Nasrrddine Youbi

Synchrony of the Central Atlantic magmatic province and the Triassic-Jurassic boundary climatic and biotic crisis

The evolution of life on Earth is marked by catastrophic extinction events, one of which occurred ca. 200 Ma at the transition from the Triassic Period to the Jurassic Period (Tr-J boundary), apparently contemporaneous with the eruption of the worlds largest known continental igneous province, the Central Atlantic magmatic province. The temporal relationship of the Tr-J boundary and the provinces volcanism is clarified by new multidisciplinary (stratigraphic, palynologic, geochronologic, paleomagnetic, geochemical) data that demonstrate that development of the Central Atlantic magmatic province straddled the Tr-J boundary and thus may have had a causal relationship with the climatic crisis and biotic turnover demarcating the boundary.

Mantle insulation beneath the West African craton during the Precambrian-Cambrian transition

At the time of the Precambrian-Cambrian transition, the West African craton underwent widespread magmatism, hydrothermal activity, and thermal rejuvenation. This tectonothermal event gave rise to an anorogenic ‘‘ring of fire’’ along the rim of this craton, following the Pan-African‐Brasiliano belt that was reactivated by extension and transtension. The thermal phenomena were due to the progressive peripheral release of mantle heat that had built up beneath this craton because of strong insulating conditions. The West African craton at the Precambrian-Cambrian transition can thus be envisioned in terms of a gigantic pressure-cooker with a thick blanketing lithospheric lid. These insulation processes triggered an unusually hot mantle that was channeled by edge-driven convection toward the peri‐West African craton extensional corridors and released through magmatic pressure-relief valves. Massive ice melting and outgassing of volcanic CO2 gave rise to a planet-scale sea-level rise, a greenhouse effect, and the end of the icehouse snowball Earth. These processes played an important role in the Phanerozoic explosion of life on Earth.

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.

The Late Triassic‐Early Jurassic Volcanism of Morocco and Portugal in the Framework of the Central Atlantic Magmatic Province: An Overview

An overview on the Late Triassic-Early Jurassic Magmatic Province of Morocco and Portugal (TJMPMP) is presented. It comprises extrusive basalts, interbedded with clastic rocks sequences preserved in elongated rift basins, and their feeder dikes and sills. Paleontologic ages range from Upper Ladinian-Lower Carnian to the Sinemurian for the sediments, while available 40 Ar/ 39 Ar analysis yield a mean age of 200±1,6 Ma for the volcanics. The volcanologic characteristics of the TJMPMP are those of continental basaltic successions. It comprises subaerial lava flows and pyroclastic deposits, sometimes deposited in lacustrine environments, and feeder dikes, constituting an interesting volcanic sub-province of the Central Atlantic Magmatic Province (CAMP) as most preserved outcrops are extrusive volcanics. These rocks correspond to Low-Ti (TiO 2 11 suggesting that their source may be within the continental lithospheric mantle. The nucleation of the rifting process may have started at two different triple junctions, an RRR junction near Florida and a RRT between Africa-Iberia-America.

Determination of uranium and thorium contents inside different materials using track detectors and mean critical angles

Abstract The critical angles of the CR-39 (θc) and LR-115 type II (θc′) solid state nuclear track detectors (SSNTD) for detecting α-particles emitted by the uranium and thorium series have been evaluated by calculating the corresponding ranges of the emitted α-particles in different material samples and in the SSNTD studied. The influence of the emitted α-particles initial and residual energies on the critical angles of the SSNTD studied has been investigated. The uranium and thorium contents of different geological samples have been evaluated by exploiting data obtained for the critical angles of the CR-39 and LR-115 type II solid state nuclear track detectors and measuring the corresponding densities of tracks.

The dawn of CAMP volcanism and its bearing on the end-Triassic carbon cycle disruption

The cause-and-effect relationship between the c. 201 Ma eruption of the Central Atlantic magmatic province (CAMP) and the end-Triassic abrupt climate change and mass extinction is at present based on controversial temporal correlations. Upper Triassic sedimentary strata underlying CAMP basalts in Morocco illustrate a clear mineralogical and geochemical fingerprint of early CAMP basaltic eruptions, namely unusually high contents of MgO (10–32 wt%) and of mafic clay minerals (11–84%). In the same rocks a coincident negative carbon-isotope excursion (CIE) is present, equivalent to the so-called ‘initial negative CIE’ recorded worldwide shortly before the Triassic–Jurassic boundary. The new data show that the onset of CAMP activity preceded the end-Triassic carbon cycle disruption and that the initial negative CIE is unequivocally synchronous with CAMP volcanism. The results of this study strongly support the hypothesis that the culmination of pollution of atmosphere and seawater by CAMP-derived volcanic gases was the proximate cause of the end-Triassic mass extinction. Supplementary material: The stratigraphic position of analysed samples, and the C-isotope, bulk-rock mineralogy, element analysis, Mg–Al–Si ternary diagram and trace-element analysis data are available at www.geolsoc.org.uk/SUP18707.

Morphology, internal architecture and emplacement mechanisms of lava flows from the Central Atlantic Magmatic Province (CAMP) of Argana Basin (Morocco)

Abstract The morphology, internal architecture and emplacement mechanisms of the Central Atlantic Magmatic Province (CAMP) lava flows of Argana Basin in Morocco are presented. The volcanic pile was produced by two volcanic pulses. The first, represented by the Tasguint Formation, corresponds to a succession of 3–13 individual flows created by 1–8 eruptions; the second, Alemzi Formation, is composed of 2–7 individual flows formed by 1–4 eruptions. These formations, geochemically distinct, are separated by thin silty or sandy horizons or by palaeosols. They include ‘compound pahoehoe flows’ and ‘simple flows’. The first type is almost exclusive of the lower formation, while the second type dominates the upper formation. The lava flows show clear evidence of endogenous growth or ‘inflation’. The characteristics of the volcanic pile suggest slow emplacement during sustained eruptive episodes and are compatible with a continental basaltic succession facies model.

End-Triassic mass extinction started by intrusive CAMP activity

The end-Triassic extinction is one of the Phanerozoics largest mass extinctions. This extinction is typically attributed to climate change associated with degassing of basalt flows from the central Atlantic magmatic province (CAMP). However, recent work suggests that the earliest known CAMP basalts occur above the extinction horizon and that climatic and biotic changes began before the earliest known CAMP eruptions. Here we present new high-precision U-Pb ages from CAMP mafic intrusive units, showing that magmatic activity was occurring ∼100 Kyr ago before the earliest known eruptions. We correlate the early magmatic activity with the onset of changes to the climatic and biotic records. We also report ages from sills in an organic rich sedimentary basin in Brazil that intrude synchronously with the extinction suggesting that degassing of these organics contributed to the climate change which drove the extinction. Our results indicate that the intrusive record from large igneous provinces may be more important for linking to mass extinctions than the eruptive record.

Application of ASTER remote sensing data to geological mapping of basement domains in arid regions: a case study from the Central Anti-Atlas, Iguerda inlier, Morocco

Satellite remote sensing is shown to provide critical support for geological and structural mapping in semiarid and arid areas. In this work, Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) data were used to clarify the geological framework of the Precambrian basement of the Iguerda Proterozoic inlier in the Moroccan Central Anti-Atlas. In this study, the interpretation of the processed digital data has been ground truthed with geological field data collected during a reconnaissance-mapping program in the Central Anti-Atlas. The Iguerda inlier offers a deeply eroded Precambrian massif dominated by a Paleoproterozoic basement composed of supracrustal metasedimentary units intruded by various Eburnian granitoids. Impressive mafic dyke swarms mainly of Proterozoic age crosscut this basement. Eburnian basement rocks are unconformably overlain by Lower Ediacaran volcanosedimentary rocks of the Ouarzazate Group and Upper Ediacaran–Lower Cambrian carbonates. The applied ASTER analyses are particularly effective in the lithological differentiation and discrimination of geological units of the Iguerda inlier. The spectral information divergence (SID) classification algorithm coupled with spectral angle mapper and maximum likelihood classification effectively discriminates between metamorphic rocks, granitoid bodies, and carbonate cover. SID classification improves geologic map accuracy with respect to the spatial distribution of plutonic bodies and metamorphic units. In addition, Paleoproterozoic granitoids have been well discriminated into separate distinct suites of porphyritic granites, granodiorites, and peraluminous leucogranite suites. This discrimination was initially identified via remote sensing analysis and later ground truthed in the field. This methodology enhances geological mapping and illustrates the potential of ASTER data to serve as a vital tool in detailed geologic mapping and exploration of well-exposed basement of arid regions, such as the Proterozoic of the Anti-Atlas Mountains of Morocco.

Low intra-crystalline closure temperatures of Cr-bearing spinels from the mantle xenoliths of the Middle Atlas Neogene-Quaternary Volcanic Field (Morocco): Mineralogical evidence of a cooler mantle beneath the West African Craton

Abstract The crystal chemistry of nine Cr-spinels from lherzolite and harzburgite xenoliths from the Middle Atlas Neogene-Quaternary Volcanic Field of Morocco have been studied by means of X-ray singlecrystal diffraction and electron microprobe analyses. Cell edges are usually within the range 8.13-8.14 Å, but there are three samples with longer a value, so that the whole analyzed series is within the range 8.1334(4)-8.2021(2) Å, while the oxygen positional parameter values are very similar ranging between 0.2626(1) and 0.2629(2) for all of them. The cation distribution shows that the crystal structure is ordered with almost all divalent cations in the tetrahedral site and trivalent cations in the octahedral site. The determined intracrystalline temperatures are in the range 550-750 °C that are the lowest values ever found for Cr-spinels from mantle xenoliths as these are usually higher than 730 °C. If we consider the behavior of some geotherms from literature, the determined temperatures are confined in a depth range of about 20-40 km. Lithospheric models for the studied area indicate that the lower crust reaches its deepest value in a range between 30 and 40 km. Consequently, we can assume that the studied xenoliths were emplaced at a “shallow” depth of about 20-30 km, just beneath the lower crust, where they were disrupted and brought to the surface from the ascending alkaline lavas. This assumption is consistent with the concomitant presence of some crustal xenoliths. It is important to notice that even in the case of a mantle xenoliths where all the silicates could be heavily altered, the presence of one single crystal of Cr-spinel and the study of its oxygen coordinates (u), inversion parameters (i), Cr content, and calculated closure temperatures can be used to validate the thermal history of the mantle xenoliths. The combined approach of structural data, intra- and inter-crystalline temperatures, and the literature geophysical data seems to be an interesting tool to assess the preexhumation history of the mantle xenoliths.