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

Abstract

The products of Montefiascone Volcanic Complex (MVC) encompass one of the most distinct association of potassic to ultrapotassic rocks of the Roman Magmatic Province (RMP), ranging in composition from trachybasalts to tephritic leucitites. New discovery of leucite melilitites, occurring as small lava flows, and of kalsilite–melilite pyroclastic ejecta, further expand the compositional range of the MVC products towards the extreme ultrapotassic compositions of the nearby Umbria-Latium Intra Apenninic Volcanism (IAV). Both lavas and ejecta are characterized by strong LILE and Th enrichments coupled with HFSE depletion, very radiogenic 87Sr/ 86Sr (0.7104–0.7106) and unradiogenic 143Nd/ 144Nd (0.51209–0.51213). Mineral and whole-rock chemistry indicate that the leucite melilites are transitional between ultrapotassic larnite-free, Roman-type magmas and kamafugites, whereas the ejecta (kalsilite melilitolites and clinopyroxene–kalsilite melilitolites) can be considered as intrusive kamafugites. Significant interactions with country rocks (mainly limestones and marls) have been excluded on the basis of trace elements and Sr–Nd isotopes, showing that low SiO 2, high (Ca+Na+K)/Al ratios and relatively high CO 2 contents (leading to crystallization of interstitial carbonates in both lavas and ejecta), are primary features of the melts which yielded the melilites and melilite-bearing ejecta. Petrogenesis of the whole range of MVC products is related to melting in the deepest part of a thinned lithosphere characterized by carbonate-bearing phlogopite–clinopyroxene veins with highly radiogenic 87Sr/ 86Sr and unradiogenic 143Nd/ 144Nd; progressive dilution of a vein-derived, K-rich (kamafugitic) end-member by a basaltic melt originated from a relatively depleted mantle would be able to explain the entire compositional range of MVC magmas. The common geochemical and isotopic signatures of MVC and IAV volcanics suggest that the same petrogenetic processes were simultaneously active in both regions, involving similar mantle domains metasomatized by carbonate–silicate or carbonatitic melts.