Mediterranean Tertiary lamproites derived from multiple source components in postcollisional geodynamics

Abstract

In the Mediterranean area, lamproitic provinces in Spain, Italy, Serbia and Macedonia have uniform geological, geochemical and petrographic characteristics. Mediterranean lamproites are SiO 2-rich lamproites, characterized by relatively low CaO, Al 2O 3 and Na 2O, and high K 2O/Al 2O 3 and Mg-number. They are enriched in LILE relative to HFSE and in Pb, and show depletion in Ti, Nb and Ta. Mediterranean lamproites show huge regional variation of Sr, Nd and 207Pb/ 204Pb isotopic values, with 87Sr/ 86Sr range of 0.707–0.722, εNd range from −13 to −3, and 207Pb/ 204Pb range of 15.62–15.79. Lamproitic rocks are derived from melts with three components involved in their origin, characterized by contrasting geochemical features which appear in 206Pb/ 204Pb, 87Sr/ 86Sr and 143Nd/ 144Nd space: (i) a mantle source contaminated by crustal material, giving rise to crust-like trace element patterns and radiogenic isotope systematics, (ii) an extremely depleted mantle characterized by very low whole-rock CaO and Al 2O 3, high-Fo olivine and Cr-rich spinel, which isotopically resembles European peridotitic massifs and lithospheric mantle; (iii) a component originating from the convecting mantle, characterized by unradiogenic 87Sr/ 86Sr and radiogenic 143Nd/ 144Nd and 206Pb/ 204Pb. These components demand multistage preconditioning of the lamproite-mantle source, involving an episode of extreme depletion, followed by involvement of terrigenous sediments, and finally interaction with melts originating from the convecting mantle, some of which are probably carbonatitic. We use our data on Mediterranean lamproites to characterize the mantle composition under the whole Alpine-Himalaya belt. Lamproites are an integral part of postcollisional volcanism, and are the most extreme melting products from a mantle which is ubiquitously crustally metasomatized. Enriched isotope signatures in Himalayan volcanics can also be explained by the involvement of subducted sediments instead of by proterozoic mantle lithosphere.