Claudio Marchesi

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.

Primitive Cretaceous island-arc volcanic rocks in eastern Cuba: the Téneme Formation

The Teneme Formation is located in the Mayari-Cristal ophiolitic massif and represents one of the three Cretaceous volcanic Formations established in northeastern Cuba. Teneme volcanics are cut by small bodies of 89.70 ± 0.50 Ma quarz-diorite rocks (Rio Grande intrusive), and are overthrusted by serpentinized ultramafics. Teneme volcanic rocks are mainly basalts, basaltic andesites, andesites, and minor dacites, and their geochemical signature varies between low-Ti island arc tholeiites (IAT) with boninitic affinity (TiO2 < 0.4 %; high field strength elements << N-type MORB) and typical oceanic arc tholeiites (TiO2 = 0.5-0.8 %). Basaltic rocks exhibit low light REE/Yb ratios (La/Yb < 5), typical of intraoceanic arcs and are comparable to Maimon Formation in Dominican Republic (IAT, pre Albian) and Puerto Rican lavas of volcanic phase I (island arc tholeiites, Aptian to Early Albian). The mantle wedge signature of the Teneme Formation indicates a highly depleted MORB-type mantle source, without any contribution of E-MORB or OIB components. Our results suggest that Teneme volcanism represents a primitive oceanic island arc environment. If the Late Cretaceous age (Turonian or early Coniacian) proposed for Teneme Formation is correct, our results indicate that the Cretaceous volcanic rocks of eastern Cuba and the Dominican Republic are not segments of a single arc system, and that in Late Cretaceous (Albian-Campanian) Caribbean island arc development is not represented only by calc-alkaline (CA) volcanic rocks as has been suggested in previous works.

Primary melt inclusions in andalusite from anatectic graphitic metapelites: Implications for the position of the Al2SiO5 triple point

Anatectic crustal xenoliths in the Miocene volcanic rocks of Mazarron (southeast Spain) contain andalusite with melt inclu- sions, an unprecedented finding. Microstructures indicate that the melt inclusions were trapped during andalusite growth. The vola- tile content of the peraluminous inclusions is too low (Cl , 0.4 wt%, F , 0.3 wt%, P 1500 ppm, B , 850 ppm) to have caused a significant reduction of the wet solidus temperature. Moreover, the presence of graphite, as observed, during partial melting is expected to have raised the temperature of the wet solidus. Melting temperatures for the inclusions—obtained from quartz-albite-or- thoclase haplogranite system (680-790 8C), Zr (620-705 8C), and light rare earth element (615-725 8C) thermometry—indicate that the stability of andalusite 1 melt is incompatible with the position of the most commonly used andalusite 5 sillimanite equilibrium, and that the Al2SiO5 triple point must be placed at higher tem- peratures and pressures.

Backarc basin inversion and subcontinental mantle emplacement in the crust: kilometre-scale folding and shearing at the base of the proto-Alborán lithospheric mantle (Betic Cordillera, southern Spain)

To constrain the latest evolutionary stages and mechanisms of exhumation and emplacement of subcontinental peridotites in the westernmost Mediterranean, we present here a detailed structural study of the transition from granular spinel peridotite to plagioclase tectonite in the western Ronda Peridotite (Betic Cordillera, southern Spain). We show that the plagioclase tectonite foliation represents an axial surface particularly well developed in the reverse limb of a downward facing moderately plunging and moderately inclined synform at the base of the Ronda massif. The fold limbs are cut by several mylonitic and ultramylonitic shear zones with top-to-the-SW sense of shear. After restoring the middle to late Miocene vertical-axis palaeomagnetic rotation and the early Miocene tectonic tilting of the massif, these studied structures record southward-directed kinematics. We propose a geodynamic model in which folding and shearing of an attenuated mantle lithosphere occurred by backarc basin inversion during late Oligocene (23–25 Ma) southward collision of the Alborán Domain with the palaeo-Maghrebian passive margin, leading to the intracrustal emplacement of peridotites in the earliest Miocene (21–23 Ma).

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

Contrasting 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 margins. Rather than convective removal or delamination of the lithospheric root, our data strongly support Alboran geodynamic models that envisage slab rollback as the tectonic mechanism responsible for the Miocene lithospheric thinning. The Ronda Cr-rich pyroxenite dikes represent the earliest unambiguous manifestation of subduction-related magmatism in the western Mediterranean and testify to the involvement of terrigenous sediments in the primitive stages of subduction.

Platinum-group elements, S, Se and Cu in highly depleted abyssal peridotites from the Mid-Atlantic Ocean Ridge (ODP Hole 1274A) : influence of hydrothermal and magmatic processes

Highly depleted harzburgites and dunites were recovered from ODP Hole 1274A, near the intersection between the Mid-Atlantic Ocean Ridge and the 15°20′N Fracture Zone. In addition to high degrees of partial melting, these peridotites underwent multiple episodes of melt–rock reaction and intense serpentinization and seawater alteration close to the seafloor. Low concentrations of Se, Cu and platinum-group elements (PGE) in harzburgites drilled at around 35–85xa0m below seafloor are consistent with the consumption of mantle sulfides after high degrees (>15–20xa0%) of partial melting and redistribution of chalcophile and siderophile elements into PGE-rich residual microphases. Higher concentrations of Cu, Se, Ru, Rh and Pd in harzburgites from the uppermost and lowest cores testify to late reaction with a sulfide melt. Dunites were formed by percolation of silica- and sulfur-undersaturated melts into low-Se harzburgites. Platinum-group and chalcophile elements were not mobilized during dunite formation and mostly preserve the signature of precursor harzburgites, except for higher Ru and lower Pt contents caused by precipitation and removal of platinum-group minerals. During serpentinization at low temperature (<250xa0°C) and reducing conditions, mantle sulfides experienced desulfurization to S-poor sulfides (mainly heazlewoodite) and awaruite. Contrary to Se and Cu, sulfur does not record the magmatic evolution of peridotites but was mostly added in hydrothermal sulfides and sulfate from seawater. Platinum-group elements were unaffected by post-magmatic low-temperature processes, except Pt and Pd that may have been slightly remobilized during oxidative seawater alteration.

Migration and accumulation of ultra-depleted subduction-related melts in the Massif du Sud ophiolite

1 pagina.-- Resumen del trabajo presentado en la 19th Annual V.M. Goldschmidt Conference, V.M. Goldschmidt Conference.

Flow in the western Mediterranean shallow mantle: Insights from xenoliths in Pliocene alkali basalts from SE Iberia (eastern Betics, Spain)

Mantle xenoliths in Pliocene alkali basalts of the eastern Betics (SE Iberia, Spain) are spinelu2009±u2009plagioclase lherzolite, with minor harzburgite and wehrlite, displaying porphyroclastic or equigranular textures. Equigranular peridotites have olivine crystal preferred orientation (CPO) patterns similar to those of porphyroclastic xenoliths but slightly more dispersed. Olivine CPO shows [100]-fiber patterns characterized by strong alignment of [100]-axes subparallel to the stretching lineation and a girdle distribution of [010]-axes normal to it. This pattern is consistent with simple shear or transtensional deformation accommodated by dislocation creep. One xenolith provides evidence for synkinematic reactive percolation of subduction-related Si-rich melts/fluids that resulted in oriented crystallization of orthopyroxene. Despite a seemingly undeformed microstructure, the CPO in orthopyroxenite veins in composite xenoliths is identical to those of pyroxenes in the host peridotite, suggesting late-kinematic crystallization. Based on these observations, we propose that the annealing producing the equigranular microstructures was triggered by melt percolation in the shallow subcontinental lithospheric mantle coeval to the late Neogene formation of veins in composite xenoliths. Calculated seismic properties are characterized by fast propagation of P waves and polarization of fast S waves parallel to olivine [100]-axis (stretching lineation). These data are compatible with present-day seismic anisotropy observations in SE Iberia if the foliations in the lithospheric mantle are steeply dipping and lineations are subhorizontal with ENE strike, implying dominantly horizontal mantle flow in the ENE-WSW direction within vertical planes, that is, subparallel to the paleo-Iberian margin. The measured anisotropy could thus reflect a lithospheric fabric due to strike-slip deformation in the late Miocene in the context of WSW tearing of the subducted south Iberian margin lithosphere.

Effects of seawater mixing on the mobility of trace elements in acid phosphogypsum leachates

This research reports the effects of pH increase on contaminant mobility in phosphogypsum leachates by seawater mixing, as occurs with dumpings on marine environments. Acid leachates from a phosphogypsum stack located in the Estuary of Huelva (Spain) were mixed with seawater to achieve gradually pH7. Concentrations of Al, Fe, Cr, Pb and U in mixed solutions significantly decreased with increasing pH by sorption and/or precipitation processes. Nevertheless, this study provides insight into the high contribution of the phosphogypsum stack to the release of other toxic elements (Co, Ni, Cu, Zn, As, Cd and Sb) to the coastal areas, as 80-100% of their initial concentrations behaved conservatively in mixing solutions with no participation in sorption processes. Stable isotopes ruled out connexion between different phosphogypsum-related wastewaters and unveiled possible weathering inputs of estuarine waters to the stack. The urgency of adopting effective restoration measures in the study area is also stressed.

New insights into the metal partitioning in different microphases of human gallstones

Chronic metal exposure, e.g. from metal mining, may cause accumulation of metals in soft and hard tissues, and in developing biomineralizations in the human body. Gallstones are biomineralizations formed in the gallbladder which are able to trap trace elements from the bile. Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) was used to analyze gallstone cross-sections to trace the elemental abundances and correlate them with the principal phases constituting gallstones, namely cholesterol, Ca bilirubinate salts, Ca carbonate, and Ca phosphate. Five different types of gallstones (pure, mixed, and composite cholesterol stones, pigment stone, and carbonate stone) were chosen according to a previous classification based on phase characterization by different spectroscopic and microscopic techniques. These data were combined with bulk solution ICP-MS/OES analyses for total elemental concentrations. The results indicated that cholesterol has a zero capacity to retain elements except for Ca. Hence, pure cholesterol stones contained the lowest bulk metal concentrations, and the metals were found in the scarce carbonate and phosphate phases in these calculi. Calcium and trace element concentrations increased in other types of gallstones along with increasing amount of bilirubinate, carbonates and phosphates; pigment stones being the most enriched in metals. Phosphates were the principal carriers of Ca, P, Na, Mg, Mn, Fe, Pb, and Cd, whereas carbonate phases were enriched in Ca, Mg, Na, and Mn in order of decreasing abundance. Bilirubinate on the other hand was enriched in Ca, Cu, Ag, and Ni. The higher trace metal affinities of bilirubinate and phosphate explain the elevated metal concentrations observed in the pigment stones. These results give new insight to the trace metal behavior in the gallstone formation and the metal accumulation in the human body, validating the possible use of these biomineralizations as a proxy for exposure to metal pollution.