MIGMATIZATION AND MELT GENERATION IN THE MONOTONOUS UNIT, MOLDANUBIAN BATHOLITH, CZECH REPUBLIC: IMPLICATIONS OF ZIRCON PB/U (SHRIMP-RG) AGES AND HF ISOTOPE COMPOSITIONS

The Monotonous Unit (or Ostrong Unit) extends from south-central Czech Republic into Austria and consists predominantly of HT-LP cordierite-sillimanite paragneisses and metagraywackes that were intruded by granites of the Moldanubian batholith. Variably migmatized paragneisses are exposed in a quarry near Vanov, along the eastern margin of the batholith. Styles of migmatization exposed in the quarry walls range from stromatic migmatite at one end, grading into migmatite with thin localized melt segregation structures and diatexite, and culminating in a large (10 m x 10 m) melt segregation (“pluton”) at the other end. The pluton is compositionally similar to leucosomes elsewhere in the quarry. Initial eHf (zircon) values range from -5.4 to +1.5 in the pluton, confirming its derivation from an isotopically evolved crustal source. Zircons from the pluton show a complex range of concordant Pb/U ages. The youngest ²⁰⁶Pb /²³⁸U ages average ~330 Ma, which is similar to a Pb/U monazite age of ~329 Ma from a migmatite stope block within the adjacent Mrákotín granite (Žák et al., 2011). Inherited zircons yielded an older range of ²⁰⁶Pb/²³⁸U ages from ~464 to 503 Ma, the youngest of which provides an upper limit to the timing of sedimentation for the paragneiss protolith. A second group of inherited zircons yielded ²⁰⁶Pb/²³⁸U ages of ~559 to 600 Ma (Cadomian-age). Similar populations of inherited zircons have been observed in plutons elsewhere in the Moldanubian batholith, including the ~303 Ma Cerínek pluton, which intruded the Mrákotín granite; it has inherited zircons with concordant ²⁰⁶Pb/²³⁸U ages from ~439 to 493 Ma and one at ~582 Ma, and initial eHf values ranging from -4.1 to +7.6. The similarities between the Cerínek pluton and the melt segregation at Vanov point to a common metapelitic source (i.e., Monotonous Unit). These results suggest that zircon may be a useful tool for determining source regions of other Late Variscan plutons in the Moldanubian batholith.