Abstract. The lower continental crust section of the Ivrea–Verbano Zone (Italian Alps)
was intruded by a ∼ 8 km thick gabbroic–dioritic body (Ivrea Mafic Complex) in the Upper Carboniferous–Lower Permian, in conjunction with the
post-collisional transtensional regime related to the Variscan orogeny. In
the deepest levels of the Ivrea Mafic Complex, several peridotite–pyroxenite
sequences considered of magmatic origin are exposed. We present here a
petrological–geochemical investigation of the peridotites from the largest
magmatic ultramafic sequence of the Ivrea Mafic Complex, locally called Rocca
d'Argimonia. In spite of the widespread subsolidus re-equilibration under
granulite facies conditions, most likely reflecting a slow cooling evolution
in the lower continental crust, the Rocca d'Argimonia peridotites (dunites
to harzburgites and minor clinopyroxene-poor lherzolites) typically retain
structures and microstructures of magmatic origin. In particular, the
harzburgites and the lherzolites typically show poikilitic orthopyroxenes
enclosing partially dissolved olivine and minor spinel. Olivine has
forsterite proportion diminishing from the dunites to the harzburgites and
the lherzolites (90 mol % to 85 mol %) and negatively correlating with δ18O (+5.8 ‰ to +6.6 ‰).
Gabbronorite dykes locally crosscut the peridotites and show millimetre-scale thick,
orthopyroxenite to websterite reaction zones along the contact with host
rocks. We propose that the Rocca d'Argimonia peridotites record a process of
reactive melt flow through a melt-poor olivine-rich crystal mush or a
pre-existing dunite. This process was most likely responsible for the
olivine dissolution shown by the poikilitic orthopyroxenes in the
harzburgites–lherzolites. We infer that the reactively migrating melts
possessed a substantial crustal component and operated at least at the
scale of ∼ 100 m.
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