Equivocal carbonatite markers in the Mantle xenoliths of the Patagonia Backarc: the Gobernador Gregores case (Santa Cruz province, Argentina).

SRef-ID: 1607-7962/gra/EGU04-A-03921.Mantle xenoliths at Gobernador Gregores consist of spinel-facies anhydrous, depleted lherzolites and harzburgites and, more abundantly, of hydrous (amphibole +/- phlogopite +/- apatite-bearing) lherzolites, harzburgites and wehrlites. The hydrous xenoliths display modal and geochemical bulk-rock and mineral phase characteristics held as indicators of carbonatitic metasomatism. However, several lines of evidence do not support carbonatitic metasomatism. For instance, a modal trend towards wehrlite is shown by hydrous xenoliths having high TiO2/Al2O3, whereas the samples with low TiO2/Al2O3 modally trend towards harzburgite. Carbonatitic geochemical features characterize both groups, but are more marked in the harzburgites of the second group. The entity of the Zr, Hf and Ti negative anomalies in bulk-rock, clinopyroxene and amphibole steadily increase at decreasing Na2O and HFSE concentrations. Steady variation trends between xenoliths do not having and having carbonatitic characteristics rules out the possibility that carbonatitic metasomatism overprinted on, or was followed by, a basaltic metasomatism that affected a previously depleted mantle lithosphere. They, rather, suggest a control by reactive porous flow of only one agent, inferred to be initially a ne-normative hydrous basalt (because of the presence of wehrlites) evolving towards silica saturation. Variation trends exhibit cusps when amphibole appears in the mode. Appearance of amphibole may explain the Ti anomaly variations, but not those of Zr and Hf. Numerical modelling (Plate Model; Vernières et al., 1997) gives results consistent with the observed geochemical features by assuming the presence of loveringite. ModestHFSE anomalies in the infiltrating melt may also be acquired during percolation in the garnet-facies peridotites beneath the spinel-facies level documented by the xenoliths sampled by the basalts, or directly from the asthenospheric melt source fertilised by component derived from the eclogitic subducted slab.ReferenceVernières J., Godard M., Bodinier J-L. (1997) J. Geophys. Res. 102, 24771-24784.