Distribution and behavior of lithium in the K-alkaline rocks from the Roccamonfina Volcano (Campania, southern Italy)

Abstract

The Li content in whole rocks, matrix and mineral separates (biotite, sanidine, leucite, phlogopite) from 58 igneous rocks of the K-alkaline suites of the Roccamonfina Volcano has been measured by atomic absorption. Among the minerals, sanidine contains as much Li as biotite x ̄ = 41.8 ppm and x ̄ = 40.2 ppm , respectively), while phlogopite and leucite have a much smaller amount ( x ̄ = 6.0 ppm and x ̄ = 2.6 ppm , respectively). Average Li distribution coefficients between minerals and groundmass are: D biot matrix Li = 0.50 , D san matrix Li = 0.84 and D leuc matrix Li = 0.06 , respectively. The Li content increases with magmatic differentiation in the rocks of the K series, showing an enrichment factor of ∼ 3.6 in the salic relative to the mafic rocks. In the high-K series the Li content increases from primitive to intermediate rocks, but shows a levelling off in the more salic rocks. Li acts incompatibly except in the most salic rocks, whose Li content was probably mainly controlled by diffusion processes through the magmas. The Li content in the Roccamonfina primitive rocks ( x ̄ = 20 ppm ) does not discriminate between the K and high-K series, and is higher than that of the “average” basalt ( x ̄ = 10 ppm ); however, it is similar to that of basalts from continental settings. This relatively high Li content in the Roccamonfina primitive rocks can be accounted for by either crustal contamination or the existence of an enriched mantle region beneath the volcano, but in both hypotheses there is a suggestion of some involvement of the lower crust in the generation of the Roccamonfina parental magmas.