Pyroxenite diversity in the subcontinental mantle: the example of the External Ligurian ophiolites (northern Apennine, Italy)

The ocean-continent transition exemplified by the External Ligurian mantle section consists of three distinct domains, developed in response to the Mesozoic extension of continental lithosphere (Ferrari et al. 2022): (1) a spinel tectonite domain (ST), characterized by sub-solidus static formation of plagioclase, (2) a plagioclase mylonite domain (PM) experiencing melt-absent deformation, and (3) a nearly undeformed domain (PI) that underwent reactive melt infiltration under plagioclase-facies conditions. All domains are characterized by the presence of distinct types of pyroxenite layers with variable origin, age and evolution. In the ST domain, spinel pyroxenites crystallized at P> 1.5 GPa from melts derived from a eclogite-bearing peridotite source, which reacted with their host peridotite to form hybrid pyroxenites, presumably in the Lower Palaeozoic (~430 Ma, Borghini et al. 2016). The PM domain locally preserves the occurrence of UHP (~2.8 GPa) garnet clinopyroxenites and websterites. The garnet clinopyroxenites were related to recycling of crustal mafic protoliths through partial melting of eclogitized gabbros, whereas the garnet websterites were interpreted as reaction products between eclogite-derived melts and peridotites. Nd-Hf-Pb isotope systematics indicate that the mafic precursors of the garnet pyroxenites experienced a long-term evolution of recycling into the mantle, developing an isotope signature close to the HIMU component of oceanic basalt sources. Re-Os isotope systematics constrains the formation of garnet pyroxenite gabbroic precursors at ~2.6 Ga. In the PI domain, spinel pyroxenites include Fe-rich clinopyroxenites and Mg-rich websterites. Reactive melt infiltration involved both peridotites and pyroxenite layers, causing extensive mineral and chemical transformations. Lower Paleozoic Re-Os model ages for the Fe-rich clinopyroxenites (~0.4-0.5 Ga) likely represent their age of crystallization, whereas a Sm-Nd isochron at ~210 Ma is interpreted to record an isotopic homogenization due to melt-rock interaction. References Borghini, G., Rampone, E., Zanetti, A., Class, C., Cipriani, A., Hofmann, A.W., Goldstein, S.L. (2016). Pyroxenite layers in the northern Apennines’ upper mantle (Italy)-generation by pyroxenite melting and melt infiltration. Journal of Petrology 57,625–653. Ferrari, E., Montanini, A., Tribuzio, R. (2022). Rifting evolution of the lithospheric subcontinental mantle: new insights from the External Ligurian ophiolites. Lithos, 410–411, 106571.