A review of petrogenesis of Mediterranean Tertiary lamproites

In the Mediterranean area, four major lamproitic provinces with uniform geological, geochemical, and petrographic characteristics are recognized: Spain, Italy, Balkans, and Turkey. Mediterranean lamproites are SiO₂-rich lamproites, characterized by low CaO, Al₂O₃, and Na ₂O, and high K₂O/Al₂O₃ and Mg-number. They are enriched in large ion lithophile elements relative to high field strength elements and in Pb, and show depletion in Ti, Nb, and Ta. The Mediterranean lamproites are characterized by a wide range of ⁸⁷Sr/⁸⁶Sr_i and¹⁴³Nd/ ¹⁴⁴Nd_i. Both intra- and interprovince variations are significant. In contrast, the Pb isotope compositions of all Mediterranean lamproite provinces are almost identical, falling within the pelagic sediment field and resembling local upper-crustal sediments and Mesozoic flysch sediments from the Tethyan Ocean. Using the Serbian lamproites as an example, we develop constraints on the mantle melting processes and geodynamic environment of the whole Mediterranean lamproitic province. Partial melting of refractory mantle material previously enriched in incompatible elements is considered to be the most likely explanation for Mediterranean lamproites. The depleted-mantle component is probably multiply depleted peridotite from above the subducting plate during Mesozoic subduction processes, which preceded collision and orogenesis. The considerable variations of Sr-Nd isotopes are explained as having been produced by vein + wall-rock melting involving metasomatic veins that were out of isotopic equilibrium with the peridotite wall rock during melting. The uniformity of the Pb isotope compositions of all Mediterranean provinces is a result of the presence of a common crustal isotopic end member in their mantle source similar to flysch sediments from the Vardar Tethyan Ocean. No generic geodynamic scenario can explain lamproites, but neither mantle plume nor subduction is essential for the initiation of the volcanism. Most of the evidence implies postcollisional tectonics, including delamination of lithospheric mantle and/or orogenic collapse, as the major causes for the volcanism.