Ronda Peridotite Research Articles

A Late Oligocene Suprasubduction Setting in the Westernmost Mediterranean Revealed by Intrusive Pyroxenite Dikes in the Ronda Peridotite (Southern Spain)

AbstractContrasting tectonic reconstructions of the westernmost Mediterranean have been proposed to explain the origin of the Alboran marine basin contemporaneously with Cenozoic convergence between the African and European plates. Cr-rich pyroxenites in the Ronda massif record the geochemical processes occurring in the subcontinental mantle of the Alboran domain in the Late Oligocene, thus constraining the geodynamic scenario of Cenozoic extension in the western Mediterranean lithosphere. Clinopyroxene in intrusive Cr-rich websterite dikes crosscutting the Ronda peridotite is strongly depleted in Nb-Ta and enriched in light rare earth elements, as typically observed in arc magmas, and is in trace element equilibrium with Neogene subduction-related lavas from the western and central Mediterranean. Sr-Nd-Pb radiogenic isotopes indicate that the mantle source of the Ronda pyroxenite dikes was contaminated by a subduction component released by detrital sediments likely deposited in passive continental margin...

The Ronda peridotite collection installed at Kyoto University Museum: its historical and todays significance

The Ronda peridotite collection installed at Kyoto University Museum: its historical and todays significance

Garnet lherzolite and garnet-spinel mylonite in the Ronda peridotite: Vestiges of Oligocene backarc mantle lithospheric extension in the western Mediterranean

Uplift and exhumation of vast exposures of diamond facies, subcontinental mantle peridotite in the Western Mediterranean arc are attributed to tectonic scenarios including pure extension, transpression or subduction followed by delamination-driven or rollback-driven stretching. In the Ronda peridotite (southern Spain) the strong overprint of low-pressure assemblages has precluded accurate determination of the pressure and temperature conditions for the onset of exhumation that formed the spinel tectonite and garnet-spinel mylonite domain in this massif. Here we report unequivocal petrographic evidence for the existence of prekinematic, coarse-grained garnet lherzolite assemblages from the garnet-spinel mylonite domain of the Ronda peridotite. Application of well-calibrated geothermobarometers yields prekinematic minimum equilibration conditions of 2.4–2.7 GPa and 1020–1100 °C, demonstrating that the Ronda peridotite equilibrated at ∼85 km depth before shearing. We also show the existence of synkinematic garnet and spinel assemblages that overprinted garnet lherzolite assemblages at 800–900 °C and 1.95–2.00 GPa. The decompressional cooling path and high pressure recorded by garnet-spinel mylonites rule out their formation by near-isobaric cooling above a subduction-collision wedge or during or after the emplacement of the peridotite massif into the crust. Ronda garnet-spinel mylonites represent the vestiges of subcontinental mantle ductile shear zones formed at early stages of lithosphere extension during backarc extension in the western Mediterranean. Southward to westward retreat of the African slab during the Oligocene-Early Miocene accounts for intense backarc lithosphere extension and development of the Ronda extensional shear zone, coeval with extreme thinning of the Alboran domain overlying crust.

Deformation partitioning during transpressional emplacement of a ‘mantle extrusion wedge': the Ronda peridotites, western Betic Cordillera, Spain

Abstract: Composite tectonic emplacement of the world's largest outcropping body of subcontinental mantle rocks, the Ronda peridotites, occurred during Early Miocene continental subduction and as a result of oblique plate convergence. Partitioning of transpressional deformation between coeval orogen-parallel wrenching and orogen-perpendicular, pure thrusting components is recorded by shear zone kinematics and dynamothermal metamorphism in the footwall to the ultramafic rocks. Left-lateral shear, characterizing the deeper, high-pressure (eclogitic) portions of the continental subduction system, propagated through the mantle into the overlying continental crust of the overriding plate, whereas top-to-the-foreland frontal thrusting dominated at the leading edge of the hot peridotite body. Top-to-the-hinterland shear along the upper contact of the peridotites is consistent with the kinematics expected for an extrusion wedge consisting of subcontinental mantle rocks. On the other hand, coeval strike-parallel extension and thinning of the crustal rocks overlying the peridotites confirms that, similarly to other Alpine–Mediterranean examples, partitioned transpression resulted in the development of a complex deformation pattern, with kinematically linked shear zones aiding exhumation. The proposed model provides a new, coherent scenario for syn-convergence peridotite emplacement and exhumation in the western Betic Cordillera.

Peri-Gondwanan provenance of pre-Triassic metamorphic sequences in the western Alpujarride nappes (Betic Cordillera, southern Spain)

The main orogenic reworking of the Betic Cordillera, in southern Spain, is a consequence of a short but severe Alpine event of Aquitanian to Burdigalian age that almost erased any evidence of pre-Mesozoic orogenies. New SHRIMP analyses on zircons from high-grade metamorphic rocks of tectonic units underlying the Ronda peridotites (Alpujarride Complex, Betic Cordillera), the largest exposed bodies of subcontinental peridotites in the world, yield mostly Neoproterozoic ( c. 624 Ma) and Paleoproterozoic (2000 to 2500 Ma) ages, with minor Variscan ( c. 292 Ma) and Miocene (20.1 to 21.9 Ma) components. The two clusters of Proterozoic ages point to provenance of detrital zircons from different source areas of the West African Craton. These new data help to improve the pre-Mesozoic geodynamic history of the Betic Cordillera, a piece of the Western Mediterranean puzzle, which can be interpreted as an accreted ribbon terrane formed along the northern margin of Gondwana.

A revised Aquitanian age for the emplacement of the Ronda peridotites (Betic Cordilleras, southern Spain)

AbstractThe hot emplacement of the Ronda peridotites (Betic Cordilleras) developed a dynamothermal aureole and partial melts that led to the intrusion of granite dykes in the peridotites. Previous geochronological data place rather broad limits for this event between 22 and 19 Ma. Analyses of neocrystalline zircon rims from large zircon populations yield a U–Pb SHRIMP age of 22.3±0.7 Ma for the dynamothermal aureole formation, and intrusion ages of granite dykes between 22.6±1.8 and 21.5±3.8 support that conclusion. Therefore, these new ages provide a more robust constraint on the hot emplacement of the Ronda peridotites at middle crustal levels.

Crystal chemistry of Cr-spinels from the lherzolite mantle peridotite of Ronda (Spain)

The crystal chemistry of some Cr-spinels from the lherzolite body of the Ronda peridotite in southern Spain has been investigated. Cell edge spans between 8.1692(2) and 8.2367(1) Å, while the oxygen positional parameter u ranges between 0.26306(7) and 0.26351(7). By using the Princivalle thermometer (1999), an intracrystalline closure temperature between 640 and 840 °C has been calculated. The higher temperatures are very close to the intercrystalline temperatures based on the olivine-spinel thermometer calculated by Woodland et al. (2006) for the Ronda orogenic lherzolites suggesting that the intracrystalline closure occurred soon after the intercrystalline closure. By comparison with Cr-spinels from lherzolite mantle xenoliths, it should be noted that: (1) the oxygen positional parameter can be linearly related to the intracrystalline temperature for both mantle peridotite and mantle xenolith Cr-spinels; (2) the intracrystalline closure temperature is reached faster and is higher in Cr-spinels in mantle xenoliths; and (3) Cr content is linearly related to u in mantle peridotite, but not in mantle xenoliths, suggesting the u value in Cr-spinels from mantle peridotite is driven solely by the chemistry of the spinels.

Melt Migration and Intrusion during Exhumation of the Alboran Lithosphere: the Tallante Mantle Xenolith Record (Betic Cordillera, SE Spain)

Microstructural and in situ mineral chemistry studies on mantle peridotite xenoliths from the Late Neogene alkaline volcanic center of Cabezo Tallante (SE Spain) reveal an exceptional record of a multi-stage history of deformation, recrystallization, melt–rock interaction and melt intrusion tracking the progressive exhumation of this lithospheric mantle sector. Xenoliths include porphyroclastic to equigranular spinel peridotites, impregnated plagioclase peridotites, and composite xenoliths made up of peridotites intruded first by gabbronorite veins and later by amphibole-bearing pyroxenites. The earliest stage involved subsolidus re-equilibration from garnet- to spinel-facies conditions, represented by rounded opx + spinel ± cpx clusters indicative of precursor garnet. The spinel-facies equilibration was followed by development of a porphyroclastic fabric, accentuated in many xenoliths by spinel trails, in response to shear deformation that may be related to the early stages of Neogene extension. Porphyroclastic spinel peridotites subsequently underwent multiple episodes of reactive porous melt percolation documented by crystallization of undeformed olivine replacing pyroxene porphyroclasts, and of undeformed poikilitic orthopyroxene at the expense of both pyroxene porphyroclasts and newly crystallized olivines. The porphyroclastic and melt–rock reaction textures are progressively obliterated by an equigranular structure developed as the result of static, possibly melt-assisted, annealing recrystallization. Clinopyroxenes in equigranular peridotites (i.e. the most equilibrated with the percolating melts) display slight light rare earth element (REE) depletion and almost flat middle to heavy REE spectra (LaN/YbN = 0·37–0·62; SmN/YbN = 0·89–1·23). Computed equilibrium liquids have an enriched tholeiitic affinity, consistent with the sub-alkaline magmatism of the Alboran Domain. Overall, the tectonic and magmatic stages recorded in spinel peridotites from Tallante are remarkably consistent with the evolution documented in the Ronda peridotites of the western Betics. Reactive porous flow and annealing recrystallization were followed by an impregnation event, documented by crystallization of interstitial (plag ± opx ± ol) aggregates in porphyroclastic and equigranular xenoliths; this indicates further exhumation to shallower depths. Diffuse melt percolation was followed by intrusion of melts with distinct chemical affinity. The first event is documented by thin gabbronoritic–noritic veins, showing opx reaction rims against the host peridotite. Comparable gabbronorites were previously ascribed to slab-derived melts. The norite veins are crosscut by centimeter-thick dikelets of amphibole pyroxenite. Geobarometric estimates and the observed crystallization order (ol–cpx–amph–plag) point to 0·7–0·9 GPa for pyroxenite intrusion. Computed melts in equilibrium with clinopyroxene show alkaline affinity, similar to the host Tallante alkali basalts. Textural and geochemical features in the xenoliths thus indicate that the progressive uplift of the Tallante lithospheric mantle was accompanied by interaction with melts of different sources, reflecting the magmatic evolution of the Alboran Domain in response to lithosphere extension and thinning leading to the formation of the Betic–Rif arc.

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.

Deformation and Reactive Melt Transport in the Mantle Lithosphere above a Large-scale Partial Melting Domain: the Ronda Peridotite Massif, Southern Spain

The Ronda peridotite massif in Southern Spain shows a well-defined 'recrystallization' front that separates a large-scale partial melting domain formed at the expense of the continental lithospheric mantle from a 'preserved' lithospheric domain. To investigate the processes allowing a transient lithosphere-asthenosphere boundary to propagate in the lithospheric mantle, we performed a joint petrostructural and geochemical study of an similar to 2.5 km(2) zone extending from the melting front to the mylonites that mark the western limit of the massif. This study emphasizes a feedback between heat transfer, melt percolation, and deformation in the lithospheric mantle. Petrographical observations and geochemical data show that heterogeneous reactive percolation of melts produced in the underlying partial melting domain led to refertilization of lithospheric peridotites up to 1.5 km ahead from the melting front, producing metre-scale layering of fertile and refractory mantle rocks. Within 800 m from the front, pre-existing garnet pyroxenite layers were partially molten and the resulting melts probably contributed to the refertilization process. Detailed structural mapping and analysis of the microstructures and crystal preferred orientations highlight the relations between reactive melt transport and deformation, and the control of the temperature gradient on both processes. Parallelism between the recrystallization front, compositional boundaries, and deformation structures, as well as variations in the deformation intensity of pyroxenes and spinels, suggest syn- to late-tectonic melt transport controlled by both the deformation and the thermal gradient. Variations in the strength of olivine crystal preferred orientations as a function of the modal and chemical composition of the spinel tectonites point to a higher contribution of diffusion to deformation in the most fertile rocks, corroborating the hypothesis that deformation occurred in presence of melt. Finally, the systematic dispersion of olivine [100] and orthopyroxene [001] axes in the foliation plane suggests a dominantly transpressive deformation regime.

Remnants of a Mesozoic Rift in a Subducted Terrane of the Alpujarride Complex (Betic Cordilleras, Southern Spain)

Amphibolite bodies with eclogite relics are interlayered at different levels in the Ojen nappe (Alpujarride Complex) below the Ronda peridotites of the Betic Cordilleras in southern Spain. We present an integrated structural, petrological, and geochemical study of such amphibolites, from which it is possible to deduce a gabbroic origin for the amphibolite protoliths, and we propose that their intrusion into the continental crust took place in a rift setting of Jurassic to lower Cretaceous age. The geochemical data are consistent with basaltic magmas produced by partial melting of an enriched or transitional mantle source (E- or T-MORB [mid-ocean ridge basalt]) and reflect plagioclase accumulation and weak crustal contamination processes. The structures associated with the rifting stages are obliterated by deformation and metamorphism related to subduction at depths of 60 km and later exhumation of the amphibolite bodies during the complex convergence of the Iberian and African plates from upper Cretaceous onward. The recognition of this Mesozoic rift is of great interest in studies of the tectonic evolution of the Betic Cordilleras and places new constraints on paleogeographic reconstructions of the western Mediterranean. We suggest that such a Mesozoic rift would correspond to the transform seaway linking the Neo-Tethys realm with the Atlantic Ocean.

Constraints on the evolutionary history of aluminous mafic rocks in the Ronda peridotite massif (Spain) from trace-element compositions of clinopyroxene and garnet

Multiple trace-element compositions of clinopyroxene and garnet in aluminous mafic layers (±corundum and/or sapphirine) alternating with peridotites from the Ronda massif, Spain, were determined by an ArF 193 nm excimer laser system coupled with a quadrupole ICP-MS. The studied aluminous mafic rock is divided into several sublayers based on the modal mineral variations. Trace-element compositions of minerals vary between sublayers. Primitive mantle-normalized trace-element distribution patterns have more significant positive Eu anomaly for both garnet and clinopyroxene in the corundum-bearing sublayers than for those in the other sublayers whereas corundum-bearing sublayers contain more abundant secondary plagioclase than corundum-free sublayers. These geochemical signatures of the corundum-bearing sublayers intercalated with corundum-free sublayers were acquired as plagioclase-rich portions of gabbroic layered sequence originated at the lower crustal to the uppermost mantle sections rather than selective accumulation of metamorphic plagioclase due to deformation during the late stage of exhumation of the massif. Partition coefficients of trace elements between clinopyroxene and garnet show systematic variations with respect to major element composition, indicating local attainment of chemical equilibrium. The trace element compositions of clinopyroxene and garnet were equilibrated with secondary metamorphic plagioclase, i.e., at granulite-facies conditions, because plagioclase is required for the geochemical balance in Sr and Eu contents of the whole-rock trace element compositions. Positive Eu anomaly of clinopyroxene (±garnet) is also found in mantle-derived aluminous eclogitic xenoliths and eclogitic mineral inclusions in diamond captured in kimberlites, and is thus a useful indicator to explore recycled crustal components in the mantle.

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

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

Deformation and kinematics in a melt-bearing shear zone from the Western Betic Cordilleras (Southern Spain)

This work deals with the structural evolution and kinematics of the basal dynamothermal aureole of the Ronda peridotites, located in the Sierra Bermeja massif of the Western Betic Cordilleras, in southern Spain. This aureole is characterized by: 1) a marked strain gradient towards the contact with the overlying peridotites; 2) the great angular dispersion, ≈90°, of stretching lineations; and 3) the intrusion of thin syn-kinematic granite sheets. These features are consistent with the aureole being a low-viscosity shear zone formed at temperatures high enough, ≈725 °C, to promote the partial melting of crustal metamorphic rocks. The high-temperature deformation is demonstrated by the activation of c-slip in quartz. We consider that the heat source came from the thrusting of the hot subcontinental mantle over metamorphic crustal rocks. The interaction of alternating events of melt-enhanced deformation and melt-enhanced embrittlement explains the heterogeneity of this ductile shear zone and the strong narrowing, to only 30 meters, of the deformed domain. The small thickness of this shear zone is in contradiction with classical conceptual models of shear zones suggesting a broadening of ductile shear zones with increasing depths and temperatures.

Strain localisation in the subcontinental mantle — a ductile alternative to the brittle mantle

It is now admitted that the high strength of the subcontinental uppermost mantle controls the first order strength of the lithosphere. An incipient narrow continental rift therefore requires an important weakening in the subcontinental mantle to promote lithosphere-scale strain localisation and subsequent continental break-up. Based on the classical rheological layering of the continental lithosphere, the origin of a lithospheric mantle shear/fault zone has been attributed to the existence of a brittle uppermost mantle. However, the lack of mantle earthquakes and the absence of field occurrences in the mantle fault zone led to the idea of a ductile-related weakening mechanism, instead of brittle-related, for the incipient mantle strain localisation. In order to provide evidence for this mechanism, we investigated the microstructures and lattice preferred orientations of mantle rocks in a kilometre-scale ductile strain gradient in the Ronda Peridotites (Betics cordillera, Spain). Two main features were shown: 1) grain size reduction by dynamic recrystallisation is found to be the only relevant weakening mechanism responsible for strain localisation and 2), with increasing strain, grain size reduction is coeval with both the scattering of orthopyroxene neoblasts and the decrease of the olivine fabric strength (LPO). These features allow us to propose that grain boundary sliding (GBS) partly accommodates dynamic recrystallisation and subsequent grain size reduction. A new GBS-related experimental deformation mechanism, called dry-GBS creep, has been shown to accommodate grain size reduction during dynamic recrystallisation and to induce significant weakening at low temperatures ( T < 800 °C). The present microstructural study demonstrates the occurrence of the grain size sensitive dry-GBS creep in natural continental peridotites and allows us to propose a new rheological model for the subcontinental mantle. During dynamic recrystallisation, the accommodation of grain size reduction by three competing deformation mechanisms, i.e., dislocation, diffusion and dry-GBS creeps, involves a grain size reduction controlled by the sole dislocation creep at high temperatures (> 800 °C), whereas dislocation creep and dry-GBS creep, are the accommodating mechanisms at low temperatures (< 800 °C). Consequently, weakening is very limited if the grain size reduction occurs at temperatures higher than 800 °C, whereas a large weakening is expected in lower temperatures. This large weakening related to GBS creep would occur at depths lower than 60 km and therefore provides an explanation for ductile strain localisation in the uppermost continental mantle, thus providing an alternative to the brittle mantle.

Timing and origin of zircon-bearing chlorite schists in the Ronda peridotites (Betic Cordilleras, Southern Spain)

This paper reports the finding of zircon-bearing chlorite schists in the south-western part of the Sierra Bermeja, the largest massif of the Ronda peridotites (Alpujarride Complex, Betic Cordilleras, Spain). The chlorite schists contain unusually high amounts of zircon and monazite, never previously recorded in the granite dykes intruded into the peridotites nor in their country rocks. Structural, petrographic, geochemical and geochronological data suggest that late high temperature fluids/melts emanating from the crustally derived granite dykes locally altered the peridotites. The large, euhedral and U-rich zircons are very different to those of the granite dykes but probably formed from late high temperature fluids/melts. U–Pb SHRIMP data of these zircons yielded an age of 21.8 ± 0.5 Ma, leading to a precise age estimation for the emplacement of the granite dykes. In the course of later serpentinization processes chlorite schist formation took place during cooling at lower temperatures (289–322 °C). Fission-track zircon ages at 19.2 ± 1.1 Ma date this episode of cooling during exhumation of the peridotites.

Tectonic implications of the granite dyke swarm in the Ronda peridotites (Betic Cordilleras, Southern Spain)

This paper deals with the origin and structural relationships of the leucocratic dykes that intruded the Ronda peridotites. Plagiogranites are frequent in ophiolitic sequences, but similar Si-rich melts are scarce in orogenic lherzolites. The Ronda peridotites could be an excellent target in which to look for Si-rich melts of mantle origin, as they form the largest mass of orogenic lherzolites in the world and contain many granite dykes. However, petrographic and geochemical data provide evidence that the dykes come from felsic magmas originated from partial melting of the continental crust. Structural studies of the deformed and undeformed dykes reveal that they are non-Andersonian dykes that exploited pre-existing fractures. Undeformed dykes display N15°E strike, whereas the deformed ones show N50°E or N100°E trends and sinistral and dextral shear senses, respectively. These structural data point to reactivation of pre-existing fractures either as tensional cracks (undeformed dykes) or transtensional shear zones (deformed dykes) during the ENE-directed thrusting of the Ronda peridotites. Consequently, the dominant N15°E trend of the undeformed dykes can be used to infer an ENE-thrusting direction for the buried basal shear zone of the Ronda peridotites.

Thermal structure of the Alboran Domain in the Rif (northern Morocco) and the Western Betics (southern Spain). Constraints from Raman spectroscopy of carbonaceous material

AbstractIn the Rif (northern Morocco) and the Western Betics (southern Spain), the Alboran Domain forms a complex stack of metamorphic nappes including mantle peridotites (Beni Bousera and Ronda). We present in this paper new temperature data obtained in the Alboran Domain based on Raman spectroscopy of carbonaceous material (RSCM thermometry). In the lower metamorphic nappes of the Alboran Domain (lower Sebtides–Alpujárrides) temperature ranges from &gt; 640 °C at the base of the metapelitic sequence to 500 °C at the top. The relationships between field isotherms and nappe structure show that peak temperatures were reached during strong ductile thinning of these nappes whereas they partly postdate this main episode in the Rif. In the upper nappes of the Alboran Domain (Ghomarides–Maláguides), generally supposed to be only weakly metamorphosed, temperatures range from ∼500 °C at their base down to &lt; 330 °C at the top. This temperature gradient is consistent with progressive Cenozoic resetting of K–Ar and 40Ar–39Ar ages. These nappes were thus affected by a significant thermal metamorphism, and the available age data in the underlying Sebtides–Alpujárrides show that this metamorphism is related to the metamorphic evolution of the whole Alboran Domain during the Late Oligocene–Early Miocene. Such thermal structure and metamorphic evolution can be explained by generalized extension in the whole Alboran Domain crustal sequence. At a larger scale, the present thermal structure of the Alboran Domain is roughly spatially consistent around the Beni Bousera peridotites in the Rif, but much more affected by late brittle tectonics around the Ronda peridotites in the Western Betics. Therefore, on the basis of the observed thermal structure, the metamorphic evolution of the Alboran Domain can be interpreted as the result of the ascent of hot mantle units contemporaneous with thinning of the whole lithosphere during an Oligo‐Miocene extensional event. The resulting structure has however been dismembered by late brittle tectonics in the Western Betics.

Corundum‐bearing mafic granulites in the Horoman (Japan) and Ronda (Spain) Peridotite Massifs: Possible remnants of recycled crustal materials in the mantle

Island ArcVolume 15, Issue 1 p. 2-3 Corundum-bearing mafic granulites in the Horoman (Japan) and Ronda (Spain) Peridotite Massifs: Possible remnants of recycled crustal materials in the mantle Tomoaki Morishita, Tomoaki Morishita Graduate School of Natural Science and Technology, Kanazawa University, Kanazawa 920-1192, Japan (email: [email protected]), Search for more papers by this authorEiichi Takazawa, Eiichi Takazawa Department of Geology, Faculty of Science, Niigata University, Niigata 950-2181, Japan,Search for more papers by this authorShoji Arai, Shoji Arai Graduate School of Natural Science and Technology, Kanazawa University, Kanazawa 920-1192, Japan (email: [email protected]), Search for more papers by this authorMasaaki Obata, Masaaki Obata Division of Earth & Planetary Sciences, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan andSearch for more papers by this authorTadahiro Kodera, Tadahiro Kodera Graduate School of Natural Science and Technology, Kanazawa University, Kanazawa 920-1192, Japan (email: [email protected]), Present address: TK service, Hakusan 924-0820, Japan.Search for more papers by this authorFernando Gervilla, Fernando Gervilla Instituto Andaluz de Ciencias de la Tierra, Universidad de Granada-CSIC, 18002 Granada, SpainSearch for more papers by this author Tomoaki Morishita, Tomoaki Morishita Graduate School of Natural Science and Technology, Kanazawa University, Kanazawa 920-1192, Japan (email: [email protected]), Search for more papers by this authorEiichi Takazawa, Eiichi Takazawa Department of Geology, Faculty of Science, Niigata University, Niigata 950-2181, Japan,Search for more papers by this authorShoji Arai, Shoji Arai Graduate School of Natural Science and Technology, Kanazawa University, Kanazawa 920-1192, Japan (email: [email protected]), Search for more papers by this authorMasaaki Obata, Masaaki Obata Division of Earth & Planetary Sciences, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan andSearch for more papers by this authorTadahiro Kodera, Tadahiro Kodera Graduate School of Natural Science and Technology, Kanazawa University, Kanazawa 920-1192, Japan (email: [email protected]), Present address: TK service, Hakusan 924-0820, Japan.Search for more papers by this authorFernando Gervilla, Fernando Gervilla Instituto Andaluz de Ciencias de la Tierra, Universidad de Granada-CSIC, 18002 Granada, SpainSearch for more papers by this author First published: 08 March 2006 https://doi.org/10.1111/j.1440-1738.2006.00517.xCitations: 1 Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article.Citing Literature Volume15, Issue1March 2006Pages 2-3 RelatedInformation

Near‐ and mid‐infrared microspectroscopy of the Ronda peridotite

Microspectroscopy is a potentially useful tool for planetary applications, as individual mineral grains can be targeted and measured in situ. A series of closely coupled experiments was designed to investigate and compare the laboratory measurement of optical properties of a peridotite sample in the macroscale and microscale. The Ronda peridotite consists largely of olivine with significant orthopyroxene and minor clinoproxene and serpentine. Microscopic transmission spectra of unmounted thin sections produce strong, well‐defined absorption features in the near infrared, and band positions can be directly compared to macroscopic and microscopic reflectance spectra. For near‐infrared reflectance, where photons are multiply scattered by different grains, measurement of a microscopic target means most photons are scattered out of the field of view and few photons return to the detector from the target area. This precludes most microspectroscopic reflectance measurements of small features in the near infrared. In the mid‐infrared, where reflectance measurements involve only surface interactions, microspectroscopic measurements detect well‐defined absorption features for most samples. Although the intensity of absorption features varies between grains, band positions are consistent with macroscopic diffuse reflectance and emittance measurements of bulk samples, and can be used to extract general chemical information such as iron/magnesium ratios of olivines.