103 research outputs found
3D gravity modelling of the Aguablanca Stock, tectonic control and emplacement of a Variscan gabbronorite bearing a Ni–Cu–PGE ore, SW Iberia
The Aguablanca stock is a Variscan mafic pluton located in the Ossa-Morena zone,
southern Iberian Massif, hosting an unusual Ni–Cu–PGE mineralization associated with magmatic
breccia pipes which intruded its northern part. The emplacement of the Aguablanca stock and the
mineralized breccia pipes are related to the activity of the Cherneca ductile shear zone, a Variscan
sinistral shear zone that favoured magma ascent through the upper crust. A detailed gravity study has
been carried out in order to investigate the 3D geometry of the Aguablanca intrusion and to get insights
about the emplacement mechanism and tectonic controls of the mineralization. The three-dimensional
gravity modelling shows that the stock has an inverted drop geometry with a feeder zone in contact
with the Cherneca ductile shear zone. The inferred orientation of the feeder zone suggests that the
emplacement probably took place along an open tensional crack formed within the strain field of the
adjacent Cherneca ductile shear zone. The modelling of the breccia pipes hosting the Ni–Cu–PGE ore
shows that they are included inside the feeder zone, thus their emplacement is probably controlled by
successive opening events of this tensional crack
Composite origin of an early Variscan transported suture: Ophiolitic units of the Morais Nappe Complex (north Portugal)
Giant magnetic anisotropy of single cobalt atoms and nanoparticles
The isotropic magnetic moment of a free atom is shown to develop giant magnetic anisotropy energy due to symmetry reduction at an atomically ordered surface. Single cobalt atoms deposited onto platinum ( 111) are found to have a magnetic anisotropy energy of 9 millielectron volts per atom arising from the combination of unquenched orbital moments (1.1 Bohr magnetons) and strong spin-orbit coupling induced by the platinum substrate. By assembling cobalt nanoparticles containing up to 40 atoms, the magnetic anisotropy energy is further shown to be dependent on single-atom coordination changes. These results confirm theoretical predictions and are of fundamental value to understanding how magnetic anisotropy develops infinite-sized magnetic particles
Neoproterozoic to early Paleozoic extensional and compressional history of East Laurentian margin sequences: The Moine Supergroup, Scottish Caledonides
Neoproterozoic siliciclastic-dominated sequences are widespread along the eastern margin of Laurentia and are related to rifting associated with the breakout of Laurentia from the supercontinent Rodinia. Detrital zircons from the Moine Supergroup, NW Scotland, yield Archean to early Neoproterozoic U-Pb ages, consistent with derivation from the Grenville-Sveconorwegian orogen and environs and accumulation post–1000 Ma. U-Pb zircon ages for felsic and associated mafic intrusions confirm a widespread pulse of extension-related magmatism at around 870 Ma. Pegmatites yielding U-Pb zircon ages between 830 Ma and 745 Ma constrain a series of deformation and metamorphic pulses related to Knoydartian orogenesis of the host Moinerocks. Additional U-Pb zircon and monazite data, and 40Ar/39Ar ages for pegmatites and host gneisses indicate high-grade metamorphic events at ca. 458–446 Ma and ca. 426 Maduring the Caledonian orogenic cycle.The presence of early Neoproterozoic silici clastic sedimentation and deformation in the Moine and equivalent successions around the North Atlantic and their absence along strike in eastern North America reflect contrasting Laurentian paleogeography during the breakup of Rodinia. The North Atlantic realm occupied an external location on the margin of Laurentia, and this region acted as a locus for accumulation of detritus (Moine Supergroup and equivalents) derived from the Grenville-Sveconorwegian orogenic welt, which developed as a consequence of collisional assembly of Rodinia. Neoproterozoic orogenic activity corresponds with theinferred development of convergent platemargin activity along the periphery of the supercontinent. In contrast in eastern North America, which lay within the internal parts of Rodinia, sedimentation did not commence until the mid-Neoproterozoic (ca. 760 Ma) during initial stages of supercontinent fragmentation. In the North Atlantic region, this time frame corresponds to a second pulse of extension represented by units such as the Dalradian Supergroup, which unconformably overlies the predeformed Moine succession
The Aguablanca Ni–(Cu) sulfide deposit, SW Spain: geologic and geochemical controls and the relationship with a midcrustal layered mafic complex
The Aguablanca Ni–(Cu) sulfide deposit is
hosted by a breccia pipe within a gabbro–diorite pluton.
The deposit probably formed due to the disruption of a
partially crystallized layered mafic complex at about 12–
19 km depth and the subsequent emplacement of melts and
breccias at shallow levels (<2 km). The ore-hosting breccias
are interpreted as fragments of an ultramafic cumulate,
which were transported to the near surface along with a
molten sulfide melt. Phlogopite Ar–Ar ages are 341–
332 Ma in the breccia pipe, and 338–334 Ma in the layered
mafic complex, and are similar to recently reported U–Pb
ages of the host Aguablanca Stock and other nearby calcalkaline
metaluminous intrusions (ca. 350–330 Ma). Ore
deposition resulted from the combination of two critical
factors, the emplacement of a layered mafic complex deep
in the continental crust and the development of small
dilational structures along transcrustal strike-slip faults that
triggered the forceful intrusion of magmas to shallow
levels. The emplacement of basaltic magmas in the lower
middle crust was accompanied by major interaction with
the host rocks, immiscibility of a sulfide melt, and the
formation of a magma chamber with ultramafic cumulates
and sulfide melt at the bottom and a vertically zoned mafic
to intermediate magmas above. Dismembered bodies of
mafic/ultramafic rocks thought to be parts of the complex
crop out about 50 km southwest of the deposit in a
tectonically uplifted block (Cortegana Igneous Complex,
Aracena Massif). Reactivation of Variscan structures that
merged at the depth of the mafic complex led to sequential
extraction of melts, cumulates, and sulfide magma. Lithogeochemistry
and Sr and Nd isotope data of the Aguablanca
Stock reflect the mixing from two distinct reservoirs, i.e.,
an evolved siliciclastic middle-upper continental crust and a
primitive tholeiitic melt. Crustal contamination in the deep
magma chamber was so intense that orthopyroxene
replaced olivine as the main mineral phase controlling the early fractional crystallization of the melt. Geochemical
evidence includes enrichment in SiO2 and incompatible
elements, and Sr and Nd isotope compositions (87Sr/86Sri
0.708–0.710; 143Nd/144Ndi 0.512–0.513). However, rocks
of the Cortegana Igneous Complex have low initial
87Sr/86Sr and high initial 143Nd/144Nd values suggesting
contamination by lower crustal rocks. Comparison of the
geochemical and geological features of igneous rocks in the
Aguablanca deposit and the Cortegana Igneous Complex
indicates that, although probably part of the same magmatic
system, they are rather different and the rocks of the
Cortegana Igneous Complex were not the direct source of
the Aguablanca deposit. Crust–magma interaction was a
complex process, and the generation of orebodies was
controlled by local but highly variable factors. The model
for the formation of the Aguablanca deposit presented in
this study implies that dense sulfide melts can effectively
travel long distances through the continental crust and that
dilational zones within compressional belts can effectively
focus such melt transport into shallow environments
The granite‑hosted Variscan gold deposit from Santo António mine in the Iberian Massif (Penedono, NW Portugal): constraints from mineral chemistry, fuid inclusions, sulfur and noble gases isotopes
The study area is located in the Central Iberian Zone, a major tectonic unit of the Iberian Massif (Variscan belt). In this region the basement is composed of Cambrian-Ordovician sedimentary and minor volcanic rocks that underwent deformation and metamorphism during the Carboniferous. These metamorphic rocks host ca. 331–308 Ma granitic plutons emplaced during the D2 extensional and D3–D4 contractional deformation phases. The gold-bearing quartz veins from the Santo António mine (Penedono region) occur in granite formed at 310.1 ± 1.1 Ma and post-dated the peak of metamorphism. Gold–silver alloy is included in quartz, but mainly occurs in spaces between grains or micro-fractures within arsenopyrite of all three generations and less in pyrite. Late sulphides and sulphosalts were deposited along fractures mainly in arsenopyrite, and locally surrounding the gold–silver alloy grains. Ferberite, scheelite and stolzite replace arsenopyrite. The abundant aqueous carbonic fluids and the occurrence of a low-salinity fluid and their minimum possible entrapment temperature of 360–380 °C suggest that this gold-forming event began during the waning stages of the Variscan orogeny. The mean δ34S values of arsenopyrite and pyrite are − 4.7‰ and − 3.8‰, respectively. He–Ar–Ne isotopic data suggest a crustal origin. The ascent of the granite magma has provided the heat for remobilization of gold, other metals and metalloids from the metamorphic rocks. This gold-arsenopyrite deposit has thus similar characteristics as other selected gold-arsenopyrite deposits from the Iberian Massif, but it contains tungstates.El área de estudio está ubicada en la Zona Centroibérica, una importante unidad tectónica del Macizo Ibérico (cinturón
varisco). En esta región el basamento está compuesto por rocas sedimentarias y volcánicas del Cámbrico-Ordovícico tectonizadas y metamorfzadas durante el Carbonífero. Estas rocas metamórfcas sirven como caja de los plutones graníticos datados
en torno a 331–308 Ma y que fueron emplazados durante la fase de deformación extensional D2 y las fases de deformación
contraccional D3 y D4. Las venas de cuarzo ricas en oro de la mina de Santo António (región de Penedono) que aparecen en
un granito datado a los 310.1 ± 1.1 Ma son posteriores al pico metamórfco regional. La aleación de oro y plata se incluye
en el cuarzo, pero se produce principalmente en los espacios entre granos o micro-fracturas dentro de arsenopirita de las
tres generaciones y menos en pirita. Los sulfuros y sulfuros tardíos se depositaron a lo largo de las fracturas principalmente
en arsenopirita, y alrededor de los granos de aleación de oro y plata. Ferberita, scheelita y la estolzita sustituyen a la arsenopirita. Los abundantes líquidos acuosos carbónicos y la presencia de un fuido de baja salinidad y su posible temperatura
de atrapamiento mínima en torno de 360-380 ºC sugieren que este evento de formación de oro comenzó durante las etapas
fnales de la orogenia varisca. Los valores medios de S de arsenopirita y pirita son − 4.7 ‰ y − 3.8 ‰, respectivamente. Los datos isotópicos de He–Ar–Ne sugieren que en el origen de los fuidos mineralizados participa la corteza continental.
El ascenso del magma granítico ha provisto el calor para la movilización del oro, otros metales y metaloides desde las rocas
metamórfcas. Este depósito de oroarsenopirita tiene así características similares a otros yaciamientos con arsenopirita y oro
del Macizo Ibérico, pero sin embargo contienen tungstates.This research was financially supported by Fundação para a Ciência e Tecnologia through the projects GOLDGranites, Orogenesis, Long-term strain/stress and Deposition of ore metals—PTDC/GEO-GEO/2446/2012: COMPETE: FCOMP-01-0124-FEDER-029192 and UID/GEO/04035/2013
Biome changes and their inferred climatic drivers in northern and eastern continental Asia at selected times since 40 cal ka bp
Recent global warming is pronounced in high-latitude regions (e.g. northern Asia), and will cause the vegetation to change.
Future vegetation trends (e.g. the “arctic greening”) will feed back into atmospheric circulation and the global climate system. Understanding the nature and causes of past vegetation changes is important for predicting the composition and distribution of future vegetation communities. Fossil pollen records from 468 sites in northern and eastern Asia were biomised at selected times between 40 cal ka bp and today. Biomes were also simulated using a climate-driven biome model and results from the two approaches compared in order to help understand the mechanisms behind the observed vegetation changes. The consistent biome results inferred by both approaches reveal that long-term and broad-scale vegetation patterns reflect global to hemispheric-scale climate changes. Forest biomes increase around the beginning of the late deglaciation, become more widespread during the early and middle Holocene, and decrease in the late Holocene in fringe areas of the Asian Summer Monsoon. At the southern and southwestern margins of the taiga, forest increases in the early Holocene and shows notable species succession, which may have been caused by winter warming at ca. 7 cal ka bp. At the northeastern taiga margin (central Yakutia and northeastern Siberia), shrub expansion during the last deglaciation appears to prevent the permafrost from thawing and hinders the northward expansion of evergreen needle-leaved species until ca. 7 cal ka bp. The vegetation climate disequilibrium during the early Holocene in the taiga-tundra transition zone suggests that projected climate warming will not cause a northward expansion of evergreen needle-leaved species
Electrospun green fibres from lignin and chitosan: a novel polycomplexation process for the production of lignin-based fibres
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