Trace Element Geochemistry of Iron + Titanium-Rich Eclogitic Rocks, Gruppo Di Voltri, Western Liguria

Abstract

New analyses of rare earth and transition elements are summarized for 29 high-pressure metamafic rocks, which occur as lenses and layers in the Beigua serpentinite of the Ligurian Alps. Included are data for 2 metarodingites, 8 eclogites, 5 glaucophanic eclogites, 7 barroisitic amphibolites and 7 porphyroblastic greenschists. Trace element data, similar to major element compositions previously presented, show broadly intergradational chemistries among the several lithologies. Rare earth distributions are relatively flat at 10-30Xchondrite, with LREE relative depletions. Transition elements exhibit a marked decrease in abundance with increasing atomic number, but patterns are characterized by positive anomalies for Ti, Mn + Fe, and Ni. The bulk-rock trace element concentrations are analogous to those of oceanic tholeiites. Similar REE patterns and overlapping ranges, and sensibly identical transition element patterns, demonstrate that a continuous spectrum of compositions is present in the analyzed rocks, thus arguing against the existence of two unrelated and chemically distinct precursor magma types, leucogabbro and melanogabbro. Combined with the discovery of eclogitic mineral assemblages developed in (Mg + Al)-rich lithologies, the hypothesis of profound post-eclogite metasomatism also may be discounted. The extent of retrograde metamorphism does not appear to correlate with trace element distributions. All analytical data are compatible with a model involving intrusion of a differentiating, consanguineous magma series into shallow levels of the Beigua mantle; earlier solidification of portions of this melt series at relatively low oxygen fugacities produced less evolved cumulates consisting principally of calcic plagioclase and magnesian olivine, whereas later, more fractionated melts precipitated abundant ilmenite. Lower REE concentrations and a positive Eu anomaly characterize the former, whereas the more evolved (Fe + Ti)-rich types have flat REE patterns and high concentrations. High-pressure subsolidus recrystallization attests to a subsequent sub-duction event, but eclogitic overprinting failed to obliterate the trace element signature of the earlier igneous fractionation.