An undeformed pyroxenite-peridotite sequence from the External Ligurian ophiolites records multiple events of melt-rock interactions.

The External Ligurian mantle sequences represent deep subcontinental lithosphere exhumed in response to Mesozoic lithospheric thinning and opening of the Jurassic Alpine Tethys. They mainly consist of spinel-plagioclase lherzolites, with tectonite to mylonitic structures, and diffuse pyroxenite layering. The deformed pyroxenites have been related to recycling of old crustal material in Lower Palaeozoic to Triassic times (e.g. Montanini et al., 2015; Borghini et al., 2016). The lherzolites of the present study (Monte Gavi) are undeformed and show evidence of melt infiltration and crystallization of plagioclase (Pl) + orthopyroxene (Cpx) at the expense of spinel (Spl) and clinopyroxene (Cpx). The lherzolites include a pyroxenite body with a thickness of 6-10 m and a length of ~ 50 m. The primary assemblage of the pyroxenite consists of Cpx and Al-Spl. Cpx is resorbed and variably replaced by Opx + Pl aggregates. Spl is extensively transformed into Ca-rich Pl + Fe-rich olivine + Cr-Spl ± ilmenite. Clinopyroxene has low Mg# (81-83) and up to 10 wt% Al2O3. Close to the main pyroxenite body, the lherzolite includes up to 10 cm-thick spinel pyroxenite layers containing Mg-rich Cpx (Mg# = 89-90) and, locally, Mg-rich olivine incorporated from the host lherzolite. REE compositions of melts in equilibrium with the preserved primary Cpx display a slight LREE enrichment and a negative HREE fractionation requiring a garnet-bearing source. The Fe-rich pyroxenite body has “melt-like” patterns of highly siderophile elements (HSE), whereas the Mg-rich pyroxenites are enriched in Os and Ir. Bulk rock 187Os/188Os ratios recalculated at the age of the Alpine Tethys opening (165 Ma) show increasingly radiogenic composition from Mg- to Fe-rich pyroxenites (187Os/188Os = 0.185-0.518). We propose that the pyroxenites formed by crystallization of Al-Fe-rich melts derived from aged pyroxenite/eclogite-rich sources. Whereas the thick pyroxenite body represents a melt-dominated system, the thin pyroxenite layers formed by melt/peridotite hybridization. Extensive replacement of the primary assemblage was most likely produced by reactive migration of depleted MORB-type melts under plagioclase facies conditions (P ~0.6 GPa). The pyroxenites preserve high T (1200-1250 °C) recorded by slowly diffusing elements like REE, presumably in response to the melt infiltration event, followed by a rapid subsolidus cooling until 900 °C during rifting-related exhumation of the mantle sequence. REFERENCES Montanini, A., Tribuzio R. (2015): Evolution of recycled crust within the mantle: Constraints from the garnet pyroxenites of the External Ligurian ophiolites (northern Apennines, Italy). Geology, 43, 911-914, 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. J.Petrol., 57, 625-653.