Beni Bousera Massif Research Articles

P-T evolution of HP migmatitic paragneiss from the Beni Bousera metamorphic unit (internal Rif, Northern Morocco) constrained by phase equilibria modelling and melt reintegration

ABSTRACT The high-pressure (HP) migmatitic paragneiss of the Beni Bousera metamorphic unit (BBMU) from the Beni Bousera Massif underwent granulite-facies metamorphism. They mainly consist of garnet, biotite, kyanite, sillimanite, cordierite, spinel, quartz, plagioclase, K-feldspar, rutile, ilmenite and graphite. These minerals form a sequence of metamorphic assemblages corresponding to five metamorphic events (M1, M2, M3, M4, and M5). The first three events correspond to a latest Carboniferous-earliest Permian metamorphic evolution, while the last two are retrograde and Alpine in age. The p-T conditions of these metamorphic events were estimated using pseudosection calculations in the MnNCKFMASTHO system. Following prograde stage conditions of 9.3–10.9 kbar and 630–665°C (M1 stage), the p-T evolution culminates in melting of the paragneiss which occurred mainly at HP conditions, within the kyanite stability field, at 11.8–14.2 kbar and 820–845°C (M2 pressure-peak); anatexis then continued at high to medium pressure (HMP), within the sillimanite stability field, at 9.9–11 Kbar and 840–860°C (M3 thermal-peak), accompanied by the production of very small proportions of melt. The prograde p-T segment of the inferred clockwise path is characterized by a pressurization related to a post-Variscan collisional event triggered by the closure of the Paleotethys Ocean, and is followed by the exhumation of the granulitic unit to mid-crustal depth, with slight coeval heating. The retrograde Alpine evolution is coeval with the final exhumation of the migmatitic paragneiss and underlying peridotites during back-arc extension related to the westward retreat of the Alpine subduction. It occurred under sub-solidus conditions from 9.5–10.7 kbar and 818–848°C (M4) to 4.9–5.85 kbar and 710–785°C (M5) and corresponds to a near-isothermal and strongly decompressive metamorphic evolution (ΔP ≈ 5 Kbar).

Spinel chromianowy z dunitów masywu Inagli i ich termobarometria tlenowa (tarcza ałdańska, platforma syberyjska)

The Inagli dunite-peridotite-shonkinite zonal-ring intrusive with platinum-chromite mineralization is located on the Aldan shield of the Siberian Platform. Considering the structure, rock composition and ore mineralization, it is similar to the platinum-bearing zonal massifs of the "Ural-Alaskan" type, but this intrusive differs from the latter in its geological position. In order to clarify the physical and chemical conditions of formation of the Inagli massif, the mineral composition of rocks, especially сhromite-containing dunites, peridotites and shonkinites, as well as platinum-chromitite ore segregations, has been studied in detail. The rocks of the Inagli massif, from dunites to shonkinites, including peridotites, clinopyroxenites, and alkaline syenites, form a single continuous series. This is confirmed by a clear dependence of the composition of olivine, pyroxene, phlogopite and chromian spinel on the content of MgO in rocks. They were formed from the initial high-potassium picrite melt, which, during rising, underwent gradual decompression solidification and formed a cylindrical diapir-like body at the near-surface level in the Early Cretaceous. This occurred as a result of subduction processes related to the formation of the Mongol-Okhotsk orogenic belt along the southern framing of the Siberian craton. The values of oxygen volatility (lgfO2) for dunites, peridotites, shonkinites, chromitites and olivine-chromite inclusions in the isoferroplatinum of the Inagli massif, calculated using the method of the olivine-spinel oxygen thermobarometer of Ballhaus-Berry-Green (BBG), form a single trend FMQ+(2-4) in the range 620-11400C, i.e. along the band by 2-4 units of lgfO2 exceeding the fayalite-magnetite-quartz (FMQ) buffer. Such a rather narrow range of variation in the values of O2 fugacity in a wide interval of temperature indicates good comparability and reliability of the data obtained. At the same time, there is a natural decrease in temperature intervals for the formation of olivine-chromite parageneses (in 0С): with isoferroplatinum – (1140-680); in chromitite segregations – (980-710); in dunites – (930-620); peridotites – (890-770) and shonkinites – (840-710). The results obtained almost completely coincide with the field of values for dunites and chromitites from the Platinum-bearing belt of the Urals, given by other researchers. In terms of redox parameters, platinum-bearing zonal ultramafic-mafic massifs of the Ural-Alaskan and Aldanian types are close to more oxidized peridotites with a long history in the lithosphere. They differ significantly from the peridotites of the Beni-Bousera massif, as well as abyssal peridotites of the oceanic ridge systems, and others, which are formed under more reduced conditions corresponding to the range between FMQ, carbon-oxygen-CO (ССО) and iron-wustite (IW) buffers.

Tectonometamorphic Evolution of the Migmatitic Paragneisses of the Filali Unit (Internal Rif, Morocco)

A lithosphere-scale extensional shear zone juxtaposes an underlying sub-continental peridotite body and overlying migmatitic paragneisses of the Filali unit in the Beni Bousera massif (Internal Rif, Morocco). Three stages are recognized in the metamorphic evolution of the aluminous paragneiss, marked by the chemical zoning of garnet porphyroblasts and the evolution of associated mineral assemblages characterized by the presence of kyanite and rutile (M1), sillimanite, k-feldspar and melt (M2), and cordierite (M3). Phase-equilibrium modeling (pseudosections) and multi-equilibrium thermobarometry point to P-T conditions of 7 kbar 750 °C and 3.5 kbar 685 °C for the M2 and M3 stages, respectively. M1 conditions of 9.3 kbar 660 °C were inferred using modeling after the reintegration of melt lost during M2 into the bulk composition. Published geochronological data suggest a Variscan age (250–340 Ma) for the M1 event, whereas M2 and M3 are Oligo-Miocene and related to the Alpine orogeny. The recorded sub-isothermal decompression is related to significant crustal attenuation in the Oligo-Miocene and is responsible for the juxtaposition of the hot asthenospheric mantle and the crustal units, causing the melting of the paragneiss. The exhumation of the gneisses by crustal extension is associated with the westward retreat of an Alpine subduction (slab rollback).

Neogene polyphase deformation related to the Alboran Basin evolution: new insights for the Beni Bousera massif (Internal Rif, Morocco)

Located in the Internal domain of the Rif belt, the Beni Bousera massif is characterized by a stack of peridotites and crustal metamorphic units. The massif is intruded by granitic dykes and affected by several normal ductile shear zones. Structural, petrological and 40Ar–39Ar dating analyses performed on these two elements highlight that (1) the granitic dykes are emplaced within major N70° to N140° trending normal faults and shear zones, resulted from an NNE-SSW extension (2) the Aaraben fault in its NE part is characterized by N70° to N150° trending ductile normal shear zones, resulted from a nearly N-S extension and (3) the age of this extensional event is comprised between 22 and 20 Ma. Available paleomagnetic data allow a restoration of the initial orientation of extension, which was nearly E-W contemporary with the Alboran Basin opening in back-arc context, during the Early Miocene. At the onset of the extension, the peridotites were somehow lying upon a partially melted continental crust, and exhumed during this event by the Aaraben Normal Shear Zone. Afterward, the Alboran Domain suffered several compressional events.

Process of serpentinization in the ultramafic massif of Beni Bousera (internal Rif, Morocco)

The Beni Bousera massif forms part of the Sebtide units in the internal Rif Mountain (Morocco). It is mainly composed of mantle peridotites surrounded by crustal metamorphic rocks (kinzigites, micaschists, and schists). The serpentinization affects all of peridotite massif to various degrees. Serpentinization is concentrated at the top of the peridotites, along the mylonitized zone, and in the NE part of the massif. It is manifested by the formation of mesh and hourglass textures along the tectonic foliation in the highly serpentinized peridotites; and brecciated texture in the least serpentinized peridotites. Pyroxene minerals are still intact hosting few serpentine veins. These petrographic features are consistent with the geochemical data, marked by the increasing of LOI and decreasing of MgO and FeO toward the top of the massif and Aaraben fault. The Raman characterization of serpentine with the brecciated mesh and hourglass textures correspond to lizardite type whereas the serpentine with the vein texture is formed by lizardite + chrysotile.

Oligo-Miocene thinning of the Beni Bousera peridotites and their Variscan crustal host rocks, Internal Rif, Morocco

Deciphering Variscan versus Alpine history in the internal Rif system is a key to constraining the tectonic evolution of the Alboran domain and hence the geodynamics of the western Mediterranean system during the Cenozoic. This study focuses on the evolution of the metamorphic envelope of the Beni Bousera massif and its relation to the underlying peridotites. Combining structural geology, metamorphic petrology, and LA-ICP-MS U-Th-Pb dating of monazite, this study contributes to the understanding of the tectonic history of the western internal Rif. The regional foliation (S2) is characterized by LP/HT mineral assemblages and obliterates a former foliation (S1) developed along a barrovian (MP/MT) metamorphic gradient. The dating of some metamorphic monazite grains from a micaschist and a migmatitic gneiss demonstrates that the crustal envelope of the peridotite recorded two distinct tectonometamorphic episodes. Data from monazite inclusions in S1 garnet suggest that the first event, D1, is older than 250-170 Ma and likely related to the Variscan collision, in agreement with the barrovian type of the metamorphic gradient. The second event, D2, is Alpine in age (at circa 21 Ma) and corresponds to a strong lithosphere thinning allowing subsequent subcontinental mantle exhumation. Such a tectonic context provides an explanation for the LP/HT metamorphic gradient that is recorded in the regional foliation of the western Betic-Rif system. This extension is probably related to a subduction slab roll-back in the western end of the Mediterranean realm during the Oligo-Miocene times. No evidences for a Tertiary HP/LT metamorphism have been identified in the studied area

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.

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.

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.

Paragenese a faible variance dans les metapelites de la serie de Filali (Rif interne marocain); description, interpretation et consequence geodynamique

Abstract The metamorphic series of Filali and the Beni Bousera massif represent the most metamorphic unit in the inner part of the Moroccan Rif. The Filali series is composed of micaschists, gneisses, migmatites and granulites wrapped around the ultramafic body of the Beni Bousera. The foliation is broadly coherent all over the massif although differences in lineation and fold axis have been found between the granulites and the micaschists-gneisses. Metamorphic grade increases continuously towards the ultramafic body; however, seven metamorphic zones which account for this increase can be defined: - chlorite zone: muscovite and chlorite underline the S1 schistosity refolded by S2. Small garnet is wrapped by or cut across S2; - chloritoid zone: chloritoid displays the same textures as chlorite and muscovite, underlining S1 and S2 schistosity. Chlorite reacts out to biotite; - staurolite+andalusite zone: staurolite, andalusite, cordierite and garnet have pre-, syn- and post- kinematic features with respect to S2. Garnet reacts out to muscovite, biotite and plagioclase; - staurolite+andalusite+kyanite zone: same textural relationships as in the previous zone. Kyanite is an additional phase, often displaying epitaxial textures with staurolite and andalusite; - andalusite+kyanite+sillimanite zone: fibrolitic sillimanite generally associated with biotite crystallizes from staurolite. However, sillimanite is also directly formed from andalusite and kyanite. As in the previous zone garnet reacts out to biotite, muscovite and plagioclase; - sillimanite+k-feldspar zone: muscovite reacts out to k-feldspar. Modes of biotite and sillimanite increase; - migmatites zone are composed by biotite, sillimanite, garnet, cordierite quartz and feldspars. Garnets do not display the reaction textures seen in the previous zones; - granulites are composed of a primary paragenesis with kyanite+k-feldspar+garnet+ or -biotite+plagioclase+rutile, partially obliterated by a secondary paragenesis with cordierite, sillimanite and spinel.

A major and trace element and strontium, neodymium, and osmium isotopic study of a thick pyroxenite layer from the Beni Bousera Ultramafic Complex of northern Morocco

Major and trace element concentrations and strontium, neodymium, and osmium isotopic compositions were determined for a series of samples from a thick, symmetrically zoned pyroxenite layer from the Beni Bousera massif. The two main rock types included in the layer, garnet pyroxenites and websterites, have distinct, well-defined elemental and isotopic compositions. The websterites are, in most respects, more refractory than the garnet pyroxenites, but, surprisingly, are more enriched in the highly incompatible elements. The observed characteristics can be explained by formation of the layer by crystal deposition along a magma conduit at medium to high pressures, given that the source and composition of the magma varies with time. Re-Os model ages for three samples of different lithology converge to about 1.3 b.y., which is interpreted as the age of formation of the layer. In contrast, Sm-Nd model ages from the layer are mutually conflicting or indeterminate, largely due to the similarity between the measured neodymium isotopic ratios and that of the depleted mantle. On the other hand, the Sm-Nd model age obtained from an extremely depleted peridotite ( 143Nd/ 144Nd = 0.51391) located about 30 m from the layer falls within error of the Re-Os ages, providing support for an extensive magmatic event at about that time. This same age was obtained by several techniques from the closely related Ronda Ultramafic Complex of southern Spain. This may suggest that the mantle lithosphere currently exposed along the southern margin of Spain and the northern margin of Morocco has been linked for over a billion years.

Diamonds, Eclogites, and the Oxidation State of the Earth's Mantle

The reaction dolomite + 2 coesite --><-- diopside + 2 diamond + 2O(2) defines the coexistence of diamond and carbonate in mantle eclogites. The oxygen fugacity of this reaction is approximately 1 log unit higher at a given temperature and pressure than the oxygen fugacities of the analogous reactions that govern the stability of diamond in peridotite. This difference allows diamond-bearing eclogite to coexist with peridotite containing carbonate or carbonate + diamond. This potential coexistence of diamond-bearing eclogite and carbonate-bearing peridotite can explain the presence of carbon-free peridotite interlayered with garnet pyroxenites that contain graphitized diamond in the Moroccan Beni Bousera massif at the Earth's surface and the preferential preservation of diamond-bearing eclogitic relative to peridotitic xenoliths in the Roberts Victor kimberlite.

Mapping present-day geochemical variations across the society hotspot

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.

Diversite des clinopyroxenites a grenat associees aux massifs ultramafiques orogeniques; eclogites, ariegites, griquaites et grospydites; une discussion

Abstract Chemical composition of rocks belonging to the garnet clinopyroxenite clan (eclogites, griquaites, grospydites) are plotted in the Al &amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; O &amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt; -FeO-MgO diagram. Compositions of eclogites from metamorphic series define a trend corresponding to liquids resulting from the low-pressure cotectic precipitation of ol+cpx+pl. These data support the igneous nature of the material and suggest that the original rocks experienced crystallization of the crust. Garnet clinopyroxenite inclusions from kimberlites and alkali basalts as well as garnet clinopyroxenite lenses from high temperature peridotite bodies define a different trend that can be interpreted as resulting from cpx+gt precipitation from a basic melt. The unusual position of these latter rocks justify that the term "griquaite" be applied to them. Corundum and/or kyanite bearing eclogites from metamorphic series as well as "grospydites" included in kimberlites or alkali basalts, or found as lenses in ultramafic bodies), plot toward the Al-rich apex of the Al-F-M diagram, but show different SiO &amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; /Al &amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; O &amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt; : the term "grospydite" appears fully justified to describe the latter, and distinguish them from the former (metamorphic) ones. The origin of grospydites is still in debate: they may be the products of high pressure crystallization of basic liquids. or of metasomatic crystallization of gabbros recycled within the mantle. The coexistence within the Beni Bousera massif in northern Morocco of griquaites and grospydites indicates the large heterogeneity of this upper mantle fragment. This heterogeneity can only be explained by a complex geodynamic evolution characterized by either 1) the succession of two distinct episodes of basic melt production; or 2) a cyclical mechanism implying the subduction in the mantle of a lithospheric fragment followed by the adiabatic ascension of this fragment towards the surface.