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

Abstract

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 °C, just below the quartz–coesite transition. The exhumation path can be traced to 10 kbar and 450–600 °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.