Abstract In the Saxothuringian Zone, a unique assemblage of high- to ultra-high-pressure and ultra-high-temperature metamorphic units is associated with 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 crystallization were reconstructed by geothermobarometry and interpreted in terms of the monazite stability field, EPMA Th–U–Pb monazite ages and garnet Y + HREE zonations. One can recognize (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 crystallized 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 characterize the tectono-metamorphic evolution of each crustal segment involved in the Variscan Orogeny.

In this chapter, we evaluate how the incorporation of H 2 O as a thermodynamic component influences phase relations in a peridotite composition. This component – present either in the form of hydrous minerals, aqueous fluids and hydrous melts, or as a structurally-bonded trace element at defect sites of nominally anhydrous minerals (NAMs) – may influence upper-mantle rheology in diverse ways. By presenting various natural cases, we identify key incorporation mechanisms and assess their role in the microstructural evolution of ultramafic rocks at different depths in the Earth’s interior. These data suggest that the influence of either aqueous fluids or hydrous melts on rheology out-matches that of NAMs or stable hydrous phases across much of the lithospheric mantle. Consequently, future research is expected to shift towards a better understanding of the transient conditions in the lithosphere that control the availability and transport of aqueous fluids and hydrous melts. These transient conditions are likely to play a more dominant role than the sole ability of hydrous defects in NAMs – a role that is currently less well-constrained experimentally – in controlling the ductile deformation of the upper mantle.