GNGTS 2024 - Atti del 42° Convegno Nazionale

Session 1.1 GNGTS 2024 Along the mylonitc horizons, the Al-rich Atg forming the matrix is afected by ductle deformaton, and OlF2 reacton bands mark a foliaton parallel to that of Atg . Ol 1 relics are also involved in the deformaton. Considering Ol 2 , EBSD maps revealed the absence of a relatonship between the metamorphic olivine ( Ol 2 ), the surrounding Al-rich Atg and the Ol 1 relics. Ol + Ti-chu + Mt + Clinochlore (C-Chl) + Di assemblage was observed within the thicker veins of the southern part of the outcrop and Ol + Ti-chu assemblage within the en-echelon veins. The contact between these two sets is sealed by Mt and fne-grained Ol arranged to form an oblique fabric. From the hand-specimen and the thin sectons the following sequence of events was determined: (i) development of horizontal Atg veins, hostng μm-sized Ol and Ti-chu grains, (ii) formaton of the Ol + Ti- chu + Atg + C-chl + Di vein subparallel to the Atg vein, decorated by locally sheared, orthogonal Atg lamellae, and of the en-echelon veins (iii) reactvaton of the veins through the formaton of Atg- bearing microcracks (iv) late veins of calcite and chrysotle crosscutng the previous structures. In-situ determinaton of trace elements through LA-ICP-MS allowed the determinaton of the sources of fuids released during the subducton process and involved in the deformaton. The geochemical analyses focused on the distributon of fuid-mobile elements (FMEs: As, Sb, Ba, W, Li, B ) within the diferent minerals, as tracers of fuid-rock interactons occurring during dehydraton reactons in subducted serpentnites. Enrichment in FMEs detected within the metamorphic olivine and in the Al- free antgorite provide evidence of infltraton of external, sedimentary-derived fuids, indicatng an opening of the system during deformaton at eclogite-facies conditons, consistently with the results reported by Scambelluri et al. (2012) and Clarke et al. (2020). These observatons suggest the possible occurrence of two stages of Atg dehydraton, and an intermediate stage of hydraton, occurring within the stability feld of Atg, Brc and Ol . The frst extensive dehydraton following oceanic serpentnizaton led to the formaton of the metamorphic olivine ( Ol 2 ) arranged along the reacton bands and the veins. The new stage of hydraton was localized along Atg -bearing microcracks formed especially within the reacton bands, which were, in turn, likely afected by dehydraton, leading to growth of granoblastc olivine ( Ol 3 ). References Behr W. M., Bürgmann R.; 2021: What’s down there? The structures, materials and environment of deep-seated slow slip and tremor. Phil. Trans. R. Soc. A 379: 20200218. doi: 10.1098/rsta.2020.0218 Clarke E., De Hoog J. C. M., Kirstein L. A., Harvey J., Debret B.; 2020: Metamorphic olivine record external fuid infltraton during serpentnite dehydraton. Geochem. Persp. Let. 16, 25–29. doi: 10.7185/geochemlet.2039 25 Hermann J., Müntener O., Scambelluri M.; 2000: The importance of serpentnite mylonites for subducton and exhumaton of oceanic crust. Tectonophysics 327, 225±238. doi: 10.1016/ S0040-1951(00)00171-2