Persistence of mantle lithospheric Re–Os signature during asthenospherization of the subcontinental lithospheric mantle: insights from in situ isotopic analysis of sulfides from the Ronda peridotite (Southern Spain)

The western part of the Ronda peridotite massif (Southern Spain) consists mainly of highly foliated spinel-peridotite tectonites and undeformed granular peridotites that are separated by a recrystallization front. The spinel tectonites are interpreted as volumes of ancient subcontinental lithospheric mantle and the granular peridotites as a portion of subcontinental lithospheric mantle that underwent partial melting and pervasive percolation of basaltic melts induced by Cenozoic asthenospheric upwelling. The Re–Os isotopic signature of sulfides from the granular domain and the recrystallization front mostly coincides with that of grains in the spinel tectonites. This indicates that the Re–Os radiometric system in sulfides was highly resistant to partial melting and percolation of melts induced by Cenozoic lithospheric thermal erosion. The Re–Os isotopic systematics of sulfides in the Ronda peridotites thus mostly conserve the geochemical memory of ancient magmatic events in the subcontinental lithospheric mantle. Os model ages record two Proterozoic melting episodes at ~1.6 to 1.8 and 1.2–1.4 Ga, respectively. The emplacement of the massif into the subcontinental lithospheric mantle probably coincided with one of these depletion events. A later metasomatic episode caused the precipitation of a new generation of sulfides at ~0.7 to 0.9 Ga. These Proterozoic Os model ages are consistent with results obtained for several mantle suites in Central/Western Europe and Northern Africa as well as with the Nd model ages of the continental crust of these regions. This suggests that the events recorded in mantle sulfides of the Ronda peridotites reflect different stages of generation of the continental crust in the ancient Gondwana supercontinent.     

Melt/peridotite interaction in the Southern Lanzo peridotite: Field, textural and geochemical evidence

This paper presents field, petrographic–structural and geochemical data on spinel and plagioclase peridotites from the southern domain of the Lanzo ophiolitic peridotite massif (Western Alps). Spinel lherzolites, harzburgites and dunites crop out at Mt. Arpone and Mt. Musinè. Field evidence indicates that pristine porphyroclastic spinel lherzolites are transformed to coarse granular spinel harzburgites, which are in turn overprinted by plagioclase peridotites, while strongly depleted spinel harzburgite and dunite bands and bodies replace the plagioclase peridotites. On the northern flank of Mt. Arpone, deformed, porphyroclastic (lithospheric) lherzolites, with diffuse pyroxenite banding, represent the oldest spinel-facies rocks. They show microstructures of a composite subsolidus evolution, suggesting provenance from deeper (asthenospheric) mantle levels and accretion to the lithosphere. These protoliths are locally transformed to coarse granular (reactive) spinel harzburgites and dunites, which show textures reminiscent of melt/rock reaction and geochemical characteristics suggesting that they are products of peridotite interaction with reactively percolating melts. Geochemical data and modelling suggest that <1–5% fractional melting of spinel-facies DMM produced the injected melts. Plagioclase peridotites are hybrid rocks resulting from pre-existing spinel peridotites and variable enrichment of plagioclase and micro-gabbroic material by percolating melts. The impregnating melts attained silica-saturation, as testified by widespread orthopyroxene replacement of olivine, during open system migration in the lithosphere. At Mt. Musinè, coarse granular spinel harzburgite and dunite bodies replace the plagioclase peridotites. Most of these replacive, refractory peridotites have interstitial magmatic clinopyroxene with trace element compositions in equilibrium with MORB, while some Cpx have REE-depleted patterns suggesting transient geochemical features of the migrating MORB-type melts, acquired by interaction with the ambient plagioclase peridotite. These replacive spinel harzburgite and dunite bodies are interpreted as channels exploited for focused and reactive migration of silica-undersaturated melts with aggregate MORB compositions. Such melts were unrelated to the silica-saturated melts that refertilized the pre-existing plagioclase peridotites. Finally, MORB melt migration occurred along open fractures, now recorded as gabbroic dikes.

Our data document the complexity of rock-types and mantle processes in the South Lanzo peridotite massif and describe a composite tectonic and magmatic scenario that is not consistent with the “asthenospheric scenario” proposed by previous authors. We envisage a “transitional scenario” in which extending subcontinental lithospheric mantle was strongly modified (both depleted and refertilized) by early melts with MORB-affinity formed by decompression partial melting of the upwelling asthenosphere, during pre-oceanic rifting and lithospheric thinning in the Ligurian Tethys realm.     

The variability of peridotite composition across a mantle shear zone (Lanzo massif, Italy): interplay of melt focusing and deformation

In this paper we present new data on the spatial variability of peridotite composition across a kilometer-scale mantle shear zone within the Lanzo massif (Western Alps, Italy). The shear zone separates the central from the northern part of the massif. Plagioclase peridotite shows gradually increasing deformation towards the shear zone, from porphyroclastic to mylonitic textures in the central body, while the northern body is composed of porphyroclastic rocks. The peridotite displays a large range of compositions, from fertile peridotite to refractory harzburgite and dunite. Deformed peridotites (proto-mylonite and mylonites) tend to be compositionally more homogeneous and fertile than weakly deformed peridotites. The composition of most plagioclase peridotites show rather high and constant (Ce/Yb) _N ratios, and Yb _N that cannot be explained by any simple melting model. Instead, refertilization modeling, consisting of melt increments from spinel peridotite sources, particularly with E-MORB melt, reasonably reproduces the plagioclase peridotite whole rock composition. Combined with constraints from Ce–Nb and Ce–Th systematics, we speculate that peridotites such as those from Lanzo record pervasive refertilization processes in the thermal boundary layer. In this scenario, mantle shear zones might act as important areas of melt focusing in the upper mantle that separates the thermal boundary layer from the conductively cooled mantle.     

Origin of the recrystallisation front in the Ronda peridotite by km-scale pervasive porous melt flow

 It is well established that porous melt flow in the upper mantle may significantly affect partial mantle melt compositions. Less well established are the length-scale of porous flow and whether porous melt flow can be a volumetrically important magmatic process. The only source for observations concerning the length-scale and nature of pervasive porous melt flow are peridotite massifs. Here we present such observations in the form of structural, and major and trace element data from peridotites of the Ronda massif, southern Spain. Trace element concentrations were obtained with high analytical precision (ICP-MS) and include trace elements rarely analysed in peridotites, such as Rb, Th, Nb and Ta. The western portion of the Ronda massif can be divided into two structural facies. The first and oldest is composed of deformed, porphyroclastic spinel peridotites, the second of virtually undeformed granular spinel peridotites. They are separated by a recrystallisation front across which grain growth of all phases occurred. The granular domain can be further subdivided into three subfacies: coarse-granular, fine-granular, and layered-granular peridotites. According to structural facies, km-scale spatial variations unrelated to Ca and Al abundances have been recognised for mg-numbers [atomic Mg/(Mg±Fe)] and incompatible elements such as rare earth elements (REE), Th and high-field-strength elements (HFSE; including Ti). Such variations are reminiscent of those commonly ascribed to mantle metasomatism, but have never been documented on the km-scale. The origin of the recrystallisation front is related to km-scale pervasive melt percolation. Feed-back processes between grain growth and melt fraction could have led to important accumulation of melt at the recrystallisation front, accomplished mainly by melting/dissolution. Variation in melt fraction across the front explains the spatial variation in the degree of recrystallisation, mg-numbers, REE fractionation, and HFSE abundances, and could account for many of the classical differences between basalts from convergent and extensional tectonic settings. Whereas the coarse-granular peridotites reflect a stage of steady-state pervasive porous melt flow, the fine- and layered-granular facies probably reflect the terminate stages of porous melt flow. Processes associated with both domains are pyroxene-forming freezing reactions at decreasing melt volumes, and progressive channelling of melt flow associated with olivine-producing reactions. Both processes show complex overprinting relationships in both time and space. Received: 10 January 1995/Accepted: 1 September 1995     

Chronology, Petrology and Isotope Geochemistry of the Erro-Tobbio Peridotites (Ligurian Alps, Italy): Records of Late Palaeozoic Lithospheric Extension

Mantle peridotites from the Erro–Tobbio (ET) ophioliticunit (Voltri Massif, Ligurian Alps) record a tectono-metamorphicdecompressional evolution, indicated by re-equilibration fromspinel- to plagioclase- to amphibole-facies conditions, andprogressive deformation from granular to tectonite to mylonitefabrics. The peridotites are considered to represent subcontinentallithospheric mantle that was tectonically denuded during riftingand opening of the Jurassic Ligurian Tethys ocean, similar tothe Northern Apennine (External Ligurides) ophiolitic peridotites.We performed chemical and isotopic investigations on selectedgranular and tectonite spinel peridotites and plagioclase tectonitesand mylonites, with the aim of defining the nature of the mantleprotoliths, and to date the onset of exhumation of the ET peridotites.Spinel- and plagioclase-bearing tectonites and mylonites exhibitheterogeneous bulk-rock major and trace element composition,despite rather homogeneous mineral chemistry, thus indicatingthat the ET mantle protoliths record a composite history ofpartial melting and melt migration by reactive porous flow.The lack of correlation between the observed geochemical heterogeneityand the structural type (granular, tectonite, mylonite) indicatesthat the inferred reactive porous flow event preceded the exhumation-relatedlithospheric history of the Erro–Tobbio mantle. The tectono-metamorphicevolution caused systematic chemical changes in minerals: (1)Al decrease in orthopyroxene; (2) Al decrease, and Cr and Tiincrease in spinels; (3) Al and Sr decrease, Cr, Ti, Zr, Sc,V and middle to heavy rare earth element increase and developmentof a negative Eu anomaly in clinopyroxene. The studied sampleshave Nd isotope compositions consistent with a mid-ocean ridgebasalt mantle reservoir. Sm/Nd isotope data on plagioclase andclinopyroxene separates (and corresponding whole rocks) fromtwo plagioclase peridotites, representative of the plagioclase-bearingmylonitic extensional shear zone, have yielded ages of 273 ±16 Ma and 313 ± 16 Ma, for the plagioclase-facies recrystallizationstage, significantly older than the expected Jurassic age. Thisindicates that the Erro–Tobbio peridotites represent subcontinentallithospheric mantle that was tectonically exhumed from spinel-faciesdepths to shallower lithospheric levels during Late Carboniferous–Permiantimes. Our results are consistent with the previously documentedevidence for an extensional regime in the Europe–Adrialithosphere during Late Palaeozoic time, and they representthe first record that extensional mechanisms were also activeat lithospheric mantle levels. KEY WORDS: plagioclase-bearing peridotites; subcontinental lithospheric mantle; mantle exhumation; Sm/Nd dating     

Subsolidus Emplacement of Mantle Peridotites during Incipient Oceanic Rifting and Opening of the Mesozoic Tethys (Voltri Massif, NW Italy)

The Erro–Tobbio (ET) peridotite (Voltri Massif, NW Italy)represents a fragment of subcontinental mantle, emplaced athigh crustal levels during rifting and opening of the Piemonte–Ligurianocean, the Alps–Apennine part of the Mesozoic Tethys.The ET peridotite is dominated by spinel-bearing lherzolites,with minor dunites, spinel websterites and plagioclase-bearinglherzolites. Granular spinel lherzolites in the ET peridotiteare transected by five generations of shear zone structures:porphyroclastic spinel-bearing tectonites, plagioclase-, hornblende-and chlorite-bearing peridotite mylonites, and serpentinitemylonites. There is a systematic correlation between the microstructuresand the composition of the constituent mineral phases. Thesecompositional trends are related to changing conditions of Pand T during the pertinent stages of syntectonic recrystallizationin the shear zones. Geothermobarometry shows that the shearzone structures developed at progressively lower P and T conditions.The P–T path obtained for the ET peridotite indicatessubsolidus uplift, from deep levels in the subcontinental mantletowards the ocean floor. Uplift is associated with limited ‘wet’partial melting. This subsolidus trajectory is particularlyconsistent with the thermal history expected for the footwallof a lithosphere-scale, dipping extensional shear zone, anddiffers from those of more oceanic peridotites showing an adiabaticuplift history at much higher temperatures presumably relatedto convective upwelling. The shear zone structures in the ETperidotites are therefore interpreted as fragments of an extensionaldetachment system. This interpretation is consistent with theoverall asymmetric architecture of the coeval passive marginsbordering the Piemonte–Ligurian ocean. It is suggestedthat the uplift of the ET peridotites occurred by tectonic denudation,in a slightly to strongly asymmetric oceanic rift. ^* Present address: Shell Int. Petroleum Maatschappij, P.O. Box 162, 2501 AN The Hague, Netherlands     

Localization of deformation and kinematic shift during the hot emplacement of the Ronda peridotites (Betic Cordilleras,southern Spain)

The Ronda peridotites form the largest mass of subcontinental mantle outcropping on land. Unlike other orogenic lherzolite massifs, the two main bodies of Ronda (the Sierra Bermeja and Sierra Alpujata massifs) are unique cases where ductile shear zones linked to the hot thrusting of mantle over continental crustal rocks are well exposed. We present a new insight into the deformation localization in these shear zones based on structural, fabric and petrological data. The Ronda peridotites show increasing deformation towards the continental footwall rocks, from porphyroclastic rocks to ultramylonites. Garnet-pyroxenites from the basal shear zone of the Alpujata massif yield ca. 1100 °C and 1.4 GPa for the mylonitization. Such conditions promoted partial melting and the formation of felsic dynamothermal aureoles from the underlying crustal rocks. Subsequent deformation is mainly localized in the dynamothermal aureoles, since they are weaker than the peridotites. Both aureoles show marked strain gradients towards the contact but record different kinematics. In Sierra Alpujata, kinematic criteria indicate a top-to-the ENE shear sense, whereas in Sierra Bermeja the felsic mylonites provide a top-to-the NNW motion. A transpressional setting is proposed to explain such kinematic shift.     

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

The Ronda orogenic peridotite (southern Spain) contains a varietyof pyroxene-rich rocks ranging from high-pressure garnet granulitesand pyroxenites to low-pressure plagioclase–spinel websterites.The ‘asthenospherized’ part of the Ronda peridotitecontains abundant layered websterites (‘group C’pyroxenites), without significant deformation, that occur asswarms of layers showing gradual modal transitions towards theirhost peridotites. Previous studies have suggested that theselayered pyroxenites formed by the replacement of refractoryspinel peridotites. Here, we present a major- and trace-element,and numerical modelling study of a layered outcrop of groupC pyroxenite near the locality of Tolox aimed at constrainingthe origin of these pyroxenites after host peridotites by pervasivepyroxene-producing, refertilization melt–rock reactions.Mg-number [= Mg/(Mg + Fe) cationic ratio] numerical modellingshows that decreasing Mg-number with increasing pyroxene proportion,characteristic of Ronda group C pyroxenites, can be accountedfor by a melt-consuming reaction resulting in the formationof mildly evolved, relatively low Mg-number melts (0·65)provided that the melt fraction during reaction and the time-integratedmelt/rock ratio are high enough (>0·1 and > 1,respectively) to balance Mg–Fe buffering by peridotiteminerals. This implies strong melt focusing caused by melt channellingin high-porosity domains resulting from compaction processesin a partial melted lithospheric domain below a solidus isothermrepresented by the Ronda peridotite recrystallization front.The chondrite-normalized rare earth element (REE) patterns ofgroup C whole-rocks and clinopyroxenes are convex-upward. Numericalmodeling 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 betweenthe light REE (LREE)-depleted composition of the initial peridotiteand that of the infiltrated melt may be distinguished in thereaction column: (1) a lower domain characterized by convex-upwardREE patterns similar to those observed in Ronda group C pyroxenite–peridotite;(2) an upper domain characterized by melts with strongly LREE-enrichedcompositions. The latter are probably volatile-rich, small-volumemelt fractions residual after the refertilization reactionsthat generated group C pyroxenites, which migrated throughoutthe massif—including the unmelted lithospheric spinel-tectonitedomain. The Ronda mantle domains affected by pyroxenite- anddunite- or harzburgite-forming reactions (the ‘layeredgranular’ subdomain and ‘plagioclase-tectonite’domain) are on average more fertile than the residual, ‘coarsegranular’ subdomain at the recrystallization front. Thisindicates that refertilization traces the moving boundariesof receding cooling of a thinned and partially melted subcontinentallithosphere. This refertilization process may be widespreadduring transient thinning and melting of depleted subcontinentallithospheric mantle above upwelling asthenospheric mantle. KEY WORDS: subcontinental mantle; refertilization; pyroxenite; peridotite; websterite; melt–rock reaction; plagioclase; trace elements     

Geodynamic Information in Peridotite Petrology

Systematic differences are observed in the petrology and majorelement geochemistry of natural peridotite samples from thesea floor near oceanic ridges and subduction zones, the mantlesection of ophiolites, massif peridotites, and xenoliths ofcratonic mantle in kimberlite. Some of these differences reflectvariable temperature and pressure conditions of melt extraction,and these have been calibrated by a parameterization of experimentaldata on fertile mantle peridotite. Abyssal peridotites are examplesof cold residues produced at oceanic ridges. High-MgO peridotitesfrom the Ronda massif are examples of hot residues producedin a plume. Most peridotites from subduction zones and ophiolitesare too enriched in SiO₂ and too depleted in Al₂O₃ to be residues,and were produced by melt–rock reaction of a precursorprotolith. Peridotite xenoliths from the Japan, Cascades andChile–Patagonian back-arcs are possible examples of arcprecursors, and they have the characteristics of hot residues.Opx-rich cratonic mantle is similar to subduction zone peridotites,but there are important differences in FeO_T. Opx-poor xenolithsof cratonic mantle were hot residues of primary magmas with16–20% MgO, and they may have formed in either ancientplumes or hot ridges. Cratonic mantle was not produced as aresidue of Archean komatiites. KEY WORDS: peridotite; residues; fractional melting; abyssal; cratonic mantle; subduction zone; ophiolite; potential temperature; plumes; hot ridges     

The Ronda Peridotite: Garnet-, Spinel-, and Plagioclase-Lherzolite Facies and the P-T Trajectories of a High-Temprature Mantle Intrusion

The Ronda peridotite is a high-temperature, alpine-type peridotiteemplaced in the internal Zone of the Betic Cordilleras, southernSpain. Using the mineral assemblages of the peridotite and maficlayers, the peridotite mass has been subdivided into 4 zonesof mineral facies: (1) garnet-lherzolite facies, (2) ariégitesubfacies of spinel-lherzolite facies, (3) seiland subfaciesof spinel-lherzolite facies, and (4) plagioclase-lherzolitefacies. It is proposed that this mineralogical zonation developedthrough a syntectic recrystallization of a hot (1100 to 1200°C),solid mantle peridotite during its ascent into the Earth's crust. Coexisting minerals from 12 peridotites covering all the mineralfacies above were analysed with an electron microprobe. Corecompositions of pyroxene porphyroclasts are constant in allmineral facies and indicate that the peridotite was initiallyequilibrated at temperatures of 1100 to 1200 °C and pressuresof 20 to 25 kb. In contrast, the compositions of pyroxene neoblastsand spinel grains (which appear to have grown during later recrystallization)are well correlated with mineral facies. They indicate thatthe recrystallization temperature throughout the mass is moreor less constant, 800 to 900 °C, but that the pressure rangesfrom 5–7 kb in the plagioclase-lherzolite facies to 12–15kb in the garnet-lherzolite facies. Therefore, variation inpressure appears to be primarily responsible for the four mineralfacies types. A pressure range of at least 5 kb appears to be too large tohave been maintained (at the same time) in a mass as small asthe Ronda peridotite. Dynamic cooling may explain the observedvariation in the recrystallization pressure; i.e. during theintrusion of the peridotite body, different parts of the bodyhave followed different P-T paths in response to different localcooling rates. Comparing the inferred P-T paths for the peridotitewith published melting temperature of peridotite and mafic rocks,it is concluded that the peridotite did not go through partialfusion during the ascent. A hypothetical, diapiric uprise thatcaused partial fusion and igneous differentiation of the mantleperidotite is considered to be a separate event prior to theascent that started from about 70 km depth in the upper mantle.Estimates of cooling rates and of Al diffusion rates in pyroxenessuggest that the ascent rate of the peridotite body was greaterthan 1 meter/year. ^* Present address: Dement of Earth Sciences, Toyama University, Gofuku, Toyama 930, Japan.     

Metamorphic evolution of garnet-spinel peridotites from the Variscan Schwarzwald (Germany)

Garnet-spinel peridotites form small, isolated, variably retrogressed bodies within the low-pressure high-temperature gneisses and migmatites of the Variscan basement of the Schwarzwald, southwest Germany. Detailed mineralogical and textural studies as well as geothermobarometric calculations on samples from three occurrences are presented. Two of the garnet-spinel peridotites have equilibrated at 680–770°C, 1.4–1.8 GPa within the garnet-spinel peridotite stability field, one of the samples having experienced an earlier stage within the spinel peridotite stability field (790°C, <1.8 GPa). The third sample, with only garnet and spinel preserved, probably equilibrated within the garnet peridotite stability field at higher pressures. These findings are in line with the distinction of two groups of ultramafic garnet-bearing high-pressure rocks with different equilibration conditions within the Schwarzwald (670–740°C, 1.4–1.8 GPa and 740–850°C, 3.2–4.3 GPa) which has previously been established (Kalt et al. 1995). The equilibration conditions of 670–770°C and 1.4–1.8 GPa for garnet-spinel peridotites from the Central Schwarzwald Gneiss Complex (CSGC) are similar to those for eclogites of the Schwarzwald and also correspond quite well to those for garnet-spinel peridotites from the Moldanubian zone of the Vosges mountains and of ecologites from the Moldanubian s.str. of the Bohemian Massif.     

Analyses d’ouvrages

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.     

Closed-system geochemical recycling of crustal materials in alpine-type peridotite

We examined aluminous mafic rock (with or without corundum or sapphirine) alternating with peridotite from the Ronda peridotite massif, southern Spain. On the basis of petrographic characteristics, these mafic rocks show a decompression history from high pressure (P > 1.5 GPa), but on the basis of their geochemical characteristics, they are crystal accumulates of plagioclase, clinopyroxene, and olivine formed within the lower crust (P < 1 GPa). A complex evolution history, including higher-pressure recrystallization after initial formation as cumulate gabbros at lower-pressure conditions, is proposed. The aluminous mafic rocks and their peridotite hosts are inferred to be recycled crustal materials now observed as centimeter-scale layered components in alpine-type peridotite. The rocks retained their original cumulate compositions; that is, their compositions were not affected by melting and metasomatic modifications during subduction, intense deformation within the upper mantle, and upwelling to the surface.     

Ultramafic nodules from Seiland,northern Norway

Peridotitic inclusions were emplaced in synorogenic basic and ultrabasic dykes during the later evolutionary stages of the Seiland petrographic province. Tectonite and cumulate types are recognized. The former show penetrative textural evidence of a series of pre-incorporation mineral assemblages due to partial equilibration of the source along a decreasing P-T gradient. Unmixing of aluminous pyroxenes and reaction with spinel resulted in the appearance of minor plagioclase, which later was replaced by hydrous minerals. The metamorphic fabric eliminates an accumulative relationship with the transporting magma. The rare cumulate-type nodules, however, retain evidence of poikilitic textures and primary plagioclase; orthopyroxene and spinel are due to subsolidus reaction of olivine and plagioclase.A high-temperature alpine peridotite rising diapirically through the upper mantle is postulated to be the parent of the tectonite-type nodules. This intrusion may be the source of the synorogenic magmatism and positive gravity anomaly of the Seiland province.     

Dunite Formation Processes in Highly Depleted Peridotite: Case Study of the Iwanaidake Peridotite, Hokkaido, Japan

Dunite formation processes in highly depleted peridotites arediscussed based upon a detailed study of the Iwanaidake peridotite,Hokkaido, Japan, which consists mainly of harzburgite with asmall amount of dunite. In the harzburgites, the Mg# [= 100x Mg/(Mg + Fe²⁺)] of olivine ranges from 91·5 to 92·5,and the Cr# [= 100 x Cr/(Cr + Al)] of spinel from 30 to 70;in the dunites, the Mg# of olivine ranges from 92·5 to94 and the Cr# of spinel from 60 to 85, respectively. The NiOwt % of olivine in harzburgites ranges from 0·38 to 0·44,and in dunites from 0·35 to 0·37. The Mg# andCr# are higher and NiO wt % is lower in the dunites than inthe harzburgites surrounding the dunites. The Mg# and Cr# exhibitnormal depletion trends expected from simple partial melting,whereas the NiO wt % shows an abnormal trend. On the basis ofmass balance calculations, dunites are considered to be derivedfrom the harzburgites by a process involving incongruent meltingof orthopyroxene (orthopyroxene olivine + Si-rich melt). Hydrousconditions were necessary to lower the solidus, and thus meltingof harzburgite was probably triggered by the introduction ofhydrous silicate melt. The dunite in this massif may have formedin the mantle wedge above a subduction zone. KEY WORDS: depleted peridotite; hydrous melt; incongruent melting; residual dunite; Iwanaidake peridotite     

Geochemistry of the Othris Ophiolite, Greece: Evidence for Refertilization?

The Othris peridotite massif, Greece, shows conflicting evidencefor a mid-ocean ridge and supra-subduction zone tectonic settingwith the presence of plagioclase peridotite that may representan area of either incomplete melt extraction, or melt impregnationand accumulation. To address these problems we focus on a 3km continuous section in the Fournos Kaïtsa area, consistingof layers of harzburgite, plagioclase harzburgite and plagioclaselherzolite with accurately known structural and petrographiccontrol. Refractory, Cr-rich spinel compositions and light rareearth element depleted clinopyroxenes in the harzburgites areconsistent with     

The Recrystallization Front of the Ronda Peridotite: Evidence for Melting and Thermal Erosion of Subcontinental Lithospheric Mantle beneath the Alboran Basin

Evidence for a major heating event accompanied by decompressionwas recently reported from crustal rocks drilled in the Alboranbasin. The metamorphic evolution recorded by these rocks impliescomplete removal of lithospheric mantle during the Cenozoic,a process that is confirmed by geophysical modelling indicatingthin lithosphere beneath the Alboran domain. In this region,the Ronda lherzolite massif (Betic Cordillera, southern Spain)provides a unique opportunity for the observation of mantleprocesses associated with lithospheric thinning. A strikingfeature of the Ronda peridotite is a narrow recrystallizationfront, which has been ascribed to kilometre-scale porous meltflow through the massif. The front separates the spinel tectonitedomain, interpreted as old, veined lithospheric mantle, fromthe granular domain where the lithospheric microstructures,mineralogical assemblages and geochemical signatures were obliteratedby grain growth coeval with pervasive infiltration of basalticmelts. On the basis of trace-element abundances in peridotitescollected over a distance of 12 km along the recrystallizationfront, our study confirms that the front is a relatively sharp(     

Ultramafic Xenoliths from the Bearpaw Mountains, Montana, USA: Evidence for Multiple Metasomatic Events in the Lithospheric Mantle beneath the Wyoming Craton

Ultramafic xenoliths in Eocene minettes of the Bearpaw Mountainsvolcanic field (Montana, USA), derived from the lower lithosphereof the Wyoming craton, can be divided based on textural criteriainto tectonite and cumulate groups. The tectonites consist ofstrongly depleted spinel lherzolites, harzburgites and dunites.Although their mineralogical compositions are generally similarto those of spinel peridotites in off-craton settings, somecontain pyroxenes and spinels that have unusually low Al₂O₃contents more akin to those found in cratonic spinel peridotites.Furthermore, the tectonite peridotites have whole-rock majorelement compositions that tend to be significantly more depletedthan non-cratonic mantle spinel peridotites (high MgO, low CaO,Al₂O₃ and TiO₂) and resemble those of cratonic mantle. Thesecompositions could have been generated by up to 30% partialmelting of an undepleted mantle source. Petrographic evidencesuggests that the mantle beneath the Wyoming craton was re-enrichedin three ways: (1) by silicate melts that formed mica websteriteand clinopyroxenite veins; (2) by growth of phlogopite fromK-rich hydrous fluids; (3) by interaction with aqueous fluidsto form orthopyroxene porphyroblasts and orthopyroxenite veins.In contrast to their depleted major element compositions, thetectonite peridotites are mostly light rare earth element (LREE)-enrichedand show enrichment in fluid-mobile elements such as Cs, Rb,U and Pb on mantle-normalized diagrams. Lack of enrichment inhigh field strength elements (HFSE; e.g. Nb, Ta, Zr and Hf)suggests that the tectonite peridotites have been metasomatizedby a subduction-related fluid. Clinopyroxenes from the tectoniteperidotites have distinct U-shaped REE patterns with strongLREE enrichment. They have ¹⁴³Nd/¹⁴⁴Nd values that range from0·5121 (close to the host minette values) to 0·5107,similar to those of xenoliths from the nearby Highwood Mountains.Foliated mica websterites also have low ¹⁴³Nd/¹⁴⁴Nd values (0·5113)and extremely high ⁸⁷Sr/⁸⁶Sr ratios in their constituent phlogopite,indicating an ancient (probably mid-Proterozoic) enrichment.This enriched mantle lithosphere later contributed to the formationof the high-K Eocene host magmas. The cumulate group rangesfrom clinopyroxene-rich mica peridotites (including abundantmica wehrlites) to mica clinopyroxenites. Most contain >30%phlogopite. Their mineral compositions are similar to thoseof phenocrysts in the host minettes. Their whole-rock compositionsare generally poorer in MgO but richer in incompatible traceelements than those of the tectonite peridotites. Whole-rocktrace element patterns are enriched in large ion lithophileelements (LILE; Rb, Cs, U and Pb) and depleted in HFSE (Nb,Ta Zr and Hf) as in the host minettes, and their Sr–Ndisotopic compositions are also identical to those of the minettes.Their clinopyroxenes are LREE-enriched and formed in equilibriumwith a LREE-enriched melt closely resembling the minettes. Thecumulates therefore represent a much younger magmatic event,related to crystallization at mantle depths of minette magmasin Eocene times, that caused further metasomatic enrichmentof the lithosphere. KEY WORDS: ultramafic xenoliths; Montana; Wyoming craton; metasomatism; cumulates; minette     

O–H isotope ratios of high pressure ultramafic rocks: implications for fluid sources and mobility in the subducted hydrous mantle

We examine the O-H isotope signatures of Alpine ultramafic rocks and eclogitic metagabbros of the Erro-Tobbio peridotite Unit (western Italian Alps), which record a subduction and exhumation cycle. Localization of subduction-related deformation along serpentinite mylonite shear zones favoured preservation of pre-subduction mantle and low temperature (oceanic) alteration assemblages within undeformed (meta)peridotite that underwent partial static recrystallization to high-pressure metamorphic parageneses. Bulk rock and mineral separate (clinopyroxene and serpentine) oxygen isotope ratios of the serpentinized mantle peridotites (5-8‰) are slightly enriched in ¹⁸O compared with those of the high-pressure metaperidotites and the serpentinite mylonites (4.4-7.6‰). The lowest values occur in high-pressure veins (3.5-5.7‰) and eclogitic metagabbros (3.1-5.3‰). These variations are comparable to variations observed in modern oceanic rocks and in non-subducted ophiolites. Preservation of pre-eclogitic '¹⁸O signatures of the Erro-Tobbio rocks and a lack of oxygen isotope re-equilibration between different shear zones imply local-scale fluid flow at low water/rock ratios and closed system behaviour during high-pressure metamorphism. Different serpentine generations show a bimodal distribution in 'D values: pre-eclogitic lizardite and chrysotile range from -102 to -77‰; high-pressure antigorite in the mylonites and in low strain metaperidotites range from -71 to -57‰ and -83 to -60‰, respectively. Comparable ranges occur in antigorite in the associated high-pressure veins, suggesting that the hydrogen signatures were acquired prior to veining. We propose that the isotopic variations reflect multiple events of fluid uptake in different geodynamic environments. The H- and O-isotope ratios in the eclogitic mylonites suggest that initial hydration occurred over a range of temperatures during local interaction with altered seawater along oceanic shear zones. The ¹⁸O-enriched and H-depleted compositions of chrysotile and lizardite in the mantle peridotites suggest that a second hydration event may have occurred as a result of interaction with metamorphic fluids at the early stages of burial in a forearc setting, where slabs undergo large-scale, low-temperature fluid fluxing. The oceanic mantle is thus a candidate for continuous hydration during its oceanic and early subduction history. The Erro-Tobbio unit thus represents an example of cycling of internally-derived fluids, whereby the different structural and textural domains behaved as relatively closed systems to fluid circulation during high-pressure metamorphism.     

The Ronda high temperature peridotite: Geochemistry and petrogenesis

The Ronda peridotite in southern Spain is a large (~300 km²) exposure of upper mantle which provides direct information about mantle processes on a scale much larger than that provided by mantle xenoliths in basalt. Ronda peridotites range from harzburgite to lherzolite, and vary considerably in major element content, e.g., Al₂O₃ from 0.9 to 4.8%, and trace element abundances, e.g., Sr, Zr and La abundances vary by factors of 20 to 40. These compositional variations are systematic and correlate with (pyroxene + garnet)/olivine ratios and olivine compositions. The data are consistent with formation of residual peridotites by variable degrees of melting (~0 to 30%) of a compositionally homogeneous peridotite. None of the peridotites have geochemical characteristics of residues formed by extensive (?5%) fractional melting and the data can be explained by equilibrium (batch) melting, possibly with incomplete melt segregation in some samples. Based on compositional differences between Ronda peridotites, the segregated melts were picritic (12–22% MgO) with relative rare earth element abundances similar to mid-ocean ridge basalt (MORB). Prior to the melting event the Ronda peridotite body was a suitable source for MORB. The compositional characteristics of Ronda peridotites are consistent with diapiric rise of a fertile mantle peridotite with relatively small degrees of melting near the diapir-wall rock interface yielding residues of garnet iherzolite, and larger degrees of melting in the diapir interior yielding residues of garnet-free peridotite. Subsequently these residual rocks were recrystallized at sub-solidus conditions (Obata, 1980), and emplaced in the crust by thrusting (Lundeen, 1978).