Contaminating melt flow in magmatic peridotites from the lower continental crust (Rocca d'Argimonia sequence, Ivrea–Verbano Zone)

Marta Antonicelli,Tong Liu,Fu-Yuan Wu,Riccardo Tribuzio

doi:10.5194/ejm-32-587-2020

Abstract

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.

Highlights

The processes driving the chemical differentiation of primitive magmas emplaced in the deep sectors of the continental crust are largely unknown
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
Based on petrographic observations and mineral/wholerock major element chemistry, we propose that the development of orthopyroxene in the Rocca d’Argimonia peridotites was related to reactive melt flow through a melt-poor olivinerich crystal mush or a pre-existing dunite

Summary

Introduction

The processes driving the chemical differentiation of primitive magmas emplaced in the deep sectors of the continental crust are largely unknown. Several recent studies have proposed that the melt evolution in large magma reservoirs predominantly occurs by reactive flow in melt-poor crystal mushes In the lower continental crust, in particular, the intrusion of primitive mantle magmas is expected to generate a region of crystal mush, in which the melt phase may originate, to a minor extent, from partial melting of the crust (see Solano et al, 2012). Buoyant melts are inferred to percolate upwards along crystal boundaries, transporting chemical components and modifying the structure and the chemistry of pre-existing crystals. Other studies support the idea that magma evolution in large magmatic systems is controlled by fractional crystallization in long-lived, liquid-dominated magma chambers Other studies support the idea that magma evolution in large magmatic systems is controlled by fractional crystallization in long-lived, liquid-dominated magma chambers (e.g. Kruger and Latypov, 2020).

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