Reconstruction of the mid-Devonian HP-HT metamorphic event in the Bohemian Massif(European Variscan belt)

Mid-Devonian high-pressure(HP)and high-temperature(HT)metamorphism represents an enigmatic early phase in the evolution of the Variscan Orogeny.Within the Bohemian Massif this metamorphism is recorded mostly in allochthonous complexes with uncertain relationship to the major tectonic units.In this regard,the MariánskéLázně Complex(MLC)is unique in its position at the base of its original upper plate(Teplá-Barrandian Zone).The MLC is composed of diverse,but predominantly mafic,magmatic-metamorphic rocks with late Ediacaran to mid-Devonian protolith ages.Mid-Devonian HP eclogite-facies metamorphism was swiftly followed by a HT granulite-facies overprint contemporaneous with the emplacement of magmatic rocks with apparent supra-subduction affinity.New Hf in zircon isotopic measurements combined with a review of whole-rock isotopic and geochemical data reveals that the magmatic protoliths of the MLC,as well as in the upper plate Teplá-Barrandian Zone,developed above a relatively unaltered Neoproterozoic lithospheric mantle.They remained coupled with this lithospheric mantle throughout a geological timeframe that encompasses separate Ediacaran and Cambrian age arc magmatism,protracted early Paleozoic rifting,and the earliest phases of the Variscan Orogeny.These results are presented in the context of reconstructing the original architecture of the Variscan terranes up to and including the mid-Devonian HP-HT event.

P-T path of metamorphism and U-Pb monazite and zircon age of the Kitoy terrane: Implication for Neoarchean collision in SW Siberian Craton

The first data on P-T metamorphic conditions coupled with U-Pb monazite and zircon age obtained for the Neoarchean Kitoy granulite-gneiss terrane(SW Siberian Craton).Alumina gneisses of the Kitoy terrane indicate two-staged metamorphic evolution.The first stage of regional metamorphism(M1)occurred at high-amphibolite facies conditions at T=780-800℃ and P=8-9 kbar.The second stage(M2)belongs to MT-HT/LP type of metamorphism with the wide temperature interval 600-750℃ and pressure 2-4 kbar.Two age peaks were established on the basis of U-Pb monazite and zircon dating in garnet-anthophyllite gneisses.Both of them correspond to the Neoarchean age:the age of M1 falls into the interval of ca.2489-2496 Ma,the age of M2-ca.2446-2456 Ma.The high-temperature metamorphism of the Kitoy block and nearly coeval granitoid magmatism can be an evidence for the Neoarchean collision in SW Siberian craton.