Detrital zircon record of shutdown and migration of Cadomian volcanic arcs in the Bohemian Massif, with implications for Ediacaran to early Cambrian plate kinematics

Abstract

This paper presents new U–Pb detrital zircon ages that constrain the age of deposition of a Cadomian intra-arc succession (Štěchovice Group in the Bohemian Massif) overlying one of the best preserved volcanic arcs (Davle volcanic complex) of the Avalonian–Cadomian belt, which rimmed the northern margin of Gondwana in the Neoproterozoic to Cambrian. The record of this active margin is well preserved, but how subduction terminated along it remains a matter of debate. Our new data from the Štěchovice Group suggest abrupt arc termination in this region followed by turbidite deposition in a short-lived basin at around 570 Ma. However, the ages of voluminous arc-derived graywackes in a neighbouring accretionary wedge suggest arc magmatism continued until ca. 527 Ma. Regional gravity highs inboard of the Davle volcanic arc indicate the presence of unexposed dense rocks that may represent the missing, post-570 Ma Cadomian arc. If so, the arc axis migrated landward, consistent with the presence of intrusive boninites within the older Davle arc, which would have then occupied a fore-arc position and must have been eroded and submerged prior to the deposition of Štěchovice Group. We suggest that the inferred arc migration beginning at around 570 Ma occurred in response to slab flattening due to the arrival of hotter oceanic lithosphere, perhaps in the form of a spreading ridge that migrated eastwards along the Avalonian–Cadomian active margin leaving a dextral transform margin in its wake. Such a ridge is thought to have first impinged on Avalonia to the west at around 600 Ma and later caused arc extinction in the Armorican Massif and Saxothuringian unit at around 570 Ma, with arc migration in the Bohemian Massif at about the same time. The ridge–trench interaction is then inferred for the Bohemian Massif in the early to middle Cambrian (at ca. 527–515 Ma), consistent with ongoing subduction in the more easterly proto-Alps until the mid-Ordovician. How the inferred migration of the ridge–trench–transform system was linked to the movement of Baltica, and whether it could have brought Baltica close enough to supply Mesoproterozoic detritus to Gondwana’s northern periphery remain open questions.