Línia Martins

The best preserved and most complete lava flow sequences of the Central Atlantic Magmatic Province (CAMP) in Morocco are exposed in the Central High Atlas and can reach up to 300 m in thickness. Four distinct formations, emplaced in subaerial environments, are classically recognized: the Lower, Intermediate, Upper and Recurrent formations. These formations are separated by paleosoils and sedimentary sequences (mudstones, siltstones, sandstones, limestones), that are in general less than two meter-thick and may exceptionally reach a thickness of 80 m, representing minor periods of volcanic quiescence. CAMP lava flows of the Central High Atlas can be grouped into two main categories: subaerial compound pahoehoe flows and simple flows. The former type is exclusively confined to the Lower and Intermediate Formations, while simple flows occur in the Upper and Recurrent Formations. The dominance of compound flows in the two lowermost units of the CAMP suggests a slow emplacement during successive sustained eruptive episodes. Instead the thick single flows characterizing the Upper and Recurrent units indicate higher effusive rates. Basaltic pillow lavas (always of short lateral extent: 10 to 100 m), showing radial jointing and vitreous rinds, identical to those found in the Western Meseta, are occasionally associated with hyaloclastites in the base of the Intermediate Formation, immediately above clastic sediments, or in the Upper Formation. The occurrence of pillow lavas does not imply a generalized subaqueous environment at the time of the lava emission. Instead, they represent subaerial flows that entered small lakes occupying depressions on the volcanic topography of the Lower and Intermediate Formations. The short lateral extent of the pillow lavas and their constant stratigraphic position, the existence of lava flows with unequivocal subaerial characteristics associated to sediments containing fossilized wood, clearly indicate onshore emplacement.

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.

The Messejana–Plasencia dyke (MPD) is one of the largest doleritic dykes of the so-called Central Atlantic Magmatic Province (CAMP). Emplaced at ∼200 Ma in the northernmost sector of this province, it crops out for some 530 km from South Portugal to Central Spain with variable width ranging from 5 to 200 m. The bulk-rock composition is that of a typical continental tholeiitic basalt and there is a remarkable geochemical homogeneity along the dyke, with only minor petrographic variations from the margin (microdolerite) to the centre (gabbro). Major and trace element and Sr–Nd–Pb isotopic compositions of the samples from the MPD suggest that they were the result of combined assimilation plus fractional crystallization (AFC). Quantitative modelling shows that the parental magma originated from a source that was ⁸⁷Sr enriched and ¹⁴³Nd depleted (⁸⁷Sr/⁸⁶Sr ∼0·7050 and ¹⁴³Nd/¹⁴⁴Nd ∼0·51244) relative to Primitive Mantle, which may represent an enriched portion of the mantle–lithosphere. The Pb isotope composition of the MPD is comparable with that of other CAMP dolerites and suggests that the enrichment of the mantle lithosphere occurred during a metasomatic event at ∼1 Ga. Before their emplacement at upper-crustal levels, the primitive magmas crystallized up to ∼30% and assimilated <10% of lower-crustal felsic granulitic rocks. Comparison of the MPD data with those from other parts of the CAMP suggests that their petrogenesis can be linked to the arrival of a Central Atlantic mantle Plume (CAP) at the base of the lithosphere before the onset of continental break-up. The presence of the CAP provides an explanation for several other geological and geochemical features of this province (e.g. the large-scale regional dyke orientations and the presence of magmatic rocks with plume-like signatures), and could have provided the heat source required for inducing melting of enriched, easily melted domains within the overlying refractory mantle lithosphere. However, the scarcity of rocks with plume-like signatures and the results of our petrogenetic modelling suggest that the CAP was only rarely the direct source of the magmas. The explanation for this is that the plume head was probably detached, cooling quickly after impingement and spreading or channelling in the form of a wide, hot sheet following favourable paths of thinned lithosphere. This scenario explains why the CAMP is mainly constituted by dykes whereas lava flows are scarce.