Multi-stage evolution of peridotites from New Caledonia: preliminary results

The Peridotite Nappe of New Caledonia, presumably of Late Cretaceous-Early Eocene age, is one of the largest and best exposed ophiolitic complex in the world. It is largely dominated by harzburgite and dunite but it also includes lherzolites in northern massifs and mafic/ultramafic cumulates in the south of the island. Although the mantle rocks have been studied for almost 30 years, their history still remains controversial, partly because of the scarcity of geochemical data and the total lack of isotopic data. Recent studies (Marchesi et al., 2009; Ulrich et al., 2010) proved that these mantle rocks have experienced a complex evolution including different phases of melting, melt-rock interaction and re-melting that led to an overall ultra-depleted composition. Here, we present the preliminary results of a petrological and geochemical study on a new set of peridotite samples from New Caledonia. Harzburgites, consisting of variable proportions of olivine (from 70-75% up to > 80 vol %), orthopyroxene (from 12-15% to 25 vol %) and Cr-rich spinel ('1% or less), are highly refractory rocks, as attested by the absence of primary clinopyroxene, very high Fo content in olivine (90.7-92.9 mol%), high Mg# in orthopyroxene ([Mg/(Mg+Fe)]= 91.0-92.7) and Cr# in spinel ([100 • Cr/(Cr+Al)]= 40-71). In contrast, lherzolites (clinopyroxene = 5-10 vol %), display a fairly fertile nature, with lower Fo in olivine (88.5-91.8 mol%), low Cr# in spinel (0.132-0.167) and relatively high Na2O (up to 0.80 wt%) and Al2O3 (3.1-6.7 wt%) contents in clinopyroxene. Secondary, interstitial and undeformed clino- and orthopyroxenes have also been observed in harzburgites. These phases testify melt percolation after partial melting and re-equilibration at lithospheric conditions. Their chemical compositions, i.e. low Al2O3 and CaO contents in orthopyroxene and very low or negligibile Na2O and TiO2 in clinopyroxene, may suggest a metasomatic origin by SiO2-rich fluids and/or depleted melts in a subduction-related setting. Mineral compositional variations (e.g. Mg# (Ol) vs Cr# (Spl) and Cr# (Spl) vs Mg# (Spl)), show that most investigated harzburgites plot in the field of SSZ (forearc) peridotites, whereas the lherzolites are more akin to (variably refertilized) abyssal peridotites. The peridotites are low strain tectonites with porphyroclastic textures partially overprinted by mosaic equigranular textures, probably recording an asthenospheric HT origin followed by sub-solidus re-equilibration. Geothermometric estimates provide temperatures of 930–1145°C and 870- 1080°C for the porphyroclastic assemblages of harzburgites and lherzolites, respectively; lower temperatures are recorded for the spinel facies recrystallization (' 830°C-980°C for both lithotypes). These preliminary results are consistent with a multi-stage history of melting, deformation, recrystallization and melt-rock interaction. Geochemical and radiogenic isotope analyses (in progress) are expected to decipher the depletion vs. refertilization evolution of the different peridotite types and provide constrains on their geodynamic significance.