Alkaline and ultramafic lamprophyres in Italy: Distribution, mineral phases, and bulk rock data

Several lamprophyre outcrops occur in Italy paralleling the European cycles. Lamprophyre occurrences from the lower Cretaceous to the Oligocene are comprised primarily of dykes and rarely lavas, generally emplaced in isolation. They are ultramafic or alkaline lamprophyres located in eight different places ranging from the eastern Alps to the areas of Tuscany, Sardinia, Abruzzi, and Puglia and exhibit similar geochemistry, suggesting that they originated from the same mantle source, despite different tectonic conditions. Lamprophyres relate to partial melting of a mantle source rich in large-ion lithophile-elements (LILEs) and C-O-H. Distribution of high-field-strength elements (HFSEs) and their ratio to LILEs depends upon the presence of specific scavenger phases, which makes the geochemistry of these and similar rocks very different with respect to that of basalts. Italian Lamprophyres preserve different specific isotopic features that can be described in term of mixing of two mantle end-members, one of which is highly radiogenic. Lamprophyre magma emplacement should occur during structural extension after major compression episodes. However, this tectonic model seems much too simple for the Italian lamprophyres and is inconsistent with the Mediterranean Tethys geodynamic evolution. On the other hand, Italian lamprophyres evolved into lamproites after the Lower Oligocene era and finally into leucitites and kamafugites (plus carbonatites), ultrapotassic rocks which could be considered anhydrous, petrologic equivalents of lamprophyres. Certainly this requires very specific source conditions, possible related to a mature stage of mantle metasomatisms triggered by repeated episodes of (alkaline) carbonatite invasion, melt extraction, and upper mantle decompression. Italian lamprophyres demonstrate, with their long-term constant geochemistry and isotopic features, that the metasomatic agent is unrelated in time and space to the relatively shorter subduction phases of the Mediterranean Sea area and better fits with the presence of a large, long-lived, pulsing deep plume.