Multiple events of melt-rock interaction recorded by undeformed spinel pyroxenites from the External Ligurian ophiolites

The External Ligurian mantle sequences are interpreted as deep subcontinental lithosphere exhumed to the ocean floor in response to Mesozoic lithospheric thinning and opening of the Jurassic Ligurian-Piedmontese basin. The sequences consist of spinel-plagioclase lherzolites with diffuse pyroxenite layers which have been related to recycling of crustal material (Montanini et al., 2012) or to eclogite-bearing peridotite sources (Borghini et al., 2016). The mantle lherzolite body considered in the present study (Monte Gavi) includes an undeformed, irregularly shaped body of spinel pyroxenites. This body has a thickness of 6-10 m, a length of about 50 m and encloses several meter-sized lherzolite lenses. Close to the main pyroxenite body, the host lherzolite frequently includes up to 10 cm thick spinel pyroxenite layers. The pyroxenites are coarse-grained and consist of clinopyroxene- and Al-spinel-rich domains. Clinopyroxene is resorbed, variably replaced by orthopyroxene + plagioclase aggregates, and locally rimmed by titanian pargasite. Spinel-rich domains are largely transformed into Ca-rich plagioclase + Fe-rich olivine + Cr-spinel ± ilmenite. Clinopyroxene locally has relatively low Mg# (≈ 83) and Cr2O3 (≤ 0.3 wt%), and up to 10 wt% Al2O3 and 1.8 wt% Na2O. The thin pyroxenite layers are characterized by Mg-rich clinopyroxene (Mg# = 0.89-0.90) and in places include forsterite-rich deformed olivine, which is interpreted to be a relic of the host lherzolite. The Fe-rich pyroxenites have basaltic, “melt-like” patterns of highly siderophile elements (HSE), whereas the Mg-rich pyroxenites are significantly enriched in Os and Ir. Bulk rock 187Os/188Os ratios recalculated for the age of the Ligurian-Piedmontese basin opening (165 Ma) vary from slightly to moderately radiogenic (0.185-0.518). We propose that the pyroxenites formed by crystallization of Al-rich melts derived by an aged pyroxenite/eclogite-rich source. In this view, the thick pyroxenite body represents a melt-dominated system, whereas the thin pyroxenite layers are hybrid rocks derived from melt/peridotite reactions. Resorption and extensive replacement of the primary clinopyroxene-spinel assemblage was most likely related to reactive migration of ultra-depleted melts under plagioclase facies conditions, during exhumation of the mantle sequence. The host lherzolites also show textural, mineralogical and geochemical evidence of melt infiltration and crystallization of plagioclase + orthopyroxene at the expense of spinel and clinopyroxene.