The peridotite-pyroxenite sequence of Rocca d'Argimonia (Ivrea-Verbano Zone, Italy): Evidence for reactive melt flow and slow cooling in the lowermost continental crust

The present study investigates the origin of a ∼ 400 m thick peridotite-pyroxenite sequence, locally known as Rocca d'Argimonia, which is encased within the lowest levels of the ∼8 km thick Mafic Complex, a gabbro-dioritic batholith from the lower continental crust section of the Ivrea-Verbano Zone (Southern Alps). The broad purpose of this work is to elucidate the evolution of mantle-derived magmas emplaced in the lowermost continental crust. We also wish to provide new geochronological constraints on the intrusion and the cooling evolution of the Lower Mafic Complex, which are still mostly unknown. Zircon grains separated from two gabbronorites enclosing the peridotite-pyroxenite sequence document a whole reset of the original U–Pb isotopic system at 286 ± 2 Ma. A Rocca d'Argimonia peridotite provided a consistent age of 296 ± 19 Ma, based on a Lu–Hf clinopyroxene-amphibole alignment (initial εHf = +7.9). These results are interpreted to date the early cooling evolution of the Lower Mafic Complex. The clinopyroxene-amphibole peridotite pair also gave a Rb–Sr alignment corresponding to an age of 250 ± 16 Ma (initial 87Sr/86Sr = 0.7045), which is reconciled with the subsequent, slow cooling evolution of the Lower Mafic Complex. The Rocca d'Argimonia peridotites (dunites to harzburgites and lherzolites) have whole-rock εNd(286 Ma) and εHf(286 Ma) values ranging from +0.4 to −1.1 and from +8.8 to +3.4, respectively, and 87Sr/86Sr(286 Ma) ratios varying from 0.7048 to 0.7060. The peridotites are crosscut by gabbronorite dykes and associated with olivine-free orthopyroxene-dominated pyroxenites. The gabbronorite dykes and the pyroxenites share the Nd-Hf-Sr signature of the peridotites. The gabbronorites enclosing the peridotite-pyroxenite sequence differ in the relatively enriched Nd-Hf-Sr isotopic fingerprint (εNd(286 Ma) = −4.5 to −5.6, εHf(286 Ma) = +0.2 to −2.5, 87Sr/86Sr(286 Ma) = 0.7073 to 0.7086). Remarkably, pyroxenes from the pyroxenites typically contain higher Cr2O3 than the peridotite counterparts (e.g., 0.23–0.37 wt% Cr2O3 in peridotite orthopyroxenes and 0.43–0.50 wt% Cr2O3 in pyroxenite orthopyroxenes). We propose that the peridotite-pyroxenite sequence formed by reactive melt percolation through an olivine-rich spinel-bearing matrix. In particular, the relatively high Cr2O3 content of the pyroxenitic pyroxenes is attributed to redistribution of Cr released by the breakdown of accessory spinel that was originally present in the olivine-dominated matrix, in response to a high melt/solid ratio. The melt residual after the crystallization of the pyroxenites continued to percolate the olivine-dominated matrix, thereby forming spinel-bearing lherzolites, harzburgites and dunites with the progression of the melt migration process. The chemically most primitive dunite records Nd-Hf-Sr isotopic disequilibrium conditions, thereby providing further information about the process of reactive melt flow. It is inferred that the original olivine-dominated matrix crystallized from a mantle magma having a slightly enriched Nd-Hf-Sr isotopic signature, whereas the percolating melts overall had a relatively depleted Nd-Hf-Sr isotopic fingerprint, despite being variably contaminated by the continental crust.