Structural and chemical variations in phlogopite fromlamproitic rocks of the Central Mediterranean region

Micas from mafic ultrapotassic rocks with lamproitic affinity from several localities of the Central Mediterranean region were studied through single-crystal X-ray diffraction (SC-XRD), electron microprobe analysis (EMPA) and Secondary Ion Mass Spectrometry (SIMS); Mössbauer Spectroscopy (MöS), when feasible, was also applied to minimise the number of unknown variables and uncertainties. Analysed lamproitic samples cover the most important Central Mediterranean type localities, fromPlan d'Albard (Western Alps) to Sisco (Corsica), Montecatini Val di Cecina and Orciatico (Tuscany, Italy) and Torre Alfina (Northern Latium, Italy). The studied crystals showdistinctive chemical and structural features; all of thembelong to the phlogopite-annite join and crystallise in the 1M polytype, except for micas from Torre Alfina, where both 1M and 2M1 polytypes were found. Studied micas have variable but generally high F and Ti contents, with Mg/(Mg + Fe) ranging from ~0.5 to ~0.9; 2M1 crystals from Torre Alfina radically differ in chemical composition, showing high contents of Ti and Fe aswell as of Al in both tetrahedra and octahedra, leading to distinctive structural distortions, especially in tetrahedral sites. SIMS data indicate that studied micas are generally dehydrogenated with OH contents ranging from ~0.2 apfu (atoms per formula unit) for Orciatico and Torre Alfina to ~1.4 for Plan d'Albard crystals; this feature is also testified by the length of the c parameter, which decreases with the loss of hydrogen and/or the increase of the F→OHsubstitution. Chemical and structural data suggest that the entry of high charge octahedral cations ismainly balanced by an oxymechanism and, to a lesser extent, by aM3+,4+-Tschermak substitution. Our data confirm that Ti preferentially partitions into theM2 site and that different Ti and F contents, as well as different K/Al values, are both dependant upon fH2O and the composition of magma rather than controlled by P and T crystallisation conditions. The obtained data help to discriminate among lamproite-like rocks formedwithin a complex geodynamic framework but still related to a destructive tectonic margin and evidence different trends for micas from the youngest Torre Alfina (Northern Latium) lamproites, referred to the Apennine orogeny and those of the older lamproites from Orciatico, Montecatini Val di Cecina (Tuscany), Western Alps, and Corsica, the latter referred to the Alpine orogeny. Phlogopite crystals fromthe older lamproites fall within the compositional and structural field ofworldwide phlogopites from both within-plate and subduction-related settings. Phlogopite from the Plio-Pleistocene lamproite-like occurrence in Tuscany and Northern Latium, despite crystals with low Mg# of the Torre Alfina rock plot well within the general field of the other crystals in less evolved samples, follows a different evolution trend similar to that of shoshonites fromTuscany and Northern Latium. On this basis, we argue that the observed differences are inherited by slight differences in the magma compositions that are related to different genetic and evolution pathways.