Common high-pressure metamorphic history of eclogite lenses and surrounding metasediments: a case study of calc-silicate reaction zones (Erzgebirge, Germany)

The Erzgebirge dome in the Central European Variscides is a stack of crustal slices including some high- and ultrahigh-pressure rocks related to continent-continent collision. One such slice is the Mica-Schist/Eclogite Unit, in which the predominantly metasedimentary country rocks appear to record maximum metamorphic pressures of at most 11-13 kbar, whereas volumetrically minor eclogite lenses within the unit indicate pressures up to 27 kbar. In the present study, the P-T evolution of rocks found in calc-silicate reaction zones between eclogite and country rock marble near the locality Stumpelfelsen, a crag composed of eclogite near the village of Hammerunterwiesenthal, has been established via conventional geothermobarometry. A metasomatic marble-eclogite interchange can be documented for the earliest stages of prograde metamorphism. Significant amounts of fluorine infiltrated the metabasic rocks from the marbles, leading to fluorine-bearing amphibole as well as phengite, and even fluorine-rich growth zones in garnet with 0.62 wt.% F and 1.2 wt.% OH. This appears to be the first description of a F-bearing member of an almandine-grossular solid solution poor in andradite component. The P-T path of the eclogite-marble reaction zone reaches a maximum pressure of 26 kbar at 520-640 degrees C, just below the quartz-coesite transition. The exhumation path can be traced to 10 kbar and 450-600 degrees C, where it is then coincident with the published P-T paths of the mica schists and orthogneisses of the Mica-Schist/Eclogite Unit. This study indicates that the Stumpelfelsen eclogite lens and the surrounding metasedimentary country rock of the area must share a common high-pressure metamorphic history. The critical question arising for future studies is how much of the Mica-Schist/Eclogite Unit has actually travelled the high-pressure eclogite route, and how much of it was never subducted to pressures greater than 11-13 kbar.