Geodynamic control on orogenic and anorogenic magmatic cycles in the Cenozoic evolution of the Western Mediterranean

The genetic relationships between orogenic and anorogenic magmatic cycles have been investigated in two Cenozoic case studies of the Western Mediterranean area: 1) in Sardinia the orogenic magmatism (38 – 12 Ma) is followed by the anorogenic volcanism, mostly since about 6 Ma; 2) in Southern Spain the anorogenic volcanism follows the orogenic cycle (20 – 6 Ma) after a gap of 0 – 4 Ma in the Calatrava and Tallante districts. For both regions the older orogenic magmatism appears to be related to a single subduction process of the Ionian oceanic lithosphere, which developed diachronously since Middle-Late Eocene, beneath the Paleo-European-Iberian continental margin, and migrated southeastwards with time up to the present position in the Eolian-Calabrian Arc and the Betic-Alboran regions (Fig 1A). Relics of subducted lithosphere are geophysically recorded as nearly-vertical single bodies down to 500 km, flattening for several hundreds kilometres under the Tyrrhenian-Sardinia and Southern Iberian margin, respectively. These relics of subducted slabs, which pond over large areas of the Mantle Transition Zone, appear to play a significant role also in the genesis of the younger anorogenic magmas, whose major volcanic fields lay above the frontal part of the subducted slab where convective instabilities and upward mantle flow components are geophysically recorded and supported by laboratory and 3D numerical models. Consequently, convective instabilities, generated as a dynamic response at the periphery of a retreated subducted slab and involving localized mantle upwellings and rising of hotter metasomatising agents, could have been fundamental factors in triggering partial melting in the overlying upper mantle/lithosphere. Due to effective slab roll-back and inter-arc extension in both Eolian-Tyrrhenian and Betic-Alboran regions, the magma sources of the previous orogenic cycles are completely replaced by “fresh” mantle diapirs from which anorogenic magmas will be generated. The widespread HIMU signature of the Mediterranean anorogenic magmatism, particularly evident in the Calatrava volcanism, is in agreement with the model above, since the HIMU mantle end-member is classically interpreted as the result of long-term recycling of oceanic basalts/gabbros (or their eclogitic equivalents) within the mantle via ancient (Pre- Paleozoic) subduction. Similarly, the geochemical component EMI, strongly represented in the Sardinian anorogenic lavas, would require the involvement of lower crust lithologies within the mantle sources, resulting from either ancient subduction or delamination of previously thickened continental crust. In this model, the on-going subduction processes of the Mediterranean chains do not provide chemical “ingredients” to the mantle sources of the anorogenic magmas, but represent fundamental factors that trigger magma-genesis remobilizing old, deep and very fertile mantle domains (Fig. 1B and C) .