On the contact relations of high-temperature peridotites in the Serrania de Ronda,Southern Spain

Large ultramafic masses along the western margin of the Alboran Sea were emplaced in two stages. The first, “hot” stage of emplacement was post-Triassic and pre-Oligocene in age. The second, “cold” stage of emplacement was of Oligo-Miocene age.The first stage caused the development of polymetamorphic aureoles in the surrounding crustal rocks. Metamorphic conditions in the contact zone of the composite aureole series changed from HP-HT to LP-HT. The HP-HT phase of metamorphism created a primary dynamo-thermal aureole. LP-HT metamorphism took place under mainly static conditions. Present contact relations are mainly defined by the amount of secondary dislocation during hot emplacement of mantle off-shoots from the base into the higher levels of the crust. Where the primary contacts between crustal and ultramafic rocks have remained undisturbed by secondary emplacement aureoles with kinzigite series were developed. Although these rocks have partly recrystallized under LP-HT conditions, their original HP-HT characteristics are largely preserved. In other localities, however, secondary dislocation brought mantle rock in contact with lower grade zones of the primary aureole and caused the development of cordierite- and feldspar-rich hornfelses and migmatites of the cordierite-feldspar hornfels series along the new contacts of the ultramafic rocks.Metapelites with composite facies series, very similar to aureole rocks of the Serranía de Ronda but not associated with high-temperature ultramafics, are found in scattered exposures along the Spanish coast east of the Serranía de Ronda over a distance of approximately 300 km.Cold thrusting during the second stage of emplacement obscured the relations between aureole and ultramafic rocks and gave rise to tectonic contacts of younger age (imbrication). In many places broad zones of mylonite and numerous serpentine lenses formed along the younger thrust planes.     

Platinum-group minerals in chromitites from the ojen lherzolite massif (Serrania de Ronda,Betic Cordillera,Southern Spain)

Summary Chromitites (Cr ores) of the Ojen lherzolite massif (Serranía de Ronda, Betic Cordillera, Southern Spain) were found to contain platinum-group minerals (PGM) as discrete inclusions in the chromite and in the associated silicates. The PGM mineralogy consists of sulfides [laurite, erlichmanite, malanite, unnamed (Ni-Fe-Cu)₂ (Ir, Rh) S₃, unidentified Pd-S], sulfarsenides (irarsite, hollingworthite, ruarsite, and osarsite), arsenides [sperrylite, unidentified (Pd, Ni)-As], one unidentified Pd-Bi compound, and native platinum group elements (PGE) consisting of Ru and Pt-Fe alloys. Textural considerations suggest that the PGE chalcogenides with S and As were formed in the high-temperature magmatic stages, as part of the chromite precipitation event (primary PGM), in contrast with the native PGE, which originated during the low-temperature serpentinization of the ultramafic host of the chromitites (secondary PGM).The primary PGM inclusions in the Ojen chromite are unusual compared with PGM inclusions in chromitites from tectonitic upper-mantle of ophiolites and other alpine-type complexes in that i) they display a great variety of mineral species sulfides, sulfarsenides and arsenides, and ii) comprise specific phases of all six PGE. The singularity of the primary PGM mineralization probably reflects high activities of both S and As during chromite precipitation at Serrania de Ronda to be related with particular physico-chemical conditions during uplifting of sub-continental, astenospheric mantle.The nature, composition, and paragenetic association of secondary PGM at Ojen confirm the relatively-high mobility of the PGE at low temperature, and indicate that remobilization can be selective under appropriate redox conditions causing separation and redistribution of the PGE in the rocks as a result of the alteration process.

---

Platingruppen-Minerale in chromititen aus dem ojen-lherzolithmassiv (Serranía de Ronda, Betische Kordillere, Süd-Spanien)

Zusammenfassung Platingruppen-Minerale in Chromititen aus dem Ojen-Lherzolithmassiv (Serranía de Ronda, Betische Kordillere, Süd-Spanien) In den Chromititen (Cr-Erzen) aus dem Ojen-Lherzolithmassiv (Serranía de Ronda, Betische Kordillere, Süd-Spanien) warden Platingruppen-Minerale (PGM) als einzelne Einschlüsse im Chromit and in den begleitenden Silikaten gefunden. Die Mineralogie der PGM setzt sich aus Sulfiden [Laurit, Erlichmanit, Malanit, einem unbenannten (Ni-Fe-Cu)₂ (Ir, Rh)S₃ und einem nicht identifizierten Pd-S], Sulfarseniden (Irarsit, Hollingworthit, Ruarsit und Osarsit), Arseniden [Sperrylit, einem nicht identifizierten (Pd, Ni)-As], einer nicht identifizierten Pd-Bi-Verbindung sowie gediegenen Platingruppen-Elementen (PGE) bestchend aus Ru and Pt-Fe-Legierungen, zusammen. Texturelle Untersuchungen haben ergeben, daß die PGE-Chalkogenide mit S und As im Zuge der Chromitfällung (primäre PGM) in den hochtemperierten, magmatischen Stadien gebildet warden, während die gediegenen PGE während der niedriggradigen Serpentini sierung des ultramafischen Nebengesteins der Chromitite (sekundäre PGM) gebildet warden.Die primären PGM-Einschlüsse in den Ojen-Chromiten sind im Vergleich zu PGM-Einschlüssen in Chromititen aus dem tektonisierten oberen Mantel in Ophiolithen und anderen alpinotypen Komplexen ungewöhnlich: i) Einerseits zeigen sie eine große Vielfalt an Mineralarten aus der Gruppe der Sulfide, Sulfarsenide und Arsenide. ii) Andererseits enthalten sie spezifische Phasen aller sechs PGE. Die Einzigartigkeit der primären PGM-Mineralisation könnte hohe Aktivitäten von S and As während der Chromit-Fällung in Serranía de Ronda widerspiegeln, die mit besonderen physiko-chemischen Bedingungen während der Hebung des subkontinentalen, asthenosphärischen Mantels zusammenhängen.Die Art, die Zusammensetzung and die paragenetische Vergesellschaftung von sekundären PGM in Ojen bestätigen die relativ hohe Mobilität der PGE bei niedriger Temperatur und zeigen, daß die Remobilisierung unter geeigneten Redox-Bedingungen selektiv wirken kann, wodurch eine Trennung und Neuverteilung der PGE in den Gesteinen als Ergebnis des Alterationsprozesses bewirkt wird.

---

---

With 7 Figures     

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

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

Abiotic methane seepage in the Ronda peridotite massif,southern Spain

Abiotic methane in serpentinized peridotites (MSP) has implications for energy resource exploration, planetary geology, subsurface microbiology and astrobiology. Once considered a rare occurrence on Earth, reports of MSP are increasing for numerous localities worldwide in low temperature, land-based springs and seeps. We report the discovery of six methane-rich water springs and two ponds with active gas bubbling in the Ronda peridotite massif, in southern Spain. Water is hyperalkaline with typical hydrochemical features of active serpentinization (pH: 10.7 to 11.7, T: 17.1 to 21.5 °C, Ca–OH facies). Dissolved CH₄ concentrations range from 0.1 to 3.2 mg/L. The methane stable C and H isotope ratios in the natural spring and bubbling sites (δ¹³C_CH4: −12.3 to −37‰ VPDB; δ²H_CH4: −280 to −333‰ VSMOW) indicate a predominant abiotic origin. In contrast, springs with manmade water systems, i.e., pipes or fountains, appear to have mixed biotic-abiotic origin (δ¹³C_CH4: −44 to −69‰; δ²H_CH4: −180 to −319‰). Radiocarbon (¹⁴C) analyses show that methane C in a natural spring is older than ca. 50,000 y BP, whereas dissolved inorganic carbon (DIC) analysed in all springs has an apparent ¹⁴C age ranging from modern to 2334 y BP. Therefore most, if not all, of the CH₄ is allochthonous, i.e., not generated from the carbon in the hyperalkaline water. Methane is also released as bubbles in natural ponds and as diffuse seepages (∼10¹–10² mg CH₄ m⁻²d⁻¹) from the ground up to several tens of metres from the seeps and springs, albeit with no overt visual evidence. These data suggest that the gas follows independent migration pathways, potentially along faults or fracture systems, physically isolated from the hyperalkaline springs. Methane does not seem to be genetically related to the hyperalkaline water, which may only act as a carrier of the gas. Gas-bearing springs, vents and invisible microseepage in land-based peridotites are more common than previously thought. In addition to other geological sources, MSP is potentially a natural source of methane for the troposphere and requires more worldwide flux measurements.     

Strain patterns of pluton emplacement in a crust undergoing non-coaxial deformation,Sierra Morena,Southern Spain

This paper investigates the problem of interference between regional and local strain fields, the latter corresponding to a pluton ballooning. Natural examples from the Sierra Morena (Southern Spain) are described. Two distinct patterns have been recognised, according to whether transcurrent motions or thrusting motions predominate in the regional deformation. The map of foliation trajectories shows typical features such as triple points and geometric continuity between pluton internal fabric and country rock cleavage even when these are oblique to the contact.A simple mathematical model of interference between ballooning and simple shear is presented. In this model, the ballooning may be circular or elliptical. The model is used to predict the position of triple points, patterns of principal trajectories and variations of finite strain. A comparison is made with plutons from the Sierra Morena.     

The late thermal history of the Ronda area, southern Spain

The Betic–Rif belt, in the western Mediterranean, experienced a pre-Alpine history and was later extensively reworked by major Alpine tectonics. There is abundant data showing that the Betic chain suffered very high cooling rates during its Alpine history, constrained mainly by geochronology using various isotopic systems and by palaeontological age determinations. In the westernmost part of the chain the high closure-temperature isotopic systems recorded Miocene high-grade metamorphism in the country rocks. In order to constrain the later stages of cooling, fission-track analysis has been applied to both zircon and apatite. The results point to extremely high rates of cooling (400 °C/Ma) between 21 and 19 Ma. Rates slowed to 100 °C/Ma for the time period 19 to about 12 Ma. The fission-track analysis also confirms the existence of an extensional tectonic stage between 19 and 17 Ma.     

Structural Petrology of the Ronda Peridotite, SW Spain: Deformation History

Solid bodies of upper-mantle peridotite, emplaced in the Betic-Rifchains of SW Spain and North Morocco, show a variety of structuresdeveloped under different metamorphic conditions. These structuresand related metamorphism reflect tectonic processes in the WestMediterranean mantle during orogeny in the Betic-Rif realm.The largest of the peridotites, the Ronda massif, has preservedthree structural domains which are spatially associated withmetamorphic domains previously defined by Obata (Journal ofPetrology, 21,533–572, 1980). These structural domainsinclude: (1) porphyroclastic spinel peridotites (spinel tectonites)and mylonitic garnet-spinel peridotites (garnet-spinel mylonites),developed during progressive strain localization at ambientconditions changing from the Arigite subfacies to garnet peridotitefacies; (2) seemingly undeformed granular peridotites, developedduring a phase of extensive recrystallization affecting thespinel tectonites and garnet-spinel mylonites at Seiland subfaciesconditions, and separated from the spinel tectonites by a well-preservedrecrystallization front which forms a marked structural, metamorphicand possibly geochemical boundary probably unique to orogenicperidotites; (3) porphyroclastic plagioclase peridotites (plagioclasetectonites) developed at the expense of the granular peridotitesduring progressive shear localization allied to ductile emplacementof the Ronda massif into the crust. Our structural and microstructural data from the Ronda massifallow us to assess the relative ages of the different metamorphicfacies seen in the West Mediterranean peridotites. In orderof decreasing relative age, these are: Arigite-subfacies, garnetperidotite facies, Seiland subfacies and plagioclase peridotitefacies. In addition, the associated microstructures providesome insight into the microphysical conditions controlling thedevelopment of the different structures and, as a result, thestructural and chemical heterogeneity of the West Mediterraneanperidotites. KEY WORDS: structural geology; peridotite; Betic Cordillera; Ronda; recrystallization; strain localization ^*Corresponding author. Present address: Philips Electron Optics BV, Applications Laboratory, Building AAE, PO Box 218, 5600 MD Eindhoven, The Netherlands     

Isotopic dating of the mylonitization of the Azuaga Group in the Badajóz-Córdoba belt,SW Spain

On the basis of Rb-Sr muscovite dating an age of about 355 Ma (Late Tournaisian/Early Visean) is assigned to the M₁ metamorphism and associated mylonitization of the ultramylonites of the Las Grullas Formation in the Azuaga Group, Badajóz-Córdoba belt. This age may possibly be related to the accretion process (suturing of the Ossa-Morena Zone to the Central Iberian Zone) thought to be responsible for the metamorphism and mylonitization. Muscovite from the Valencia de Las Torres Formation displays a somewhat lower Rb-Sr age of about 337 Ma. The corresponding K-Ar ages indicate that after the termination of the metamorphism it took some 20 Ma before the rocks had cooled below the closure temperature of muscovite to K-Ar. Whole-rock Rb-Sr data indicate that the Las Grullas ultramylonites were probably derived from Ordovician granitoids.

---

Zusammenfassung Aufgrund von Rb-Sr-Datierungen in Muskoviten ist für die M₁Metamorphose und die damit verbundene Mylonitisierung der Ultramylonite der Las Grullas-Formation in der Azuaga-Gruppe, Badajóz-Córdoba-Gürtel, ein Alter von ca. 355 Ma (Spätes Tournais/Frühes Visé) festgestellt worden. Dieses Alter könnte mit dem Akkretionsproze\ (Sutur der Ossa-Morena-Zone zur Zentral-Iberischen-Zone) verbunden sein, der für Metamorphose und Mylonitisierung verantwortlich gemacht wird.Muskovit der Valencia de Las Torres-Formation spiegelt ein etwas geringeres Rb-Sr-Alter von ca. 337 Ma wieder. Die entsprechenden K-Ar-Alter zeigen, da\ es nach dem Ende der Metamorphose einige 20 Ma gedauert hat, bis die Gesteine unter die Schlie\ungstemperatur des Muskovits für K-Ar abgekühlt waren. Rb-Sr-Daten von Gesamtgesteinsproben zeigen, da\ die Las Grullas-Ultramylonite wahrscheinlich aus ordovizischen Granitoiden entstanden sind.

---

Resumen Dataciones Rb-Sr en moscovitas de alrededor de 355 Ma (Tournaisiense tardío a Viseense) se asignan al metamorfismo M₁ y se asocian a la milonitización de ultramilonitas de la Formación Las Grullas en el Grupo de Azuaga, Eje Badajóz-Córdoba. Esta edad podría posiblemente datar el proceso de acreción (sutura de la Zona de Ossa-Morena a la Zona Centro-Ibérica) interpretado como responsable del metamorfismo y milonitización. Moscovita de la Formación Valencia de Las Torres indice una edad Rb-Sr un poco más bajo de cerca de 337 Ma. Las correspondientes edades K-Ar indican que después de la terminación del metamorfismo, se tardó cerca de 20 Ma antes que las rocas se enfriaran por debajo de la temperatura de cierre de la moscovita para K-Ar. Datos Rb-Sr de roca total indican que las ultramilonitas de Las Grullas posiblemente se deriven de granitoides ordovicicos.

---

Rb/Sr m₁ Las Grullas Azuaga, Badajoz-Cordiba 355 — / /-. . Rb/Sr , Valencia de Las Torres , , 337 . , , / , , , , k/Ar . Rb/Sr , Las Grallas, , .

     

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

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

Palaeomagnetic study of the Ronda peridotites (Betic Cordillera, southern Spain)

A palaeomagnetic study of the Ronda peridotites (southern Spain) has been carried out on 301 samples from 20 sites, spread along the three main outcrops of the ultrabasic complex: Ronda, Ojén and Carratraca massifs. Different lithologies and outcrops with different degrees of serpentinization have been sampled and analysed. Rock magnetic experiments have been carried out on a representative set of samples. These measurements include: Curie curves, hysteresis cycles, isothermal remanent magnetization (IRM) acquisition curves, thermal demagnetization of IRM imparted along three orthogonal axes and magnetic bulk susceptibility. Results indicate that magnetite is the main magnetic mineral present in the samples. Stepwise thermal and alternating field (AF) demagnetization of the natural remanent magnetization (NRM) reveals the presence of a characteristic remanent magnetization (ChRM) carried by magnetite, and in some sepentinized samples, a northward component with variable unblocking temperatures up to 250–575 °C. The appearance and the relative intensity of this northward component are strongly related to serpentinization degree. Taking into account the geological history of the peridotites, the ChRM has been considered as a thermo-chemical remanent magnetization acquired during the first serpentinization phase associated to the post-metamorphic cooling of this unit. On the basis of radiometric and fission track analysis, the ChRM is proposed to have been acquired between 20 and 17–18 Ma. The inclination of the mean direction of the ChRM statistically coincides with the expected inclination for stable Iberia during the Oligocene–Miocene. The declination of the ChRM differs from the expected declination, indicating clockwise block rotations of 41±12° about vertical axes since the cooling of the peridotites. When applying a compositional layering correction, the ChRM directions fail to pass this kind of fold test, thus, the compositional layering was not a palaeohorizontal during ChRM acquisition time. Normal and reversed polarities of the ChRM are reported, showing that at least one reversal of the Earth's magnetic field took place during ChRM acquisition process. A tentative polarity zonation within the peridotitic outcrops is also suggested. No evidence is found from these data for the previously proposed simultaneity between post-metamorphic cooling and rotation of the peridotites.     

Serpentinization-driven extension in the Ronda mantle slab (Betic Cordillera,S. Spain)

We investigate the stress regimes acting during serpentinization and faulting of the largest known subcontinental lithospheric peridotite body, namely the Ronda peridotites (Betic Cordillera, S. Spain). Petrological and structural analyses on serpentinites grown along fault planes crosscutting the peridotite slab, reveal that they were developed during three superposed stress tensors: the oldest one (E1) is characterized by NW–SE sub-horizontal compression; the intermediate one consists in NE–SW to ENE–WSW extension with orthogonal compression (E2); and the youngest one (E3) shows a sub-vertical maximum stress axis and NW–SE sub-horizontal extension. During serpentinization, maximum and minimum stress axes flip between a NW–SE horizontal position and a vertical one in the whole peridotite body (E1 and E3), while E2 represents an intermediate stress stage. Field relationships and previous petrological and geochronological data indicate that serpentinization and associated stress tensors are coeval with intrusive leucogranite dikes crosscutting the peridotites, thus constraining these processes to 19–22 Ma and occurring at upper continental crust depths (P < 4 kbar). Gravity data reveal that the average density of the Ronda mantle slab (~ 2.7–2.8 g/cm³) shows a negligible contrast with the surrounding crustal rocks, thus suggesting that the peridotite body is serpentinized in a great proportion. Our preferred tectonic model to account for the evolution of the Ronda peridotites in the upper crust considers that E1 compression was linked to the collision of the Alborán continental domain with the Iberian passive margin during the Gibraltar Arc formation. Subsequently, the sudden onset of extension recorded within the peridotite slab (E2 and E3) was favored by serpentinization-driven buoyancy.     

Lower Cretaceous charophytes from the Serrania de Cuenca,Iberian chain: Taxonomy,biostratigraphy and palaeoecology

The assemblages of fossil charophyte fructifications from the La Huérguina Formation at the La Huérguina stratotype and the Las Hoyas section, along with some other samples from isolated outcrops are composed of Atopochara trivolvis var. triquetra, Globator maillardii var. trochiliscoides, Globator maillardii var. biutricularis var. nov., Clavator harrisii var. reyi, Ascidiella cruciata and Mesochara harrisii. Some reworked utricles of Ascidiella iberica var. iberica were also found. The in situ assemblage belongs to the Cruciata-Paucibracteatus Biozone, which is Late Barremian–Early Aptian in age. These results confirm that the entire deposition of the La Huérguina Formation took place within this biozone and not earlier, as previously thought. The top of the unit can be limited to the uppermost Barremian with biostratigraphic data from ostracods. Globator maillardii var. biutricularis var. nov. is defined as the end form for the Globator lineage according to present knowledge. It is characterised by its unique utricle morphology, showing a basal ring representing a second, external utricular layer. This layer shows a primitive structure, reminiscent of G. maillardii var. mutabilis, in contrast to the internal utricle, which is more derived and similar to G. maillardii var. trochiliscoides. In comparison with other non-marine formations of the same age in the Iberian Chain and in Europe, the charophyte assemblages from the La Huérguina Formation appear to be relatively poor and monotonous, suggesting that some of the species found elsewhere never reached this part of the basin owing to the brief development of non-marine facies there. The main differences in composition between the samples studied are indicative of the palaeoecological conditions. Atopochara trivolvis triquetra was found to be dominant in shallow lacustrine facies and Globator maillardii var. trochiliscoides and var. biutricularis are associated with temporary lakes from a well-drained palustrine area. Clavator harrisii and Mesochara harrisii thrived on floodplains whereas Ascidiella cruciata grew in palustrine environments with significant edaphic activity.     

OSL dating of the Middle Palaeolithic Hotel California site,Sierra de Atapuerca,north‐central Spain

In this study we present optically stimulated luminescence (OSL) dating results obtained at one of the most important open‐air Middle Palaeolithic sites in the Sierra de Atapuerca foothills – Hotel California. We also assess the possibility of obtaining extended‐range OSL chronologies for a nearby Middle Pleistocene fluvial deposit using several novel methods, namely OSL dating of individual quartz ‘supergrains’, multi‐grain aliquot thermally transferred OSL (TT‐OSL) dating and the first application of a single‐grain TT‐OSL dating procedure. Four single‐grain OSL ages constrain the Middle Palaeolithic occupation of Hotel California to between 71±6 and 48±3 ka. The Hotel California single‐grain equivalent dose (D_e) distributions are highly overdispersed and contain several dose populations, which are probably attributable to post‐depositional sediment mixing, partial bleaching and intrinsic scatter. The reliability of multi‐grain aliquot OSL dating is compromised by the complex underlying D_e dispersion affecting these samples, as well as by biasing multi‐grain averaging effects. Extended‐range OSL and TT‐OSL chronologies for the nearby Pico River terrace are consistent with each other and with broad independent age control. These experimental approaches yield a weighted average age of 348±16 ka for terrace T_A9 of the Arlanzón River sequence. Our results highlight the benefits of comparing ages obtained using several OSL methodologies to improve the robustness of luminescence chronologies. They also demonstrate the potential that single‐grain OSL techniques offer for establishing improved age constraints on the many other Middle Palaeolithic sites found at Atapuerca and elsewhere across north‐central Spain.     

Material Transfer and Local Equilibria in a Zoned Kelyphite from a Garnet Pyroxenite, Ronda, Spain

A zoned kelyphite after garnet, from a garnet pyroxenite layer,the Ronda peridotite. Spain, has been studied and the mechanismof kelyphite formation is discussed. The kelyphite is an extremelyfinegrained symplectitic mixture of orthopyroxene, spinel, olivine,plagioclase, and ilmenite. It is concentrically zoned, formingthree mineralogical subzones. They are, from adjacent to a garnetgrain toward a clinopyroxene side, zone I (orthopyroxene+spinel+ plagioclase), zone II (olivine+spinel+plagioclase), and zoneIII (olivine+plagioclase). The analysis of phase equilibriashows that this mineralogical zonation can develop stably asa result of the presence of chemical potential gradients. Onthe basis of microprobe chemical analyses for each zone, materialtransfer across the zone that took place during the kelyphitizationwas quantitatively evaluated, and by locating the initial grainboundary between garnet and clinopyroxene grains and by writingmetasomatic reactions for each zone boundary, a simple dynamicmodel for the kelyphite formation is proposed. The kelyphiteformation probably took place when the host Ronda peridotiteascended from the upper mantle to the crust. It involved a co-operativebreakdown of the garnet and aluminous clinopyroxene, being accompaniedby a material transfer across the zone boundaries. By examiningthe Fe-Mg partitioning between olivine, spinel, and orthopyroxenein the kelyphite and by examining the Al content of the orthopyroxene,an attainment of local equilibrium has been confirmed, and thephysical conditions of the kelyphite formation have been estimatedto be 620–700C and 4–8 kbar.     

Isotopic dating of the Chengjiang Fauna-bearing horizon in Central Yunnan Province, China

Twenty black shale samples, which are free from the influence of weathering, were collected from the Chengjiang Fauna-bearing horizon, central Yunnan Province, yielding a Pb-Pb isochron age of 534±60 Ma. Although this age is younger than both the Rb-Sr isochron age and 40Ar-39Ar age, it should represent the lower isotopic age limit of the Chengjiang Fauna.     

Palaeomagnetism of the Palaeoproterozoic ultramafic intrusion near Lake Konchozero,Southern Karelia,Russia

New palaeomagnetic results are presented from the recently dated Palaeoproterozoic ultramafic Konchozero sill, and associated basalts (three sites, 38 oriented samples). Three stable components of remanence have been isolated during thermal and alternating field demagnetisation. The component I, with a mean direction of D=103°, I=40°, k=18, α₉₅=11° (N=11 samples), pole position of 14°S, 282°E, has been obtained from the unaltered deeper part of the sill and from baked schists. The study of the baked contact confirms the conclusion that component I is supposed to be primary and corresponds to the Sm–Nd age of the sill of 1974±27 Ma. The palaeopole of component I is not consistent with the accepted Fennoscandian apparent polar wander path (APWP) for the period 2120–1880 Ma, and for that part the Fennoscandian APWP should be revised. Two other components (component II: D=349°, I=39°, k=35, α₉₅=6°, N=19 samples, pole position 49°N, 231°E; and component III: D=17°, I=41°, k=44, α₉₅=5°, N=19 samples, pole position 50°N, 190°E) fit the APWP well, with palaeomagnetically estimated ages of ca. 1860 and 1760 Ma respectively.     

Isotopic relationships of volatile and lithophile trace elements in continental ultramafic xenoliths

The concentrations and isotopic compositions of Sr, Nd, Pb, He and C have been determined for suites of xenoliths from Bullenmerri (Australia), Ichinomegata (Japan), Geronimo (Arizona), and East Africa. The wehrlites and pyroxenites from Bullenmerri have Sr, Nd and Pb isotopic compositions that are generally similar to those found for alkali basalts in the region. The spinel lherzolites, in contrast, have higher ⁸⁷Sr/⁸⁶Sr and ²⁰⁶Pb/²⁰⁴Pb and lower ¹⁴³Nd/¹⁴⁴Nd ratios. Whereas the isotopic compositions of He are generally within the range of mid-ocean-ridge basalts (MORB) and do not covary with those of other trace elements, there is an apparent correlation between the ¹³C/¹²C and ¹⁴³Nd/¹⁴⁴Nd ratios for each of the two petrologic groups. These relationships, if substantiated for other xenolith suites, greatly limit the possible mechanisms for generating lithophile and volatile isotopic variations in the continental lithosphere. The helium isotopic compositions for all of the xenoliths fall within the range for MORB. This includes those from Ichinomegata, suggesting that the lower ³He/⁴He ratios found for He sampled at the surface at subduction zones result from mixing mantle He with near-surface crustal He rather than with subducted radiogenic He. Measured C isotopic compositions (relative to Peedee belemnite) for the Ichinomegata xenoliths include values that are both lighter and heavier than those in MORB, and are compatible with contributions from subducted carbon. The Nd and Sr isotopic compositions of the Ichinomegata xenoliths exhibit a correlation over a substantially greater range of values than typically observed for other light-rare-earth-element (LREE)-depleted xenoliths, and include more radiogenic Sr and less radiogenic Nd compositions. The carbon isotopic compositions found for the East African and Geronimo xenoliths extend to values that are lighter than those typically found for MORB.     

Ductile-to-brittle transition in shear during thrust sheet emplacement,Southern Appalachian thrust belt

Mesoscopic structures in anchimetamorphic (T = 200–300°C) strata of the Pulaski thrust sheet, Southern Appalachian thrust belt, developed in progressive, heterogeneous simple shear near the ductile-to-brittle transition. Shear (γ≤3) was localized in weak, anisotropic pelitic rocks (Rome Formation) along the base of this 5–11 km thick thrust sheet. Folds, which vary from upright and open to isoclinal and NW-facing, developed during ductile shearing and display a correlation between tightness and axial-surface dip. Movement along brecciated thrust zones, which evolved progressively from zones of greatest ductile strain, resulted in low-angle truncation of fold axis trends, coaxial refolding of earlier structures, and imbrication of the thrust sheet.Transient variations in fluid pressure (P_f) controlled the mechanical behavior of the thrust sheet. Systematic veins imply P_f >σ₃ + T (T = tensile strength) during ductile deformation, whereas later non-systematic vein arrays in high strain zones record periods of nearly hydrostatic stress. Elevated P_f, which led to fracturing, dilation, and fault initiation, appears confined to pelitic zones within the Rome Formation. This, coupled with decreasing temperature, resulted in the transition from ductile folding to brittle thrusting. Changing physical conditions probably reflect erosional unroofing during uplift and late Paleozoic thrust sheet emplacement.     

Fast switch from extensional exhumation to thrusting of the Ronda Peridotites (South Spain)

The Alboran Domain, situated at the western end of the Mediterranean subduction system, is characterized by the Ronda Peridotites, one of the world's largest exposures of sub‐continental mantle. Using U–Pb (LA‐ICP‐MS) and Ar–Ar dating, we precisely dated two tectonic events associated with the Tertiary exhumation of the Ronda Peridotites. First, shearing along the Crust–Mantle Extensional Shear Zone caused, at ca. 22.5 Ma, mantle exhumation, local partial melting in the deep crust and coeval cooling in the upper crust. Second, the Ronda Peridotites Thrust triggered the final emplacement of the peridotites onto the continental crust at c. 21 Ma, as testified by granitic intrusions in the thrust hangingwall. The tectonic evolution of the western Alboran Domain is therefore characterized by a fast switch from continental lithospheric extension in a backarc setting, with sub‐continental mantle exhumation, to a rift inversion by thrusting driven by shortening of the upper plate.     

Isotopic dating and structural relationships of granitoids and greenstones in the East Pilbara,Western Australia

A structural RbSr isotopic study has been made on two small areas in the eastern Pilbara block, Western Australia. The sites were chosen because they showed good interrelationships of granitoid and greenstone units. At Warrery Gap, on the western side of the Corunna granitic dome, the acid volcanics of the Duffer Formation (at the top of the lower Talga-Talga Subgroup of the Warrawoona Group) show some updating, but a good 3506 ± 62 Ma isochron, with initial ratio (IR) of 0.7006 ± 0.0011, was recognized, consistent with the 3452 ± 16 Ma zircon measurement of Pidgeon (1978b).Both a penetrative D₁ deformation, and the doming of D₂ preceded intrusion of late tectonic granitoids, dated at 3270 ± 22 Ma (IR = 0.7015 ± 0.0003), into both the Duffer Formation and the overlying pillow lavas of the Salgash Subgroup. Thus, the Salgash Subgroup is much older than suggested by Glikson (1979) and the stratigraphic succession cannot contain a hiatus between the apparently conformable Talga-Talga and Salgash Subgroups of the magnitude he proposed.The granitic domes clearly owe their form to the D₂ deformation rather than to batholitic intrusion, but near horizontal structural lineations suggest that they were not formed by diapiric movements. A granodiorite and pink feldspar granite from just within the Corunna granitic dome are slightly deformed: pooled isochrons indicate an age of 3232 ± 27 Ma but different IRs of 0.7032 and 0.7009, respectively. At Tambourah, in the eastern Shaw granitic dome, local D₂ also deforms an intrusive microadamellite of age 3087 ± 34 Ma and IR = 0.7103 ± 0.0057. There is therefore a real spread in ages of D₂ granitoids and D₂ deformation between about 3300 and 3100 Ma.Layered megacrystic gneiss, at Tambourah, also intruded by the microadamellite, contains a nebulous foliation argued to be local S₁. Layered and homogeneous megacrystic gneiss produce updated RbSr total rock isochrons of 2995 ± 95 and 2779 ± 38 Ma, respectively. The primary age of these D₁ gneisses is clearly greater than that of the D₂ granitoids and is probably indicated by Pidgeon's (1978c) zircon age of 3417 ± 40 Ma from the Shaw granitic dome. If so, Hickman's (1975) “Migmatite Suite” contains both D₁ gneisses and D₂ granitoids separated in age at Tambourah by ca. 300 Ma, although neither appears to be older than the lower part of the preserved layered sequence. The protocrust on which that sequence was deposited has yet to be directly identified.