Two new Emsian (Early Devonian) U–Pb zircon ages from volcanic rocks of the Rhenish Massif (Germany): implications for the Devonian time scale

Abstract

So far, only few biostratigraphically well-constrained isotopic ages constitute the geochronological framework of the Devonian period. To expand this sparse dataset, two new U–Pb isotope dilution–thermal ionization mass spectrometry (ID-TIMS) zircon ages were obtained from volcanic rocks of the Rhenish Massif (Germany). These rocks are interbedded in Emsian (Lower Devonian) marine sedimentary successions that are well documented by index fossils. Given that the U–Pb zircon data represent primary crystallization and eruption ages, they provide an important contribution to the Devonian time scale. The older sample, a stratiform volcaniclastic layer (Hans-Platte) is collected from Bundenbach (Hunsruck), the type locality of the Lower Emsian Hunsruck Slate. A cluster of five coherent single-zircon analyses obtained from this layer yielded a weighted mean 206 Pb/ 238 U age of 407.7 ± 0.7 Ma, which is regarded as the time of eruption of the pyroclastic rock. Biostratigraphically, the layer can be assigned to the upper part of the Polygnathus excavatus conodont zone. The younger U–Pb age was obtained from the Global Stratotype Section and Stratigraphic Point section at Wetteldorf (Eifel), where the K-bentonite ‘Hercules I’ is intercalated 13 m below the Lower–Middle Devonian boundary in the uppermost part of the Polygnathus costatus patulus conodont zone. Some of the 19 single-zircon analyses obtained from this bentonite appear to document significant influences of inheritance. The 13 youngest points are, however, concordant and form an elongated cluster along concordia with 206 Pb/ 238 U ages between 392.2 and 407.7 Ma. Given the (risky) assumption that this age scattering is exclusively caused by varying amounts of inheritance, the youngest point of this cluster represents the analysis with the least or no inheritance. In this case, its 206 Pb/ 238 U age of 392.2 ± 1.5 Ma should approach the eruptional age of the bentonite. The ages presented here in combination with U–Pb ID-TIMS zircon and monazite ages from other workers allow a recalibration of the early Emsian to early Eifelian interval of the Devonian time scale. The result is a duration of the Emsian stage (excluding the basal Emsian P. kitabicus conodont zone) of more than 17.2 Ma and an Emsian–Eifelian (= Early–Middle Devonian) boundary age of 391.8 ± 0.4 Ma.