Petrochronology by EPMA and automated SEM in the Saxothuringian high pressure nappes of the central and western Erzgebirge

The Saxothuringian Zone of the Variscan orogen is composed of autochthonous and allochthonous domains. Dating of metamorphic events in the domains of the Saxonian Granulite Massiv, and the Münchberg, Frankenberg, and Erzgebirge nappe units is critical for resolving the complex geodynamic evolution during the Variscan orogeny. The in-situ chemical Th-U-Pb monazite (Th, U, Si, LREE, Y, Ca)PO4 dating by electron probe microanalysis (EPMA) has demonstrated its high potential to resolve polyphase metamorphism. The method is based on the premise that monazite inherits negligible amounts of common Pb and that the radiogenic Pb is retained due to very low diffusion rates even at high T [1]. A monazite dating routine, enclosing the analysis of HREE, was performed with a JEOL-8530F, producing 100 - 200 single analyses per sample. Also, energy dispersive x-ray mapping (GXMAP) by automated SEM was used for semiquantitative identification of garnet zonation pattern. Quantitative chemical compositions of garnet and related plagioclase, biotite and muscovite were then measured by EPMA for geothermobarometric estimates by cation exchange and net transfer reactions. Monazite is abundant in lenses of granulitic garnet gneisses ("saidenbachites") in the central Erzgebirge UHP unit. The monazite ThO2*-PbO data straightly define isochrons at around 335±3 Ma. High pyrope (27 mol%) garnet crystallised at 830 °C/19 kbar [2]. Despite such high T, the monazite Y contents are low. In the intercalated MP micaschists, the monazite ThO2*-PbO isochrones appear more diverse, between 334±4 and 344±5 Ma. Furthermore, monazite has been studied in the micaschists and related phyllites of the western Erzgebirge. The monazite ThO2*-PbO data define isochrones between 323±10 and 360±10 Ma, with most samples around 340 Ma, interpreted as the ages of Variscan regional metamorphism. Several samples bear an older minor monazite population at ages between 415 to 432 Ma. Special regard has been dedicated to metapelites within the Pöhla mineralisation. The Eibenstock granite Th-U-Pb monazite isochrone is at 321±2 Ma. In non-mineralised micaschists older monazites (329±8 Ma), and in mineralised parts younger (309±6 Ma) and hydrothermal low-Th monazites are observed.

[1] Montel, J.-M., Foret, S., Veschambre, M., Nicollet, C., Provost, A. (1996): Electron microprobe dating of monazite. - Chem. Geol., 131: 37-51.
[2] Tichomirowa, M., Whitehouse, M., Gerdes A., Schulz, B. (2018): Zircon (Hf, O isotopes) as melt indicator: Melt infiltration and abundant new zircon growth within melt rich layers of granulite-facies lenses versus solid-state recrystallization in hosting amphibolite-facies gneisses (central Erzgebirge, Bohemian Massif). - Lithos 302–303.