Blueschist-facies metamorphism in the Kaczawa Mountains (Sudetes, SW Poland) of the Central-European Variscides: P-T constraints from a jadeite-bearing metatrachyte

Sodic pyroxene is reported from an Ordovician metatrachyte of the Kaczawa Mountains, SW Poland. Its composition ranges from Jd(0.98)Ae(0.02) to Jd(0.15)Ae(0.85). Relict jadeite and phengite (up to 3.75 Si atoms per fomula unit) belong to the peak-pressure assemblage of an early HP-LT event. Later greenschist-facies stages are represented by riebeckite, biotite, chlorite, low-Si potassic white mica and actinolite. P-T pseudosections calculated for the range 200-450 degrees C, 3-13 kbar allow evaluation of the conditions of formation of jadeite in the metatrachyte and derivaton of a P-T path. Considering the position of prograde, peak and retrograde metamorphic assemblages and respective mineral compositions, we can derive the following equilibration stages: 8.5 +/- 0.5 kbar, 270 +/- 20 degrees C for the pressure maximum, 6.0 +/- 1.0 kbar, 310 +/- 20 degrees C for the temperature maximum and 3.5 +/- 0.5 kbar, 280 +/- 20 degrees C as well as <3.5 kbar, <280 degrees C for the retrograde stages. The metamorphic gradient for the peak-pressure is estimated at similar to 10 degrees C/km, which is typical of a subduction setting involving subducted continental crust, in particular of an exhumation channel within a collision zone of a microplate. Based on earlier structural observations, the ESE-oriented subduction in the NE Bohemian Massif was confined with WNW thrusting and followed by extension and ESE backward normal faulting during Devonian-Early Carboniferous times.