Petrogenesis of Potassium-rich Lavas from the Roccamonfina Volcano, Roman Region, Italy

Abstract

The Roccamonfina volcano in the Roman Region of Italy is characterized by a High K series of ne-and lc-normative leucite-bearing lavas and a Low K series of ne-to Qz-normative olivine basalts, trachybasalts, and biotite augite latites. The lavas of the High K series are enriched in K, P, Ba, Ce, Rb, Sr, Th, and Zr relative to the Low K lavas, and there is an almost parallel enrichment of K and associated elements in the primitive (i.e. most basic) lavas. The volcanoes of the Roman Region exhibit a wide variation in the level of enrichment of K and associated elements and it can be shown that the compositional variation can be related to two main differentiation processes. An intermediate- or high-pressure process produces the parental magmas with specific levels of enrichment in K and associated elements while the evolution towards salic derivatives is controlled by low-pressure crystal-liquid fractionation. Crustal assimilation and vapour transfer processes, and partial melting of a primitive upper mantle composition, cannot account for the variation observed in the primitive lavas of the Roman Region nor for the geochemical characteristics of potassic undersaturated rocks in general. The level of enrichment of K and associated elements may, however, be controlled by high-pressure eclogite fractionation, zone-refining or mantle wall-rock reaction.