Petrogenesis of orenditic and kamafugitic rocks from Central Italy

In central Italy, ultra potassic rocks having an orenditic or lamproitic and kamafugitic affinity are found associated with potassic (KS) and highly potassic (HKS) volcanic rocks of the Roman Province. Orenditic rocks (OREN) are silica oversaturated, intermediate in composition and have a high (71-80) value of Mg# [100 Mg/(Mg + FeZ+)], high KINa, and high abundances of incompatible elements, Cr and Ni. Kamafugitic rocks (KAM) are ultrabasic, strongly silica undersaturated, and show the highest Ca(%), Mg# (75-81) and K/Na in the Roman province; incompatible elements are highly enriched; Ni and Cr range from 79 to 153 and from 40 to 830 ppm, respectively. Orenditic rocks have higher Sr-isotope ratio (0.71256-0.71715) and lower abundance of Sr (402-847 ppm) with respect to KAM (87Sr/86Sr 0.71037-0.71120; 1724-3704 ppm Sr). Derivation of OREN from KAM or HKS magmas by assimilation of crustal material, although consistent with Sr isotopic variations, is ruled out by the high Mg#, Ni and Cr of OREN, as well as by a large number of other geochemical and petrological data. The genetic model proposed suggests that KAM and OREN were generated by melting at different depths of a residual, phlogopite-bearing upper mantle enriched in LILE and radiogenic Sr. Enrichment was provided by addition of liquids derived by melting of sediments carried down by subduction processes that were active under the Appennines during Tertiary times. The strong degree of silica undersaturation and the very high Mg# exclude significant interaction of KAM magmas with crustal rocks. Contamination of OREN magma with significant amounts of crustal material is possible only if parental magma had extremely high Mg, Ni, and Cr, as found in kimberlites and some highMg lamproites which, however, are not observed in the Italian peninsula.