Timing of Variscan syn-collisional metamorphism constrained by Lu-Hf and Sm-Nd garnet petrochronology (The Tatra Mountains, Western Carpathians)

The Variscan high-pressure (HP) eclogite and medium pressure (MP) metapelitic gneisses from the Tatra Mts. crystalline basement (Western Carpathians, Slovakia) were dated by means of the Lu-Hf and Sm-Nd methods. The Lu-Hf and Sm-Nd garnet dating of mafic eclogite and metapelitic gneisses provides a new set of precise ages on Variscan syn-collisional metamorphism in the Western Carpathians. The published P-T conditions document peak eclogite facies metamorphism at 15-17 kbar and 650-695 degrees C, whereas garnet-sillimanite gneisses reached their climax at 5-6 kbar and 650-700 degrees C. Lu-Hf bulk garnet dating gave the precise ages of 354.5 +/- 1.2 Ma for eclogite, while the ages of 346.7 +/- 1.0 Ma and 345.9 +/- 1.0 Ma were determined for metapelites. The Sm-Nd dating of the same garnet fractions from metapelites gave the ages 344.9 +/- 2.6 and 345.4 +/- 9.3 Ma. Both the Sm-Nd and Lu-Hf ages are indistinguishable within their uncertainties, and so, when considering the major and trace element zonation in garnet, we interpret them as dating the time of prograde garnet crystallization. The data therefore indicates high-pressure, eclogite facies metamorphism in the Tournaisian, and medium-pressure metamorphism in Visean times. We interpret the eclogite facies metamorphism as a record of initial Variscan continental collision in the Early Carboniferous. Early metamorphism in the eclogite facies is a deeper expression of the subducting slab, and thus, garnet started to grow in eclogite ca. 10 Myr earlier than that in the sillimanite-bearing gneiss in a shallower level.