Petrogenesis of some Ligurian peridotites—II. Rare earth element chemistry

REE compositions of nine partially serpentinized Iherzolitic rocks from Liguria have been investigated: eight samples are from the Erro-Tobbio thrust sheet of the Gruppo di Voltri (western Liguria); one sample is from an allochthonous eastern Liguria mass within the External Ligurides. As previously described (Ernst and Piccardo, 1979, Geochim. Cosmochim. Acta43, 219–237), all specimens are spinel (±plagioclase) Iherzolites. They possess major element bulk rock chemistries which are more or less ‘primitive’ or only slightly depleted compared to estimated upper mantle compositions. Mineral chemistries, varying insignificantly from rock to rock, evidently reflect mantle equilibration in the range 1000–1150°C and 14–16 kbar; pervasive retrograde effects testify to later decompression at shallower levels characterized by temperatures exceeding a typical intraplate oceanic geothermal gradient.The investigated lherzolitic samples show concordant REE patterns, LREE-depleted with respect to chondrites. A wide range in values is observed among the LREE: the observed depletion of LREE in most of the samples is accompanied by a Ce-negative anomaly. Inasmuch as the analyzed samples have been affected to a variable extent by serpentinization, REE mobility during hydrous alteration is treated employing both a theoretical approach and natural evidence.Effects of partial melting prior to serpentinization (resulting chiefly in depletion in LREE) can be rationalized by postulating a two-times chondritic REE abundance in the original unaltered peridotitic material and by considering the HREE contents to have been unaffected by hydrous alteration. When comparison is made with REE compositions for residual material after different degrees of partial melting (computed using an equilibrium, non-modal partial melting model and selected REE partition values), the analyzed Ligurian samples show effects of incipient melting on only a small scale, i.e. 1–5%. Judging from their major and REE compositions, the investigated lherzolites cannot have produced substantial amounts of tholeiitic melt, hence they cannot have provided the primary magmas for the oceanic crustal sequences so widespread within the Northern Apennine and Gruppo di Voltri ophiolitic terranes.