Petrogenetic relationships between melilitite and lamproite

A detailed petrological study of the S. Venanzo olivine melilitite and Cupaello kalsilitite, located at the NE margin of the Roman Comagmatic Region of Italy has provided new data to evaluate their genetic relationships and related tectonic/magmatic conditions. Early crystallized olivine (Fo₉₂) from S. Venanzo is compatible with crystallization from near primary mantle melts, while late-crystallized olivine from both rocks (Fo_88–79) reflects primarily the high Ca of the host liquids, attained under the volcanic crystallization regime. Magnesiochromite inclusions in the early-crystallized olivine are consistent with near-primary melts close to lamproite in composition. Nepheline and kalsilite from both lavas contain high alkalis (+Ca), relative to Al, and thus correspond more closely with nepheline compositions from carbonatite-related assemblages, than with those from a wide compositional range of Alban Hills lavas. Coexisting melilite has high (Na+K)/Al, reflecting the Peralkalinity Index of the bulk rocks. Diopside and phlogopite from both lavas are characteristic of lamproites and groundmass kimberlites in their high Mg/(Mg+Fe²⁺) ratio (0.86–0.95; 0.80–0.90, respectively) and T-site (Si+Al) deficiencies. Götzenite, Ca₂NaZr, Ti]Si₂O₇(O, OH, F)₂, identified in both lavas, is typical of nephelinite-ijolite assemblages. On the other hand, khibinskite, K₄Zr₂Si₄O₁₄, found in the Cupaello lava, may be regarded as a Si-poor variety of wadeite, a mineral characteristic of lamproites. Clinopyroxene and monticellite, coexisting as late-crystallized phases in both lavas, suggest a common P-T liquid path of thermal descent in the system CaMgSi₂O₇–CO₂ in the presence of excess CO₂, but with different intersections with the åkermanite stability field. Substantial differences in SiO₂ saturation combined with high Mg number and liquidus temperatures experimentally determined at atmospheric pressure in both lavas (1276° C and 1260° C, respectively) indicate that a parent-daughter relationship is unlikely under the volcanic P-T regime. In La Roche's Rm-Ri-Rs diagram, the S. Venanzo composition fall close to the Oldoinyo Lengai alkalic pyroxenite trend of Donaldson and Dawson, while the Cupaello compositions follow a lamproitic trend, consistent with that indicated by the quartz-normative glass of the Gaussberg lava. Mantle compositions corresponding to wehrlite-clinopyroxenite and enriched in H₂O, CO₂, F, and LILE, are favoured as potential sources for the lavas. Their origin is probably related to subcrustal fluid transfer promoted by the Tyrrhenian mantle doming.