MINERALOGY AND PETROLOGY OF THE POLINO MONTICELLITE CALCIOCARBONATITE(CENTRAL ITALY)

Two small diatremes, about 0.25 my old, cut through Liassic limestones about 1 km NNE of the village of Polino (Long. 12 degrees 50'54''E-La t. 42 degrees 35'34''N; Central Italy). The material filling the large r diatreme is mainly composed of a tuffisite with abundant lapilli sho wing concentric structure. Both unaltered country-rocks and massive hy pabyssal carbonatite occur in the tuffisite as angular clasts and bloc ks, from a few mm up to more than 1 m in diameter. The Polino rock occ urs in a strongly-potassic igneous district (Umbria Latium Ultra-alkal ine District) which comprises phonolitic pyroclastic rocks and very ra re kamafugitic lavas. Massive carbonatite blocks have an average mode of 53% Sr-Ba-rich calcite, 23% Fe-monticellite, 9% Th-perovskite plus Ti-magnetite, 6% Cr-phlogopite, 6% forsteritic olivine, about 2% Zr-sc horlomite and ca. 1% Si-CO-OH apatite. Perovskite, schorlomite, and ap atite form cognate phases, whereas olivine and phlogopite, often repla ced by monticellite, occur as nodules and as discrete grains with comp ositions and deformation features typical of mantle xenocrysts found i n alkali basalts and ultramafic rocks. High modal content of Ca-carbon ate, high Sr, Ba and LREE contents of calcite, the presence of rare mi nerals peculiar to carbonatitic rocks and an essential amount of monti cellite indicate classification of the Polino rock as a monticellite c alciocarbonatite. The Polino rock represents a carbonatitic melt stron gly contaminated by mantle-crystal debris. It displays unusual geochem ical features having trace elements closer to those of the regional-as sociated kamafugitic rocks rather than to those of common carbonatites .