New evidence of blueschist facies rocks and their geotectonic implication for Variscan suture(s) in the Bohemian Massif

Abstract

AbstractBlueschist facies rocks, exposed within consolidated continental blocks, provide some of the best evidence for the existence of previous suture(s). They usually occur as lenses or layers embedded within greenschist or amphibolite facies rocks and indicate reequilibration at medium‐ to low‐pressure conditions. In the Bohemian Massif, a few occurrences of blueschists have been reported, and here, new evidence of high‐pressure (HP) metamorphism in various lithologies is presented that suggests a larger extent of blueschist facies rocks along the northern border of this Massif. An earlier blueschist facies metamorphism is documented by inclusions of glaucophane in garnet, epidote and titanite from metabasites along with zoned white mica having a phengitic core and a muscovite rim in metapelites and orthogneisses. The estimated P–T conditions, obtained using pseudosections and mineral isopleths, correspond to blueschist and low‐temperature eclogite facies conditions (1.1–2.0 GPa at 350–550 °C). Together with medium‐temperature eclogites from different units in the Bohemian Massif they indicate a geothermal gradient of 8–10 °C km−1, which is typical for cool subduction. Radiometric dating on phengite from metapelites confirms an early Palaeozoic cooling age of c. 360 Ma for this HP metamorphic event. The presence of blueschist facies rocks, their P–T relations and age constraint together with those from eclogite facies rocks allows us to locate the Variscan suture, which straddles the SE margin of Saxothuringian Zone from Erzgebirge to Sudetes, and its possible continuation to the Moldanubian Zone, where eclogite facies and UHPM rocks are abundant.