Olivine-rich troctolites from Ligurian ophiolites (Italy): evidence for impregnation of replacive mantle conduits by MORB-type melts

The gabbroic plutons of the Internal Ligurian ophiolites (Northern Apennines, Italy) include olivine-rich troctolite bodies up to 80 m thick. The rounded to embayed morphology of olivine (Fo = 87-88 mol %) and spinel from the olivine-rich troctolites, together with the high Mg-number (88-90) and Cr contents (2400-3000 ppm) of associated poikilitic clinopyroxene, indicates their formation through reaction between an olivine-spinel matrix and an infiltrating melt. This interpretation is corroborated by the occurrence of plagioclase-rich veins that exhibit diffuse margins against the host olivine-rich troctolites and centimetre-scale spinel-bearing dunitic clots, which are interpreted as the crystallization products of the infiltrating melts and relics of pre-existing olivine-rich material, respectively. The incompatible element signature of clinopyroxene from the olivine-rich troctolites shows that the infiltrating melts were chemically similar to mid-ocean ridge basalt (MORB). Spinel from the olivine-rich troctolites contains inclusions of Ti-pargasite/kaersutite, phlogopite/aspidolite and orthopyroxene; inclusions of ilmenite and loveringite are also locally present. These spinels most probably formed from hybrid melts derived from interaction between melts rich in incompatible elements and new injections of primitive melt. We propose that this interaction occurred in the mantle, in melt conduits of replacive origin near the base of the conductive thermal boundary layer, and led to formation of a matrix made up of olivine and minor inclusion-bearing spinel. This matrix was subsequently impregnated by MORB-type melts saturated in plagioclase + clinopyroxene to produce the olivine-rich troctolites. The amount of melt added to the olivine-spinel matrix to produce the olivine-rich troctolites is estimated to be ~25 wt%.