Ronda Peridotite Research Articles

Heavy Minerals Distribution and Provenance in Modern Beach and Fluvial Sands of the Betic Cordillera, Southern Spain

This study uses heavy detrital minerals to determine actualistic fluvial and beach sand provenance across the Betic Cordillera (Spain), along the coast from Almeria to Marbella. The Betic Cordillera, primarily composed of metamorphic rocks to the east, supply an assemblage dominated by almandine and graphite, with a longshore dispersal from Almeria to Malaga. Buergerite and hypersthene indicate the provenance of calcalkaline lavas east of Cabo de Gata. The western part of the Betic Cordillera, which comprises the Ronda Peridotite Complex, supplies a chromite and diopside assemblage, with a dispersal from Marbella to Algeciras. Considering these mineralogical suites, the effects of source rock compositions and weathering are evaluated. The heavy mineral species mirror the mineralogy of the source rocks of local outcrops and wider source terranes. The fluvial heavy mineral suites do not differ significantly from those in the beaches except for some unstable species. Unstable species such as olivine, pyroxene, and amphibole do not show evidence of loss because of elevated topography and semiarid climate, which do not affect heavy minerals. This contribution also evaluates the potential of some heavy detrital species as ideal pathfinders in searching for diamonds.

Using U-Pb detrital zircon systematics to constrain the architecture and provenance of the Ronda peridotites’ crustal hanging wall, S Spain

Detrital zircon geochronology is frequently used for sediment provenance analysis in orogenic belts, but it can also be applied to locate the boundary between basement and sedimentary cover in successions and tectonometamorphic units where the paleontological record is missing, and primary stratigraphic relationships have been erased by intense deformation and/or metamorphism. This is critical to differentiate metamorphic mineral associations, P-T conditions and deformational events associated with superposed orogenic cycles. In this work, detrital zircon geochronology has been applied to the Ronda peridotites’ crustal hanging wall (hinterland of the Alpine Betic Cordillera, southern Spain), apparently made up of two major tectonic units – the Jubrique unit and the Maláguide complex – separated by a major Alpine tectonic contact camouflaged in between low-grade metamorphic rocks.Our results show that pre-Paleozoic peaks (c. 650, 1000, 2000, and 2600 Ma) are common along the entire studied crustal sequence, and Paleozoic ages are only recorded in the upper part of the Jubrique unit (c. 290, 350, and 475 Ma) and in the Maláguide complex (c. 350 and 475 Ma). Thus, the Paleozoic rocks of the Maláguide complex, constituting the highest crustal unit overlying the Ronda peridotites, represent syn-to-late orogenic Variscan sediments deformed and slightly metamorphosed prior to the Alpine cycle. In contrast, the underlying Jubrique unit is made of a post-Variscan sedimentary cover deposited on a Variscan basement, with the contact between both (hidden unconformity?) located within low- to medium-grade schists. These data support the presence of a major tectonic contact between the Maláguide complex (Late Carboniferous maximum depositional ages) and the Jubrique unit (Early Permian maximum depositional ages), i.e. the Maláguide complex would have overthrust the Jubrique unit during the Alpine orogeny. These results indicate a Permian or younger exhumation/emplacement up to crustal levels of the Ronda peridotites subcontinental mantle slab.Altogether, these detrital zircon age spectra support a relatively, but not entirely local derivation of the studied crustal sequences (fragments of the Iberian Massif) and suggest some Mesozoic drifting with respect to its present position. Nonetheless, the different age distribution of the Paleozoic populations between the Jubrique (Alpujárride complex) and Maláguide samples suggests that the former deposited relatively close to other samples from the eastern-central Alpujárride complex and Nevado-Filábride complex, which, in turn, show strong similarities with the Cantabrian zone and NE Iberia (Iberian Massif). In contrast, the Maláguide samples were probably deposited close to the Ossa-Morena Zone (Iberian Massif) . Thus, Alpujárride/Nevado-Filábride and Maláguide complexes would have been juxtaposed during the Triassic-Cretaceous Neotethys rifting/drifting and/or the subsequent Alpine collisional events.

Tectono-metamorphic interaction of upper mantle peridotites and lower crustal units during continental rifting in the western Betic Cordillera

Recent geological mapping conducted in the Ronda Peridotites (Betic Cordillera, S Spain) has revealed a systematic field correlation between lower crustal metamorphic units and different tectono-metamorphic domains of the ultramafic massif. Mylonitic and highly tectonized Spl ± Grt peridotites (i.e., Grt-Spl mylonite and Spl tectonite domains), which are considered to be derived from a thick continental lithosphere, are in contact with garnet-bearing gneisses (i.e., kinzigites) of the Jubrique unit along a narrow but continuous mylonitic shear zone. Phase equilibrium calculations indicate that the metamorphic rocks of the Jubrique unit are consistent with an initial continental setting characterized by normal crustal thicknesses, which underwent two melting events. The first melting occurred at 0.9–1.0 GPa / 770–800 °C and resulted in 13 %melt, while the second one took place at shallower crustal conditions (0.4–0.5 GPa and 710–765 °C) and led to more restricted melt production (2–3 %melt). In contrast, the Spl ± Pl peridotites (Pl-tectonite domain), which are stable only at shallowest mantle levels within a highly extended continental lithosphere, are consistently found exposed in contact with heterogeneous granites and migmatites of the Guadaiza unit. According to new thermodynamic modeling, the Guadaiza metamorphic rocks record a single melting event characterized by a theoretical melt production of 6 to 11 % at the base of a very thin continental crust (ca. 0.3 GPa and 675–710 °C). This process was likely facilitated by influx of external water, necessary to generate high melt fractions observed in the diatexites. The systematic correlation observed between crustal metamorphic units, consistently overlaying the mantle rocks, and specific ultramafic domains of the Ronda massif, suggests that their juxtaposition primarily resulted from the severe extension of the continental lithosphere.U-Pb radiometric dating of zircons from gneisses, migmatites, and heterogeneous granites in the middle crustal rocks of the Guadaiza unit indicates that extensional processes, crustal anatexis, and melt stagnation occurred at around 280 Ma. Comparison of these new radiometric ages with previous results from the Jubrique unit suggests that a Permian high-temperature / low- to medium-pressure event uniformly affected the crustal units over the Ronda Peridotites. This event coincided with the formation of characteristic ultramafic mineral assemblages within the Ronda massif and provides evidence for the interaction between upper mantle rocks and lower- to mid-crustal metamorphic rocks during this period.

Combining methane clumped and bulk isotopes, temporal variations in molecular and isotopic composition, and hydrochemical and geological proxies to understand methane's origin in the Ronda peridotite massifs (Spain)

In serpentinised peridotite and ultramafic rock systems, methane (CH4) origin is frequently considered abiotic, but variable microbial and thermogenic components can also exist. Typically, the origin of CH4 is studied using bulk, 13C/12C and 2H/H isotopic composition, molecular gas composition, occasionally radiocarbon (14C), microbiology and geological context. Recent advances in CH4-clumped isotope methods have yielded novel insights into the formation of CH4: nonetheless, their interpretation in natural gas samples is often uncertain and requires additional research. Here, we study the origin of the gas released in hyperalkaline (pH > 10) springs in the Ronda Peridotite Massifs (southern Spain), combining bulk and clumped CH4 isotopes with molecular gas composition, hydrochemical (Total Organic Carbon and Platinum Group Elements in water), geothermal and geo-structural data. Five springs analysed in 2014 have been re-examined for changes in gas chemistry over time, and three newly discovered gas-bearing springs are analysed for the first time. Regardless of whether springs have microbial or abiotic isotopic fingerprints, we find that bulk CH4 isotopes are fairly stable over a seven-year period. This suggests that the CH4 source(s) or postgenetic processes (such as oxidation and diffusion) have not undergone significant temporal changes. Major variations in H2 and CH4 concentrations in certain springs may be the result of changes in gas pressure and migration intensity. Paired CH4 clumped isotopes (Δ12CH2D2 - Δ13CH3D) were analysed in two bubbling springs, where the presence of CH4 can be interpreted as non-microbial based on 13C enrichment, absence of 14C, and the presence of ethane and propane. However, these isotopes are in disequilibrium, which prevents the quantification of the gas formation temperature. Within the Δ12CH2D2 - Δ13CH3D diagram, the data lie within both the microbialgenic zone, suggested by previous authors, and the abiotic zone that results combining data from laboratory gas synthesis and other natural gas samples. Therefore, attributing a microbial origin to CH4 based only on clumped isotopes is less definite than previously assumed. The amount of Total Organic Carbon appears to be correlated with the origin of CH4, as it is higher in 13C-depleted CH4 samples and lower in 13C-enriched samples. Palladium (Pd) and Rhodium (Rh) dissolved in water (the more soluble Platinum Group Elements) can be a proxy for the chromitite ore deposits contained in plagioclase tectonite layers throughout the investigated area, which may act as catalysts for abiotic CO2 hydrogenation. Clumped isotope disequilibrium and the reported absence of diffuse CH4-bearing fluid inclusions in the peridotites appear to rule out high temperature gas genesis in post-magmatic inclusions. These observations, along with the moderate temperatures at the base of the peridotite massifs and the consistent occurrence of gas along tectonic contacts between serpentinised (H2-bearing) peridotite and carbon-bearing rocks, are compatible with the theory of low-temperature CO2 hydrogenation.

Melt-enhanced strain localization and phase mixing in a large-scale mantle shear zone (Ronda peridotite, Spain)

Abstract. Strain localization in upper-mantle shear zones by grain size reduction and the activation of grain-size-sensitive deformation mechanisms is closely linked to phase mixing. With its mylonitic grain size (50–100 µm) and well-mixed phase assemblage, the kilometer-scale shear zone at the northwestern boundary of the Ronda peridotite is, in this respect, no exception. In transects across the high-strain mylonitic into the low-strain tectonitic part of this shear zone, the following four dominant microstructural domains were identified: (1) olivine-rich matrix, (2) mixed matrix, (3) neoblast tails of clinopyroxene porphyroclasts, and (4) neoblast tails of orthopyroxene porphyroclasts. In these domains, phase mixing and its impact on strain localization were investigated by a combination of microstructural (optical microscopy), textural (EBSD), and geochemical (EPMA) analysis. The dominant microstructural domain of all samples is the mixed matrix composed of olivine, orthopyroxene, and clinopyroxene. Its homogenous distribution of interstitial pyroxenes contradicts mechanical mixing. Instead, extensive phase mixing under near-steady-state conditions is documented by the constant grain size and by phase boundary percentages > 60 % for the entire mylonitic unit and all the microstructural domains. Lobate phase boundaries, homogenous phase mixing, and secondary-phase distribution, as well as continuous geochemical trends that are independent of the microstructural domain, point to a reaction-driven, metasomatic formation of the mixed matrix and pyroxene porphyroclast tails in the entire shear zone. An OH-bearing metasomatism by small fractions of evolved melts is indicated by amphibole abundance in pyroxene neoblast tails, olivine B-type-crystallographic-preferred orientations (CPOs), and the microstructural consistency of the garnet–spinel (grt–spl) mylonites from both major peridotite massifs of the Gibraltar arc, Ronda, and Beni Bousera (Morocco). The established syn-deformational temperature of 800–900 ∘C at 1.95–2.00 GPa suggests that the metasomatism did not reset the equilibrium temperatures. Consistent geochemistry and phase assemblage in mylonites and tectonites but a change from equiaxial (tectonites) to wedge-shaped pyroxenes aligned parallel to the foliation (mylonites) point to a pre- to syn-deformational metasomatism, with the potential annealing of the tectonites. For the mylonitic mixed matrix, wedge-shaped pyroxenes, and neoblast tail formation in pyroxene porphyroclast stress shadows point to the activity of incongruent dissolution–precipitation creep. Apart from the dissolution–precipitation creep, strong CPOs of all major phases (ol, opx, and cpx) suggest dislocation creep as being the major deformation mechanism in the entire shear zone.

Provenance Analysis of the Ojén Nappe and Its Implication for the Geodynamic History (Alpujárride Complex, Betic Cordilleras, Spain)

The Ojén nappe, an allochthonous unit that underlies the Ronda peridotites (Betic Cordilleras), is composed of Triassic marbles overlying a metapelitic sequence. In order to carry out an LA-ICP-MS U-Pb age determination and provenance analysis, detrital zircon grains from two quartzites interlayered within marbles and metapelites were extracted. The obtained results yield the youngest zircon population of 254 ± 3 Ma (Late Permian) with a mean Th/U ratio of 0.34 that supports both the Permian–Triassic age of the marble member and the felsic magma nature for zircon-bearing protoliths. The Permian zircons exhibit tight age peaks in three main populations of 292 ± 2 Ma, 278 ± 3 Ma, and 254 ± 3 Ma, in agreement with the main age populations reported from rhyolites and shallow crustal basaltic–andesite subalkaline rocks emplaced in transtensional Permian basins of the Variscan Orogen during the break-up of Pangea. Moreover, the analyzed detrital zircon grains yield age distributions with clear Tonian–Stenian (ca. 950–975 Ma), Ediacaran–Cryogenian (ca. 560–615 Ma), Ordovician (ca. 460–465 Ma), and Cisuralian peaks (ca. 280–290 Ma) and smoothed Mesoproterozoic peaks that point to a weak and residual Cadomian peri-Gondwanan terrane inheritance. The identified Middle Ordovician detrital zircon populations (460–465 Ma) strengthen the hypothesis that the Alborán domain would be located along the southern passive margin of the European Hunic superterrane.

Trace and rare earth element distribution in hyperalkaline serpentinite-hosted spring waters and associated authigenic carbonates from the Ronda peridotite

The Ronda peridotite in southern Spain is subject to low-temperature serpentinization by circulating groundwaters that produces hyperalkaline fluids of high pH and low dissolved inorganic carbon content (DIC). When these waters resurface as hyperalkaline springs they take up atmospheric carbon dioxide (CO2), which triggers the precipitation of travertine carbonates. Spring waters at Baños del Duque, Fuente Romana, and Fuente Amargosa show typical chemical features of hyperalkaline springs such as high pH and low DIC, yet exhibit an unusual distribution of rare earth elements (REEs) enriched in lanthanum (La) and cerium (Ce). In order to constrain changes in spring water chemistry and carbonate precipitation during atmospheric CO2 uptake, a reaction path model was set up to illustrate that the distribution of REE species in the high pH and low DIC water is strongly dependent on REE complexation by hydroxide and carbonate species. Reaction path modeling results show that during spring fluid emission and CO2 uptake, REE speciation in the fluids shifts from a dominance of hydroxide to carbonate complexes. The complexation of rare earth elements in the high pH, low DIC fluids strongly depends on the DIC content of the respective fluid. The REE distribution of travertine deposits associated with the hyperalkaline springs deviates from that in the fluids, suggesting that travertines precipitated from fluids different to those currently emitted from the springs.

Peri-Gondwanan Provenance and Geodynamic Evolution of The Guadaiza Nappe (Alpujarride Complex, Betic Cordilleras, Spain): Insights on The Paleotethyan Paleogeography

Based on the LA-ICP-MS U-Pb zircon ages of four metamorphic samples, we discuss the geochronology and provenance of the Guadaiza nappe, an allochthonous unit that underlies the Ronda peridotites (Betic Cordilleras, Spain). The Guadaiza nappe is composed of Triassic marbles overlying a metapelitic sequence with schists and migmatites. Zircons from a quartzite interlayered with the marbles yield a maximum depositional age of ca. 289 Ma that supports the Triassic age. The idiomorphic morphology of these Paleozoic zircon crystals and the lack of late-Variscan metamorphism (ca. 300 Ma) supports a proximal source area, and suggests that the marbles were discordantly deposited over the metapelitic sequence, along the northern margin of the Alboran microplate. The zircon patterns from the metapelitic sequence mainly yield Paleoproterozoic (ca. 1.6–2.5 Ga), Tonian–Stenian (ca. 1000 Ma), Ediacaran–Cryogenian (ca. 600 Ma) and Paleozoic (ca. 500 Ma) age clusters. These results suggest provenance from areas within the triangle bound by the West African Craton, the Metasaharan Craton and the Hun Superterrane during the Paleotethys opening (Silurian–Devonian). A Silurian–Carboniferous deposition age for the schist protoliths is constrained by the youngest detrital zircon population (ca. 443 Ma) and the Variscan age of their migmatization by an additional peak of around 299 Ma in the migmatites.

Genesis and evolution of the San Manuel iron skarn deposit (Betic Cordillera, SW Spain)

The San Manuel magnesian skarn is an iron deposit hosted in dolomitic marbles from a tectonic slice imbricated within the Ronda peridotites, in the westernmost part of the Betic Cordillera, Spain. According to the dominant mineral assemblage, the skarn is subdivided into three different zones, (1) forsterite ± calcite skarn, (2) calcite ± chlorite ± serpentine skarn, and (3) Ca-amphibole skarn. The main ore in the skarn is a ∼ 2.5 m thick, massive ore body situated in the middle of the sequence. In this paper, we firstly report a comprehensive major to trace element composition, texture, microstructure, and mineralogy characterization for zoned magnesioferrite-magnetite grains of the San Manuel deposit using a combination of (1) laser ablation inductively coupled plasma mass spectrometer, (2) focused ion beam combined with transmission electron microscopy, and (3) electron back-scattered diffraction. We have defined four different magnesioferrite-magnetite generations. A complete sequence of zoning includes cores of magnesioferrite (Mag-1; MgO up to 10.6 wt%) overprinted by three successive generations of magnetite, namely Mag-2, Mag-3, Mag-4. Mag-2 (MgO < 4 wt%), hosts composite forsterite ± calcite ± chlorite inclusions, consistently with high Si, Ca, and Sr (average: 8204 ppm, 8980 ppm, and 49 ppm respectively) contents detected by in situ laser ablation inductively coupled plasma (LA-ICP-MS). Mag-3 replacing former Mag-1 and Mag-2 includes nanometric spinel and gahnite exsolutions detected by focused ion beam combined with a transmission electron microscope (FIB-TEM), which is consistent with its high Al, Ti, V, and Ga (average: 5073 ppm, 368 ppm, and 20 ppm, respectively) trace element concentration. Mag-4 is the Fe-richest magnetite (up to 94.16 wt% FeOtotal) forming the outermost rims in magnetite grains, and exhibiting the lowest total trace element contents. Approaches in temperature estimations employing magnetite-spinel exsolutions in Mag-3 suggest that the minimum temperature of the prograde stage reached temperatures below 700 °C, whereas Mag-4 should be formed during the retrograde stage. Magnetite microstructure studied by electron backscatter diffraction (EBSD) suggests Mag-4 formation under fluid-assisted dynamic conditions, which is consistent with the tectonic evolution of the emplacement. We propose that the San Manuel deposit formed by pulsed hydrothermal fluids derived from anatexis of crustal rocks during peridotite emplacement, promoting re-equilibration processes that led to the magnesioferrite-magnetite zoning.

Exhumation of the Ronda Peridotite During Hyper‐Extension: New Structural and Thermal Constraints From the Nieves Unit (Western Betic Cordillera, Spain)

AbstractThe Ronda peridotite (Betic Cordillera, Southern Spain) is the largest alpine‐type peridotite massif worldwide. Yet, the emplacement mechanism of these mantle rocks is still a highly debated topic. In this study, we aim at better constraining their context of exhumation by focusing on the Nieves Unit, which is mostly composed of Mesozoic metasediments displaying HT metamorphism and deformation along the contact with peridotite. The topmost parts of the peridotites and the bottom parts of the Nieves Unit share the same planar/linear fabric parallel to the contact accompanied by component of sinistral kinematic indicators. Raman Spectroscopy on Carbonaceous Material geothermometry then shows a contraction of peak‐temperature conditions within metasediments of the Nieves Unit, as well as a north‐eastward decrease along strike, highlighting a strong thinning of the unit and a significant obliquity of the isograds to the contact with the peridotites. Local magnetite ores and ophicalcite bodies are finally observed nearby the contact, suggesting a hyper‐extension context for the mantle exhumation. Indeed, the Nieves Unit, the Ronda peridotite and the Jubrique Unit, where a strong thinning has also been documented, are all in contact at a triple junction between the western contact of the peridotite and the Nieves/Jubrique contact. Altogether, these features support the idea that the western shear zone limiting the peridotites from the Nieves and Jubrique units is a tilted large‐scale extensional detachment that has exhumed mantle rocks from beneath the continental basement (Jubrique Unit) and its Mesozoic cover (Nieves Unit) during an episode of hyper‐extension.

Developing Atom Probe Tomography of Phyllosilicates in Preparation for Extra‐Terrestrial Sample Return

Hydrous phyllosilicate minerals, including the serpentine subgroup, are likely to be major constituents of material that will be bought back to Earth by missions to Mars and to primitive asteroids Ryugu and Bennu. Small quantities (&lt; 60 g) of micrometre‐sized, internally heterogeneous material will be available for study, requiring minimally destructive techniques. Many conventional methods are unsuitable for phyllosilicates as they are typically finely crystalline and electron beam‐sensitive resulting in amorphisation and dehydration. New tools will be required for nanoscale characterisation of these precious extra‐terrestrial samples. Here we test the effectiveness of atom probe tomography (APT) for this purpose. Using lizardite from the Ronda peridotite, Spain, as a terrestrial analogue, we outline an effective analytical protocol to extract nanoscale chemical and structural measurements of phyllosilicates. The potential of APT is demonstrated by the unexpected finding that the Ronda lizardite contains SiO‐rich nanophases, consistent with opaline silica that formed as a by‐product of the serpentinisation of olivine. Our new APT approach unlocks previously unobservable nanominerals and nanostructures within phyllosilicates owing to resolution limitations of more established imaging techniques. APT will provide unique insights into the processes and products of water/rock interaction on Earth, Mars and primitive asteroids.

Isotopic Signatures of Carbon in the ‘Los Pobres’ Graphite Mine, Ronda, Spain

AbstractGraphite formation temperatures in the ‘Los Pobres’ mine within the Ronda peridotite, Spain, previously reported to be between 770 and 820 °C, have been reinterpreted based on new temperature measurements using Raman spectroscopy. Additional in situ and bulk stable carbon isotopic measurements and fluid inclusion studies contributed to improved understanding of parts of the graphite formation process. Raman spectroscopy revealed that the formation of the ‘Los Pobres’ graphite extends to temperatures as low as 500 °C, indicating a broader temperature range than previously reported. Stable carbon isotopes and temperature estimates suggest two different crystallization events, followed by a late hydrothermal alteration of the host rock. The first event occurred at temperatures higher than ∼600°C, in which crystalline graphite was formed with a mixed 13C composition as a result of the mixing of two different carbon-bearing sources. The second graphite formation event took place below ∼600°C, within the same system, but with lower purity and crystallinity of the graphite. In the third event, the temperature decreased to less than 550 °C, and hydrothermal fluids altered the host rock, precipitating silica and iron oxides in veins penetrating both the host rock and the deposited graphite.

Geochemistry and origin of zircon in chlorite schists of the Ronda peridotites (Betic Cordilleras, southern Spain)

AbstractThis work deals with scarce chlorite schists scattered through the Ronda peridotites (Betic Cordilleras, Spain). These schists have unusually high zircon contents, which contrast with the usual lack of this mineral in ultramafic rocks. From field data and detailed petrographic, geochemical, and geothermometric studies, we focused on the origin of the zircon, a relevant issue for the interpretation of geochronological results. The chlorite schists appear as concordant sheets with granite dikes and as blackwall zones between dikes and serpentinized peridotites. As the intrusion age of the dikes and chlorite schist zircon crystallization (ca. 22 Ma) is slightly older than the age of serpentinization and related chlorite schist formation (ca. 19 Ma), we propose that the chlorite schists are tied to the intrusion of the granite dikes and the subsequent serpentinization of peridotites. Trace and rare earth elements alone are not indicative of the magmatic or hydrothermal origin of the zircon, but the combination of information about zircon morphology, melt inclusions, geothermometry, and the structural relationships between granite dikes and chlorite schists points to late magmatic melts for the zircon origin. We suggest that high-temperature melts saturated in F and Cl acted as Zr carriers under low-pH conditions. A change of the pH conditions, due to hydrothermal alkaline fluids incoming for the concomitant peridotite serpentinization, would have led to zircon crystallization and concentration at the apical zones of the dikes, and to rodingitization before the extensive observed chloritization.

Geochemistry and mineralogy of serpentinization-driven hyperalkaline springs in the Ronda peridotites

We present a detailed study of the water geochemistry, mineralogy and textures in serpentinization-related hyperalkaline springs in the Ronda peridotites. Ronda waters can be classified into hyperalkaline fluids and river waters that are broadly similar to Ca2+–OH- and Mg2+–HCO3- water types described in serpentinite-hosted alkaline springs elsewhere. At the discharge sites of the fluids (fractures or human made outlets) and ponds along the fluid flow paths, the fluids are hyperalkaline (10.9 < pH < 12) and characterized by low Mg and high Na, K, Ca, and Cl- concentrations. River waters, occurring near the spring sites, are mildly alkaline (8.5 < pH < 8.9) and enriched in Mg and DIC compared to Na, K, Ca and Cl-. The chemistry of Ronda Mg–HCO3 river waters is likely due to the hydrolysis of ferromagnesian peridotite minerals in equilibrium with the atmosphere by infiltrated meteoric water and shallow groundwater in the serpentinized peridotite. The Ronda Ca–OH hyperalkaline fluids are generated by the combination of low temperature serpentinization reactions from infiltrated surface Mg–HCO3 river waters —or Ca–HCO3 waters from near karst aquifers— and deep carbonate precipitation isolated from atmospheric CO2. Mass balance calculations indicate that the weathering of Ca-bearing peridotite silicates, such as diopside, is a feasible source of Ca in Ronda Ca–OH hyperalkaline fluids; however, it requires steady-state dissolution rates substantially greater than those determined experimentally.Travertine, crystalline crusts and sediment deposits comprise the types of solid precipitates observed in Ronda hyperalkaline spring sites. Calcite, aragonite, dolomite and Mg-Al-rich clays are the main mineral phases identified in the spring sites. As illustrated in the Baños del Puerto spring site, (i) calcite-dominated precipitation is due to uptake of atmospheric CO2 by Ca–OH hyperalkaline fluids during discharge, and (ii) aragonite-dominated precipitation is due to mixing of Ca–OH hyperalkaline fluids with Mg–HCO3 river waters. Aragonite and dolomite contents increase away from the springs and toward the river waters that uniquely reflects the effect of Mg ions on the precipitation of aragonite versus calcite. Other potential factors controlling the precipitation of these CaCO3 polymorphs are the Mg/Ca ratio, the CO2 content, and the temperature of the fluids. Dolomite forms during lithification of travertine due to periodic flooding of river water combined with subsequent evaporation.

Western Mediterranean Subcontinental Mantle Emplacement by Continental Margin Obduction

AbstractObduction is the tectonic process that results from thrusting of an oceanic lithosphere section (ophiolite) onto a continent. In contrast, thrusting of subcontinental mantle, observed in several mountain belts, remains a major unknown of plate tectonics. In the western Mediterranean, the Ronda and Beni Bousera peridotites are the largest worldwide subcontinental mantle exposure. From a geological point of view, the Ronda peridotites are exhumed by hyperstretching of the continental lithosphere in a back‐arc immediately followed by thrusting, explaining their present‐day position inside the Alboran crust. Using 2‐D and 3‐D modeling of new gravimetric data combined with local seismic tomography, we show that the Ronda peridotites are rooted inside the Alboran mantle along the entire Gibraltar arc. On these bases, we propose that the emplacement of the Ronda peridotites occurred in a back‐arc setting and corresponds to the thrusting of an entire hyper‐stretched continental margin onto a continent, a process that we define as continental margin obduction. This results from two successive deformation events: continental upper plate extension driven by slab roll‐back, immediately followed by upper plate shortening, likely triggered when a buoyant continental domain enters the subduction. We propose that this process affected the entire western Alboran domain, with peridotite bodies embedded within the crust along the whole Gibraltar Arc. We suggest that other examples such as the Alps Ivrea mantle body could likely represent continental margin obduction at the onset of continental collision.

Implication of the geology of the Guadaiza and Verde valleys (Malaga Province, Betic Cordillera) on the position of the Ronda peridotites and the structure of the Alpujárride Complex

In the valleys of Guadiaza and Verde (the latter in its area of Hoyo del Bote), the tectonic window of the Guadaiza unit has been previously defined based on the interpretation that a formation existing there was a dynamothermal aureole formed under thrusting Ronda peridotites, this being interpreted as having occurred during the early Miocene. Nevertheless, the analysis of the geometrical relationship between this formation and the peridotites indicates that the cited formation is not situated under the peridotites, but above them and, moreover, the rocks of this supposed aureole present a lithological transition to others of Permian age, which are also situated over the peridotites. As a result, the supposed tectonic window does not exist and the number of tectonic units existing in the western part of the Alpujarride Complex diminishes. It is also clear that the peridotites of this region reached the surface during the Paleozoic.

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

AbstractThe 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.

Fluid-assisted strain localization in the shallow subcontinental lithospheric mantle

We report microstructural evidence for fluid-assisted ductile strain localization in a ≤50m-wide mylonitic–ultramylonitic shear zone in the Ronda Peridotite massif, Southern Spain. Strain localization occurred at relatively low pressure (<0.8GPa) and moderate temperature (750–1000°C). Initial deformation by dislocation creep resulted in formation of mylonites. Focusing of aqueous fluids in the shear zone favored the activation of dissolution–precipitation creep, resulting in further strain localization. This process is recorded by two generations of ultramylonitic bands composed of fine-grained, well-mixed olivine–orthopyroxene aggregates. Microstructural observations in the ultramylonites suggest alternating dissolution and precipitation of olivine and orthopyroxene, which may be explained by local changes in silica molality of the percolating fluid (disequilibrium and mass transfer at scales > mm). In the mylonites, olivine shows a crystal preferred orientation (CPO) coherent with dominant (001)[100] glide, probably due to the presence of interstitial fluids during deformation. In the ultramylonites, olivine CPO is weak to very weak, consistently with a decreasing contribution of dislocation creep to deformation. In contrast, fine-grained orthopyroxene in both mylonites and ultramylonites displays a clear CPO characterized by a [001] maximum normal to the foliation, which is not consistent with dislocation glide in any known slip system for orthopyroxene. We interpret this CPO as formed by oriented crystallization during dissolution–precipitation. In the present study, dissolution–precipitation creep predominates only in small-scale ultramylonite bands due to limited fluid availability and localized dynamic permeability. However, this process may be important in intermediate temperature domains of subduction zones, where it may lead to a feedback between strain localization and fluid transport.

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 19–22Ma and occurring at upper continental crust depths (P<4kbar). Gravity data reveal that the average density of the Ronda mantle slab (~2.7–2.8g/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.

Using nanogranitoids and phase equilibria modeling to unravel anatexis in the crustal footwall of the Ronda peridotites (Betic Cordillera, S Spain)

Anatexis in the crustal footwall of Ronda peridotites (Betic Cordillera, S Spain) is apparently related to the hot emplacement of this mantle slab over metasedimentary rocks. In this study, we combine the analysis of melt inclusions (MI) and phase equilibria calculations on quartzo-feldspathic mylonites (former migmatites) occurring at the contact with the mantle rocks, in the region of Sierra Alpujata (Ojén unit). The goal is to better characterize anatexis in these rocks and to provide new constraints on the geodynamic evolution of the crustal footwall. Such data are important for understanding the mechanisms of crustal emplacement of the mantle slice. The quartzo-feldspathic mylonites are characterized by the mineral assemblage Qtz+Pl+Kfs+Sil+Grt+Ilm+Bt±Ap±Gr. Clusters of MI are observed both at the core and toward the rim of peritectic garnet. In each cluster, MI range from totally glassy to nanogranitoids, consisting of Qtz+Kfs+Bt+Ms+Pl aggregates. The trapped melt is leucogranitic and peraluminous with variable Na2O/K2O values and low H2O contents (≈2–4wt%). Phase equilibria modeling in the MnO–Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2–TiO2–O2–C (MnNCaKFMASHTOC) system with graphite-saturated fluid constrains the P–T conditions of melting at ≈6kbar, ≈820°C. MI data support the fluid-absent character of melting. The investigated MI represent the primary anatectic melts produced during prograde anatexis of the host rocks via biotite dehydration melting. Field, compositional, and textural observations indicate that mylonitic migmatites represent strongly deformed former diatexites. The comparison between the new data and some recently published information on migmatites located further from the contact with the peridotites and toward the bottom of the crustal footwall, raises some important issues which question the previously proposed geodynamic models for this region. Among them, (i) the crustal footwall at Sierra Alpujata constitutes an inverted metamorphic sequence, not only in terms of temperature but also in pressure, and (ii) the Ojén unit does not seem to represent a coherent high-P portion of a continental subduction system.