João Mata

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.

Petrology of ultramafic xenoliths from Madeira Island

Ultramafic xenoliths from Madeira island are divided into dunite/websterite/wehrlite/clinopyroxenite (DWWC) and harzburgite/lherzolite suites; the harzburgite/lherzolite xenoliths show abundant deformational features and are more refractory (Fo = 90–91) than the DWWC suite (Fo = 77–88). DWWC xenoliths are spinel-bearing olivine ± orthophyroxene cumulates with intercumulus clinopyroxene and rare plagioclase, amphibole and phlogopite. Mineral chemistry and geothermobarometric data indicate that DWWC xenoliths crystallized at 1150–1300 °C from Madeiran alkalic basalts and accumulated in magma reservoir(s) located 36–45 km beneath the island. The harzburgite/lherzolite xenoliths are composed of olivine + orthopyroxene + spinel ± clinopyroxene ± (rare) phlogopite and display alkali feldspar or clinopyroxenite veins and crystal aggregates. The complex thermal evolution recorded by these xenoliths and the close similarity of clinopyroxene REE contents and calculated f O 2 values in both harzburgites and DWWC cumulates are attributed to recent infiltration of the harzburgites by melts trapped or crystallized within the mantle; these features, and the refractory bulk chemistry of the harzburgite/lherzolite suite, support the interpretation that these xenoliths represent depleted oceanic lithosphere variously modified by magmatism associated with the genesis of Madeira island. The association of these upper mantle xenoliths with cumulates crystallized from Madeiran magmas (DWWC) suggests that the harzburgite/lherzolite suite originated in the uppermost mantle above magma storage zone(s), probably near the boundary between the mantle and the overlying oceanic crust.

Alternating crustal architecture in West Iberia: a review of its significance in the context of NE Atlantic rifting

The crustal architecture of the Western Iberian Margin is investigated so that the relationship between synrift intra-plate segmentation, magmatic events and subsequent margin convergence can be discussed in the context of the evolution of the Central–North Atlantic Ocean. The evidence in this paper indicates distinct crustal architectures for NW Iberia (lower-plate) and SW Iberia (upper-plate), as shown by (1) the different geometries of the rotated tilt-blocks across thinned continental crust, (2) the distinct dips of deep crustal detachments and (3) the presence of first-order strike-slip zones bounding different crustal segments. We demonstrate that a switch in the dip direction of the crustal detachment is accomplished by accommodating persistent transcurrent deformation along first-order transfer zones and associated uplifted hinges, both of which accommodated synrift displacement and post-rift margin inversion. This alternating architecture explains why recurrent magmatism is conspicuously located on the upper-plate margin, which we consider to have favoured, in discrete pulses, the emplacement of magma. Finally, the crustal detachment underlying the upper-plate is interpreted to have been reactivated since the Eocene as the weak zone accommodating ocean–continent convergence and the putative onset of margin subduction. Supplementary material: Summary of the three main magmatic events on the onshore and offshore West Iberia and their associated occurrences, location and magmatism type, in relation with the syn- to post-rift evolution of the margin are available at https://doi.org/10.6084/m9.figshare.c.3591209

Noble Gas Constraints on the Origin of the Azores Hotspot

Noble gas geochemistry is a valuable tool for assessing the nature of contributors to magma mantle sources. In this chapter, we analyse previously published data regarding helium and neon in the Azores to discuss the origin of the Azores archipelago. After the pioneering works of Kurz et al. (1982a, 1990) examining helium isotopic variations along the Mid-Atlantic Ridge (MAR) between 28°N and 53°N and on Sao Miguel Island, a systematic study was conducted by Moreira et al. (1999) on several Azores islands. These authors analysed He isotopic ratios from phenocrysts collected in five islands of the archipelago. Helium isotopic data from Terceira (minimum 4He/3He ratio of ≈63,700; R/Ra ≈11.3) were interpreted as the presence of a relatively primitive component in the mantle source, whereas the radiogenic 4He/3He ratios found at Sao Miguel (from 121,600 to 276,800; R/Ra from ≈5.9 to ≈2.6) have been interpreted as resulting from the melting of an enriched and ancient recycled material. Moreira and Allegre (2002) analysed noble gas isotopic data for MORB glasses dredged along a MAR segment (from 21.25°N to 39.9°N), which also includes the Azores triple junction area. The obtained 4He/3He ratios decrease from 90,000 at 37°N to 75,000 at 38.5°N and later increase to 100,000 at 40.5°N. The low 4He/3He ratio measured on the ridge at 38.5°N (76,000; R/Ra = 9.5) was interpreted as the result of present-day interaction between the ridge source and the Azores plume (see also Madureira et al. 2014), as sampled by lava erupted at Terceira and Sao Jorge islands (the latter with 4He/3He ratios down to 40,000; R/Ra ≈18). Madureira et al. (2005) focused their study on Terceira Island, for which He and Ne isotopic ratios were determined from olivine phenocrysts. Ne isotopic data corroborated the presence of a relatively primitive component in the Terceira mantle source (21Ne/22Necorr = 0.052), which is, however, dominated by a MORB-type component. Jean-Baptiste et al. (2009) presented helium isotope data for thermal waters and gas emissions sampled at Terceira, Graciosa and Sao Miguel islands, as well as Faial, Pico and Flores islands. The results were interpreted as the presence of relatively primitive He isotopic ratios at Terceira (4He/3He ≈53,500; R/Ra ≈13.5), which also extend to Graciosa Island (4He/3He ≈64,500; R/Ra ≈11.2). These values contrast with those obtained by these authors for Sao Miguel Island, which are significantly more radiogenic than those typical of MORB (4He/3He from ≈120,400 to ≈138,900; R/Ra from 5.2 to 6.0) and are in agreement with the helium measurements in lavas from the eastern part of Sao Miguel (Moreira et al. 2012). Noble gas geochemistry suggests, despite the strong dilution of its primitive signature by MORB-like material, that a lower mantle-derived mantle plume is located under the central group of islands, particularly under Sao Jorge, in agreement with seismic tomographic images. The peculiar helium isotopic ratios observed in Sao Miguel lavas can be explained by different scenarios invoked for the Sr–Pb isotopic and trace element systematics, which suggest that the Sao Miguel source contains recycled mafic material ~3 Ga in age (e.g., Beier et al. 2007; Elliott et al. 2007).

Exhumation of a migmatite complex along a transpressive shear zone: inferences from the Variscan Juzbado–Penalva do Castelo Shear Zone (Central Iberian Zone)

High-grade metamorphic rocks associated with S-type granites are recorded in the Central Iberian Zone, Iberian Variscides. Though most of these occur as inferred metamorphic core complexes affiliated with detachment faults, others, such as the Figueira de Castelo Rodrigo–Lumbrales Anatectic Complex, crop out between low-grade metamorphic rocks separated by steeply-dipping strike-slip shear zones, such as the Juzbado–Penalva do Castelo Shear Zone. Our structural analysis has been able to constrain two major deformation stages during the Variscan D3: (a) D3a ductile deformation event, with clear sinistral kinematic criteria; and (b) D3b thrusting ductile–brittle deformation event. The petrological investigation confirmed the jump in metamorphic grade between the host rocks and the anatectic complex. P–T estimates on calc-silicate rocks interlayered with the metapelites of the anatectic complex provided minimum metamorphic peak conditions of T = 761 ± 50°C and P = 5.0 ± 1.0 kbar. However, petrological modelling results show that P–T conditions must have exceeded T > 800°C. Both structural and geothermobarometric data support a two-step model for the exhumation of the Anatectic Complex, including a 5 – 8 km vertical exhumation along a 65 – 100 km horizontal displacement due to a simple shear-dominated transpression mechanism during the Variscan D3 events. Supplementary material: The petrology data, mineral chemistry analyses of the calc-silicate units and EPMA analytical conditions, and the P–T modelling methodology can be found at https://doi.org/10.6084/m9.figshare.c.3785648

Evidences for multiple remagnetization of Proterozoic dykes from Iguerda inlier (Anti-Atlas Belt, Southern Morocco)

No paleomagnetic data exist for Paleo-Mesoproterozoic times of the West African Craton (WAC). Therefore, paleogeographic reconstructions for such old geological times are difficult to constrain. Gaps on the sedimentary record and intense remagnetizations are the major problems that paleomagnetic studies come across. Recent geochronological results for dyke swarms that intrude several Proterozoic inliers of WAC in the Anti-Atlas Belt (southern Morocco) revealed ages between Paleoproterozoic and early Neoproterozoic, opening for the first time a window of opportunity to conduct paleomagnetic studies and tentatively infer about the paleoposition of WAC during Proterozoic. On this scope we conducted a paleomagnetic study on seven Proterozoic dykes of the Iguerda inlier. The meaning of the obtained paleomagnetic directions was evaluated by rock magnetic and mineral analyses, complemented by petrographic observations. Our samples record the presence of a complex history of remagnetization, mostly assigned to several Phanerozoic thermal/chemical events, in particular to the late stages of Pan African orogeny (s.l.), to the Late Carboniferous Variscan orogeny, and even to more recent events. The recognized remagnetization processes are related to widespread metamorphic events under greenschist facies followed by low-temperature oxidation, both responsible for the formation of new magnetic phases, like magnetite and hematite. These events obliterated the primary (magmatic) thermo-remanent magnetization and promoted multiple remagnetizations of the dykes, thermally and chemically. For only one dyke the presence of primary magnetization is possible to infer, though not to confirm, and would place WAC at an equatorial position around 1750 Ma.

2D and 3D resistivity tomography of the Suímo garnet-bearing dyke, Lisbon Volcanic Complex, Portugal: a case study

Upper Cretaceous volcanic activity in the West Iberian Margin produced a singular garnet-bearing basaltic dyke occurring on the northern slope of the Su?mo hill near Lisbon. 2D and 3D resistivity tomography have been carried out to elucidate the properties of the dyke regarding the horizontal and vertical subsurface extension after historical mining. 2D large-scale resistivity profiles (GA-1 and GA-2) were measured. Ten sub-parallel 2D resistivity sections were measured in small scale to perform a 3D resistivity model. All measurements were carried out using a pole?dipole array. The basaltic dyke, nowadays not cropping out, is detected as a low resistivity layer, extending to a depth of at least 40?m. The width of the dyke ranges from 150 to about 40?m to the south. A high resistivity limestone unit bounds the dyke from east and west sides. Some karstification features can be easily observed in the limestone unit, forming very high resistivity zones. The obtained results allow the prediction that for sampling of in situ basaltic rock the extraction of an ?20?m thick layer of loose material would be necessary. The central garnet-rich zone of the dyke would be only reached at depths higher than ?45?m.