Phantom plumes in Europe and the circum-Mediterranean region

Abstract

Anorogenic magmatism of the circum-Mediterranean area (Tyrrhenian Sea, Sardinia, Sicily Channel and Middle East) and of continental Europe (French Massif Central, Eifel, Bohemian Massif and Pannonian Basin) has been proposed to be related to the presence of one or more mantle plumes. Such conclusions based on geochemical data and seismic tomography are not fully justified because: 1) a given chemical and isotopic composition of a magma can be explained by different petrogenetic models; 2) a given petrogenetic process can produce magmas with different chemical and isotopic composition; 3) tomographic studies do not furnish unique results (i.e., different models give different results); 4) the commonly adopted interpretation of seismic wave velocity anomalies exclusively in terms of temperature is not unique -velocities are dependent also on other parameters such as composition, melting, anisotropy and anelasticity. Tomography and geochemistry are powerful tools but must be used in an interdisciplinary way, in combination with geodynamics and structural geology. Alone they cannot provide conclusive evidence for or against the existence of mantle plumes.The existence of large and/or extensive thermal anomalies under Europe is here considered unnecessary, because other models, based on the existence of upper mantle heterogeneity, can explain the major, trace-element, and isotopic variability of the magmas. Volcanism in central Europe (the French Massif Central, Germany and the Bohemian Massif) is concentrated in Cenozoic rifted areas and is here interpreted as the result of passive asthenosphere upwelling driven by decompression. Similarly, anorogenic magmatism in Sardinia, the Tyrrhenian Sea and the Pannonian Basin is explained as the result lithospheric stretching in a back-arc geodynamic setting. The most important factors determining the locus and, in part, the 2 geochemical characteristics of magmatic activity are the Moho and the lithosphere/asthenosphere boundary depths. Where both are shallowed by tectonic processes (e.g., in rift zones or back-arc basins) passive upwelling of asthenospheric mantle can explain the magmatic activity.

What ‘anorogenic’ igneous rocks can tell us about the chemical composition of the upper mantle: case studies from the circum-Mediterranean area

“…This feature is particularly important, considering that the temperature regime responsible for mantle partial melting should have been only a few tens degrees higher than the solidus temperature (see discussion in Lustrino & Carminati, 2007), thereby allowing mobilization of only the volumes of mantle with the lowest solidus temperature (that is, the plum part of the plum-pudding system). According to the SUMA model, the smaller the degree of melting, the more it is probable that extreme isotopic compositions should be expected.…”

“…The possibility of reconciling the existence of CiM-ACI activity with thermal anomalies (mantle plumes) has been taken into consideration by several authors who proposed various thermo-chemical-rheological models involving the existence in single cases of tabular, narrow, fat, head-free, cold, intermittent, depleted, lazy, super, finger-like, patchy, fossil, channelled, recycled, not-very-energetic, deflected, dehydrated, wet, thin cactus, 'splash' or 'baby' mantle plumes (see the review of the existing models in Lustrino & Carminati, 2007;www.mantleplumes.org). In a few cases (e.g.…”

“…Goes, Spakman & Bijwaard, 1999;Ritter, 2007). As outlined also in Lustrino & Carminati (2007), the geochemical similarity among two or more igneous rocks reflects only the existence of similar petrogenetic processes and mantle sources with similar chemical and mineralogical composition. Granet, Wilson & Achauer, 1995;Wilson & Patterson, 2001;Lustrino & Wilson, 2007).…”

Petrogenesis of Cenozoic volcanic rocks in the NW sector of the Gharyan volcanic field, Libya

“…Much more common are HIMU-like compositions, whose high TiO 2 concentration and peaks at Nb\Ta, troughs at K and Pb and fractionated REE in primitive mantle-normalized diagrams are the most peculiar features compared with magmas emplaced in subduction-related settings (e.g., Prytulak and Elliott, 2007;Pilet et al, 2010, 2011, andreferences therein). These HIMU-like elemental characteristics are commonly found both in oceanic and continental mid-plate settings, especially in the circum-Mediterranean area (e.g., Wilson and Bianchini, 1999;Wilson and Downes, 2006;Lustrino and Carminati, 2007;Lustrino and Wilson, 2007;Bianchini et al, 2008;Lustrino, 2011;Avanzinelli et al, 2012). The major element composition, together with the primitive mantle-normalized incompatible element pattern and the Sr\Nd isotopic composition of the Gharyan mafic rocks resemble HIMU-like compositions typical of mid-plate magmas.…”

Gravity modeling of the lithosphere in the Calatrava Volcanic Province (Spain): geodynamic implications.

“…12C). This mechanism was used to explain other smallvolume intraplate volcanism in the European Cenozoic Volcanic Province in central Europe (c.f., baby-plume, hot finger, finger plume; Lustrino and Carminati, 2007). This hypothesis consists of an asthenospheric upwelling that yields a lithospheric mantle thinning beneath the CVP.…”