The origin of lamproites revisited: a Mediterranean perspective

Lamproites show strong enrichment in IE combined with variable and extreme radiogenic isotope compositions. General agreement exists that a normal four-phase peridotitic mantle cannot represent their source, because a K-rich hydrous mineral, in most models phlogopite, is required. The origin of the metasomatism responsible for the enrichment is, however, a puzzling issue, as lamproites typically show an “ancient” isotopic signature unrelated to known tectonic events: either the metasomatism is old and convective mantle-derived, or young but includes an old component such as terrigenous sediment. This view serves ultimately for a general distinction between anorogenic and orogenic lamproites. In the Mediterranean area, lamproites are an integral part of Tertiary postcollisional volcanism within the Alpine-Himalaya belt, resulting from interplay of subduction/collisional and postcollisional/extensional processes. Lamproites are derived from melts that are mixtures of three mantle components, traceable in Sr-Nd-Hf-Pb isotope space: (i) a mantle source contaminated by crustal material, (ii) an ultra-depleted mantle component derived from oceanictype mantle accreted to older lithosphere, and (iii) a component originating from the convecting mantle. 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. Although the majority of Mediterranean lamproites have an orogenic affinity with radiogenic 87Sr/86Sr, 207Pb/204Pb, and unradiogenic 143Nd/144Nd and 206Pb/204Pb, and high LILE/HFSE ratios, the lamproites from few localities exhibit geochemical features characteristic for anorogenic lamproites, with unradiogenic 87Sr/86Sr, 207Pb/204Pb, and radiogenic 143Nd/144Nd and 206Pb/204Pb, coupled with smooth IE patterns with low LILE/HFSE ratios and high concentrations of Nb and Ti. The co-existence of anorogenic and orogenic lamproites in a single volcanic province suggests that the geodynamic distinction of lamproites based on geochemistry only is problematic. In our contribution we review all available data of global lamproites and extend our multicomponent approach. Special emphasis is put on radiogenic isotope data in order to discuss the role of the upper crustal and convecting mantle components.