The circum-Mediterranean anorogenic Cenozoic igneous province

Abstract

During the Cenozoic widespread anorogenic magmatism, unrelated to recent supra-subduction zone modification of its mantle source, developed within the Mediterranean and surrounding regions; this is referred to collectively as the CiMACI (Circum-Mediterranean Anorogenic Cenozoic Igneous) province. On the basis of a comprehensive review of published and new major and trace element and Sr–Nd–Pb isotopic data (more than 7800 samples) for the magmatic rocks, a common sub-lithospheric mantle source component is identified for most of the region. This has geochemical affinities to the source of HIMU oceanic island basalts and to the European Asthenospheric Reservoir (EAR) and the Low Velocity Component (LVC) of previous workers; we refer to this as the Common Mantle Reservoir (CMR). Global and local seismic tomography studies of the mantle beneath the CiMACI province have revealed a range of P- and S-wave velocity anomalies, some of which have been related to the presence of mantle plumes. Detailed local tomography experiments in the Massif Central of France and the Eifel region of central Germany suggest that, locally, there are diapiric upwellings rooted within the upper mantle which induce adiabatic decompression melting and magma generation. These velocity anomalies can be interpreted as evidence of mantle temperatures up to 150 °C hotter than the ambient mantle. However, the seismic attenuation could also be attributable to the presence of fluid or partial melt and significant thermal anomalies are not necessary to explain the petrogenesis of the magmas. A model is proposed in which the geochemical and isotopic characteristics of the sub-lithospheric mantle beneath the CiMACI province reflect the introduction of recycled crustal components (derived from both oceanic and continental lithosphere) into the ambient depleted upper mantle. This sub-lithospheric mantle is subsequently partially melted in a variety of geodynamic settings related to lithospheric extension, continental collision and orogenic collapse, and contemporaneous subduction, slab roll-back and slab-window formation. On the basis of this in-depth geochemical and petrological study of the CiMACI province, we consider that there is no need to invoke the involvement of anomalously hot mantle (i.e., the presence of a single or multiple deep mantle plumes) in the petrogenesis of the magmas. If, however, we adopt a more permissive definition of “mantle plume”, allowing it to encompass passive, diapiric upwellings of the upper mantle, then we can relate the CiMACI province magmatism to multiple upper mantle plumes upwelling at various times during the Cenozoic. To avoid confusion we recommend that such upper mantle plumes are referred to as diapiric instabilities.