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 Provi nce of Central Italy; the volcanic activity took place in an extension al, post-collisional setting during Late Pleistocene, giving rise to l ava flows and pyroclastic deposits. The extrusive products consist of moderately to strongly undersaturated K-rich lavas ranging in composit ion from trachybasalts through leucite basanites and leucititic tephri tes to tephritic leucitites. They show the typical geochemical and iso topic characters of the Roman potassic magmas, i.e., low TiO2, low K2O /Al2O3 strong enrichment in LILE, high LILE/HFSE ratios, highly radiog enic Sr-87/Sr-86 ratios (0.71005-0.71112) and unradiogenic Nd-143/Nd-1 44 (0.51209-0.51229, corresponding to epsilon(Nd) = -10.7 to -6.8). La rge chemical variations have been recognized within the Montefiascone volcanics, resulting both from the occurrence of different primary mag mas and shallow-level fractionation processes. The differentiation mai nly took place by means of closed-system fractional crystallisation wi th local influence of crustal assimilation. The leucite basanites repr esent primary mantle magmas which did not yield derivative products, w hereas the leucititic tephrites, tephritic leucitites and trachybasalt s comprise highly differentiated rocks strongly depleted in compatible elements and enriched in LILE. Fractional crystallisation dominated r espectively by clinopyroxene + leucite and clinopyroxene + plagioclase yielded the most evolved tephritic leucitites and trachybasalts. In c ontrast, assimilation of metamorphic basement rocks characterized by h ighly radiogenic Sr-87/Sr-86 is needed to explain the moderate increas e of the Sr-87/Sr-86 ratio within the leucititic tephrites. The geoche mical and isotopic signatures shown by the Montefiascone primary magma s require a clinopyroxene- and phlogopite-rich mantle source; in parti cular, partial melting of a veined lithospheric mantle can account for the occurrence of different primary magmas characterized by relativel y 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 k amafugitic and carbonatitic rocks of the adjoining ultra-alkaline Umbr ia-Latium district. (C) 1998 Elsevier Science B.V. All rights reserved .