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

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.

Formation of Amphibole‐Bearing Peridotite and Amphibole‐Bearing Pyroxenite Through Hydrous Melt‐Peridotite Reaction and In Situ Crystallization: An Experimental Study

“…The authors thank Joseph Boesenberg for assistance in microprobe analyses and Riccardo Tribuzio for bringing our attention to the work of Antonicelli et al. (2020). The authors also wish to thank Associate Editor John Lassiter and two anonymous reviewers for their detailed comments and constructive suggestions which helped to improve this manuscript.…”

“…We have demonstrated that hydrous basalt-peridotite reaction and in situ crystallization can produce spatially associated amphibole-bearing gabbronorite-orthopyroxenite-peridotite sequence at conditions examined. One field example is from the lower crust section of the Ivrea-Verbano Zone in Italian Alps described in details by Antonicelli et al (2020). In the deepest levels of the Ivrea Magmatic Complex, harzburgites are intruded by amphibole gabbronorite dikes, with millimeter-scale thick contact zones of orthopyroxenite toward the harzburgite and hornblende websterite toward the gabbronorite.…”

“…One field example is from the lower crust section of the Ivrea‐Verbano Zone in Italian Alps described in details by Antonicelli et al. (2020). In the deepest levels of the Ivrea Magmatic Complex, harzburgites are intruded by amphibole gabbronorite dikes, with millimeter‐scale thick contact zones of orthopyroxenite toward the harzburgite and hornblende websterite toward the gabbronorite.…”

“…In the deepest levels of the Ivrea Magmatic Complex, harzburgites are intruded by amphibole gabbronorite dikes, with millimeter‐scale thick contact zones of orthopyroxenite toward the harzburgite and hornblende websterite toward the gabbronorite. The harzburgites are amphibole‐bearing, and contain poikilitic orthopyroxenes enclosing partially dissolved olivine and minor spinel (Antonicelli et al., 2020). Amphibole, orthopyroxene, and clinopyroxene have increasing Mg# and Cr# from gabbronorite to pyroxenites and harzburgite, and spinel have an increasing Cr# from websterite to orthopyroxenite and harzburgite.…”

Mobilisation of deep crustal sulfide melts as a first order control on upper lithospheric metallogeny

“…Mantle-derived mafic magmas that accumulated at the base of the continental crust, at a depth of 15–25 km 16 comprise the Mafic Complex, subdivided into a lower Layered Series (Cyclic Units), Main Gabbro, and an upper Diorite Unit 17 . The Cyclic Units are made up of layered mafic-ultramafic cumulates with intercalated septa of migmatised metasedimentary rocks 18 and despite the effects of exhumation, retain structures and microstructures of magmatic origin 19 . The Main Gabbro, volumetrically the most significant unit of the Mafic Complex, is overlain by the Diorite Unit, which is in direct contact at its upper margins with the metasediments of the Kinzigite Formation (Fig.…”