Crystallysation condition and genesis of peralkaline magmas from Pantelleria, Italy: an integrated petrological and crystal chemical study

Pantelleria, the type locality of pantellerite, is a Late-Pleistocene magmatic system with a bimodal association of alkali basalts and peralkaline rocks showing a large compositional gap at SiO2=50–67 wt.%. Major and trace element data show a strong change in all element patterns with two distinct liquid lines of descent for the mafic and the felsic end-members for many elements (Al2O3, FeOtot, Ba, and Cr). The mineral chemistry data also show compositional gaps for many mineral phases, similar to those observed in the whole-rock data. Trace element models show that a trachytic melt can be generated from a basaltic parental magma either by a two step fractional crystallisation process, or by 30% partial melting of gabbroic cumulate. Pantelleritic magmas are derived by fractional crystallisation of mainly K-feldspar from a trachytic end-member. The crystal-chemical parameters of the clinopyroxene crystals show two clearly distinct patterns for the mafic and felsic rocks, which are unlikely to be generated by continuous crystal fractionation process from basalt to pantellerite. Clinopyroxene crystals in the mafic rocks show trends comparable with patterns observed for some Italian potassic suites (e.g., Sabatini volcano), with an increase of Al in the T site and Ca in M2 with differentiation. On the other hand, crystals in the peralkaline felsic rocks follow the patterns of the African Na-rich products (e.g., Nyambeni, Boseti) with Si increasing in T and Na in M2. Chemical parameters of the host rocks plotted vs. structural data of the clinopyroxene crystals divide mafic and felsic rocks into two well distinguished groups of undersaturated and oversaturated magmas, but do not explain the transition between the two groups. The data suggest that simple fractional crystallisation by itself cannot explain the generation of peralkaline magmas in Pantelleria starting from parental basalts. Other processes such as partial melting of gabbroic cumulates are more viable and have to be considered in the genesis of peralkaline magmas.