Lu-Hf garnet geochronology of eclogites from the Balma Unit (Pennine Alps): implications for Alpine paleotectonic reconstructions

Abstract

Three samples of eclogite from the Balma Unit, an ophiolite sheet on top of the Monte Rosa Nappe in the Pennine Alps, were investigated in terms of their P-T evolution, geochemistry, and Lu-Hf geochronology. The paleogeographic origin of this unit is controversial (North Penninic vs. South Penninic). It has been interpreted as a piece of Late Cretaceous oceanic crust, on the basis of ca. 93 Ma U-Pb SHRIMP ages of synmagmatic zircon cores in an eclogite. Trace element and isotope data suggest a mid ocean ridge (MOR) rather than an intraplate or OIB setting for the protoliths of the eclogites. Electron microprobe analyses of representative garnets show typical prograde zoning profiles. Estimated peak metamorphic temperatures of 550–600 Cº most likely did not exceed the closure temperature of the Lu-Hf system. Hence, Lu-Hf ages most likely reflect garnet growth in the studied samples. To minimize inclusion effects on age determinations, a selective digestion procedure for garnet was applied, in which zircon and rutile inclusions are not dissolved. The ages obtained for three samples, 42.3 ± 0.6 Ma (MSWD: 0.47), 42 ± 1 Ma (MSWD: 3.0) and 45.5 ± 0.3 Ma (MSWD: 0.33), are younger than all Lu-Hf ages reported so far for South Penninic Units. Metamorphic zircon domains of the 42.3 Ma sample (PIS1) were previously dated by U-Pb SHRIMP at 40.4 ± 0.7 Ma, indicating that the growth of metamorphic zircon post-dated the onset of garnet growth.

These new data put important constraints on the paleogeographic reconstruction of the Alps. The MORB character of the rocks, together with their previously published protolith age, imply that oceanic spreading was still taking place in the Late Cretaceous. This supports a North Penninic origin for our samples because plate tectonic models predict Cretaceous spreading in the North Penninic but not in the South Penninic Ocean. If the Balma Unit is indeed North Penninic, the new Lu-Hf data, in combination with published geochronological data, require that two independent subduction zones consumed the South and North Penninic oceans.