PETROGENESIS OF LEUCITE-BEARING LAVAS IN THE ROMAN VOLCANIC REGION, ITALY - SABATINI LAVAS

The petrogenesis of leucite-bearing lavas from the Roman Region, Italy, is considered in the light of the petrochemistry and mineralogy of the Sabatini lavas, believed to be transitional between the mafic Alban Hills leucitites and the differentiated Vico and Vulsini suites. The Sabatini leucitites are a close variant of the Alban Hills analogues and are probably related to the same cycle of partial melting in the mantle, The prevalent compositional variation, from leucite-tephrite to leucite-phonolite compositions, conforms to the well-known variation of the Vesbian lavas and is attributed to crystal-liquid processes under a low-pressure volcanic regime. The differentiation trends of both the Sabatini and Vico lavas are related to Sabatini parental leucitite compositions, which interacted with recurrent pulses of leucitite liquid during differentiation in a shallow-level environment. The mineral chemistry of the Sabatini lavas reveals extensive and complex chemical zoning in both early and late-crystallized phases. Among the latter, poikilitic feldspars with exceptionally high Sr content (SrO=2-3% wt.) are reported. Early-crystallized phases, notably feldspars and pyroxene, are xenocrystal and indicate significant contamination of the lavas with disequilibrated mineral compositions. Consequently, genetic considerations based on isotopic data from these lavas are suspect. A review of these data is made and it is proposed that the mantle source material of the Roman Region lavas evolved as an open system under the influence of local concentrations of volatile and other mobile elements (Bailey, 1964, 1972, 1974). © 1979 Springer-Verlag.