Roberto Oyarzun

A first insight into mercury distribution and speciation in soils from the Almadén mining district, Spain

Abstract Almost no environmental data on mercury distribution and speciation in soils have been published so far for the Almaden mining district (central Spain), despite its huge size and historic importance. The mercury distribution in soils of the district reveals the existence of high and extremely high mercury values (up to ∼9000 ppm Hg). The Hg-thermodesorption curves for soils from a decommissioned metallurgical precinct (Almadenejos) and a phytoremediation site show that mercury occurs in the forms of cinnabar and as mercury bound to organic matter. The TEM-EDX study of the highly contaminated anthrosols from Almadenejos (samples with Hg >5000 ppm) shows the existence of cinnabar particles adsorbed to the surface of chlorite grains. Given the generally pyrite-poor character of the ores, and the presence of carbonates in the host rocks, cinnabar solubilization is limited, which in turn mitigates environmental hazards in the district. The only by-product of cinnabar leaching in the mineral dumps is schuetteite (Hg 3 SO 4 O 2 ). Preliminary results on local plants ( Asparagus acutifolius , Dittrichia graveolens , Marrubium vulgare ) show that mercury gets incorporated to roots, stems and leaves, with values of up to about 300 ppm Hg.

Sr and O isotope constraints on source and crustal contamination in the high-K calc-alkaline and shoshonitic neogene volcanic rocks of SE Spain

The Neogene volcanic province of SE Spain NVPS is characterized by calc-alkaline CA , high-K calc-alkaline KCA , . . . shoshonitic SH , ultrapotassic UP , and alkaline basaltic AB volcanic series. All these series, except the AB, have high LILErLREE, LILErHFSE and BrBe ratios and high but variable Sr, Pb and O isotope compositions. The KCA and SH lavas contain metapelitic xenoliths whose mineralogical and chemical composition are typical of anatectic restites. The geochemical characteristics of CA, KCA, SH and UP series suggest that they originated from the lithospheric mantle, previously contaminated by fluids derived from pelagic sediments. Additionally, the presence of restite xenoliths in the KCA and SH lavas indicates some sort of interaction between the mantle-derived magmas and the continental crust. Trace element and isotope modeling for the KCA and SH lavas and the restites, point towards the existence of two mixing stages. During the first stage, the lithospheric mantle was contaminated by 1-5% of fluids derived from pelagic sediments, which produced

Alkalic (ocean-island basalt type) and calc-alkalic volcanism in the Mexican volcanic belt: A case for plume-related magmatism and propagating rifting at an active margin?

The Mexican volcanic belt has been traditionally regarded as a classic case of subduction-related calc-alkalic volcanism. However, a series of geologic, geophysical, and petrological arguments makes this simple relationship doubtful. A seismic gap beneath the belt, a large-scale mantle anomaly, a graben triple-junction domain, and the presence of volumetrically important oceanic-island basalt (OIB) volcanism throughout the belt suggest a more complex tectonic scenario involving plume- and subduction-related processes. We here propose a model involving the development of a propagating rift opening from west to east in response to plume activity. The process started in Miocene time within the western sector of the belt (Guadalajara) and gave rise to a graben triple junction and OIB-type and calc-alkalic volcanism. Extension and volcanism proceeded to the east, giving rise to progressively younger ages for the initiation of OIB-type volcanism: (1) Miocene in the west (e.g., Guadalajara), (2) Pliocene in the central zone (e.g., Michoacan-Guanajuato), and (3) Quaternary farther east (e.g., Chichinautzin). Geochemical evidence suggests that part of the modern calc-alkalic volcanism (e.g., Chichinautzin) may be derived from magma mixing between the OIB mafic magmas and silicic, crust-derived magmas. However, we do not preclude some influence of the subducting slab in the generation of other (e.g., Jorullo) calc-alkalic volcanic rocks. Our model suggests a currently unrooted upper plume attached to the subcontinental lithosphere, which defines a hot zone beneath the Mexican volcanic belt.

Opening of the central Atlantic and asymmetric mantle upwelling phenomena: Implications for long-lived magmatism in western North Africa and Europe

The Mesozoic to present evolution of the central Atlantic realm is interpreted as a two-stage tectonomagmatic scenario involving long-lived asymmetric mantle upwelling phenomena and magmatism within its eastern margin. The first, a pre-drift tholeiitic stage of Triassic-Jurassic age, resulted from the interaction between several elements: (1) a thinned, weakened corridor along the collapsed southern branch of the Variscan belt between eastern North America and western North Africa; (2) a central Atlantic plume located in the triple junction between Africa, North America, and South America; (3) a progressive asymmetric continental breakup between northwest Africa and North America characterized by detachment systems; and (4) a highly thinned European realm pervaded by rift-type basins that we interpret as a large thin-spot-type domain. These conditions would have induced north-northeast–directed large-scale sublithospheric plume channeling from the central Atlantic plume site to the European large thin-spot, leading to widespread early tholeiitic magmatism (Triassic-Jurrassic) within a giant irregular zone of ∼3000 × 4000 km. Plume activity and chaneling continued afterward during a Cretaceous to present second tectonomagmatic stage (passive margin alkaline stage) leading to the onset of alkaline magmatism along a general north-northeast trend in the eastern Atlantic margin (Late Cretaceous) and Europe (Paleocene–Oligocene). As a whole, a north-northeasst–directed propagating magmatic vector can be defined from the Mesozoic to the present.

Neogene extensional collapse in the western Mediterranean (Betic-Rif Alpine orogenic belt): Implications for the genesis of the Gilbraltar Arc and magmatic activity

The Betic-Rif orogenic belt crops out in the western Mediterranean and constitutes the westernmost part of the Alpine edifice. The building up of this belt by compressional nappe tectonics occured during a series of Alpine deformational episodes. Subsequent Neogene extension resulted from the collapse of the previous compressional edifice through major extensional detachment systems. This episode was accompanied by cafe-alkaline to shoshonitic and lamproitic volcanism, tectonic denudation of metamorphic core complexes, and emplacement of peridotite bodies as a final consequence of the evolution of detachment systems. Furthermore, we relate the generation of the Gibraltar Arc and the opening of the Alboran Sea to this major extensional event. Finally, two main stages of tectonic denudation of these detachment systems are defined: (1) Nevado Filabride stage, with ex-humation of metamorphic core complexes, and (2) Ronda-Beni Boussera stage, with a more advanced degree of denudation, leading to the exhumation of upper mantle peridotites, which may be tentatively termed mantle core complexes.

Tectonics and volcanism of Sierra Chichinautzin: extension at the front of the Central Trans-Mexican Volcanic Belt

Abstract Because of its recent activity and position at the southern magmatic front of the Trans-Mexican Volcanic Belt (TMVB), the Sierra Chichinautzin volcanic field (SCN) is a key area for the understanding of this controversial volcanic province. Volcanic activity has built more than 220 monogenetic volcanoes (shields, scoria cones, thick lava flows, and hydromagmatic structures) during the last 40,000 years, for a total volume of about 470 km 3 . The SCN basalts are geochemically similar to OIBs, while the intermediate and felsic volcanic rocks show a calc-alkaline trend and abundant evidence for magma mixing. The structural analysis of this volcanic field and surrounding areas has been based on field data, satellite images, and a method for detecting volcanic center alignments. The tectonic data, together with geophysical evidence, confirm active general N–S extensional conditions with a strike–slip component for the SCN area, the same structural setting that prevails in the rest of the Central TMVB. Extensional tectonics, a negative regional Bouger gravity anomaly, a low-velocity mantle, high heat flow, and shallow seismicity suggest a rift-type setting involving the upwelling of anomalous mantle beneath the Central TMVB. The combined petrological, structural and geophysical arguments support that the SCN volcanism is rift-related, and rule out processes involving the subduction of the Cocos plate, which casts further doubts on the standard subduction model for the TMVB volcanism.

“Mantle core complexes” and Neogene extensional detachment tectonics in the western Betic Cordilleras, Spain: an alternative model for the emplacement of the Ronda peridotite

Abstract An alternative model for the emplacement of the Ronda peridotite (Betic Alpine orogenic belt, southern Spain) is proposed, involving mantle uprising during a major Neogene extensional event. The development of a detachment system, and its further evolution by isostatic rebound and upward arching, would have triggered the tectonic denudation of these ultramafic bodies, forming what may be termed “mantle core complexes”. Mantle core complexes would represent a final step in the core-complex forming processes once a complete separation of the upper and lower blocks has been achieved in a detachment system, marking probably the transition from an intracontinental to a proto-oceanic rift environment.

Permo-Carboniferous volcanism in Europe and northwest Africa: a superplume exhaust valve in the centre of Pangaea?

Abstract The Permo-Carboniferous transition in the European-northwest African province was characterised by widespread volcanism (calc-alkaline with crustal and/or mantle lithospheric characteristics followed by alkaline/subalkaline with HIMU-type signature) with a maximum in the Stephanian-Autunian (Late Pennsylvannian to Early Permian). Extrusion of volcanics was accompanied by massive S-type synextensional granitic intrusions and hydrothermal mineralisation. The geotectonic framework involved the gravitational collapse of the Variscan Belt by extensional detachment tectonics, and its final disruption by wrench faulting. The characteristics of this volcanism are explained within progressively evolving extensional processes. It is suggested that this Permo-Carboniferous Pangaean volcanic province might be interpreted in terms of a superplume impinging on the base of the lithosphere. This model envisages that magmatism acted as an exhaust valve releasing the heat accumulated beneath the Pangaean supercontinent by insulation and blanketing processes which triggered large-scale mantle-wide upward convection and general instability of the supercontinent.

Extensional tectonics in the central Iberian Peninsula during the Variscan to Alpine transition

Abstract The passage from the Variscan cycle to the Early Alpine framework in the central part of the Iberian Peninsula can be explained in terms of a transitional process involving four clearly differentiated tectonic episodes. 1. (1) A first Variscan compressional stage (VI, Middle Devonian to Early Carboniferous) dominated by compressional conditions leading to the building-up of the orogenic edifice. The stress regime was relevant to what might be called “Variscan-type” compression (E-W-oriented). This stage was characterized by major Himalayan-type tectonics with frontal nappes, thrusts, overturned folds, lateral transcurrent ramps, and localized anatectic magmatism. Minor synorogenic extension and plutonism was also recorded during this stage in the Tormes Granitic Dome. 2. (2) A second Variscan stage (V2, Early to Middle Carboniferous) was characterized by increasing extensional conditions leading to widespread plutonism (adamellites, granodiorites). Wanning compressional conditions were restricted to the eastern and southern realms of central Iberia (the eastern part of the Spanish Central System, and the Toledo Mountains). 3. (3) A third stage, here defined as Late Variscan (LV), developed from Middle Carboniferous to Early Permian, as a result of N-S late-orogenic extension. This episode is relevant to detachment tectonics and the gravitational collapse of the Variscan orogenic edifice under combined simple/ pure-shear conditions. Plutonism (granites and leucogranites) was still of major importance. Early Permian andesitic to dacitic volcanism and sedimentary basins developed within the eastern part of the Spanish Central System. 4. (4) A fourth stage, here defined as Early Alpine (EA, Early Permian to Triassic) marks the onset of the Alpine framework. This stage was characterized by what might be called an “Early Alpine-type” regional stress regime i.e. E-W extension and N-S compression, within a simple-shear model, and resulted in the configuration of the Iberian Peninsula into two contrasted realms: a western inherited Variscan block, and an eastern Alpine block subjected to post-orogenic extension. Elements developed during this event include N-S high-angle normal faults, NW-SE and NE-SW conjugate strike-slip faulting, and asymmetric rifting involving listric low-angle detachments.

The late Oligocene−Miocene opening of the North Balearic Sea (Valencia basin, western Mediterranean) a working hypothesis involving mantle upwelling and extensional detachment tectonics

Abstract The North Balearic Sea (Valencia basin) is one of the western Mediterranean basins that opened during the late Oligocene-Miocene. However, in contrast with neighbouring basins, the Valencia basin is floored by highly attenuated continental crust. The geological and geophysical characteristics of this realm indicate that extensional tectonics and mantle instability played a major role in the opening of the basin and consequent SE-directed drifting of its eastern flank (Balearic Rise). A model for the generation of this basin is proposed, involving the intrusion of asthenospheric mantle and subsequent extensional detachment tectonics during late Oligocene-Miocene times. This model accounts for the asymmetrical rifting of the North Balearic Sea, and the clockwise drifting/rotation of the Balearic Rise.