GNGTS 2024 - Atti del 42° Convegno Nazionale

Session 1.1 GNGTS 2024 Figure 1. Erro-Tobbio Unit P-T path. The blue rectangle marks the possible conditons of formaton of the metamorphic olivine veins and reacton bands The N-S trending mylonitc horizons include: (i) type 1 mylonites, composed of a planar foliaton marked by olivine reacton bands (OlF2), and (ii) type 2 mylonites, displaying a chaotc structure. The southern part of the outcrop contains, in additon, sub-horizontal, cm-thick veins of Ol + Ti-chu + Chl , and conjugated right stepping en-echelon arrays of Ol + Ti-chu veins. At the microscale, the meta-peridotte matrix within the undeformed domains consists of coarse Al- rich Atg statcally replacing mantle olivine ( Ol 1 ) and clinopyroxene ( Cpx ). In these domains, OlF1 is arranged into conjugate Ol , Ti-chu reacton bands, and radial aggregates, in which Al-free Atg replaces Ol 2 along a network of microcracks. These reacton bands include relics of coarse Al-rich Atg and Brc , suggestng that the formaton of Ol 2 localized along Brc -rich zones. Coronitc granoblastc olivine grows at the contact between Brc and Atg , refectng the dehydraton reacton Atg + Br → Ol + H 2 O ; a similar mineral phase ( Ol 3 ) was observed within previous Ol 2 sites within the reacton bands. This phase is characterized by a Mg/Si rato typical of olivine, stll, TEM analysis will be performed to unambiguously identfy this phase. Ol 1 relics appear structurally linked to the fne-grained Ol 2 that consttutes the reacton-bands. EBSD maps acquired on a reacton band, reveal epitaxial growth of Ol 2 over Atg 1 and Ol 1 . Ol 2 grains are iso-oriented with Atg 1 and Ol 1 in areas close to the Ol 1 relics, and the misorientaton of the Ol 2 grains increases with the distance from the Ol 1 relics.