Sediment subduction and the source of potassium in orogenic leucitites

Leucitites from the Italian volcanic province show marked enrichment of the LIL elements over the high-field-strength elements, indicative of a subduction-related origin. In addition, they have high and variable delta (super 18) O and high REE contents, but negative Eu anomalies despite the absence of plagioclase phenocrysts in the most primitive rocks. Despite the similarity of Sr- and Nd-isotope ratios to lamproites, trace-element variations do not suggest these rocks as a source for K-enrichment. It is suggested that the break in subduction through arc-continent collision, coupled with a ready supply of sediment, is the controlling factor for the production of orogenic potassic magmas. Stabilization of phlogopite in a hybridized mantle above the subduction zone is regarded as an essential aspect of the petrogenesis of the orogenic leucitites.