Alkaline Rocks and Carbonatites of the World

Setup during HiTech AlkCarb: an online database of alkaline rock and carbonatite occurrences

Roman Region, Italy

In the Roman Igneous Province (Tolfa, Ceriti, Vulsini, Latera, Cimini, Vico, Sabatini and Alban Hills volcanic complexes, Fig. 1) within  pyroclastic flows and opening-vent breccia occur several types of ejecta. They range for  dimensions from 20 to 40 cm and consist of crystalline basement fragments (very rare), sedimentary rocks, older volcanic rocks (tuffs end lavas), skarn, marbles and hornfelds (rare), sub-volcanic/plutonic such as alkali syenites (ubiquitous), foiditic syenites and glimmerites (eg. Parodi et al., 1994; Schingaro et al., 2001). The skarn/syenite ratio is also very variable. At Bassano Romano (BR in table), along with rare occurrence of metamorphosed carbonates (marbles) and aphyric hornfelds, the ejecta are of 5% skarns and 70% syenites plus 30% of volcanic rocks. At Sacrofano crater (Sa in table) skarn are dominat and alkali syenites are about 10% of the total ejecta (mostly volcanic rocks). On the contrary at the Vico volcano skarns are virtually absent whereas alkali syenites and foiditic syenites are about 30% of the total ejecta. At Pianciano quarry alkali syenites are 5% of the total in a leucititic pyroclstic flow.
Skarns have in general essential gehlenite or olivine-cpx-mica association with vesuvianite and cuspidine (Federico and Peccerillo, 2002). Alkali syenites have fine/medium sized equigranular texture, with perthitic K-feldspar and little content foid, variable mica, clinopyroxene and amphibole. Baddeleyite, britholite, allanite-(Ce), hellandite, gadolinite plus pyrochlore, perovskite, apatite and vanadium mineral are found in melanocratic alkali syenites and foiditic syenites or missourite/fergusite varieties (eg. Oberti et al., 1999; Della Ventura et al., 2002). Pyrochlore is in general up to 1% and is associated with REE-phases. Real italites or leucocratic leucititolites ejecta are very rare in the volcanoes north of Rome but are common in the Alban Hills were they are associated to melilitolites (Stoppa et al., 2003) and kalsilitolites containing U-Th varieties of REE phases and pyrochlore (eg. Federico et al., 1994; Federico and Peccerillo, 2002). The ejecta may shows sign of hydrothermal/metasomatic actions represented by mineralised veins.

Stoppa et al., 2016 State of the art: Italian carbonatites and their potential for critical-metal deposits, Gondwana Research

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