Blueschist-facies metamorphism in the Kaczawa Mountains (Sudetes, SW Poland) of th

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• journal_title：Mineralogical Magazine
• Contributor：R. Kryza ; A. P. Willner ; H.-J. Massonne ; A. Muszyński ; H.-P. Schertl
• Publisher：Mineralogical Society of Great Britain and Ireland
• Date：2011-
• Format：text/html
• Language：en
• Identifier：10.1180/minmag.2011.075.1.241
• journal_abbrev：Mineralogical Magazine
• issn：0026-461X
• volume：75
• issue：1
• firstpage：241
• section：Letter

摘要

<p id="p-1">Sodic pyroxene is reported from an Ordovician metatrachyte of the Kaczawa Mountains, SW Poland. Its composition ranges from Jd_0.98Ae_0.02 to Jd_0.15Ae_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°C, 3–13 kbar allow evaluation of the conditions 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°C for the pressure maximum, 6.0±1.0 kbar, 310±20°C for the temperature maximum and 3.5±0.5 kbar, 280±20°C as well as <3.5 kbar, <280°C for the retrograde stages. p><p id="p-2">The metamorphic gradient for the peak-pressure is estimated at ~10°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. p>