Geochemistry of micas from the Finero spinel-lherzolite,Italian Alps

The Finero lherzolite is distinct amongst the tectonically emplaced slices of mantle in the Ivrea Zone (Italian Alps) for its abundant coarse phlogopite. An average composition (SiO₂ 39.9, TiO₂ 0.97, Al₂O₃ 16.0, Cr₂O₃ 1.16, FeO 2.73, MgO 24.5, NiO 0.16, BaO 0.31, Na₂O 0.58, K₂O 8.7, Rb₂O 0.056, Cl 0.03, F 0.10 wt.%) is similar in Fe, Cr, Ni, Ba and F/Cl to primary-textured micas from coarse garnet-lherzolite xenoliths from S. Africa, but is higher in Ti, Na, Rb, and Al, and lower in halogens. The distinct values of Ti and Fe for five specimens of Finero peridotites demonstrate local spatial variation. The overall ranges of TiO₂ (0.5–1.7) and FeO (2.3–3.6) fall within the range for secondary-textured micas in peridotite xenoliths from S. Africa. The Finero micas are lower in both K/Rb and K/Ba than the primary and secondary micas from S. Africa, and their mean values of K/Rb (110–220) and K/Ba (15–39) are lower than for almost all bulk rocks, but fit well with the ranges of 109–180 and 12–49 for the high-K lavas of the Roman region.Although all evidence is indicative rather than conclusive, the chemical properties of the Finero micas are consistent with introduction of an alkaline phase into peridotite during or before emplacement of the Finero complex from the upper mantle into the crust, and the coarse, partly-deformed textures can be explained by incomplete metamorphic equilibration during prolonged deformation. The alkaline phase is tentatively attributed to the uppermost mantle.