Petrology and geochemistry of ultrapotassic rocks from the Montefiascone Volcanic Complex (Central Italy): magmatic evolution and petrogenesis

The Montefiascone Volcanic Complex belongs to the Roman Magmatic Province of Central Italy; the volcanic activity took place in an extensional, post-collisional setting during Late Pleistocene, giving rise to lava flows and pyroclastic deposits. The extrusive products consist of moderately to strongly undersaturated K-rich lavas ranging in composition from trachybasalts through leucite basanites and leucititic tephrites to tephritic leucitites. They show the typical geochemical and isotopic characters of the Roman potassic magmas, i.e., low TiO2, low K2O/Al2O3 strong enrichment in LILE, high LILE/HFSE ratios, highly radiogenic Sr-87/Sr-86 ratios (0.71005-0.71112) and unradiogenic Nd-143/Nd-144 (0.51209-0.51229, corresponding to epsilon(Nd) = -10.7 to -6.8). Large chemical variations have been recognized within the Montefiascone volcanics, resulting both from the occurrence of different primary magmas and shallow-level fractionation processes. The differentiation mainly took place by means of closed-system fractional crystallisation with local influence of crustal assimilation. The leucite basanites represent primary mantle magmas which did not yield derivative products, whereas the leucititic tephrites, tephritic leucitites and trachybasalts comprise highly differentiated rocks strongly depleted in compatible elements and enriched in LILE. Fractional crystallisation dominated respectively by clinopyroxene + leucite and clinopyroxene + plagioclase yielded the most evolved tephritic leucitites and trachybasalts. In contrast, assimilation of metamorphic basement rocks characterized by highly radiogenic Sr-87/Sr-86 is needed to explain the moderate increase of the Sr-87/Sr-86 ratio within the leucititic tephrites. The geochemical and isotopic signatures shown by the Montefiascone primary magmas require a clinopyroxene- and phlogopite-rich mantle source; in particular, partial melting of a veined lithospheric mantle can account for the occurrence of different primary magmas characterized by relatively constant Sr- and Nd-isotopic compositions. Depleted mantle Nd-model ages (1.1-1.5 Ga) suggest that the mantle enrichment may be a very old event, unrelated to the subduction that preceded the Roman magmatism. This hypothesis is further supported by the strong similarity in age and isotopic composition between the Montefiascone volcanics and the kamafugitic and carbonatitic rocks of the adjoining ultra-alkaline Umbria-Latium district. (C) 1998 Elsevier Science B.V. All rights reserved.