MEDITERRANEAN TERTIARY LAMPROITES: MULTICOMPONENT MELTS IN POSTCOLLISIONAL GEODYNAMICS

In the Mediterranean area, lamproitic provinces in Spain, Italy, Serbia, Macedonia and Turkey have uniform geological, geochemical and petrographic characteristics. Mediterranean lamproites are SiO₂-rich, characterized by low CaO, Al₂O₃ and Na₂O, and high K₂O/Al₂O₃ and Mg-number. They are enriched in LILE relative to HFSE, and show depletion in Ti, Nb and Ta. They show huge variation of Sr, Nd and ²⁰⁷Pb/²⁰⁴Pb isotopic values, not observed in any other terrestrial mantle-derived magmas, with ⁸⁷Sr/⁸⁶Sr range from 0.707-0.722, εNd range from -13 to -2, and ²⁰⁷Pb/²⁰⁴Pb range from 15.62-15.79. Lamproites are derived from multi-component melts, and three components are involved in their origin: (i) a mantle source contaminated by crustal material, giving rise to crust-like trace element patterns and radiogenic isotope systematics; a spatial isotopic difference is an evidence of involvement of crustal components of different age and provenance, with western Mediterranean lamproites having more radiogenic Sr and ²⁰⁷Pb/²⁰⁴Pb, and unradiogenic Nd isotopes due to involvement of Hercynian crust in their mantle source, while in eastern Mediterranean lamproites less radiogenic Sr and Pb, and more radiogenic Nd isotopes are due to involvement of younger crust related with the closure of the Tethys ocean; (ii) an ultra-depleted mantle characterized by very low whole-rock CaO and Al₂O₃, high-Fo olivine and Cr-rich spinel, which isotopically resembles European SCLM generally; it is probably due to accretion of island-arc oceanic lithospheric blocks during Alpine collisional processes; (iii) a third component originating from the convecting mantle. These facts demand a multistage preconditioning of lamproite-mantle source, involving an episode of (extreme) depletion in Mesozoic, followed by involvement of the terrigenous sediments, and finally interaction with the melts ultimately originating from the convecting mantle which happened in Tertiary. We use our data on Mediterranean lamproites to characterize the mantle composition under the Alpine-Himalaya belt. Being an integral part of postcollisional volcanism, lamproites are the most extreme melting products from the mantle which is ubiquitously crustally metasomatized along the whole Alpine-Himalayan belt.