Leucite-bearing (kamafugitic/leucititic) and –free (lamproitic) ultrapotassic rocks and associated shoshonites from Italy: constraints on petrogenesis and geodynamics

Sandro Conticelli, Riccardo Avanzinelli, Massimo Mattei, Marinella A. Laurenzi, Simone Tommasini, Elena Boari, Leone Melluso, Giulia Perini, Guido Giordano, and Francesca Cifelli

In Italy and surroundings, leucite-free (i.e., lamproites), leucite-bearing (i.e., kamafugites, leucitites, plagioclase-leucitites), and haüyne-bearing (i.e., haüynites, haüyne-leucitites) ultrapotassic igneous rocks have been recorded from Oligocene to present in association with shoshonitic, and high-K calc-alkaline volcanic rocks.

The oldest outcrops of ultrapotassic and related rocks are found within the western Alps in the form of lamprophyric to calc-alkaline dykes intruded during the Oligocene. Four different magmatic provinces, characterised by the association of ultrapotassic igneous rocks with shoshonitic to calc-alkaline series, are also found along the Tyrrhenian margin of the peninsula. These rocks have been produced from Miocene to Holocene with an eastward/southeastward migration with time. Leucite-free silica-rich ultrapotassic lamproitic rocks are restricted to the early stages of magmatism, whereas ultrapotassic leucite-bearing rocks to the middle and late stages.

Mafic ultrapotassic igneous rocks are enriched in incompatible trace elements, with variable fractionation of Ta, Nb, and Ti with respect to Th and Large Ion Lithophile elements, and variable enrichment in radiogenic Sr and Pb and unradiogenic Nd. These characteristics are reconducted to sediment recycling within the upper mantle via subduction. Recycling of carbonate-rich pelites plays an important role in the genesis of leucite-bearing magmas.

Large volume of metasomatic components is predicted to be accommodated within a vein network in the sub-continental lithospheric mantle. Partial melting of the vein generates ultrapotassic magmas, either lamproitic or kamafugitic. Increased interaction between the metasomatic veins and the surrounding mantle dilutes the alkaline component producing shoshonites and high-K calc-alkaline suites. The addition of a further subduction-related component shortly before magma generation is required to explain the isotopic composition of rocks from the Neapolitan district, together with the probable arrival of a within-plate component from the Adria mantle through slab-tear.