region and confirms the existence of several faults, localized or inferred, from former geological studies. This study suggests a large extension of the Beni Bousera peridotites on the offshore from Jebha to Ceuta, and highlights an extended anomaly related to deeper crops probably ophiolite remnant of the suture zone.

The helium flux from the continents and ubiquity of low-3He/4He recycled crust and lithosphere

New helium isotope and trace-element abundance data are reported for pyroxenites and eclogites from South Africa, Siberia, and the Beni Bousera Massif, Morocco that are widely interpreted to form from recycled oceanic crustal protoliths. The first He isotope data are also presented for Archaean peridotites from the Kaapvaal (South Africa), Slave (Canada), and Siberian cratons, along with recently emplaced off-craton peridotite xenoliths from Kilbourne Hole, San Carlos (USA) and Vitim (Siberia), to complement existing 3He/4He values obtained for continental and oceanic peridotites.Helium isotope compositions of peridotite xenoliths vary from 7.3 to 9.6RA in recently (<10kyr) emplaced xenoliths, to 0.05RA in olivine from cratonic peridotite xenoliths of the 1179Ma Premier kimberlite, South Africa. The helium isotope compositions of the peridotites can be explained through progressive sampling of 4He produced from radiogenic decay of U and Th in the mineral lattice in the older emplaced peridotite xenoliths. Ingrowth of 4He is consistent with generally higher 4He concentrations measured in olivine from older emplaced peridotite xenoliths relative to those from younger peridotite xenoliths. Collectively, the new data are consistent with pervasive open-system behaviour of He in peridotite xenoliths from cratons, mobile belts and tectonically-active regions. However, there is probable bias in the estimate of the helium isotope composition of the continental lithospheric mantle (6.1±2.1RA), since previously published databases were largely derived from peridotite xenoliths from non-cratonic lithosphere, or phenocrysts/xenocrysts obtained within continental intraplate alkaline volcanics that contain a contribution from asthenospheric sources. Using the new He isotope data for cratonic peridotites and assuming that significant portions (>50%) of the Archaean and Proterozoic continental lithospheric mantle are stable and unaffected by melt or fluid infiltration on geological timescales (>0.1Ga), and that U and Th contents vary between cratonic lithosphere and non-cratonic lithosphere, calculations yield a 3He flux of 0.25–2.2atoms/s/cm2 for the continental lithospheric mantle. These estimates differ by a factor of ten from non-cratonic lithospheric mantle and are closer to the observed 3He flux from the continents (<1atoms/s/cm2).Pyroxenites and eclogites from the continental regions are all characterized by 3He/4He (0.03–5.6RA) less than the depleted upper mantle, and relatively high U and Th contents. Together with oceanic and continental lithospheric peridotites, these materials represent reservoirs with low time-integrated 3He/(U+Th) in the mantle. Pyroxenites and eclogites are also characterized by higher Fe/Mg, more radiogenic Os–Pb isotope compositions, and more variable δ18O values (∼3‰ to 7‰), compared with peridotitic mantle. These xenoliths are widely interpreted to be the metamorphic/metasomatic equivalents of recycled oceanic crustal protoliths. The low-3He/4He values of these reservoirs and their distinctive compositions make them probable end-members to explain the compositions of some low-3He/4He OIB, and provide an explanation for the low-3He/4He measured in most HIMU lavas. Continental lithospheric mantle and recycled oceanic crust protoliths are not reservoirs for high-3He/4He and so alternative, volumetrically significant, He-rich reservoirs, such as less-degassed (lower?) mantle, are required to explain high-3He/4He signatures measured in some intraplate lavas. Recycling of oceanic crust represents a fundamental process for the generation of radiogenic noble gases in the mantle, and can therefore be used effectively as tracers for volatile recycling.

Vegetation of Beni Bousera (northern Morocco) ultramafic soils and adjacent non-ultramafic soils in relation to edaphic factors

Vegetation and soil sampling were undertaken in 47 relevés in two different sites in Beni Bousera (northern Morocco), 32 relevés were on ultramafic sites (peridotite) and 15 were on adjacent non-ultramafic soils (mica-schist). Soil composition of exchangeable elements (nickel, calcium, magnesium, calcium : magnesium ratio, iron, copper) and physicochemical characteristics (e.g. pH, cation exchange capacity, conductivity, carbonate) were investigated and the species composition and cover were recorded. Although there was a significant difference between the two types of soil shown by Student’s t-test and principal component analysis, the ‘serpentine factor’ can be considered moderate on the basis of the observed heavy metal concentrations and of the related calcium : magnesium ratio. Correspondence and canonical analysis were used to detect the principal factors associated with gradients in species composition. This analysis showed a clear differentiation of vegetation among the plant communities. Preferential or strict serpentinophyte species were recognised, whereas no hyperaccumulator species have yet been found.

New constraints on the evolution of the Gibraltar Arc from palaeomagnetic data of the Ceuta and Beni Bousera peridotites (Rif, northern Africa)

The Betic Cordillera and the Moroccan Rif together form one of the smallest and tightest orogenic arcs on Earth and almost completely close the Mediterranean to the west. For the explanation of the geodynamic evolution of the mountain belt, palaeomagnetic data that generally found clockwise block rotations in the Iberian and anticlockwise rotations in the Moroccan part of the mountain belt, have played a key role in recent works. This palaeomagnetic study has found new constraints on the rotations and timing of the peridotitic bodies outcropping in the key position at the westernmost margin of the mountain belt, in Ceuta and Beni Bousera (Rif, northern Africa).Detailed thermal demagnetization of 115 individually oriented samples from 14 sites was combined with rock magnetic and scanning electron microscopic experiments to analyze the magnetic mineralogy responsible for the remanences and the mechanisms and relative times of their acquisition. In Ceuta, up to three magnetic components, and in Beni Bousera, up to two magnetic components have been found, that are all to be interpreted as chemical remanent magnetizations (CRM). The data suggests the following succession of geodynamic events affecting the peridotites until recent times: (1) after their exhumation and subsequent cooling about 20Ma ago, they recorded a characteristic remanent magnetization of both normal and reversed polarities, carried by (pseudo-)single-domain magnetite grains; (2) after their dismembering, the Ceuta peridotites were tilted southward by 22–34° about a horizontal or tilted axis (up to plunge 50°) with an azimuth of 72–145° and the Beni Bousera peridotites were rotated anticlockwise by 72.3±12.1° about a vertical axis and (3) both recorded another magnetic signal of normal polarity only, carried by multi-domain magnetite grains; and finally (4) the Ceuta peridotites rotated anticlockwise by 19.7±5.9° about a vertical axis.This study provides the first palaeomagnetic data for the Ceuta peridotites that, with their tilt and recent small net rotation, had a distinct geodynamic evolution from the large net rotations about vertical axes in Beni Bousera and Ronda (Betic Cordillera). Moreover, earlier palaemagnetic data for Beni Bousera is improved, as mixed polarities have been found in the older of the remanences for the first time, and its interpretation as a CRM changes the rotation timing that was proposed previously. The sequence of events exposed in this work are important constraints that need to be incorporated in any geodynamic model of the evolution of the Betic–Rifean mountain belt.

Amphibole genesis in pyroxenites from the Beni Bousera peridotite massif (Rif, Morocco): Evidence for two different metasomatic episodes

Abstract The presence of variable amounts of amphibole ± phlogopite in a garnet websterite and a garnet clinopyroxenite from the Beni Bousera peridotite massif provides evidence for post-formation metasomatism. Textural observations associated with major- and trace-element mineral compositions allowed us to distinguish two metasomatic episodes, which occurred at different stages of the Beni Bousera massif evolution. The garnet websterite has recorded interaction with LREE-rich silicate melts before the uplift of the massif. Amphibole/clinopyroxene and amphibole/garnet trace-element ratios closely approach partition coefficient values, indicating that chemical equilibrium was attained between amphibole and pyroxenite matrix minerals. The geochemical signatures of the putative alkaline interacting melts are similar to those of recent basaltic magmas erupted in Morocco, suggesting a common peridotite mantle source. In contrast, amphibole from the garnet clinopyroxenite is in chemical disequilibrium with the pyroxenite matrix minerals. In this clinopyroxenite the crystallization of amphibole and plagioclase occurred at lower T (and P) conditions, most probably during the ascent of the Beni Bousera massif and its emplacement into the crust. The melt responsible for this later metasomatic episode was LREE-depleted and HREE-enriched, suggesting that it resulted from decompression melting of a garnet-bearing source (with garnet as a melting phase), similar to the garnet-bearing pyroxenites outcropping in the Beni Bousera massif.

Crustal attenuation as a tracer for the emplacement of the Beni Bousera ultramafic massif (Betico-Rifean belt)

The study of petrology (fieldwork, petrography, and phase diagram modeling) and structural data of the metapelitic granulites and the southern, high-temperature exposed peridotites in the Beni Bousera massif (northern Morocco), combined with results from previous regional studies of the Alboran, suggest a new emplacement mechanism for the mantle rocks in the Betico-Rifean belt. We document two key metamorphic episodes in the granulites within a temperature window of 710–830 ± 50 °C: (1) An earlier prograde high-pressure period (from 9 ± 1.0 to 12 ± 1.0 kbar) characterized by the assemblage garnet + biotite + kyanite + K-feldspar + rutile. Pressure differences of ~3 kbar are found over a continuous crustal section of ~1.5 km of exposed granulites that indicate a significant crustal attenuation during exhumation of the ultramafic rocks; and (2) a later post-kinematic low-pressure (5 ± 0.8 kbar) symplectic assemblage of cordierite + spinel + plagioclase + sillimanite. At the scale of the entire Betico-Rifean belt, two main contacts are observed as mirror images in both sides of the Alboran Sea: (1) the long axis of the high-temperature ductile contact between granulites and peridotites occurs in the west side of the Beni Bousera and Ronda massifs, coupled with (2) the consistent high-angle, east-dipping normal fault in the east parts of the belt massifs. The integration of the petrologic results with information on the rotation of both contacts reveals ductile deformation in the lower crust related to the emplacement of the ultramafic rocks in the Betico-Rifean belt along deep-reaching normal faults. The presence of the early high-temperature contact suggests that it was originally a shallow, west-dipping detachment fault developed in a back-arc environment of the east-dipping, retreating subduction zone (current western part of the Gibraltar arc). This scenario is in concordance with the tectonic evolution in western Italy, where anticlockwise Pleistocene rotations associated with northeast-directed thrusting in the Apennines—and coeval with the southeastward motion in the Calabria-Peloritani terrane—were triggered by retreat and rollback of the Adriatic-Ionian slab toward the southeast during the northwest-directed subduction beneath the Calabrian arc.

The Beni Bousera Peridotite (Rif Belt, Morocco): an Oblique-slip Low-angle Shear Zone Thinning the Subcontinental Mantle Lithosphere

Detailed structural and petrological mapping in the Beni Bousera peridotite (Rif Belt, northern Morocco) shows that this orogenic peridotite massif is composed of four tectono-metamorphic domains with consistent kinematics, marked by a pervasive, shallowly dipping foliation with a NW-SE stretching lineation that progressively rotates towards a NNE-SSW orientation in the lowermost part of the massif. From top to bottom, these domains are garnet-spinel mylonites, Ariegite subfacies fine-grained porphyroclastic spinel peridotites, Ariegite-Seiland subfacies porphyroclastic, and Seiland subfacies coarse-porphyroclastic to coarse-granular spinel peridotites. Microstructures and crystal preferred orientations point to deformation dominantly by dislocation creep in all domains, but the continuous increase in average olivine grain size indicates decreasing plastic work rates from top to bottom. This evolution in deformation conditions is consistent with the change in synkinematic pressure and temperature conditions, from 900°C at 2*0 GPa in garnet-spinel mylonites to 1150°C at 1*8 GPa in the Seiland domain. A pervasive diffuse dunitic-websteritic layering subparallel to the foliation suggests deformation in the presence of melt in the Seiland domain. Gravitational instabilities owing to local melt accumulation may account for <200 m wide areas exhibiting a vertical lineation in this domain. To account for the consistent kinematics and the tectono-metamorphic evolution, which implies a temperature gradient of c. 125°C km−1 preserved across the Beni Bousera massif, we propose that the entire massif records the functioning of a low-angle shear zone, a few kilometres wide, which accommodated exhumation of the base of the lithosphere from ∼90 to ∼60 km depth. Partial melting in the Seiland domain may be explained by fast decompression of the footwall, without the need for exotic heat sources. Moreover, if the present-day orientation of the shear zone is similar to that when it was active in the mantle, the stretching lineations at high angle to the metamorphic gradient imply that shearing parallel to the trend of the belt accompanied thinning; that is, a transtensional deformation of the margin.

Metamorphic and magmatic overprint of garnet pyroxenites from the Beni Bousera massif (northern Morocco): Petrography, mineral chemistry and thermobarometry

A detailed mineralogical and textural study of two pyroxenites from the Beni Bousera massif, a garnet clinopyroxenite (GP) and a garnet clinopyroxenite containing graphite pseudomorphs after diamond (GGP), indicates a strong metamorphic overprint associated with the massif exhumation. In both pyroxenites, the primary assemblage [Cpx(I)+Gt(I)±Opx] originally stable with diamond, records temperatures in excess of 1200°C. Along the exhumation path, Cpx(I) has decomposed according to the reaction:CpxI=0.55–0.60CpxII+0.30–0.35OpxII+0.03–0.05GtII+0.03–0.5Pl.This breakdown reaction occurred under sub-solidus conditions in at least two stages, which first led to Cpx and Opx exsolution lamellae and second to garnet crystallization at the expense of the newly formed Cpx lamellae. Successive garnet generations, Gt(II), can be distinguished by their REE content; primary garnets Gt(I) have the highest HREE content. Secondary garnets, Gt(II), have grown in the 850–950°C temperature range. We show that these conditions are below the blocking temperature of Mg–Fe exchange between garnet and pyroxene (ca. 1050°C) and above the blocking temperature of Mg–Fe interdiffusion in garnet. Consequently, the original composition of the secondary garnet has not been modified upon further cooling; equilibrium with the appropriate pyroxene can be used to retrieve meaningful P–T conditions. This is however not the case for pairs of pyroxene inclusion and their porphyroclastic garnet host, formed earlier in the Beni Bousera history (i.e. at higher grade) and for which decoupling between Al diffusion and (Mg, Fe) interdiffusion (in pyroxene) has occurred. In that particular case of decoupling, conventional thermometry which yield apparent conditions around 2GPa and 1000–1100°C based on Al-content in pyroxene on one hand and Mg–Fe partitioning between the pyroxenite silicates on the other, will intrinsically fail at providing reliable P and T estimates. Detailed chemical inspection of Gt–Cpx–Opx inclusions in the graphite pseudomorphs indicates that (1) these silicates are genetically related to the same minerals in the bulk rocks and (2) they were chemically isolated from the bulk rock (included in diamond and/or graphite). Finally, the pyroxenite samples from Beni Bousera have recorded two major events pertaining to the late evolution of this massif: (1) the increase of the water activity in GP as shown by the crystallization of pargasitic amphibole which suggests the proximity to crustal units, and (2) a temperature increase up to ca. 1050°C, which led to water-present partial melting of both GP and GGP.

Partition Coefficients of Platinum Group and Chalcophile Elements Between Arsenide and Sulfide Phases as Determined in the Beni Bousera Cr-Ni Mineralization (North Morocco)

Abstract The partition coefficients of platinum group elements (PGE) and chalcophile elements Au, Re, Ag, Se, Bi, Te, and Sb, between arsenide and sulfide phases (DAs/sulf) have been estimated by measuring in situ concentrations of these elements using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) in coexisting arsenide and sulfide minerals from the Beni Bousera Cr-Ni mineralization (North Morocco). Previous experimental studies and observations on the distribution of PGE in a number of As-rich, Ni-Cu-PGE ore deposits have shown that arsenide minerals may play an important role controlling the distribution of these metals in magmatic sulfide systems. However to date, there is no comprehensive study quantifying the partitioning behavior of these elements when arsenide minerals crystallize either directly from a sulfide melt or from an arsenide melt previously segregated by immiscibility from a sulfide melt. The Beni Bousera mineralization represents an excellent natural laboratory to evaluate these partition coefficients because maucherite (Ni11As8) coexists in equilibrium with pyrrhotite, pentlandite, and chalcopyrite in form of globules mostly associated with pyrrhotite, and arsenide and sulfide minerals account for the bulk of the PGE (with the exception of Pt) and chalcophile elements in the samples. The laser ablation analyses reveal that maucherite is strongly enriched in all chalcophile elements, except Se, relative to sulfide minerals. The calculated DPGEAs/sulf are the following: DIrAs/sulf = 920 DRhAs/sulf = 620, DPtAs/sulf = 330, DPdAs/sulf = 250, DOsAs/sulf = 140, and DRuAs/sulf = 50. For the rest of elements, the obtained values are the following: DSbAs/sulf = 890, DTeAs/sulf = 190, DBiAs/sulf = 50, DReAs/sulf = 6, DAuAs/sulf = 310, DAgAs/sulf = 4, and DSeAs/sulf = 0.6. These results clearly highlight the strong affinity of PGE for arsenide phases and the importance of these phases as potential carriers of PGE in Ni-Cu-PGE ore deposits.

Crustal xenoliths from Tallante (Betic Cordillera, Spain): insights into the crust–mantle boundary

AbstractThe volcano of Tallante (Pliocene) in the Betic Cordillera (Spain) exhumed a heterogeneous xenolith association, including ultramafic mantle rocks and diverse crustal lithologies. The latter include metagabbroids and felsic rocks characterized by quartz-rich parageneses containing spinel ± garnet ± sillimanite ± feldspars. Pressure–temperature estimates for felsic xenoliths overlap (at 0.7–0.8 GPa) those recorded by the mantle-derived peridotite xenoliths. Therefore, we propose that an intimate association of interlayered crust and mantle lithologies characterizes the crust–mantle boundary in this area. This scenario conforms to evidence provided by the neighbouring massifs of Ronda and Beni Bousera (and by other peri-Mediterranean deep crust/mantle sections) where exhumation of fossil crust–mantle boundary reveals that this boundary is not sharp. The results are discussed on the basis of recent geophysical and petrological studies emphasizing that in non-cratonic regions the crust–mantle boundary is often characterized by a gradational nature showing inter-fingering of heterogeneous lithologies. Silica-rich melts formed within the crustal domains intruded the surrounding mantle and induced metasomatism. The resulting hybrid crust–mantle domains thus provide suitable sources for exotic magma types such as the Mediterranean lamproites.

Fission track thermochronology of the Beni Bousera peridotite massif (Internal Rif, Morocco) and the exhumation of ultramafic rocks in the Gibraltar Arc

The Beni Bousera peridotite massif and its metamorphic surrounding rocks have been analyzed by the fission track (FT) method. The aim was to determine the cooling and uplift history of these mantle and associated crustal rocks after the last major metamorphic event that dates back to the Lower Miocene–Upper Oligocene time (∼22–24 Ma). The zircon FT analyses give an average cooling—i.e., below 320 °C—age of ∼19.5 Ma. In addition, the apatite FT data give an average cooling—i.e., below 110 °C—age of ∼15.5 Ma. Taking into account the thermal properties of the different thermochronological systems used in this work, we have estimated a rate of cooling close to 50 °C/Ma. This cooling rate constrains a denudation rate of about ∼2 mm year−1 from 20 to 15 Ma. These results are similar to those determined in the Ronda peridotite massif of the Betic Cordilleras documenting that some ultrabasic massifs of the internal zones of the two segments of the Gibraltar Arc have a similar evolution. However, Burdigalian sediments occur along the Betic segment (Alozaina area, western Betic segment) unconformably overlying peridotite. At this site, ultramafic rock was exposed to weathering at ages ranging from 20.43 to 15.97 Ma. Since the Beni Bousera peridotite was still at depth until 15.5 Ma, we infer that no simple age projection from massif to massif is possible along the Gibraltar Arc. Moreover, the confined fission track lengths data reveal that a light warming (∼100 °C) has reheated the massif during the Late Miocene before the Pliocene–Quaternary tectonic uplift.

Deformation processes and rheology of pyroxenites under lithospheric mantle conditions

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.

Petrogenesis of Pyroxenites and Melt Infiltrations in the Ultramafic Complex of Beni Bousera, Northern Morocco

Petrogenesis of Pyroxenites and Melt Infiltrations in the Ultramafic Complex of Beni Bousera, Northern Morocco

Melting textures and microdiamonds preserved in graphite pseudomorphs from the Beni Bousera peridotite massif, Morocco

Over 30 graphite aggregates that represent pseudomorphs after diamond were manually extracted from a garnet pyroxenite layer in the Beni Bousera peridotite massif, northern Morocco. The inclusions present in the aggregates were characterized by combining scanning electron microscopy, micro-Raman spectroscopy and cathodoluminescence. Large composite clinopyroxene–orthopyroxene–garnet inclusions ( ca . 500 μm across) are common in the core of the graphite aggregates. Silicate films with a thickness of a few micrometres occur intercalated between graphite flakes throughout each aggregate. They are of basaltic composition and are interpreted as partial melts formed by in situ melting of the large composite inclusions and, possibly, of the host pyroxenite, during the Beni Bousera massif uplift. In addition, various solid inclusions composed of chlorides, sulphates and carbonates are found to be evenly distributed irrespective of the graphite aggregate texture (coarse in the core, in some instances fine-grained on the rim). Diamond crystals, 0.5–2 μm in size, were also observed in several aggregates, apparently included in large graphite flakes, and were characterized using cathodoluminescence and Raman micro-spectroscopies. They are interpreted as relics of large mantle-stage diamonds, now heavily graphitized. This finding confirms earlier propositions that the graphite aggregates in Beni Bousera and Ronda garnet pyroxenites are pseudomorphs after diamond and raises questions on the kinetics of graphitization.

Oligo-Miocene exhumation of the Beni-Bousera peridotite through a lithosphere-scale extensional shear zone

New structural data and P–T estimates of syn-deformational assemblages within the Beni Bousera peridotites and their crustal envelope are used to explain their Alpine exhumation. The Beni Bousera peridotites occur as thin sheets within high grade crustal units of the lower Sebtides (inner Rif, Morocco) and are composed of weakly deformed spinel lherzolite in the core of the massif and garnet-spinel mylonite at the rim. The main foliation trajectories in both the peridotites and overlying crustal units show systematic rotation towards their mutual contact, indicating a kilometer-scale top to the NW shearing with a dextral component along this crust/mantle contact. Widespread top to the NW shear criteria within the crustal units overlying the peridotite support this feature. Available ages constrain the development of the main foliation in both the peridotites and crustal rocks between 25 and 20 Ma. New P–T data from the peridotites show that deformation occurs during decompression from ≈ 22 kbar, 1050°C to ≈ 9-15 kbar, 800°C. As a consequence, exhumation of the Beni Bousera peridotites takes place during the Oligo-Miocene lithosphere thinning in the footwall of a lithospheric extensional shear zone. The exceptional preservation of garnet within the mylonitic peridotites results from rapid cooling of the border of the massif due to the juxtaposition with colder crustal rocks along this shear zone. Uplifting of the hot mantle rocks simultaneously induces high temperature metamorphism in the overlying crustal units. These new findings allow us to reconstruct the deformation history of the Beni Bousera region and the Alboran domain in the framework of the western Mediterranean geodynamics during the last 40 Myrs.

Melting textures and microdiamonds preserved in graphite pseudomorphs from the Beni Bousera peridotite massif, Morocco

Melting textures and microdiamonds preserved in graphite pseudomorphs from the Beni Bousera peridotite massif, Morocco

Timing and modes of granite magmatism in the core of the Alboran Domain, Rif chain, northern Morocco: Implications for the Alpine evolution of the western Mediterranean

[1] The Betic-Rif orogen forms the western termination of the Alpine orogenic system in the Mediterranean region. The precise timing, structural evolution, and distribution of high-grade metamorphic units (Alpine versus pre-Alpine) in the inner zones of the orogen (Alboran Domain) remain controversial issues. In this paper we report occurrence of distinct generations of peraluminous granitic bodies intruded within Beni Bousera peridotites and their amphibolite-to-granulite facies envelope, in the core of the Alboran Domain of the Rif chain (northern Morocco). These granitic bodies are central to the reconstruction of the high-grade evolution of the Alboran Domain because they provide first-order structural markers to assess the P-T-t deformation history of the high-grade terranes. Here we document the petrography and structural relationships with the host rocks and constrain the timing of granite emplacement using laser ablation–inductively coupled plasma–mass spectrometry U-Pb zircon and/or monazite dating, complemented by 40Ar/39Ar dating. The results indicate that granite emplacement occurred in two major episodes of anatectic magmatism, during the Hercynian (circa 300 Ma) and Alpine (circa 22 Ma) periods, respectively. These data (1) provide conclusive evidence for an important phase of Hercynian magmatism and high-grade metamorphism in the Alboran Domain and (2) permit a revaluation of the significance of the high-grade early Miocene event documented in the Alboran Domain in terms of a late stage, thermal pulse that reworked a polymetamorphic (Hercynian and Alpine) nappe pile. These results provide new constraints for construction of a feasible tectonometamorphic model for the Alpine evolution of the western Mediterranean.

The U, Th and Pb elemental and isotope compositions of mantle clinopyroxenes and their grain boundary contamination derived from leaching and digestion experiments

Minerals from peridotites are known to be affected by trace element contamination on their grain boundaries. In this contribution we investigate the extent and origin of exogenous contamination associated with mantle clinopyroxenes from various localities (Middle Atlas, Beni Bousera [Morocco], Pyrenean Massif, Massif Central [France]) and test the efficacy of different leaching methodologies used to remove this contamination. In doing so we present new U–Th–Pb (double-spike) isotope and trace elemental data of clinopyroxenes and their leachates from spinel-facies sub-continental lithospheric mantle (SCLM, n = 18). Sequential leaching and dissolution of one clinopyroxene separate shows that multiple and short leaching attacks with dilute HCl at moderate temperatures (e.g., 120 °C) interspersed with rigorous ultra-pure water washes do not induce elemental fractionation and are sufficient to remove grain-boundary contamination. Short attacks with very dilute mixtures of HF and HCl induced strong parent/daughter elemental fractionation and significant elemental loss prior to the clinopyroxene digestion with HF/HNO 3. Such leaching is not suitable for studies investigating parent/daughter elemental or isotope ratios of mantle clinopyroxenes. Fluoride co-precipitates that formed in the presence of moderately dilute HF, used during leaching and typical HF/HNO 3 digestions, are an important sink for all trace elements studied here and lock up at least 60% of the trace elements considered. A suite of clinopyroxene–leachate pairs from Moroccan peridotites confirm extreme grain-boundary contamination and show that up to 65% of U, 82% of Th and 91% of Pb, respectively, are of exogenous origin. Pb isotopes of all leachates considered and nearly all reconstructed unleached clinopyroxenes have highly positive Δ7/4 and 207Pb/ 204Pb– 206Pb/ 204Pb systematics reminiscent of enriched mantle (particularly EM II), whereas the corresponding extensively leached clinopyroxenes have very different Pb isotope systematics (verified by replicate digestions). Furthermore, the leachates are often marked by remarkably high 232Th/ 238U (leachate ≫ bulk silicate Earth) in addition to 238U/ 204Pb and 232Th/ 204Pb that are seemingly consistent with depleted mantle (DM) and high 238U/ 204Pb (HIMU) mantle mixing. After leaching, clinopyroxenes may have highly elevated 238U/ 204Pb and 232Th/ 204Pb that are distinct from typical DM- and HIMU mantle, whereas corresponding leachates bear strong resemblance to anthropogenic Pb recovered from rainwater, snow and aerosols. In the light of the great potential of Pb isotope contamination associated with mantle clinopyroxene we have compiled literature data of mantle peridotites. These data seem to suggest that the SCLM is nearly devoid of HIMU signatures but ostensibly represents an important repository for enriched mantle signatures. However, the dominance of enriched mantle is not verified when 87Sr/ 86Sr and 208Pb/ 204Pb ratios for the same database are considered. We highlight that the scarcity of HIMU-like SCLM and the alleged dominance of EM in continental roots may be reconciled if the current Pb isotope database of SCLM contains samples that were insufficiently leached prior to digestion and thus are contaminated by anthropogenic Pb. We stress the importance of replicate digestions, adequate leaching and leachate analysis to verify the U–Th–Pb elemental and isotope systematics of any given mantle (-derived) sample.

Origin and significance of spinel and garnet pyroxenites in the shallow lithospheric mantle: Ultramafic massifs in orogenic belts in Western Europe and NW Africa

Anhydrous spinel and garnet pyroxenites form a small (< 10%) but integral part of the shallow sub-continental lithospheric mantle. They occur as layers or dykes within tectonically-emplaced ultramafic massifs and display lithologies that range from orthopyroxenite through websterites to clinopyroxenite, with or without olivine. The ultramafic massifs in orogenic belts of western Europe and Morocco contain many different varieties of mantle pyroxenites that show a wide range of major and trace element compositions. Their bulk compositions tend to vary between at least three end-members: two of these are similar to the consitutent clinopyroxene and orthopyroxene, while the third component has concentrations of MgO, CaO, Al 2O 3, TiO 2 and Na 2O that are close to basaltic levels and may indicate the former presence of trapped basaltic melt. These pyroxenites are probably cumulates in the broadest sense. Nevertheless, some pyroxenites from Beni Bousera and Ronda have whole rock compositions that are similar to that of bulk oceanic crust. Most mantle pyroxenites have LREE-depleted whole-rock REE patterns, although with lower absolute concentrations than MORB or bulk oceanic crust, and most lack Eu anomalies. Trace element patterns in clinopyroxenes from pyroxenites are also usually LREE-depleted. Most clinopyroxenes show depletions in Zr, Hf and Sr, although those from the Cabo Ortegal complex (NW Spain) display LREE-enrichment and significant Sr peaks, attributed to the influence of subduction-related fluids or melts. δ 18O values for clinopyroxenes from mantle pyroxenites in ultramafic massifs tend to be in the range 5.0–5.8‰, i.e. typical mantle values. Rare garnet pyroxenite layers from a few massifs show higher values that reflect recycling of a crustal component. Similarly, most Sr, Nd and Pb isotopic ratios from pyroxenites fall in the depleted mantle field, although with a slightly wider range of values than is typical for shallow sub-continental mantle peridotites from Europe. Most mantle pyroxenites are best explained as products of crystal accumulation of mantle-derived magmas, together with variable amounts of trapped interstitial magma. Others may be the products of interaction between magma and peridotite or older pyroxenite wall-rock. Only samples from a few massifs (e.g. garnet pyroxenites from Beni Bousera and Ronda) display strong evidence of being related to recycled oceanic crust or melts of oceanic crust. Even fewer mantle pyroxenites show good evidence for derivation either by incipient melting of the host peridotite or by metamorphic segregation. Pyroxenites are low-MgO, high CaO, high Al 2O 3 lithologies that provide a well-documented example of enrichment of the lithospheric mantle. If such material were recycled into the asthenosphere, mantle pyroxenites might melt preferentially to the host peridotite. However, the precise role of mantle pyroxenites in magmagenesis could be difficult to track, as many of their isotope and trace element compositions (e.g Sm/Nd ratios) are often similar to those of their host mantle peridotites.

Origin of eclogite and garnet pyroxenite from the Moldanubian Zone of the Bohemian Massif, Czech Republic and its implication to other mafic layers embedded in orogenic peridotites

The granulite terrane of the Czech part of the Gfohl unit includes numerous small bodies of mantle derived peridotite, some of which contain layers or lenses of eclogite and garnet pyroxenite. These eclogitic rocks have generally been considered to be high-pressure crystal cumulates formed in the upper mantle. We present new analyses of whole-rock major and trace element contents for three kynanite-quartz eclogite samples taken from the Nove Dvory garnet peridotite body. Integrating these data with previously published analyses from the literature on eclogitic rocks from this terrane, we demonstrate that a magnesian group of eclogites, including these three new samples, were originally formed as cumulus gabbros, which were later transformed to eclogites in the mantle. A gabbroic origin for some mafic layers (Type II) has been advocated for other orogenic peridotites, such as Beni Bousera (Morocco), Ronda (Spain), and Horoman (Japan). By comparing these sets of data with those from the Bohemian Massif, we propose a simple method of identifying groups of metagabbros by utilizing MgO-normalization in oxide ratio plots for whole-rock major element analyses.