First Lu-Hf ages of garnet-bearing lithologies from the Saxonian Granulite Massif

The Saxonian Granulite Massif is an antiformal gneiss dome with granulite facies rocks in the core, enwrapped by uppermost amphibolite to lower greenschist facies metasediments. Their subduction and exhumation histories during the Variscan orogeny are still poorly understood. Samples of two garnet-bearing serpentinites and one eclogite from the dome-core, as well as one garnet-cordierite gneiss from the lowermost schist-cover were dated using Lu-Hf chronometry.

The granulitic core mostly consists of leucocratic granulites with scarce and laterally discontinuous slivers of mafic and ultramafic rocks. All samples were taken at the largest occurrence of ultramafics at the Rubinberg, where granofelsic eclogites (CPx, Grt, Pl, Rt) are found as two 1 m-thick, foliation parallel layers within foliated garnet-bearing serpentinites (Grt, relics of CPx, Amp, Ol, Spl). Garnet is better preserved within eclogites than in serpentinites. Within eclogites, EPMA major element zoning profiles of garnet blasts show a prograde metamorphic path, partially modified by diffusional relaxation leading to flattening of chemical zoning profiles. Complete homogenization of major elements is characteristic for all pyrope-rich, strongly kelyphitic garnets in the serpentinites. Garnet blasts within the garnet-cordierite gneiss (Pl, Grt, Sil, Bt, Crd, Qtz, Ilm, Spl) reveal complex compositional zoning profiles with a prograde metamorphic zonation pattern in the core and a secondary increase of Ca and Mn in the rims.

The eclogite yielded a Lu-Hf age of 347.5±6.7 Ma, which is interpreted to indicate the timing of peak metamorphism within the granulitic core, predating the retrograde exhumation-related fabrics. Ages of serpentinites and gneisses are still pending.