Geochemical data of Al-rich diopside pyroxenites from the Premosello mantle peridotite massif, Ivrea-Verbano Zone, Southern Alps

Abstract

Pyroxenites of different generations and composition are usually found within orogenic peridotite massifs and in mantle xenoliths entrained in volcanic rocks. Orogenic peridotite massifs, however, offer great advantages over xenolith studies because structural relationships which formed in the mantle before exhumation are often preserved, and crosscutting relationships between dykes of different generations and composition can be readily observed. Numerous orogenic peridotite massifs occur in the Ivrea-Verbano Zone (IVZ) in the western Southern Alps, providing petrologists, geochemists and geophysicists a natural laboratory to study and understand Earth's mantle processes and evolution. We here report new geochemical data for peculiar Al-rich diopside pyroxenites which crosscut the Premosello mantle peridotite massif in central IVZ close to the transition to the continental crust (i.e., the Moho region). The pyroxenite is composed of Al-rich clinopyroxene (Cpx), spinel (Sp), and amphibole (Amph), with subordinate amounts of olivine (Ol), and occasional orthopyroxene (Opx) as an accessory phase. Electron microprobe (EMP) and laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis were performed to measure the major and trace elements contents of the mineral phases of the pyroxenite. Major element composition of each mineral phase is characterized by uniform Mg# (Cpx: 0.88-0.90; Ol: 0.87-0.88; Sp: 0.73-0.77; Amph: 0.84-0.87; Opx: 0.87) and high Al₂O₃ contents (except in Ol). The trace element composition of Cpx and Amph from the Al-rich diopside pyroxenite shows strong rare earth elements (REE) fractionation and enrichments in LREEs and MREEs over the HREEs, and is distinct from other pyroxenite compositions (i.e., Al-augite and Cr-diopside pyroxenites) reported from the Premosello and other IVZ lherzolitic peridotite massifs. The geochemical data presented herein, therefore, offer valuable insights into the compositional variability and formation processes of mantle pyroxenites, and may contribute to unravelling the broader evolutionary history of the Earth's subcontinental lithospheric mantle, in particular at Moho levels.