Evidence of glaucophane-schist facies metamorphism in the East Karkonosze complex, West Sudetes, Poland

Abstract

In the East Karkonosze complex (Karkonosze = Riesengebirge), which occurs at the northern margin of the Bohemian massif, rocks of the glaucophane-schist facies and transitions between the glaucophane-schist facies, greenschist facies and epidote-amphibolite facies are present. They belong to the Leszczyniec Volcanic Formation (LVF) of Cambrian/Ordovician age and to the mainly metasedimentary Czarnow Schist Formation (CSF) of Ordovician/Silurian age. Similar high-pressure, low-temperature rocks occur in the southern Karkonosze and in the Kaczawa Mountains within metavolcanic formations of approximately the same age. Petrographic and electron probe studies show complex relationships between minerals including chemical zoning. In the East Karkonosze three stages of metamorphism pre-dating contact metamorphism by late Variscan (lowermost Upper Carboniferous) granite intrusion were distinguished [stage 1: ocean floor, amphibolite facies (observed only in part of the LVF); stage 2: high-pressure, low-temperature, variably glaucophane-schist facies, high-pressure greenschist facies and epidote-amphibolite facies; stage 3: medium-pressure greenschist facies accompanied by strong deformations]. Glaucophane-schist facies rocks formed in stage 2 survived the later stages of metamorphism only in the southern part of East Karkonosze, i. e. in Lasocki Range and Rýchory. Using the Maruyama et al. (1986) geobarometer the glaucophane-bearing rocks formed at 6.5–7 Kb, those with crossite at 5–6 Kb and rocks with magnesioriebeckite/riebeckite at 4–5 Kb. Other estimates for glaucophane-bearing rocks give somewhat higher values of pressure, i. e. 7–12 Kb at temperatures between 300 and 530°C. The highest temperatures are recorded in the glaucophane- and garnet-bearing rocks. Stilpnomelane may occur in all of these rocks. The subduction/obduction episode responsible for this high-pressure, low-temperature metamorphism is considered to have taken place in the early Variscan, although no geochronology is yet available to confirm this.