Electron probe petrochronology of monazite- and garnet-bearing metamorphic rocks in the Saxothuringian allochthonous domains (Erzgebirge, Granulite and Münchberg massifs)

Abstract

In the Saxothuringian Zone an unique assemblage of high to ultra-high pressure and ultra-high temperature metamorphic units is associated to medium-to-low pressure and temperature rocks. The units were studied in a campaign with garnet and monazite petrochronology of gneisses, micaschists and phyllites, and monazite dating in granites. P-T path segments of garnet crystallisation were reconstructed by geothermobarometry and interpreted in terms of monazite stability field, EPMA-Th-U-Pb monazite ages, and garnet Y+HREE zonations. One can recognise (1) Cambrian plutonism (512-503 Ma) with contact metamorphism in the Münchberg Massif. Subordinate monazite populations may indicate a (2) widespread but weak Silurian (444-418 Ma) thermal event. A (3) Devonian (389-360 Ma) high pressure metamorphism prevails in the Münchberg and Frankenberg Massifs. In the ultra-high pressure and high pressure units of the Erzgebirge the predominant (4) Carboniferous (336-327 Ma) monazites crystallised at the decompression paths. In the Saxonian Granulite Massif, prograde-retrograde P-T paths of cordierite-garnet gneisses can be related to monazite ages from 339 to 317 Ma. A (5) local hydrothermal overprint at 313-302 Ma coincides partly with post-tectonic (345-307 Ma) granite intrusions. Such diverse monazite age pattern and P-T-time paths characterise the tectono-metamorphic evolution of each crustal segment involved in the Variscan Orogeny. Supplementary material at https://doi.org/10.6084/m9.figshare.c.6793959