Magnetic fabric from Quaternary volcanic edifices in the extensional Bransfield Basin: Internal structure of Penguin and Bridgeman islands (South Shetlands archipelago, Antarctica)

Studying the magnetic fabric in volcanic edifices, particularly lava flows from recent eruptions, allows us to understand the orientation distribution of the minerals related to the flow direction and properly characterize older and/or eroded flows. In this work, the magnetic fabric from recent (Quaternary) lava flows (slightly inclined in seven sites and plateau lavas in two sites), pyroclastic deposits (two sites from a scoria cone) and volcanic cones, domes and plugs (three sites) from Penguin and Bridgeman islands, located in the Bransfield backarc basin, are presented. The volcanism in the two islands is related to rifting occurring due to the opening of the Bransfield Strait, between the South Shetlands archipelago and the Antarctic Peninsula. The direction of flow of magmatic material is unknown. Rock magnetic analyses, low temperature measurements and electron microscope observations (back-scattered electron imaging and Energy Dispersive X-ray analyses) reveal a Ti-poor magnetite (and maghemite) as the main carrier of the magnetic fabric. Hematite may be present in some samples. Samples from the centre of the lavas reveal a magnetic lineation either parallel or imbricated with respect to the flow plane, whereas in the plateau lavas the magnetic lineation is contained within the subhorizontal plane except in vesicle-rich samples, where imbrication occurs. The magnetic lineation indicates a varied flow direction in Bridgeman Island with respect to the spreading Bransfield Basin axis. The flow direction in the plateau lavas on Penguin Island is deduced from the imbrication of the magnetic fabric in the more vesicular parts, suggesting a SE-NW flow. The volcanic domes are also imbricated with respect to an upward flow, and the bombs show scattered distribution

El magmatismo alcalino del Triásico Superior en el NO de la Cadena Ibérica (España)

We have carried out a complete study of the Upper-Triassic alkaline magmatism on the northwestern margin of the Iberian Chain. This magmatism is composed of mafic sills, which intrude the Keuper facies in two geographic sectors: the Cameros Massif (NW) and the Moncayo Massif (SE). In both of these sectors, the rocks are characterised by an intense alteration (spilitisation). The field relationships point to an Upper Triassic age for the studied sills, given that: 1) interactions are recognised between the magma and wet, unconsolidated host sediments, suggesting that the magma was emplaced during or shortly after the deposition of the Keuper facies; 2) in the Moncayo sector a conglomerate bed that includes igneous clasts overlaies the sills and passes upwards to the Norian-Rhaetian Imón Fm. According to the petrological and geochemical features of the sills, an internal differentiation can be recognised inwards, with the developments of three zones: chilled margins, central facies and pegmatoid facies. The rocks are porphyritic and are mainly composed of phenocrysts of pseudomorphed olivine and microcrysts of plagioclase, opaque minerals and minor clinopyroxene. Vesicles and xenoliths of the host sedimentary rocks are also recognised, especially at the chilled margins. The mineral assemblage is constant across the sills but the mineral proportions vary among the different zones. The sills are basic to intermediate rocks. They are classified as basalts to dacites with an alkaline geochemical affinity. According to the trace element contents, this magmatism is related to an enriched sub-lithospheric mantle source affected by crustal contamination. Considering the Upper Triassic magmatisms from southwestern Europe, two types can be distinguished. On the one hand, the magmatisms in the Catalonian Coastal Ranges and SE France are paleogeographically located in the inner platform and are related to deep, astenospheric mantle sources with no evidence of crustal contamination. On the other hand, the magmatisms in the NW Iberian Chain –this study-, the External Betics and the Brescian Prealps are situated in a more litoral position and are related to a shallower, lithospheric mantle with evidence of crustal contamination. These differences could be related to the thickness of the continental crust, which might be thinner in the inner areas of the platform.Se ha realizado un estudio completo del magmatismo alcalino del Triásico Superior del margen noroccidental de la Cadena Ibérica. Este magmatismo consta de sills máficos emplazados entre los materiales sedimentarios de la Facies Keuper en dos sectores: Macizo de Cameros (NO) y Macizo del Moncayo (SE). En ambos sectores, las rocas estudiadas se caracterizan por una intensa alteración (espilitización). Las relaciones de campo apuntan a una edad de los sills estudiados de Triásico Superior dado que: 1) se reconocen interacciones entre el magma y el sedimento encajante sin consolidar, que sugieren su emplazamiento coetáneo con el depósito de la facies Keuper o poco posterior y, 2) se observa, en el sector de Moncayo, un nivel conglomerático con cantos de rocas ígneas similares a las que forman los sills, depositado entre los sedimentos argilíticos que se disponen por encima de los sills y siempre, bajo la Fm. Imón de edad Noriense terminal a Retiense inferior. Las características petrológicas y geoquímicas de los sills sugieren una diferenciación hacia el interior de los mismos, reconociéndose tres zonas: borde enfriado, facies central y facies pegmatoide. Las rocas son porfídicas y tienen una asociación mineral común formada por fenocristales de olivino alterado y una mesostasia compuesta por microcristales de plagioclasa, opacos y clinopiroxeno. También se reconocen vesículas y enclaves de la roca sedimentaria encajante, especialmente en el borde enfriado. Las proporciones modales varían entre las tres facies reconocidas. Los sills están formados por rocas básicas a intermedias, clasificadas desde basaltos a dacitas, con afinidad alcalina. De acuerdo a sus contenidos en elementos traza, este magmatismo está relacionado con una fuente mantélica sublitosférica enriquecida, afectada por contaminación cortical. Considerando los magmatismos del Triásico Superior del suroeste de Europa, pueden establecerse dos tipos. Por un lado, los magmatismos aflorantes en las Cadenas Costero Catalanas y el Sur de Francia estaban paleogeográficamente situadas hacia zonas internas de la plataforma y se relacionan con un manto profundo, astenosférico, sin evidencias de contaminación cortical. Por el contrario, los magmatismos de las áreas más occidentales y septentrionales (NO de la Cadena Ibérica –este estudio-, Zonas Externas de las Béticas y Prealpes de Brescia), próximas al litoral paleogeográfico, se relacionan con un manto más superficial, litosférico y muestran evidencias de contaminación cortical. Todas estas diferencias observadas pueden relacionarse con variaciones del espesor de la corteza continental, más adelgazada hacia las zonas internas de la plataforma

Primeros datos del magmatismo pérmico medio-superior del SE de la Cordillera Ibérica: caracterización y comparación con magmatismos contemporáneos del Tethys occidental

A multiple basic to intermediate sill is reported for the first time in the south-eastern Iberian Ranges. It is composed of several tabular to irregular levels intercalated within the fluvial sediments of the Alcotas Formation (Middle-Upper Permian). The sill could represent the youngest Paleozoic subvolcanic intrusion in the Iberian Ranges. The igneous rocks are classified as basaltic andesites. They show a subophitic microstructure constituted by plagioclase (An62 – An6), augite (En48Wo44Fs7 –En46Wo39Fs15), pseudomorphosed olivine, minor amounts of oxides (magnetite and ilmenite) and accessory F-apatite. According to the mineralogy and whole-rock composition, their geochemical affinity is transitional from subalkaline to alkaline. Radiometric dating of the sill is not feasible due to its significant alteration. Field criteria, however, suggest an emplacement coeval to the deposition of the Alcotas Formation (Middle-Upper Permian). This hypothesis is supported by the transitional affinity of these rocks, similar to other Middle-Upper Permian magmatisms in the western Tethys, e.g., from the Pyrenees. Taking into account their isotopic signature (εSr: -6.8 to -9.2; εNd: +1.7 to +8.3), an enriched mantle source with the involvement of a HIMU component has been identified. This interpretation is supported by the trace element contents. Some of these HIMU characteristics have been recognised in the Middle-Upper Permian magmatisms of the Central Pyrenees (Anayet Basin) and the High Atlas (Argana Basin). However, none of these source features are shared with other Middle-Upper Permian magmatisms of the western Tethys (Catalonian Coastal Ranges, Corsica-Sardinia and southern France), nor with the Lower Permian magmatism of the Iberian Ranges. These differences support the presence of a heterogeneous mantle in the western Tethys during the Permian.Se describe por primera vez en el sudeste de la Cordillera Ibérica un sill múltiple de carácter básico a intermedio. Está compuesto por varios cuerpos tabulares a irregulares intercalados entre los sedimentos de origen fluvial de la Formación Alcotas (Pérmico Medio-Superior). El sill podría representar la intrusión subvolcánica paleozoica más reciente en la Cordillera Ibérica. Estas rocas subvolcánicas se clasifican como andesitas basálticas. Muestran una textura subofítica constituida por plagioclasa (An62 – An6), augita (En48Wo44Fs7 –En46Wo39Fs15), pseudomorfos de olivino, minerales opacos (magnetita e ilmenita) y F-apatito accesorio. De acuerdo con su composición mineral y de roca total, su afinidad geoquímica es transicional entre subalcalina y alcalina. La datación radiométrica del sill no es posible debido a su elevado grado de alteración. No obstante, los criterios de campo sugieren un emplazamiento contemporáneo con el depósito de la Formación Alcotas (Pérmico Medio-Superior). Esta hipótesis está apoyada por la afinidad transicional de estas rocas, similar a otros episodios magmáticos del Pérmico Medio-Superior en el Tethys occidental, como los que afloran en los Pirineos. Teniendo en cuenta su signatura isotópica (εSr: -6.8 a -9.2; εNd: +1.7 a +8.3), se propone un origen a partir de un manto enriquecido, con la participación de un componente de tipo HIMU. Esta interpretación está apoyada por sus contenidos en elementos traza. Algunas de estas características del protolito han sido reconocidas en los magmatismos del Pérmico Medio-Superior del Pirineo (cuenca del Anayet) y del Alto Atlas (cuenca de Argana), pero no son habituales en otros magmatismos de edad Pérmico Medio-Superior del Tethys occidental (Cadenas Costero Catalanas, Córcega-Cerdeña y Sur de Francia), ni en el magmatismo Pérmico Inferior de la Cordillera Ibérica. Estas diferencias apoyan la presencia de un manto heterogéneo en el Tethys occidental durante el Pérmico

Transition between Variscan and Alpine cycles in the Pyrenean-Cantabrian Mountains (N Spain): Geodynamic evolution of near-equator European Permian basins

In the northern Iberian Peninsula, the Pyrenean-Cantabrian orogenic belt extends E-W for ca. 1000 km between the Atlantic Ocean and Mediterranean Sea. This orogen developed from the collision between Iberia and Eurasia, mainly in Cenozoic times. Lower-middle Permian sediments crop out in small, elongated basins traditionally considered independent from each other due to misinterpretations on incomplete lithostratigraphic data and scarce radiometric ages. Here, we integrate detailed stratigraphic, sedimentary, tectonic, paleosol and magmatic data from well-dated lithostratigraphic units. Our data reveal a similar geodynamic evolution across the Pyrenean-Cantabrian Ranges at the end of the Variscan cycle. Lower-middle Permian basins started their development under an extensional regime related to the end of the Variscan Belt collapse, which stars in late Carboniferous times in the Variscan hinterland. This orogenic collapse transitioned to Pangea breakup at the middle Permian times in the study region. Sedimentation occurred as three main tectono-sedimentary extensional phases. A first phase (Asselian-Sakmarian), which may have even started at the end of the Carboniferous (Gzhelian) in some sections, is mainly represented by alluvial sedimentation associated with calc-alkaline magmatism. A second stage (late Artinskian-early Kungurian), represented by al-luvial, lacustrine and palustrine sediments with intercalations of calc-alkaline volcanic beds, shows a clear up-ward aridification trend probably related to the late Paleozoic icehouse-greenhouse transition. The third and final stage (Wordian-Capitanian) comprised of alluvial deposits with intercalations of alkaline and mafic beds, rarely deposited in the Cantabrian Mountains, and underwent significant pre-and Early Mesozoic erosion in some segments of the Pyrenees. This third stage can be related to a transition towards the Pangea Supercontinent breakup, not generalized until the Early/Middle Triassic at this latitude because the extensional process stopped about 10 Myr (Pyrenees) to 30 Myr (Cantabrian Mountains). When compared to other well-dated basins near the paleoequator, the tectono-sedimentary and climate evolution of lower-middle Permian basins in Western and Central Europe shows common features. Specifically, we identify coeval periods with magmatic activity, extensional tectonics, high subsidence rates and thick sedi-mentary record, as well as prolonged periods without sedimentation. This comparison also identifies some evolutionary differences between Permian basins that could be related to distinct locations in the hinterland or foreland of the Variscan orogen. Our data provide a better understanding of the major crustal re-equilibration and reorganization that took place near the equator in Western-Central Europe during the post-Variscan period

New lithostratigraphy for the Cantabrian Mountains: A common tectono-stratigraphic evolution for the onset of the Alpine cycle in the W Pyrenean realm, N Spain

The Pyrenean-Cantabrian Orogen arose through the collision of the Iberian and Eurasian plates, mostly in Cenozoic times. This orogen comprises two main mountain ranges, the Pyrenees to the east, and the Cantabrian Mountains to the west. To date, the early Alpine tectono-sedimentary phases preserved in the Cantabrian Mountains, of Permian and Triassic age, have been considered independently from the same phases in neighbouring basins of SW Europe, and even from the eastern part of the same orogeny (the Pyrenean orogeny). In consequence, the beginning of the Alpine cycle in the Cantabrian Mountains has been interpreted within a specific geodynamic context, far from the general evolutionary phases of the western Peri-Tethys basins. Through detailed field work, including geological mapping, sedimentology, lithostratigraphy and petrology of volcanic rocks, and new palaeontological data, here we define several new lithostratigraphical formations and five new tectono-sedimentary cycles (TS I-V) for the initial phases of evolution of the Mesozoic Basque-Cantabrian Basin, interrupted by periods of tectonic stability. To complete this information, we include data from an onshore borehole (Villabona Mine) and two offshore boreholes constrained by 2D reflection seismic profiles acquired in the North Iberian continental platform. The main tectono-sedimentary cycles, related to the deposition of five major identified lithostratigraphic units, can be described as follows: TS I (late Gzelian-early Asselian), relating to the late Variscan deformation and preserved in a single outcrop in all the Cantabrian Mountains (San Tirso Formation). This formation is constituted by medium-distal alluvial fan deposits in which humid intervals predominate, forming some thin coal beds. TS II (Asselian-Sakmarian), a post-Variscan extensional phase with associated calc-alkaline magmatism, represented by profuse volcanic and volcanosedimentary intercalations in the early Permian sedimentary basins (Acebal Formation) and small plutons in surrounding areas. TS III (Kungurian), or reactivation of the post-Variscan extension leading to alluvial and lacustrine carbonate sedimentation in arid climate conditions, which do not change during the rest of the Permian and Triassic periods (Sotres Formation). A generalized karstification in the basin represents the end of Permian deposition, followed by an interruption in sedimentation longer than 30 Myr. The Permian tectono-sedimentary cycles (TS II and TS III) are contemporary with Variscan belt collapse and the basins are controlled by extensional reactivation of NE-SW and E-W Variscan structures, and NW-SE late Variscan structures. TS IV (late Anisian–middle Carnian), renewed sedimentation in more extensive basins, precursors of the great Mesozoic Basque-Cantabrian Basin. This cycle is represented by fluvial deposits (Cicera Formation, or Buntsandstein facies), which are interrupted by the first Mesozoic marine ingression (Rueda Formation, or Muschelkalk facies). TS V (Norian-Rhaetian), or shallow marine carbonate deposits (Transición Formation) related to increasingly compartmentalized sub-basins, controlled by normal faults. This final TS is broadly connected with different basins of the western Peri-Tethys domain. The identification of units TS I-V in the Cantabrian Mountains along with the volcanic character of TS II, all indicate the development of a common post-Variscan to early Alpine tectono-sedimentary evolution for the whole Pyrenean-Cantabrian realm

Vulcanismo cuaternario de la Isla Decepción (Antártida): Una signatura relacionada con la subducción de la Fosa de las Shetland del Sur en el dominio de tras-arco de la Cuenca de Bransfield

Deception Island shows a volcanism related to the Phoenix Plate subduction and roll-back under South Shet- land Block in the present times. The development of the island is related to the evolution and collapse of a volcanic caldera, and this study is focused on the petrology, mineralogy and geochemistry of the post-caldera rocks. We have made a study of the lava flows, dikes and the youngest historic eruption in 1970. These rocks range from dacite to rhyolite and have a microporphyritic texture with olivine and minor clinopyroxene. A pre-caldera basaltic andesite has also been studied. It has a microporphyritic texture with clinopyroxene.The intermediate and acid compositions alternating in the volcanostratigraphic sequence suggest either mafic recharge events or melt extraction from different levels in the deep magmatic system. All the studied compo- sitions share a subduction-related signature similar to other magmatics from the Bransfield Basin. However, compositional differences between pre-caldera and post-caldera rocks indicate a different magma source and depth of crystallisation. According to the geothermobarometric calculations the pre-caldera magmas started to crystallise at deeper levels (13.5–15 km) than the post-caldera magmas (6.2–7.8 km). Specifically, the post- caldera magmas indicate a smaller influence of the subducting slab in the southwestern part of the Bransfield Basin in respect to the available data from other sectors as well as the involvement of crustal contamination in the genesis of the magmas

Data for: Sector-zoned clinopyroxene as a recorder of magma history, eruption triggers, and ascent rates

Supplementary Tables 1-