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Journal of Petrology Volume 42 Number 1 Pages 141-158 2001
© Oxford University Press 2001
The Recrystallization Front of the Ronda Peridotite: Evidence for Melting and Thermal Erosion of Subcontinental Lithospheric Mantle beneath the Alboran Basin
1LABORATOIRE DE TECTONOPHYSIQUE, UMR 5568, INSTITUT DES SCIENCES DE LA TERRE, DE L’EAU ET DE L’ESPACE DE MONTPELLIER, CNRS & UNIVERSITÉ DE MONTPELLIER 2, CASE 49, PLACE E. BATAILLON, 34095 MONTPELLIER CEDEX 05, FRANCE
2INSTITUTO ANDALUZ DE CIENCIAS DE LA TIERRA, CSIC & UNIVERSIDAD DE GRANADA, FACULTAD DE CIENCIAS, FUENTENUEVA S/N, 18002 GRANADA, SPAIN
Revised typescript accepted June 27, 2000
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ABSTRACT |
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TOP
ABSTRACT
INTRODUCTIONEvidence for a major heating event accompanied by decompression was recently reported from crustal rocks drilled in the Alboran basin. The metamorphic evolution recorded by these rocks implies complete removal of lithospheric mantle during the Cenozoic, a process that is confirmed by geophysical modelling indicating thin lithosphere beneath the Alboran domain. In this region, the Ronda lherzolite massif (Betic Cordillera, southern Spain) provides a unique opportunity for the observation of mantle processes associated with lithospheric thinning. A striking feature of the Ronda peridotite is a narrow recrystallization front, which has been ascribed to kilometre-scale porous melt flow through the massif. The front separates the spinel tectonite domain, interpreted as old, veined lithospheric mantle, from the granular domain where the lithospheric microstructures, mineralogical assemblages and geochemical signatures were obliterated by grain growth coeval with pervasive infiltration of basaltic melts. On the basis of trace-element abundances in peridotites collected over a distance of 12 km along the recrystallization front, our study confirms that the front is a relatively sharp (
400 m) geochemical discontinuity at the scale of the Ronda massif. Compared with the spinel tectonites, the coarse-granular peridotites are more homogeneous, more refractory in terms of major elements and more depleted in incompatible trace elements. These features are consistent with a process involving partial melting, kilometre-scale migration of melts by diffuse porous flow and limited melt extraction (2·5–6·5%). Hence, the Ronda recrystallization front is interpreted as the narrow boundary of a partial-melting domain (the coarse-granular peridotites) formed at the expense of subcontinental lithospheric mantle (the spinel tectonites). The existence of melt-consuming reactions in the transitional peridotites, a few hundred metres ahead of the melting front, demonstrates that the front was thermally controlled. This implies that a smooth thermal gradient existed across the Ronda massif during the development of the recrystallization front. Differences in pyroxene compositions on either side of the front may be explained by a transient heating event at 
1200°C (
1·5 GPa) coeval with partial melting. Consistent with the geodynamic scenario proposed for the Alboran domain during the Cenozoic, the evolution of the Ronda recrystallization front is considered as an example of thermal erosion and partial melting of lithospheric mantle above upwelling asthenosphere. KEY WORDS: asthenosphere–lithosphere interaction; partial melting; Ronda peridotite; subcontinental mantle; trace elements
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INTRODUCTION |
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Removal of lithospheric mantle above upwelling asthenosphere and mantle plumes is now recognized as an important mechanism for lithospheric thinning in the early stages of continental rifting (e.g. Menzies et al., 1993
; Griffin et al., 1998). Most often, this process has been ascribed to mechanical erosion of conductively heated, lower lithosphere by convective mantle (Monnereau et al., 1993; Davies, 1994; Ruppel, 1995). Moreover, upon heating and/or decompression, the lithospheric peridotites may be partially molten (McKenzie & Bickle, 1988) and infiltrated by reactive, deep-seated melts (Kelemen et al., 1995). This process—which has been referred to as ‘chemical erosion’ by Van der Wal & Bodinier (1996) and Bedini et al. (1997)—may obliterate previous microstructures, mineral assemblages and geochemical signatures inherited from long-term residence of peridotites in subcontinental lithospheric mantle (Garrido & Bodinier, 1999). In turn, melt–rock reactions and magma ponding at the base of the thermal boundary layer (e.g. Xu et al., 1998) may possibly weaken the lower lithosphere, favour its removal by mantle convection, and initiate feedback mechanisms involving further thermal or chemical erosion.
Recently, evidence for a major heating event accompanied by decompression was reported from crustal rocks drilled in the Alboran basin in the westernmost Mediterranean Sea (Soto & Platt, 1999). For Platt et al. (1998), the metamorphic evolution recorded by these rocks implies complete removal of lithospheric mantle during the Cenozoic, a process that is confirmed by geophysical modelling indicating thin lithosphere beneath this basin (Torné et al., 2000). In this region, the Ronda lherzolite massif (Betic Cordillera, southern Spain) provides a unique opportunity for the on-land study of the mantle processes associated with lithospheric thinning. A well-known characteristic of the Ronda peridotite is the existence of a kilometre-scale structural, petrological and geochemical zoning (Obata, 1980; Frey et al., 1985; Reisberg et al., 1989; Van der Wal & Bodinier, 1996; Van der Wal & Vissers, 1996; Garrido & Bodinier, 1999) (Fig. 1). On the basis of correlations between geochemistry, microstructures and petrological facies in pyroxenites and peridotites, Van der Wal & Bodinier (1996), and Garrido & Bodinier (1999) have ascribed the zoning of Ronda to kilometre-scale porous melt flow through the massif. A striking feature related to this event is a narrow (400 m) recrystallization front (Fig. 1) separating the ‘spinel tectonite domain’, interpreted as old, veined lithospheric mantle, from the ‘granular domain’ where lithospheric microstructures, mineralogical assemblages and geochemical signatures have been obliterated by textural coarsening and re-equilibration with pervasive basaltic melt.
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The Ronda recrystallization front can be followed over a distance of 20 km in the field. Van der Wal & Vissers (1993, 1996) have already highlighted the importance of the front at regional scale and its probable connection with asthenospheric upwelling in the Alboran domain. As the front represented—shortly before the emplacement of the massif—the sharp boundary between lithospheric mantle rocks and a mantle domain extensively percolated by basaltic melts, it is clearly pivotal to the understanding of lithosphere–asthenosphere interaction. Van der Wal & Bodinier (1996) have provided an overview of textural and geochemical variations on either side of the front, in the spinel tectonite and in the whole granular domain (i.e. the coarse-granular, fine-granular and layered-granular subdomains). In addition, they have proposed a scenario to explain the overall chemical variations in the Ronda massif, involving spatial and temporal variations of interstitial melt fraction (‘porosity’) and melt–rock reactions.
This study is focused on the geochemical modifications coeval with the generation of the recrystallization front and particularly on those involved in the development of the coarse-granular peridotites at the expense of the spinel tectonite domain. The aims of this study are (1) to evaluate the lateral continuity of geochemical variations across the Ronda recrystallization front, over a distance of 12 km, (2) to further constrain the nature of melt processes at and adjacent to the front, and (3) to investigate whether peridotite minerals have registered a thermal event contemporaneous with the development of the recrystallization front.
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THE RONDA RECRYSTALLIZATION FRONT |
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The Ronda peridotite (southern Spain) is the largest outcrop (300 km2) of subcontinental mantle peridotites exposed at the Earth’s surface. A well-known characteristic of this massif is a kilometre-scale petrological zoning (Fig. 1). From the NNW to the SSE of the massif, Obata (1980)
mapped four metamorphic facies recording decreasing equilibration pressures: (1) a garnet lherzolite facies; (2) an ariegite sub-facies of the spinel lherzolite facies; (3) a seiland sub-facies of the spinel lherzolite facies; (4) a plagioclase lherzolite facies (Fig. 1). On the basis of overprinting relationships, Van der Wal & Vissers (1993, 1996) have recently identified three tectono-metamorphic domains of decreasing relative age (Fig. 1):
- the spinel tectonite domain corresponds to the garnet lherzolite facies and the ariegite sub-facies of Obata (1980). It is made of porphyroclastic spinel peridotites and mylonitic garnet–spinel peridotites, with subordinate amounts of garnet-bearing mafic layers. The spinel tectonites represent old lithospheric mantle, isolated from the convective mantle at 1·36 Ga (Reisberg & Lorand, 1995).
- The granular peridotite domain broadly coincides with the seiland sub-facies of Obata (1980). It is composed of unfoliated spinel peridotites and contains various types of pyroxenites, generally devoid of garnet (Garrido & Bodinier, 1999). This domain derived from the spinel tectonites by pervasive annealing and grain growth of silicates and spinel. These textural changes occur across a relatively narrow recrystallization front (400 m), which was developed as a result of kilometre-scale porous melt flow, shortly before the emplacement of the massif in the crust (Van der Wal & Bodinier, 1996). To be consistent with previous work, we will used here the term ‘recrystallization front’ to denote the sharp increase in grain size (coarsening) of peridotite and mafic layer minerals observed in the field at this interface. However, in the light of the physical processes involved in the development of the front (grain growth, annealing and/or melt-enhanced sintering), we note that the term ‘coarsening front’ is a more appropriate textural term, as recrystallization—as used in the material science literature—implies grain-size reduction relative to the former microstructure.
- The plagioclase tectonite domain is the youngest of the three tectono-metamorphic domains and corresponds to the plagioclase lherzolite domain of Obata (1980). It is composed of porphyroclastic plagioclase peridotites, with subordinate mylonites and layers of spinel–plagioclase–olivine websterites. The development of this domain is related to the crustal emplacement of the massif.
The recrystallization front is a striking feature of the Ronda massif. It can be followed over a distance of 20 km and is generally oriented parallel to the foliation trend of the spinel tectonites (Fig. 1). Similar to the spinel tectonite foliation, which dips 70–80° to the NW, the front is almost vertical. However, Van der Wal & Vissers (1993, 1996) have noted local obliquity between the front and the spinel tectonite foliation, especially between the Anicola and Monte de los Reales summits (Fig. 1). Across the front, all the microstructures of the spinel tectonites were overprinted. The strongly foliated spinel tectonites were transformed to coarse-granular peridotites. Simultaneously, the minerals were re-equilibrated in an open system at high melt–rock ratio (Van der Wal & Bodinier, 1996) and the Al-rich, garnet pyroxenites were replaced by Al-poorer spinel websterites (Garrido & Bodinier, 1999). For Van der Wal & Bodinier (1996), the recrystallization front underlines the maximum extent of basaltic melt percolation in the massif during a late event of lithospheric thinning. During this event, the front represented a permeability barrier for basaltic melts, which were accumulated in the coarse-granular peridotites. However, volatile-rich small melt fractions with a refractory (Mg-rich, Al-poor), calc-alkaline composition were infiltrated beyond the recrystallization front and invaded the spinel tectonite domain (Garrido & Bodinier, 1999). These melts are ascribed to olivine- and melt-consuming reactions associated with porous melt flow in Ronda, during and after the development of the recrystallization front.
Several studies have revealed important trace-element and isotopic variations in Ronda peridotites and pyroxenites (Frey et al., 1985; Suen & Frey, 1987; Reisberg et al., 1989; Van der Wal & Bodinier, 1996; Garrido & Bodinier, 1999). Some of these geochemical variations coincide with the petrological–structural zoning of the massif and were probably generated by the magmatic event inferred for the development of the recrystallization front.
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SAMPLING AND TEXTURAL GROUPS |
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For this study, 35 samples of peridotites were collected along six sections, a few hundred metres long, across the recrystallization front (Fig. 1). In addition, one sample (RX30) was collected at the southern end of the front, near the locality of Casa del Teniente. The sampling localities cover a distance of
12 km along the front. In addition, we collected five samples in the spinel tectonite domain (samples RX37, RX38, RX39, RX41 and RX42), at a greater distance from the front than any of the samples studied by Van der Wal & Bodinier (1996). These samples belong to the garnet lherzolite facies mapped by Obata (1980) but they are devoid of garnet. The studied peridotites can been classified in three textural groups, which are closely related to the tectonic domain in which they occur (Fig. 2):
- (1) porphyroclastic peridotites, representing the deformed peridotite facies predominant in the spinel tectonite domain (Fig. 2a) (see also Van der Wal & Vissers, 1996).
- (2) Transitional peridotites, which define the recrystallization front itself (Fig. 2b–e). This textural group occurs in a narrow band, a few hundred metres thick, between the porphyroclastic peridotites of the spinel tectonite domain to the NW and the recrystallized, coarse-granular peridotites to the SE (Fig. 1). Texturally, the transitional peridotites vary between two extremes:
- (2a) one end-member is similar to the porphyroclastic peridotites in the field, but its texture is distinguished by the presence of undeformed aggregates of clinopyroxene (cpx). These aggregates, made of equant (0·5 mm) cpx grains with a cloudy appearance (Fig. 2b and c), coexist with clean cpx neoblasts—produced by recrystallization of porphyroclasts—and deformed cpx porphyroclasts (Fig. 2b and d). Obata (1980) had already described these aggregates and pointed out their unusual ‘turbid’ appearance (Fig. 2c)—owing to the numerous tiny inclusions—and their strong chemical disequilibrium with clinopyroxene neoblasts and porphyroclasts in the same thin section. He showed that these cloudy cpx aggregates are distinctly more aluminous and less calcic and therefore record a high-temperature event. This textural facies occurs in contact with the spinel tectonites, along the northwestern border of the recrystallization front. However, it is also found locally within the spinel tectonite domain, a few hundred metres ahead of the recrystallization front [e.g. sample R243 of Obata (1980) and sample DR93.7 of Van der Wal & Bodinier (1996); Fig. 1].
- (2b) The other end-member of the transitional peridotites resembles the coarse-granular peridotites (described below), but the spinel grains preserve ‘holly-leaf’ shapes typical of the porphyroclastic facies and are still aligned in trails, parallel to the foliation of the spinel tectonites (Fig. 2e). This textural facies is found along the southeastern border of the recrystallization front, in contact with the coarse-granular peridotites—which also contain isolated relics of this facies.
Most samples collected across the recrystallization front are intermediate between these two extremes. They generally contain undeformed aggregates of cpx, but the ‘secondary’ origin of these aggregates (i.e. subsequent to the deformation registered by the spinel tectonites) is often obscured by pervasive recrystallization of silicate minerals.
- (3) Coarse-granular peridotites characterized by very coarse and completely annealed microstructures (Fig. 2f). This textural facies defines within the granular domain the coarse-granular subdomain, which is spatially associated with the recrystallization front (Van der Wal & Bodinier, 1996).
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ANALYTICAL METHODS |
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Twenty-four peridotites representative of textural types and sampling localities have been analysed by electron microprobe (EPMA) at Service Commun Microsonde Sud, Université Montpellier II, using a Camebax SX100. Both orthopyroxene (opx) and cpx are zoned from core to rim, and the cores contain pervasive exsolution lamelli. Compositions of pyroxene cores before unmixing were estimated for four samples from this study, plus one sample from Van der Wal & Bodinier (1996)
. The samples include two spinel tectonites (RX39 and RX41), one transitional peridotite representing the less recrystallized end-member of this group [characterized by a porphyroclastic texture and the presence of secondary cpx (DR93.7)], one transitional peridotite representing the most recrystallized end-member [characterized by a coarse-granular texture with preserved spinel trails (RX11)], and one coarse-granular peridotite (RX35).
The calculations were performed with the total inversion method of Tarantola & Valette (1992) applied to a mass-balance equation relating (1) electron probe microanalysis (EPMA) data for unmixed pyroxene cores and exsolution lamellae, (2) EPMA data for pyroxene rims and (3) bulk pyroxene major-element compositions obtained by inductively coupled plasma–atomic emission spectrometry (ICP-AES) analyses of hand-picked, pyroxene separates. A priori values were estimated for exsolution and crystal rim proportions by back-scattered electron imaging and EPMA traverses across pyroxenes, respectively, and fixed within large error ranges. The a posteriori results indicate 7 ± 2% of opx exsolutions in cpx cores for the spinel tectonites, 14 ± 3% for the less recrystallized transitional peridotite and 17 ± 3% for the most recrystallized transitional and the coarse-granular peridotite samples. The opx cores have a low proportion of cpx exsolutions (2·6 ± 1·0%) without significant variations according to textural facies. The a posteriori rim proportions vary from 10·5 ± 2% for opx in the transitional and coarse-granular samples (RX11 and RX35) to 16 ± 2·5% for cpx in the spinel tectonites and in the less recrystallized transitional peridotite (RX39, RX41 and DR93.7). In situ pyroxene analyses by EPMA and bulk pyroxene analyses by ICP-AES, as well as compositions of pyroxene cores reconstituted by the inverse method, are given in Table 1 for the five samples processed with this method.
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Whole-rock and pyroxene separate analyses of major elements were performed by ICP-AES at CEREGE, Université Aix–Marseille III. Transition metals (Sc, Ti, Co, Ni, Cu and Zn) and trace elements [Rb, Sr, Y, Zr, Nb, Cs, Ba, rare earth elements (REE), Hf, Ta, Pb, Th and U] were determined by ICP–mass spectrometry (ICP-MS) at ISTEEM, Université Montpellier II, using a VG-PQ2 spectrometer. In addition to the 24 samples analysed by EPMA, ICP-AES and ICP-MS, a second set of 14 samples was analysed by ICP-MS to further constrain the spatial variations of trace elements across the recrystallization front. For trace-element analyses, we followed the method described by Ionov et al. (1992). The precision of Montpellier ICP-MS analyses is in the range 1–5% for most elements, except for Sm, Gd and Hf (5–6%), and Eu and Ta (9%) (Godard et al., 2000). The accuracy of our ICP-MS analyses can be evaluated from three duplicate analyses of the international standard UB-N performed during this study and reported in Table 2.
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Modal proportions were calculated with the total inversion method applied to a mass-balance equation relating major-element compositions of whole rocks and minerals. Modal, major-element and trace-element compositions are reported in Table 2 for 10 samples collected along two sections across the front (samples RX8–RX 12 and RX25–RX29; Fig. 1). The dataset can be obtained from the corresponding author upon request.
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MINERAL CHEMISTRY AND GEOTHERMOMETRY |
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Single olivine and spinel grains are homogeneous, and grains within a sample have similar compositions; however, there are significant compositional differences between samples in a sampling cross-section. The whole dataset shows Fo variations in the range 0·881–0·924 for olivine, and cr-number in the range 0·06–0·50 for spinel. These variations mostly reflect whole-rock compositional variations. The cr-number in spinel, for example, is significantly higher in the harzburgites than in the lherzolites from the same cross-section (e.g. cr-number is 0·69 for the harzburgite RX42, compared with 0·21 for the lherzolite RX4).
Opx and cpx display strong core to rim zoning of Al and Ca, as well as pervasive exsolution of pyroxene and/or spinel. Obata (1980) ascribed the chemical intra-grain disequilibrium shown by Ronda pyroxenes to late cooling of the massif during its emplacement into the crust. Otherwise, similar to olivine and spinel, opx and cpx display variations between samples that reflect whole-rock variations. The Al content of the opx cores, for instance, is lower in the harzburgite sample RX42 (Al2O3 = 3·5%; unmixed host composition of the opx core) than in the lherzolite RX41 from the same section (Al2O3 = 5·2%). However, the reconstituted cpx cores show significant variations of Ca related to textural facies that cannot be explained by whole-rock variations (Table 1). The spinel tectonites show cpx with Ca-rich cores (19·6–19·8 CaO wt %) compared with cpx of transitional and coarse-granular peridotites (17·6–17·7 CaO wt %).
Following Obata (1980), the integrated chemical composition of pyroxene cores in Ronda peridotites records the ‘primary’ metamorphic conditions of peridotites at upper-mantle conditions. Accordingly, the chemical variations shown by the reconstituted composition of pyroxenes may provide insight into the equilibration temperatures related to the development of the recrystallization front. With this aim, we have estimated the equilibration temperature in five samples using the compositions of unmixed cores (Table 1) and the three following thermometric formulations involving pyroxenes: (1) the Ca-in-cpx [‘BKN’ formulation of Brey & Köhler (1990)] and (2) the Ca-in-opx geothermometers [‘Ca-in-opx’ formulation of Brey & Köhler (1990)] based on the two-pyroxene solvus; and (3) the Al-in-opx geothermometer based on the Al solubility in orthopyroxene in the spinel lherzolite facies [opx–sp formulation of Witt-Eickschen & Seck (1991)]. The temperature estimates are shown in Fig. 3, as well as the temperature standard deviations obtained from the error propagation of core compositions in the thermometric formulations (see Fig. 3 caption for further details of the calculation). The three formulations yield similar temperatures for the two samples of spinel tectonites, indicating relatively low temperatures in the range 1050–1100°C. The BKN formulation provides significantly higher temperatures for the three transitional and coarse-granular peridotites (1180–1225°C). The Ca-in-opx formulation also provides similar results for the transitional and coarse-granular peridotites, but the temperature range is lower (1140–1150°C). Finally, the opx–sp formulation yields a temperature similar to that obtained with the Ca-in-opx method for the coarse-granular peridotite (1155°C ± 33°C), but provides lower temperatures—indistinguishable from those obtained for the spinel tectonites—for the two transitional peridotites (1050–1110°C). In general, our results indicate that the development of the recrystallization front was associated with a heating event in the temperature range of 1180–1225°C, as recorded by the BKN geothermometer in the transitional and coarse-granular peridotites. It is worth noting that the temperature of 1200°C inferred for this event is close to the anhydrous solidus of peridotites at 1·5 GPa (e.g. Takahashi & Kushiro, 1983). This pressure range roughly coincides with the transition of ariegite to seiland subfacies observed in the Ronda mafic layers at the recrystallization front (Obata, 1980; Garrido & Bodinier, 1999).
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computed from the error propagation in the geothermometric formulation of the a posteriori variances of compositions obtained in the inverse mass-balance calculation. Symbols as in Fig.
If the temperatures obtained with the BKN method for the various peridotite textural facies were considered as actual differences in temperatures across the recrystallization front (400 m), they would imply an unrealistic, steep geothermal gradient of 0·25°C/m. The close relationship between the higher temperatures recorded by the BKN geothermometer and the textural variations ascribed to melt-enhanced recrystallization by Van der Wal & Bodinier (1996) leads us to suggest that the temperature variations recorded in peridotites across the recrystallization front have a kinetic origin. In fact, the three geothermometric formulations are characterized by distinct diffusion kinetics. Experimental (Brady & McCallister, 1983; Dimanov et al., 1996) and petrological observations (Witt-Eickschen & Seck, 1991, and references therein) indicate decreasing diffusion coefficient (D) in the order Dcpx(Ca–Mg) > Dopx(Ca) >> Dopx(Al). During a fast, transient-heating event, inter-mineral re-equilibration would have been enhanced in the transitional and coarse-granular peridotites, as a result of the presence of a relatively high fraction of interstitial melt. As discussed below, dissolution–precipitation reactions involving cpx, together with the faster diffusion of Ca in cpx, may account for the higher temperatures provided by the BKN thermometer. The lower temperatures recorded by the other geothermometers (especially by the opx–sp method for the transitional peridotites) would reflect disequilibrium owing to slower solid diffusion of exchanges involved in these reactions, compared with critical rates of melt processes (Bédard, 1989; Qin, 1992; Iwamori, 1993). Thus, the lower diffusivity of Ca in opx and larger grain size of opx (Van der Wal & Bodinier, 1996) probably limited the equilibration of this mineral and would account for the lower temperatures obtained with the Ca-in-opx formulation. Likewise, inter-mineral equilibration of aluminium was possibly limited by solid diffusion in spinel, which would explain the low temperatures yielded by the opx–sp method for the transitional peridotites. Dissolution–precipitation reactions affecting spinel in the coarse-granular peridotites may have released this blockage and accounted for the high opx–sp temperature measured in this textural group (Fig. 3).
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GEOCHEMISTRY |
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Major elements and transition metals
The studied peridotites display a wide range of modal and major-element compositions comparable with the variation range reported by Frey et al. (1985)
for the whole Ronda massif and by Van der Wal & Bodinier (1996) for the spinel tectonite and the coarse-granular domains (olivine = 55–77 wt %, cpx = 1·5–16 wt %, MgO = 39–46 wt %, Al2O3 = 0·9–4 wt % and CaO = 0·5–3·6 wt %). Similar to other mantle peridotites world-wide (e.g. McDonough & Frey, 1989, and references therein), these samples show positive correlations between Al, Ca, Na, Ti, Sc and Cu, and negative correlations between these elements and Mg, Co and Ni. Fe and Zn do not show significant correlations with the other elements. The mg-number [= Mg/(Mg + 
Fe) cationic ratio] varies from 89·2 to 92·8 and is roughly correlated with olivine proportion.
Our sampling across the recrystallization front reveals subtle differences between the spinel tectonites and the transitional peridotites, and more important variations between the transitional peridotites and the coarse-granular ones (Table 2 and Fig. 4). In contrast to the spinel tectonites, which include extremely variable compositions, from very fertile lherzolites (CaO/MgO > 0·08; Fig. 4a) to refractory harzburgites (CaO/MgO < 0·02), the transitional peridotites tend to be more homogeneous and dominated by mildly fertile lherzolites (CaO/MgO = 0·06–0·08). The coarse-granular peridotites also tend to be more homogeneous than the spinel tectonites, but they are dominated by more refractory compositions (CaO/MgO = 0·02–0·04), compared with the transitional peridotites. In general, the three textural groups show important overlaps of their major-element compositions. The whole range of CaO/MgO, for instance, is 0·012–0·102 for the spinel tectonites, 0·030–0·083 for the transitional peridotites and 0·009–0·090 for the coarse-granular peridotites (Fig. 4a). However, using conventional box-plots it can be shown that these overlaps are mostly caused by the ‘very extreme value’ of two samples from the transitional and coarse-granular peridotites (Fig. 4b; see figure caption for more details about the design of box plots). RX4 is a transitional peridotite and corresponds to the only harzburgite in this group. It is distinguished from the other samples of this group by its lower CaO/MgO ratio (0·03) (Fig. 4b). Conversely, RX35 is a coarse-granular peridotite distinguished from the other samples of this group by its fertile composition and higher CaO/MgO ratio (0·09) (Fig. 4b). Modal calculations performed for RX35 with reconstituted pyroxene compositions indicate a very high cpx/opx ratio for this sample (0·96 ± 0·17, compared with 0·66 ± 0·12 for the other coarse-granular peridotites). In this respect, sample RX35 is probably akin to the fine-granular subdomain. This subdomain overprinted the coarse-granular subdomain during melt-consuming, cpx-forming reactions in the waning stages of melt percolation in the Ronda subdomain that resulted in overall fertilization of the peridotites (Van der Wal & Bodinier, 1996). This subdomain is especially well developed to the NW of Monte de los Reales, where it almost overprints the recrystallization front [see fig. 1 of Van der Wal & Bodinier (1996)]. Sample RX35 was collected in this area (Fig. 1) and it may then be transitional to the fine-granular facies.
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µ2 (i.e. the distributions have different means). An 
of 0·05 (corresponding to a 0·95 confidence interval) was used in all tests. R and FR stand respectively for ‘reject’ and ‘fail to reject’ the null hypothesis as conventionally used in statistical significance tests. If H0 is rejected, the averages of CaO/MgO differ between the two textural groups for the given
To assess whether the variations of the average CaO/MgO ratios observed between the three textural groups are statistically meaningful, we have applied a two-sample t-test using our dataset and including the analyses reported by Van der Wal & Bodinier (1996) for the spinel tectonites and coarse-granular samples (Fig. 4; further information about the statistical method is given in the figure caption). The t-test results indicate that the transitional peridotites are significantly more fertile (CaO/MgO = 0·072 ± 0·009) than the spinel tectonite (CaO/MgO = 0·048 ± 0·027) and the coarse-granular textural facies (CaO/MgO = 0·030 ± 0012). Conversely, the coarse-granular facies is significantly more refractory than the two other facies (Fig. 4). The high average fertility and the elevated CaO/MgO ratio of the transitional peridotites is probably connected with the presence of secondary cpx aggregate characteristics of this textural facies (Figs. 2b and c). In good agreement with this textural observation, modal calculations based on reconstituted pyroxene compositions indicate higher cpx/opx ratios for the two transitional peridotites (DR93.7 and RX11; cpx/opx = 0·6 ± 0·15), compared with the two spinel tectonites (RX39 and RX41; cpx/opx = 0·4 ± 0·09). We have also conducted a statistical F-test to evaluate the meaning of the differences of the CaO/MgO variances between the textural groups. The results of the F-test and further information on its computation are provided in Table 3. The F-test corroborated that the CaO/MgO ratio of transitional and coarse-granular facies (Fig. 4b) is significantly more homogeneous than that of the spinel tectonites.
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Rare earth elements
The chondrite-normalized REE patterns of the analysed peridotites are shown in Fig. 5 grouped according to textural facies. Overall, the REE variations display features similar to those previously described in orogenic peridotites, including Ronda (Frey et al., 1985
; McDonough & Frey, 1989; Van der Wal & Bodinier, 1996). These include: (1) systematic, positive correlations of heavy REE (HREE) and medium REE (MREE) as a function of Ca and Al, and less well-defined correlations of light REE (LREE) with these elements; (2) depletion of chondrite-normalized LREE relative to HREE in lherzolites; (3) lesser depletion of LREE, or LREE enrichment relative to HREE, in harzburgites (samples RX42, RX5 and RX21). These variations have been discussed in previous studies and generally interpreted in terms of partial melting, magma transport and melt–rock interactions (e.g. Frey et al., 1985; Navon & Stolper, 1987; Bodinier, 1988; McDonough & Frey, 1989; Takazawa et al., 1992; Godard et al., 1995; Van der Wal & Bodinier, 1996; Vernières et al., 1997; and references therein).
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Besides these general features, our data reveal a marked change in REE contents and ratio of LREE to MREE across the recrystallization front (Figs 5–7). These variations were noted by Van der Wal & Bodinier (1996)
, but our detailed sampling across and along the front allows us to better localize the REE discontinuities. Similar to major elements, REE show subtle differences between the spinel tectonites and the transitional peridotites, and more important variations between the transitional peridotites and the coarse-granular ones.
Ahead of the recrystallization front, the spinel tectonites are characterized by a wide HREE gap between 0·2 x chondrites and 1·2 x chondrites [see also fig. 5a of Van der Wal & Bodinier (1996)]. This HREE gap also coincides with a gap in the fertility degree of the peridotites, between harzburgites (cpx 5%) and relatively fertile lherzolites (cpx 10%). In general, the spinel tectonites have slightly more variable MREE and HREE contents than the other two textural groups (Table 3; Fig. 6). The transitional peridotites show slightly higher MREE and HREE contents than the spinel tectonites (Figs 5 and 6). A harzburgite (RX4) and a very refractory harzburgite (RX21) have extremely low values of REE compared with other transitional peridotites (Fig. 6). Sample RX21 also has extremely high LREE/MREE ratio. On the other hand, sample RX34 has a relatively high CeN/SmN ratio and is characterized by a relatively flat REE pattern compared with the other transitional peridotites (Fig. 5). Excluding these extreme values, the transitional peridotites show, on average, a significantly smaller CeN/SmN ratio (Fig. 7) and have a much more homogeneous LREE/MREE ratio than the spinel tectonites.
The coarse-granular peridotites differ markedly from the other two textural facies with respect to REE content and LREE/MREE fractionation (Figs 5–7). In particular, their HREE contents are mostly restricted to a relatively narrow range that coincides with the HREE gap observed in the spinel tectonites, between 0·2 x chondrites and 1·2 x chondrites (Fig. 5). However, with the exception of two outliers [samples RX35 and DR93.5, which are probably more akin to the fine-granular facies of Van der Wal & Bodinier (1996), as discussed above], the coarse-granular peridotites are significantly more depleted in all REE than the other two textural facies (Fig. 6) and display lower CeN/SmN ratio than spinel tectonites (Fig. 7). On average, the coarse-granular peridotites are significantly more homogeneous in terms of REE contents than spinel tectonites and transitional peridotites, but show a range of CeN/SmN variation similar to that of transitional peridotites (Table 3; Fig. 7).
As they are correlated with Ca content, variations of HREE and MREE abundances across the front (Table 2) are essentially controlled by the fertility of the peridotites, which increases from the spinel tectonites to the transitional peridotites, and then decreases abruptly in the coarse-granular subdomain (Fig. 4a). The higher HREE and MREE contents of the transitional peridotites (Fig. 6) are in good agreement with cpx-forming reactions as previously inferred from their greater CaO/MgO ratio (Fig. 4). In part, the LREE contents also reflect the variations in peridotite fertility. However, the mean CeN/SmN ratio shows an overall decrease across the recrystallization front (Fig. 7), unrelated to peridotite fertility, and it is significantly more homogeneous in the transitional and coarse-granular peridotites compared with spinel tectonites (Table 3).
Other trace elements
The patterns of trace elements normalized to primitive-mantle values (PM) are shown in Fig. 8 for two sections across the recrystallization front (samples RX8–RX12 and RX25–RX29; Fig. 1 and Table 2). These examples indicate that the overall trace-element signatures are broadly similar in the three textural facies, all samples showing similar depletion of Ba, Th, U, Nb and Ta relative to LREE and selective enrichment of Rb (± Ba) relative to Th. They have subtle to significant positive U anomalies that vary in amplitude along the front. The selective enrichment of Rb (± Ba, ± U) relative to Th is observed in all textural facies of the Ronda massif and is considered as a primary, mantle signature, as it is observed in acid-leached mineral separate and in situ analyses of silicates (Garrido et al., 2000). Although broadly correlated with LREE, the highly incompatible elements (Rb to U) and Sr do not decrease as much as the LREE across the recrystallization front. This is particularly well illustrated by the PM-normalized, trace-element patterns of samples RX8–RX12 (Fig. 8), which show only a restricted variation of Rb, Ba, Th and U, compared with LREE. In addition, the lesser depletion of Sr is indicated by the existence of a positive anomaly of this element on the trace-element patterns of most of the coarse-granular peridotites. Accordingly, the coarse-granular peridotites tend to show higher ratios of the highly incompatible elements and Sr to LREE than the other facies. In contrast, Zr and Hf tend to be slightly more depleted than MREE in the coarse-granular peridotites (Fig. 8).
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One of the most striking features of the trace-element variations across the Ronda recrystallization front is the abrupt change in the Nb/Ta ratio occurring within a distance of 200 m between the transitional and the coarse-granular peridotites (Fig. 8). The spinel tectonites and transitional peridotites show a systematic depletion of Nb relative to Ta (Fig. 8), with Nb/Ta ratios (mean values 11·7 and 9·2, respectively) lower than the PM values [17·3 after Sun & McDonough (1989)]. In contrast, the coarse-granular peridotites have flat Nb–Ta segments on the PM-normalized diagram and Nb/Ta ratios close to the PM values (mean value 15·9).
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NATURE OF MELT PROCESSES RELATED TO THE DEVELOPMENT OF THE RECRYSTALLIZATION FRONT |
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Van der Wal & Bodinier (1996)
have demonstrated that the Ronda recrystallization front was developed in an open system. To account for the geochemical differences between peridotites collected on either side of the front, they proposed a model involving kilometre-scale melt percolation through the massif. In this model, the recrystallization front was interpreted as a permeability barrier between two distinct porous-flow domains: (1) the spinel tectonite domain representing old lithospheric mantle traversed by small fractions of volatile-rich melts and (2) the coarse-granular (recrystallized) domain, percolated by high fractions of basaltic melts. The latter is considered to be produced by in situ partial melting, with a contribution of a deep-seated melt component (Garrido & Bodinier, 1999). In contrast, the volatile-rich, small melt fractions are interpreted as residual liquids after incomplete solidification of basaltic melts along and ahead of the recrystallization front.
As it was based on samples collected at some distance on either side of the recrystallization front, the study of Van der Wal & Bodinier (1996) does not directly bear on the melt processes that occurred at the front itself and were intimately connected with peridotite recrystallization. On the basis of our detailed sampling, we show that both partial melting and melt-consuming reactions were contemporaneous across the recrystallization front, although spatially separated by a few hundred metres. The extraction of partial melts from the coarse-granular peridotites is indicated by the abrupt decrease of peridotite fertility degree (Fig. 4) and REE contents in the few hundred metres (200 m) separating this facies from the transitional peridotites (Fig. 6). Further evidence for this process includes lower CeN/SmN ratios in the coarse-granular and transitional peridotite relative to spinel tectonites (Fig. 7). Thus, the LREE/MREE values of individual samples may be used to visualize the two-dimensional REE variations related to partial melting in the SW of Ronda. CeN/SmN mapping, for instance, indicates that the Ronda recrystallization front constitutes a melting front, over a distance of at least 12 km (Fig. 9). Using conventional melting models, the percentage of melt extraction can be estimated from the difference in Yb concentration between the coarse-granular peridotites and the other peridotite facies. Because the HREE are less sensitive than LREE to melting (i.e. batch, fractional or incremental melting; Shaw, 1970; Johnson et al., 1990) and/or melt porous flow models (e.g. Godard et al., 1995; Ozawa & Shimizu, 1995; Vernières et al., 1997; Suhr, 1999), these trace elements are good indicators of the degree of melt extraction in peridotites. In addition, these elements are sufficiently incompatible to be almost insensitive to the nature of the melting reaction as long as garnet is not involved (e.g. Kinzler & Grove, 1992; Baker & Stolper, 1994; Walter et al., 1995; Kinzler, 1997; Niu, 1997). For the calculation, we have used the experimental HREE mineral–melt partition coefficients published by Kelemen et al. (1993). Using the mean Yb content of the spinel tectonites as the source composition [including the dataset of Van der Wal & Bodinier (1996)], the estimated melt extraction degree varies between 2·5% for fractional melting and 3·5% for batch melting. It is possible that the spinel tectonites did not represent the actual source of the coarse-granular peridotites, as the two facies are separated by the more fertile, transitional peridotites. As discussed below, transitional peridotites were probably fertilized by synchronous melt-consuming reactions, ahead of the peridotite melting front. If the mean Yb content of the transitional facies is taken as the source composition, the degree of melt extraction is 5·5–6·5%.
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The textural evidence of secondary cpx (Fig. 2b and c) and high average fertility of transitional peridotites—compared with the other facies (Fig. 4)—may indicate melt-consuming melt–rock reactions. Several features, such as their low LREE/MREE ratio compared with the spinel tectonites (Fig. 7), suggest that the reaction involved partial melts infiltrated from the coarse-granular domain. Previous studies have reported textural, mineralogical and geochemical evidence for melt-consuming reactions in Ronda (Van der Wal & Bodinier, 1996; Garrido & Bodinier, 1999). These reactions involve precipitation of cpx + spinel (or cpx + opx + spinel) at the expense of opx (or olivine) and occurred either pervasively, forming the fertilized fine-granular lherzolites (Van der Wal & Bodinier, 1996), or locally, forming cpx-rich mafic layers (type-C spinel websterites of Garrido & Bodinier, 1999). As stated above, these melt-consuming reactions have been ascribed to late percolation stages, when melts were progressively ‘drained’ from the granular domain during cooling of the Ronda massif (Van der Wal & Bodinier, 1996). However, in terms of space and time relationships with the recrystallization front, the melt–rock reaction revealed by the present study is markedly different from those previously envisioned for Ronda. First, it occurred in a different structural domain, i.e. in the transitional peridotites intercalated between the spinel tectonite domain and the coarse-granular subdomain, whereas the other reactions are restricted to the fine-granular and layered-granular subdomains. Second, the presence of undeformed cpx aggregates in porphyroclastic, transitional peridotites (Fig. 2b and c) indicates that the reaction has overprinted the deformation of the spinel tectonite domain, and was thereafter overprinted by grain coarsening associated with the coarse-granular peridotites. In contrast, the reactions ascribed to late percolation stages overprint the coarse-granular subdomain. Therefore, the melt-consuming reaction inferred for the transitional peridotites was broadly coeval with the evolution of the recrystallization front and probably occurred in an aureole a few hundred metres thick ahead of the melting front.
The existence of clinopyroxene, ‘melt freezing’ reactions ahead of the recrystallization front probably implies that a smooth thermal gradient existed across the Ronda massif during the formation of this front. It also suggests that the front was basically an isotherm close to anhydrous peridotite solidus (T 1200°C) as deduced from our thermometric study. This explanation differs from Van der Wal & Bodinier’s interpretation of the recrystallization front as a permeability barrier created by a feedback between melt fraction and grain growth; yet correlation between chemical variations and recrystallization across the front confirms that grain coarsening was related to the presence of melt.
Finally, the sharp variation of Nb/Ta ratio at the melting front, from sub-chondritic values in the spinel tectonites and transitional peridotites to near-chondritic values in the coarse-granular domain (Fig. 8), is striking. A possible explanation for this feature would be a change from ‘lithospheric’ conditions, under which Nb–Ta would be dominantly controlled by very small amounts of Ti-oxides precipitated from volatile-rich small volume melts (Bodinier et al., 1996), to more ‘asthenospheric’ conditions, under which the Ti-oxides would be dissolved into the basaltic partial melt and Nb–Ta redistributed between silicate phases. Bedini et al. (1997) have shown with numerical modelling that the gradual solidification of partial melts in the lithospheric mantle (i.e. down a conductive thermal gradient) may result in residual, small melt fractions strongly enriched in large ion lithophile elements. In Ronda, this hypothesis can account for the trace-element variation observed in the harzburgites of the spinel tectonite domain as a function of distance from the front [fig. 5a of Van der Wal & Bodinier (1996)].
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CONCLUSIONS AND GEODYNAMIC IMPLICATIONS |
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Geochemical variations in peridotites across the Ronda recrystallization front are consistent with partial melting (Fig. 9), kilometre-scale migration of melts by diffuse porous flow and limited melt extraction (< 5%) during the development of the front. These geochemical variations indicate that the recrystallization front is the narrow boundary of a partial melting domain (the coarse-granular peridotites) formed at the expense of subcontinental lithospheric mantle (the spinel tectonites). The presence of secondary cpx related to melt-consuming reactions in the transitional peridotites, a few hundred metres ahead of the melting front, demonstrates that the development of the front was thermally controlled. It implies that a smooth thermal gradient existed across the Ronda massif during the formation of the recrystallization front. The melting event inferred from the geochemical variation of peridotites across the front is recorded by differences in pyroxene compositions on either side of the front, probably implying a rapid, transient heating event at (1200°C and
1·5 GPa. The sharpness of geochemical and textural variations occurring at the Ronda recrystallization front strongly indicates coupling between melting and textural coarsening processes during asthenosphere–lithosphere interaction. Given the importance of grain size and melt fraction in the rheology of peridotites (e.g. Hirth & Kohlstedt, 1995), it is likely that such a front may induce a significant drop in viscosity of the recrystallized lithosphere, constituting a potential mechanical boundary for the removal of lithosphere by the underlying, convective asthenosphere.
It is logical to ascribe this thermal event to the late geodynamic evolution of the extensional Alboran sea basin (Van der Wal & Vissers, 1993; Comas et al., 1999; and references therein). This thermal event occurred in the late geodynamic evolution of Ronda and was aborted by the final emplacement of the massif into the crust along extensional detachments (Van der Wal & Vissers, 1993). The P–T evolution of metamorphic crustal rocks drilled in the floor of the Alboran sea basin (Soto & Platt, 1999) is consistent with complete removal of the lithospheric mantle beneath the centre of the basin (Platt et al., 1998). This interpretation is supported by recent geophysical data indicating significant lithospheric thinning beneath the Alboran sea basin (Torné et al., 2000). The Ronda recrystallization front indicates that this heating event caused pervasive shallow melting of the thinned lithospheric roots beneath the Alboran basin. On the other hand, the particular geodynamic evolution of the extensional Alboran basin accounts for the preservation of the singular petrological zoning of the Ronda peridotite massif. The Ronda peridotite then offers a unique place to investigate melt and rheological processes related to asthenosphere–lithosphere interaction beneath rapidly extending (upwelling rates of 0·4 cm/yr; Platt et al., 1998) continental regions, and probably slow-spreading oceanic environments.
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ACKNOWLEDGEMENTS |
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This paper has greatly benefited from detailed and constructive reviews by Peter Kelemen, Martin Drury and Fred Frey. Peter Kelemen is also acknowledged for his editorial advice. We thank Liliane Savoyant and Simone Pourtales for their assistance during ICP-MS analyses, and Claude Merlet for helping with microprobe analyses. This work was financed by an international programme of scientific co-operation ‘PICS 275’ between the ISTEEM (Montpellier) and the IACT (Granada)—funded by CNRS and Ministère des Affaires Étrangères (France), and CSIC (Spain)—and by the Spanish DGES project PB97-1211. Financial support for Xavier Lenoir came from a French MENRT fellowship, and for Carlos J. Garrido from a European Commission TMR30 Marie Curie fellowship (ERBFMBICT972120).
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FOOTNOTES |
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*Corresponding author. Present address: Laboratoire de Géologie, Université du Maine, Avenue Olivier Nlessiaen, 72085 Le Mans Cedex 9, France Telephone: 02-43-83-32-37. Fax: 02-43-83-37-95. E-mail: xavier.lenoir{at}univ-lemans.fr
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V. Soustelle, A. Tommasi, J. L. Bodinier, C. J. Garrido, and A. Vauchez Deformation and Reactive Melt Transport in the Mantle Lithosphere above a Large-scale Partial Melting Domain: the Ronda Peridotite Massif, Southern Spain J. Petrology, July 1, 2009; 50(7): 1235 - 1266. [Abstract] [Full Text] [PDF]
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 H.-F. Zhang, S. L. Goldstein, X.-H. Zhou, M. Sun, and Y. Cai Comprehensive refertilization of lithospheric mantle beneath the North China Craton: further Os-Sr-Nd isotopic constraints Journal of the Geological Society, March 1, 2009; 166(2): 249 - 259. [Abstract] [Full Text] [PDF]
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 E. Rampone and G. Borghini Melt migration and intrusion in the Erro-Tobbio peridotites (Ligurian Alps, Italy): Insights on magmatic processes in extending lithospheric mantle European Journal of Mineralogy, August 1, 2008; 20(4): 573 - 585. [Abstract] [Full Text] [PDF]
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J.-L. Bodinier, C. J. Garrido, I. Chanefo, O. Bruguier, and F. Gervilla Origin of Pyroxenite-Peridotite Veined Mantle by Refertilization Reactions: Evidence from the Ronda Peridotite (Southern Spain) J. Petrology, May 1, 2008; 49(5): 999 - 1025. [Abstract] [Full Text] [PDF]
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S. Sano and J.-I. Kimura Clinopyroxene REE Geochemistry of the Red Hills Peridotite, New Zealand: Interpretation of Magmatic Processes in the Upper Mantle and in the Moho Transition Zone J. Petrology, January 1, 2007; 48(1): 113 - 139. [Abstract] [Full Text] [PDF]
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A. MONTANINI, R. TRIBUZIO, and R. ANCZKIEWICZ Exhumation History of a Garnet Pyroxenite-bearing Mantle Section from a Continent-Ocean Transition (Northern Apennine Ophiolites, Italy) J. Petrology, October 1, 2006; 47(10): 1943 - 1971. [Abstract] [Full Text] [PDF]
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 S. Pilet, J. Hernandez, F. Bussy, and P.J. Sylvester Short-term metasomatic control of Nb/Th ratios in the mantle sources of intraplate basalts Geology, February 1, 2004; 32(2): 113 - 116. [Abstract] [Full Text] [PDF]
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D. IONOV Chemical Variations in Peridotite Xenoliths from Vitim, Siberia: Inferences for REE and Hf Behaviour in the Garnet-Facies Upper Mantle J. Petrology, February 1, 2004; 45(2): 343 - 367. [Abstract] [Full Text] [PDF]
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J. P. Platt, J.P. Platt, T.W. Argles, A. Carter, S.P. Kelley, M.J. Whitehouse, and L. Lonergan Exhumation of the Ronda peridotite and its crustal envelope: constraints from thermal modelling of a P-T-time array Journal of the Geological Society, September 1, 2003; 160(5): 655 - 676. [Abstract] [Full Text] [PDF]
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J.P. Platt, M.J. Whitehouse, S.P. Kelley, A. Carter, and L. Hollick Simultaneous extensional exhumation across the Alboran Basin: Implications for the causes of late orogenic extension Geology, March 1, 2003; 31(3): 251 - 254. [Abstract] [Full Text] [PDF]
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