Transitional blueschist–epidote amphibolite facies metamorphism in the Frankenberg massif, Germany, and geotectonic implications

Abstract

Bimodal metavolcanic rocks, granitic gneisses and metasediments are associated in the Frankenberg massif, Germany. These rocks are faulted against underlying very low‐grade Palaeozoic sequences and adjacent metamorphic complexes of the Variscan basement. The granitic gneisses record an Rb–Sr whole‐rock isochron age of 461±20 Ma that is taken as at least a minimum protolith age. The bimodal meta‐igneous suites are interpreted to have formed during rifting of the Gondwana continental margin in the Cambro‐Ordovician. The various metamorphic units have all experienced a common P–T history. The peak‐pressure stage is constrained to around 490–520 °C and 10–14 kbar (10–12 kbar being most realistic). The metamorphism proceeded along a clockwise P–T path towards conditions of around 580–610 °C and 7–8.5 kbar at the thermal peak followed by a final low‐pressure overprint which spanned amphibolite facies to prehnite–actinolite facies temperatures. Owing to a secondary Rb–Sr whole‐rock isochron age of 381±24 Ma, interpreted to date the retrograde stage, the whole metamorphic cycle in the Frankenberg massif is ascribed to the late Silurian–early Devonian high‐pressure event widely recorded in the European Variscides. The antiformal complexes bordering the Frankenberg massif underwent a well‐documented early Carboniferous metamorphism, suggesting that the Frankenberg massif constitutes a klippe which was overthrust towards the end of this second metamorphic cycle.