Search

The Skoura inlier in the Moroccan High Atlas consists of thick Precambrian-Paleozoic formations. It displays, in the western part near Aguerzega village, a volcano-sedimentary succession of about 400 m thickness. It includes a variety of welded and non-welded ignimbrites, lapilli-tuff fallout deposit, coherent lava, sedimentary mass flow deposit and fluvial sediments, which are grouped into three lithofacies associations, corresponding to different depositional environments. The volcanic activity in the studied area represents successive volcanic eruptions, alternating with phases of quiescence (sedimentary beds). The Ediacaran volcano-sedimentary succession of the Skoura inlier represents an evolution in four stages in a subaerial environment. The geochemical results obtained from Skoura volcanic rocks display high silica-rich composition ranging from dacite to rhyolite. Major and trace elements show high-K calc-alkaline and alkali-calcic to alkaline affinities and reflect an active continental margin signature. In addition, the studied rocks show enriched LILE compared to HFSE with negative Nb-, Ta- and Ti anomalies. This is consistent with a post-collisional setting, documented for the final phase of the Pan-African orogeny. Laser ablation U-Pb zircon dating on three rhyolitic ignimbrites from the base and top of two profiles, yielded ages of 580 ± 7 Ma, 574 ± 14 Ma and 571 ± 4 Ma. These ages compare well with the lower Ouarzazate Supergroup, widespread in the Anti-Atlas, as well as in the High Atlas and with rare exposures in the Meseta domain in northern Morocco.

The Late Cretaceous (~78 Ma) Poços de Caldas massif is the largest alkaline complex in Brazil and the second in the world by extension (>800 km2). It is considered the westernmost outcrop of the Cabo Frio magmatic lineament, in the northern sector of Serra do Mar potassic-ultrapotassic igneous province, central-eastern Brazil. The outcropping rocks are peralkaline phonolites (~80%) and nepheline syenites (~15%) with rarer (

Post-Variscan Early Permian magmatism is widespread from Corsica to the Eastern Alps. After the formation of the Variscan orogenic belt between Laurussia and Gondwana, felsic and mafic bodies were emplaced during an extensional tectonic phase. This study focuses on a mafic dyke swarm that intruded in the region of Ajaccio (Corsica, France) and on a gabbroic intrusive complex outcropping in the Eastern Alps (Bressanone/Brixen, South Tyrol, Italy) both of which were emplaced during this extensional event. New U-Pb data from zircon show that both of these intrusions were emplaced at ca. 282 Ma. Most Ajaccio dykes display a calc-alkaline affinity and are characterized by enrichment of LILE (Large Ion Lithophile Elements) and LREE over HREE (Light and Heavy Rare Earth Elements) and HFSE (Nb, Ta, Hf, Zr; High Field Strength Elements). Two dykes show tholeiitic affinity and are characterized by marked LREE and Th-U depletion and by minor negative Nb-Ta anomalies on multi-element diagrams, which is typical for within-plate basalts. Bressanone/Brixen gabbros display similar geochemical features to the Ajaccio calc-alkaline samples with LREE enrichment over MREE (Medium Rare Earth Elements) and HREE, and HFSE depletion. Such characteristics are also confirmed by trace element data obtained by LA-ICP-MS on clinopyroxene and plagioclase crystals. The calc-alkaline to tholeiitic dykes are characterized by enriched to depleted Sr-Nd-Pb isotopic compositions, respectively. Some Ajaccio dykes and the Bressanone/Brixen gabbros are clearly affected by crustal contamination. However, the combination of trace element and isotopic variations suggests that mantle source heterogeneities were also involved. Isotopic compositions of the Ajaccio dykes reveal the presence of two distinct mantle source signatures: an enriched mantle (EM) component, which dominates the composition of the calc-alkaline-like suite, while a DMM (Depleted MORB Mantle) component dominates the source of the tholeiitic suite. The EM mantle source likely formed during Variscan subduction, however the composition of the tholeiitic suite requires a source composed dominantly of depleted mantle, with a minor contribution from both the subducting slab and supra-subduction mantle wedge. The mantle source of the tholeiitic magmas possibly originated from greater depths (compared to the calc-alkaline-like suite) during the post-orogenic extensional phase potentially due to break-off and/or roll-back of the Variscan slab or mantle flow from the front or margins of the subducted slab. Notably, coeval Permian mafic intrusive bodies from throughout Corsica (Bocca di Tenda, Porto, Pila Canale) and from the Australpine and Southalpine Alps intruded at a similar time and have geochemical features that are very similar to the Ajaccio dyke swarm and Brixen gabbro. This indicates that a widespread Permian magmatic province developed in a post-orogenic extensional tectonic setting at the margin of the former Variscan belt. During the Middle-Late Triassic magmatism with similar enriched geochemical signatures occurred in the Southern Alps, Dinarides, Australpoine domains and at the end of the Triassic over central Pangea (Central Atlantic Magmatic Province). This would highlight the importance that the Variscan orogeny had in the development of a more fertile mantle.

Pyroxenite layers embedded within peridotite represent widespread lithological mantle heterogeneities and are potential components in the mantle source of many oceanic basalts. They can be generated by a variety of magmatic and metamorphic processes. However, in most natural samples (especially in ultramafic massifs), their primary characteristics are partially or completely erased by later processes (e.g. metamorphism, metasomatism or partial melting). Here we investigate a suite of pyroxenites from the External Liguride Jurassic ophiolites (Northern Apennines, Italy). These are spinel-bearing websterites and clinopyroxenites, partially recrystallized under plagioclase-facies conditions, and occur as centimetre-scale layers parallel to the tectonite foliation of their host peridotites. The pyroxenites have bulk-rock Mg-numbers from 74 to 88 and display rather constant light rare earth element (LREE) depletion relative to middle REE (MREE) (LaN/SmN ¼ 015-035), but variable MREE-heavy REE (HREE) fractionation, with some having markedly positive HREE slopes (SmN/YbN¼030-096). The HREE enrichment, coupled with high Zr and Sc contents in clinopyroxene porphyroclasts from spinel-bearing domains, provides strong evidence that garnet was present in the precursor mineral assemblages. Mass-balance calculations suggest that the pyroxenites originally contained up to about 40 vol. % garnet, indicating that they originated by segregation of melts at relatively high pressure (P>15 GPa). The parental melts of the pyroxenites have reacted to some extent with the host peridotite during mantle infiltration. Lack of olivine in the primary mineral assemblage and the presence of orthopyroxene-rich rims along the contact with the wall-rock peridotites suggest that the pyroxenites crystallized from silica-rich melts. These probably had REE patterns and Sr-Nd isotope compositions similar to those of enriched mid-ocean ridge basalt. We propose that the pyroxenites originated from melts derived from a hybrid eclogite-bearing peridotite source, which subsequently reacted with their host peridotite to form 'secondary pyroxenites'. The existence of such pyroxenites has been invoked in current models of basalt petrogenesis. During later decompression, the pyroxenites experienced recrystallization at spinel-facies conditions, at 12-15 GPa and minimum temperatures of 950-1000C, and partial re-equilibration in the low-pressure plagioclase facies. The latter event is dated by internal Sm-Nd isochrons at 178 (68) Ma and is associated with Mesozoic exhumation during extension of the Tethys lithosphere.