Hot plutonism in a cold accretionary wedge: What terminated the Cadomian orogeny along the northern periphery of Gondwana?

Abstract

The late Ediacaran to Cambrian end of the Cadomian accretionary orogeny was likely a complex cascade of events that culminated in the opening of the Rheic Ocean and formation of a passive margin along the northern Gondwana periphery. However, the geodynamic causes and kinematics of this protracted transition remain elusive. To address this issue, we examine the emplacement and tectonic setting of the ca. 524–523 Ma, shallow-level Kdyně gabbro to diorite pluton in the southwestern part of the Bohemian Massif as it represents the earliest event following the end of the Cadomian trench-sediment accretion at around 527 Ma. Our structural analysis combined with investigation of the anisotropy of magnetic susceptibility (AMS) suggests that the pluton was emplaced via magma wedging, some minor lateral expansion, and magmatic stoping, when the ascending magma utilized a network of orthogonal extension fractures (∼NNE–SSW and ∼WNW–ESE). Furthermore, the pluton is interpreted as an apical part of a dome located in an extensional stepover between two dextral transtensional zones, forming a large-scale structure resembling a metamorphic core complex. Nevertheless, our data imply that final pluton emplacement was controlled by magma buoyancy and less so by regional tectonic deformation. We suggest that these structural features together with the remarkably short time span between the end of Cadomian accretion and the earliest plutonism in the overall cold forearc region are compatible with slab break-off as the potential geodynamic cause of the necessary rapid heat input into the base of the host accretionary wedge. Taken together, the high-temperature events following Cadomian accretion, represented by the Kdyně pluton and other Cambrian plutons as young as ca. 505 Ma, suggest that magmatism may have been an important geodynamic driver of the Cambro–Ordovician rifting of northern Gondwana and that the rifting mode was active during at least the initial stages before being overridden by the slab pull force of the subducting Iapetus Ocean.