Composite melt-rock interactions in the lowermost continental crust: insights from a dunite-pyroxenite-gabbronorite association of the Mafic Complex from the Ivrea-Verbano Zone (Italian Alps)

Abstract

The processes leading to the building of the continental crust through magmatic underplating are fundamentally unknown, mainly because of the rare accessibility to deep level sections of the continental crust. The Italian Alps expose the Permian Mafic Complex, an 8 km-thick gabbronorite-diorite batholith that intruded the lower continental crust during the post-Variscan transtensional tectonics. We present here a petrological and geochemical study of a concentric dunite-pyroxenite-gabbronorite association, called Monte Mazzucco sequence, enclosed at deep levels of the Mafic Complex, thereby allowing us to provide new insights into the magmatic processes driven by emplacement of mantle melts in deep crustal continental areas. The studied sequence includes a ~60 m thick dunite lens, in which olivine (82 mol% forsterite) is associated with accessory Cr-spinel including blebs and lamellae made up of magnetite. The dunite lens is permeated by mm- to cm-scale thick magmatic veins, which range in composition from hornblende lherzolite to olivine hornblendite and hornblende websterite. The lens is mantled by a m-scale ring consisting of amphibole-bearing (≤1 vol%) websterite, and the websterite ring is in turn enclosed by amphibole-free gabbronorites. Both magmatic veins within the dunites and mantling websterites typically include an oxide association of Al-spinel and magnetite. Remarkably, the hornblende websterite veins and the mantling websterites are typically plagioclase-free and include clinopyroxene and amphibole with chondrite-normalized rare earth element patterns characterized by negative Eu anomaly. The mantling websterites display a subtle, gradual outward decrease of Mg# for orthopyroxene, clinopyroxene and accessory olivine, coupled with an increase of the negative Eu anomaly in clinopyroxene and amphibole. The enclosing gabbronorites are amphibole-free and have a chemically evolved signature. We propose a petrogenetic scenario including two major events of melt-dunite interaction. The first resulted from focused reactive melt infiltration and formed the magmatic veins within dunites. The hornblende lherzolite and the olivine hornblendite veins were produced by focused reactive melt migration through dunite grain boundaries, involving dissolution of olivine and recrystallization of Cr-spinel into Al-spinel and magnetite, whereas the hornblende websterite veins crystallized from melts penetrating through narrow fractures and recording earlier plagioclase fractionation. Most likely, the infiltrating melts were overall derived from an evolving H2O-rich magma emplaced below the dunite body. The second event of melt-dunite reactive interaction developed the websterite ring around dunites. We envision that the outermost domain of the dunite body was replaced by websterites in response to reaction with an invading H2O-poor melt, which had previously undergone plagioclase fractionation. The dunite replacement occurred under dynamic conditions, which promoted the reaction progress, thereby leading to total or almost total dissolution of precursor olivine, and started to form the lens shape of the Monte Mazzucco ultramafic association. The gabbronorites closely adjacent to the websterite ring represent the crystallization products of the invading melt.