The evolution of the subcontinental mantle beneath the Central Iberian Zone: Geochemical tracking of its mafic magmatism from the Neoproterozoic to the Cenozoic

Abstract

Continental lithosphere is modified over eons in response to large-scale tectonic processes, such as rifting and collisions. The lithospheric mantle beneath central Iberia was affected by multiple Neoproterozoic to Cenozoic events including two orogenic cycles. However, the details of this evolution, and the relationship of the Iberian lithosphere to that beneath central Europe remains poorly constrained. We have compiled a trace element and isotope (Sr-Nd-Pb) data set (n = 230) regarding the main ten mafic magmatic episodes that intruded the central-western Iberia basement during a 575 Ma age range. In this long period, two orogenic events with accompanying mafic calc-alkaline magmatism (Cadomian and Variscan cycles) were followed by eight intraplate, alkaline or tholeiitic, magmatic inputs. The two orogenic episodes have subordinate mafic intrusions associated to a more voluminous felsic magmatism, which highlights both the major intracrustal recycling promoted and the significant crustal thickening accompanying these convergence tectonic events. Nevertheless, only minor crustal subduction signatures within deep mantle are observed in mafic magmas or mantle xenoliths after those two orogenic cycles, and the existence of an old (pre-Neoproterozoic) enriched subcontinental mantle beneath central Iberia is suggested. Mantle-derived magmas from central European basement terranes (Bohemian to Armorican-French Massif Central massifs) show a marked contrast of geochemical tracers during the same long time record. They show more juvenile and depleted mantle sources during the pre-Variscan times indicating their farther distance to continental Gondwana than the studied central Iberian terrane. Thus, the change in Nd isotopes and model ages associated with the Variscan orogeny is more abrupt in central European terranes. This collisional event was followed by the arrival of broadly similar mafic magmas in both southwestern and central European terranes, implying substantial crust-mantle interaction across a wide geographical area. The post-Variscan mafic magmatism, mainly from Cretaceous time, suggests a lithospheric mantle rejuvenation by widespread asthenospheric upwelling throughout these western circum-Mediterranean areas. However, the presence of minor episodes of intraplate K-rich alkaline magmatism involving enriched lithospheric mantle sources in the southernmost Mediterranean Europe marks a contrast, suggesting a complex lithospheric/asthenospheric mantle flow regime in these areas during post-collisional convergence between Africa and Eurasia.