Assessing the Valais ocean, Western Alps: U-Pb SHRIMP zircon geochronology of eclogite in the Balma unit, on top of the Monte Rosa nappe

Abstract

Recent tectonic studies suggest that the Balma unit, a nappe sliver of eclogite and serpentinite directly overlying the Monte Rosa nappe in the Western Alps, represents a remnant of the Cretaceous Valais ocean. In order to test this hypothesis, U-Pb SHRIMP dating was carried out on zircons from an eclogite of the Balma unit. For the eclogite studied, PT conditions of 500–590 °C, 13–14.5 kbar (minimum P) are inferred by geothermobarometry. Oscillatory zoned (co-magmatic) zircon domains yielded a 206 Pb/ 238 U weighted mean age of 93.4 ± 1.7 Ma, interpreted as the time of magmatic crystallization of the gabbroic protolith. Metamorphic zircon domains yielded a 206 Pb/ 238 U weighted mean age of 40.4 ± 0.7 Ma. These domains contain inclusions of rutile and garnet displaying a composition similar to the rim of the matrix garnet. ’In situ’ LA-ICPMS trace/REE analyses revealed low Th, Nb and Ta contents of the metamorphic zircon domains indicating concurrent growth with the HP phases clinozoisite (a sink for Th) and rutile (a sink for Nb, Ta). The lack of a negative Eu anomaly in the chondrite-normalized REE pattern of metamorphic zircon points to the absence of plagioclase at the time of metamorphic zircon formation. Thus, formation of metamorphic zircon took place probably at the peak of metamorphism or the early retrograde stage at HP. The 40.4 ± 0.7 Ma age is therefore ascribed to HP metamorphism. The 93.4 ± 1.7 Ma protolith age of the eclogite dated is identical to protolith ages reported for metabasic rocks of the Chiavenna ophiolites (Central Alps), attributed to the Valais ocean. The 40.4 ± 0.7 Ma HP metamorphism is marginally older than the HP metamorphism in the Valais-derived Antrona ophiolites, Western Alps, and in the Chiavenna ophiolites, Central Alps. The data provide a correlation between these units and indicate that the Cenomanian-Turonian ocean floor formation in the Valais ocean is more widespread than previously assessed. Our geochronological data (both the 93.4 ± 1.7 Ma time of crystallization and the 40.4 ± 0.7 Ma time of metamorphism) independently arrive at the conclusion of the tectonic studies that the Balma unit is part of the Valais ocean. The 40.4 ± 0.7 Ma age of metamorphism of the Balma unit fits well the model suggesting that the Valais rocks were metamorphosed earlier than those of the European margin ( ca. 35 Ma) and later than the Piemont-Ligurian oceanic rocks ( ca. 44 Ma). The Balma unit very likely represents part of the youngest oceanic crust in the Valais ocean.