Petrology of melilite-bearing rocks from the Montefiascone Volcanic Complex (Roman Magmatic Province): new insights into the ultrapotassic volcanism of Central Italy

The products of Montefiascone Volcanic Complex (MVC) encompass one of the m ost distinct association of potassic to ultrapotassic rocks of the Roman Ma gmatic Province (RMP), ranging in composition from trachybasalts to tephrit ic leucitites. New discovery of leucite melilitites, occurring as small lav a flows, and of kalsilite-melilite pyroclastic ejecta, further expand the c ompositional range of the MVC products towards the extreme ultrapotassic co mpositions of the nearby Umbria-Latium Intra Apenninic Volcanism (IAV). Bot h lavas and ejecta are characterized by strong LILE and Th enrichments coup led with HFSE depletion, very radiogenic Sr-87/Sr-86 (0.7104-0.7106) and un radiogenic Nd-143/Nd-144 (0.51209-0.51213). Mineral and whole-rock chemistry indicate that the leucite melilites are tr ansitional between ultrapotassic larnite-free, Roman-type magmas and kamafu gites, whereas the ejecta (kalsilite melilitolites and clinapyroxene-kalsil ite melilitolites) can be considered as intrusive kamafugites. Significant interactions with country rocks (mainly limestones and marls) have been exc luded on the basis of trace elements and Sr-Nd isotopes, showing that low S iO2, high (Ca + Na + K)/Al ratios and relatively high CO, contents (leading to crystallization of interstitial carbonates in both lavas and ejecta), a re primary features of the melts which yielded the melilites and melilite-b earing ejecta. Petrogenesis of the whole range of MVC products is related to melting in th e deepest part of a thinned lithosphere characterized by carbonate-bearing phlogopite-clinopyroxene veins with highly radiogenic Sr-87/Sr-86 and unrad iogenic Nd-143/Nd-144; progressive dilution of a vein-derived, K-rich (kama fugitic) end-member by a basaltic melt originated from a relatively deplete d mantle would be able to explain the entire compositional range of MVC mag mas. The common geochemical and isotopic signatures of MVC and IAV volcanic s suggest that the same petrogenetic processes were simultaneously active i n both regions, involving similar mantle domains metasomatized by carbonate -silicate or carbonatitic melts. (C) 2001 Elsevier Science B,V. All rights reserved.