Geochemistry of the early Jurassic Messejana- Plasencia dyke (Portugal-Spain): Implications on the origin of the Central Atlantic magmatic province

Abstract

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.