Spinel Websterites Research Articles

Diversity of origin and geodynamic evolution of the mantle beneath the Variscan Orogen indicating rapid exhumation within subduction-related mélange (Moldanubian Zone, Bohemian Massif)

A variable assemblage of ultrabasic rocks along with minor eclogites investigated on a small area of a few km2 (Gföhl unit, Moldanubian Zone of the Bohemian Massif) reflects incorporation of contrasting mantle domains within a subduction-related tectonic mélange during the Variscan orogeny. Based on mineral composition, whole-rock chemistry, isotopic signatures, and pressure-temperature (P–T) estimates, four principal lithological types have been distinguished (1) spinel harzburgite (2) garnet lherzolite (3) spinel websterite and (4) eclogite. Spinel harzburgite, exclusively associated with HT migmatized gneisses, corresponds to the refractory oceanic lithosphere, as demonstrated by whole-rock composition (low Al2O3, CaO), chemistry of spinel (Cr# <0.4) and Al-rich pyroxene (Al2O3 up to 9 wt%). By contrast, garnet lherzolite, predominantly enclosed in HP felsic granulite, represents a fragment of the subcontinental mantle wedge, considering the strong fertile character (high Al2O3, TiO2, Yb) and spinel chemistry (Cr# >0.5). Websterites likely represent products of decompression partial melting of the asthenospheric mantle with a variable input of crustal component, whereas eclogites correspond to HP crystal cumulates from partial melts migrating through the Variscan mantle wedge. Both peridotite protoliths experienced various degrees of secondary refertilization, recorded as a cryptic metasomatic overprint, due to interaction with subduction-related silicate melts, from which numerous websterite and rare eclogite layers crystallized. The secondary mantle refertilization via melt-peridotite reaction is well-documented by decreasing bulk Mg# along with MgO/SiO2, elevated Al2O3/SiO2, TiO2 and FeOtot contents, and isotopic composition (87Sr/86Sr338 ~ 0.7051). Moreover, the positive correlation between highly fluid-immobile incompatible trace elements (e.g. Ti, Sc, V, Zr, Yb) and distribution of REE and Li is consistent with the melt refertilization trend.

CORUNDUM-BEARING VEINS IN CHLORITITE FROM THE ETIROL-LEVAZ AUSTROALPINE CONTINENTAL SLICE (VALTOURNENCHE, AOSTA, ITALY)

The high-pressure (HP) continental Etirol-Levaz slice (ELS) is exposed on the right side of the Valtournenche (Aosta) at the contact between the overlying blueschist-facies Combin Zone and the underlying eclogite-facies Zermatt-Saas Zone. The ELS contains a layered metagabbro showing a complex evolution, which includes the pre-Alpine high-temperature (HT) recrystallization of the igneous mineralogies, and the polyphase Alpine metamorphism, initially at high-pressure (HP) quartz eclogite-facies, and greenschist-facies (GS) retrogression.In this paper, an unusual corundum-bearing ultramafic rocks associated with metagabbros are described. The studied sample is a Mg-chloritite with relict green spinel partly replaced by corundum, Mg-beltrandoite-2N3S (a new mineral of the hogbomite supergroup), Mg-chlorite and dolomite. The rock is crossed by mm- to cm-thick veins composed of coarse-grained corundum + Mg-chlorite + dolomite. P-T phase-diagram projections indicate that the corundum-bearing assemblages formed in the presence of a water-rich fluid (X(CO2) ≤ 0.04), during retrograde decompressional evolution. This stage follows the prograde HP-peak of the associated eclogites. The newly inferred prograde-to-retrograde P-T path suggests that the ELS and the underlying Zermatt-Saas Zone shared a common Alpine metamorphic evolution.The detailed study of the relict minerals preserved in the chloritite indicates its pre-Alpine protolith, a green spinel websterite, and its evolution, characterized by a high-T recrystallization of the original igneous asemblages. During the Alpine orogeny, the spinel websterite experienced metasomatic hydration that converted the original igneous rock into a chloritite.

>1.8 Ga cold subduction of lithospheric mantle: Evidences from the Fengzhen olivine-bearing garnet pyroxenite xenoliths in Trans-North China Orogen

Whether modern-style subduction exists in Paleoproterozoic has been hotly debated because of the scarcity of robust petrological evidence. Here, we present a comprehensive study of olivine-bearing garnet pyroxenite xenoliths hosted in the Paleoproterozoic Fengzhen carbonatite. The carbonatite is located in the conjugate area between the Paleoproterozoic Trans-North China Orogen (TNCO) and the Khondalite Belt in North China Craton (NCC) with a dated age of 1810 ± 3 Ma. Petrographically, four-stages of evolution have been identified in the studied olivine-bearing garnet pyroxenite: 1) the formation of the protolith spinel websterite (S1), 2) the prograde metamorphism of spinel-lherzolite facies to garnet-lherzolite facies (M1), 3) retrograde metamorphism to Ariegite subfacies (M2) with formation of symplectite-I, and 4) Seiland subfacies (M3) forming symplectite-II. The clinopyroxene in the xenoliths display high Mg# (Mg2+/(Mg2++Fe2+)*100 = 89–94), strongly depleted HREE (heavy REE) and HFSE (high field strength elements; e.g., Nb, Zr, Ti) and enriched LREE (light REE) and LILE (large ion lithophile elements; e.g., Th and U). Similarly, the whole-rock chemistry exhibits high Mg# (85–88) and high Al2O3 + CaO (20.2–21.9 wt%) with enrichment in LREE and LILE (Th, U) and depletion in HREE and HFSE (e.g., Nb, Ta, Zr, Hf, Ti). The geochemical signatures imply that they might originate from the refractory mantle wedge in subduction zone and have been metasomatized by crust-derived melts. Major mineral thermobarometry yields the peak PT conditions of 26–33 kbar and 890–962 °C corresponding to a cold subduction geothermal gradient (307 ± 35 °C/GPa) with a depth up to 110 km for olivine-bearing garnet pyroxenites, which is also consistent with the results by zirconium-in-rutile and REE thermobarometers. This studies suggest that that the modern-style subduction of the lithospheric mantle was initiated at least since 1.8 Ga.

Chromium isotope heterogeneity in the mantle

To better constrain the Cr isotopic composition of the silicate Earth and to investigate potential Cr isotopic fractionation during high temperature geological processes, we analyzed the Cr isotopic composition of different types of mantle xenoliths from diverse geologic settings: fertile to refractory off-craton spinel and garnet peridotites, pyroxenite veins, metasomatised spinel lherzolites and associated basalts from central Mongolia, spinel lherzolites and harzburgites from North China, as well as cratonic spinel and garnet peridotites from Siberia and southern Africa. The δ53CrNIST 979 values of the peridotites range from −0.51±0.04‰ (2SD) to +0.75±0.05‰ (2SD). The results show a slight negative correlation between δ53Cr and Al2O3 and CaO contents for most mantle peridotites, which may imply Cr isotopic fractionation during partial melting of mantle peridotites. However, highly variable Cr isotopic compositions measured in Mongolian peridotites cannot be caused by partial melting alone. Instead, the wide range in Cr isotopic composition of these samples most likely reflects kinetic fractionation during melt percolation. Chemical diffusion during melt percolation resulted in light Cr isotopes preferably entering into the melt. Two spinel websterite veins from Mongolia have extremely light δ53Cr values of −1.36±0.04‰ and −0.77±0.06‰, respectively, which are the most negative Cr isotopic compositions yet reported for mantle-derived rocks. These two websterite veins may represent crystallization products from the isotopically light melt that may also metasomatize some peridotites in the area. The δ53Cr values of highly altered garnet peridotites from southern Africa vary from −0.35±0.04‰ (2SD) to +0.12±0.04‰ (2SD) and increase with increasing LOI (Loss on Ignition), reflecting a shift of δ53Cr to more positive values by secondary alteration.The Cr isotopic composition of the pristine, fertile upper mantle is estimated as δCr53=−0.14±0.12‰, after corrections for the effects of partial melting and metasomatism. This value is in line with that estimated for the BSE (−0.12±0.10‰) previously.

Fractionation of highly siderophile elements in refertilized mantle: Implications for the Os isotope composition of basalts

Highly fertile lherzolite and spinel websterite in the Ronda peridotite massif are enriched in Pt and Pd compared to Os, Ir, Ru and Re. The fractionation of the highly siderophile elements (HSE) in these rocks was produced by reaction of melt with their peridotite and pyroxenite precursors. Modeling indicates that upon reaction primary HSE hosts were dissolved and magmatic sulphides precipitated, largely erasing the original HSE signature of reacting protoliths. The budget of HSE in the melt was controlled by sulphide–silicate partitioning or entrainment of molten sulphide in silicate melt. Rhenium was likely removed from precursor peridotite and pyroxenite by previous melt extraction. Refertilization of peridotite by melts increased Al2O3, CaO, Pt, Pd contents and 187Os/188Os beyond typical values for mantle peridotite and the primitive upper mantle. The interaction of melts with residual peridotite and pyroxenite, as illustrated in the Ronda massif, can generate radiogenic-Os-enriched domains in the mantle with high melt productivity, higher Pt/Os and Pt/Re than common peridotite, and lower Re/Os than typical pyroxenite. As also previously proposed for pyroxenites, these “hybrid” domains can account for the difference in Os isotope compositions between many basalts (e.g., MORB and OIB) and common depleted mantle peridotites. Moreover, the presence of these enriched hybrid components in the mantle, evolved over sufficient timescales and in geologically reasonable proportions, may account for the 186Os–187Os enrichment of plume lavas without requiring a chemical contribution from the core.

Mantle refertilization by melts of crustal-derived garnet pyroxenite: Evidence from the Ronda peridotite massif, southern Spain

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 ∼1.5GPa 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.

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.

Melt transport and deformation history in a nonvolcanic ophiolitic section, northern Apennines, Italy: Implications for crustal accretion at slow spreading settings

[1] Field observations and petrological and geochemical data are used to constrain a conceptual model for the formation of a gabbro-peridotite section from Ligurian ophiolites (Italy). The studied section is attributed to an intraoceanic domain of the Jurassic Ligurian-Piedmontese basin and is characterized by the lack of a basalt layer, similar to nonvolcanic segments from (ultra)slow spreading ridges. The proposed model shows a “hot” lithospheric evolution in which melt transport in the mantle under spinel to plagioclase facies conditions occurred mostly in the form of grain-scale porous flow. We recognize a series of melt/peridotite interaction events, either diffuse or channeled, which modified the composition of the moderately depleted precursor mantle. In particular, localized infiltrations of MORB-type melts gave rise to formation of spinel websterite layers close to the lithosphere-asthenosphere boundary. The peridotite-websterite association was involved in a spinel facies deformation attributed to emplacement of asthenospheric material at the base of the lithosphere. The “hot” lithospheric evolution is followed by an evolution characterized by melt transport through fractures, which started with crystallization of melt into troctolite to olivine gabbro dikes. Both mantle structures and gabbroic dikes are locally crosscut by gabbroic sills. As the mantle section cooled significantly, the dip of the melt migration structures evolved from subvertical to subhorizontal. The growth of a gabbroic pluton (up to ∼400 m thick) that is intruded into the mantle sequence is attributed to accretion of gabbroic sills. The tectonomagmatic history recorded by the gabbroic pluton after its solidification is characterized by ductile shearing developed from near-solidus to amphibolite facies conditions.

Formation of pyroxenite layers in the Totalp ultramafic massif (Swiss Alps) – Insights from highly siderophile elements and Os isotopes

Pyroxenitic layers are a minor constituent of ultramafic mantle massifs, but are considered important for basalt generation and mantle refertilization. Mafic spinel websterite and garnet–spinel clinopyroxenite layers within Jurassic ocean floor peridotites from the Totalp ultramafic massif (eastern Swiss Alps) were analyzed for their highly siderophile element (HSE) and Os isotope composition. Aluminum-poor pyroxenites (websterites) display chondritic to suprachondritic initial γ Os (160 Ma) of −2 to +27. Osmium, Ir and Ru abundances are depleted in websterites relative to the associated peridotites and to mantle lherzolites worldwide, but relative abundances (Os/Ir, Ru/Ir) are similar. Conversely, Pt/Ir, Pd/Ir and Re/Ir are elevated. Aluminum-rich pyroxenites (clinopyroxenites) are characterized by highly radiogenic 187Os/ 188Os with initial γ Os (160 Ma) between +20 and +1700. Their HSE composition is similar to that of basalts, as they are more depleted in Os, Ir and Ru compared to Totalp websterites, along with even higher Pt/Ir, Pd/Ir and Re/Ir. The data are most consistent with multiple episodes of reaction of mafic pyroxenite precursor melts with surrounding peridotites, with the highest degree of interaction recorded in the websterites, which typically occur in direct contact to peridotites. Clinopyroxenites, in contrast, represent melt-dominated systems, which retained the precursor melt characteristics to a large extent. The melts may have been derived from a sublithospheric mantle source with high Pd/Ir, Pt/Ir and Re/Os, coupled with highly radiogenic 187Os/ 188Os compositions. Modeling indicates that partial melting of subducted, old oceanic crust in the asthenosphere could be a possible source for such melts. Pentlandite and godlevskite are identified in both types of pyroxenites as the predominant sulfide minerals and HSE carriers. Heterogeneous HSE abundances within these sulfide grains likely reflect subsolidus processes. In contrast, large grain-to-grain variations, and correlated variations of HSE ratios, indicate chemical disequilibrium under high-temperature conditions. This likely reflects multiple events of melt–rock interaction and sulfide precipitation. Notably, sulfides from the same thick section for the pyroxenites may display both residual-peridotite and melt-like HSE signatures. Because Totalp pyroxenites are enriched in Pt and Re, and depleted in Os, they will develop excess radiogenic 187Os and 186Os, compared to ambient mantle. These enrichments, however, do not possess the requisite Pt–Re–Os composition to account for the coupled suprachondritic 186Os– 187Os signatures observed in some Hawaiian picrites, Gorgona komatiites, or the Siberian plume.

Origin of Pyroxenite-Peridotite Veined Mantle by Refertilization Reactions: Evidence from the Ronda Peridotite (Southern Spain)

The Ronda orogenic peridotite (southern Spain) contains a variety of pyroxene-rich rocks ranging from high-pressure garnet granulites and pyroxenites to low-pressure plagioclase^spinel websterites. The ‘’ part of the Ronda peridotite contains abundant layered websterites (‘group C’pyroxenites), without significant deformation, that occur as swarms of layers showing gradual modal transitionstowardstheirhost peridotites. Previousstudieshave suggested that these layered pyroxenites formed by the replacement of refractory spinel peridotites. Here, we present a major- and trace-element, and numerical modellingstudy ofa layered outcrop ofgroup C pyroxenite near the locality of Tolox aimed at constraining the origin of these pyroxenites after host peridotites by pervasive pyroxene-producing, refertilization melt^ rock reactions. Mg-number [¼ Mg/(Mg þ Fe) cationic ratio] numerical modelling shows that decreasing Mg-number with increasing pyroxene proportion, characteristic of Ronda group C pyroxenites, can be accounted for by a melt-consuming reaction resulting in the formation of mildly evolved, relatively low Mg-number melts (� 0� 65) provided that the melt fraction during reaction and the time-integrated melt/rock ratio are high enough (40� 1and41, respectively) to balance Mg^Fe buffering by peridotite minerals. This implies strong melt focusing caused by melt channelling in high-porosity domains resulting from compaction processes in a partial melted lithospheric domain below a solidus isotherm represented by the Ronda peridotite recrystallization front.The chondrite-normalized rare earth element (REE) patterns of group C whole-rocks and clinopyroxenes are convexupward. Numerical modeling of REE variations in clinopyroxene produced by a pyroxene-forming, melt-consuming reaction results in curved trajectories in the (Ce/Nd)N vs (Sm/Yb)N diagram (where N indicates chondrite normalized). Based on (Ce/Nd)N values, two transient, enriched domains between the light REE (LREE)depleted composition of the initial peridotite and that of the infiltrated melt may be distinguished in the reaction column: (1) a lower domain characterized by convex-upward REE patterns similar to those observed in Ronda group C pyroxenite^peridotite; (2) an upper domain characterized by melts with strongly LREE-enriched compositions.The latterare probably volatile-rich, small-volume melt fractions residual after the refertilization reactionsthatgeneratedgroup C pyroxenites, which migrated throughout the massifcincluding the unmelted lithospheric spinel-tectonite domain. The Ronda mantle domains affected by pyroxenite- and dunite- or harzburgite-forming reactions (the ‘layered granular’subdomain and ‘plagioclase-tectonite’domain) are on average morefertilethan the residual,‘coarsegranular’subdomain at the recrystallization front. This indicates that refertilization traces the moving boundaries of receding cooling of a thinned and partially melted subcontinental lithosphere. This refertilization process may be widespread during transient thinning and melting of depleted subcontinental lithospheric mantle above upwelling asthenospheric mantle.

Pyroxenites from the Southwest Indian Ridge, 9-16 E: Cumulates from Incremental Melt Fractions Produced at the Top of a Cold Melting Regime

The Southwest Indian Ridge (SWIR) at 9–16°E and 52–53°S is characterized by ultra-slow, oblique spreading and contains one of the few documented occurrences of pyroxenite veins associated with abyssal peridotites. The origin of these uncommon lithologies is still debated. We present a detailed study (including electron microprobe and laser ablation inductively coupled plasma mass spectrometry) of spinel websterites collected during Cruise 162, Leg 9, of the R.V. Knorr. Rare earth element patterns in clinopyroxenes (Cpx) lead us to discard a possible origin of the pyroxenites as residues from partial melting of garnet pyroxenites (i.e. relics of a layered mantle protolith). Their composition and cumulate texture (when not obscured by mylonitization related to emplacement on the seafloor) are better interpreted in terms of fractional crystallization from a basaltic melt at relatively high pressure. Evidence for a high pressure of crystallization includes the lack of plagioclase in the cumulate assemblage and the high Al2O3 contents of the pyroxenes: up to 5 wt % in orthopyroxene (Opx) and up to 7 wt % in Cpx. These values are among the highest reported for pyroxenes in a mid-ocean ridge setting. Sub-solidus breakdown of spinel to plagioclase (now altered) is observed in one sample, providing a rough estimate of the final equilibration pressure of these cumulates, around 0· 6–0· 7 GPa (plagioclase–spinel transition for a bulk pyroxenite composition). The inferred pyroxenite parent melts were close to equilibrium with the associated residual peridotites; some samples have a slightly evolved composition in terms of the Mg-number [Mg/(Mg + total Fe)]. These parental melts had major and trace element compositions consistent with a mid-ocean ridge basalt (MORB) affinity, although they were not rigorously identical to MORB. Among other characteristics, these melts were relatively depleted in highly incompatible elements. We propose that they correspond to the latest, shallowest, incremental melt fractions produced during fractional decompression melting of a normal MORB (N-MORB) mantle source. These melts experienced fractional crystallization as soon as they segregated from the peridotite matrix, moved upward, and crossed the lithosphere–asthenosphere boundary (defined here as the base of the conductive lid). As a consequence, these shallow melt fractions produced beneath mid-ocean ridges did not fully mix with melt fractions produced and extracted at greater depths. Our study provides concrete evidence for the actuality of pyroxene crystallization in melt channels beneath mid-ocean ridges at relatively high pressures, a process frequently invoked to account for the ‘pyroxene paradox’ in MORB petrogenesis.

What is the tectono-metamorphic evolution of continental break-up: The example of the Tasna Ocean–Continent Transition

The break-up of continental lithosphere in magma-poor margins is accompanied by the exhumation of mantle and crustal rocks in the footwall of large-scale detachment faults. Although these structures have been described from many modern and ancient margins, little is known about how they accommodate strain and evolve in time and space during continental break-up. The Tasna Ocean–Continent Transition (OCT) in southeastern Switzerland is one of the rare examples where such detachment faults are exposed and can be observed on a kilometre scale. In this paper we describe the deformation structures and their evolution observed along detachment faults in the Tasna OCT. Our results show that continental break-up was attained by a series of detachment faults. These detachment faults accommodated extensional strain in fault zones formed by a localized core zone that is surrounded by a several tens to hundred metres wide damage zones. The core zone corresponds to the zone of highest strain and is well defined structurally by the occurrence of gouges and/or foliated cataclasites and physically by separating a hanging wall from a footwall. Deformation in the fault zones occurred under greenschist facies to seafloor conditions and within the stability field of serpentine. U/Pb ages on zircon from a garnet-bearing pegmatite cross cutting high-temperature shear zones (upper amphibolite facies and higher) provide Carboniferous ages and demonstrate that these shear zones are neither kinematically nor genetically related to the detachment faults observed in the Tasna OCT. Ar/Ar ages on phlogopite from spinel websterite suggest that mantle exhumation occurred during Middle Jurassic time. Our data show that the detachment faults observed in the Tasna OCT formed during latest rifting, post-date major thinning of the crust and onset of mantle serpentinization. These results compare well with those from the deep Iberia margin. Our observations support the idea that rifting leading to continental break-up is a multi-phase process, and that serpentinization is the consequence rather than the reason for strain localization at non-volcanic margins. Apart from the more general implications for the tectonic evolution of continental break-up, our results have some important consequences for the palaeogeographic reconstruction of certain Alpine domains and question the existence of an independent Early Cretaceous Valais ocean in the Alpine realm.

Aluminous websterite and granulite xenoliths from the Chyulu Hills volcanic field, Kenya: gabbro–troctolitic cumulates subjected to lithospheric foundering

Whole rock major and trace element abundances in aluminous garnet–spinel websterite, sapphirine-bearing Mg–Al granulite and hibonite-bearing Ca–Al granulite xenoliths from the Chyulu Hills volcanic field, Kenya, suggest that the samples represent a meta-igneous suite linked by fractionation. The incompatible major element contents increase from the websterites to the Mg–Al granulites and further to the Ca–Al granulites. High bulk rock Mg#s and very low concentrations of most incompatible trace elements indicate that the rocks are cumulates rather than crystallized melts. Elevated Ni abundances, impoverishment in Cr and HFSE and high contents of normative plagioclase and olivine in the granulites indicate that their protoliths were similar to troctolite. The textures and metamorphic reaction paths recorded in the granulites suggest igneous emplacement in the crust and cooling from igneous to ambient crustal temperatures accompanied or followed by compression. For the websterite xenoliths, there is an apparent contradiction between the results of P–T calculations that suggest high P and T of crystallization of early generation pyroxenes and elevated P–T conditions during final equilibration (1.4–2.2 GPa/740–980°C) on the one hand and the positive Eu anomaly that suggests shallow-level plagioclase accumulation on the other hand. This contradiction can be reconciled by a model of compression of a plagioclase-bearing (gabbroic) protolith to mantle depths where it recrystallized to an ultramafic assemblage, which requires foundering of dense lower crustal material into the mantle.

Geochemistry of pyroxenitic and hornblenditic xenoliths in alkaline lamprophyres from the Spanish Central System

The alkaline lamprophyres and diabases of the Spanish Central System carry a heterogeneous suite of xenoliths which includes scarce pyroxenitic and hornblenditic types that can be divided in two groups: (a) pyroxenite xenoliths, including spinel clinopyroxenites and spinel websterites with granoblastic textures, and (b) hornblende-bearing clinopyroxenites and hornblendites (here after called hornblenditic xenoliths) characterised by the presence of Ti-rich kaersutitic amphibole and magmatic textures. Both groups of xenoliths can be assigned to the Al-augite series of Wilshire and Shervais (1975) [Wilshire, H.G., Shervais, J.W., 1975. Al-augite and Cr-diopside ultramafic xenoliths in basaltic rocks from western United States. Phys. Chem. Earth 9, 257–272] with Al-rich and Cr-poor mafic phases. Clinopyroxenes show a very similar trace element composition in all of the ultramafic xenoliths, characterised by convex-upward chondrite-normalised REE patterns and low contents of incompatible elements such as Rb, Ba, Th and Nb. Kaersutite in the amphibole-bearing xenoliths shows a similar convex-upward REE pattern as clinopyroxene. Whole-rock and mineral geochemistry support an origin as cumulates from alkaline to subalkaline melts for most of the pyroxenites and hornblendites that have been studied. The Sr–Nd isotope ratios of pyroxenite xenoliths display two extreme compositional poles: one clinopyroxenite plots in the OIB field towards depleted values ( 87Sr/ 86Sr = 0.7028 and εNd = 6.2), whereas the other pyroxenites plot in enriched lithospheric fields (0.705 to 0.706 and −2.8 to −3.4, respectively), which implies that different magmas have been involved in their genesis. The hornblenditic xenolith suite has a very homogeneous isotopic composition, close to the isotopically depleted values of high εNd and low 87Sr/ 86Sr ratios of one of the pyroxenite xenoliths. Some of these ultramafic xenoliths fall within the isotopic compositional range of their host alkaline dykes, which also define a bipolar compositional field, suggesting that most of them are cogenetic with the lamprophyres. P–T estimates yield temperatures in the range of 970–1080 °C and pressures mainly from 0.9 to 1.2 GPa for pyroxenites, whilst hornblenditic xenoliths give lower (and probably underestimated) pressures (0.7–0.9 GPa). This pressure range is in agreement with pyroxenites being formed by an underplating event at the upper mantle–lower crust boundary, whereas pressure estimates for hornblenditic xenoliths suggest equilibration within the lower crust.

Ultrahigh-pressure corundum-rich garnetite in garnet peridotite, Sulu terrane, China

Corundum-rich garnetite occurs as an isolated lens in a garnet peridotite body in the Donghai area of the Sulu ultrahigh-pressure (UHP) terrane. This rock consists of garnet and corundum, along with minor crack-related zoisite, pargasite, Mg-staurolite, Mg-chloritoid, sapphirine and chlorite. Pyropic garnet (Prp54–63Grs26–36Alm10–12) exhibits a sinusoidal REE pattern, positive Ta, Pb, and negative Nb, Ti anomalies due to metasomatism. Reddish corundum contains 1.1–1.7 wt% Cr2O3, and shows three oriented sets of exsolved rutile needles. Both garnet and corundum contain inclusions of apatite, Mg-allanite (MgO>4 wt%), and Ni-Fe sulfides formed as trapped Ni-Fe-S melt. The protolith of the corundum-rich garnetite could have been spinel websterite formed in the upper mantle. Both the websterite and the host garnet peridotite were subjected to subduction-zone UHP metamorphism at ≥800 °C and >4 GPa. Crack-related hydrous phases were formed by fluid infiltration during exhumation.

Magmatic modification and metasomatism of the subcontinental mantle beneath the Vitim volcanic field (East Siberia): evidence from trace element data on pyroxenite and peridotite xenoliths from Miocene picrobasalt

The genesis of several groups of pyroxenite xenoliths from Miocene picrobasalt of the Vitim volcanic field are considered on the basis of petrology and mineral chemical data, including trace element analyses of minerals by ion probe. The pyroxenites and related xenoliths can be classified into three groups consisting of 10 sets of xenoliths: Group I (Cr-diopside series) is made up by three subdivisions; (Ia) common Cr-diopside garnet and spinel websterites, (Ib) Al-poor Cr-diopside websterites, (Ic) modally metasomatized Cr-diopside lherzolites. Group II (Al-augite series) consists of four further xenolith types; (IIa) Cr-rich and (IIb) Cr-poor megacrystic pyroxenite assemblages, (IIc) Al-rich garnet clinopyroxenites, and (IId) amphibole and/or phlogopite-bearing rocks and associated orthopyroxenites. Group III consists of xenoliths all characterized by Ca-rich clinopyroxene (garnet clinopyroxenite and gabbros, garnet granulites and spinel websterites) thought to have a similar depth of origin near the crust–mantle boundary. Trace element data and textural criteria allow the distinction of three melt types operating in the mantle: (1) Cr-rich melt migrating along small fractures and forming Cr-diopside pyroxenite veins as the result of percolation fractionation; (2) picrobasaltic to alkaline basaltic melt fractionating in large hydraulic fractures or magma chambers and producing host volcanics, clinopyroxene megacrysts and megacrystic pyroxenites (Groups IIa–IIb); and (3) high field strength element (HFSE)-enriched smaller volume melts, which formed the hydrous mineral-bearing xenoliths of Groups I and II (Ic and IId). The megacrystic pyroxenites (IIa–IIb) were crystallized in magma chambers or large fractures at various depths during multistage polybaric fractionation. The high-temperature Cr-rich websterites (Group IIa) correspond to the deepest level at near 30 kbar. The garnet websterites and megacrystic clinopyroxenites crystallized at intermediate levels, whereas the most differentiated megacrystic clinopyroxenites with ilmenite and phlogopite inclusions are from the shallowest levels at 15–11 kbar. The HFSE-rich melt resulted in amphibole- and phlogopite-rich assemblages crystallized in veins and interstitially in peridotites, and the development of ilmenite–phlogopite-bearing orthopyroxenites in a reaction zone between these. Pyroxenites of Groups I and II represent samples of an intensely veined sub-Vitim lithosphere: the preservation of textural and chemical disequilibrium indicates that the enrichment immediately preceded xenolith sampling and is related to the upward and outward migration of the asthenosphere–lithosphere boundary beneath the developing Baikal Rift. The formation of lower pressure xenoliths (Group III) is related to both of the first two melt types mentioned above. Spinel websterites of Group III are genetically related to the Group Ia melt crystallized near the crust–mantle boundary, and the mafic garnet granulites of Group III to intermediate fractionation products (gabbro-norites) of similar melts. Garnet gabbros are associated with the most fractionated megacrystic pyroxenites.

Diversity of Mafic Rocks in the Ronda Peridotite: Evidence for Pervasive Melt-Rock Reaction during Heating of Subcontinental Lithosphere by Upwelling Asthenosphere

melt fractions that percolated pervasively throughout the massif and On the basis of their structural and compositional characteristics, had a refractory, calc-alkaline character. Locally, these melts were mafic rocks in the Ronda peridotite are classified into four groups: tapped into cracks forming intrusive dykes. 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

MANTLE PERIDOTITES OF GRANULITE - GNEISS COMPLEX AS FRAGMENTS OF ARCHEAN (?) OPHIOLITES IN THE BAIKAL REGION (RUSSIA)

The Sharyzhalgay block, located in the south-western Baikal region, is an exposure of the Precambrian basement at the margin of the Siberian Platform. It chiefly consists of granitoids (80%) that host relic granulite facies rocks (20%). Metamorphism under granulite facies conditions took place about 2.4-2.5 Ga ago. Petrological and geochemical data on the granulites suggest protoliths of predominantly MORB-like basalt (~70%) and minor sedimentary rocks. Granulites also include ultramafic rocks. Relics of protogranular texture, high XMg in rocks and rock-forming minerals, occurrence of depleted harzburgites, mantle compositional trend of spinels, REE patterns, and thermobarometry data allow us to interpret the provenance of Saramta peridotites as mantle material depleted by partial melting in the stability field of spinel. Peridotites are cut by dikes of layered wehrlite, spinel websterite and garnet websterite. Spinel websterites have a chemistry and mineralogy of Cr-Di series. Their origin inferred to be the result of mineral segregations from percolating melt within the upper mantle. Garnet websterites differ from Al-augite pyroxenites by lower contents of TiO2 and Al2O3, and higher SiO2. Incongruent melting of Cr-Di spinel websterites on the scheme Cpx+ Opx+ Sp ® Mg-richer Ol+ Cr-richer Sp+L produced melts and restite wehrlites. Later on the melt crystallized in situ into garnet websterites with magmatic textures. Regional granulite-facies metamorphism at temperatures of 700-800°C and a pressure of 5-6 kb produced exsolution lamellae of spinel in pyroxene, kelyphite Sp+Opx+Pl rims on garnet, and zoning of orthopyroxene in garnet websterite. Tectonitic restite harzburgites with ultramafic dikes are correlative with the mantle parts of ophiolite sections. All these facts allow us to interpret relic granulite-facies rocks as fragments of Precambrian ophiolites.

Garnet-spinel-pyroxenite xenoliths from hyblean plateau (South-eastern Sicily, Italy)

Rare garnet-spinel pyroxenite xenoliths occur in some basaltic tuff-breccia levels of Miocene age from the Valle Guffari (Hyblean Plateau, Sicily), together with a number of spinel-bearing mantle xenoliths. The garnet-bearing pyroxenites may be divided into two groups (“a” and “b”) on textural and mineralogical bases. Garnet-bearing spinel websterites with a fully recrystallized texture represent the first group (a). Here the garnet (Py54.5 A1m32 Gr13.5), with a diffuse kelyphitic alteration, forms a reaction corona between coarse spinel grains and the in contact pyroxenes. The transition from the spinel-pyroxenite to the garnet-pyroxenite field may depend on isobaric cooling from higher (magmatic?) temperatures. Garnet-pyroxene geothermometry indicates that the last equilibration most probably occurred at P = 1.0 GPa (ca.), T = 750 °C (ca). The second lithotype (b) is an orthopyroxene-bearing garnet-spinel clinopyroxenite, exhibiting a complex texture. It consists of zones of coarse clinopyroxene grains enclosing euhedral spinel passing to zones where tiny rounded crystals of the same pyroxene and spinel are enclosed in relatively large patches of extensively kelyphitisized garnet (Py64.8 Alm25.6 Gr9.6). Garnet also occurs as inclusion-free grains up to 4 mm in diameter. P-T calculations give significantly higher values than for the former case (a). The origin of the b-type garnet may also depend on subsolidus reaction of spinel and pyroxenes after an isobaric cooling from still higher temperatures, but a primary magrnatic origin might also be possible, especially for the “granular” garnets. P-T estimates for both the pyroxenite types closely match a steady geotherm for 100 mW/m2 surface heat flow. Such a relatively intense heat flow may suggest the occurrence of huge masses of hot magma intruding the Hyblean lithospheric mantle and lower crust at different levels.

Re-Os Systematics in Orogenic Peridotite Massifs and Constraints on the Petrogenesis of Pyroxenites

Orogenic lherzolites are considered to represent large portions of the upper mantle that have been tectonically emplaced in the crust. They are composed of lherzolites, harzburgites, dunites and small proportions of pyroxenites of variable thickness and composition. The study of such massifs has provided much information on the spatial distribution of geochemical variations in the upper mantle. Because of the extreme compatibility of osmium and its resistance to secondary effects like metasomatism and retrograde alteration, The Re-Os isotopic system provides an additional perspective to help understanding the origin of orogenic lherzolites massifs. Here we investigate, Re-Os concentrations and isotopic compositions of peridotites and pyroxenites from three ultramafic massif: Beni-Bouzra (North Morocco), Ronda (South Spain) and Lherz (French Pyrenees). The Os isotopic ratios (1870S/1860S) for the peridotites range from 0.951 to 1.102 in good agreement with data published previously and with upper mantle value, while the concentration range from 0.531 to 8.740 ppb and is in general higher than those encountered in mantle xenoliths. It is widely admitted that orogenic massif come from the asthenospheric mantle that accreted into the lithosphere sub-continental (SCLM) via limited diapiric upwelling. Subsequently the correlation between Os isotopic ratios and Mg# or A1203 may result from ancient melt extraction (Fig. 3). Our study of mafic layers has focused on two familly of pyroxenites: spinel websterites and garnet pyroxenites. The garnet pyroxenites are more differentiated than spinel websterites and have high concentration in A1203, CaO end V and low concentration in MgO, Ni and Co. The osmium isotopic ratios of garnet pyroxenites range from 1.752 to 12.352 (Fig. 2). These values are higher than those encountered in the websterites (from 1.058 to 1.904) and are the result of a large Re/Os fractionation (160 to 900). Among the mafic samples, Os isotopic ratios are found to decrease strongly with increasing Os concentration. The negative correlation obtained between Re/Os and Mg# confirms the incompatible nature of Re. The pyroxenites data of Beni-Bouzra show a positive correlation (R 2 = 0.97, Fig. 1) between 1870s/1860s and lS7Re/lS6Os ratios which yield an apparent 'age' of 700 _+ 55 Ma. This age is in agreement with the model age obtained with 2~176 systematics for the same massif (590 +_ 150 Ma; Hamelin and Allbgre, 1988) and may reflect the time when was incorporated in the non-convecting SCLM. This event is younger than the stabilisation of, the Ronda ultramafic massif deduced for Os isotopes(~ 1.2 Ga, Reisberg and Lorand, 1995). The high Os isotopic ratios in mafic layers suggesting that this material cannot represent recent partial melting, or cumulates derived from recent melt of the surounding ultramafic rocks. They provide evidence for a process that occurred at least hundreds of millions of years ago. Comparison of the Os and Pb data for the peridotites and pyroxenites with the range observed in oceanic basalts shows that the radiogenic ratios in basalts could be produced by a mixture ofperidotitepyroxenite in the source which lends support to the hypothesis that the lithologies found in orogenic massifs are representative of the upper mantle, the 'marble cake' model (All~gre and Turcotte, 1986). However, the geochemical characteristics of pyroxenites, the zonation observed in some mafic layers and the positive correlation observed between the model age versus thickness (Fig. 4) suggests that this material represent high-pressure crystallization from melts of subducted oceanic crust (Pearson et al., 1993). Although in the basis of FeO-MgO-AI203 systematics, small proportion of Beni-Bouzra pyroxenites may represent subducted, metamorphosed oceanic crust directly recycled into the mantle (Kornprobst et al., 1991).