Assessing the Valais Ocean, Western Alps; U-Pb SHRIMP zircon geochronology of eclogite in the Balma Unit, on top of the Monte Rosa Nappe

For an eclogite of the Balma unit, a sliver of eclogite and serpentinite directly overlying the Monte Rosa nappe, P-T conditions of 500-590 degrees C, 13-14.5 kbar were inferred from geothermobarometry on zircons. Oscillatory zoned (co-magmatic) zircon domains yielded a (super 206) Pb/ (super 238) U mean age of 93.4 + or - 1.7 Ma, interpreted as the time of crystallization of the gabbro protolith. Metamorphic zircon domains yielded a (super 206) Pb/ (super 238) U mean age of 40.4 + or - 0.7 Ma. LA-ICP-MS trace and REE analyses revealed low Th, Nb and Ta contents for the metamorphic zircon domains, indicating concurrent growth with the high-P phases clinozoisite (a sink for Th) and rutile (a sink for Nb, Ta). It is considered that the formation of the metamorphic zircon probably took place at the peak of metamorphism of the early retrograde stage at high pressures (i.e. at 40.4 + or - 0.7 Ma). The 93.4 + or - 1.7 Ma protolith age of the eclogite is identical to protolith ages reported for the metabasic rocks of the Chiavenna ophiolites, and the 40.4 + or - 0.7 high-P metamorphism is marginally older than that in the Valais-derived Antrona ophiolites. The geochronological data both for the time of crystallization and the time of metamorphism indiate that the Balma unit is part of the Valais ocean.