Assembly of the Variscan Orogenic Wedge in the Bohemian Massif: Monazite U‐Pb Geochronology of the Tectonic Events Recorded in Saxothuringian Metasediments

Abstract

AbstractThe geochronology of metasediments incorporated to orogenic wedges provides an important key in understanding the early evolution of collisional systems. This study reveals the timing of Variscan processes in the Saxothuringian orogenic wedge, reflecting transition from oceanic to continental subduction and collision. In situ monazite U‐Pb geochronology and Rare Earth Elements (REE) geochemistry were performed in the Erzgebirge Crystalline Complex on phyllites and micaschists surrounding the ultra‐high‐pressure (UHP) core of the Erzgebirge dome. The resulting ages and REE patterns were linked to the individual tectonometamorphic events and revealed that the hanging‐wall phyllites experienced prograde metamorphism around ∼350 Ma, followed by exhumation at ∼345–340 Ma. The oldest age (∼339 Ma) recorded in garnet cores in the foot‐wall micaschists is considered as an upper age limit for their prograde metamorphism, while matrix monazite ages of ∼330 Ma reflect a significant resetting of the monazite age system. Spatial distribution of metamorphic isograds and ages indicates a phase of accretion of continental material resulting in an inverted metamorphic field gradient in the wedge between ∼360 and ∼340 Ma. This phase was followed by exhumation of a significant portion of buoyant subducted continental material leading to massive ductile thinning of the wedge around ∼335 Ma. Finally, a late Variscan intracontinental deformation was responsible for heterogeneous reactivation and final exhumation of the wedge at ∼330 Ma. It is newly shown that the Saxothuringian wedge can be divided into a younger inner part, formed by micaschists and UHP rocks, and an older outer part, formed by phyllites.