Tracking the magmatic flow in a dyke-sill hybrid system using a multi-method approach (AMS, SPO, X-ray micro-CT) for petrofabrics characterization (Lessines, Belgium)

Understanding how magmas are transported and collected within the crust is crucial for constraining the dynamic of shallow plumbing volcanic systems and associated hydrothermal activity. This study focuses on the Lessines dioritic intrusion exposed in the SW margin of the Brabant Massif in Belgium. The kilometric subvolcanic body was emplaced around 419 Ma and is thought to result from the emplacement of multiple sills which intruded a lithostratigraphic discontinuity within Upper Ordovician sedimentary units. Our study aims to constrain how magmatic flow is recorded through different fabrics, how this flow varies across the solidified magmatic intrusion and how primary fabrics can be affected by subsequent hydrothermal overprint.</p><p>The petrofabric of 40 oriented diorite samples was investigated with a multi-methods approach: (i) Anisotropy of Magnetic Susceptibility (AMS) along with K-temperature curves determined using low field KLY-4S Kappabridge susceptibilimeter (at LIENS lab, University of La Rochelle, France), (ii) Shape Preferred Orientations (SPO) of melanocratic phenocrysts (pseudomorphosed amphibole and biotite) as well as leucocratic phenocrysts (quartz and sericitized felspars s.l.) determined by the Intercepts method applied on optical scans of three adjacent cut faces of each sample, (iii) X-ray micro-CT scanning of five selected samples using the HECTOR device at UGCT lab (Ghent University, Belgium).</p><p>AMS and melanocratic fabrics SPO are mainly marked by prolate shaped ellipsoids. Both subsets show similar and homogeneous orientation of their structures through the studied area, with E-W striking foliations dipping 70° to the North to subvertical. Leucocratic petrofabric SPO shows more heterogeneous distribution with a similar E-W to N120-striking foliations but generally subhorizontal to low dipping structures (< 30°). This discrepancy is thought to be due to differential record of the subvolcanic phenocrysts during the ultimate emplacement and solidification of the Lessines magmatic body. These results combined to field observations (e.g., enclave orientations, columnar joints, borehole logs) suggest that the Lessines intrusion is a complex dyke-sill hybrid system, made of a main subvertical dyke-like structure that fed lateral sills bodies

Geodynamic evolution of a Neoproterozoic island arc complex (Anti-Atlas, Morocco) : tracking intra-oceanic arc growth and accretion processes

Les reliques néoprotérozoïques d’un système d’arc intra-océanique affleurent dans l’Anti-Atlas au Sud du Maroc, dans les fenêtres du Sirwa et de Bou Azzer. Les deux zones présentent des ensembles fortement tectonisés représentés au Nord par une séquence ophiolitique d’arrière-arc charriée sur des complexes d’arc accrétés au Sud. Ces complexes d’arc (complexes de Tachakoucht, Tazigzaout et Bougmane) sont composés de gneiss granodioritiques et d’amphibolites dont les protolithes, datés entre 750 et 730 Ma, montrent des signatures typiques d’arcs océaniques. Ces derniers ont été enfouis, déformés et métamorphisés sous des conditions de MP-MT (700°C – 8 kbar) à Tachakoucht et de HP-MT à Bougmane (750°C – 10 kbar) avant que des magmas basiques hydratés (hornblendites, gabbros à hornblende) d’affinités d’arcs océaniques ne les intrudent successivement à ~700 et ~650 Ma. Ces épisodes magmatiques ont bouleversé le régime thermique de l’arc entrainant la granulitisation et la fusion partielle des roches encaissantes à plusieurs niveaux crustaux de l’arc et générant des magmas granodioritiques à granitiques recoupant la section crustale du paléo-arc et de l’ophiolite sus-jacente. Cette étude de terrain, pétrologique, géochimique et géochronologique a mis en évidence trois épisodes de magmatisme océanique d’arc sur une période de plus de 120 Ma (de 760 à 640 Ma). La croissance de ce paleo-arc fut contrôlée par les phases successives d’alimentation magmatique mais aussi par des processus tectoniques d’épaississement en domaine intra-océanique et ce, avant l’obduction de l’arc sur le Craton Ouest africain autour de 630-600 Ma.Relics of an intra-oceanic arc system are exposed in the Anti-Atlas in southern Morocco, in the Sirwa and Bou Azzer windows. Both of these areas form a highly tectonized patchwork made of a back-arc ophiolitic sequence to the north thrusted onto accreted arc complexes to the south. These arc complexes (Tachakoucht, Tazigzaout and Bougmane complexes) are made of granodioritic gneisses and amphibolites with typical oceanic arc signature and for which igneous ages range from 750 to 730 Ma. These magmas were buried, deformed and metamorphosed under MP-MT conditions in Tachakoucht (700°C - 8kbar) and HP-MT in Bougmane (750°C - 10 kbar) prior to several magmatic events dated at 700 and 650 Ma and the intrusion of hydrous basic magmas (hornblende gabbros, hornblendites) with oceanic arc signatures. This episodic magmatism strongly perturbed the thermal regime of the arc leading to the granulitization of the host rocks at different levels of the arc crust and to the genesis of intermediate to felsic magmas (granodioritic to granitic). These ones have been segregated through the crustal section intruding both stacked paleo-arc and ophiolitic remnants. This field, petrological, geochemical and geochronological study established that oceanic arc magmatism in the Anti-Atlas occurred in three flare-ups on a 120 Ma long time span (760 to 640 Ma). The growth of the arc was controlled and driven both by successive magmatic inputs and intra-oceanic tectonic thickening processes while final collision of the intraoceanic system with the West African Carton occurred later, around 630-600 Ma

Print Your Compass: Using 3D Printed Geological Compasses for Teaching and Research Purposes

International audienc

Rapport de la mission archéologique de Piantarella (2018)

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Arclogites in the Subarc Lower Crust: Effects of Crystallization, Partial Melting, and Retained Melt on the Foundering Ability of Residual Roots

ABSTRACT Thick-crusted (>45 km) Cordilleran arcs exhibit cyclic processes including periods of magmatic quiescence interspersed with pulses of high-flux magmatism. Most models assume that during high-flux events, fractional crystallization and partial melting within the deep crustal hot zone generate a dense (>3.4 g/cm3) arclogitic subarc root that can readily founder into the mantle. Yet these models do not consider that: (1) the retention of low-density melt within the subarc root, and (2) the protolith lithology of the restitic portion of the subarc root may greatly impact the density evolution of the root and its susceptibility to foundering. In this article, we first address the effect of retained melt on the foundering ability of the subarc root by calculating the density and time for foundering of melt-bearing arclogitic residue at 1.5, 2, and 2.5 GPa. We find that melt volumes >10–18% are required to stabilize the root within the lower crust; melt volumes below this threshold lower the viscosity of the residue so much so as to decrease the time for foundering by an order of magnitude. We then constrain through phase equilibria modeling the effect of partial melting of different lower crustal protoliths on the density of the restitic subarc root. To do this, we model the density and mineralogical evolution of restites in equilibrium with their derivative melts during open-system, isobaric partial melting of typical crustal assemblages from 600–1100°C at 1.5 (~50 km) and 2 GPa (~65 km). In our models, typical end-member assemblages in a lower crustal hot zone include basalt, metapelite, and metagraywacke. We find that melt-depleted restites derived from basaltic compositions are amphibole-bearing arclogites with densities conducive to foundering, which at 2 GPa can occur even in the presence of the coexisting hydrous felsic residual melt. Foundering of the amphibole ± melt-bearing root may refertilize the mantle wedge and induce melting of the surrounding asthenosphere as well as the arclogitic mass. However, if temperatures in a 50-km deep subarc hot zone are not sufficient to drive melt depletion of basaltic restites, these dense (3.1–3.3 g/cm3) residues are gravitationally stable, increasing the density of the lower crust and lowering the elevation of the arc. In comparison, partial melting of metasedimentary country rock produces alkali feldspar-rich residues that never achieve densities conducive to foundering. Thus, if high-flux events are driven by the influx of melt-fertile lithosphere beneath the arc as envisioned by the Cordilleran cycle model, then partial melting of the metasedimentary portion will generate low-density residues that remain in the lower crust and contribute to the thickness, geochemistry, and seismic structure of the bulk arc

Supplemental Material: Europium anomalies in detrital zircons record major transitions in Earth geodynamics at 2.5 Ga and 0.9 Ga

Additional figures, datasets with a compilation of detrital zircon data (U-Pb and trace element) and references; the detailed statistical treatment for zircon U-Pb and trace composition as well as the parameters and solid solution models used for thermodynamic modelling; and links to a GitHub repository that contains two Python codes built to perform bootstrap analysis and timeseries plotting on the zircon U-Pb and trace elements dataset. </p

New Timing and Depth Constraints for the Catalina Metamorphic Core Complex, Southeast Arizona

The Santa Catalina-Tortolita-Rincon Mountains of Southeast Arizona are a classic metamorphic core complex (MCC) and represent footwall exposures of crustal rocks exhumed by a detachment system. This study presents new evidence for the formation of the majority of ductile deformation during the Eocene (similar to 46 Ma), synchronous with the emplacement of the regionally significant Wilderness Sills Suite (57-45 Ma). The evidence is provided by Eocene U-Pb ages of syn- to late kinematic dikes emplaced in the principal ductile mylonitic fabric of the Catalina forerange, earlier than the brittle normal fault system and the formation of Tucson basin beginning with the latest Oligocene. Well-documented shear sense indicators may not reflect extension at that time (Eocene), but more likely the direction of crustal flow now rotated during later extension. Muscovite-plagioclase Rb-Sr isochron ages of three mylonitic rocks are all clustered around 34 Ma, which is inferred to be the last age when these rocks were being deformed under ductile conditions following the emplacement of the Wilderness Sills Suite and various related dikes. Biotite-plagioclase Rb-Sr ages on the same rocks demonstrate that the section cooled below similar to 300 degrees C at 25-26 Ma during the development of normal faulting. Normal faulting was synchronous with the emplacement of the Catalina Intrusive Suite. New U-Pb age results for Catalina Intrusive Suite indicate a combined mean age of 24.9 Ma. Chemical compositions of hornblende-plagioclase pairs were obtained on six Catalina Intrusive Suite samples; depth estimates for the emplacement of the Catalina Intrusive Suite average of about 6 km. These results suggest that the exposed Catalina ductile detachment system was at about 5 km beneath the surface at 25 Ma. These new data bring new light into the development of this core complex and suggest that the similarity in orientations ofprincipalductile and brittle fabrics at the Catalina MCC locality are coincidental. Neither was the principal ductile fabric developed during the low-angle normal faulting of the latest Oligocene nor was this exposed section a midcrustal one at that time. Transient, pluton emplacement-enhanced, and extension-related ductile deformation at shallow crustal levels operated locally at similar to 25 Ma but that does not account for the development of the majority of the Catalina MCC mylonites.6 month embargo; first published online 15 August 2020This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Geoarchaeological non-invasive techniques to study quarrying activity of the Corscican granites

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Les carrières de granite des îles Cavallo et San Bainzu (archipel des Lavezzi, Corse), en usage sous l'antiquité romaine.

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