MELT-PERIDOTITE MULTISTAGE INTERACTION AT MANTLE CONDITIONS: PETROLOGICAL ANO GEOCHEMICAL EVIDENCES FROM SAPPHIRINE-APATITE-CALCITE-BEARING GABBROIC DYKES FROM THE FINERO PHLOGOPITE PERIDOTITE (IVREA-VERBANO ZONE)

The Finero Phlogopite-Peridotite (FPP) is a mantle uni! outcropping in the northernmost pari of the Ivrea-Verbano Zone (IVZ, Southern Alps). Multistage pervasive lo channelled meli migrations had completely recrystallized the entire FPP. The main metasomatic event pervasively formed an association of amphibole-rich phlogopite harzburgite with subordinated phlogopite-pyroxenites which do not show geochemical gradients (Zanelli et al., 1999). Channelled migrations lately formed dunite bodies, sometimes containing stratiform chromitites and, more rarely, pyroxenite layers similar lo those associated lo phlogopite harzburgite. Several other lithologies, showing geochemical gradients with rocks of the main FPP association and characterized by the presence of apatite sometimes associated lo carbonates (i.e. dolomite and calcite), are subordinated in volumes and abundances. Commonly these lithologies occur as dykes or veins along deformation zones. Geochronological data from apatite-calcite zircon syenites and apatite-dolomite wehrlites provide Triassic ages assumed lo document the lime of the meltlfluid migrations. Notwithstanding the apparent mineralogica! and chemical differences with the main lithologic sequences, apatite-carbonates-bearing rocks have been frequently interpreted as cogenetic lo phlogopite harzburgites and related lo the main metasomatic event. Recently, apatite-calcite-bearing gabbroic dykes randomly crosscutting the FPP lithologic associations were recognized as possibly the las! (or one of the las!) melt migration event within the mantle unii (Giovanardi et al., 2013). The dykes show symmetrical internal layering formed by melanocratic bands towards the host peridotite dominated by titanian pargasite and a centrai leucocratic zone dominated by plagioclase. Magmatic sapphirine occurs in plagues al the contaci of the leucocratic zone within the melanocartic bands. New field, petrographic and geochemical studies were conducted lo constrain the gabbroic veins intrusion and their genetic relationships with other FPP metasomatic events. Petrographic evidences, major and trace element data and the O isotopic composition of such gabbroic veins indicate that they formed al shallow mantle conditions by multistage fractional crystallisation of a migrating meli unrelated lo those forming the harzburgite-pyroxenite association and the dunite bodies. However, local strong enrichments in LILE, LREE and 1i180 in vein minerals confirm that such melt was deeply modified by interaction with the host phlogopite peridotite. However, the amphiboles in textural equilibrium with sapphirine show a marked M-H REE and Y depletion associated lo a marked positive Eu anomaly, which suppor! meli evolution through plagioclase assimilation. The genetic relationships with other intrusive events recorded by the FPP and the associated crustal sequence will be addressed with the aim of placing piace new constraints on the petrologic and geodynamic evolution of the IVZ.